Reproductive Neuroendocrinology Neuroendocrinology 2000;72:91–101 Maternal Behavior Stimulates c-fos Activity within Estrogen Receptor Alpha-Containing Neurons in Lactating Rats Joseph S. Lonstein a Béatrice Gréco a Geert J. De Vries a Judith M. Stern b Jeffrey D. Blaustein a a Center for Neuroendocrine Studies, University of Massachusetts, Amherst, Mass., and b Department of Psychology, Busch Campus, Rutgers University, New Brunswick, N.J., USA Received: December 28, 1999 Accepted after revision: May 23, 2000 Joseph S. Lonstein Center for Neuroendocrine Studies, University of Massachusetts Tobin Hall, Box 37720 Amherst, MA 01003-7720 (USA) Tel. +1 413 545 0524, Fax +1 413 545 0996, E-Mail lonstein@cns.umass.edu ABC Fax + 41 61 306 12 34 E-Mail karger@karger.ch www.karger.com © 2000 S. Karger AG, Basel Accessible online at: www.karger.com/journals/nen Key Words c-fos W Immunocytochemistry W Lactation W Maternal behavior W Gonadal steroid receptors W Amygdala W Preoptic area W Bed nucleus of the stria terminalis W Habenula Abstract Estradiol and other hormones are thought to be critical for the onset, but not maintenance, of maternal behavior in rats. Maternal behavior is instead maintained postpar- tum by tactile stimulation that dams receive during inter- actions with pups, and many neural sites implicated in the control of maternal behavior show elevated c-fos activity in response to this stimulation. Many of these sites also contain neurons that express the alpha sub- type of the estrogen receptor (ER·). Because of possible interactions between tactile stimulation from pups, c-fos, and ER· in the lactating rat brain, we determined if popu- lations of cells that show increased c-fos activity after maternal behavior in lactating rats also contain ER·. Dams were separated from their pups for 48 h beginning on day 5 postpartum. On day 7 postpartum, experimen- tal dams were reunited with pups and mother-litter inter- actions were observed for 60 min. Control dams received no pup stimulation. Subjects were sacrificed 60 min later and brain sections were double immunolabeled for the Fos and ER· proteins. As expected, the number of ER·- immunoreactive (ER·-ir) neurons did not differ between the two groups in the eight areas analyzed (lateral region of the lateral septum, posterodorsal medial amygdala, dorsal and ventral medial preoptic area, dorsal and ven- tral bed nucleus of the stria terminalis, lateral habenula, and ventrolateral caudal periaqueductal gray). Consis- tent with previous reports, maternal dams had 2- to 7- fold more Fos-immunoreactive (Fos-ir) neurons in these sites compared with nonstimulated controls. Maternal dams had significantly more Fos-ir neurons that also contained ER·-ir in all sites, with the greatest increases in the ventral medial preoptic area, lateral habenula, and ventral bed nucleus of the stria terminalis. Between F25 and 45% of the Fos-ir cells in the sites examined also expressed ER·. Thus, a substantial number of neurons that are genomically activated during maternal behavior contain ER·, raising the possibility that the postpartum display of maternal behavior can be influenced by ER· activity. Copyright © 2000 S. Karger AG, Basel Adult female rats are typically not maternal until undergoing the fluctuations in ovarian and pituitary hor- mones associated with gestation and parturition [1–3]. Ovarian hormones secreted during mid- to late pregnancy may be particularly important for the onset of nurturance [4], and a regimen of hormones mimicking their pattern of release induces maternal responding in virgin albino female rats [5]. Estradiol may be especially critical for this