Cannabinoid receptor modulation of synapses received by cerebellar Purkinje cells

The high density of cannabinoid receptors in the cerebellum and the degradation of motor coordination produced by cannabinoid intoxication suggest that synaptic transmission in the cerebellum may be strongly regulated by cannabinoid receptors. Therefore the effects of exogenous cannabinoids on synapses received by Purkinje cells were investigated in rat cerebellar slices. Parallel fiber-evoked (PF) excitatory postsynaptic currents (EPSCs) were strongly inhibited by bath application of the cannabinoid receptor agonist WIN 55212-2 (5 microM, 12% of baseline EPSC amplitude). This effect was completely blocked by the cannabinoid CB1 receptor antagonist SR 141716. It is unlikely that this was the result of alterations in axonal excitability because fiber volley velocity and kinetics were unchanged and a cannabinoid-induced decrease in fiber volley amplitude was very minor (93% of baseline). WIN 55212-2 had no effect on the amplitude or frequency of spontaneously occurring miniature EPSCs (mEPSCs), suggesting that the effect of CB1 receptor activation on PF EPSCs was presynaptically expressed, but giving no evidence for modulation of release processes after Ca(2+) influx. EPSCs evoked by climbing fiber (CF) stimulation were less powerfully attenuated by WIN 55212-2 (5 microM, 74% of baseline). Large, action potential-dependent, spontaneously occurring inhibitory postsynaptic currents (sIPSCs) were either severely reduced in amplitude (<25% of baseline) or eliminated. Miniature IPSCs (mIPSCs) were reduced in frequency (52% of baseline) but not in amplitude, demonstrating suppression of presynaptic vesicle release processes after Ca(2+) influx and suggesting an absence of postsynaptic modulation. The decrease in mIPSC frequency was not large enough to account for the decrease in sIPSC amplitude, suggesting that presynaptic voltage-gated channel modulation was also involved. Thus, while CB1 receptor activation reduced neurotransmitter release at all major classes of Purkinje cell synapses, this was not accomplished by a single molecular mechanism. At excitatory synapses, cannabinoid suppression of neurotransmitter release was mediated by modulation of voltage-gated channels in the presynaptic axon terminal. At inhibitory synapses, in addition to modulation of presynaptic voltage-gated channels, suppression of the downstream vesicle release machinery also played a large role.