We investigated the mechanisms of presynaptic inhibition of GABAergic neurotransmission by group III metabotropic glutamate receptors (mGluRs) and GABAB receptors, in dopamine (DA) neurons of the substantia nigra pars compacta (SNc). Both the group III mGluRs agonist L-(+)-2-amino-4- phosphonobutyric acid (AP4, 100 μM) and the GABAB receptor agonist baclofen (10 μM) reversibly depressed the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) to 48.5 ± 2.7 and 79.3 ± 1.6% (means ± SE) of control, respectively. On the contrary, the frequency of action potential-independent miniature IPSCs (mIPSCs), recorded in tetrodotoxin (TTX, 1 μM) and cadmium (100 μM) were insensitive to AP4 but were reduced by baclofen to 49.7 ± 8.6% of control. When the contribution of voltage-dependent calcium channels (VDCCs) to synaptic transmission was boosted with external barium (1 mM), AP4 became effective in reducing TTX-resistant mIPSCs to 65.4 ± 3.9% of control, thus confirming a mechanism of presynaptic inhibition involving modulation of VDCCs. Differently from AP4, baclofen inhibited to 58.5 ± 6.7% of control the frequency mIPSCs recorded in TTX and the calcium ionophore ionomycin (2 μM), which promotes Ca2+-dependent, but VDCC-independent, transmitter release. Moreover, in the presence of α-latrotoxin (0.3 nM), to promote a Ca 2+-independent vesicular release of GABA, baclofen reduced mIPSC frequency to 48.1 ± 3.2% of control, while AP4 was ineffective. These results indicate that group III mGluRs depress GABA release to DA neurons of the SNc through inhibition of presynaptic VDCCs, while presynaptic GABAB receptors directly impair transmitter exocytosis.
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