Photoperiod-Induced Neuroplasticity in the Circadian System

Abstract

Seasonal changes in light exposure have profound effects on behavioral and physiological functions in many species, including effects on mood and cognitive function in humans. The mammalian brain's master circadian clock, the suprachiasmatic nucleus (SCN), transmits information about external light conditions to other brain regions, including some implicated in mood and cognition. Although the detailed mechanisms are not yet known, the SCN undergoes highly plastic changes at the cellular and network levels under different light conditions. We therefore propose that the SCN may be an essential mediator of the effects of seasonal changes of day length on mental health. In this review, we explore various forms of neuroplasticity that occur in the SCN and other brain regions to facilitate seasonal adaptation, particularly altered phase distribution of cellular circadian oscillators in the SCN and changes in hypothalamic neurotransmitter expression.

Figure 1

Representative confocal microscopic image of neuropeptide expression in the mouse suprachiasmatic nucleus (SCN). Slice was obtained from a CD1 mouse and processed for double immunofluorescence labeling for AVP (red) and VIP (blue). Scale bar 75 μm. AVP: arginine vasopressin; VIP: vasoactive intestinal peptide.

Figure 2

Proposed model for photoperiod-dependent cell rearrangement in the SCN. (a) Schematic of coronal SCN slice showing approximate spatial distributions of AVP, GRP, VIP, and astroglia. GABAergic inputs tend to result in excitatory responses under long photoperiod (left), while responses tend to be inhibitory under short photoperiod (right). (b) Relative mRNA expression levels of Per2, Rev-erbα, AVP, Bmal1, and VIP throughout a circadian cycle under long (left) photoperiod or short (right) photoperiod. Adapted from Ecker et al. [48] and Aton et al. [64]. AVP: arginine vasopressin; Bmal1: brain and muscle ARNT-like protein 1; GABA: gamma-aminobutyric acid; GRP: gastrin-releasing peptide; Per2: period 2; VIP: vasoactive intestinal peptide.

Figure 3

Schematic view highlighting certain ipRGC (blue) and SCN (orange) projections and areas that demonstrate rhythmic activity (clock symbol). Many areas receive input from both ipRGCs and the SCN, such as the VLPo, SPZ, LH, and LHb. The SCN receives input from the IGL and DRN (dashed lines). Amyg: amygdala; DRN: dorsal raphe nuclei; Hipp: hippocampus; IGL: intergeniculate leaflet; ipRGC: intrinsically photosensitive retinal ganglion cell; LC: locus coeruleus; LH: lateral hypothalamus; LHb: lateral habenula; NAc: nucleus accumbens; PVN: paraventricular nucleus; SCN: suprachiasmatic nucleus; Sept: septal area; SPZ: subparaventricular zone; VLPo: ventrolateral preoptic area; VTA: ventral tegmental area.