Lateral Parabrachial Afferent Areas and Serotonin Mechanisms Activated by Volume Expansion Lisandra Oliveira Margatho, 1 Andrea Godino, 2 Fabı ´ola Raquel Teno ´ rio Oliveira, 3 Laura Vivas, 2 * and Jose ´ Antunes-Rodrigues 1 1 Department of Physiology, School of Medicine of Ribeira ˜o Preto, University of Sa ˜o Paulo (USP), Sa ˜o Paulo, Brazil 2 Instituto de Investigacio ´n Me ´dica Mercedes y Martı ´n Ferreyra (INIMEC-CONICET), Co ´ rdoba, Argentina 3 Department of Physiology, Federal University of Para, Para, Brazil Recent evidence has shown that the serotonergic mech- anism of the lateral parabrachial nucleus (LPBN) partici- pates in the regulation of renal and hormonal responses to isotonic blood volume expansion (BVE). We investi- gated the BVE-induced Fos activation along forebrain and hindbrain nuclei and particularly within the seroto- nergic clusters of the raphe ´ system that directly project to the LPBN. We also examined whether there are changes in the concentration of serotonin (5HT) within the raphe ´ nucleus in response to the same stimulus. With this purpose, we analyzed the cells doubly labeled for Fos and Fluorogold (FG) following BVE (NaCl 0.15 M, 2 ml/100 g b.w., 1 min) 7 days after FG injection into the LPBN. Compared with the control group, blood volume- expanded rats showed a significant greater number of Fos-FG double-labeled cells along the nucleus of the solitary tract, locus coeruleus, hypothalamic paraven- tricular nucleus, central extended amygdala complex, and dorsal raphe ´ nucleus (DRN) cells. Our study also showed an increase in the number of serotonergic DRN neurons activated in response to isotonic BVE. We also observed decreased levels of 5HT and its metabolite 5- hydroxyindoleacetic acid (measured by high-pressure liquid chromatography) within the raphe ´ nucleus 15 min after BVE. Given our previous evidence on the role of the serotonergic system in the LPBN after BVE, the present morphofunctional findings suggest the exis- tence of a key pathway (DRN-LPBN) that may control BVE response through the modulation of 5HT release. V V C 2008 Wiley-Liss, Inc. Key words: Fos and Fluorogold immunoreactivity; dorsal raphe ´; nucleus; lateral parabrachial nucleus; serotonin The lateral parabrachial nucleus (LPBN), a struc- ture lying dorsolateral to the superior cerebellar peduncle, has been shown to play an important role in the control of body fluid balance (Edwards and Johnson, 1991; Menani et al., 1995, 1996, 1998, 2000; Ohman and Johnson, 1995; Colombari et al., 1996), and it is connected with hindbrain and forebrain areas that are activated by changes in body fluid volume (Saper and Loewy, 1980; Ciriello et al., 1984; Fulwiler and Saper, 1984; Jhamandas et al., 1996). Recently, Margatho et al. (2007) have shown evidence for serotonergic mecha- nisms in the LPBN involved in the control of atrial natriuretic peptide (ANP) and oxytocin secretion as well as in the excretion of sodium, potassium, and water in response to isotonic blood volume expansion (BVE). In addition, there is evidence suggesting the exis- tence of a serotonergic pathway with cell bodies in the raphe ´ system and with terminals within the LPBN involved in the regulatory control of water/sodium intake and excretion (Petrov et al., 1992; Menani et al., 1996, 1998, 2000; Margatho et al., 2007). Previous stud- ies also indicate that renal and hormonal responses to BVE are strongly influenced by central serotonergic mechanisms (Reis et al., 1991, 1994). For example, in- tracerebroventricular administration of serotonin (5HT) or the 5HT 2 receptor agonist 2,5-dimetoxi-4-iodoam- phetamine hydrobromide (DOI) increases urinary so- dium excretion in water-loaded rats (Reis et al., 1991). On the other hand, electrolytic lesions of dorsal raphe ´ nucleus (DRN), one of the major serotonergic groups of Contract grant sponsor: Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP); Contract grant number: 03/00327-8; Contract grant number: 02/02464-0 (to L.O.M.); Contract grant sponsor: Con- selho Nacional de Desenvolvimento a Pesquisa (CNPq); Contract grant number: 478981-4; Contract grant sponsor: CONICET (A.G.); Contract grant number: PIP5802; Contract grant sponsor: ANPCyT. *Correspondence to: Laura Vivas, PhD, Instituto de Investigacio ´n Me ´dica Mercedes y Martı ´n Ferreyra (INIMEC-CONICET), Casilla de Correo 389-5000, Co ´rdoba, Argentina. E-mail: lmvivas2003@yahoo. com.ar or lvivas@immf.uncor.edu Received 14 December 2007; Revised 28 March 2008; Accepted 8 May 2008 Published online 5 August 2008 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/jnr.21806 Journal of Neuroscience Research 86:3613–3621 (2008) ' 2008 Wiley-Liss, Inc.