A pervasive mechanism for analgesia: activation of GIRK2 channels

Abstract

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, alpha adrenergic, muscarinic cholinergic, gamma-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.

Figure 1

Deletion of GIRK2 channel produces a small thermal hyperalgesia, which is greater in females than males. (A) Response latency as a function of hot plate temperature for females, n = 10 for each genotype. There is a significant effect of the mutation. (B) Response latency as a function of hot plate temperature for males, n = 7–8 for each genotype. There is no significant effect of the mutation. (C) Response latencies for a large number of mice at a hot plate temperature of 52.5°C, n = 193–200 for each group. There is a significant effect of sex and mutation. Summary of statistics: (A and B) Wild-type mice showed an effect of gender (P = 0.0002) and temperature (P < 0.0001). GIRK2 KO showed no dependence on gender, only on temperature (P < 0.0001). Females showed effect of genotype (P < 0.0001) and temperature (P < 0.0001). Males showed dependence only on temperature (P < 0.0001) and not genotype. At 49°C (P < 0.001) and 49.5°C (P < 0.05), GIRK2 knockout females showed shorter latency than wild-type females (Bonferroni post hoc analysis). (C) There are strong effects of genotype (P < 0.0001) and gender (P < 0.0001).

Figure 2

Deletion of GIRK2 channels reduces the antinociception (hot plate response given as percent of maximum possible effect) induced by ethanol, oxotremorine, baclofen, and clonidine. (A and B) Ethanol antinociception in females and males, n = 14–18 animals for each group. (C and D) Oxotremorine analgesia in females and males, n = 12–16 animals for each group. (E and F) Baclofen analgesia in females and males, n = 10–16 animals for each group. (G and H) Clonidine antinociception in females and males, n = 15–16 for each group. *, P < 0.05; **, P < 0.01; wild-type mice are different from saline control. &, P < 0.05; &&, P < 0.01; knockout mice are different from saline control (Dunnett's post hoc test). Summary of statistics. Ethanol, wild-type mice showed dependence on sex (P = 0.014) and treatment (P < 0.0001). Mutant mice showed no dependence on sex and only marginal dependence on treatment (P = 0.043). Oxotremorine, wild-type mice showed a trend for an effect of gender (P = 0.065); mutant mice had no effect of gender. Baclofen, wild-type mice show no gender dependence (P = 0.16), but mutant mice show a dependence on gender (P = 0.03). Clonidine, no dependence on gender in wild-type or mutant mice.

Figure 3

GIRK2-null mice show marked reduction of nicotine and WIN 55,212-2-induced antinociception but no change in ketamine action (hot plate response given as a percent of maximum possible effect). (A and B) Nicotine antinociception in females and males, n = 10–16 for each group. (C and D) WIN 55,212-2 antinociception in females and males, n = 10–15 for each group. (E and F) Ketamine antinociception in females and males, n = 10–16 for each group. *, P < 0.05; **, P < 0.01; ***, P < 0.001; wild-type mice are different from saline control. &, P < 0.05; &&&, P < 0.001; knockout mice are different from saline control (Dunnett's post hoc test). Summary of statistics: Nicotine, effect of sex in wild-type (P = 0.03) but not in GIRK2 knockout mice. WIN 55,212-2, no dependence on gender in wild-type or mutant mice.

Figure 4

SIA depends upon both sex and mutation. GIRK2-null mice do not display SIA after swimming in warm water but show increased SIA after swimming in cold water. (A and B) Warm water, wild-type females show greater SIA than males, and deletion of GIRK2 blocks SIA; n = 15–16 for each group. (C and D) Cold water, wild-type females and males show similar SIA, but deletion of GIRK2 reduces SIA in females and increases it in males; n = 15–16 for each group. *, P < 0.05; **, P < 0.01; wild-type mice are different from saline control. &&, P < 0.01; knockout mice are different from saline control (Dunnett's post hoc test). Summary of statistics: Warm water, wild-type mice show strong gender (P = 0.015) and time (P = 0.0006) dependence. GIRK2-null mutant mice show no gender or time dependence. Cold water, no gender dependence but dependence on time (P = 0.02) for wild-type mice. For GIRK2 mice, there are effects of gender (P = 0.002) and time (P < 0.0001).