Calbindin-D28k immunoreactivity in the suprachiasmatic nucleus and the circadian response to constant light in the rat

Recent studies in the hamster have led to the discovery that the expression of the calcium binding protein, calbindin-D28k, is a defining feature of n eurons in the suprachiasmatic nucleus involved in the regulation of circadi an rhythms by environmental light.(2,18,19,32) To study further the involve ment of calbindin-D28k, we examined the effect of exposure to constant ligh t on calbindin-D28k immunoreactivity in the suprachiasmatic nucleus of inta ct rats and of rats treated neonatally with the retinal neurotoxin, monosod ium glutamate, Exposure to constant light is known to disrupt circadian rhy thms in rodents and we found previously that treatment with monosodium glut amate selectively prevents the disruptive effect of constant light on circa dian rhythms in rats.(7,9) In the present study we found that exposure to l ight suppresses calbindin-D28k expression in the ventrolateral retinorecipi ent region of the suprachiasmatic nucleus of rats and that neonatal treatme nt with monosodium glutamate blocks the suppressive effect of constant ligh t on calbindin-D28k expression. These findings are consistent with the prop osed role of calbindin-D28k in photic signaling in the suprachiasmatic nucl eus,(32) and point to the possibility that suppression of calbindin-D28k ex pression is linked to the mechanism by which constant light disrupts circad ian rhythms. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights r eserved.