The G-protein coupled cannabinoid receptors CB₁ and CB₂ are molecular targets for endocannabinoids and are also modulated by exogenous synthetic agents and phytocannbinoids derived from Cannabis sativa. We have recently shown that the phytocannabinoid Δ⁹-tetrahydrocannabinvarin (Δ⁹-THCV) acts to increase inhibitory neurotransmission in synapses between interneurones and Purkinje cells (IN-PC) in the mouse cerebellum, suggesting either an antagonist or inverse agonist effect at CB₁ receptors (Ma et al., BPS Oxford Meeting 2006 abstract). Additionally, Δ⁹-THCV has recently been reported to act as a CB₁ receptor antagonist in membranes from whole mouse brain (Thomas et al., 2005). In the current study, the effect of Δ⁹-THCV on [³⁵S]GTPγS binding in mouse cerebellar membranes was investigated and compared with synthetic CB₁-selective agents, to complement electrophysiological findings and investigate mode of action in a specific brain area. Cerebellar membranes were prepared from male TO mice (3-5 weeks old). [³⁵S]GTPγS binding assays were performed to establish log concentration-response curves (10pM-10μM) for the synthetic CB₁ receptor agonists WIN55212-2 and CP55940, the antagonist/inverse agonist AM251, and Δ⁹-THCV. The effects of AM251 (100pM-10nM) or Δ⁹-THCV (100nM-5μM) on log concentration-response curves for WIN55212-2 were also investigated. AM251 and Δ⁹-THCV (Figure 1) alone showed no concentration dependent reduction of [³⁵S]GTPγS binding. AM251 and Δ⁹-THCV (Figure 1) produced rightward shifts in the log concentration-response curve for WIN55212-2. Preliminary Schild analysis gave apparent K_B values of 63 pM (AM251) and 68 nM (Δ⁹-THCV). These results suggest that AM251 and Δ⁹-THCV act as antagonists rather than inverse agonists at CB₁ receptors in the cerebellum. Therefore, the observed increase in inhibitory neurotransmission at IN-PC synapses by AM251 and Δ⁹-THCV may be due to antagonism of CB₁ receptors (which will block endocannabinoid-induced decreases in in neurotransmission) in the cerebellum.