GABAergic network from AVP neurons to VIP neurons in the suprachiasmatic nucleus sets the activity/rest time of the circadian behavior rhythm

The central circadian clock of the suprachiasmatic nucleus (SCN) is a network composed of multiple types of {gamma}-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 by a vesicular GABA transporter (Vgat) gene deletion 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 match the behavioral rest time. Conversely, eliminating functional GABAA receptors (GABAAR) in VIP neurons by in vivo genome editing reduced locomotor activity level and the activity time, and lengthened the high Ca2+ duration in VIP neurons. Optogenetic activation of AVP neurons in vivo increased Ca2+ in VIP neurons during the night. A similar Ca2+ response of VIP neurons to AVP neuronal activation was also observed in SCN slices and was inhibited by a GABAAR antagonist, gabazine. Importantly, gabazine application alone raised the baseline Ca2+ in VIP neurons, suggesting a tonic depression of these neurons by GABA. Moreover, AVP neuronal activation decreased Ca2+ in non-AVP neurons located between AVP- and VIP-rich regions in the SCN. These results suggest that GABA from AVP neurons disinhibits VIP neurons indirectly by suppressing other intermediate GABA neurons to set the behavior activity/rest time precisely.