*Corresponding author e-mail: andrelbpontes@yahoo.com.br Introduction: The core nucleus of the mammalian circadian timing system (CTS) is the hypothalamic suprachiasmatic nucleus (SCN) which receives a strong serotonergic projection from the raphe. Serotonin is involved in the fine adjustments at several brain centers and in the SCN these actions are reported as been primarily mediated by the receptor type 5-HT 1 . In rodents, there is a consensus that the general 5-HT projection pattern arrives at the ventral portion of the SCN, mismatching the arriving fibers of the retinohypothalamic tract. Our objective was to locate of 5-HT 1A and 5-HT 1B within the SCN, and comparing their location with the retinal and serotonergic projection in the marmoset SCN. Methods: Four marmosets were deeply anesthetized to receive a unilateral intraocular injection of CTb. After 5-7 post-injection days, the marmosets were reanesthetized and perfused transcardially with phosphate-buffered saline, followed by paraformaldehyde. The brains were removed from the skull and postfixed for 2-4 h. Brain blocks containing the SCN were frozen and sectioned in the coronal plane (30 µm). Sections were sequentially collected into six separated compartments. To each animal, one in each six series was used to reveal immunoreactivity for CTb, and three others were used for the immunohistochemical detection of the 5-HT, 5-HT 1A , and 5-HT 1B . The two remaining series were employed to perform double labeling experiments combining serotonin receptors label and calbindin (CB) label. The sections were processed immunohistochemically against CTb, 5-HT 1A , 5-HT 1B . For the double labeling, the tissue was simultaneously incubated with an antibody against one of the two 5-HT subtype receptors joint with mouse-anti-CB. The sections were examined under brightfield illumination and epifluorescence. Results: In a New World primate species, Callithrix jacchus , the occurrence of the positive labeling for the subtypes 5-HT 1A and 5-HT 1B are mostly at the dorsal SCN. We also confirm the dualistic nature of the placement of the 5-HT-positive fibers in the SCN of Callithrix , with higher density in their dorsal and ventral contours, with a core devoid of any labeling. Conclusion: We believe that such display is highly suggestive that the exclusively dorsal distribution of these SCN arriving projections observed in this species holds a distinct serotonergic characteristic, performed by the 5-HT 1 family subtype receptors. These data confirm that Callithrix jacchus is an important experimental subject in the study of the serotonergic circuitry involved in the generation and modulation of circadian rhythms. 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
Pontes et al. (Thu,) studied this question.