Suckling-induced Fos activation and melanin-concentrating hormone immunoreactivity during late lactation R.D. Alvisi a,1 , G.B. Diniz b,1 , J.M. Da-Silva b , J.C. Bittencourt b,c , L.F. Felicio a, ⁎ a Department of Pathology, School of Veterinary Medicine, University of São Paulo, São Paulo 05508-270, Brazil b Laboratory of Chemical Neuroanatomy, Department of Anatomy, Institute of Biomedical Sciences, University of São Paulo, São Paulo 05508-000, Brazil c Center for Neuroscience and Behavior, Institute of Psychology, University of São Paulo, Sao Paulo 05508-030, Brazil abstract article info Article history: Received 3 September 2015 Received in revised form 6 February 2016 Accepted 9 February 2016 Available online 10 February 2016 Aims: Melanin-concentrating hormone (MCH) is implicated in the control of food intake, body weight regulation and energy homeostasis. Lactation is an important physiological model to study the hypothalamic integration of peripheral sensory signals, such as suckling stimuli and those related to energy balance. MCH can be detected in the medial preoptic area (MPOA), especially around the 19th day of lactation, when this hormone is described as displaying a peak synthesis followed by a decrease after weaning. The physiological significance of this phenom- enon is unclear. Therefore, we aimed to investigate hypothalamic changes associated to sensory stimulation by the litter, in special its influence over MCH synthesis. Main methods: Female Wistar rats (n = 56) were euthanized everyday from lactation days 15–21, with or without suckling stimulus (WS and NS groups, respectively). MCH and Fos immunoreactivity were evaluated in the MPOA and lateral and incerto-hypothalamic areas (LHA and IHy). Key findings: Suckling stimulus induced Fos synthesis in all regions studied. An increase on the number of suckling-induced Fos-ir neurons could be detected in the LHA after the 18th day. Conversely, the amount of MCH decreased in the MPOA from days 15–21, independent of suckling stimulation. No colocalization between MCH and Fos could be detected in any region analyzed. Significance: Suckling stimulus is capable of stimulating hypothalamic regions not linked to maternal behavior, possibly to mediate energy balance aspects of lactation. Although dams are hyperphagic before weaning, this behavioral change does not appear to be mediated by MCH. © 2016 Published by Elsevier Inc. Keywords: Suckling stimulus Maternal behavior Fos Medial preoptic area Lateral hypothalamic area Incerto-hypothalamic area 1. Introduction Melanin-concentrating hormone (MCH) is a nonadecapeptide that was first discovered in the fish class Teleostei and later described in mammals [1–4]. In the mammalian brain, MCH is synthesized mainly in cell bodies of hypothalamic sites such as in the lateral hypothalamic area (LHA), perifornical area and, to a lesser degree, in the incerto- hypothalamic area (IHy) [5]. Exclusively during the lactation period, MCH synthesis can be detected in neurons of the medial preoptic area, raising from the 5th to the 19th day and decreasing concomitantly to the weaning of the pups [6, 7]. The exact role played by MCH in physio- logical alterations of motherhood is still unclear. The projections of MCH-immunoreactive [MCH-ir] neurons are broad, reaching for in- stance the median eminence, hippocampal formation, prefrontal cortex, periaqueductal gray matter, lateral part of the medial mammillary nucleus, nucleus accumbens, and medial septal nucleus. Thus, LHA and IHy can be considered integrative centers that are optimally positioned to influence a wide range of systems and functions, not simply as direct modulators of effector and motor functions [8]. Melanin-concentrating hormone is involved in the regulation of body weight, acting on G- protein-coupled receptors similarly to other central and peripheral peptides, such as orexin, cholecystokinin, and bombesin [9–11]. Additionally, intra-medial preoptic area injections of MCH altered monoamine concentrations in this area and stimulated sexual behavior and luteinizing hormone secretion, as well as a decrease in appetitive components of maternal behavior of dams [12–14]. The present study evaluated the daily influence of the suckling stimulus on MCH and Fos immunoreactivity (Fos-ir) in the LHA, MPOA, and IHy during late lactation. 2. Materials and methods 2.1. Animals Adult Wistar rats (n = 56) were mated with approximately 90 days old at the beginning of the experiments. The rats were obtained from the Department of Pathology, School of Veterinary Life Sciences 148 (2016) 241–246 ⁎ Corresponding author at: Department of Pathology, School of Veterinary Medicine, University of São Paulo, São Paulo ZIP:05508-900, Brazil. E-mail address: lfelicio@usp.br (L.F. Felicio). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.lfs.2016.02.038 0024-3205/© 2016 Published by Elsevier Inc. Contents lists available at ScienceDirect Life Sciences journal homepage: www.elsevier.com/locate/lifescie