δ-OPIOJD RECEPTOR MOBILIZATION OF INTRACELLULAR CALCIUM IN SH-SY5Y CELLS: LACK OF EVIDENCE FOR 8 RECEPTOR SUBTYPES

Abstract

δ Opioid receptors mobilize intracellular Ca²⁺ in SH-SY5Y cells when applied in the presence of muscarinic agonists. In the presence of 1 μM carbachol the putative δ1 receptor agonist DPDPE elevated [Ca²⁺]i with and EC50 of 11 nM, the putative δ2 agonist deltorphin II elevated [Ca²⁺]i with an EC50 of 14 nM. The maximal elevations of [Ca²⁺]i caused by both agonists were similar (44 ± 3 nM for DPDPE and 44 ± 8 nM for deltorphin II). When maximally effective concentrations of DPDPE (1 μM) and deltorphin II (1 μM) were applied together the elevation of [Ca²⁺]i was no greater than with either agonist alone. The putative δ1 selective antagonist BNTX and the putative δ2 selective antagonist naltriben both reduced the elevations of [Ca²⁺]i caused by DPDPE (30 nM) and deltorphin II (10 nM) by at least 50 % at concentrations of less than 10 nM. The elevations of [Ca²⁺]i caused by either DPDPE (1 μM) or deltorphin II (1 μM) in the presence of carbachol declined rapidly with continued opioid exposure; after 5 minutes of agonist application the [Ca²⁺]ihad returned to preopioid levels. The desensitization appeared to be homologous because application of DAMGO or NPY at the end of 5 minute exposure to a δ agonist still evoked a robust elevation of [Ca²⁺]i. In this acute desensitization paradigm the crossdesensitization between DPDPE and deltorphin II was complete. Thus in SH-SY5Y cells δ receptor mediated elevation of [Ca²⁺]i desensitizes rapidly and is mediated by a population of receptors which does not discriminate between agonists and antagonists that are purportedly selective for δ1 or δ2 receptors.