Effects of social cues on GnRH-I, GnRH-II, and reproductive physiology in female house sparrows (Passer domesticus) Tyler J. Stevenson a, * , George E. Bentley b , Takayoshi Ubuka b , Lutgarde Arckens c , Elizabeth Hampson a , Scott A. MacDougall-Shackleton a a Department of Psychology and Graduate Program in Neuroscience, University of Western Ontario, USA b Department of Integrative Biology and Helen Wills Neuroscience Institute, University of California, Berkeley, USA c Department of Animal Physiology and Neurobiology, K.U. Leuven, Belgium Received 10 October 2007; revised 21 December 2007; accepted 8 January 2008 Available online 26 January 2008 Abstract In all vertebrates, at least two forms of gonadotropin-releasing hormone (GnRH) are present: GnRH-I and GnRH-II. GnRH-I directly influences the reproductive axis whereas the function of GnRH-II is less clear. The present experimental objectives were to determine the effect(s) of male social cues on the peripheral and neural responses of female house sparrows (Passer domesticus). We hypothesized that male breeding status would significantly influence the amount of immunoreactive GnRH-II in female house sparrow brains. In order to test this hypothesis, females were caged with a breeding male, a non-breeding male, or caged alone. The presence of breeding males did not significantly influence ovary development, luteinizing hormone, or estradiol levels, but male presence increased female body mass, and male presence and condition interacted to influence ovarian follicle size. Using immunocy- tochemistry, GnRH-I and GnRH-II immunoreactivity was measured in order to evaluate the neuroendocrine response to breeding status in males. When females were housed with breeding males, there were stable numbers of immunoreactive GnRH-I and -II cells but significantly lower amounts of immunoreactive GnRH-I fibre staining within the preoptic area compared to females housed with non-breeding males. Moreover, immunoreactive GnRH-II fibres in the preoptic area, ventromedial nucleus, and medial septum were significantly greater in females housed alone in chamber with non-breeding males. The data demonstrate that the GnRH system in songbirds is modulated by social context. These finding provide novel insight into the mechanisms involved with regulating avian reproductive physiology. Ó 2008 Elsevier Inc. All rights reserved. Keywords: GnRH-II; LHRH; Reproduction; Breeding; Social cues; Immunocytochemistry 1. Introduction Social cues have a wide variety of effects on physiology. In many seasonally breeding animals photoperiod is the primary factor that controls seasonal changes in reproduc- tive physiology, but social cues are critical for the fine tun- ing of reproduction. Social cues can influence peripheral reproductive physiology such as hormone levels and thus modify hormone-dependent behaviors (e.g., Hinde, 1965; Hinde and Steel, 1976). In addition, social cues can influ- ence central physiology by modifying gonadotropin-releas- ing hormone (GnRH) neurosecretory cells through increased synthesis in the brain (e.g., White et al., 2002; Burmeister and Wilczynski, 2005). In this study, we explore the effects of visual and acoustic social cues on central and peripheral reproductive physiology of female house spar- rows (Passer domesticus). 0016-6480/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.ygcen.2008.01.015 * Corresponding author. Present address: Department of Psychological and Brain Sciences, The Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA. E-mail address: tsteve13@jhu.edu (T.J. Stevenson). www.elsevier.com/locate/ygcen Available online at www.sciencedirect.com General and Comparative Endocrinology 156 (2008) 385–394