Acetylcholine responses in synaptically active neurons in mouse ventral mesencephalon cultures

Acetylcholine (ACh) sensitivity was examined in neurons in dissociated cultures from fetal ventral mesencephalon (14-16 days gestational age); many neurons probably originate in substantia nigra. Direct responses to iontophoretically applied ACh were recorded by intracellular microelectrodes in 53 cells and indirect responses were recorded from 58 synaptically connected cells. Cultures (24-50 days old) were studied in growth media enriched with serum (10%) and calcium (6.8 mM). Direct responses: ACh caused a slow depolarization (seconds) in 94% (50/53); in 30% of these cells (15/50) the depolarization exceeded action potential threshold. Hyperpolarizing responses occurred in 4% (2/53) and another cell showed reduced action potential firing. Indirect responses: Application of ACh to adjacent cells caused an increase in postsynaptic potential (PSP) activity which was excitatory in 66% (38/58) and inhibitory in 17% (10/58). A reduced level of PSP activity occurred in 7% (4/58) of cells exhibiting excitatory PSPs and in 10% (6/58) of cells with inhibitory connections. Indirect responses were blocked by tetrodotoxin. ACh receptor types: Responses to ACh were predominantly muscarinic (77%, 10/13). Nicotinic responses were present in the remaining 23% (3/13).