The pharmacology of pholcodine

Sections

Introduction
Part one. Nomenclature
Part two. Physico-chemical study
Part three. Acute toxicity
Part four. Chronic toxicity and addiction liability
Part five. Pharmacodynamic effects on anaesthetized animals
Part six. Effect on the central nervous system
Part seven. Discussion
Summary and conclusions

Details

Author: Raymond Cahen
Pages: 19 to 37
Creation Date: 1961/01/01

The pharmacology of pholcodine

M.D., Ph.D Raymond Cahen Formerly Chief of Laboratory, Faculty of Medicine, Paris; Visiting Professor at Yale University; Scientific Director of the Lafon Research Centre, Maisons-Alfort, France

Introduction

Since the dangers of morphine addiction were observed, chemists have redoubled and consistently pursued their efforts to discover a substitute for morphine which would not produce addiction phenomena.

Hundreds of derivatives have been synthetized and studied from the pharmacological point of view. Nevertheless, none of those which have an analgesic effect comparable with that of morphine lacks addiction-producing properties.

β-4-morpholinylethylmorphine is one of a small group of non-addiction-producing narcotic drugs. J. Delay in Paris and H. Isbell in Lexington demonstrated that this derivative produces neither tolerance nor addiction. On the basis of these observations, the Expert Committee of the World Health Organization on Drugs liable to produce Addiction have classified β-4-morpholinylethylmorphine in group II of the 1931 Convention. The international non-proprietary name of "pholcodine" was given to β-4-morpholinylethylmorphine by the World Health Organization.

The pharmacodynamic study of pholcodine has been succinctly summarized by Eddy [ (21)] . Chabrier, Giudicelli & Thuillier [ (15)] have established that pholcodine has a low toxicity in mice; that it has an inhibiting effect on the respiratory and pain centres in rabbits; and that it has no contracturating effect on a guinea-pig's isolated intestine. The same authors have pointed out the anti-tussive effects of pholcodine, which have also been observed in cats by May & Widdicombe [ (43)] and Green & Ward [ (31)] . Clinical tests have fully confirmed these results (Giudicelli, Chabrier & Thuillier [ (28)] ). The analgesic effect observed in rabbits [ (14)] has, however, been described by Eddy [ (21)] as much less than that of codeine in the case of mice.

When studying the homologues of codeine in 1955 we observed in various laboratory animals certain pharmacological effects of pholcodine which had not yet been demonstrated. This led us to make a systematic study of pholcodine; it is well known that no new drugs can be accepted in therapy on the basis of experiments conducted on one animal species only and on the basis of a single test.

This study therefore relates to the investigation of the acute and chronic toxicity of pholcodine, and to a comparative study of its effects and those of morphine and codeine on the cardio-vascular, respiratory and intestinal systems and on the central nervous system, from both the physiological and the psychological points of view.

Our sincere thanks are due to our assistants, Christie Krementz, Edouard Grosinsky and Peter Parisek at Hillside (USA) and Simone Aubron, Jacqueline Clavel, Jacques Chariot and André Pessonnier at Maisons-Alfort, whose technical help has made the success of our experiments possible.

Part one. Nomenclature

The wide variety of names under which narcotic drugs appear in trade and technical literature is well known. In order to remedy this confusion, which complicates the task of the control authorities, a list was drawn up by the United Nations Secretariat (40). The common appellation of β-4-morpholinylethylmorphine is pholcodine.

The list of the fourteen synonyms for "pholcodine" is as follows: beta-4-morpholinylethylmorphine; ethnin; ethnine; folcodina; homocodeine;

6-hydroxy-N-methyl-3-(2-morpholinoethoxy)-4,5 epoxymorphinen-7;

MEM,

3-(2-morfolinoetossi)-4,5-epossi-6-idrossi-N-metil-7-morfinene;

β-morfolinoetylmorfin; morfolinylaetylmorfin; morpholinylethylmorphine; morpholinyl ethyl morphine (Fr.); pholcodin; pholcodinum.

In addition to the alkaloid, the Multilingual List of Narcotic Drugs under International Control also mentions pholcodine hydrochloride (chlorhydrate de pholcodine) and pholcodine phenylacetate (phenylacetate de pholcodine), the synonym for which is hibernyl.

The general appellation of "pholcodine" has been adopted by the Committee for the French Pharmacopoeia.

Part two. Physico-chemical study

The physico-chemical data established by Chabrier and his collaborators [ (13)] 1 may be briefly summarized as follows:

Pholcodine is an amino alkyl-ether of morphine, resulting from the substitution of the radical morpholinylethyl for the phenol hydrogen of morphine. Its developed structure compared with that of codeine is as follows:

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I. PHYSICAL PROPERTIES

According to Chabrier [ (15)] , pholcodine appears in the form of colourless micro-crystals, with a melting point varying between +90° and 91°.It is odourless, with a slightly bitter taste and is soluble in 24.1 parts water (20°), 8.3 parts water (100°), 2.06 parts alcohol at 95° and 1.52 parts chloroform.

Morpholinylethylmorphine is laevogyrate in a 2% alcohol solution (αD = -94.5°).

II. CHEMICAL PROPERTIES

Pholcoline is a base, with an alkaline reaction, but cannot be accurately titrated by acidimetric means, using coloured

1

We wish to thank Mr. Charles Genot and Mr. René Giudicelli for kindly communicating this information to us

indicators. With methyl iodide in an acetonic solution, it forms a di-iodomethylate with a melting-point of +240°; at 100°, it loses its water of crystallization and becomes syrupy. When heated to a temperature of 150°with concentrated hydrochloric acid, it hydrolyses less easily and forms a smaller quantity of apomorphine than codeine and dionine.

Pholocdine

III. ANALYTICAL PROPERTIES

Pholcodine reacts to the usual reactors of alkaloids. Warmed on a water-bath with concentrated sulphuric acid, it assumes a brownish-purple colour. If two drops of concentrated sulphuric acid and a trace of sodium selenite are added to a few milligrammes of pholcodine, a greenish-blue-green colour develops. A saturated solution of pholcodine does not turn amber-coloured when hydriodic acid is added, as in the case of morphine.

Part three. Acute toxicity

Chabrier, Giudicelli & Thuillier [ (15)] noticed that in mice pholcodine has a lower intravenous, intraperitoneal and subcutaneous toxicity than the most widely used cough sedatives.

We have pursued this study on three animal species - rats, mice and dogs - using the intraperitoneal, gastric and subcutaneous methods of administration.

Our tests were based on comparison with codeine phosphate.

The alkaloids were administered by different methods in doses increasing in logarithmic proportions. In order to determine the average effective dose, we used the graphic method of Litchfield & Wilcoxon [ (41)] , which made it possible to determine also the standard error and confidence limits of 19/20 probability, the accuracy of the method by the X2 test and the slope of the curve. The results were deemed acceptable when the P probability was less than or equal to 0.02. These results are summarized in table I, which also gives the results obtained by Chabrier and his colleagues [ (15)] . Finally, the toxicity of pholcodine was determined in relation to codeine, the latter being taken as equal to 1.

TABLE I

Comparative acute toxicity of pholcodine and of codeine determined according to various methods of administration

 

Pholcodine

Codeine phosphate

Codeine

 
Animals
Number
Method of administration
LD 50mg/kg
19/20 Confidence limits
LD 50
19/20 Confidence limits
LD 50
Relative toxicity
Rats
147
Subcutaneous
0.750
_
0.530
_
0.40 1.88
  186
Gastric
0.205
_
0.47
_
0.33 0.62
  80 ,, 0.114 0.071 0.49 0.395 0.34 0.34
        0.42   0.607
_
 
Mice
222
Subcutaneous
1.02
_
0.218   0.153 6.6
 
?
Chabrier
1.01
_
    0.134 7.6
  230
Gastric
1.48
_
0.335   0.249 5.95
  60 ,, 1.86 1.56 0.47 0.41 0.33 5.65
 
-
,,   2.11   0.53    
 
?
Chabrier
1.70         7.25
  60
Intraperitoneal
0.59 0.49 0.155 0.026 0.108 6
        0.70   0.911    

I. RATS

Experiments were conducted on male albino Wistar rats.

  1. Subcutaneous method. Codeine base is 1.9 times more toxic than pholcodine. Pholcodine is 1.9 times less toxic than codeine base.

  2. Gastric method. In this case pholcodine is more toxic than codeine base.

In a first series of experiments it was found that the toxicity of codeine is 0.62 that of pholcodine. In a second series it was found that the toxicity of codeine is 0.34 that of pholcodine.

II. MICE

Female Webster mice were used.

  1. Subcutaneous method. Pholcodine is 6.6 times less toxic than codeine. The results confirm those obtained by Chabrier [ (15)] , who observed that pholcodine was 7.6 times less toxic than the codeine base.

  2. Gastric method. Pholcodine is 5.65 to 5.95 times less toxic than codeine. Our results agree with those obtained by Chabrier [ (15)] .

  3. Intraperitoneal method. Pholcodine is six times less toxic than codeine.

To sum up, the toxicity of pholcodine is low. It is less toxic than codeine, except when administered gastrically in rats. In that particular case pholcodine was found to be more toxic than codeine. Working on phenytoine with Good-man [ (29)] , we observed a similar inversion of toxicity in the same animal by the same method of administration. These phenomena have been explained by the limited gastrointestinal absorption capacity of rats. These facts confirm the importance of not limiting the determination of toxicity to a single animal species and to a single method of administration.

III. DOGS

Doses of pholcodine varying between 7.5 mg and 150 mg/kg were administered to 12 mongrel dogs of both sexes, weighing between 6 and 12 kg. None of these doses proved lethal. On the other hand, a dose of 150 mg/kg of codeine phosphate proved lethal. A 300 mg/kg dose of pholcodine given to two dogs was fatal to one of the animals and a dose of 450 mg was toxic to both the animals treated.

It would therefore seem that pholcodine is an alkaloid with low toxicity. Nevertheless, in order to determine with certainty its harmlessness over a long period of administration, a systematic study had to be made to determine its chronic toxicity, in accordance with principles recently summarized [ (6)] .

Part four. Chronic toxicity and addiction liability

The subacute toxicity of pholcodine has been determined by Chabrier, Giudicelli & Thuillier [ (15)] in mice by intraperitoneal injection for 30 consecutive days (except Sundays) of a single dose of 0.05 g/kg; the authors observed no abnormal symptoms.

Our experiments in chronic toxicity were carried out on another animal species, young albino Wistar rats under the age of puberty, by daily administration for 90 days. This species, which has been widely studied in determining the chronic toxicity of various drugs, lends itself to the establishment of very accurate criteria of intolerance and toxicity. The gastric method of administration was used, as is nearly always done in clinical tests.

I. CHRONIC TOXICITY

A . Method

The method used has been described previously (6). Briefly, the three principles are as follows: (1) experimentation on young rats during the period of their most rapid growth when they are most sensitive to toxic substances; (2) use of animals with a constant sensitivity, preferably animals of the same stock from a pure-strain colony; (3) administration of a non-toxic dose corresponding to a therapeutic effect.

B . Results

  1. Mortality.All theanimals, with the exception of a female rat exposed to a 0.02% concentration of pholcodine, survived throughout the whole experimental period.

  2. General condition. All theanimals were in excellent condition throughout the whole experiment. No loss of appetite was observed.

  3. Growth.All the animals concerned gained weight steadily. No difference was noted between them and the control animals.

  4. Haematology.The haematological study included a haemoglobin count, a red and white blood-corpuscle count and calculation of the leucocytary formula. This study was made after three months' repeated administration to the various animal; treated and was compared with control animals of the same sex. The blood was taken by puncturing the caudal vein.

It would appear that the red and white blood-corpuscle count is identical with that of the control animals and that the extreme figures are also identical; in fact, there is no difference between the haemoglobin counts of the control animals and those treated. The leucocytary formula is not altered by chronic administration of pholcodine.

  1. Histopathology.The animals were killed after three months' chronic administration. Microscopic study showed no difference between the animals treated and the control animals.

II. ADDICTION LIABILITY

The main question on which the admission of a morphine derivative into therapy depends is whether or not the product is liable to produce addiction.

Addiction comprises three basic phenomena [ (7)] : first, the tolerance gradually acquired by the organism, so that its reaction to therapeutic effects is reduced; secondly, habit, or the state of dependency on or enslavement to toxic substances; and thirdly, abstinence symptoms on the abrupt withdrawal of the drug.

Whereas the first phenomenon may be demonstrated easily on various laboratory animals, the second and the third are observed only among primates, by using complex techniques requiring special equipment, which has been evolved by Seevers & Deneau (51). All three phenomena have been studied clinically in the case of pholcodine.

A . Tolerance

Tolerance phenomena have been studied by Giudicelli and his collaborators. No decrease in anti-tussive action developed in the patients given pholcodine regularly for several months.

In three cases, daily doses of pholcodine of 40, 50 and 60 mg produced a sedative effect on the cough. In the case of another group of ten patients, 60 mg of pholcodine a day administered over a period exceeding three to four months remained effective against cough even after eight months' prolonged administration.

B . Addiction Liability and Withdrawal Symptoms

The absence of addiction liability and withdrawal symptoms was established by Professor Delay. These results were confirmed at Lexington by Dr. Isbell.

According to Delay, eight volunteer patients did not feel depressed or euphorized by a single subcutaneous dose of 100 mg of pholcodine; 120 mg given to three patients caused headache in one case and vomiting in the others.

The incidence of withdrawal symptoms was studied according to the Lexington method. Six addicts physically dependent on morphine were abruptly withdrawn and, in the acute stage of abstinence syndrome, they received, in two cases, 50 mg and in others 100 mg of pholcodine. Apart from a decrease in the respiratory rate, no changes in the withdrawal symptoms were noticed. This is a definitive demonstration of negative addiction-liability.

These results were confirmed by Dr. Isbell at the Lexington Addiction Research Center. In morphine addicts, pholcodine subcutaneously injected in doses of 50 to 100 mg failed to produce either euphoria, myosis, nausea or vomiting. Oral no doses of 400 mg in two patients and subcutaneous doses of 400 mg in two others had no effect.

Six morphine addicts stabilized on daily doses of between 120 and 300 mg of morphine were abruptly withdrawn and given pholcodine four hours after the abstinence syndrome appeared. They received total doses of 3,500 mg and 5,200 mg orally distributed over a 40-hour period. No change in the withdrawal symptoms was observed.

The methods of administration were conditioned by the fact that single large doses of pholcodine did not affect the abstinence syndrome and were irritating when administered subcutaneously.

The results obtained by the two experts thus coincide. According to Professor Delay, pholcodine does not produce either physical dependence or addiction and Dr. Isbell is of opinion that pholcodine should be considered as having less addiction liability than codeine. This view has been corro- borated by the Expert Committee of the World Health Organization. The report 2of this Committee states" [Pholcodine] is not more liable than codeine to produce addiction and is therefore not assimilable to the drugs mentioned in article I, paragraph 2, sub-group (a) of group I of the 1931 Convention ... [but is] assimilable to the drugs mentioned in group II of that Convention."

2

WHO Technical Report Series, No. 57. See Bulletin on Narcotics, vol. IV, No. 3.

Part five. Pharmacodynamic effects on anaesthetized animals

In 1950 Chabrier and his collaborators [ (15)] , using rabbits only, showed that pholcodine is hypotensive, inhibits the pain and respiratory centres and depresses the cough centres.

Unlike morphine and codeine, pholcodine, used on a guinea pig's isolated intestine, produces no increase in tonus and no change in rate and volume.

In mice, pholcodine is less convulsion-producing than morphine and codeine and, in particular, it produces no catatonus of the tail, known as the Straub phenomenon.

Subsequent pharmaco-dynamic studies on the anti-tussive action of pholcodine in cats were made in 1954 by May & Widdicombe [ (43)] and confirmed in 1955 by Green & Ward [ (31)] . Recently, in 1955, Eddy [ (21)] , in a systematic study of morphine derivatives, demonstrated by his jumping mouse test that pholcodine has only a very slight analgesic effect, ten times less than that of codeine.

Our experiments comprise a systematic study of the effects of pholcodine on different systems and organs in anaesthetized dogs, rabbits, cats and rats. We used non-toxic doses of pholcodine exclusively and compared them with morphine and codeine. The study will be completed by a further chapter (part six) on experiments on unanaesthetized animals.

I. CARDIO-VASCULAR EFFECTS A. Method

The cardio-vascular effects of pholcodine have been studied by the classical method, using normo-tensive dogs anaesthetized with pentobarbital and not given artificial respiration.

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FIGURE I Effect of pholcodine on a dog bi-vagotomized at the neck

Graph a: Effect on carotid pressure of intravenous injection of 4 mg/kg of pholcodine (P).

Graph b: Effect observed after ten minutes.

Graphs c and d: Comparison of cardiac pulsations registered on kymograph operating at higher speed.

Graph c: Before pholcodine.

Graph d: Ten minutes after pholcodine.

B. Results

In all the seven dogs which were given 4 mg/kg intravenously, it was found that, after a 15 to 20 minute latent period, there was an 85% to 91% fall in carotid or femoral pressure from the original level maintained for 20 to 30 minutes. The phenomenon is subject to tachyphylaxy and is therefore comparable to the phenomenon noted in 1933 by Schmidt & Livingstone [ (50)] in the case of morphine.

In order to compare the effect of pholcodine quantitatively with that of morphine, the action of the two alkaloids was tried successively on the same animal. Owing to the appearance of cross tolerance in the case of morphine, it was necessary either to use new animals or to use the same ones again after an interval of a week; this could be done in the case of four dogs. It was observed that the tensional action of pholcodine could be superposed on that of morphine.

C. Nature of Hypotension

Apart from the central mechanism already considered in connexion with morphine by Schmidt & Livingstone [ (50)] and demonstrated for morphine and its derivatives by Feld-berg & Paton [ (23)] , Schachter [ (49)] and Nasmyth & Stewart [ (45)] , the question arises whether the hypotensive effect of pholcodine does not originate partly from the cholinergic vascular periphery, or whether it is not engendered by the release of a histamine.

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FIGURE II Effect of morphine on a dog bi-vagotomized at the neck

Graph a: Effect on carotid pressure and respiration of intravenous injection of 2 mg/kg of morphine hydrochloride (M).

Graph b: Effect observed after eight minutes.

Graphs c and d: Comparison of cardiac pulsations registered on a kymograph operating at higher speed.

Graph c: Before morphine.

Graph d: Ten minutes after morphine.

  1. Cholinergic Effect

The effects of vagotomy and atropinization have been studied.

  1. Effect of vagotomy. Comparison between a normal dog and one which has been bi-vagotomized at the neck shows that the hypotensive effect of pholcodine is not appreciably changed by vagotomy (Fig. II).

It has also been confirmed that vagotomy does not change the hypotensive effect of morphine.

  1. Effect of atropinization. The intravenous injection of a 2 mg/kg dose of atropine sulphate, which completely blocks the hypotensive and cardiac effect of a 4 mg intravenous dose of acetylcholine, does not appreciably alter the hypotensive and cardiac effect of pholcodine. Accordingly, this hypotensive effect does not seem to be cholinergic in character.

2 . Release of Histamines

The hypotensive effect of pholcodine might conceivably be explained by the release of a histamine.

The release of histamine by morphine had already been presumed in 1917 by Sollmann & Pilcher [ (54)] . The fact was confirmed in 1950 by Nasmyth & Stewart [ (45)] , who established that this effect was diminished by anti-histamine agents. The presence of circulatory histamine was also established by Feldberg & Paton [ (24)] and confirmed by Evans [ (22)] . We were able to prove that pholcodine releases a histamine by using three methods: (a) inhibition by anti-histamine agents, (b) appearance of oedemas and histamine reactions, and (c) determination of circulatory histamine.

  1. Inhibition by anti-histamine agents. Hypotension produced by the intravenous injection of 4 mg/kg of pholcodine is appreciably less if an anti-histamine, such as mepyramine hydrochloride (10 mg/kg), has been previously administered.

  2. Appearance of oedemas and inflammatory reactions. In dogs, intravenous injection of 4 mg/kg and subcutaneous injection of 1 to 5 mg/kg of pholcodine produce a reddish-purple discoloration of the skin and a visible oedema of the lips, muzzle, ears and pads.

In rats, oral administration of 0.1 g to 0.4 kg and subcutaneous injection of 0.5 to 2 mg/kg also produce an inflammatory reaction accompanied by oedema of the snout, tongue, scrotum and paws. This syndrome is related to that produced by intra-peritoneal injection of egg-white (Selye) and injection of substances releasing either endogenous histamine (Halpern) (32), or 5-hydroxytryptamine (dextrane, derivative 48/80).

The intensity of the reaction may be determined quantitatively by the Parratt & West (47) method, after intravenous injection of Evan's blue, which accumulates in the erythe-matopapular area. The intensity of the blueing of the tissue spaces and the degree of oedema are recorded by allotting different marks

Our experiments with rats treated with between 0.25 and 2 mg of pholcodine and morphine yielded effects identical in intensity and duration. No effects were observed with 0.6 g/kg of codeine per os.

Differentiation between the release of histamines, the appearance of 5-hydroxytryptamine and capillary permeability phenomena remains to be established.

  1. Quantitative determination of circulating histamines. The quantity of histamine present in the blood in the period following injection of pholcodine to a dog was estimated by determining the contracting effect of the dog's plasma on a guinea-pig's isolated ileum.

The plasma taken from the femoral artery of a dog before administration of pholcodine revealed practically no histamine action. When the plasma was taken 3 minutes after intravenous injection of 4 mg/kg of pholcodine, a contraction corresponding to 8 of histamine was observed. The contraction still corresponded to 8 of histamine after 6 minutes, to 5 of histamine after 10 and 15 minutes, and to only 0.5 after 30 minutes. Practically no contraction of the guinea-pig's ileum was observed when using plasma taken 60 minutes after the pholcodine injection. These results were confirmed on three other dogs, which received the same dose of 4 mg/kg of pholcodine. A comparative experiment on three dogs with morphine showed that histaminemia stood at a level of only 3 / cm 3 three minutes after injection of morphine. Moreover, it disappeared only 6 minutes after injection.

The results differ quantitatively from those observed in rats. This yet further confirms the impossibility of limiting experimentation to a single pharmacological test and to a single animal species. The only problem lies in deciding on which animal species to base results so as to extrapolate them to man.

Experiments on man have confirmed that release of histamine is much more intense after subcutaneous injection of pholcodine than of morphine. Pholcodine also produces an intense and intolerable pruritus.

  1. Peripheral Vaso- dilatory Effect

It is known that even such potent and specific histamine liberators as Dews' (19) 48/80 do not achieve their hypotensive effects solely through release of histamine. Dews has shown that the hypotension produced by 48/80 is inhibited by a sympatho-mimetic drug having no anti-histamine action: this is methoxamine, or (2,5-dimethoxyphenyl)-hydroxyisopropylamine, studied by De Beer (17).

Possession of a sample of methoxamine 3enabled us to study the effect of this peripheral vaso-constrictor on dogs subjected to the hypotensive action of 4 mg/kg of pholcodine. We found that the hypotensive effect of pholcodine, which is only slightly decreased by an injection of 10 mg/kg of mepyramine, is, at least temporarily, inhibited by injection of methoxamine.

2

We wish to thank Doctor Edwin De Beer of Tuckahoe (USA) for letting us have a sample of his product.

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FIGURE III Modifying effect of an anti-histamine and a peripheral vasoconstrictor on the hypotensive effect of pholcodine

Graph a: Injection of 4 mg/kg of pholcodine (P).

Graph b: Injection of 10 mg/kg of mepyramine hydrochloride (CM).

Graph c: Injection of 1 mg of methoxamine (MET).

From top to bottom, carotid pressure, respiration. Time in ten seconds.

To sum up, apart from the central hypotensive effect which it has in common with morphine, pholcodine exercises a peripheral hypotensive action which is non-cholinergic in character and is due partly to the release of histamine and partly to a peripheral vaso-dilatory action; these effects seem to be more intense and of longer duration in dogs treated with pholcodine.

II. RESPIRATORY ACTION

Chabrier and his collaborators [ (15)] , operating exclusively on anaesthetized and non-anaesthetized rabbits, observed that pholcodine decreased the respiratory rate and volume in all cases. The effect is therefore similar to that of morphine.

Our study related to the respiratory action on an anaesthetized dog of non-lethal doses of pholcodine, as compared with morphine. In addition, some experiments were conducted on anaesthetized rabbits.

A. Method

Pholcodine and morphine hydrochloride were injected intravenously into dogs anaesthetized either with chloralose or with pentobarbital and into rabbits anaesthetized with urethane.

Respiratory movements were registered either by means of a Marey capsule attached to the trachea, or by using the Anderson [ (1)] apparatus, which measures the respiratory rate and volume and, hence, the respiratory volume per minute. A device allowing the air exhaled to escape through a valve prevents accumulation of CO 2

B. Results

(a) Respiratory frequency. The depressant action of morphine on respiratory frequency was confirmed. In anaesthetized dogs this action is progressive, reaching a maximum 35 minutes after injection. The doses (1 to 3 mg/kg) injected intravenously produced none of the rhythmic irregularities which appear in the case of large doses.

On the other hand, injection of 4 mg/kg doses of pholcodine produced an increase in respiratory frequency in all the anaesthetized dogs, after a brief period of apnoea. The increase reached its maximum level after 30 minutes.

In rabbits anaesthetized with urethane, on which a decrease of respiratory frequency under morphine was confirmed (50% after injection of a 2 mg/kg dose), pholcodine in 5 mg/kg doses also decreased the respiratory rate by over 50%. These results confirm the observations of Chabrier [ (15)] .

(b) Respiratory volume per minute. In twelve dogs anaesthetized with pentobarbital 2 Mg/kg doses of morphine lowered the respiratory volume per minute by an average of 35%. The maximum effect became apparent after half an hour.

On the contrary, doses of 4 to 10 mg/kg of pholcodine received by eight dogs, also anaesthetized with pentobarbital, raised the respiratory volume per minute by 30 to 40%.

In the case of four other dogs anaesthetized with chloralose doses of 1 to 4 mg/kg of morphine lowered the respiratory rate per minute by 28 to 55% (an average of 35%), whereas, on the contrary, the respiratory volume per minute of six dogs anaesthetized with chloralose and treated with 1 to 5 mg/kg pholcodine increased by 68% or did not change. 4

The difference between the respiratory effects of pholcodine in rabbits and in dogs has not been explained. It confirms once again the necessity of not limiting pharmacological experiments to a single animal species.

III. GASTRO-INTESTINAL EFFECTS

Chabrier and his collaborators [ (14)] , in studying pholcodine on a guinea-pig's isolated intestine in comparison with codeine and morphine, noted that, while the latter two alkaloids

________

4

It would have been interesting to study the respiratory effect in unanaesthetized dogs. Apart from the technical difficulties, however, the study was rendered impossible by the fact that small and medium doses, whether of morphine or of pholcodine, lower neither the frequency nor the volume of respiration and produce a lengthy polypnea, comparable to that observed in dogs after a long run.

increased the intestinal tonus, pholcodine induced no change in the rate and volume of the movements of the intestine or in its tonus.

The importance of this observation as regards its bearing on the therapeutic effect of pholcodine has led us to supplement this study by a more physiological experiment than that on the isolatedintestine - namely, on the intestine in situ of dogs and rats.

A . Intestines insitu of Dogs

It is common knowledge that morphine induces an increase in the tonus of the small intestine, as Krueger [ (38)] has clearly shown; this effect can go as far as contracture and is accompanied by an increase in the extent and frequency of peristaltic movements.

In registering peristaltism by means of a balloon and following the experimental methods described by Krueger [ (39)] at the level of the jejunum and the duodenum, we were able to reproduce the phenomenon of contracture and increased peristaltism by injecting intravenously into chloralized dogs doses of 0.5 mg to 2 mg/kg of morphine hydrochloride. In all cases, there was a brief initial spasm, followed by transitory relaxation and then by another increase in tonus. On the contrary, in the case of six dogs anaesthetized with chloralose, doses varying between 0.5 mg and 4 mg/kg of pholcodine provoked no intestinal stimulation; in three cases a decrease of tonus was noted, and in three cases there was no effect. This difference of behaviour between morphine and pholcodine confirms Chabrier's observations and may explain the presence or absence of constipating effects.

B . Intestines insitu of Rats

Again using an in situ method, an attempt was made to ascertain the gastro-intestinal effect of pholcodine by testing propulsive gastro-intestinal activity, which can be done in an unanaesthetized animal. This method, already described [ (8)] , consists in measuring the rate of progress of the intestinal chyme, which may be followed by administering a piece of animal charcoal in gum arabic. The animal is killed after a certain time; the distance separating the pyloric sphincter from the most advanced section reached by the coloured indicator is measured. The theory is that a constipating drug slows down the propulsive gastro-intestinal action.

Whereas a 17 mg/kg dose of codeine decreases the intestinal transit by 50%, a dose of pholcodine ten times as strong (180 Mg/kg) is required to bring about the same decrease, half an hour after administration of the alkaloids. The same test conducted one hour after administration shows the same 50% decrease of the transit, either with 19 mg/kg of codeine or with 100 mg/kg of pholcodine. Accordingly, codeine would seem to be five times more constipating than pholcodine.

C . Counting the Number of Rats' Excreta

In a last series of experiments, an attempt was made to determine the effects of codeine and pholcodine with greater accuracy by using a modified form of the Eddy [ (20)] technique for rabbits, which we adapted to rats.

By this technique the excreta of rats treated with varying doses of pholcodine and codeine are counted: in relation to control animals, the decrease of excreta is measured 22 hours after gastric administration of 33 mg/kg of codeine phosphate; 270 mg/kg of pholcodine, a nine times stronger dose, is required to obtain the same effect.

In conclusion, if the results in rats can be extrapolated to man, it may be assumed that pholcodine is much less constipating than codeine.

It is well known that the therapeutic use of codeine for adults and children is greatly limited by its constipating effects.

Part six. Effect on the central nervous system

In the pharmacology of morphine derivatives, particularly pholcodine, it is important to consider separately the neuro-physiological aspects and the psychological aspects, which may be studied on unanaesthetized animals.

The primary action of morphine is neuro-physiological. In man, with the exception of a direct effect on the smooth muscles, all the effects of therapeutic doses of morphine relate exclusively to the central nervous system. The same applies to animals.

Although the psychological aspects are noteworthy, they are much more difficult to study, even in man,since only tests such as Rorschach's can be used. In the case of animals, the approach is still more difficult, and has only recently been tackled by psychologists such as Brady [ (5)] and Hunt [ (36)] , and by neuro-pharmacologists such as Wikler [ (58)] . These methods are essentially based on the abolition or reduction of conditioned reflexes; they have enabled us to compare the action of pholcodine with that of codeine and morphine.

I. NEURO-PHYSIOLOGICAL STUDY

The five effects considered were analgesic effect, anti-tussive effect, anticonvulsant effect, effect on general behaviour and the response to tests of ataraxics.

A. Analgesic Action

The close structural analogy of pholcodine with morphine and codeine has stimulated various researchers to study its analgesic effect.

Using Bovet's [ (4)] method on a rabbit's ear, Chabrier [ (15)] and his collaborators observed an inhibiting action on the pain centre. A dose of 0.5 mg/kg of pholcodine exercises an analgesic effect always at least equal and sometimes superior to that of codeine, but less than that of morphine. With larger doses the phenomena remain qualitatively the same, but the differences between the action of the three alkaloids become less and less apparent as the doses are increased.

On the other hand, by using the test of pain stimulation by heat conduction on mice, Eddy [ (21)] observed that the analgesic action of pholcodine was much less than that of morphine and codeine; pholcodine proved to be one-tenth as effective as codeine.

Using heat contact on a rat's tail, according to the Wolfe & Macdonald [ (61)] method, we observed no rise in the pain threshold, even in the case of a 450 mg/kg dose administered orally, which was toxic for 25% of the rats.

A 30 mg/kg dose of codeine used as a reference standard protected 50% of the rats against painful radiation; however, codeine is not toxic in this dose.

Using radiant heat according to the Cahen, Epstein & Krementz method [ (10)] , whereby a radiant heat stimulus is applied to a blackened area of the skin of a rat's back we noted no rise in the pain threshold, even with oral and subcutaneous doses of 450 mg/kg. Thus, even a toxic dose of pholcodine does not raise the pain threshold.

B. Anti-tussive Action

The anti-tussive action of pholcodine demonstrated by Giudicelli, Chabrier & Thuillier [ (28)] has been confirmed by pharmacological experiments and clinical study.

The effect was produced by using the method of inhibiting the stimulating effect of lobeline on the cough centre in man.

After having stabilized the subjects by injecting intravenously the dose of lobeline required to induce a cough, the authors determined the dose of pholcodine which blocks the cough reflex when injected 30 minutes before the threshold-dose of lobeline. Comparable anti-tussive action may be observed after subcutaneous injection of 80 to 100 mg of pholcodine, or 100 to 150 mg of codeine.

Another method has been used in the case of cats by May & Widdicome [ (43)] . The cough reflex is produced in the anaes-thetized animal either by mechanical irritation of the trachea with a polythene catheter, or by inhalation of sulphurous anhydride. The comparative action of pholcodine and codeine differs according to the kind of test used. In the first case, pholcodine is three times more active than codeine, and in the second it is, on the contrary, only half as active.

Green & Ward [ (31)] , using the same animal and either the same mechanical and chemical stimulants or electrical excitation of the superior laryngeal nerve, observed that in all three methods pholcodine proved twice as active as codeine.

In view of these somewhat conflicting results, we resumed efforts to determine the anti-tussive effect of pholcodine, using anaesthetized cats, and, in order to avoid interference from the anaesthetic, chronic dogs.

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FIGURE IV

Technique for the determination of the antitussive effect in cats

Cats.The cats, slightly anaesthetized with pentobarbital (30 mg/kg administered intraperitoneally), were subjected either to mechanical stimulation by passing a polythene catheter down the trachea, or to chemical stimulation. For this purpose, we substituted for sulphurous anhydride, which is difficult to handle, an inhalation of ammonia, which had given Hoglund & Michaelsson [ (35)] good results in man. The forced inhalation and exhalation resulting from the cough reflex were registered pneumographically.

Each of these stimuli was applied once and repeated at 15-minute intervals, until a constant response was obtained.

The stimulation was repeated after intravenous administration of codeine or pholcodine (2 to 8 mg/kg). We then determined the dose which appreciably decreased the response to induced coughing by more than 50% of the response.

The experiment was carried out on 9 cats, and an attempt was made to compare the effect of the two drugs on them. The results varied from one cat to another, but seemed to remain constant for the same animal. The cats were kept alive after the experiment and again subjected to experiments ten days later.

The average anti-tussive dose of codeine administered intravenously was 2.3 kg ±.17 and that of pholcodine was 3.5 ± .3, in the case of mechanical stimulation; the dose was 2.8 ± .19 of codeine, and 3.43 ± .24 of pholcodine in the case chemical stimulation. The differences were significant (P <0.01) in all the tests. Thus, pholcodine is less active than codeine.

Chronic dogs. We followed the Stefko & Benson [ (56)] technique, in which a tussigenic area of the tracheal submucosa of a dog is stimulated by means of embedded electrodes. After surgical recovery, the exteriorized leads are connected to an electrical stimulating device. The threshold of tussive excitability, characterized by a distinct cough of moderate intensity, was thus determined.

The constancy of the threshold once established, the drug was administered and the percentage of inhibition determined at intervals of 15 to 20 minutes. The animals could be used again, but not more than once a week. On the other hand, sclerification phenomena sometimes prevented an animal from being used for over 3 or 4 weeks.

From tests carried out on 7 dogs, it may be concluded that codeine phosphate, which is inactive in subcutaneous doses of 0.5 g/kg, produces a 40% inhibition in doses of 1 mg/kg. The inhibition reaches 70% at 2 mg and 90% at 4 mg/kg. In the same dogs a dose of 1 mg of pholcodine produced an inhibition of 40%, a dose of 2 mg, 80%, and a dose of 4 mg, an average of 100%. Thus, by this method there seems to be no significant difference between the anti-tussive effect of pholcodine and of codeine administered subcutaneously.

To sum up, the anti-tussive activity of pholcodine in comparison with that of codeine seems to differ according to the authors and the technique used. The action is probably partly central in origin. Since pholcodine exercises a moderately depressant respiratory action, it may be supposed that its anti-tussive action differs from that of morphine and codeinein that it is not exercised directly on the respiratory centre, but rather on a relay of the reflex arc.

Whatever the method of action of pholcodine and the relationship of its activity with that of codeinemay be, clinical tests in France and the United Kingdom have established the value of pholcodine as a cough medicine.

C. Anti-convulsant Action

Classical pharmacological treatises agree in admitting that morphine potentializes the central convulsant agents and cannot be used in anti-convulsant therapy; conversely, a central stimulant such as strychnine cannot be used for treating morphine addiction.

Hazelton & Koppanyi [ (33)] have, moreover, observed that in mice morphine and codeine increase not only the convulsant effects, but also the toxicity of pentylenetetrazol.

We studied the problem in rats, using two convulsant agents, pentylenetetrazol and strychnine sulphate, and observed not only the effects of analgesic and sub-toxic doses, but also the effect of very small doses of morphine, codeine and various synthetic derivatives, such as dl-methadone, keto-bemidone, pethidine hydrochloride and l-isomethadone. We then compared the effects of these alkaloids with that of phol-codine.

(a) Inhibition of the convulsive crisis was studied in rats by means of a technique previously described (9). The percentage of animals protected by a preliminary administration of alkaloids was measured against the effect of an intraperitoneal injection of 75 mg/kg of pentylenetetrazol, which was an active dose for 100% of the control animals.

The results show that morphine and all its derivatives present no anticonvulsant effect in analgesic doses and that only very small doses of codeine, morphine and pethidine exercise a protective or anti-convulsant effect (20%).

Pholcodine provides a striking contrast [ (11] ). It exercises a much higher anti-convulsant action, which increases with the doses and can attain up to 90% protection. The protective dose for 50% of the animals is 200 mg, which produces an effect similar to that obtained with 120 mg/kg of meprobamate. Comparable results were obtained by using a subcutaneous 1.25 mg/kg dose of strychnine as an anti-convulsant.

(b) Protection against a toxic dose. In rats, subcutaneous doses of 25 to 450 mg/kg of pholcodine are extremely effective in preventing the lethal effects of pentylenetetrazol (75 mg/kg, or E.D.35; 100 mg/kg, or E.D.100) and of strychnine (1.25 mg/kg, or E.D.70). In this connexion the action of meprobamate confirms Berger's results with mice. Massive doses of morphine, codeine and morphine derivatives, on the other hand, increase the toxicity of pentylenetetrazol and strychnine. Only small doses of morphine and codeine are protective [ (11)] .

To sum up, pholcodine, as opposed to morphine, codeine and synthetic morphine derivatives, undoubtedly blocks the convulsion-producing and toxic effects of pentylenetetrazol and strychnine.

D. General Behaviour

Thorough observation can produce valuable results as regards the action of a medicament on the central nervous system. We began this study with various laboratory animals: mice, rats, dogs, cats.

(a) Mice.Chabrier and his collaborators [ (15)] observed that pholcodine did not produce tail catatonus. We have confirmed these observations. Pholcodine forms a striking contrast with morphine and its synthetic derivatives.

Subcutaneous doses of up to 400 mg/kg of pholcodine and oral doses of 900 mg/kg do not produce tail catatonus (Straub reaction). No stimulant, depressant or hypnotic effect characterized by the loss of erection power is observed. On the other hand, an intra-peritoneal dose of 90 to 100 mg/kg of codeine, a dose of 6 mg/kg of ketobemidone and d-methadone, administered in the same manner, and a dose of l0 mg/kg of morphine administered subcutaneously produce tail erection, a phenomenon which Wickler attributes to the freeing of the sub-cortical mechanisms [ (59] .

In the case of all these alkaloids this reaction is accompanied by general stimulation and at times convulsant phenomena. Morphine-dosed animals, when placed in a glass jar, run round in circles. On the other hand, mice to whom pholcodine is administered remain calm.

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FIGURE V

Effect of morphine compared with that of pholcodinc on the Straub reaction

Left: Effect on two mice of a subcutaneous dose of 6 mg/kg of morphine
Right: Effect of a subcutaneous dose of 400 mg/kg of pholcodine.
  1. Rats.Pholcodine has an effect on rats only if a dose of at least 300 mg/kg is administered subcutaneously: this pro-duces a slackening of muscles and limp paralysis.

  2. Dogs.Pholcodine has an effect onty if a dose of at least 150 mg/kg is administered subcutaneously. A 300 mg/kg dose produces ataxia and general depression. On the other hand, a subcutaneous dose of 75 mg/kg of codeine produces stimulation and convulsions. A dose of 150 mg/kg is lethal.

  3. Cats. Rosenbaum [ (48)] , in 1879, was the first to describe the stimulating action of morphine on cats, which has been described as cat madness. All the authors who later confirmed this action, known as morphinic madness, seem to have confused under this name two basically different phenomena: one, which is common to the sympatho-mimetic poisons and consists of mydriasis and pilo-erection, and another, which seems to be specifically produced by morphine in this animal species and which expresses itself in motorial excitement of a violent, recurrent and disorderly character, consisting of apparently purposeless movements which seem to be connected with hallucination phenomena.

We find it odd that Claude Bernard (3), who had already studied the action of morphine on cats in 1864, did not describe these phenomena.

In sixteen cats we found motorial excitement and hallucination phenomena appearing regularly 30 to 45 minutes after a subcutaneous injection of 15 mg/kg and 20 mg/kg of morphine hydrochloride. On the other hand, with .1 mg/kg and .2 mg/kg doses we observed symptoms of depression where the animal sat quietly in the corner of its cage as long as it was not disturbed by the experimenter. With doses of .5 mg/kg, 1 mg/kg, 2 mg/kg and 5 mg/kg, motorial excitement is preceded by signs of depression, but indications of contentment or sociability are also noticeable: purring or rubbing against the cage. However, the objectivity of these tests is still questionable.

When the same cats were given subcutaneous injections of pholcodine in 5 mg/kg, 30 mg/kg, 60 mg/kg and 160 mg/kg doses the only effect produced was calmness and indifference.

Pilo-erection and mydriasis were noted in all cats given morphine and pholcodine in any size of doses. On the other hand, motorial excitation, which commonly occurs when large doses of morphine are given, was not noted in the case of pholcodine.

These phenomena of motorial excitation and morphinic hallucination which we noticed constantly in the case of morphine, codeine and methadone are a striking contrast with the calm and indifference produced in cats after subcutaneous doses of pholcodine in 5 mg/kg, to 30 mg/kg and even larger doses.

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FIGURE VI Comparison between the calming effect of pholcodine and morphine madness of a cat on the same animal

Left: Effect of a subcutaneous dose of 30 mg/kg of pholcodine.
 
Right: Effect of a subcutaneous dose of 15 mg/kg of morphine.

E . Responses to Ataraxic Tests

The above observations have drawn our attention to the fact that it might be interesting to make a systematic study of pholcodine in comparison with morphine by using certain tests which have served to detect ataraxics.

The following tests were used: potentialization of the hypnotic effect, modification of the electro-encephalogram, diminution of motility, interaction with psycho-mimetic stimulants, inhibition of morphinic madness.

  1. Potentialization of the hypnotic effect. Although this method is far from being specific, we studied it in the case of pholcodine administered to mice and dogs [ (11)] .

The results show that doses of pholcodine which are not hypnotic significantly increase the hypnotic action of hexobarbital of sodium characterized by the disappearance of erection reflexes (table II).

The action increases with the doses of pholcodine. It is not, however, greater than that produced by codeine and much less than that produced by chlorpromazine. A prolongation of the hypnotic action of hexobarbital is also noted in dogs which have been given either a subcutaneous injection of 30 mg/kg of pholcodine or a subcutaneous injection of 7.5 mg/kg of codeine administered half an hour before an intravenous injection of 50 mg/kg of hexobarbital.

(b) Modification of the electro-encephalogram. The action of morphine on the electrical activity of the cortex has been clearly demonstrated both in man and animals.

We studied the comparative effect of pholcodine and morphine in rats.

Many years ago, Wikler [ (12)] and I showed that in the case of a non-anaesthetized rat a dose of morphine not exceeding 20 mg/kg, which has an obvious analgesic effect, does not make any change in the electro-encephalogram. Doses of 20 to 50 mg/kg increase the "burst" amplitude and diminish the frequency which is followed by the periodical appearance of waves of great amplitude - 10 to 20 cycles per second - which are repeated for several hours. These changes are comparable with those observed in man during natural sleep or sleep induced by barbiturates. Absolutely

TABLE II

Prolongation of the hypnotic effect of hexobarbital by prior administration of pholcodine and codeine

Group of experiments
Medicament
Dose
Manner of adminis-tration
Number of mice
Intervals before hexobarbital (minutes)
Duration of sleep (minutes)
Standard error
t.
Probability
I
Physiological serum
0.1 cc/10 g
Sub-cutaneously
10 30 19 2.36
-
-
 
Pholcodine
75 mg/kg
,, 10 30 26 3.50 1.67 0.1
 
"
150 mg/kg
,, 10 30 53 10.15 3.27 0.01
 
Codeine (phosphate)
15 mg/kg
,, 10 30 32 5.40 2.20 0.04
II
Physiological serum
0.1 cc/10 g
Sub-cutaneously
10 30 31 2.61
-
-
 
Pholcodine
300 mg/kg
,, 10 60 61 5.63 4.84 0.01
  ,,
300 mg/kg
Orally
10 60 43 6.01 1.84 0.09
 
Codeine (phosphate)
30 mg/kg
Sub-cutaneously
10 60 61 7.64 3.72 0.01
III
Physiological serum
0.4 cc/10 g
Orally
10 60 40 4.86
-
-
 
Pholcodine
300 mg/kg
,, 10 30 55 6.83 0.1 0.1
 
"
600 mg/kg
,, 10 30 65 8.32 2.60 0.02
 
"
600 mg/kg
,, 10 60 60 8.83 1.98 0.06
IV
Physiological serum
0.1 cc/10 g
Sub-cutaneously
10 30 26 3.54
-
-
 
Chlorpromazine
0.5 mg/kg
,, 10 30 31 4.48 0.88 0.42
  ,,
1 mg/kg
,,
10 30 41 4.40 2.68 0.02
  ,,
5 mg/kg
,, 10 30 88 7.45 7.96 0.01

similar effects have been observed by Wikler & Altschul [ (60)] in a normal dog or a dog curarized with both morphine and methadone, and in man by Gibbs & Maltby [ (27)] .

Subcutaneous injections of 5 to 100 mg/kg of pholcodine produce changes in rats quite comparable with those caused by morphine.

(c) Decrease of motility. Spontaneous motility was recorded in the case of mice which were given morphine, pholcodine or codeine by making a comparison with control animals which were given physiological serum. The number of circuits was determined by making the animals pass in front of a beam of light operating a photo-electric cell, connected with an electric meter.

Morphine injected subcutaneously in very small doses of 20 to 200 decreases motility but motility is greatly increased when analgesic doses of 0.50 mg to 1 mg/kg of morphine are administered. Pholcodine, on the other hand, administered in doses of 25 to 400 mg, decreases motility in mice.

Qualitatively, this action is comparable to that of chlorpromazine. Quantitatively, however, pholcodine is 100 times less active than chlorpromazine which, when administered in doses of 2 mg/kg, decreases motility by 80 per cent.

(d) Interaction with psycho-mimetic stimulants. The Fellows & Cook test [ (25)] was used on mice. These writers observed that mescaline, when administered to mice, considerably increases spontaneous scratching, but only in the case of mice. Chlorpromazine reduces this effect. Having confirmed this reaction in the case of many ataraxics we studied it in the case of pholcodine.

Morphine administered in doses of 0.02 mg to 1 mg/kg and pholcodine administered in doses ranging from 25 to 400 mg/kg block the response to mescaline proportionately as the dose is increased.

The significance of these results is still unknown. It cannot be claimed that the inhibition of this response to mescaline is connected with what is called ataraxic activity.

(e) Inhibition of morphinic madness. Two groups of American pharmacologists, Seifter and his collaborators [ (52] ) on the one hand, and Loewe [ 42] on the other, have shown independently that in the complex of morphinic madness in the cat motor stimulation is blocked by chlorpromazine and phenothiazine derivatives (promazine). On the other hand, this effect is not general; not only is it not blocked by reserpine but it is even increased. (e) Inhibition of morphinic madness. Two groups of American pharmacologists, Seifter and his collaborator [ 52] ) on the one hand, and Loewe [ 42] on the other, have shown independently that in the complex of morphinic madness in the cat motor stimulation is blocked by chlorpromazine and phenothiazine derivatives (promazine). On the other hand, this effect is not general; not only is it not blocked by reserpine but it is even increased.

We have been able to confirm that benactyzine (1 mg/kg) and also chlorpromazine (5 mg/kg), administered subcutaneously 30 minutes before morphine, block motor stimulation in cats, without modifying the mydriasis and sympathetic reactions and the vomitive effect.

Pholcodine, on the other hand, when administered orally in doses of 60 to 80 mg/kg, or subcutaneously in doses of 30 to 60 mg/kg, is inactive.

II. PSYCHOLOGICAL STUDY

In order to be complete, this study should include the examination of the psychological aspects of the effects of pholcodine on the central nervous system. This study has been made by studying, on the one hand, the spontaneous behaviour of the cat and, on the other, the action on the conditioned reflexes of the rat.

A . Spontaneous Behaviour of the Cat

Norton (46) has recently thought out and developed a method which enables various modifiers of the central nervous system to be studied in cats.

For this purpose, he singled out five types of behavioural pattern in the cat; they are (1) sociability, (2) state of satisfaction - contentmentc (3) excitement, (4) defensive hostility, (5) aggressive hostility.

After making friends with the animals, which are kept in conditions as physiologically favourable as possible - one animal to a large cage - the observer tries to give each animal marks (scores) for the various types of behaviour. In the case of sociability, the observer watches to see whether as soon as the cage is opened, the animal tries to approach the person making the experiment, mewing and holding its tail erect. Contentment is shown by the cat purring, licking its fur and rubbing the observer's hand. Over-excitement is shown by high-pitched mewing, pilomotor reaction, mydriasis, hyperactivity and tail-lashing. Defensive hostility is exhibited when the cat growls and cowers in the corner of the cage. On the other hand, aggressive hostility is marked by attempts to bite and scratch and to jump up and seize anything within reach.

In most of these cases, the "scores" remain unchanged for the same animal, but some change as the experiment proceeds and, in that case, the animals must be eliminated. Moreover, in the case of ataraxic substances, it also seems necessary to eliminate animals which exhibit too little hostility.

The administration of various ataraxics, hypnotics and analeptics acting on the central nervous system enabled Norton & De Beer (46) to observe qualitative and quantitative changes.

We were able to confirm their results so far as chlorpromazine is concerned; this, administered in doses of 15 mg/kg, increases sociability, but does not change the state of contentment and slightly increases hyperexcitability. Chlorpromazine definitely decreases defensive hostility but increases aggressive hostility.

According to the same writers, and we have confirmed this, morphine administered in doses of 15 to 20 mg/kg considerably increases hyperexcitability and contentment. It increases hostility.

Our study of pholcodine, given in doses varying between 15 and 40 mg/kg, shows that it causes decreased sociability, produces no change in contentment, very definitely decreases hyperexcitability and defensive hostility; while aggressive hostility increases. Thus, the action of pholcodine seems to resemble much more closely that of chlorpromazine than of morphine. It acts in an absolutely opposite manner to morphine so far as hyperexcitability is concerned.

We cannot, however, emphasize too strongly the subjective character of the method, although it has been followed in our laboratory independently by at least two different observers, using the "blind test" method in which the person making the experiment does not know what medicament is being used. The greatest caution must still be recommended in extrapolating such laboratory results to clinical practice as a whole.

B. Action on the Conditioned Reflex

Wikler and his collaborators [ (34)] had observed that analgesic doses of morphine decrease the disruptive effects produced by fear of the suffering resulting from a painful electric shock. Wilker [ (60)] was also able to demonstrate that morphine administered to a rat reduces the anxiety associated with the anticipation of pain. Using a Skinner box, he caused an emotional response conditioned by an electric shock (response assimilated to fear). The motivating recompense is supplied by the administration of nourishment to a rat which has been deprived of food for 48 hours.

Our own experiments consisted in conditioning a rat to associate the painful reaction provoked by an electric discharge with the noise of a buzzer. We used a simplified form of the method used by Dews [ (18)] on pigeons, and Cook [ (16)] on rats.

The apparatus consists of a rectangular plywood box 29 cm wide and 48 cm high, sound-proofed with Isorel.

The floor of the box is a metal grid composed of rods 7 mm in diameter and 2 cm apart. These rods can be connected with an electric current of 110 volts.

Inside the box there is a buzzer with a high-pitched sound. A pole 2.5 cm in diameter is attached to one side of the cage, its lower end being 2 cm above the metal floor of the box.

The rat very soon learns to avoid the electric shock (conditioned reflex) by clambering up the pole. After a period varying from 3 to 15 days the rat becomes conditioned to the noise of the buzzer and the latter only has to ring for the rat to clamber up the pole.

Cook & Weidley [ (16)] showed that chlorpromazine deconditions certain rats, the percentage of animals deconditioned increasing with the dose. The response to the conditioned stimulus (sound of a buzzer) disappears. Under the influence of pholcodine a rat is also deconditioned to the conditioned stimulus of the sounding of a buzzer but not to that of an electric shock.

After having verified that physiological serum is incapable of deconditioning rats, our experiments showed that morphine in doses varying between 2 and 10 mg deconditions a rat, the effect being proportionate to the dose. The active dose in 50 per cent of the animals treated was 10 mg administered subcutaneously.

Pholcodine administered in doses of 50 to 400 mg/kg also deconditions rats.

Part seven. Discussion

As long ago as 1869 Claude Bernard explained the action of morphine as a dual one - " numbing and stimulating".

The results achieved by a group of chemists and pharmacologists who made a comparative study of morphine with the alkaloid derivatives and more than 125 synthetic derivatives have made it possible to determine the relation between chemical structure and pharmacological action. In particular, the work of N. B. Eddy & Small [ (53)] shows that the blocking of the phenolic hydroxyl of morphine increases convulsant action and decreases all other action (analgesic, respiratory depressant, spasmogenic, intestinal). On the other hand, the blocking of the alcoholic hydroxyl increases toxicity, convulsant and other morphinic action. From this it follows that the phenolic radical of morphine is responsible for analgesic, respiratory depressant and constipating action, that alcoholic hydroxyl tends to neutralize these actions and that it is essential to have the two hydroxyl groups combined in order to prevent the appearance of stimulant properties.

Our pharmacological study of pholcodine seems to prove that there is an exception to this rule. Pholcodine, which is the result of substituting an atom of phenolic hydrogen of morphine, does not increase stimulant action, on the contrary it reduces it. Pholcodine is not only non-convulsant; it is an anti-convulsant. It does not produce the Straub reaction in mice or morphinic madness in cats.

It is none the less true that, as in the case of codeine, the blocking of the phenolic hydroxyl of pholcodine decreases its analgesic, respiratory depressant and intestinal spasmogenic action.

Tatum & Seevers [ (55)] explained morphinomania as the progressive predominance of the stimulating action of morphine, which the depressant action is not sufficient to "balance ". Our experimental study of pholcodine shows that it lacks the stimulant component of morphine, which explains the absence of drug-addiction-producing effects in pholcodine. It is true, however, that since pholcodine is not an analgesic, it could not a priori have addiction-producing properties.

Another conclusion may be drawn from our experiments. Pholcodine is histamino-releasing to a much greater degree than morphine and codeine. It may be that the intensification of this action is linked to the morpholinylethyl radical.

Briefly, a comparison of morphine and pholcodine does not show that the replacement of an atom of phenolic hydrogen in morphine atom by a morpholinylethyl radical results in the transformation or dissociation of the pharmacological action. The duality of the stimulant and depressant effects of morphine is replaced by depressant action only.

Summary and conclusions

  1. Pholcodine is an amino-alkylether of morphine resulting from the replacement of phenolic hydrogen by a morpholylethyl radical.

  2. The acute toxicity of pholcodine when administered to rats, mice and dogs is low and, except when administered orally to rats, is lower than that of codeine.

  3. Continued experiments with rats show the absence of any toxic action and of sensibilization phenomena after repeated doses.

  4. The clinic has established that pholcodine does not cause drug-addiction.

  5. Pholcodine administered intravenously lowers arterial pressure in a normo-tensive dog. Only a small part of this action is of central origin. It is due essentially to a peripheric vaso-dilation and to a freeing of histaminic substances. It is not of a cholinergic nature.

  6. Pholcodine differs from morphine and codeine in that it does not depress the respiratory volume or amplitude in

dogs. On the contrary, it increases the amplitude and the respiratory volume per minute.

  1. Unlike morphine and codeine, pholcodine produces no intestinal spasm, and decreases neither the amplitude nor the peristaltic tonus in dogs. In rats the depressant action of the propulsive intestinal activity of pholcodine is ten times lower than that of morphine.

  2. Pholcodine has almost no analgesic action.

  3. Pholcodine is an anti-tussive like codeine and morphine.

  4. Pholcodine does not cause tail catatonus in mice, nor morphinic madness in cats.

  5. Pholcodine is an anti-convulsant, unlike morphine, codeine and morphine derivatives, which are all convulsants.

  6. Pholcodine responds positively to certain tests for ataraxics.

To sum up, pholcodine, as compared with morphine, appears, when administered in therapeutic doses, not to have any central stimulant action.

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Professor H. T. Baggesgaard-Rasmussen