Olfactory Bulb Cells Generated in Adult Male Golden Hamsters Are Specifically Activated by Exposure to Estrous Females Liyue Huang 1 and Eric L. Bittman 2 Department of Biology, Center for Neuroendocrine Studies, and Program in Neuroscience and Behavior, The University of Massachusetts, Amherst, Massachusetts 01003 Received August 7, 2001; revised November 10, 2001; accepted November 19, 2001 Two experiments were carried out to test whether cells which are born in adulthood and migrate to the olfactory bulb of adult male golden hamsters are activated during sexual behaviors, to determine the time course over which such responsiveness appears, and to ask whether activation is specific to sexual cues. In the first experi- ment, adult male hamsters were injected with 5-bro- modeoxyuridine (BrdU, 50mg/kg b.w.) 3 times over the course of one week in order to mark dividing cells. Ten days, three weeks, or seven weeks after the first BrdU injection, the animals were allowed to mate with an estrous female for half an hour before being sacrificed. Confocal analysis of fluorescent immunostaining of BrdU and c-Fos first revealed dual labeled cells in the olfactory bulb 3 weeks after injection of the thymidine analog. In order to determine whether the activation of these newly generated cells is specific to sexual cues, we next compared the incidence of c-Fos expression in newborn (BrdU positive) cells among male hamsters ex- posed to an estrous female, an aggressive male, a cot- ton swab containing vaginal secretion from an estrous female hamster (FHVS), a cotton swab containing pep- permint, or a cotton swab containing distilled water. In the mitral and glomerular layers of the accessory olfac- tory bulb, animals exposed to an estrous female had significantly more double labeled cells than did those given other treatments ( p < 0.01). In the mitral layer of the main bulb, animals exposed to an estrous female had a significantly higher percentage of double labeled cells than those of other groups, except those exposed to an aggressive male ( p < 0.05). No double labeled cells were seen in medial preoptic area (MPOA), medial nucleus of the amygdala (Me), the bed nucleus of the stria ter- minalis (BNST), or the hypothalamus. Our results indi- cate that cells born in adulthood are more responsive to cues arising from estrous females than other stimuli, and thus may participate in sociosexual behaviors. © 2002 Elsevier Science (USA) Key Words: olfactory bulbs; BrdU; neurogenesis; c- Fos; sexual behavior. INTRODUCTION New cells continue to be generated in adulthood in specific regions of vertebrate brain. In canaries and zebra finches, the seasonal recruitment of new neu- rons into the hyperstriatum ventrale is believed to be functionally important in the annual reemergence of song (Nottebohm et al., 1986). In adult rodents, cells continue to be generated in the subependymal zone (SEZ) of the lateral ventricle and subgranular layer of the dentate gyrus (Peretto et al., 1999). Cells born in the SEZ travel along the rostral migratory stream to both the main and accessory olfactory bulbs, where they differentiate into interneurons (Lois and Alvarez- Buylla, 1994). In contrast to our understanding of the role of adult-born neurons in the bird song system, however, the functional significance of the incorpora- tion of large populations of new cells into the rodent olfactory bulb is unknown. We recently documented adult neurogenesis and the existence of a rostral migratory stream in male Syrian hamsters (Huang et al., 1998). Chemosensory control of sexual behavior is particularly well studied in this species. Input from both the main and vomer- onasal olfactory epithelia is relayed to a well-charac- terized set of telencephalic and diencephalic structures in order to coordinate mounting, intromission, and ejaculation (Wood and Newman, 1995). The activation of several of the brain regions critical for male repro- 1 Current address: Pfizer Global Research & Development, 2800 Plymouth Road, Ann Arbor, MI 48105. 2 To whom correspondence and reprint requests should be ad- dressed. E-mail: elb@bio.umass.edu. Hormones and Behavior 41, 343–350 (2002) doi:10.1006/hbeh.2002.1767, available online at http://www.idealibrary.com on 0018-506X/02 $35.00 © 2002 Elsevier Science (USA) All rights reserved. 343