Morphine analgesia and tolerance: a possible role of cyclic AMP


Effect of morphine sulfate in tro on the conversion of 3H-adenosine to 3H-cyclic AMP by rat brain tissue slices Tissues were incubated for 30 minutes at 37 oC. a


Pages: 9 to 12
Creation Date: 1977/01/01

Morphine analgesia and tolerance: a possible role of cyclic AMP

Department of Pharmacology, West Virginia University, Morgantown, West Virginia (USA). J.A. THOMAS
Department of Pharmacology, West Virginia University, Morgantown, West Virginia (USA). G.M. LING UN
Division of Narcotic Drugs, UN Office at Geneva (Switzerland).

There are at least two major facets of the action of morphine-like agents on the central nervous system. These are firstly, the immediate effects of the drugs on the neuronal plasma membrane, leading to changes in electrical activity and consequent changes in perception and behaviour, and secondly, the long-term effects of these agents on the structure and/or the enzymic complement of the cells leading to tolerance and physical dependence. These facets are poorly understood at the molecular level, thus making it exceedingly difficult to design new analgesics or to devise rational and effective approaches to the pharmacological management of drug dependent individuals.

There has been considerable research pertaining to molecular mechanisms of synaptic transmission. The results implicate adenosine 3', 5' - monophosphate (cyclic AMP) as a second messenger for certain neurotransmitter substances in the central nervous system. Early studies revealed that cyclic AMP and the enzyme catalyzing its synthesis, adenylate cyclase, were present in the brain at exceptionally high levels (Sutherland, Rall and Menon, 1962). It was also found that the activity of adenylate cyclase was increased by such catecholamine neurotransmitters as epinephrine and norepinephrine (Klainer et al., 1962). Moreover it is known that the rate of spontaneous discharge by the cerebellar Purkinje cell of the rat can be decreased by stimulation of the nucleus locus coeruleus and by direct iontophoretic application of either cyclic AMP or catecholamines (Siggins, Henriksen and Landis, 1976). These findings, which seem to implicate cyclic AMP in central neurotransmission, led to investigations of the relationships of narcotics and cyclic AMP in the brain. Investigators have also examined the effects of cyclic AMP on narcotic analgesia as well as the effects of narcotic drugs on brain levels of cyclic AMP and adenylate cyclase. Studies have also examined the role of this cyclic nucleotide in the development of narcotic tolerance and physical dependence.

The administration of exogenous cyclic AMP generally seems to antagonize morphine analgesia. Ho, Loh and Way (1972) found that mice pre-treated with cyclic AMP required higher doses of morphine (more than twice as much) to delay the tail-flick response to heat. Manipulating (i.e. raising or lowering) endogenous cyclic AMP levels with pharmacological agents also modifies the analgesic actions of morphine. In mice the reduction of brain cyclic AMP levels by either propranolol or imidazole increased the duration and intensity of action of a single injection of morphine (Contreras, Castillo and Quijada, 1972). In this study, theophylline, a phosphodiesterase inhibitor and hence an agent that can elevate cyclic AMP, did not inhibit morphine analgesia. Subsequently, however, theophylline was reported to have the ability to antagonize morphine analgesia in mice (Ho, Loh and Way, 1973 a).

Some compounds chemically related to cyclic AMP also seem to inhibit the acute antinociceptive effects of morphine. In mice, dibutyryl cyclic AMP blocks morphine analgesia (Ho, Lob and Way, 1973 a). Gourley and Beckner (1973) found that adenine, adenosine and non-cyclic adenine nucleotides were less effective inhibitors of morphine action than cyclic AMP. Adenosine derivatives (e.g. 2'-deoxyadenosine, 8-bromoadenosine) and other purines (e.g. hypoxanthine, uridine, cytidine) failed to disturb morphine analgesia.

Since cyclic AMP is thought to be involved in the mediation of noradrenergic transmission, drugs which affect monoamine metabolism have been studied in animals concomitantly receiving cyclic AMP. These studies involved pretreatment of animals with both cyclic AMP and drugs affecting monoamines, followed by an examination of the analgesic activity of morphine (Ho, Loh and Way, 1973a). In animals treated with pargyline, a monoamine oxidase inhibitor, injections of cyclic AMP failed to interfere with the analgesic actions of morphine. However when brain dopamine was elevated by the administration of L-dopa, cyclic AMP effectively blocked morphine analgesia; similarly, analgesia was blocked by cyclic AMP after serotonin levels were elevated by injections of tryptophan. Conversely, cyclic AMP injections failed to alter the analgesic dose of morphine after brain norepinephrine levels were increased by administration of dihydroxyphenylserine.

Several investigators have studied the changes in endogenous cyclic AMP induced by morphine. Shahid-Salles (1977) has found that a single subcutaneous injection of morphine (10 mg/kg) significantly elevated cyclic AMP levels in the rat cerebral cortex, but not in hypothalamus. Morphine also failed to alter cyclic AMP either in rat corpus striatum or nucleus accumbens (Carenzi, et al., 1975). In addition changes in tissue cyclic AMP levels also might result from the direct effects of morphine on those enzymes which catalyze the synthesis or degradation of this narcotic analgesic. For example, a single injection of mor phine can produce a 50 per cent increase in adenylate cyclase activity in homogenates of rat cerebral cortex (Chou, Ho and Loh, 1971). Cortical adenylate cyclase can also be stimulated by morphine in vitro. No stimulation of adenylate cyclase was observed in homogenates of cerebellum or hypothalamus regardless of whether morphine was administered in vivo or in vitro. Shahid-Salles et al., (1975) reported that conversion of 3H-adenosine to 3H-cyclic AMP by slices of cerebral cortex was enhanced in rats pre-treated with morphine. The in vitro application of morphine also stimulated 3H-cyclic AMP formation by tissue slices of the cortex but not by those of the hypothalmus, see table, page 10. Stimulation of adenylate cyclase and inhibition of phosphodiesterase in rat corpus striatum have been observed one hour after a single injection of morphine (Puri, Cochin and Volicer, 1975). These effects would be expected to raise cyclic AMP levels in this region of the brain, but no increase was evident twenty minutes after injection of the same doses (Carenzi et al., 1975).

Effect of morphine sulfate in tro on the conversion of 3H-adenosine to 3H-cyclic AMP by rat brain tissue slices Tissues were incubated for 30 minutes at 37 oC. a

Hypothalamus H-cyclic AMP (dmp/mg protein)
Morphine (M)Cerebral cortexHypothalamus
Cerebral cortex
10500 ± 550a
1050 ± 110
1 x 10- 3
11030 ± 600
1200 ± 95
11 195 ± 179
1 121 ± 127
2 x 10- 3
13 688 ± 476
1 159 ± 65
10742 ± 367
997 ± 57
5 x 10- 3
16731 ± 400 b
1227 ± 125
a. X± S.E.M. (N=5)
b. P ? 0.001

Shahid-Salles et al. (1975).

Is there a possible role for cyclic AMP in the development of tolerance and physical dependence? Injections of cyclic AMP reportedly accelerate the development of tolerance to morphine's analgesic effects and of physical dependence as indicated by withdrawal jumping precipitated by naloxone (Ho, Loh and Way, 1972, 1973 b). In addition, subcutaneous implantation of a morphine pellet for three days can elevate the endogenous formation and levels of cyclic AMP in central nervous system tissue. Shahid-Salles (1977) showed a significant increase in cyclic AMP in the rat cerebral cortex after this treatment. Likewise, there was an increase in the formation of 3H-cyclic AMP from 3H-adenosine by cerebral cortex slices (Shahid-Salles et al., 1975). In contrast, however, Chou, Ho and Loh (1971) reported a decrease in adenylate cyclase activity in homogenates of cerebral cortex from rats three days after morphine pellet implantation.

It is of interest that morphine pellets of similar size produced apparently opposite effects in these two studies (Chou, Ho and Loh, 1971 and Shahid-Salles et al., 1975) even though implanted for the same duration. While the underlying reasons for the difference are not readily apparent, the role of the cell membrane may be pivotal. Membrane associated adenylate cyclase activity certainly could be different in the two different preparations. Chou, Ho and Loh used homogenized tissues, while the investigation of Shahid-Salles were performed on intact cortical slices. In the latter studies the cell membrane of the slices was essentially intact, while in the homogenates they were largely disrupted. Moreover, phos-phodiesterase activity might also be changed by homogenization.

These observations indicate that morphine-like drugs can influence enzymes that synthesize and degrade cyclic AMP in areas of the central nervous system. Similar effects of opiate drugs have been noted in non-nervous tissues, especially in hormone-sensitive organs (cf. Thomas, Shahid-Salles and Donovan, 1976), but morphine-cyclic AMP interactions seem to be more prominent in the central nervous system. Further investigation of these interactions may lead to better understanding not only of narcotic analgesia, but also of tolerance and physical dependence.


Carenzi, A., D.L. Cheney, E. Costa, A. Guidotfi and G. Racagni, Actions of opiates, antipsychotics, amphetamine, and apomorphine on dopamine receptors in rat striatum: In vivo changes of 3', 5'-cyclic AMP content and acetylcholine turnover rate, Neuropharmacol. 14:927-939, 1975.

Chou, W.S., A.K.S. Ho and H.H. Loh, Effect of acute and chronic morphine and norepinephrine on brain adenyl cyclase activity, Proc. West. Pharmacol. Soc. 14:42-46, 1971.

Contreras, E., S. Castillo and L. Quijada, Effect of drugs that modify 3', 5'-AMP concentrations on morphine analgesia, J. Pharm. Pharrnac. 24:65-66, 1972.

Gourley, D.R.H. and S.K. Beckner, Antagonism of morphine analgesia by adenine, adenosine, and adenine nucleotides. Proc. Soc. Exptl. Biol. Med. 144:774-778, 1973.

Ho, I.K., H.H. Loh and E.L. Way, Effect of cyclic AMP on morphine analgesia, tolerance and physical dependence, Nature. 238:397-398, 1972.

Ho, I.K., H.H., Loh and E.L. Way, Cyclic adenosine monophosphate antagonism of morphine analgesia, J. Pharmacol. Exptl. Therap. 185:336-346, 1973 a.

Ho, I.K., H.H. Loh and E. L. Way, Effects of cyclic 3', 5'-adenosine monophosphate on morphine tolerance and physical dependence, J. Pharmacol. Exptl. Therap. 185:347-357, 1973 b.

Klainer, L.M., Y.-M. Chi, S.L. Freidberg, T.W. Rall and E.W. Sutherland, Adenyl cyclase IV. The effects of neurohormones on the formation of adenosine 3', 5'-phosphate by preparations from brain and other tissues, J. Biol. Chem. 237:1239-1243, 1962.

Puri, S.K., J. Cochin and L. Volicer, Effect of morphine sulfate on adenylate cyclase and phosphodiesterase activities in rat corpus striatum, Life Sci. 16:759-768, 1975.

Shahid-Salles, Kh. S., Doctoral Dissertation, Dept. of Pharmacology, West Virginia University (in press), 1977.

Shahid-Salles, Kh. S., B.K. Colasanti, C.R. Craig and J.A. Thomas, Changes in cerebral cortical cyclic AMP formation in the rat after acute and chronic treatment with morphine, Fed. Proc. 34:713, 1975.

Siggins, G.R., S.J. Henriksen and S.C. Landis, Electro-physiology of Purkinje neurons in the weaver mouse: Iontophoresis of neurotransmitters and cyclic nucleotides, and stimulation of the nucleus locus coeruleus., Brain Res. 114:53-69, 1976.

Sutherland, E.W., T.W. Rall and T. Menon, Adenyl cyclase I. Distribution, preparation and properties. J. Biol. Chem. 237:1120-1227, 1962.

Thomas, J.A., Kh. Shahid-Salles and M.P. Donovan, "Effects of narcotics on the reproductive system" in Advances in Sex Hormone Research, III, J.A. Thomas and R.L. Singhal (eds.) (in press), 1976.