Participation of opioid and serotoninergic systems in prolactin secretion induced by hypothalamic action of estradiol

The aim of the present study was to determine the central effect of estradiol (E2) on the pattern of secretion of prolactin (PRL) in virgin rats and the participation of opioid and serotoninergic systems in the regulation of this secretion. Bilateral cannulae containing E2 (group E) or cholesterol (group C) were implanted in the arcuate nucleus on the day of estrus (day 0). Blood samples were obtained at 09.00, 14.00, or 18.00 h on days 1, 3, 6, or 9. All rats were blood sampled once. In group E, the PRL levels at 09.00 h on days 1 and 3 were similar to those from group C. However, higher values were obtained at 14.00 and 18.00 h, thus showing a diurnal rhythm with low levels in the morning and high values during the afternoon. No rhythm in PRL secretion was observed on days 6 and 9 in group E in which serum PRL was similarly increased with respect to group C at all times. The progesterone (P) levels paralleled PRL concentration, being significantly higher in group E at 18.00 h on day 1, at 14.00 and 18.00 h on day 3, and at all three times on days 6 and 9; the P measurements were consistent with luteotropic actions of PRL. Naloxone (NAL; 2 mg/kg i.p.) was injected at 17.30 h on days 3, 6, or 17, and 30 min later the animals were blood sampled. p-Chlorophenylalanine (pCPA; 200 mg/kg s.c.) was administered at 07.00 h on days 2, 5, or 16, and blood samples were taken 35 h later. Control E rats were injected with vehicle and blood 18.00 h on days 3 and 6 was not modified by pretreatment with NAL or pCPA. Serum P was significantly reduced after pCPA administration on days 3 and 6 and after NAL only on day 6. The increase in PRL at 18.00 h on day 17 induced by E2 was dramatically enhanced by NAL or pCPA, while these treatments did not significantly modify serum P levels. Our results indicate an inhibitory influence from both opioid and serotoninergic systems on PRL secretion induced by the long-term application of E2 in the arcuate nucleus.