Retinal input to the sleep-active ventrolateral preoptic nucleus in the rat.

Light-dark periodicity and luminance play important roles in regulating sleep-wake cycles. We recently identified a discrete cluster of sleep-active cells in the ventrolateral preoptic nucleus that we hypothesize plays a central role in sleep regulation. These ventrolateral preoptic nucleus neurons are GABAergic and galaninergic and innervate the monoaminergic cell groups that contribute to the ascending arousal system. To determine whether there are retinal inputs to sleep-active ventrolateral preoptic nucleus neurons, we injected 5 microl of 1% cholera toxin B subunit into one eye in a series of rats. We found anterogradely labeled axons in the ventrolateral preoptic nucleus bilaterally, with a contralateral predominance. Using Fos to identify sleep-active neurons, we found that the cholera toxin B subunit-labeled retinal fibers gave rise to terminals that were distributed among the Fos-positive, sleep-active ventrolateral preoptic nucleus neurons. Combining cholera toxin B subunit staining with galanin immunoreactivity, we found that the retinal terminals formed appositions with the cell bodies and proximal dendrites of galaninergic ventrolateral preoptic nucleus neurons. Following Fluorogold injection into the ventrolateral preoptic nucleus region, we found retrogradely labeled neurons in the superior part of the peripheral retina contralateral to the injection site and in the superior temporal quadrant of the peripheral retina ipsilateral to the injection site. The size of labeled retinal ganglion cells suggested that they belonged to the type III or W type, which provide luminance input, e.g., to the suprachiasmatic nucleus. The retinal projection to the ventrolateral preoptic nucleus may provide an anatomical substrate by which luminance levels may directly influence sleep.