Nicotine Selectively Enhances NMDA Receptor-Mediated Synaptic Transmission during Postnatal Development in Sensory Neocortex

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Abstract
The neurotransmitters acetylcholine (ACh) and glutamate have been separately implicated in synaptic plasticity during development of sensory neocortex. Here we show that these neurotransmitters can, in fact, act synergistically via their actions at nicotinic ACh receptors (nAChRs) and NMDA receptors, respectively. To determine how activation of nAChRs modifies glutamatergic EPSPs, we made whole-cell recordings from visualized pyramidal neurons in slices of rat auditory cortex. Pulsed (pressure) ejection of nicotine onto apical dendrites selectively enhanced EPSPs mediated by NMDA receptors without affecting AMPA/kainate (AMPA/KA) receptor-mediated EPSPs. The enhancement occurred during a transient, postnatal period of heightened cholinergic function [neurons tested on postnatal day 8–16 (P8–16)], and not in the mature cortex (>P19). Three related findings indicated the mechanism of action: (1) The specific α7 nAChR antagonist methyllycaconitine citrate (MLA) blocked the effect of nicotine; (2) pulsed nicotine did not enhance postsynaptic depolarizations induced by iontophoretically applied NMDA; and (3) bath exposure to nicotine for several minutes produced apparent nAChR desensitization and precluded enhancement of EPSPs by pulsed nicotine. Together, the data suggest that nicotine acts at rapidly desensitizing, presynaptic α7 nAChRs to increase glutamate release onto postsynaptic NMDA receptors. The synergistic actions mediated by α7 nAChRs and NMDA receptors may contribute to experience-dependent synaptic plasticity in sensory neocortex during early postnatal life.