The brain's calendar: neural mechanisms of seasonal timing

Abstract

The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal component of the mammalian biological clock, the neural timing system that generates and coordinates a broad spectrum of physiological, endocrine and behavioural circadian rhythms. The pacemaker of the SCN oscillates with a near 24 h period and is entrained to the diurnal light-dark cycle. Consistent with its role in circadian timing, investigations in rodents and non-human primates furthermore suggest that the SCN is the locus of the brain's endogenous calendar, enabling organisms to anticipate seasonal environmental changes. The present review focuses on the neuronal organization and dynamic properties of the biological clock and the means by which it is synchronized with the environmental lighting conditions. It is shown that the functional activity of the biological clock is entrained to the seasonal photic cycle and that photoperiod (day length) may act as an effective zeitgeber. Furthermore, new insights are presented, based on electrophysiological and molecular studies, that the mammalian circadian timing system consists of coupled oscillators and that the clock genes of these oscillators may also function as calendar genes. In summary, there are now strong indications that the neuronal changes and adaptations in mammals that occur in response to a seasonally changing environment are driven by an endogenous circadian clock located in the SCN, and that this neural calendar is reset by the seasonal fluctuations in photoperiod.

The regulation of neuroendocrine function: Timing is everything

“…Several hypotheses have been proposed to account for how the SCN and its targets track time on a seasonal basis Hofman, 2004;Lincoln et al, 2003). In the SCN of Syrian and Siberian hamsters, photoperiod alters the duration of clock and clock-controlled gene expression, while the amplitude of gene expression is influenced by photoperiod in the pars tuberalis Messager et al, 2000).…”

Two cysteine substitutions in the MC1R generate the blue variant of the arctic fox (Alopex lagopus) and prevent expression of the white winter coat

“…The pacemaker of the SCN oscillates with near 24 h period, and coordinates a broad spectrum of physiological, endocrine and behavioral circadian rhythms. Consistent with its role in circadian timing, several experiments have provided evidence that SCN is the locus of the brains endogenous calendar, enabling organisms to anticipate seasonal changes (for review see [8]). Mechanisms of how the circadian rhythm of SCN is translated into seasonal variation and how seasonal variation in coat color is functionally regulated are not well understood.…”

Temporal organization of activity in the brown bear (Ursus arctos): roles of circadian rhythms, light, and food entrainment

“…Moreover, there are accompanying changes in the daily light-to-dark cycle that are transformed into a photoperiod-dependent hormonal signal in the form of melatonin secretion by the pineal gland (25). In mammals, this photoperiod transduction requires, in part, the integration of photic information from the retina with an endogenous pacemaker, or circadian clock, in the brain to organize the daily cycle of melatonin (31,56).This pacemaker, or circadian (about 24 h) clock, resides in the suprachiasmatic nucleus (SCN) of the hypothalamus and generates an endogenous oscillation with a period slightly different from 24 h via a molecular transcription-translation …”

“…Environmental light serves as the primary effector of synchronization between the biological clock and the external photocycle; this function is mediated by the SCN via a diverse set of SCN neural projections (31,64). The endogenous clock synchronizes to the external clock by being reset (or phase-shifted) each day to match the 24-h photocycle.…”