δ-opioid receptor mobilization of intracellular calcium in SH-SY5Y cells: Lack of evidence for δ-receptor subtypes

δ-opioid receptor agonists mobilize intracellular Ca²⁺([Ca²⁺](i)) in SH-SY5Y cells when applied in the presence of muscarinic agonists. The putative δ1 receptor agonist [D-Pen²,D-Pen⁵]-enkephalin (DPDPE) elevated [Ca²⁺](i) with an EC₅₀ of 11 nM and 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 not different, nor were maximally effective concentrations of DPDPE (1 μM) and deltorphin II (1 μM) applied together more effective than either agonist applied alone. The elevations of [Ca²⁺](i) caused by DPDPE (1 μM) and deltorphin II (1 μM), in the presence of carbachol, desensitized rapidly with continued opioid exposure and the cross-desensitization between DPDPE and deltorphin II was complete. The putative δ1 selective antagonist 7-benzylidenenaltrexone (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 greater than 50% at concentrations of less than 10 nM. In SHSY5Y cells δ-receptor mediated elevation of [Ca²⁺](i) is mediated by a population of receptors, which does not discriminate between agonists and antagonists purportedly selective for δ1 or δ2 receptors.