Recent molecular studies suggest that the expression levels of δ and γ2 GABAA receptor subunits regulate the balance between synaptic and extrasynaptic GABA neurotransmission in multiple brain regions. We investigated the expression of GABAAδ and GABAAγ2 and the functional significance of a change in balance between these subunits in a robust local GABA network contained within the suprachiasmatic nucleus of the hypothalamus (SCN). Muscimol, which can activate both synaptic and extrasynaptic GABAA receptors, injected into the SCN during the day phase advanced the circadian pacemaker whereas injection of the extrasynaptic GABAA superagonist THIP had no effect on circadian phase. In contrast, injection of either THIP or muscimol during the night was sufficient to block the phase shifting effects of light. Gene expression analysis of the whole SCN revealed different temporal patterns in GABAAδ and GABAAγ2 mRNA expression. When examined across all subregions of the SCN, quantitative immunohistochemical analysis found no significant variations in GABAAδ protein immunoreactivity (IR), but did find significant variations in GABAAγ2 protein-IR in hamsters housed in either light-dark cycles or in constant darkness. Remarkably, significant interactions in the ratio of GABAAδ:GABAAγ2 subunits between lighting condition and circadian phase occurred only within one highly discrete anatomical area of the SCN; a region that functions as the input for lighting information from the retina. Taken together these data support the hypothesis that the balance between synaptic and extrasynaptic GABAA receptors determines the functional response to GABA, and that this balance is differentially regulated in a region specific manner.
Significance Statement GABA neurotransmission is mediated primarily by GABAA receptors. These receptors are composed of different combinations of five subunits that determine their pharmacological properties and subcellular location. Differences in the expression of GABAA receptors that contain the γ2 subunit versus those that contain the δ subunit may regulate the balance between synaptic and extrasynaptic GABA neurotransmission. We report here that expression of the γ2 and the δ subunits are differentially regulated within the circadian pacemaker in the suprachiasmatic nucleus (SCN) and evidence that the balance between synaptic and extrasynaptic GABAA receptors determine the functional response to GABA, and that this balance is regulated in a site-specific manner within the SCN.
The authors declare no competing financial interests.
National Institutes of Health grant R01NS078220 to HEA and F32NS092545 to JCW.