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

The anterograde neuronal transport of Cholera toxin B subunit (CTB) was use d in this study to label the termination of retinal afferents in the hypoth alamus of the tree shrew Tupaia belangeri. Upon pressure-injection of the s ubstance into the vitrous body of one eye, a major projection of the retino hypothalamic tract (RHT) was found to the hypothalamic suprachiasmatic nucl ei (SCN). Although the innervation pattern was bilateral, the ipsilateral S CN received a somewhat stronger projection. Labeling was also found in the supraoptic nucleus and its perinuclear zone, respectively, mainly ipsilater ally as well as in the bilateral para- and periventricular hypothalamic reg ions without lateral predominance. In the raphe region, scattered fibers an d terminals were seen in the dorsal and median raphe nuclei. CTB-immunoreac tive structures were observed neither in the locus ceruleus nor in vagal nu clei. Our results, partly in contradiction to earlier studies using differe nt tracing techniques in another tree shrew species (Tupaia glis), reveal t hat hypothalamic nuclei, in particular the SCN, are contacted by retino-aff erent fibers which are thought to mediate the effects of light to the endog enous 'clock' and to parts of the neuroendocrine system. (C) 2000 Elsevier Science B.V. All rights reserved.