Effects of cannabis resin on learning by repetition in the rat

Sections

Materials and methods
Results and conclusions
Summary

Details

Author: A. PASQUALE, , G. COSTA, , A. TROVATO
Pages: 55 to 61
Creation Date: 1978/01/01

Effects of cannabis resin on learning by repetition in the rat

A. PASQUALE,
G. COSTA,
A. TROVATO
Institute of Pharmacology and Pharmacognosy, Messina University

It is well known that the active constituents of cannabis, particularly Δ 9-tetrahydrocannabinol Δ 9-THC), have an appreciable influence on the behaviour of guinea-pigs [ 1] - [ 14] and induce changes in the metabolism of the CNS. Not all the molecular mechanisms of the action of cannabis resin are clear, but a number of authors habitually link them with alteration of the metabolism of the biogenic amines. It is true that either the resin or Δ 9-THC causes changes in the levels of serotonin (5-HT) [ 15] - [ 19] , noradrenaline (NA) [ 16] , [ 17] , [ 20] , 5-hydroxyindolacetic acid (5-HIAA) and normetanephrine [ 21] in many species of animal. Our own earlier research has shown that, in the rat, cannabis resin at a dose corresponding to 14 mg of Δ 9-THC per kg causes significant increases in the levels of cerebral 5-HT, 5-HIAA, NA and homovanillic acid (HVA) and is capable of preventing the drop in these levels caused by audiogenic stress (22-24). The influence of cannabis on behaviour is linked by some authors [ 25] - [ 26] with the size of the dose and may be associated with variations in the metabolism of the transmitter substances.

The purpose of the experiments described in this report was to study the effects of different doses of cannabis resin on learing by repetition in a water-filled maze and the conditioned avoidance reflex in the rat.

Materials and methods

The rats used in our experiments were selected at random from among albino adult males of the Wistar-Glaxo breed, with a weight of 140-160 g. All were kept under constant conditions both before and during the experiments.

The cannabis resin employed was first subjected to analysis by gas chromatography [ 27] , [ 28] , which showed that it contained 7.85 per cent of L-Δ 9-tetra-hydrocannabinol, 10 per cent of cannabidiol, and 12.3 per cent of cannabinol. It was administered by intraperitoneal injection, in a solution of neutral olive oil, in doses corresponding to a Δ9-THC content of 0.5 or 5 mg/kg of body weight.

Effects of cannabis resin on the conditioned avoidance reflex (percentage of shocks avoided)

Session

Controls

Cannabis dose A

Cannabis dose B

1 29.8 31.4 30.7
2 41.6 52.9 44.2
3 53.3
77.3 *
39.8
4 58.9
79.5*
31.1*
5 63.4
81.8 *
28.9 *

* P < 0.05 by comparison with controls;

Dose A = dose corresponding to 0.5 mg of r 9-THC per kg;

Dose B = dose corresponding to 5 mg of r 9-THC per kg.

Water-filled maze

Figure 1 shows the layout of the water-filled maze which was used. The entire complex was filled to a height of 25 cm with water which was maintained at a temperature of 15° C. The means of exit was a metal grid resting on the bottom of the maze and inclined at 45° to the horizontal. Runs through the maze were timed; on completing them, the rats left the maze via the ramp and were wiped dry and returned to their cages.

Each experiment lasted five days. On the first day, a number of animals were picked out at random and only those which proved unable to find the exit from the maze within five minutes of being placed in the "entrance" compartment were retained for use in the experiments. The animals which remained after this eliminatory stage were divided into three groups of 12 rats each: the first two groups each received a different dose of cannabis, while the third, which was used as a control, received only neutral olive oil. The animals were put through the maze twice a day, with the cannabis being administered one hour before the morning test and one hour after the afternoon test. In addition to timing each animal's runs through the maze, we noted the number of errors it made by entering compartments A, B, C, D, E or F or moving in the opposite direction to thc exit.

Our statistical analysis of trip times and errors was based on the averages for the two daily runs; in order to achieve improved stabilization of the variance, all the data were subjected to logarithmic transformation. The significance of the difference between the results of the treated groups and those of the control group was assessed using Student's test for variables not presented in pairs; we considered as significant cases where P<0.05.

FIGURE I - Plan of the water-filled maze

Full size image: 11 kB, FIGURE I - Plan of the water-filled maze

Conditioned avoidance reflex

The experiments were carried out using three groups of 10 animals, which received either the resin in oil or simply olive oil one hour before each test, in the same doses and the same manner as the animals used in the maze tests. The animals were trained individually using an automatic conditioner (Basile Comerio Automatic Reflex Conditioner). The latter comprises an acrylic resin cage divided in half by a partition containing a hole which permits movement from one compartment to the other. The floor consists of a metal grid through which an electric shock may be transmitted. There is a signal lamp in the centre of the cage. Each rat was placed in the cage and allowed 10 minutes to adapt itself to its surroundings before being exposed to the light stimulus for six seconds. When an animal failed to move into the opposite compartment within three seconds after the signal light was switched on, it was subjected to an electric shock transmitted through the flooring grid. Each animal spent one session a day in the cage over a period of five consecutive days. It was subjected to the light stimulus 40 times during each session, at a rate of two stimulations per minute. The cannabis was administered in the doses mentioned above, with each animal being given its first injection one hour before it was placed in the conditioner and the second 12 hours later.

We expressed the results obtained as a percentage of the shocks avoided, which we took to be indicative of the degree of conditioning. The significance of the difference between the groups was assessed on the basis of Student's test; we took to be significant cases in which P<0.05.

Results and conclusions

The results obtained showed that the lower dose of cannabis resin (corresponding to 0.5 mg of Δ 9-THC per kg) led to improved learning, with reductions in the time taken to move through the water-filled maze (figure 2) and in the number of errors (figure 3). Similarly, the percentage of shocks avoided in the CAR experiments was appreciably higher than for the animals in the control group, demonstrating an increase in activity with regard to the reflex conditioned by the light stimulus (see table).

On the other hand, the larger dose of resin (corresponding to 5 mg of Δ 9-THC per kg) occasioned a slight but insignificant improvement in the animals' performance on the first day of treatment and a clear decline in their learning capacity on succeeding days, irrespective of the test method concerned.

These results are confirmed by the observations of Devis et al. [ 25] and Grisham et al. [ 26] : when administered to rats in small doses, THC causes slight excitation and an increase in spontaneous motility, but when given in higher doses, it has the opposite effect.

The two-phase effects occasioned by THC have also been demonstrated in relation to other parameters: Sofia [ 29] claims that, with rats, doses of THC of between 0.5 and 1 mg/kg cause hyperthermia, whereas higher doses, of 4-8 mg/kg, cause marked hypothermia.

Holtzman et al. [ 16] and Ho et al. [ 19] have also found two-phase effects on the metabolism of the cerebral monoamines, with an increase in the levels of 5-HT and a fall in the levels of NA after the administration of r 9-THC in doses of between 5 and 50 mg/kg of body weight, and opposite effects on the levels of these two transmitters at doses of 2 mg/kg.

FIGURE 2

Full size image: 18 kB, FIGURE 2

Effects of different doses of cannabis resin on learning, as assessed by the water-filled maze technique:

Average times for the two runs per day by the animals retained after the elimination test(s);

Dose A = dose corresponding to 0.5 mg of Δ 9-THC per kg;

Dose B = dose corresponding to 5 mg of Δ 9-THC per kg;

* P<0.05 by comparison with controls.

Finally, Dolby et al. [ 30] have observed in rats that doses of Δ 9-THC of between 0.1 and 1 mg/kg cause a slight rise in the level of cerebral c-AMP, whereas doses of between 2 and 10 mg/kg cause a decline in the level of this nucleotide.

These observations lead us to consider that the effects of cannabis on the learning ability of rats which we found by using differing techniques and differing doses are probably attributable to complex reciprocal action between the active constituents of the resin, particularly THC, and the metabolisms of the cerebral monoamines and c-AMP.

FIGURE 3

Full size image: 17 kB, FIGURE 3

Effects of different doses of cannabis resin on learning, as assessed by the water-filled maze technique; average number of errors during the two runs per day by the animals retained after the elimination test(s);

Dose A = dose corresponding to 0.5 mg of Δ 9-THC per kg;

Dose B = dose corresponding to 5 mg of Δ 9-THC per kg;

* P<0.05 by comparison with controls.

Summary

Studies were made of the effects of cannabis resin in differing doses on a type of learning by repetition, using the water-filled maze technique, and on the conditioned avoidance reflex in the rat.

When administered at a dose corresponding to 0.5 mg of Δ 9-THC per kg, the resin led to improved learning in both the types of test employed; opposite effects were observed with the higher dose (corresponding to 5 mg of Δ 9-THC per kg).

These effects are probably attributable to complex reciprocal action between the active constituents of the resin, particularly THC, and the metabolisms of the cerebral monoamines and c-AMP.

References

001

R.L. Foltz, A.F. Fentiman, Jr., E.G. Leighty, J.L. Walter, H.R. Drewes, W.E. Schwartz, T.F. Page, Jr., and E. B. Truitt, Jr. Science, N.Y. 168, 844, 1970.

002

L.E. Hollister. Science, N.Y. 172, 21, 1971.

003

M. Perez-Reyes, M.C. Timmons, M.A. Lipton, K.H. Davis, and M.E. Wall. Science, N.Y. 177, 633, 1972.

004

L. Lemberger, R.E. Crabtree, and H.M. Rowe. Science, N.Y. 173, 62, 1972.

005

S.H. Burstein, F. Menezes, E. Williamson and R. Mechoulam. Nature, Lond. 225, 87, 1970.

006

M. Galanter, R.S. Wyatt, L. Lemberger, H. Weingartner, R.B. Vaughan and W.T. Roth. Science, N.Y. 176, 934, 1972.

007

L. Lemberger, N.R. Tamarkin, J. Axelrod, I.J. Kopin. Science, N.Y. 177, 72, 1971.

008

R. Mechoulan. Science, N.Y. 168, 1159, 1970.

009

I.M. Nilsson, S. Agurell, J.L.G. Nilsson, A. Ohlsson, F. Sandberg and M. Wahlquist. Science, N.Y. 168, 1228, 1970.

010

H. Isbell, C. Gorodetzsky, D. Janinski, V. Claussen, F. Spulak and F. Korte. Psychopharmacologia 11, 184, 1967.

011

L. Hollister, R. Richards and H. Gillespie. Clin. Pharmac. Ther. 9, 783, 1968.

012

Y. Gaoni and R. Mechoulam. J. Am. Chem. Soc. 86, 1646, 1964.

013

H.D. Christensen, E.I. Freudenthal, J.T. Gidley, R. Rosenfeld, G. Boegli, L. Testino, D.R. Brine, C.G. Pitt and M.E. Wall. Science, N.Y. 172, 165, 1971.

014

R. Mechoulam, A. Sani, H. Edery and Y. Grunfield. Science, N.Y. 169, 611, 1970.

015

B.C. Bose, A.Q. Saifi and A.W. Bhagwat. Arch. int. Pharmacodyn. Ther. 147, 291, 1964.

016

D. Holtzman, R.A. Lovell, J.J. Jaffe and D.X. Freedman. Science, N.Y. 163, 1464, 1969.

017

B.L. Welch, A.S. Welch, F.S. Messina and H.J. Berger. Res. Commun. Chem. Path. Pharmac. 2, 382, 1971.

018

R.D. Sofia, B.N. Dixit and H. Barry III. Life Sci. Pt. I, 10, 425, 1971.

019

B.T. Ho, D. Taylor, G. E. Fritchie, L. F. Englert and W. M. McIsaac. Brain. Res. 38, 163, 1972.

020

J.J. Schildkraut and D.H. Efron. Psychopharmacologia 20, 191, 1971.

021

B.T. Ho, D. Taylor, L.F. Englert and W.M. McIsaac. Brain Res. 31, 233, 1971.

022

G. Costa, R. Costa de Pasquale and C. Scarpignato. Il Farmaco Ed. Prat. 32, 180, 1977.

023

G. Costa and C. Scarpignato. Il Farmaco Ed. Prat. 32, 152, 1977.

024

A. De Pasquale, G. Costa and C. Scarpignato. Rivista di Farmacologia e Terapia 8, 71, 1977.

025

W. Davis, J. Moreton, W. King and H. Pace. Res. Commun. Chem. Path. Pharmac. 3, 29, 1972.

000

Effects of cannabis resin on learning in the rat 61

026

M.C. Grisham and D.P. Ferraro. Psychopharmacologia 27, 163, 1972.

027

P.P. Lerner. The precise determination of tetrahydrocannabinol in marihuana and hashish, Bulletin on Narcotics, XXI: 3, 41, 1969.

028

P.S. Fetterman, E.S. Kelt, C.W. Waller, O. Guerrero, N. J. Doorembos and M.W. Quimby. J. Pharm. Sci. 60, 1247, 1971.

029

R.D. Sofia. Res. Commun. Chem. Path. Pharmac. 4, 281, 1972.

030

T.W. Dolby and L.J. Kleinsmith. Biochem. Pharmacol. 23, 1817, 1974.