MUSCARINIC IPSPS IN RAT STRIATAL CHOLINERGIC INTERNEURONS

1. Intracellular recordings were made from neurones in slices of rat s triatum in vitro. 2. The forty-nine neurones studied were immunoreacti ve for choline acetyltransferase and had the electrophysiological char acteristics typical of large aspiny interneurones. 3. Focal stimulatio n of the slice elicited a hyperpolarizing inhibitory postsynaptic pote ntial in thirty-five neurones. This IPSP lasted 0.5-1 s and reversed p olarity at a membrane potential which was dependent on the logarithm o f the extracellular potassium concentration. 4. The IPSP was reversibl y blocked by scopolamine and methoctramine, which has some selectivity for the M-2 subtype of muscarinic receptor. It was unaffected by 6-cy ano-7-nitroquinoxaline-2,3-dione (10 rho M), DL-2-amino-phosphonovaler ic acid (30 mu M) and bicuculline (30 mu M). 5. Exogenous acetylcholin e and muscarine also hyperpolarized the neurones, and this was blocked by methoctramine but not by pirenzepine, which is an M-1 receptor-sel ective antagonist. 6. The findings demonstrate that muscarinic IPSPs o ccur in the central nervous system. The IPSP may mediate an 'autoinhib ition' of striatal cholinergic neurone activity.