Coital activation of the hypothalamo-hypophyseal ovarian axis (HOA) is well documented in rabbits, but coital excitation of the hypothalamo- hypophyseal testicular axis (HTA) is less well described. We and others have postulated that the response of the HOA to coitus, as characterized by a dramatic release of hypothalamic GnRH, may be mediated by an increase in norepinephrine (NE) neuronal activity. Herein, we studied selective HOA and HTA responses in New Zealand White rabbits before, during, and after coitus. Firstly, we determined the effects of microdialysis (μD) and blood-sampling methods on plasma LH and testosterone (T) patterns in male rabbits during sexual performance. Subsequently, we compared the patterns of release in GnRH and norepinephrine (NE) from the arcuate nucleus-median eminence (AME) at 10- min intervals with changes in plasma LH levels in copulating male and female rabbits. Lastly, in 2.5-min AME μD samples from females immediately after coitus, we measured NE and GnRH concentrations to determine whether NE release precedes that of GnRH. Tethered, freely moving rabbits were exposed to their partners for 10 min at the end of the third (10-min sampling for 5- 7 h) or second (2.5-min sampling for 4 h) hour. Data from individuals that did not mate during the 10-min of pairing in the 5- to 7-h sampling trials were included as a control group (sham-mated). The results showed no changes (P > 0.05) in plasma LH and T in either mated (LH: pre, 0.13 ± 0.08 ng/ml; post, 0.15 ± 0.03 ng/ml; T: pre, 2.39 ± 1.20 ng/ml; post, 0.85 ± 0.26 ng/ml) or sham-mated males (LH: pre, 0.21 ± 0.08; post, 0.25 ± 0.10 ng/ml; T: pre, 1.46 ± 0.51 ng/ml; post, 1.40 ± 0.38 ng/ml). Likewise, coitus did not alter patterns of AME-NE (pre, 0.47 ± 0.25; post, 0.56 ± 0.25 ng/ml) and GnRH (pre, 0.61 ± 0.45; post, 0.74 ± 0.32 pg/ml) in mated or sham- mated males. The constant HTA activity during coitus in males appears to be independent of experimental manipulation per se because LH and T levels between μD (0.18 ± 0.05 and 1.72 ± 0.85 ng/ml, respectively) and non-μD (0.16 ± 0.05 and 1.52 ± 0.36 ng/ml, respectively) rabbits were not different (P > 0.05). In contrast to males, females displayed unambiguous and simultaneous increases in NE (P < 0.05) and GnRH (P < 0.01) release from the AME within 10-20 min after coitus; these elevated concentrations in μD samples lasted for 3-4 h. Microdialysis NE levels averaged 0.02 ± 0.01 ng/ml before mating, whereas postcoital values averaged 0.09 ± 0.01 ng/ml. GnRH levels were 1.04 ± 0.56 and 11.78 ± 5.06 pg/ml before and after coitus, respectively. Concomitant increases in plasma LH levels were also observed after coitus in these female rabbits. Moreover, measurements of NE and GnRH in 2.5-min μD samples revealed that the postcoital increase in NE preceded that in GnRH by 2.5-7.5 min (P < 0.05). The results suggest that neuroendocrine circuits in the two sexes of the New Zealand White rabbit respond differently to genital stimulation. In male rabbits, coitus does not alter patterns of AME NE or GnRH secretion, nor does it change the circulating levels of plasma LH or T. Conversely, in females, coitus induces the rapid release of hypothalamic NE, GnRH, and pituitary LH. The increase in coitally induced NE occurs before the rise in GnRH, which supports the hypothesis that NE is a critical neurochemical in coital activation of GnRH neurons.
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