Intracellular calcium concentration ([Ca 2ϩ ] 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 1 receptor agonists potentiate NMDA receptor-mediated neuronal activity in the hippocampus and recruit Ca 2ϩ -dependent second messenger cascades (e.g., protein kinase C; PKC) in brainstem motor structures. The present study therefore assessed whether the potentiating action of 1 agonists on the NMDA response observed in the hippocampus involves the regulation of [Ca 2ϩ ] i and PKC. For this purpose, [Ca 2ϩ ] i changes after NMDA receptor activation were monitored in primary cultures of embryonic rat hippocampal pyramidal neurons using microspectrofluorometry of the Ca 2ϩ -sensitive indicator Fura-2/acetoxymethyl ester in the presence of 1 agonists and PKC inhibitors. We show that successive activations of the 1 receptor by 1-min pulses of (ϩ)-benzomorphans or (ϩ)-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 [Ca 2ϩ ] i , whereas 1,3-di-o-tolyl-guanidine, a mixed 1 / 2 agonist, did not significantly modify the glutamate response. Both potentiation and inhibition were prevented by the selective 1 antagonist N, N-dipropyl-2-[4-methoxy-3-(211phe-nylethoxy) phenyl]-ethylamine monohydrochloride (NE-100). Furthermore, only (ϩ)-benzomorphans could induce [Ca 2ϩ ] 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 (ϩ)-benzomorphans on the glutamate response. Our results provide further support for a general mechanism for the intracellular 1 receptor to regulate Ca 2ϩ -dependent signal transduction and protein phosphorylation.