Developmental patterning of peptide transcription in the central circadian clock in both sexes

Arginine vasopressin: Critical regulator of circadian homeostasis

Circadian rhythms optimally regulate numerous physiological processes in an organism and synchronize them with the external environment. The suprachiasmatic nucleus (SCN), the center of the circadian clock in mammals, is composed of multiple cell types that form a network that provides the basis for the remarkable stability of the circadian clock. Among the neuropeptides expressed in the SCN, arginine vasopressin (AVP) has attracted much attention because of its deep involvement in the function of circadian rhythms, as elucidated in particular by studies using genetically engineered mice. This review briefly summarizes the current knowledge on the peptidergic distribution and topographic neuronal organization in the SCN, the molecular mechanisms of the clock genes, and the relationship between the SCN and peripheral clocks. With respect to the physiological roles of AVP and AVP-expressing neurons, in addition to a sex-dependent action of AVP in the SCN, studies using AVP receptor knockout mice and mice genetically manipulated to alter the clock properties of AVP neurons are summarized here, highlighting its importance in maintaining circadian homeostasis and its potential as a target for therapeutic interventions.

Circadian neurogenetics and its implications in neurophysiology, behavior, and chronomedicine

The circadian rhythm affects multiple physiological processes, and disruption of the circadian system can be involved in a range of disease-related pathways. The genetic underpinnings of the circadian rhythm have been well-studied in model organisms. Significant progress has been made in understanding how clock genes affect the physiological functions of the nervous system. In addition, circadian timing is becoming a key factor in improving drug efficacy and reducing drug toxicity. The circadian biology of the target cell determines how the organ responds to the drug at a specific time of day, thus regulating pharmacodynamics. The current review brings together recent advances that have begun to unravel the molecular mechanisms of how the circadian clock affects neurophysiological and behavioral processes associated with human brain diseases. We start with a brief description of how the ubiquitous circadian rhythms are regulated at the genetic, cellular, and neural circuit levels, based on knowledge derived from extensive research on model organisms. We then summarize the latest findings from genetic studies of human brain disorders, focusing on the role of human clock gene variants in these diseases. Lastly, we discuss the impact of common dietary factors and medications on human circadian rhythms and advocate for a broader application of the concept of chronomedicine.

The transcription factor VAX1 in VIP neurons of the suprachiasmatic nucleus impacts circadian rhythm generation, depressive-like behavior, and the reproductive axis in a sex-specific manner in mice

Background

The suprachiasmatic nucleus (SCN) within the hypothalamus is a key brain structure required to relay light information to the body and synchronize cell and tissue level rhythms and hormone release. Specific subpopulations of SCN neurons, defined by their peptide expression, regulate defined SCN output. Here we focus on the vasoactive intestinal peptide (VIP) expressing neurons of the SCN. SCN VIP neurons are known to regulate circadian rhythms and reproductive function.

Methods

To specifically study SCN VIP neurons, we generated a novel knock out mouse line by conditionally deleting the SCN enriched transcription factor, Ventral Anterior Homeobox 1 (Vax1), in VIP neurons (Vax1Vip; Vax1fl/fl:VipCre).

Results

We found that Vax1Vip females presented with lengthened estrous cycles, reduced circulating estrogen, and increased depressive-like behavior. Further, Vax1Vip males and females presented with a shortened circadian period in locomotor activity and ex vivo SCN circadian period. On a molecular level, the shortening of the SCN period was driven, at least partially, by a direct regulatory role of VAX1 on the circadian clock genes Bmal1 and Per2. Interestingly, Vax1Vip females presented with increased expression of arginine vasopressin (Avp) in the paraventricular nucleus, which resulted in increased circulating corticosterone. SCN VIP and AVP neurons regulate the reproductive gonadotropin-releasing hormone (GnRH) and kisspeptin neurons. To determine how the reproductive neuroendocrine network was impacted in Vax1Vip mice, we assessed GnRH sensitivity to a kisspeptin challenge in vivo. We found that GnRH neurons in Vax1Vip females, but not males, had an increased sensitivity to kisspeptin, leading to increased luteinizing hormone release. Interestingly, Vax1Vip males showed a small, but significant increase in total sperm and a modest delay in pubertal onset. Both male and female Vax1Vip mice were fertile and generated litters comparable in size and frequency to controls.

Conclusion

Together, these data identify VAX1 in SCN VIP neurons as a neurological overlap between circadian timekeeping, female reproduction, and depressive-like symptoms in mice, and provide novel insight into the role of SCN VIP neurons.