Abstract

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 σ₁ 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 σ₁ 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 σ₁ agonists and PKC inhibitors. We show that successive activations of the σ₁ 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) concommitantly 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 σ₁/σ₂ agonist, did not significantly modify the glutamate response. Both potentiation and inhibition were prevented by the selective σ₁ 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 (Gö-6976) prevented the potentiating effect of (+)-benzomorphans on the glutamate response. Our results provide further support for a general mechanism for the intracellular σ₁ receptor to regulate Ca²⁺-dependent signal transduction and protein phosphorylation.