Ultrastructural plasticity in the rat suprachiasmatic nucleus. Possible involvement in clock entrainment

Dec 15, 2007Glia

Tiny structural changes in the rat brain’s internal clock that may help reset daily rhythms

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Abstract

At nighttime, glial coverage of VIP neuron dendrites increased by 29%, while axon terminal coverage of these neurons decreased by up to 43%.

The suprachiasmatic nucleus (SCN) shows rhythmic structural changes over a 24-hour cycle.
Increased glial coverage of VIP neuron dendrites at night is associated with decreased axon terminal coverage.
Conversely, glial coverage of AVP neuron dendrites decreased at night, with increases in axon terminal coverage.
Daily fluctuations in glial fibrillary acidic protein (GFAP) expression reflect structural plasticity in the SCN.
GFAP rhythm is dependent on light exposure, suggesting its role in synchronizing the SCN with the light/dark cycle.
Circulating glucocorticoid hormones are necessary for maintaining normal GFAP rhythm amplitude.

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Circadian rhythms in mammals are synchronized to the light (L)/dark (D) cycle through messages relaying in the master clock, the suprachiasmatic nucleus of the hypothalamus (SCN). Here, we provide evidence that the SCN undergoes rhythmic ultrastructural rearrangements over the 24-h cycle characterized by day/night changes of the glial, axon terminal, and/or somato-dendritic coverage of neurons expressing arginine vasopressin (AVP) or vasoactive intestinal peptide (VIP), the two main sources of SCN efferents. At nighttime, we noted an increase in the glial coverage of the dendrites of the VIP neurons (+29%) that was concomitant with a decrease in the mean coverage of the somata (-36%) and dendrites (-43%) of these neurons by axon terminals. Conversely, glial coverage of the dendrites of AVP neurons decreased (-19%) with parallel increase in the extent of somatal (+96%) and dendritic (+52%) membrane appositions involving these neurons. These plastic events were concomitant with daily fluctuations in quantitative expression of glial fibrillary acidic protein (GFAP), which were then used as an index of structural plasticity. The GFAP rhythm appeared to be strictly dependent on light entrainment, indicating that structural reorganization of the SCN may subserve synchronization of the clock to the L/D cycle. Other results presented reinforced this view while showing that circulating glucocorticoid hormones, which are known to modulate photic entrainment, were required to maintain amplitude of the GFAP rhythm to normal values.

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Ultrastructural plasticity in the rat suprachiasmatic nucleus. Possible involvement in clock entrainment

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