*Corresponding author e-mail: twylabarros@hotmail.com Introduction: The paraventricular nucleus of the thalamus (PVT) is the most dorsal and widest component of the midline thalamic group. It plays a role in the integration of multiple sensorymotor, somatovisceral and motivational-emotional systems, besides to be involved in the circadian timing system regulation. The PVT, particularly its anterior portion, is the largest extrahypothalamic target of the efferent connections of the suprachiasmatic nucleus (SCN). Besides the SCN, the PVT also receives afferent terminals from the thalamic intergeniculate leaflet, another circadian center related to synchronization of the circadian rhythms. By considering these connections, the PVT is usually conceived as an output pathway of the circadian timing system. The PVT is derived from the epithalamus together with the pineal gland, responsible by melatonin production, which is essential to control of seasonal behaviors and activityrest phase signal. Methods: The aim of this work was to describe the cytoarchitecture and neurochemical profile of the PVT of two native species of the Brazilian Northeast region, the rodent rock cavy ( Kerodon rupestris ), and the primate common marmoset ( Callithrix jacchus ). The pattern of the retinal projections to the midline thalamus of both species was also compared. Cytoarchitectonic and immunohistochemical techniques and the cholera toxin subunit B (CTb) as an anterograde tracer were used in this study. Results: Enkephalin (ENK)-, neuropeptide Y (NPY)-, and parvalbumin (PV)-immunoreactive fibers and terminals and calbindin (CB)- and calretinin (CR)-immunoreactive perikarya were detected in the rock cavy PVT. ENK-, NPY- and CB- immunoreactive fibers and terminals and CR-immunoreactive perikarya, fibers and terminals were observed in the common marmoset PVT, in which no PV-immunoreactivity was detected. CTb-immunoreactive fibers and terminals, indicating a retinal input, were observed in the caudal rock cavy PVT. In the common marmoset, besides the PVT, CTb-labeled terminals extend to other midline thalamic nuclei, such as rhomboid, interanteromedial, and reuniens. Conclusion: This work allows to establish a comparative description between two typical Brazilian species, defining the cytoarchitectonic and neurochemical profile of the PVT and the pattern of the retinal connection with the midline thalamic nuclei. Support: CNPq, CAPES, FINEP, PROPESQ-UFRN. Publication History Article published online: 16 June 2026 © 2009. Brazilian Sleep Academy. This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/) Thieme Revinter Publicações Ltda. Rua Rego Freitas, 175, loja 1, República, São Paulo, SP, CEP 01220-010, Brazil
Sousa et al. (Thu,) studied this question.