THE INTERACTION BETWEEN OPIOID AND MUSCARINIC RECEPTORS IN THE MOBILIZATION OF INTRACELLULAR CALCIUM IN SH-SY5Y CELLS

Abstract

Agonists at μ and δ opioid receptors mobilize intracellular Ca²⁺ in SH-SY5Y cells when applied in the presence of muscarinic agonists. The δ receptor agonist DPDPE elevated [Ca²⁺]i to a similar extent and with identical potency (EC₅₀ 11 nM) when applied in the presence of either 1 μM or 100 μM carbachol. The μ agonist DAMGO was less potent than DPDPE in elevating [Ca²⁺]i when applied in the presence of 1 μM and 100 μM carbachol. In our preparations of SH-SY5Y cells muscarinic receptor activation was an absolute requirement for opioid elevation of [Ca²⁺]i. When the muscarinic receptor antagonist atropine (10 μM) was applied to cells at the same time as the opioid agonist the opioid failed to increase [Ca²⁺]i. If we mimicked the consequences of muscarinic receptor activation by elevating intracellular Ca²⁺ with maitotoxin and activating protein kinase C with phorbol ester, opioids alone failed to induce any elevation of [Ca²⁺]i. Similarly, in the presence of agents that block elements of the Gq/phospholipase C signal transduction cascade (either caffeine or U-73122) neither carbachol nor opioid in the presence of carbachol elevated [Ca²⁺]i. Muscarinic receptor activation also depletes intracellular Ca²⁺ stores which leads to the activation of a store depletion-dependent Ca²⁺ entry pathway in SH-SY5Y cells. We have measured the Ca²⁺ entry through this pathway and opioids, either alone or in the presence of muscarinic agonist, failed to alter the Ca²⁺ entry observed. Thus in SH-SY5Y cells opioid receptor activation mobilizes intracellular Ca²⁺ stores, inhibits Ca²⁺ entry through N-type voltage dependent Ca²⁺ channels but does not appear to modulate store depletion-dependent Ca²⁺ entry.