Rat drinking rhythms: Central visual pathways and endocrine factors mediating responsiveness to environmental illumination

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Continuous environmental illumination (L-L) suppressed water consumption in Sprague-Dawley rats. Bilateral transection of the inferior accessory optic tracts (IAOT) completely abolished this effect of light on drinking. On the other hand, bilateral ablation of the primary optic tracts (POT) increased the suppressive effect of light on water intake. POT rats were impaired in the acquisition of a light-dark discrimination in a Y-maze, whereas IAOT animals performed normally. These results suggest that the inferior accessory optic tracts mediate the inhibitory effects of excessive light on drinking and that the visual pathways which underlie pituitary responsiveness to illumination are distinguishable from those which mediate visually guided behavior. Pinealectomy did not attenuate the effect of L-L on drinking, but unilateral blinding diminished and hypophysectomy completely eliminated the suppressive effects of light. Rats maintained in a 12:12 light-dark (L-D) cycle displayed nocturnal rhythms in water intake; when the illumination cycle was inverted (D-L), they required 6–9 days to completely re-entrain drinking. Bilateral interruption of the inferior accessory and primary optic tracts and pinealectomy did not interfere with nocturnal drinking rhythms in L-D nor with their re-entrainment in D-L. Hypophysectomy attenuated nocturnal eating and drinking rhythms, and unilateral enucleation as well as hypophysectomy retarded re-entrainment in D-L. The effects of combined bilateral transection of the IAOT and POT were described; a drinkometer analysis of water intake confirmed results obtained with less refined measures of rhythmicity. Our findings indicate that excessive illumination influences the neuroendocrine drinking system of the albino rat via a completely decussated visual pathway. We propose that neither the primary nor the accessory optic tracts mediate the entrainment of drinking to the day-night cycle; this function may be fulfilled by a direct retino-hypothalamic pathway.