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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.

No abstract

Ewes supposedly need to be separated from rams before male stimuli can increase gonadotrophin secretion and induce ovulation. In the present study, we investigated the LH response of ewes to 'novel' and 'familiar' rams after varying periods of separation. In Experiment 1, ewes (n = 8 per treatment) were separated from familiar rams for 15 min or 1 month and then exposed to either familiar rams, novel rams or novel wethers. After 15 min or 1 month of separation, exposure to novel rams increased pulsatile LH secretion (P < 0.05) and induced an LH surge in all ewes whereas exposure to familiar rams or novel wethers had no effect on LH secretion (P > 0.1). After 1 month of separation, re-exposure to the same familiar rams increased pulsatile LH secretion (P < 0.05) in six of eight ewes, but only induced an LH surge in two of eight ewes. In Experiment 2, familiar rams were removed and returned after 15 min, 1 day or 17 days (n = 5 per treatment). None of these treatments affected LH secretion. We conclude that separation of ewes from rams is a prerequisite for familiar rams to increase LH secretion, but is not necessary if the rams are novel.

No abstract