The phytocannabinoid Δ9-tetrahydrocannabivarin modulates synaptic transmission at central inhibitory synapses

Yu-Ling Ma, Benjamin J. Whalley and Gary J. Stephens. School of Pharmacy, University of Reading, Reading, UK.

The cannabinoid receptors CB1 and CB2 are G protein-coupled receptors, predominantly linked via Gi/o subunits to the inhibition of adenylate cyclase. In addition to being the molecular targets for endocannabinoids, cannabinoid receptors can also be activated by phytocannabinoids present in Cannabis sativa. In the cerebellum, an important centre for movement and balance, CB1 receptors localised to basket cell interneurone (IN) terminals onto Purkinje cells (PCs) modulate inhibitory GABAergic transmission (Diana et al., 2002). The phytocannabinoid, Δ9-tetrahydrocannabivarin (THCV) has been recently shown to have functional effects in the periphery (Thomas et al., 2005); in the current study, the effects of THCV were examined at IN-PC synapses and actions compared with synthetic receptor agents to reveal the first reports of functional central effects for THCV.

Whole cell voltage-clamp recordings of miniature inhibitory postsynaptic currents (mIPSCs) were made from PCs in parasagittal cerebellar slices prepared from 3-5 week old male TO mice (10-20 g) in accordance with Home Office-approved procedures. The CB1/CB2 agonist, WIN55,212-2 (5μM), the selective CB1 antagonist, AM251 (2μM) or THCV (58μM ) were bath applied in the presence of TTX, NBQX and CGP 55845. Drugs were made as 1000 x stock solutions in vehicle (DMSO (WIN55,212-2 AM251 and CGP 55845); ethanol (THCV) or water (TTX, NBQX)) and dissolved to final concentrations in standard aCSF. Changes in mean mIPSC amplitude and frequency were analysed at steady-state; holding potential was -70mV and experiments performed at RT.

WIN55,212-2 caused a reduction in mean mIPSC frequency of 37.7 ± 2.1 % (n=25, paired t-test P<0.001). The WIN55,212-2-induced reduction was reversed by AM251; in 8 cells tested, mIPSC frequency typically was increased above control levels (138 ± 9.6 %, n=8; ANOVA plus Tukey’s HSD test P<0.01). In addition, AM251 applied alone caused a further significant increase in mIPSC frequency from control values (143 ± 8.7 %, n=6; paired t-test P<0.001). The WIN55,212-2-induced reduction was also reversed by THCV; in 6 cells tested, mIPSC frequency typically was increased above control levels (158 ± 15 %, n=6; ANOVA plus Tukey’s HSD test P<0.05). In addition, THCV applied alone caused a further significant increase in mIPSC frequency from control values (199 ± 32 %, n=6; paired t-test P<0.001). In all cases, mean mIPSC amplitude was not significantly changed throughout experiments, consistent with a presynaptic site of action.

These data are consistent with THCV acting at cannabinoid receptors at central IN-PC synapses. Overall, the increase in GABA release caused by THCV or AM251 suggests either an antagonism of basal inhibition (e.g. caused by endocannabinoid release) or an inverse agonist effect at CB1 receptors in this preparation.

Diana MA et al. (2002). J Neurosci 22: 200-208.
Thomas A et al. (2005). Br J Pharmacol 146: 917-926.

 Work supported by The Wellcome Trust. THCV was a kind gift from GW Pharmaceuticals.