Induction of δ-Opioid Receptor Function in the Midbrain after Chronic Morphine Treatment

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Abstract
δ-Opioid receptor (DOPr) activation fails to produce cellular physiological responses in many brain regions, including the periaqueductal gray (PAG), despite neural expression of high densities of the receptor. Previous histochemical studies have demonstrated that a variety of stimuli, including chronic morphine treatment, induce the translocation of DOPr from intracellular pools to the surface membrane of CNS neurons. PAG neurons in slices taken from untreated mice exhibited μ-opioid receptor (MOPr) but not DOPr-mediated presynaptic inhibition of GABAergic synaptic currents. In contrast, after 5-6 d of chronic morphine treatment, DOPr stimulation inhibited synaptic GABA release onto most neurons. Shorter exposure to morphinein vitro(upto 4 h) orin vivo(18 h) did not induce functional DOPr responses. DOPr-mediated presynaptic inhibition could not be induced in slices from untreated animals by increasing synaptic activityin vitrousing high extracellular potassium concentrations or activation of protein kinase A. Induction of functional DOPr signaling by chronic morphine required MOPr expression, because no DOPr receptor responses were observed in MOPr knock-out mice. DOPr agonists also had no effect on miniature IPSCs in β-arrestin-2 knock-out mice after chronic morphine. These results suggest that induction of DOPr-mediated actions in PAG by chronic morphine requires prolonged MOPr stimulation and expression of β-arrestin-2.