Citation

The circadian timing system (CTS) is composed of a set of neural structures responsible for the generation and modulation of the circadian rhythms. The suprachiasmatic nucleus (SCN) of the hypothalamus is the structure that generates the circadian rhythmicity, with the 24h environmental light-dark cycle (LD) being its main synchronizer. SCN generates the expression of circadian rhythms in various diencephalic, telencephalic, and forebrain neural structures. Recently, the concept of “extended STC” has been proposed, considering that there are approximately 100 neural structures that could compose this system. Of the several regions involved in this context, the subparaventricular zone (SPVZ), that receives significant input from the SCN, has been highlighted, although not yet studied in primate species such as the Callithrix jacchus. The aim of our study is to characterize its cytoarchitecture and neurochemical composition, in addition to mapping retinal afferents to the SPVZ of marmosets. For this, frontal cuts of the marmoset brain were submitted to staining using the Nissl method, as well as immunohistochemical techniques. From our results, we could characterize the SPVZ cytoarchitecture in this species through the Nissl method and immunoreactivity for neuronal specific nuclear (NeuN) and glial fibrillary acid (GFAP) proteins. We identified SPVZ in the region located between the dorsal and caudal portions of the SCN, and ventral portions of the paraventricular nucleus of the hypothalamus (PVN), distributing laterally to the III ventricle, occupying periventricular portions. The neurochemical content of calcium-binding proteins demonstrates a more pronounced presence of calbindin (CB +) immunoreactive neurons when compared to the immunoreactive neurons to calretinin (CR), and non-marking to parvalbumin (PV). The CB + neurons present a dorsolateral, and occupy the ventromedial, ventrolateral and perisuprachiasmatic ventrolateral regions of the SPVZ. It was also possible to observe in the ventromedial portion of SPVZ immunoreactive neural fibers to neuropeptide Y (NPY), decarboxylase of glutamic acid (GAD) enzyme, and substance P (SP), whereas in the ventral and dorsal portions of SPVZ we observed 5-HT immunoreactive fibers. The glutamate receptor subunit 1 (GluR1) is strongly present in the dorsal SPVZ. We also observed a bilateral distribution of immunoreactive retinohypothalamic tract axon terminals (RHT) to the b subunit of cholera toxin (CTb), which demonstrates the projection of the retina to the ventral and ventrolateral perisuprachiasmatic portion. Although we did not perform functional experiments, the clear presence of CB in the SPVZ, its intimate relation with the SCN and PVN, together with the data available in the literature, places the SPVZ in marmosets as an important region of the extended CTS.