Variations of serum prolactin (PRL) levels and activities of tuberoinfundibular dopaminergic (TIDA) neurons during the afternoon of ovariectomized (OVX) rats treated with or without estrogen were determined in this study. Long-term OVX rats treated with or without polyestradiol phosphate (0.1 mg/rat, s.c.) were decapitated every hour from 10.00 to 19.00h(except 11.00 and 13.00 h). Serum PRL and median eminence (ME) dihydroxyphenylacetic acid (DOPAC) or dihydroxyphenylalanine (DOPA) levels were determined by radioimmunoassay and high performance liquid chromatography plus electrochemical detection, respectively. A prominent PRL surge started and peaked around 14.00–15.00h, and remained significantly higher than levels of 10.00 and 12.00 h throughout the afternoon. Significant decreases of ME DOPAC and DOPA concentrations were also observed between 14.00 and 19.00 h. In OVX rats with no estrogen replacement, no PRL surge was observed and the changes of ME DOPAC concentrations during the afternoon were not significant except for that at 17.00 h. The ME DOPA accumulation, however, exhibited significantly lower levels from 14.00 to 19.00 h than that at 12.00 h, indicating that an endogenous rhythm for DA synthesis existed in OVX rats. In estrogen-treated OVX rats bearing bilateral lesions of the suprachiasmatic nuclei, both changes in serum PRL level and TIDA neuron activity were abolished. We conclude that an endogenous rhythm of the activities of TIDA neurons may exist in both OVX and OVX plus estrogen-treated rats. The rhythm is regulated by the suprachiasmatic nuclei and may be amplified by estrogen for the induction of PRL surge.

Central nervous system regions were examined in long term ovariectomized rats to determine if they are involved in the estrogen-induced afternoon surge in plasma PRL. Adult female rats were ovariectomized 2-3 weeks before bilateral radiofrequency or electrolytic lesions of the brain were placed on day 0. In short term lesion studies, catheterizations and sc injections of polyestradiol phosphate (PEP) were done after the lesion was made; blood sampling was performed on day 2, 3, 4, or 6. In long term lesion studies, the catheterization and PEP injection were done on day 21; blood was collected on day 28. In short term experiments, extensive lesions in the medial preoptic area/suprachiasmatic nuclei (MPO/SCN) completely blocked the PEP-induced afternoon PRL surges sampled on days 2, 3, 4, and 6, while bilateral lesions in the corticomedial amygdala (CMA) had no effect. Discrete bilateral lesions of either MPO or SCN eliminated the afternoon PRL surge on day 6. Discrete, yet complete, lesions of the ventromedial nuclei of the hypothalamus also blocked the PRL surge; however, lesions in the dorsomedial nuclei of the hypothalamus increased, the magnitude of the afternoon PRL surge. In long term studies, lesions of the CMA delayed and attenuated the PEP-induced PRL surge, and lesions of the stria terminalis for 4 weeks had a similar effect. As in the short term lesion studies, long term lesions of the MPO/SCN eliminated the daily rhythm of PRL secretion, although small sporadic rises in plasma PRL levels could be observed throughout the sampling period. It can be concluded that structural integrity of the MPO/SCN and ventromedial hypothalamic nuclei is essential for the estrogen-induced afternoon PRL surge; destruction of the dorsomedial hypothalamic nuclei can increase the magnitude of the afternoon PRL surge; and the CMA is not essential for induction of the PRL surge even though removing its neural input to the hypothalamus for an extended period can delay the onset of and suppress the magnitude of hormone release.

A diurnal change of the tuberoinfundibular dopaminergic (TIDA) neuron activity in ovariectomized rats treated with or without estrogen was recently reported, and the endogenous cholinergic system may be responsible for its induction. Whether a similar rhythm exists in intact female or male rats was the focus of this study. TIDA neuron activity was assessed by measurement of the precursor or metabolite of dopamine in the median eminence by HPLC with electrochemical detection. In intact female Sprague-Dawley rats, diurnal changes in TIDA neuron activity were observed during all stages of the estrous cycle, i.e., proestrus, estrus, and diestrus 1, and they were nearly identical. No such rhythm, however, was observed in intact male rats. Castration alone decreased and increased basal TIDA neuron activity in female and male rats, respectively, and estrogen treatment increased the activity in both sexes. The diurnal changes in TIDA neuron activity, however, were observed only in the female rats, not the male rats, regardless of castration or estrogen treatment. Serum prolactin levels in the male rats exhibited no diurnal changes either, irrespective of whether the animals had been castrated or treated with estrogen. Central administration of mecamylamine (1 micrograms/3 microliters per rat, i.c.v.), a nicotinic receptor antagonist, in the morning (at 1000 h) or afternoon (at 1500 and 1700 h) induced a differential effect on the TIDA neuron activity of intact female rats, i.e., no effect in the morning and stimulation in the afternoon; it had no effect on the TIDA neuron activity in the male rats regardless of the injection time. In castrated male rats treated with estrogen, however, mecamylamine treatment further increased TIDA neuron activity, but still with no diurnal difference. In summary, a sexual difference in the diurnal changes of TIDA neuron activity was observed, and these changes may be under differential control by the cholinergic system.

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In the intact cycling female rat, there is a surge of plasma PRL during the afternoon of proestrus. Ovariectomy on diestrous day 1 eliminated the PRL surge completely, and injection of 100 micrograms polyestradiol phosphate, a long-acting estrogen, not only maintained the surge, but amplified and prolonged it. Bilateral implantation of an estradiol (E2)-containing cannula [diluted 1:4 with cholesterol (C)] in the medial preoptic area (MPO), but not in the cerebral cortex (CC), also maintained the surge. In long term ovariectomized rats, bilateral implantation of E2-containing cannulae (1:5, 1:10, 1:20, and 1:200) in the corticomedial amygdala (CMA) or ventromedial nucleus of the hypothalamus or a singular implantation in the third ventricle failed to induce a PRL surge 12 days later. Similar singular implantation in the anterior pituitary (AP) increased the basal levels of plasma PRL compared to those in C-implanted controls, but no surge was evident. Only bilateral implantation of E2 in the MPO induced a small but significant rise of plasma PRL at 1700 and 1900 h. Using higher concentration implants with a higher E2 concentration (1:4) in the MPO and sampling at shorter intervals, significant afternoon PRL surges were induced on days 2-4. However, systemic effects of E2, i.e. vaginal cornification and uterine weight enlargement, were also evident. Similar implants in other brain regions had the same results. Further increasing the E2 to C dilution from 1:10 to 1:200 eliminated the systemic effect of E2 implants, while the PRL surge-inducing ability persisted. It appears that the highest diluted E2 implants (1:150 and 1:200) gave the highest PRL response and persisted for the greatest number of days. Using the highly diluted E2 implants (1:100 and 1:200) in various brain regions, the MPO and the ventromedial hypothalamus were the most sensitive areas in inducing the PRL surge; the other areas studied, including suprachiasmatic nuclei, CMA, AP, and CC were ineffective. In conclusion, highly diluted E2 implants in the brain appear to be effective in obtaining a specific effect on the afternoon PRL surge; the CMA, suprachiasmatic nuclei, AP, and CC are not estrogen feedback sites for the induction of PRL release, and the MPO was the most sensitive area of the estrogen action in the brain regions examined for the induction of the afternoon PRL surge.