Abstract
The brain's master circadian pacemaker resides within the hypothalamic suprachiasmatic nucleus (SCN). SCN clock neurons are entrained to the day/night cycle via the retinohypothalamic tract and the SCN provides temporal information to the central nervous system and to peripheral organs that function as secondary oscillators. The SCN clock-cell network is thought to be the hypothalamic link between the retina and descending autonomic circuits to peripheral organs such as the adrenal gland, thereby entraining those organs to the day/night cycle. However, there are at least three different routes or mechanisms by which retinal signals transmitted to the hypothalamus may be conveyed to peripheral organs: 1) via retinal input to SCN clock neurons; 2) via retinal input to non-clock neurons in the SCN; or 3) via retinal input to hypothalamic regions neighboring the SCN. It is very well documented that light-induced responses of the SCN clock (i.e., clock gene expression, neural activity, and behavioral phase shifts) occur primarily during the subjective night. Thus to determine the role of the SCN clock in transmitting photic signals to descending autonomic circuits, we compared the phase dependency of light-evoked responses in the SCN and a peripheral oscillator, the adrenal gland. We observed light-evoked clock gene expression in the mouse adrenal throughout the subjective day and subjective night. Light also induced adrenal corticosterone secretion during both the subjective day and subjective night. The irradiance threshold for light-evoked adrenal responses was greater during the subjective day compared to the subjective night. These results suggest that retinohypothalamic signals may be relayed to the adrenal clock during the subjective day by a retinal pathway or cellular mechanism that is independent of an effect of light on the SCN neural clock network and thus may be important for the temporal integration of physiology and metabolism.
Highlights
The suprachiasmatic nucleus (SCN) of the anterior hypothalamus contains the primary circadian oscillator in the central nervous system
Light stimulation during the subjective day (CT2 and CT4) produced no change in the transcripts of Per1, Per2 or c-fos in the SCN compared to animals maintained in the dark, whereas during the subjective night (CT23) light-evoked an increase in the SCN expression of all three genes (Fig. 1A)
These results are consistent with numerous previous reports indicating that light-induced effects on SCN gene expression and behavior are restricted to the subjective night [22]
Summary
The suprachiasmatic nucleus (SCN) of the anterior hypothalamus contains the primary circadian oscillator in the central nervous system. An interconnected network of autonomous clock cells within the SCN is entrained to the day/night cycle via retinal signals that daily reset clock gene oscillations and neuronal firing of SCN cells [1]. One postulated means by which the SCN regulates peripheral oscillators such as the adrenal gland is by way of descending autonomic circuits. In this manner, the SCN pacemaker may entrain the adrenal molecular clockwork to the light:dark cycle, gating responsiveness to adrenocorticotropic hormone (ACTH) and thereby generating an appropriately phased daily rhythm in adrenal corticosterone secretion [3,4,5,6]. Light evokes an increase in corticosterone secretion independent of a detectable increase in plasma ACTH, highlighting the critical role played by the sympathetic splanchnic nerve in conveying retinal signals to the adrenal [9]
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