NEONATAL MONOSODIUM GLUTAMATE TREATMENT PREVENTS EFFECTS OF CONSTANT LIGHT ON CIRCADIAN TEMPERATURE RHYTHMS OF ADULT-RATS

Housing rats under continuous illumination (LL) disrupts circadian rhy thms controlled by a pacemaker located in the hypothalamic suprachiasm atic nucleus (SCN). The neural mechanisms underlying this effect are n ot well understood. The present study examined the effects of LL on ci rcadian rhythms and on light-induced expression of Fos protein in the SCN, intergeniculate leaflet (IGL), and ventrolateral geniculate nucle us (vLGN) in adult rats treated neonatally with monosodium glutamate ( MSG). Such treatment is known to lead to acute degeneration of retinal ganglion cells. Despite degeneration of the optic nerve, neonatal MSG treatment (2 mg/g SC on postnatal days 1,3,5,7, and 9) had no effect on daily temperature rhythms in the adult animal under a light-dark cy cle. However, the disintegration of circadian rhythms under LL conditi ons observed in adult rats treated neonatally with 10% saline was prev ented in MSG-treated rats. Furthermore, neonatal MSG treatment attenua ted light-induced expression of Fos protein in the IGL and vLGN, but n ot in the SCN. These data suggest that neonatal MSG treatment alters t he response of the circadian system to LL and that cells within the IG L/vLGN region may mediate this response.