Δ9-Tetrahydrocannabivarin (Δ9-THCV) increases inhibitory Neurotransmission in the mouse cerebellum

Yu-Ling Ma, Imogen Dennis, Samantha Weston, Ben Whalley, Gary Stephens. University of Reading, Reading, United Kingdom.

The phytocannabinoid Δ9-tetrahydrocannabivarin (Δ9-THCV) is a propyl analogue of the cannabinoid CB1 receptor agonist, Δ9-tetrahydrocannabinol. Δ9-THCV has recently been reported to be a competitive CB1 and CB2 antagonist, but also to have non-CB1 receptor-mediated agonist effects in smooth muscle (Thomas et al., 2005). Here, we describe the first functional studies of effects of Δ9-THCV in the CNS. We have used single and multi-electrode electrophysiological recording in acute brain slices, in combination with GTPγS binding assays in isolated membranes, to examine the mode of action of Δ9-THCV in the cerebellum. We compare Δ9-THCV effects to those of synthetic agents AM251 and SR141617A, reported to act as antagonists/inverse agonists at CB1 receptors.

Using whole-cell patch clamp recordings from Purkinje cells in mouse (male TO strain, 3-5 weeks old) cerebellar brain slices, we have recently shown that the CB1 agonist WIN-55,212-2 caused a reduction in mean miniature inhibitory post-synaptic current (mIPSC) frequency (Ma et al. BPS Oxford meeting 2006 abstract). The WIN-55,212-2–induced reduction was increased to levels above control by subsequent application of AM251 or Δ9-THCV. Moreover, when applied alone, AM251, SR141716A or Δ9-THCV all acted to increase mIPSC frequency. We have now made extracellular multi-electrode array (8x8 electrode array) recordings of spontaneous activity in the same preparation. Recording electrodes were positioned in close proximity to the Purkinje cell layer in cerebellar slices and the effects of AM251 (2 μM) or Δ9-THCV (5 μM) were examined (n=6 slices for all data, minimum 6 electrodes’ data from each slice, significance by Mann-Whitney U test). Both agents reduced spontaneous excitatory spike firing frequency (Δ9-THCV: 25 ± 12% of control; AM251: 71 ± 9% of control; both P<0.05).

In parallel studies, we measured the effects of synthetic CB receptor agents and Δ9-THCV on [35S]GTPγS binding to mouse cerebellum membranes (see Dennis et al., Abstract 0063, this meeting). Log concentration-response curves for WIN55,212-2 indicated adose-dependent agonist action. WIN55, 212-2 curves were shifted rightward by AM251 (100pM-10nM) or Δ9-THCV (100nM-5μM) with KB values of 28 pM (AM251) and 7 nM (Δ9-THCV), suggesting CB1 antagonism. AM251 and Δ9-THCV (at supra-μM concentrations only) also depressed basal [35S]GTPγS binding, suggesting some inverse agonist activity.

Overall, we provide evidence that Δ9-THCV possesses both CB1 receptor antagonist and inverse agonist activity. Functionally, Δ9-THCV increases GABA release onto Purkinje cells in the cerebellum and this action correlates with a reduction in spontaneous excitatory transmission. The potent effect of Δ9-THCV on network activity may suggest a differential effect at inhibitory vs excitatory synapses.

Thomas et al. (2005) Br J Pharmacol 146: 917-926