Involvement of endogenous opioidergic neurons in modulation of prolactin secretion in response to mating in the female rat

Mating in female rats induces an acute prolactin (PRL) release within 60 mi n and twice-daily surges of PRL throughout the first 10 days of pregnancy t o maintain luteal function. Little is known about the brain mechanism where by the vaginocervical stimulation is processed to induce PRL release. Our r ecent results revealed an increase in Fos expression in the arcuate nucleus (ARC) following mating in the intact estrous rat, suggesting that a neuron al network in the brain area may participate in conveying and integrating t he genitosensory stimulation. To further investigate the phenotype of activ ated neurons in the ARC, the present study examined whether beta-endorphin (beta-END) and/or dopamine (DA) neurons are activated by mating, and if so, whether activation is involved in the mating-induced acute release of PRL and the establishment of the twice-daily surges of PRL. In experiment 1, pr oestrous rats receiving intromissions (mated group) from males or mounts wi thout intromission (mounted group) were sacrificed along with rats taken di rectly from their home cage (control group) 60 min after the beginning of m ating or mounting. Expression of Fos in beta-END neurons and expression of fos-related antigen (FRA) in DA neurons, which were labeled by tyrosine hyd roxylase (TH) antibody in the ARC were examined by double-label immunocytoc hemistry. In experiment 2, proestrous females with indwelling atrial cathet ers were mated with males. Naloxone (10 mu l/min, 2 mg/10 min), an opiate a ntagonist, or saline was infused before, during and after mating. Blood sam ples were collected during the mating session and also at several times 3 d ays after mating. The results showed that mating induced a significant incr ease in the percentage of beta-END/Fos colabeled neurons and a significant decrease in the number of beta-END cells in all subdivisions of the ARC. In contrast, neither the percentage of FRA/TH colabeled cells nor the number of TH cells was influenced by mating. Mating induced an acute increase in P RL release in saline-treated control animals within 30 min and a subsequent diurnal surge (18.00 h) and a nocturnal surge of PRL (2.00 h) 3 days after mating. Naloxone infusion during mating blocked the mating-induced acute P RL response and the diurnal surge of PRL 3 days after mating, but affected neither the nocturnal surge of PRL nor the incidence of pregnancy. These re sults demonstrate that(1) beta-END neurons but not DA neurons in the ARC ar e activated in response to mating in proestrous rats, and (2) the mating-in duced activation of beta-END neurons may participate in the acute response of PRL release to mating and the memory mechanism for the establishment of the diurnal PRL surge, but not the nocturnal PRL surge in early pregnancy. These results lead to a conclusion that endogenous opioid peptides may be i nvolved in the neuronal transmission of genitosensory stimulation to induce PRL secretion. Copyright (C) 2000 S. Karger AG, Basel.