Effect of genetic and pharmacological blockade of GABA receptors on the 5-HT2C receptor function during stress

Abstract

Serotonin (5-HT)2C receptors play a role in psychoaffective disorders and often contribute to the antidepressant and anxiolytic effects of psychotropic drugs. During stress, activation of these receptors exerts a negative feedback on 5-HT release, probably by increasing the activity of GABAergic interneurons. However, to date, the GABA receptor types that mediate the 5-HT2C receptor-induced feedback inhibition are still unknown. To address this question, we assessed the inhibition of 5-HT turnover by a 5-HT2C receptor agonist (RO 60-0175) at the hippocampal level and under conditions of stress, after pharmacological or genetic inactivation of either GABA-A or GABA-B receptors in mice. Neither the GABA-B receptor antagonist phaclofen nor the specific genetic ablation of either GABA-B1a or GABA-B1b subunits altered the inhibitory effect of RO 60-0175, although 5-HT turnover was markedly decreased in GABA-B1a knock-out mice in both basal and stress conditions. In contrast, the 5-HT2C receptor-mediated inhibition of 5-HT turnover was reduced by the GABA-A receptor antagonist bicuculline. However, a significant effect of 5-HT2C receptor activation persisted in mutant mice deficient in the α3 subunit of GABA-A receptors. It can be inferred that non-α3 subunit-containing GABA-A receptors, but not GABA-B receptors, mediate the 5-HT2C -induced inhibition of stress-induced increase in hippocampal 5-HT turnover in mice.

Keywords: GABA-A α3 subunit; GABA-B; bicuculline; knockout; phaclofen.

Figure 1. 5-HT_2C receptor-mediated inhibition by RO 60-0175 of the stress-induced increase in 5-HT turnover in BALB/c and C57BL/6J mice

A) Effect of restraint stress on the 5-HT turnover [5-HIAA/5-HT ratio; means ± S.E.M.] in the hippocampus of C57BL/6J (n= 14–17) and BALB/c (n= 20–29) WT mice. B) The effects of RO 60-0175 (RO; 3 mg/kg i.p.) are presented as a percentage (means ± S.E.M.) of the 5-HIAA/5-HT ratio of saline-treated mice of the C57BL/6J (n=14–16) and BALB/c (n=22–31) strain. ***p<0.001, two-tail Student’s t-test.

Figure 2. Consequences of pharmacological or genetic inactivation of specific GABA receptors on 5-HT_2C receptor-mediated inhibition of 5-HT turnover in stressed mice

The effect of RO 60-0175 (RO; 3 mg/kg, i.p.) on 5-HT turnover under conditions of stress was determined in mice with genetic ablation of either GABA-B1a, GABA-B1b or GABA-Aα₃ receptor subunit and in WT mice after pharmacological blockade of GABA-A or GABA-B receptors with bicuculline (8 mg/kg, i.p.) or phaclofen (2 mg/kg, i.p.), respectively. Results are presented as a percentage of the 5-HIAA/5-HT ratio in saline-treated mice of the same genotype (means ± S.E.M.; n= 4–8). * p≤0.05, ** p<0.01, one-tail Student’s t-test.

Figure 3. Schematic representation of GABAergic interneuron-mediated inhibition of serotonergic neuron activity

GABAergic interneurons are known to inhibit 5-HT neuronal activity via GABA-A receptors (1). Because of 5-HT release during stress, 5-HT_2C receptors expressed by GABAergic interneurons are activated (2), which leads to a GABA-mediated negative feedback control of 5-HT neuron during stress (Mongeau et al., 2010). Glutamatergic (Glu) projections from forebrain (cortex) areas also excite GABAergic interneurons (3), but inhibitory terminal GABA-B1a subunit containing heteroreceptors exert a presynaptic modulation on these projections (4). Accordingly, the excitatory influence of cortical glutamatergic projections onto GABA interneurons is larger in GABA-B1a^−/− mice, which would account for a reduced 5-HT turnover in these mutants.