Rapid Induction of Cell Proliferation in the Adult Female Ungulate Brain (Ovis aries) Associated with Activation of the Reproductive Axis by Exposure to Unfamiliar Males1

Abstract

In many species, the reproductive centers of the brain are profoundly affected by sociosexual stimuli. This is particularly evident in female ungulates such as sheep, in which exposure to males switches them from reproductively quiescent to fertile. In two experiments with female sheep, we tested whether the brain centers that control gonadotropin-releasing hormone (GnRH) neuronal activity respond differentially to ''novel'' vs. familiar males and whether the neuroendocrine response is associated with increased cell proliferation in the hippocampus, a site associated with memory formation. In experiment 1, groups of 10 female sheep that had previously been habituated to males for 3 mo were re-exposed to familiar males or were exposed to novel males. Only the novel males increased luteinizing hormone (LH) pulse frequency, indicating stimulation of GnRH neuronal activity. In experiment 2, groups of six female sheep were injected with bromodeoxyuridine (BrdU) and then maintained in isolation from males or exposed to novel males. Two days later, the hippocampus and hypothalamus were removed and processed for fluorescence immunohistochemistry. Again, exposure to males increased LH pulse frequency. Most important, male exposure also doubled the number of BrdUpositive cells in the dentate gyrus of the hippocampus. No BrdUpositive cells were detected in the hypothalamus. We conclude that the stimulus from novel males switches on the reproductive centers of the brain of female sheep and rapidly doubles the rate of cell proliferation in the hippocampus. The rapidity of this response contrasts with rodents, in which several days of exposure to male pheromones seem necessary for an effect on neurogenesis.

New understanding of an old phenomenon: uncontrolled factors and misconceptions that cast a shadow over studies of the ‘male effect’ on reproduction in small ruminants

“…This was borne out by studies in mice showing that male pheromones that help females to distinguish between subordinate and dominant males induce neurogenesis in the dentate gyrus of the hippocampus, a brain region fundamentally associated with memory (61). We also found that the acute LH response of ewes to novel rams is associated with a rapid (within 2 h) and robust increase in the rate of cell proliferation in the dentate gyrus of the hippocampus (14). Importantly, the odour of the newborn lamb induces cell division in the same region of the brain, indicating that neurogenesis is involved in both cases of the formation of olfactory memory (62).…”

“…These observations were mirrored in goats when Véliz et al (11) showed that maintenance of does with bucks did not reduce the proportion of does that ovulated in response to novel bucks. In a recent study, we directly measured the neuroendocrine response of females to novel males and showed that exposure to novel rams increased tonic secretion of LH and induced an LH surge in all ewes (13,14). Importantly, this series of neuroendocrine events did not occur in ewes reexposed to familiar rams after only 15 min of separation ( Figure 2).…”

“…The male symbol and arrow at 0 min indicates the time of male exchange. Redrawn after Hawken et al(14).…”

Noncanonical Sites of Adult Neurogenesis in the Mammalian Brain

“…One study did not identify any BrdU þ cells in the hypothalamus of sheep despite detecting BrdU þ cells in the SGZ and using higher intraperitoneal (i.p.) doses of BrdU (Hawken et al 2009). The reason for this discrepancy remains unclear.…”

MRI Techniques and New Animal Models for Imaging the Brain

“…In sheep, cell proliferation, evaluated by bromodeoxyuridine (BrdU) incorporation, has been observed in the dentate gyrus of the hippocampus of ewes exposed to a novel male (Hawken et al 2009). Using BrdU incorporation and cellular biomarkers such as doublecortin or glial fibrillary acid protein (for review Sierra et al 2011), it has been demonstrated that cell proliferation is down-regulated in the subventricular zone, the dentate gyrus and the main olfactory bulb at parturition and during interactions with the young (Brus et al 2010, Fig.…”