Bernard Barcikowski

Natural and Melatonin-Stimulated Changes in the Circadian Rhythm of Prolactin Secretion in the Ewe during Seasonal Anestrus

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.

Genistein, a Phytoestrogen, Effectively Modulates Luteinizing Hormone and Prolactin Secretion in Ovariectomized Ewes during Seasonal Anestrus

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 µg/100 µl/h (total 4 µg, n = 7) and 10 µg/100 µl/h (total 40 µg, 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.

Effect of melatonin on daily LH secretion in intact and ovariectomized ewes during the breeding season.

This study was conducted to find out whether daily LH secretion in ewes may be modulated by melatonin during the breeding season, when the secretion of both hormones is raised. Patterns of plasma LH were determined in luteal-phase ewes infused intracerebroventricularly (icv.) with Ringer-Locke solution (control) and with melatonin (100 microg/100 microl/h). Response in LH secretion to melatonin was also defined in ovariectomized (OVX) ewes without and after estradiol treatment (OVX+E2). Basal LH concentrations by themselves did not differ significantly before, during and after both control and melatonin infusions in intact, luteal-phase ewes. However, single significant (P<0.05) increases in LH concentration were noted during the early dark phase in the control and 1h after start of infusion in melatonin treated ewes. In both OVX and OVX+E2 ewes, melatonin decreased significantly (P<0.01, P<0.05, respectively) mean plasma LH concentrations as compared to the levels noted before the infusions. In OVX+E2 ewes, a single significant (P<0.05) increase in LH occurred 1h after start of melatonin treatment, similarly as in luteal-phase ewes. No significant differences in the frequencies of LH pulses before, during and after melatonin infusion were found in all treatments groups. In conclusion, melatonin may exert a modulatory effect on daily LH secretion in ewes during the breeding season, stimulating the release of this gonadotropin in the presence of estradiol feedback and inhibiting it during steroid deprivation. Thus, estradiol seems to be positively linked with the action of melatonin on reproductive activity in ewes.

Melatonin Modulation of the Daily Prolactin Secretion in Intact and Ovariectomized Ewes

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 µg/100 µl/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.

Effect of prolactin infused into the third ventricle on LH secretion in follicular-phase and ovariectomized ewes.

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.

Calcium channel blocker prevents stress-induced activation of renin and aldosterone in conscious pig

SummaryA considerable amount of data suggest the involvement of calcium-mediated processes in the activation of the renin-angiotensin-aldosterone (RAA) cascade. To investigate the effect of calcium-channel inhibition on the RAA system, we studied 21 conscious pigs. Blood renin and aldosterone levels increased by subjecting animals to 24 hours of immobilization stress. Renin and aldosterone levels were repeatedly measured by radio-immunoassay in blood samples taken periodically over 24 hours from a chronically implanted arterial cannula. Pretreatment of the animals (N = 11) with nisoldipine, 2 × 20 mg p.o. daily for 2 days before and on the day of immobilization, transiently attenuated the stress-induced increase of plasma renin activity and completely prevented the rise of aldosterone, as compared to nontreated controls (N = 10). The finding that nisoldipine suppresses RAA activation induced by a nonpharmacologic stimulus in the conscious intact animal may have clinical implications.

LOCAL UTERO-OVARIAN RELATIONSHIPS

This chapter discusses a study to examine local utero–ovarian relationships. The study examined the roles of oestradiol (E 2 -17β) and progesterone (P) in controlling the luteolytic release of prostaglandin F 2α (PGF 2α ) from the ovine uterus. It was observed that peaks of E 2 -17β secretion occurred at intervals throughout the ovine estrus cycle, but it was only around the time of CL regression that peaks of PGF 2α secretion from the uterus were associated with peaks of E 2 -17β from the ovary. Experiments were conducted to examine the effect of exogenous E 2 -17β on PGF 2α secretion by infusing physiological amounts of E 2 -17β into the arterial supply of the autotransplanted uterus at different times of the ovine estrus cycle. It was observed that E 2 -17β had no effect on PGF 2α secretion early in the cycle, whereas on day 14, there was a 50- to 100-fold increase, an effect that could be decreased by indomethacin. This latter finding suggests that E 2 -17β promotes the de novo synthesis of PGF 2α rather than simply the release of stored material.