Protein kinase C-dependent potentiation of intracellular calcium influx by sigma1 receptor agonists in rat hippocampal neurons

Intracellular calcium concentration ([Ca2+]i) plays a major role in neuronal excitability, especially that triggered by the N-methyl-d-aspartate (NMDA)-sensitive glutamatergic receptor. We have previously shown that sigma1 receptor agonists potentiate NMDA receptor-mediated neuronal activity in the hippocampus and recruit Ca2+-dependent second messenger cascades (e.g., protein kinase C; PKC) in brainstem motor structures. The present study therefore assessed whether the potentiating action of sigma1 agonists on the NMDA response observed in the hippocampus involves the regulation of [Ca2+]i and PKC. For this purpose, [Ca2+]i changes after NMDA receptor activation were monitored in primary cultures of embryonic rat hippocampal pyramidal neurons using microspectrofluorometry of the Ca2+-sensitive indicator Fura-2/acetoxymethyl ester in the presence of sigma1 agonists and PKC inhibitors. We show that successive activations of the sigma1 receptor by 1-min pulses of (+)-benzomorphans or (+)-N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1-ethyl-but-3-en-1-ylamine hydrochloride (JO-1784) concomitantly with glutamate time dependently potentiated before inconstantly inhibiting the NMDA receptor-mediated increase of [Ca2+]i, whereas 1,3-di-o-tolyl-guanidine, a mixed sigma1/sigma2 agonist, did not significantly modify the glutamate response. Both potentiation and inhibition were prevented by the selective sigma1 antagonist N,N-dipropyl-2-[4-methoxy-3-(211phenylethoxy) phenyl]-ethylamine monohydrochloride (NE-100). Furthermore, only (+)-benzomorphans could induce [Ca2+]i influx by themselves after a brief pulse of glutamate. A pretreatment with the conventional PKC inhibitor 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo [2,3-a] pyrrolo [3,4-c] carbazole (G?-6976) prevented the potentiating effect of (+)-benzomorphans on the glutamate response. Our results provide further support for a general mechanism for the intracellular sigma1 receptor to regulate Ca2+-dependent signal transduction and protein phosphorylation.