Fast excitatory nicotinic transmission in the chick lateral spiriform nucleus

The lateral spiriform nucleus (SpL) in the chick mesencephalon contains fun ctional nicotinic receptors and receives a cholinergic fiber projection. We now use double-label immunohistochemistry to demonstrate that choline acet yltransferase-immunopositive fibers in the SpL and in the cholinergic fiber tract lateral to the nucleus are associated with fibers expressing the alp ha 5 and/or alpha 3 nicotinic receptor subunits as determined by mAb35 immu noreactivity. This morphological evidence suggests that there might be syna pses between the cholinergic fibers and the dendrites of SpL neurons. Whole -cell recordings from SpL neurons in current-clamp mode revealed EPSPs evok ed by stimulation of the cholinergic fiber tract lateral to the SpL. These EPSPs increased in amplitude in the presence of bicuculline. Further additi on of the nicotinic antagonist dihydro-beta-erythroidine (DH beta E) to the buffer significantly attenuated them. Almost all of the remaining EPSP was blocked by 6,7-dinitroquinoxaline-2,3-dione. In the presence of an antagon ist cocktail that isolated the nicotinic responses, a fast, monosynaptic ni cotinic EPSP or EPSC was evoked. In some neurons, the nicotinic EPSP result ed in the generation of an action potential. The nicotinic nature of the ev oked response was confirmed by blockade of the EPSPs or EPSCs with nicotini c antagonists, including DH beta E, D-tubocurare, and mecamylamine. The nic otinic response was insensitive to low concentrations (10-100 nM) of methyl lycaconitine, indicating that typical alpha 7-containing receptors were not involved. The results demonstrate that endogenously released acetylcholine generates EPSPs that can elicit action potentials by acting at postsynapti c nicotinic receptors on SpL neurons.