Opioid-induced inhibition of voltage-gated calcium channels parallels expression of ω-conotoxin-sensitive channel subtype during differentiation of NG108-15 cells

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 μM) 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 μM and blocked by 100 μM 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 μM nitrendipine inhibited Ca²⁺ influx by at least 65% at all stages of differentiation, while sensitivity to ω-conotoxin GVIa (ω-CgTx) did not appear until day 3. ω-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 ω-CgTx sensitive voltage-gated Ca²⁺ channels.