I have suggested that G_βγ subunits dissociated from G_q/11 linked with M₃-receptors reduce the GABA release through an effect on the release machinery, when M₃-muscarinic ACh receptors at the presynaptic terminal of a putative striato-nigral projection fiber are activated. In order to get some more evidences on this hypothesis, I analyzed the effects of Calphostin C (1 μM), an inhibitor of protein kinase C, ω-Conotoxin GVIA (1 μM), a selective N-type Ca²⁺ channel blocker and BAPTA-AM (30 μM), a membrane permeable Ca²⁺ chelator of intracellular store. All these drugs reduced the amplitude of IPSC. Muscarine significantly inhibited these reduced IPSCs in the solutions with the drugs. Thus it was again indicated that the activation of protein kinase C, the Ca²⁺ influx to the presynaptic terminals and the Ca²⁺ release from intracellular store sites at the presynaptic terminals were not the main mechanisms of the muscarinic inhibition of IPSC. This could insist on the justice of the hypothesis mentioned above. Muscarine also reduced the amplitude of the mono- and poly-synaptic glutamatergic EPSCs onto SNr GABA neurons evoked by the electrical stimulation to subthalamic nucleus (STN). This result suggested that muscarine inhibited the glutamatergic EPSCs onto not only SNr GABA neurons but also STN neurons, since it have been reported that the polysynaptic EPSC onto SNr GABA neurons was generated the recurrent glutamate synapses in the STN. [J Physiol Sci. 2007;57 Suppl:S152]