Parasympatholytic activity of (-)-Δ9-trans- Tetrahydrocannabinol in mongrel dogs

EUROPEAN JOURNAL OF PHARMACOLOGY 19 (1972) 301-304. NORTH-HOLLAND PUBLISHING COMPANY

Short communication PARASYMPATHOLYTIC

A C T I V I T Y O F ( - ) - A9 . T r a n s -

TETRAHYDROCANNABINOL

IN M O N G R E L

DOGS

lcilio CAVERO, Joseph P. BUCKLEY and Bhagavan S. JANDHYALA Department o f Pharmacolog3,, School o f Pharmacy, University of Pittsburgh, Pittsburgh, Pa. 15213, U.S.A.

Received 27 March 1972

Accepted 26 May 1972

I. CAVERO, J.P. BUCKLEY and B.S. JANDHYALA, Parasympatholytic activi O, of(-)-~x9-trans-tetrahydrocan nabinol in mongrel dogs, European J. Pharmacol. 19 ( 1972) 301 - 304. (- )-A g-trans-Tetrahydrocannabinol (A9-THC) (0.312-5 mg/kg) shifted the frequency-response curves of vagal stimulation to the right and attenuated salivation induced by chorda tympani stimulation in dogs. This parasympatholytic activity of Ag-THC was neither atropine-like in nature nor was it due to ganglionic blockade. It is postulated that A 9-THC may interfere with the release of acetylcholine. ,~9-Tetrahydrocannabinol Stellate ganglia

Vagus Carbachol

Chorda tympani Cervical spinal section

1. INTRODUCTION

2. MATERIALS AND METHODS

(-)-Ag-trans-tetrahydrocannabinol (A9-THC), a constituent of marihuana, was recently shown to affect neither the chorda tympani-stimulated salivary secretion nor the bradycardia in response to vagal stimulation in cats (Gill et al., 1970). However, several compounds related to Ag-THC have been reported to interfere with the bradycardia elicited by electrical stimulation of the vagus in dogs (Garriot et al., 1968). Similar effects were confirmed with A9-THC in a preliminary study from this laboratory (Kubena et al., 1971). In addition, A9-THC was demonstrated to reduce acetylcholine output from parasympathetic nerve endings of guinea-pig ileum (Gill et al., 1970; Layman and Milton, 1971) and to inhibit the responses to acetylcholine in the same preparation (Layman and Milton, 1971). The present investigation deals with the effects of Ag-THC on certain parasympathetically innervated organs in mongrel dogs.

Mongrel dogs (8 12 kg) were anesthetized with sodium pentobarbital, 35 mg/kg, i.v., and placed under artificial respiration (Harvard Model 607). The peripheral trunck of the right vagus was stimulated with platinum electrodes at supramaximal voltage ( 4 - 8 V; pulse duration, 5 msec)for a period of 15 sec with various frequencies ( 1 . 5 - 2 4 Hz). Heart rate responses to vagal stimulation were obtained prior to and 1 5 - 3 0 min following the administration of selected doses of ethanolic solution of AU-THC, 0.312, 2.5 and 5 mg/kg, into the femoral vein. The above procedure was repeated in a group of dogs receiving 2.5 mg/kg of A9-THC in which bilateral vagotomy plus section of the spinal cord at C 2 - 4 level were performed. In 4 additional dogs the chorda tympani was electrically stimulated (2--6 V; D = 1 msec; F = 5 - 2 0 Hz) for 30 sec and the amount of saliva from the catheterized submaxillary duct was weighed before and after the administration of A9-THC, 2.5

302

I. Cavero et aL, Parasyrnpatholytic effects o f A 9- THC

mg/kg, i.v. The alterations in heart rate and salivary secretion to carbachol, 0.25, 0.5/~g/kg, were also investigated in the above preparations• The preganglionic and postganglionic branches to the right stellate ganglion were stimulated ( 2 - 8 V; D = 1 msec; F = 0 . 5 - 1 6 Hz) and the increase in heart rate was recorded to assess the effects of A9-THC, 1 and 2.5 mg/kg on ganglionic transmission. Statistical analysis of pairs of regression lines was conducted as described by Conolly et al. (1971).

3.

ward the right indicating an inhibition of responses. In order to verify the possibility that this vagal antagonistic effect of A 9 - T H C w a s not secondary to marked reduction in heart rate elicited by the compound itself (A H.R. -+ S.E.M.; - 4 8 . 5 -+ 8.7 after 5 mg/kg; 30.0 +- 9.0 after 2.5 mg/kg; 22.7 -+ 8.8 after 0.312 mg/kg) bilateral vagotomy and cervical cord section at C2 - 4 level were performed. This procedure in addition to lowering the heart rate (A H.R. = --46.5 -+ 5.6) also eliminated the bradycardia produced by A9-THC. The frequency-bradycardia curve to vagal stimulation after cervical cord section demonstrated a significant parallel shift to the right when compared with control (fig. 1). However, following the administration of A 9 - T H C to this group of dogs, the frequency response curve was moved significantly further to the right indicating a direct vagolytic effect of this compound (fig. 1).

RESULTS

Figure 1 summarizes the bradycardia induced by vagal stimulation prior to and 1 5 - 3 0 rain following the administration of selected doses of A9-THC. The 3 curves after A9-THC were significantly shifted to-

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Fig. 2. Effects of 2x9-THC, 0.312, 2.5 and 5 mg/kg, i.v., on the decrease in heart rate (H.R.) (X ± S.E.M.) to electrical stimulation of the right vagus t r u n k in dogs tn = 6 per group). The control responses for the 3 groups (n = 18) were pooled together. An asterick (*) indicates that the f r e q u e n c y response curves do n o t depart from parallelism and are significantly non-coincident with the control.

I. Cavero et al., Parasympatholytic effects of/X9-THC

Table 1 Effects of A9-THC, 2.5 mg/kg, i.v., on the salivary output (mg) induced by electrical stimulation of the chorda tympani and by carbachol in dogs (n = 4).

303

A9-THC failed to alter the negative chronotropic effects produced by carbachol; 0.25 and 0.5 /ag/kg, i.v., of carbachol reduced the heart rate by 13.7 + 2.4 and 24.5 -+ 5.2 beats/min respectively, and after A9-THC, 2.5 mg/kg, i.v., these changes were 16.2 + 3.7 and 27.7 + 9.27 (n = 5). Table 1 reports the amount of saliva secreted from the submaxillary glands following stimulation o f the chorda tympani. Salivary secretion elicited by nerve stimulation was inhibited by Ag-THC, but no such inhibition was found on the carbachoMnduced salivation. F r e q u e n c y - r e s p o n s e curves to stimulation o f the right stellate ganglion before and after zX9-THC overlapped indicating that no impairment o f transmission across the ganglia occurred.

The parasympatholytic effect of A9-THC was further confirmed by the ability of this compound to significantly reduce the salivary secretion induced by electrical stimulation of the chorda tympani. It is evident that these effects were not due to a blockade of muscarinic receptors since A9-THC failed to alter the effects of carbachol on heart rate or salivation. It seems improbable that A 9-THC might possess a specific parasympathetic ganglionic blocking property, since this compound did not alter the transmission across the stellate ganglia in this study or the superior cervical ganglia in cats {Layman and Milton, 1971). It has recently been reported that A9-THC reduces the amount of acetylcholine released in transmurally stimulated ileum of guinea pigs (Layman and Milton, 1971; Gill et al., 1970). A similar mechanism may be suggested to explain the present findings. Our observations do not support the previous findings of Gill et al. {1970) who failed to observe parasympatholytic activity of A9-THC in cats under chloralose anesthesia. This discrepancy may be due to a difference in species. Johnson and Domino (1971) reported that marihuana smoking produces certain atropine-like effects in man such as dryness of the mouth and tachycardia. Some of these effects may be explained by the result of this investigation. In conclusion, these studies indicate that A9-THC possesses parasympatholytic activity and that this effect is neither due to alteration in ganglionic transmission nor due to blockade of muscarinic receptors. It is postulated that A9-THC may interfere with the release of acetylcholine from postganglionic parasympathetic neuronal sites.

4. DISCUSSION

ACKNOWLEDGEMENTS

This investigation demonstrated a dose-dependent vagolytic activity o f Ag-THC in dogs under pentobarbital anesthesia. These effects were observed in animals in which A9-THC produced bradycardia and to a lesser degree in spinalectomized animals, in which A9-THC failed to induce bradycarida. Thus, it is apparent that the ability of A9-THC to cause a parallel shift of the f r e q u e n c y - r e s p o n s e curve to the right (fig. 2) was due to a direct vagal inhibitory action o f this compound as well as due to its ability to lower heart rate.

This research was supported by P.H.S.P. Training Grant GM 1217-07. The authors express their tanks to Dr. E. Gyang for his excellent assistance during various procedures. A 9-THC was kindly supplied by N.I.M.H.

Frequency (Hz) 5 10 20

Saliva (mg) (X ± S.E.M.) Control 485.5 ± 78.9 895.0± 48.8 1373.0 ± 67.1

Treated 95.5~- 59.2a 440.0± 74.2a 961.0-+ 135.0a

Carbachol (~g/kg, i.v.) 0.25 0.5

90.0± 68.6 652.5 ± 245.7

112.5 ± 37.5 1052.5 -+ 321.5

a p < 0.05 (paired t-test).

REFERENCES Conolly, M.E., D.S. Davies, C.T. Dollery and C.F. George, 1971, Resistance to />receptor stimulants (a possible explanation for the rise in asthma deaths), Brit. J. Pharmacol. 43, 389.

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L Cavero et aL, Parasympatholytic effects o f ~ 9-THC

Garriott, J.C., R.B. Forney, F.W. Hughes and A.B. Richards, 1968, Pharmacologic properties of some cannabis related compounds, Arch. Intern. Pharmacodyn. 171,425. Gill, E.W., W.D.M. Paton and R.G. Pertwee, 1970, Preliminary experiments on the chemistry and pharmacology of cannabis, Nature (London) 228, 134. Johnson, S. and E.F. Domino, 1971, Some cardiovascular

effects of marihuana smoking in normal volunteers, Clin. Pharmacol, Therap. 12, 762. Kubena, R.K., I. Cavero, B.S. Jandhyala and J.P. Buckley, 1971, Certain respiratory and cardiovascular effects of A 9-THC in dogs, The Pharmacologist 13, 311 (Abs.). Layman, J.M. and A.S. Milton, 1971, Some actions of /x ltetrahydrocannabinol and cannabidiol at cholinergic junctions, Brit. J. Pharmacol. 41,739P.