Opioid regulation of intracellular free calcium in cultured mouse dorsal root ganglion neurons

Opioid agonists induced an increase in the intracellular free calcium concentration ([Ca²⁺]_i) or an inhibition of K⁺ (25 mM)-stimulated increase in [Ca²⁺]_i in different subsets of mouse dorsal root ganglion (DRG) neurons. The total neuronal population was grouped into three classes according to somatic diameter and defined as small (<16 μm), intermediate (16–25 μm), or large (>25 μm) neurons. Substance P-like immunoreactivity was detected mainly in the small and intermediate neurons. The δ, κ, and μ opioid receptor agonists [D-Ser², Leu⁵]enkephalin-Thr (DSLET), U69593, and [D-Ala², MePhe⁴, Gly-ol⁵]enkephalin (DAMGO) each induced a transient increase in [Ca²⁺]_i in a small fraction (<30%) of neurons. The increases in [Ca²⁺]_i were blocked by the opioid antagonist naloxone. The dihydropyridine-sensitive calcium channel blocker nifedipine also blocked the increase in [Ca²⁺]_i induced by 1 μM DSLET. The rank order of potency (percentage of cells responding to each opioid agonist) was DSLET > U69593 > DAMGO. The opioid-induced increase in [Ca²⁺]_i was observed mainly in large neurons, with a low incidence in small and intermediate neurons. Opioid agonists also caused inhibition of K⁺-stimulated increases in [Ca²⁺]_i, which were blocked by naloxone (1 μM). Inhibition of the K⁺-stimulated increase by 1 μM DSLET or U69593 was greater in small and intermediate neurons than in large neurons. © 1996 Wiley-Liss, Inc.