NMDA-receptor-dependent synaptic activation of voltage-dependent calcium channels in basolateral amygdala

Afferent stimulation of pyramidal cells in the basolateral amygdala produced mixed excitatory postsynaptic potentials (EPSPs) mediated by N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors during whole cell current-clamp recordings. In the presence of GABA, receptor blockade, the mixed EPSPs recruited a large "all-or-none" depolarizing event. This recruited event was voltage dependent and had a distinct activation threshold. An analogous phenomenon elicited by exogenous glutamate in the presence of retrodbroxin (TTX) was blocked by Cd2+, suggesting that the event was a Ca2+ spike. Selective glutamatergic blockade revealed that these Ca2+ spikes were recruited readily by single afferent stimulus pulses that elicited NMDA EPSPs. In contrast, non-NMDA EPSPs induced by single stimuli failed to elicit the Ca2+ spike even at maximal stimulus intensities although these non-NMDA EPSPs depolarized the soma more effectively than mixed EPSPs. Elongation of non-NMDA EPSPs by cyclothiazide or brief trains of stimulation were also unable to elicit the Ca2+ spike. Blockade of K+ channels with intracellular Cs+ enabled single non-NMDA EPSPs to activate the Ca2+ spike. The finding that voltage-dependent calcium channels are activated preferentially by NMDA-receptor-mediated EPSPs provides a mechanism for NMDA-receptor-dependent plasticity independent of Ca2+ influx through the NMDA receptor.