*Corresponding author: rovenabio@yahoo.com.br Introduction: The circadian timing system (CTS) is composed by a set of structures that shows endogenous oscillation, responsible for the maintenance of some biological rhythms, despite of constant environmental conditions. The CTS of primates is comprised by the suprachiasmatic nucleus (SCN) of the hypothalamus, which is the central pacemaker, and the pregeniculate nucleus (PGN) of the thalamus, a component homologous to the intergeniculate leaflet of rodents, that acts as a second photic pathway entrainment to the SCN and also participate of non-photic synchronization mechanisms. NeuN is a nuclear neuron specific marker, widely employed to distinguish neurons and glia cells. The glia cells are identified by glial fibrilary acidic protein (GFAP). In this way, our objective was to charaterize some components of CTS of the common marmoset ( Callithrix jacchus ), a New World primate, utilizing NeuN and GFAP immunoreactivity. Methods: Adult male marmosets were previously anesthetized and them submitted to transcardiac perfusion. Then, the brains were removed from the skull, frozen and sectioned on a sliding microtome in coronal sections of 30 pm. The sections were processed immunohistochemically against NeuN and GFAP. Afterwards the sections were mounted on glass slides and observeds by optic microscopy. Results: We observed that the PGN was densely marked by NeuN and GFAP immunorreactivity, delimiting this region. On the other hand in the SCN, NeuN was strongly founded in ventral portion, with a pale immunoreactivity in the dorsal part. In this region the GFAP immunoreactivity was inverse to the NeuN. Conclusion: So we conclude that PGN, possibly, have a similar number of neurons and glia cells, and both proteins can be used to delimit this region. The SCN can be divided in two portions through the analysis of NeuN and GFAP. 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
Engelberth et al. (Thu,) studied this question.