The relationships between the release of LHRH, β-endorphin (β-END) and noradrenaline (NA) from the hypothalamic infundibular nuclei/median eminence (NI/ME) during the periovulatory period in the ewe was studied. Neurohormone release was assayed in perfusates collected from the NI/ME via push-pull cannulae. LHRH concentrations in perfusates ranged from below detectable values (5 pg) to 50 pg and from 15 to 240 pg/20 min perfusate on the days of proestrus and estrus, respectively. β-END concentrations in perfusates ranged from 320 to 6,000 pg on the day of proestrus and fell to a range between 100 and 380 pg/20 min perfusate on the day of estrus. The NA content of perfusates ranged from an undetectable level to 0.9 × 10⁴ pg/perfusate during proestrus, and rose from 1.0 × 10⁴ to 6.6 × 10⁴ pg/perfusate shortly before the preovulatory release of LHRH and LH. On the basis of the present observations, the following sequence of events leading to the massive LH ovulatory surge in the sheep is suggested: (1) increased secretion of β-END in the NI/ME on the day of proestrus generates an increase in the releasable pool of LHRH through inhibition of LHRH release; (2) on the day of estrus a decreased release of β-END allows the expression of NA activity in the NI/ME and the augmentation of NA tone facilitates the release of newly accumulated LHRH; (3) the resultant intensified LHRH output with its significantly changing pattern of release triggers the preovulatory surge of LH.

Diurnal variations in the plasma cortisol level were studied in anoestrous, pro-oestrous and pregnant ewes subjected to weak electric stimulation of the forelimbs 9 h daily for 3 consecutive days. In non-pregnant ewes the cortisol level rose on each of the 3 days when the stimulation was applied and then decreased on the day following the stimulation. A similar decrease in plasma cortisol concentrations in pregnant ewes appeared on the second day of footshocking. The acrophase of the circadian rhythm on electrostimulation days was synchronous with the time of application of footshocks; therefore, in stimulated ewes it was significantly accelerated compared to the prestimulatory day. A decrease in the plasma cortisol level in pro-oestrous and pregnant ewes was accompanied by disappearance of its normal rhythmicity. Since a normal plasma cortisol response to exogenous corticotrophin was noted after 3 days of foot\x=req-\ shocking it seems unlikely that the decrease in the cortisol level after prolonged stress was caused by exhaustion of the adrenal cortex. Some central mechanisms which could account for the biphasic changes in the plasma cortisol level and for disturbances of the hormone diurnal rhythmicity under conditions of prolonged stress are discussed.Although considerable attention has been given to the effect of acute stress on the secretion of hvpophvsial and adrenal hormones, the influence of prolonged or chronic stress has been studied much less extensively, and contradictory results are re¬ ported in the literature. Dallman & Jones (1973) described, in immobilized male rats, a biphasic Requests for reprints to Dr. E. Domanski. variation of the plasma corticosterone level consist¬ ing of a rise during the first hour of restraint and a decrease after 4 h. On the other hand, Mikulaj et al. (1974) andTaché et al. (1976) showed that longterm stressing procedures increased adrenocortical activity in rats when applied for several days. In particular. Taché et al. (1976) found a progressive elevation of the plasma corticosterone level in male rats which were immobilized 8 h daily for 6 days.In previous studies we found that prolonged footshock disturbed oestrous cycles when admini¬ stered during 24 h to female rats (Chomicka et al., unpublished data) or during 9 h dailv up to 4 days in sheep (Przekop et al. 1984). The effect of stress on reproductive function was found to depend on the oestrous phase of its administration. These results prompted us to compare the effect of prolonged footshock on plasma cortisol levels, the classical index of responsiveness to stress, in anoestrous, pro-oestrous and pregnant ewes. Materials and Methods AnimahTwenty-six Polish Merino ewes 2.5-3.5 years old were used in this study. Thev were kept singly in 2 x 2 m compartments under natural light conditions, fed hay and concentrates and water ad libitum. In winter ambient temperature was maintained at 14 ± 2°C. Each animal bore an indwelling catheter in the jugular vein for drawing blood samples

Using the labeled and unlabeled immunoperoxidase metods, the distribution and concentration (1) of immunoreactive LFRH-material in the hypothalamus, and (2) of gonadotropic hormones in the adenohypophysis of the ewe were determined during various reproductive stages, including two phases of the estrous cycle, anestrus, lactation, and the state after ovariectomy. The concentration of LHRH-immunoreactive material varied in particular regions of the median eminence (ME) and was closely dependent on the physiological state. The immunoreactive material was most abundant in hypothalami of lactating animals, exceeding gradually the corresponding deposits in ovariectomized animals, in ewes during the 16th day of the estrous cycle (before ovulation), and in anestrous ewes. A severe depletion of LHRH from the rostral and central parts of the ME was observed 24 h after ovulation. This depletion was accompanied by a degranulation of LH- and prolactin-producing cells in the adenophypophysis. It is postulated that LHRH is stored in the ME of the sheep in all examined endocrine stage. Only circumscribed regions of the ME take part in the cyclic release of LHRH during the ovulatory phase; this suggests a functional differentiation of the nerve terminals in this neurohemal area.

Lesions made in the anterior medial basal hypothalamus (MBH) in pregnant or lactating ewes caused lack of development of the mammary gland and depressed milk yield, a decrease in plasma prolactin concentration and structural changes in the prolactin cells as manifested by a lack of the expected degranulation (lack of the hormone release) or by atrophic changes and diminished cell granulation (or granule synthesis). Lesions made in the caudal MBH during pregnancy advanced development of the mammary gland, induced a rapid increase in the plasma prolactin concentration and caused extensive degranulation of prolactin cells. These results indicate that at least two functional systems controlling the secretion and production of prolactin exist in the hypothalamus of sheep: an anterior system which is stimulatory and a caudal which is inhibitory.

Serotonin or melatonin was infused into the 3rd cerebral ventricle (3rd V) or into the medial basal hypothalamus (MBH) of both intact ewes and ewes with lesions of the anterior hypothalamic area (AHA). Prolongation of the estrous cycle and delay of ovulation were obtained after infusions of melatonin or serotonin in both the intact and lesioned ewes. Statistically, serotonin proved to be more effective in this respect than melatonin in the intact ewes. The infusions also blocked the preovulatory peak of LH. Furthermore, the infusion of serotonin blocked estrous behavior, while that of melatonin did not affect this function. The effect of indoleamie infusions in the lesioned ewes, in which the inhibitory action of the AHA over the transmission of gonadotrophic releasing hormones (G-RH) had been eliminated, seems to indicate that the inhibitory action of indoleamines on the release of these neurohormones is being displayed at the level of the MBH.