Neuroglial and synaptic rearrangements associated with photic entrainment of the circadian clock in the suprachiasmatic nucleus

Dec 15, 2010The European journal of neuroscience

Changes in brain support cells and connections linked to light resetting the body’s internal clock in the main time-keeping center

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Abstract

In rats, synaptic inputs to vasoactive intestinal peptide (VIP) neurons in the suprachiasmatic nucleus (SCN) increased during the day, while inputs to vasopressin (AVP) neurons remained unchanged.

The SCN undergoes neuroglial architectural rearrangements throughout the 24-hour light-dark cycle.
VIP neurons in the SCN are major integrators of photic signals and exhibit increased synaptic input during daytime.
AVP neurons do not show changes in synaptic input levels between day and night.
Glutamatergic and nonglutamatergic axons contribute to synaptic remodeling of VIP neurons.
Daily fluctuations in plasma glucocorticoid hormones may influence SCN neuroglial plasticity through the rhythmic release of serotonin.

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Rhythmic biological functions in mammals are orchestrated by a circadian timekeeper in the suprachiasmatic nucleus of the hypothalamus (SCN) which precisely adjusts clock outputs to solar time through the process of photic synchronization. Entrainment to the 24-h light-dark cycle is known to act on the molecular loops which trigger circadian oscillations but is also thought to involve day-night adjustments in the intercellular phasing of the multiple component SCN oscillators. This view is supported by data showing that the SCN undergoes important rearrangements of its neuroglial architecture throughout the 24-h cycle. The present paper highlights our data showing in rat that the two main sources of SCN efferents, composed of neurons synthesizing either vasopressin (AVP) or vasoactive intestinal peptide (VIP), are differentially involved in day-night SCN neuroglial plasticity. We found that the synaptic inputs received by the VIP neurons, which are major integrators of photic signals in the retinorecipient SCN subregion, increased during the day while those received by the AVP neurons remained unchanged at day and night. Glutamatergic axons, known to convey photic information from the retina, together with nonglutamatergic axons, contribute to the synaptic remodellings on VIP neurons. Experimental data providing strong indication that these plastic events may subserve synchronization of the clock to the light-dark cycle and that the daily fluctuations of plasma glucocorticoid hormones may act as temporal endocrine signals that may modulate SCN neuroglial plasticity through the rhythmic release of serotonin are also reviewed.

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Neuroglial and synaptic rearrangements associated with photic entrainment of the circadian clock in the suprachiasmatic nucleus

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