G protein-coupled inwardly rectifying potassium channels (GIRKs) provide a common link between numerous neurotransmitter receptors and the regulation of synaptic transmission. We asked whether GIRKs specify a single behavioral action that is produced by drugs acting on the diverse receptors coupled with GIRKs. By using GIRK2-null mutant mice, we found marked reduction or complete elimination of the antinociceptive (hot plate test) effects of ethanol, oxotremorine, nicotine, baclofen, clonidine, and the cannabinoid receptor agonist WIN 55,212. However, ketamine analgesia remained intact. For most drugs, there was a sex difference in antinociceptive action, and the impact of deletion of the GIRK2 channel was less in female mice. The deletion of the GIRK2 channel blocks the opioid-dependent component of stress-induced analgesia (SIA), whereas nonopioid SIA was not changed. We propose that opioid, ␣ adrenergic, muscarinic cholinergic, ␥-aminobutyric acid-B, and cannabinoid receptors are coupled with postsynaptic GIRK2 channels in vivo. Furthermore, this pathway accounts for essentially all of the antinociceptive effects in males, although females appear to recruit additional signal transduction mechanisms for some analgesic drugs.C hemically and pharmacologically diverse drugs reduce nociceptive responses in humans and in animal models of acute pain. Many of these drugs act through G protein coupled receptors and affect multiple pre-and postsynaptic signaling pathways (1, 2). A key question is whether there is a single G protein signal transduction pathway that is important for antinociceptive actions of many different drugs. One candidate for such a signaling system is the G protein-coupled inwardly rectifying potassium channel (GIRK). This assumption is based in part on the observation that weaver mutant mice have a mutation in GIRK2 and display reduced analgesia after either morphine or -opioid agonist (Ϫ)-U-50488 or ethanol administration (3, 4). However, the weaver mutant is not ideal for assessing the role of GIRK2 because the mutation alters the ion selectivity of GIRK2 and produces neuronal degeneration rather than a simple loss of GIRK2 function (5). Targeted mutation has produced mice lacking the GIRK2 protein; these mice lack postsynaptic responses to several neurotransmitters known to act through G protein coupled receptors, but retain normal presynaptic actions of these receptors (6-8). These mice are viable and similar to wild-type controls in many behavioral tests (9-11) and provide a unique approach to defining the role of GIRK2 in drug actions.GIRKs are activated by M 2 muscarinic, ␣ 2 adrenergic, D 2 dopaminergic, histamine, 5HT 1A , A 1 adenosine, ␥-aminobutyric acid (GABA)-B, , , and ␦ opioid, and somatostatin receptors (12, 13). Because many of these neurotransmitter systems are implicated in antinociception, we tested pharmacologically diverse analgesics in GIRK2 knockout mice by using the hot plate test. Because of known sex differences in sensitivity to druginduced analgesia (14), we compared...

Mice lacking either the ␣1 or ␤2 subunit of the GABA A receptor were tested for ethanol, saccharin, or quinine consumption, ethanol-conditioned place preference, ethanol-conditioned taste aversion, ethanol-simulated motor activity, and handlinginduced seizures following chronic consumption of an ethanol liquid diet. The ␣1 null mutants showed decreased ethanol and saccharin consumption, increased aversion to ethanol, and a marked stimulation of motor activity after injection of ethanol.The ␤2 null mutants showed decreased consumption of saccharin and quinine, but not ethanol. Surprisingly, neither mutant showed marked changes in handling induced seizures before or after withdrawal of ethanol. The unique effects of deletion of these two GABA A receptor subunits on ethanol responses are discussed in terms of the distinct changes in different populations of GABA A receptors.

Enhancement of the activation of GABA A receptors is a common feature of many sedative and hypnotic drugs, and it is probable that the GABA A receptor complex is a molecular target for these drugs in the mammalian central nervous system. We set out to elucidate the role of the two predominant (␣ 1 and ␤ 2 ) subunits of GABA A receptor in sedative drug action by studying mice lacking these two subunits. Both ␣ 1 (Ϫ/Ϫ) and ␤ 2 (Ϫ/Ϫ) null mutant mice showed markedly decreased sleep time induced by nonselective benzodiazepine, flurazepam, and GABA A agonist, 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol. The sleep time induced by the ␤-selective drug etomidate was decreased only in ␤ 2 (Ϫ/Ϫ) knockout mice. In contrast, ␣ 1 (Ϫ/Ϫ) mice were more resistant to the ␣ 1 -selective drug zolpidem than ␤ 2 (Ϫ/Ϫ) or wild-type animals. Knockout mice of both strains were similar to wild-type mice in their responses to pentobarbital. The duration of loss of the righting reflex produced by ethanol was decreased in male mice for both null alleles compared with wild-type mice, but there were no differences in ethanol-induced sleep time in mutant females. Deletion of either the ␣ 1 or ␤ 2 subunits reduced the muscimolstimulated 36 Cl Ϫ influx in cortical microsacs suggesting that these mutant mice have reduced number of functional brain GABA A receptors. Our results show that removal of either ␣ 1 or ␤ 2 subunits of GABA A receptors produce strong and selective decreases in hypnotic effects of different drugs. Overall, these data confirm the crucial role of the GABA A receptor in mechanisms mediating sedative/hypnotic effects.

These studies show that GIRK2 deletion reduced ethanol's impact in tasks that are commonly used to index the drug's rewarding and aversive effects. These findings could reflect either a learning/memory deficit or decreased sensitivity to ethanol's motivational effects in null mutant mice. The latter interpretation is more consistent with previous data showing that knockout mice consume higher doses of ethanol than wild type mice.

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