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Vaginocervical stimulation received either during mating or by artificial mechanical means has been shown to induce FOS expression in medial amygdala, preoptic area, hypothalamus, and midbrain of female rats. While mat-ing-induced increases in FOS-like immunoreactivity (FOS-IR) have been shown to require intromissive stimulation from males, the pattern of FOS-IR in animals receiving numbers of intromissions across a range relevant to the induction of the prolactin surges of early pregnancy has not been explored. Experiment 1 examined brain FOS-IR following 15 mounts without intromission or 5, 10, or 15 intromissions in ovariectomized females treated with estrogen and progesterone; these treatments are known to be less than or more than sufficient to trigger prolactin surges in cycling females. FOS was expressed in a graded fashion in the medial amygdala with respect to the numbers of intromissions received and in an all-or-nothing manner in preoptic area, bed nucleus of the stria terminalis, and ventromedial nucleus of the hypothalamus. In experiment 2, 15 intromissions induced expression of another immediate-early gene, egr-1 in each of these same areas as well as in a second division of the bed nucleus of the stria terminalis and in the paraventricular nucleus of the hypothalamus. These studies demonstrate that mating is differentially effective in inducing FOS expression in responsive brain areas and point to the medial amygdala as a site in which summation of intromissive stimulation may occur. Furthermore, the induction of EGR-1 may be a more sensitive marker for mating-induced neural activation in these areas than is FOS.

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In female rats, genitosensory stimulation received during mating initiates twice-daily prolactin (PRL) surges, a neuroendocrine response that is the hallmark of early pregnancy or pseudopregnancy (P/PSP). Nocturnal and diurnal PRL surges are expressed repeatedly for up to 2 weeks after copulation, suggesting that a neuroendocrine memory for vaginocervical stimulation (VCS) is established at the time of mating. These studies investigated whether the processing and retention of VCS involves acute glutamatergic activation or de novo protein synthesis within the medial nucleus of the amygdala (MEA), a VCS-responsive brain site that is implicated in P/PSP initiation. Pharmacological activation of the MEA with the glutamate agonist, NMDA, initiated nocturnal PRL surges, causing a PSP state in females that had not received VCS. P/PSP initiation by mating was prevented by intra-amygdalar infusion of the NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid (AP-5), provided that it was administered before mating. AP-5 treatment also disrupted mating-induced c-fos expression in the principle bed nucleus of the stria terminalis and the ventrolateral division of the ventromedial hypothalamic nucleus, but not in the medial or anteroventral periventricular preoptic nuclei. Neither P/PSP nor downstream cellular activation was prevented when a protein synthesis inhibitor, anisomycin, was administered to the MEA. The results indicate that MEA cells are critical to the early processing of VCS through NMDA channel activation, rapidly conveying information to downstream hypothalamic cell groups that modulate neuroendocrine function.

Hormonal conditions that elicit lordosis in female rats are ineffective in males, suggesting that this behavior is actively suppressed in males. Previous studies theorize that serotonergic and gamma-aminobutyric acidergic (GABA) inputs to the ventrolateral division of the ventromedial nucleus of the hypothalamus (VMNvl) may contribute to lordosis inhibition in males. Using triple-label immunofluorescent techniques, the present studies explored potential sex differences in the density of these projections within three hypothalamic sites: the VMNvl, the arcuate nucleus (ARC), and the dorsomedial nucleus of the hypothalamus. Antibodies directed against HuC/D, estrogen receptor (ER)-alpha and either serotonin (5-HT) or the gamma-aminobutyric acid synthetic enzyme glutamic acid decarboxylase-65 were used to compare the densities of glutamic acid decarboxylase (GAD)-65- and 5-HT-containing fibers in each brain area, the percentage of VMNvl HuC/D immunoreactive (ir) neurons that contained ERalpha, and the percentage of HuC/D and ERalpha double-labeled cells receiving apparent contacts from 5-HT fibers between adult, gonadectomized male and female rats. The densities of VMNvl and ARC 5-HT immunolabeled fibers were significantly higher in the males, and the percentage of VMNvl HuC/D-ir neurons containing ERalpha was significantly higher in the females. The percentage of HuC/D-ir cells contacted by 5-HT fibers was significantly higher in the males, compared with the females, but there was no sex difference in the proportion of those cells receiving contacts that were ERalpha-ir. Neonatal administration of estradiol but not genistein masculinized 5-HT content in the adult female VMNvl, but the percentage of HuC/D-ir cells colabeled with ERalpha was not significantly affected by treatment. A similar, but not statistically significant, pattern was observed in the ARC. These findings suggest that the development of serotonergic inputs to the male VMNvl is orchestrated by neonatal estradiol exposure. The hormone-dependent organization of these 5-HT projection patterns may be an important developmental mechanism accounting for sex-specific behaviors in adulthood.