Synaptic mechanisms of NMDA-mediated hyperpolarization in lateral amygdaloid projection neurons

Synaptic mechanisms underlying NMDA-mediated responses of neurons in the guinea pig lateral amygdala (AL) were investigated in in vitro slice preparations. Local application of NMDA resulted in initial hyperpolarization of pyramidal-like spiny cells (projection neurons), followed by prolonged depolarization. The slow depolarization represented a direct postsynaptic effect of NMDA, whereas the initial hyperpolarization was induced presynaptically through activation of GABAergic interneurons and was sensitive to blockade by tetrodotoxin as well as the GABA(A)-receptor antagonist bicuculline. Application of NMDA resulted in AP-5-sensitive, lasting depolarization also in putative interneurons of the AL suggesting direct activation of GABAergic interneurons by NMDA. These data indicate that interneurons in the rat lateral amygdala possess functional NMDA receptors, which may contribute to the predominantly inhibitory synaptic responses in amygdaloid neurons following activation through afferent input systems.