GABA from vasopressin neurons regulates the time at which suprachiasmatic nucleus molecular clocks enable circadian behavior

Abstract

The suprachiasmatic nucleus (SCN), the central circadian pacemaker in mammals, is a network structure composed of multiple types of γ-aminobutyric acid (GABA)-ergic neurons and glial cells. However, the roles of GABA-mediated signaling in the SCN network remain controversial. Here, we report noticeable impairment of the circadian rhythm in mice with a specific deletion of the vesicular GABA transporter in arginine vasopressin (AVP)-producing neurons. These mice showed disturbed diurnal rhythms of GABAA receptor-mediated synaptic transmission in SCN neurons and marked lengthening of the activity time in circadian behavioral rhythms due to the extended interval between morning and evening locomotor activities. Synchrony of molecular circadian oscillations among SCN neurons did not significantly change, whereas the phase relationships between SCN molecular clocks and circadian morning/evening locomotor activities were altered significantly, as revealed by PER2::LUC imaging of SCN explants and in vivo recording of intracellular Ca2+ in SCN AVP neurons. In contrast, daily neuronal activity in SCN neurons in vivo clearly showed a bimodal pattern that correlated with dissociated morning/evening locomotor activities. Therefore, GABAergic transmission from AVP neurons regulates the timing of SCN neuronal firing to temporally restrict circadian behavior to appropriate time windows in SCN molecular clocks.

Arginine-Vasopressin Expressing Neurons in the Murine Suprachiasmatic Nucleus Exhibit a Circadian Rhythm in Network Coherence In Vivo

“…AVP-specific Per2::luc rhythms in SCN slices illustrate the presence of functional molecular clocks[26]. Imaging from SCN slices from AVPEluc/+ mice demonstrates the rhythmic transcription of avp in the SCN[54], and there is a circadian rhythm in the secretion of vasopressin[55, 56] and GABA[34] from AVP neurons in the SCN. Using longitudinal calcium imaging, we find that stable circadian rhythmicity is not observed in all AVP neurons.…”

“…The apparently stochastic nature of rhythmic activity in AVP neurons presents a challenge given the importance of rhythmicity in AVP neurons. Loss of rhythmic GABA release from AVP neurons resulted in abnormal SCN firing and locomotor activity [34], and AVP-specific knockout of bmal1 results in mice that have significantly longer activity periods in constant darkness, decreased light responsiveness, and that also rapidly re-entrained to 4-hour phase advances [24]. Taken together, these results suggest that while some aspect of rhythmicity in AVP neurons must be important for circadian regulation, perhaps stable rhythmic activity of individual neurons is not needed.…”

“…Observation of VIP neuronal gene expression and intracellular calcium via fiber photometry of fluorescence show robust population level rhythms in calcium, Per1, Per2, and Cry1 in vivo [23, 32]. This approach highlights the importance of VIP neurons for photic entrainment[23, 33], and the importance of GABA release from AVP neurons in regulating the timing of firing of other SCN neurons[34]. However, fiber photometry is a population measure that does not provide the cellular resolution needed to enumerate how neural networks are formed from individual neurons.…”

Astrocyte Gliotransmission in the Regulation of Systemic Metabolism

“…These effects seem to be driven by the inability of astrocytes to control extracellular GABA content [155,157,159]. Considering that the vast majority of neurons in the SCN are GABAergic [160] and the synchronization of clock neurons in the SCN highly depends on GABAergic transmission [161,162], astrocytes may exert relevant modulation on the inhibitory circuitry dictating circadian oscillations via GABA homeostasis regulation. Indeed, the cooperative orchestration of the activity fluctuations of neurons and astrocytes in the SCN governing the circadian rhythm has gained new insights since the observation that neurons are active during the active phase of the circadian rhythm whereas astrocytes are active during the resting phase, as evidenced by Ca 2+ measurements [158].…”

Conditional Knockout of Bmal1 in Corticotropin-Releasing Factor Neurons Does Not Alter Sleep–Wake Rhythm in Mice

“…For instance, several neuropeptides, such as arginine vasopressin, vasoactive intestinal peptides, and gastrin-releasing peptides are involved in the SCN. Importantly, almost all SCN neurons are GABAergic, and this inhibitory signal is suggested to be involved in the regulation of circadian outputs such as sleep and wake cycles (Ono et al, 2019(Ono et al, , 2020Maejima et al, 2021). Circadian period, amplitude, and robustness are different in individual SCN neurons without neuronal networks when studied in a dispersed cell culture (Welsh et al, 1995).…”