Anterograde tracing of retinal afferents to the tree shrew hypothalamus and raphe.

The anterograde neuronal transport of Cholera toxin B subunit (CTB) was used in this study to label the termination of retinal afferents in the hypothalamus of the tree shrew Tupaia belangeri. Upon pressure-injection of the substance into the vitreous body of one eye, a major projection of the retinohypothalamic tract (RHT) was found to the hypothalamic suprachiasmatic nuclei (SCN). Although the innervation pattern was bilateral, the ipsilateral SCN received a somewhat stronger projection. Labeling was also found in the supraoptic nucleus and its perinuclear zone, respectively, mainly ipsilaterally as well as in the bilateral para- and periventricular hypothalamic regions without lateral predominance. In the raphe region, scattered fibers and terminals were seen in the dorsal and median raphe nuclei. CTB-immunoreactive structures were observed neither in the locus ceruleus nor in vagal nuclei. Our results, partly in contradiction to earlier studies using different tracing techniques in another tree shrew species (Tupaia glis), reveal that hypothalamic nuclei, in particular the SCN, are contacted by retino-afferent fibers which are thought to mediate the effects of light to the endogenous 'clock' and to parts of the neuroendocrine system.