In this study, two circadian related centers, the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) were evaluated in respect to their cytoarchitecture, retinal afferents and chemical content of major cells and axon terminals in the rock cavy (Kerodon rupestris), a Brazilian rodent species. The rock cavy SCN is innervated in its ventral portion by terminals from the predominantly contralateral retina. It also contains vasopressin, vasoactive intestinal polypeptide and glutamic acid decarboxilase immunoreactive cell bodies and neuropeptide Y, serotonin and enkephalin immunopositive fibers and terminals and is marked by intense glial fibrillary acidic protein immunoreactivity. The IGL receives a predominantly contralateral retinal projection, contains neuropeptide Y and nitric oxide synthase-producing neurons and enkephalin immunopositive terminals and is characterized by dense GFAP immunoreactivity. This is the first report examining the neural circadian system in a crepuscular rodent species for which circadian properties have been described. The results are discussed comparing with what has been described for other species and in the context of the functional significance of these centers.

The thalamic paraventricular nucleus (PVT) receives afferents from numerous brain areas, including the hypothalamic suprachiasmatic nucleus (SCN), considered to be the major circadian pacemaker. The PVT also sends projections to the SCN, limbic system centers and some nuclei involved in the control of the Sleep-Wake cycle. In this study, we report the identification of a hitherto not reported direct retinal projection to the PVT of the rock cavy, a typical rodent species of the northeast region of Brazil. After unilateral intravitreal injections of cholera toxin subunit B (CTb), anterogradely transported CTb-immunoreactive fibers and presumptive terminals were seen in the PVT. Some possible functional correlates of the present data are briefly discussed, including the role of the PVT in the modulation of the circadian rhythms by considering the reciprocal connections between the PVT and the SCN. The present work is the first to show a direct retinal projection to the PVT of a rodent and may contribute to elucidate the anatomical substrate of the functionally demonstrated involvement of this midline thalamic nucleus in the modulation of the circadian timing system.