A direct pretectosuprachiasmatic projection in the rat

Abstract

The major afferent projections of the suprachiasmatic nuclei originate in the retina and the intergeniculate leaflet of the lateral geniculate nucleus and are important in the entrainment of endogenous circadian rhythms. A characteristic feature of the suprachiasmatic nucleus and the intergeniculate leaflet of the thalamus is that they are bilaterally innervated from the retina. However, parts of the olivary and posterior pretectal nuclei have been shown to be bilaterally innervated from the retina as well. We therefore aimed to explore whether these two nuclei, in the rat, were anatomically related to the suprachiasmatic nucleus. The anterograde neuronal tract-tracer, Phaseolus vulgaris-leucoagglutinin, was injected iontophoretically into different pretectal nuclei. Pretectal injections centered only in the medial part of the pretectum, i.e. involving the olivary and posterior pretectal nuclei, gave rise to a substantial bilateral innervation of the suprachiasmatic nucleus. From the site of injection, Phaseolus vulgaris-leucoagglutinin-immunoreactive nerve fibers coursed laterally and rostrally into the optic tract, and within the optic tract and chiasm, under the diencephalon to penetrate dorsally into the suprachiasmatic nucleus. Varicose Phaseolus vulgaris-leucoagglutinin-labeled nerve fibers were found exclusively in the ventrolateral part of the suprachiasmatic nucleus, mostly on the ipsilateral side. To determine the precise location of the projecting neurons, the retrograde tracer Cholera toxin, subunit B, was iontophoretically injected into the suprachiasmatic nucleus. The presence of labeled neurons scattered in both the posterior and olivary pretectal nuclei was observed. Most neurons were found in the medial part of the posterior pretectal nucleus and in the dorsomedial part of the olivary pretectal nucleus ipsilateral to the injection, confirming the anterograde tracing studies. These neuroanatomical tract-tracing results reveal the presence of a novel pathway linking the pretectum and the suprachiasmatic nucleus, potentially involved in phase-shifting mechanisms.