Depolarization-induced increases in intracellular free calcium concentration ([Ca2+]i) were measured in single NG108-15 cells using indo-1 based microfluorimetry. In cells differentiated for 6 days in serum-free forskolin (5 microM) supplemented media, application of micromolar concentrations of [D-Ala2-D-Leu5]enkephalin (DADLE) inhibited Ca2+ influx mediated by voltage-gated Ca2+ channels. Inhibition of 50 mM K(+)-induced Ca2+ influx by DADLE was concentration-dependent over the range of 0.1 to 10 microM and blocked by 100 microM naloxone. Differentiation increased the amplitude of depolarization-induced [Ca2+]i transients from 78 +/- 21 nM in undifferentiated cells to 1,282 +/- 318 nM after 6 days. One microM nitrendipine inhibited Ca2+ influx by at least 65% at all stages of differentiation, while sensitivity to omega-conotoxin GVIa (omega-CgTx) did not appear until day 3. omega-CgTx inhibited a dihydropyridine-sensitive Ca2+ channel. DADLE inhibition of Ca2+ channels did not appear until 3 days of differentiation. Thus, opioid inhibition of depolarization-induced Ca2+ influx paralleled the expression of omega-CgTx sensitive voltage-gated Ca2+ channels.