Physiological responses of the circadian clock to acute light exposure at night

Abstract

Circadian rhythms in physiological, endocrine and metabolic functioning are controlled by a neural clock located in the suprachiasmatic nucleus (SCN). This structure is endogenously rhythmic and the phase of this rhythm can be reset by light information from the eye. A key feature of the SCN is that while it is a small structure containing on the order of about 20,000 cells, it is amazingly heterogeneous. It is likely that anatomical heterogeneity reflects an underlying functional heterogeneity. In this review, we examine the physiological responses of cells in the SCN to light stimuli that reset the phase of the circadian clock, highlighting where possible the spatial pattern of such responses. Increases in intracellular calcium are an important signal in response to light, and this increase triggers many biochemical cascades that mediate responses to light. Furthermore, only some cells in the SCN are actually endogenously rhythmic, and these cells likely do not receive strong direct input from the retina. Therefore, this review also considers how light information is conveyed from the retinorecipient cells to the endogenously rhythmic cells that track circadian phase. A number of neuropeptides, including vasoactive intestinal polypeptide, gastrin-releasing peptide and substance P, may be particularly important in relaying such signals, but other neurochemicals such as GABA and nitric oxide may participate as well. A thorough understanding of the intracellular and intercellular responses to light, as well as the spatial arrangements of such responses may help identify important pharmacological targets for therapeutic interventions to treat sleep and circadian disorders.

Environmental Circadian Disruption Worsens Neurologic Impairment and Inhibits Hippocampal Neurogenesis in Adult Rats After Traumatic Brain Injury

“…The environmental light/dark cycle plays a pivotal role in modulating circadian rhythm in mammals (Antle et al 2009; Zelinski et al 2014). Thus, disruption of cyclic light conditions has a variety of effects on physiology and behavior (Panda et al 2002; Roenneberg et al 2013; Stevens et al 2013; Tataroglu et al 2004).…”

The dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

“…There are a number of excellent reviews of SCN neuroanatomy that discuss the issues related to SCN subdivisions, so we provide only a brief background and discussion here (Moore et al, 2002; Hastings and Herzog, 2004; Lee et al, 2003; Antle et al, 2009; Morin, 2007; Antle and Silver, 2005; Yan et al, 2007; Yan, 2009; Evans, 2016; Evans and Gorman, 2016). Historically, each side of the bilaterally paired SCN has been divided into two regions (although a third division has been described as well) (Morin, 2007).…”

Linking neural activity and molecular oscillations in the SCN

“…This is a central feature of entrainment, that is, the process by which circadian oscillators are synchronized to the environment. One way to consider how the membrane and the molecular clockwork are coupled is therefore by examining the signalling mechanisms through which light resets the circadian system 9,10 .…”