Temporospatial characteristics of light-induced Fos immunoreactivity in suprachiasmatic nuclei are not modified in Syrian hamsters treated neonatally with monosodium glutamate

Abstract

Neonatal treatment of rodents by intraperitoneal injections of monosodium glutamate (MSG) destroys many retinal ganglion cells whose neurons belong to the circadian system; howertheless, adults always synchronize their locomotor activity rhythm (LAR) to the light/dark cycle. Recent studies have shown that light-induced phase shifts of LAR are associated with the c- fos induction in suprachiasmatic nuclei (SCN) of nocturnal rodents. In this study, the circadian system was analyzed in treated and control hamsters maintained in constant darkness and exposed to light at circadian times (CTs) 13 and 18 during subjective night, 1 and 6 h after the onset of LAR. The period of the LAR and delay (CT13) and advance (CT18) phase shifts of LAR were not significantly different between MSG-treated and control hamsters. Temporospatial variations of Fos induction after light exposure were similar in both MSG-treated and control hamsters although the total number of Fos immunoreactive (Fos-ir) nuclei in the SCN was always lower in treated hamsters. However, the decrease in Fos-ir was significant only for the caudal third of the SCN of treated hamsters, the part where retinal afferents are most dense. The effect of light exposure on Fos expression in SCN of MSG-treated and control hamsters was the same at CT13 and CT18: (1) Fos-ir nuclei were significantly more numerous at CT18 than at CT13 in the rostral SCN; (2) dorsal Fos-ir cells were observed in the SCN only at CT18; (3) a ventral subgroup expressed Fos protein in intermediate SCN only at CT13. This study demonstrates that MSG-treatment does not significantly modify the phase-shifting effects of light on either the LAR or the associated cellular activation.