Muscarinic receptors depress gabaergic synaptic transmission in rat midbrain dopamine neurons

The effects of muscarine and nicotine on evoked and spontaneous release of GABA were studied using intracellular and whole-cell patch-clamp recordings from rat midbrain dopamine neurons in an in vitro slice preparation. Musca rine (30 mu M) reversibly depressed the pharmacologically isolated inhibito ry postsynaptic potential evoked by local electrical stimulation. The maxim al inhibition of the inhibitory postsynaptic potential amplitude was 39.6 /- 5%. This depressant effect of muscarine was blocked by the M3/M1 recepto r antagonist 3-diphenylacetoxy-N-methylpiperidine methiodide (100 nM), but was slightly affected by the M1/M3 receptor antagonist pirenzepine (1 mu M) . In addition, muscarine decreased the frequency of the miniature synaptic currents without any effect on their amplitude. Moreover, muscarine did not change the GABA-induced hyperpolarization, indicating that its effect on t he inhibitory postsynaptic potential is mediated by presynaptic receptors. On the contrary, the cholinergic agonist nicotine did not change the freque ncy or the amplitude of the spontaneous glutamatergic and GABAergic synapti c currents. Our data indicate that a prevalent activation of presynaptic M3 muscarinic receptors inhibits the GABA-mediated synaptic events, while the activation of nicotinic receptors does not affect the release of glutamate and GABA on midbrain dopamine neurons. (C) 2000 IBRO. Published by Elsevier Science Lt d.