GABAA receptor subunit composition regulates circadian rhythms in rest-wake and synchrony among cells in the suprachiasmatic nucleus

The mammalian circadian clock located in the suprachiasmatic nucleus (SCN) produces robust daily rhythms including rest-wake. SCN neurons synthesize and respond to gamma- aminobutyric acid (GABA), but its role remains unresolved. We tested the hypothesis that gamma 2- and delta- subunits of the GABAA receptor in the SCN differ in their regulation of synchrony among circadian cells. We used two approaches: 1) shRNA to knock- down (KD) the expression of either gamma 2 or delta subunits in the SCN or 2) knock- in mice harboring a point mutation in the M2 domains of the endogenous GABAA gamma 2 or delta subunits. KD of either gamma 2 or delta subunits in the SCN increased daytime running and reduced nocturnal running by reducing their circadian amplitude by a third. Similarly, delta subunit knock- in mice showed decreased circadian amplitude, increased duration of daily activity, and decreased total daily activity. Reduction, or mutation of either gamma 2 or delta subunits halved the synchrony among, and amplitude of, circadian SCN cells as measured by firing rate or expression of the PERIOD2 protein, in vitro. Surprisingly, overexpression of the gamma 2 subunit rescued these phenotypes following KD or mutation of the delta subunit, and overexpression of the delta subunit rescued deficiencies due to gamma 2 subunit KD or mutation. We conclude that gamma 2 and delta GABAA receptor subunits play similar roles in maintaining circadian synchrony in the SCN and amplitude of daily rest-wake rhythms, but that modulation of their relative densities can change the duration and amplitude of daily activities.