FAST SYNAPTIC SIGNALING BY NICOTINIC ACETYLCHOLINE AND SEROTONIN 5-HT3 RECEPTORS IN DEVELOPING VISUAL-CORTEX

Cholinergic and serotonergic fiber systems invade the developing visua l cortex several weeks before eye opening; both transmitters have been implicated in plasticity of neocortical circuits. These transmitters have been presumed to act predominantly through second messenger-coupl ed receptors, because fast cholinergic or serotonergic neurotransmissi on has never been observed in neocortex. However, acetylcholine and se rotonin also act on ligand-gated ion channels; the nicotinic acetylcho line receptor and the serotonin 5-HT3 receptor respectively. Here, usi ng whole-cell patch-clamp techniques in developing ferret visual corte x, we pharmacologically isolated fast, spontaneous, and evoked choline rgic and serotonergic synaptic events in pyramidal cells and interneur ons of all cortical layers. The number of cells receiving such inputs increased with the ingrowth of thalamic afferents, and the frequencies of the spontaneous events increased at eye opening. Thus, both acetyl choline and serotonin can mediate fast synaptic transmission in the vi sual cortex; the early onset of these mechanisms suggests a role durin g initial stages of circuit formation and during subsequent experience -dependent remodeling of cortical connections.