Absence and rescue of morphine withdrawal in GIRK/Kir3 knock-out mice

Although morphine induces both analgesia and dependence through μ-opioid receptors (MORs), the respective contributions of the intracellular effectors engaged by MORs remain unknown. To examine the contribution of G-protein-gated inwardly rectifying K⁺ (GIRK, Kir3) channels to morphine dependence and analgesia, we quantified naloxone-precipitated withdrawal behavior and morphine analgesia using GIRK knock-out (^-/-) mice. The morphine withdrawal syndrome was strongly attenuated, whereas morphine analgesia was mostly preserved in mice lacking both GIRK2 and GIRK3 (GIRK2/3 ^-/-mice). In acute slices containing the locus ceruleus (LC) from GIRK2/3^-/- mice, the increase in spontaneous firing typically associated with morphine withdrawal was absent. Moreover, although morphine elicited normal presynaptic inhibition in the LC, postsynaptic GIRK currents were completely abolished in GIRK2/3^-/- mice. Altogether, these data suggested that morphine-evoked postsynaptic inhibition of the LC was required for the induction of dependence. Consistent with this hypothesis, morphine withdrawal behavior was rescued in GIRK2/3^-/- mice by ablation of adrenergic fibers using the neurotoxin N-(2-chloroethyl)-N-ethyl-2- bromobenzylamine. Our data suggest that inhibition of adrenergic tone is required for the induction of dependence, and that channels containing GIRK2 and GIRK3 serve as an inhibitory gate.