*Corresponding authors e-mail: ioacilgermano@hotmail.com Introduction: The biological rhythms are expressed in all organic structures even in the absence of external stimuli. In mammals the circadian cycle is governed by a central pacemaker, which has been pointed out in the course of decades of research as being the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN receives photic information from the retina by the retinohypothalamic tract, and non-photic inputs from other pathways, to adjust the behavior to the light-dark cycle. The thalamic intergeniculate leaflet (IGL) originates one of these pathways that acts on the SCN modulating its function. Both SCN and IGL, as detected in several mammal species, are known to be influenced by pathways originated in the midbrain raphe, which utilizes serotonin (5-HT) as neurotransmitter. The objective of this study was to identify the serotonergic groups in the brain stem and the distribution of serotonergic terminals on the SCN and the IGL of the rock cavy ( Kerodon rupetris ), a rodent native of the northeastern Brazil, whose locomotor activity is mostly crepuscular. Methods: Two animals were used, which after pre-anesthesia with atropine sulfate and tramadol, were anesthetized with a mixture of ketamine, xylazine and diazepan and followed by transcardiac perfusion. After that, the fixed brains were removed from the skull to be sectioned one in sagittal and the other in coronal sections on sliding freezing microtome. The sections were collected in six compartments, one of the which was stained by Nissl method with thionin and another was submitted to 5-HT immunohistochemistry. 5-HT-IR perykaria were identified forming several cluster cells along the midline of the brainstem, recognized as raphe nuclei. A dense 5-HT-IR terminal plexus distribution of 5-HT terminals was seen in the ventral portion of the SCN along its entire rostrocaudal extent and filling all IGL. Conclusion: Because 5-HT is a neurotransmitter present in large quantity in the raphe nuclei, it is believed that the SCN and the IGL receive 5-HT modulatory influence from those nuclei. However additional studies with neuronal tracers are need to demonstrate such connections, as well as to establish the source of the projection from raphe nuclei to SCN and IGL. Support: CNPq, CAPES, FINER 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
Soares et al. (Thu,) studied this question.