The intergeniculate leaflet does not mediate the disruptive effects of constant light on circadian rhythms in the rat

Prolonged constant light exposure causes disruptions in circadian rhythms, resulting in splitting of circadian activity rhythms in hamsters and arrhyt hmicity in rats. Hamsters with lesions of the thalamic intergeniculate leaf let do not exhibit constant light-induced disruptions in rhythmicity. We ha ve shown that circadian rhythms of rats with monosodium glutamate-induced n eurotoxic damage to visual pathways persist under constant light, and hypot hesized that monosodium glutamate damaged the retinogeniculate pathway, thu s preventing constant light- induced arrhythmicity. The present study demon strates, however, that the intergeniculate leaflet does not mediate these e ffects in rats. Rats with bilateral electrolytic intergeniculate leaflet le sions showed the same rate of disruption of circadian temperature rhythms a s did sham-operated animals, housed under constant light. We also show that , unlike intergeniculate leaflet-lesioned rats, rats treated neonatally wit h monosodium glutamate exhibit neuropeptide Y fiber staining in the suprach iasmatic nucleus, indicating that the geniculohypothalamic tract is functio nally intact following monosodium glutamate treatment. Taken together, these data demonstrate that the disruption of circadian rhy thms during constant light exposure is not mediated directly via the genicu lohypothalamic tract in rats. Whether this disruption in rhythmicity result s from effects of constant light exposure on the circadian pacemaker, or is a direct effect of light on body temperature, is unknown. Retinal or colli cular damage in monosodium glutamate-treated rats may render these animals insensitive to the disruptive effects of constant light. (C) 1999 IBRO. Pub lished by Elsevier Science Ltd.