*Corresponding author e-mail: janaina siqueira@hotmail. com Introduction: The circadian timing system (CTS) contains as major centers the hypothalamic suprachiasmatic nucleus (SCN) and the thalamic intergeniculate leaflet (IGL). Both play a pivotal role in generation and modulation of circadian rhythms. The SCN receives environmental inputs, such the light-dark information, through direct retinal projections. The IGL acts as a non-photic mediator and center of a second photic entrainment pathway of the SCN. The aim of this work was to verify, by using immunohistochemical techniques, the NeunN distribution within SCN and IGL, comparing three rodents species: Wistar rat (Ratus norvegicus), mice (Mus musculus) and rock cavy (Kerodon rupestris). The last one is a rodent originating from Brazil northeast, wich one shows a activity pattern predominant crepuscular. Methods: The animals were previously anesthetized and perfused transcardially with saline containing heparine, following by 4% paraformaldehyde solution in phosphate buffer 0, 1M, pH 7, 4. The brains were removed, frozen and sectioned on a sliding microtome in coronal plane (30 µm). The sections were collected in six series. Then, sections of a series were submitted to Nissl method of staining (Thionin) for cytoarchitectonic delimitation. Another series were processed immunohistochemically (ABC protocol) to reveal the presence of NeuN. Results: Comparing the species, we can say that exist a scarce NeuN labeling in the SCN of mice and rock cavy, in wich we founded a predominant labeling in the ventral part. In the rat we found the strongest labeling comparing to other species studied, although also in the ventral portion of the SCN. About IGL, was observed a evident cellular labeling in the three species studied. Conclusion: Considering the CTS in the three studied rodent species, the NeuN-immunoreactivity is a good marker for all IGL, but only for the ventral SCN. 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
Borda et al. (Thu,) studied this question.