A GABAergic network from AVP- to VIP-neurons in the suprachiasmatic nucleus sets the timing of circadian behavior rhythms

The central circadian clock of the suprachiasmatic nucleus (SCN) consists of a network of multiple types of γ-aminobutyric acid (GABA)-ergic neurons and glial cells. However, the precise role of GABAergic transmission in the SCN remains unclear. In this study, we investigated the GABAergic regulation from arginine vasopressin (AVP)-producing neurons in the SCN shell to vasoactive intestinal polypeptide (VIP)-producing neurons in the SCN core. Blocking GABA release from AVP neurons via deletion of the vesicular GABA transporter (Vgat) gene lengthened the activity time (the interval between the onset and offset of locomotor activity) and shortened the duration of high Ca2+ activity in VIP neurons to correspond to the behavioral rest time. Conversely, eliminating functional GABAA receptors (GABAAR) in VIP neurons by in vivo genome editing reduced morning locomotor activity level and shortened the activity time, while lengthening the high Ca2+ duration in VIP neurons. Optogenetic activation of AVP neurons in vivo increased Ca2+ levels in VIP neurons during the night; this effect was significantly reduced in AVP neuron-specific Vgat-deficient mice. A similar Ca2+ response in VIP neurons following AVP neuronal activation was observed in SCN slices and was inhibited by the GABAAR antagonist gabazine. Importantly, gabazine application alone elevated baseline Ca2+ levels in VIP neurons, suggesting tonic GABA-mediated inhibition of these neurons. Moreover, AVP neuronal activation decreased Ca2+ levels in non-AVP neurons located between AVP- and VIP-rich regions of the SCN. These results suggest that GABA released from AVP neurons indirectly disinhibits VIP neurons by suppressing intermediate non-AVP neurons, thereby precisely setting behavioral activity/rest time.