Opioid tolerance in periaqueductal gray neurons isolated from mice chronically treated with morphine

The midbrain periaqueductal gray (PAG) is a major site of opioid analgesic action, and a significant site of cellular adaptations to chronic morphine treatment (CMT). We examined μ-opioid receptor (MOP) regulation of voltage-gated calcium channel currents (I _Ca) and G-protein-activated K channel currents (GIRK) in PAG neurons from CMT mice. Mice were injected s.c. with 300mg kg ^-1 of morphine base in a slow release emulsion three times over 5 days, or with emulsion alone (vehicles). This protocol produced significant tolerance to the antinociceptive effects of morphine in a test of thermal nociception. Voltage clamp recordings were made of I _Ca in acutely isolated PAG neurons and GIRK in PAG slices. The MOP agonist DAMGO (Tyr-D-Ala-Gly-W-Me-Phe-Gly-ol enkephalin) inhibited I _Ca in neurons from CMT mice (230 nM) with a similar potency to vehicle (150 nM), but with a reduced maximal effectiveness (37% inhibition in vehicle neurons, 27% in CMT neurons). Inhibition of I _Ca by the GABA _B agonist baclofen was not altered by CMT. Met-enkephalin-activated GIRK currents recorded in PAG slices were significantly smaller in neurons from CMT mice than vehicles, while GIRK currents activated by baclofen were unaltered. These data demonstrate that CMT-induced antinociceptive tolerance is accompanied by homologous reduction in the effectiveness of MOP agonists to inhibit ICa and activate GIRK. Thus, a reduction in MOP number and/or functional coupling to G proteins accompanies the characteristic cellular adaptations to CMT previously described in PAG neurons.