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Searching for the role of prolactin (PRL) in controlling gonadotropic axis activity in sheep, we studied the effects of prolonged, intracerebroventricular (i.c.v.) PRL infusion on luteinizing hormone (LH) secretion and catecholaminergic activity in the hypothalamic infundibular nuclei/median eminence (IN/ME) in sexually active ewes during the periovulatory period. Three groups of animals received the following treatments: 1). i.c.v. infusion of PRL at a dose of 200 microg/day (Lower dose, n = 5); 2). i.c.v. infusion of PRL at a dose of 400 microg/day (Higher dose, n = 6), and 3). i.c.v. infusion of the vehicle (control, n = 5). Each dose of PRL was infused in a pulsatile manner, 4 x 50 microg/h and 4 x 100 microg/h, in 30-min intervals, respectively, during four consecutive days before oncoming ovulation. The estrous behavior of ewes following treatments was also monitored as a determinant of the GnRH/LH surge. Two series of blood collections were made in every ewe, the first on the day preceding the infusion (day 0 of the experiment), the second on the day after the infusion (day 5 of the experiment). In addition, on day 5 of the experiment, perfusions of the IN/ME were made by the push-pull method, either in control or lower dose-treated animals. It was shown that a significant (p < 0.01, p < 0.001) increase in tonic LH secretion during the periovulatory period remained in ewes irrespective of the kind of infusion. No statistical differences were found in LH pulse frequency, amplitude, or in the length of the pulse when compared with values from day 0 and 5 of the experiment within each group. A significant (p < 0.001) increase in IN/ME perfusate concentrations of dopamine and noradrenaline metabolites was noted in PRL-treated ewes in comparison with those in the control. The estrous behavior in PRL-treated animals was delayed for a few days, 3.80 +/- 0.80 days at the lower dose (p < 0.01), and 2.83 +/- 0.98 days at the higher dose (p < 0.05) in comparison with the control, 0.20 +/- 0.20 days. These data indicate that maintenance of an increased PRL concentration within the central nervous system (CNS) for a few days before oncoming ovulation has no inhibitory effect on tonic LH secretion. A few-day shift of the preovulatory GnRH/LH surge, as determined by estrous behavior, might, however, be a consequence of the PRL-induced increase in catecholamine turnover in the IN/ME.

This study tested a hypothesis that the enhancement of the prolactin (PRL) concentration within the central nervous system (CNS) disturbs pulsatile luteinizing hormone (LH) and growth hormone (GH) secretion in rams that are in the natural breeding season. A 3h long intracerebroventricular (icv.) infusion of ovine PRL (50 microg/100 microl/h) was made in six rams during the daily period characterized by low PRL secretion in this species (from 12:00 to 15:00 h); the other six animals received control infusions during the same time. Blood samples were collected from 9:00 to 18:00 h at 10 min intervals. A clear daily pattern of LH secretion was shown in control animals, with the lowest concentration at noon and an increasing basal level around the time of sunset (P < 0.001). No significant changes in LH concentration occurred in PRL-infused animals and the concentration noted after infusion of PRL was significantly (P < 0.05) lower than after the control infusion. The frequency of LH pulses tended to decrease in rams after PRL treatment. The changes in LH secretion clearly carried over to the secretion of testosterone in the rams of both groups. The GH concentrations changed throughout the experiment in both groups of rams, being higher after the infusions (P < 0.001). However, the mean GH concentration and GH pulse amplitude noted after PRL infusion were significantly lower (P < 0.001 and P < 0.05, respectively) from those recorded in the control. The continued fall in PRL secretion observed in rams following PRL infusion (P < 0.05 to P < 0.001) indicates a high degree of effectiveness of exogenous PRL at the level of the CNS. In conclusion, maintenance of an elevated PRL concentration within the CNS leads to disturbances in the neuroendocrine mechanisms responsible for pulsatile LH and GH secretion in sexually active rams.

Through binding with estrogen receptors, phytoestrogens, plant-derived estrogen-like compounds, affect numerous reproductive functions. It is not known whether these compounds are capable of evoking effective changes in luteinizing hormone (LH) and prolactin (PRL) secretion in ewes by acting directly within the central nervous system (CNS). The hypothesis studied was that genistein, infused for several hours into the third ventricle, could immediately affect LH and PRL secretion in ovariectomized (OVX) ewes during seasonal anestrus. Two doses of genistein, 1 microg/100 microl/h (total 4 microg, n = 7) and 10 microg/100 microl/h (total 40 microg, n = 7), were infused intracerebroventricularly from 12.00 to 16.00 h and blood samples were collected from 8.00 to 20.00 h at 10-min intervals. Randomly selected ewes were infused with a vehicle (control, n = 5). The mean plasma LH concentration in control ewes was significantly (p < 0.01) higher during infusion of the vehicle than before the infusion. It remained on an insignificantly changed level after the infusion. The frequency of LH pulses in control ewes did not differ significantly before, during, or after vehicle infusion. In ewes infused with a lower dose of genistein, plasma LH concentrations decreased significantly (p < 0.001) after the infusion, as compared with the values noted before and during genistein infusion. Only a tendency towards a decrease in LH pulse frequency occurred after infusion of a lower dose of genistein. In ewes infused with a higher dose of genistein, the plasma LH concentration decreased significantly (p < 0.01) after phytoestrogen administration as compared with the values noted before and during infusion. The frequency of LH pulses was also significantly (p < 0.01) lower after genistein administration. Because the changes in PRL secretion were more dynamic in response to genistein infusion, the statistical analysis included 2-hour periods. The mean plasma PRL concentration in control animals was significantly enhanced (p < 0.01) only during the first 2-hour period of sampling. After that it decreased and remained on an unchanged level up to the end of sampling. Similar changes in PRL secretion were observed in both experimental groups before genistein infusion. In contrast, significant (p < 0.01 to p < 0.001) increases in PRL concentration were noted regularly during and shortly after the genistein infusion in either low-dose or high-dose genistein-infused ewes, compared with the concentrations noted before genistein treatment. Plasma PRL concentrations during and after genistein infusion in both experimental groups were also significantly higher than the control (p < 0.01 to p < 0.001). The presented data demonstrate that genistein, a phytoestrogen, may effectively modulate LH and PRL secretion in OVX ewes by acting within the CNS., (Copyright 2004 S. Karger AG, Basel)

This study tested a hypothesis that an acute enhancement of prolactin concentration within the central nervous system (CNS) would affect the LH secretion in ewes, depending on the level of endogenous estrogens in the organism. A 3-h long intracerebroventricular (icv.) infusion of ovine prolactin was made in late follicular-phase ewes, experiment 1, and in ovariectomized (OVX) ewes (experiment 2). No significant differences were found in mean LH concentrations and LH peak number before, during and after prolactin administration (50 microg/100 microl/h) in intact cyclic ewes. No diurnal rhythm in LH was detected in prolactin-infused ewes. From the two doses of prolactin used in OVX ewes (25 and 50 microg/100 microl/h) only the lower dose suppressed significantly the mean plasma LH concentration after the infusion, compared to those noted before (P < 0.01) and during (P < 0.001) prolactin treatment. Prolactin had no effect on LH pulse frequency in OVX ewes, however, a tendency to decrease in LH peak number was observed after administration of a lower dose. Plasma prolactin levels decreased significantly (P < 0.01 and P < 0.001) after the icv. infusion in all groups, indicating a high degree of effectiveness for exogenous prolactin at the level of the CNS., (Copyright 2002 Elsevier Science B.V.)

This study tested the hypothesis that the dopaminergic and opioidergic systems are not involved in the short-term stimulatory action of melatonin (MLT) on the secretion of prolactin in anestrous ewes. Thus, MLT should stimulate prolactin release after blockade of either dopamine (DA) or opiate receptors with specific antagonists at the level of the pituitary gland and central nervous system (CNS), respectively. During afternoon intracerebroventricular (icv.) infusion of MLT, the mean plasma prolactin concentration increased significantly (P < 0.001) as compared with the concentrations noted before and during the infusion of the vehicle (veh.). As a result of subcutaneous (s.c.) injection of sulpiride (SULP, DA antagonist), an increase in plasma prolactin concentration was observed, followed by a gradual decrease during the icv. infusion of the vehicle. MLT infused icv. significantly increased (P < 0.001) the secretion of prolactin in SULP + MLT-treated ewes, as compared with the concentration of prolactin noted during infusion of the vehicle in SULP + veh.-treated ewes. Naloxone (NAL, opioid antagonist) infused icv. did not significantly affect the secretion of prolactin, however, a significant (P < 0.01) increase in the concentration was observed after the infusion. In MLT + NAL-treated ewes, the plasma prolactin concentration increased significantly (P < 0.001) during the infusion, as compared with the concentration noted before and that in NAL-alone infused ewes. These results demonstrate that melatonin stimulates prolactin release after the pharmacological exclusion of the dopaminergic input with the DA antagonist sulpiride and also despite the presence of DA activity in the hypothalamus after NAL treatment. Secondly, endogenous opioid peptides are not a major component of this melatonin action.

The aim of this study was to investigate whether melatonin might modulate the daily prolactin secretion in the ewe during a period of ovarian activity and, if so, whether this modulatory action of melatonin was related to the presence of estradiol in the organism. Ewes in the late follicular and luteal phase, as well as overiectomized ewes without (OVX) and after 7 days of estradiol injections (OVX+E2) were examined. Melatonin was infused into the third brain ventricle (100 microgram/100 microliter/h) from 14.00 to 18.00 h. The concentration of prolactin increased significantly during the infusion of melatonin in late follicular-phase ewes, but not in luteal-phase ewes, as compared to the concentration before the infusion: range from 204.0 +/- 31.7 to 272.2 +/- 50.1 ng/ml vs. range from 68.2 +/- 31.8 to 94.7 +/- 33.1 ng/ml (mean +/- SEM, n = 4, p < 0.01) and to the concentration noted during control infusions: range from 130.0 +/- 58.0 to 179.3 +/- 55.6 ng/ml (mean +/- SEM, n = 4, p < 0.05). In ovariectomized ewes, the concentration of prolactin during infusion of melatonin increased significantly, unrelated to the presence of estradiol, as compared to the concentration before infusion: range from 136.7 +/- 20.3 to 260.0 +/- 11.6 ng/ml vs. range from 41.6 +/- 2.6 to 152.3 +/- 14.6 ng/ml in OVX ewes (mean +/- SEM, n = 4, p < 0.01) and range from 161.5 +/- 66.5 to 250.2 +/- 24.3 ng/ml vs. range from 61.2 +/- 1.7 to 159.2 +/- 43.3 ng/ml in OVX+E2 ewes (mean +/- SEM, n = 4, p < 0.01). Concentrations during infusion of melatonin in OVX and OVX+E2 ewes were also significantly higher than during the control infusions: range from 7.2 +/- 1.7 to 22.2 +/- 4.1 ng/ml (mean +/- SEM, n = 4, p < 0.001). These results indicate that melatonin may affect the daily secretion of prolactin in ewes during the breeding season, and suggest that the variable response of prolactin to the melatonin signal in intact and ovariectomized ewes relates to the interaction between both ovarian steroids - estradiol and progesterone - and the prolactin-releasing factor.

The relations between the circadian rhythms of melatonin and prolactin, and the effect of melatonin infused into the third ventricle or the mediobasal hypothalamus (MBH) on prolactin secretion and dopamine (DA) release were studied in anestrous ewes under increasing daylength conditions. The decreased amplitude and duration of nocturnal melatonin secretion were accompanied with changes in the daily pattern of prolactin secretion. Marked peaks of prolactin occurred after sunrise and melatonin decreases in February, as well as significant evening or nocturnal peaks under long day conditions between April and July. Melatonin infused into the third ventricle evoked an abrupt increase in the concentration of prolactin after 30 min, and the enhanced prolactin level was significantly higher than during the control infusion (range from 204 +/- 75 to 248 +/- 48 ng/ml vs. 128 +/- 68 to 149 +/- 93 ng/ml, mean +/- SD). The concentrations of DA in MBH perfusates decreased during perfusion of melatonin but to a degree similar to that noted in untreated ewes. These data suggest that short-term infusions of melatonin stimulate the secretion of prolactin in the ewe under increasing daylength conditions, and that this effect is not mediated by changes in DA release.

Secretion of all the pituitary hormones undergoes marked circadian and seasonal changes. The rhythmicity of these changes is controlled by the circadian pacemaker system and the pineal gland transmitting daylength information to the neuroendocrine axis via the secretion of melatonin. This article presents data on the effects of the short-term melatonin administration into the third brain ventricle on prolactin, beta-endorphin and luteotropin secretion in ewes kept under the increasing and decreasing daylength conditions. Additional emphasis is given to dopamine and LHRH release in the mediobasal hypothalamus under the melatonin treatment by the push-pull method. The long-term and short-term actions of melatonin on the hormonal status in ewes is also discussed.

Plasma prolactin concentrations were determined by radioimmunoassay during oestrous cycles and around the time of oestrus in different types of primitive gilts: Vietnamese, Zlotnicka and wild-boar X domestic pig hybrids. The animals were bled without stress from an indwelling arterial catheter. The following results were obtained: (1) in all gilts the main prolactin peak was observed at Day 15 or 16 of the oestrous cycle; (2) Vietnamese and hybrid gilts showed a second smaller prolactin surge after (Day 2) or before (Day 17) oestrus; (3) base levels of prolactin during the oestrous cycle were 14.8 +/- 0.93 ng/ml (Vietnamese gilts), 13.2 +/- 1.05 ng/ml (Zlotnicka gilts) and 15.6 +/- 2.01 ng/ml (hybrid gilts). The 15-16-day prolactin peaks reached maximum values of 36.4, 43.4 and 56.5 ng/ml respectively.