Role of G(q) protein in behavioral effects of the hallucinogenic drug 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane

Abstract

Extensive evidence suggests that 5-HT2 receptors may play a role in mental disorders including schizophrenia. In addition, several studies indicate that G(q)-coupled 5-HT(2A) receptors are likely targets for the initiation of events leading to the hallucinogenic behavior elicited by lysergic acid diethylamide (LSD), (+/-)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), and related drugs. However, 5-HT(2A) receptors couple to other G proteins in addition to G(q) protein. To evaluate the role of the G(q) signaling pathway in DOI-induced behaviors, we utilized two behavioral models of 5-HT(2A) receptor activation: induction of head-twitches by DOI, a common response to hallucinogenic drugs in rodents, and DOI elicited anxiolytic-like effects in the elevated plus maze. Experimental subjects were genetically modified mice [Galpha(q)(-/-)] in which the G(q) alpha gene was eliminated. Galpha(q)(-/-) mice exhibited a decrease in DOI-induced head-twitches, when compared to wild-type littermates. In addition, the DOI-induced increase in anxiolytic-like behavior was abolished in Galpha(q)(-/-) mice. These results, combined with our finding that DOI-induced FOS expression in the medial prefrontal cortex was also eliminated in Galpha(q)(-/-) mice, suggests a key role for G(q) protein in hallucinogenic drug effects.

Fig. 1

Effect of pretreatment with MDL100907 on DOI-induced behaviors. A Anxiolytic-like effects of DOI (2.5 mg/Kg, i.p. 30 min pre-test) are prevented by pretreatment with the 5-HT_2A receptor antagonist MDL100907 (0.25 mg/Kg, i.p. 50 min pre-test) B DOI-induced head-twitches counted during a 30 min observation period were inhibited by pretreatment with MDL100907 (0.25 mg/Kg, i.p. 20 min pre-test). Data shown as means ± S.E.M., n= 6 per group. (*)p<0.05, (***)p<0.001 relative to saline control group determined by a one-way ANOVA.

Fig. 2

Quantitative analysis of FOS-Li positive nuclei in the mPFC of C57BL/6 mice. A Dose response of DOI-induced FOS expression in the mPFC, n=3. B DOI (2.5 mg/Kg, i.p.) induced FOS-Li expression is inhibited by pretreatment with MDL100907 (0.25mg/kg, i.p. 20 min pre-DOI). Data shown as means ± S.E.M., n= 6. (**) p<0.01 relative to saline (Sal-Sal) determined by a one-way ANOVA.

Fig. 3

Anxiolytic-like activity of DOI on the elevated plus maze is absent in mice deficient for Gα_q. A Time spent in the open arms, and B % time spent in the open arms, as a percentage of time spent in the open and close arms (center excluded) during the 5 min test session following administration of DOI (2.5 mg/kg, i.p. 30 min pre-test) or ethanol (1.5 g/kg, i.p. 5 min pre-test). C Exploratory activity measured in total distance traveled during the elevated plus maze test. Data shown as means ± S.E.M., n= 7–12 per group. (*) p<0.05, (**) p<0.01 relative to saline control group determined by a two-way ANOVA.

Fig. 4

DOI-induced head-twitch response is reduced in Gα_q knockouts. Mice were injected with DOI (1.0 mg/Kg or 2.5 mg/Kg, i.p.) prior to a 30 min observation period. A Number of head twitches in 5 min bins (open symbols represent WT mice, closed symbols represent Gα_q(−/−) littermates). Main effect of genotype, p<0.0001. B Each column represents the total number of head-twitches counted during 30-min test time. Data shown as means ± S.E.M., n= 6–7 per group. (*) p<0.05, (***) p<0.001 relative to respective wild-type control groups determined by a two-way ANOVA..

Fig. 5

Quantitative analysis of number of FOS-Li positive nuclei in the medial prefrontal cortex of wild-type and Gα_q(−/−) mice. Values are numbers of FOS-Li positive cells (mean ± S.E.M. within area of analysis; n=6). DOI-induced FOS expression in the medial prefrontal cortex is abolished in Gα_q(−/−) mice. (***) p<0.001 relative to saline control group determined by a two-way ANOVA.