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.

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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

The effect of prolonged intermittent infusion of beta-endorphin or naloxone into the third cerebral ventricle in ewes during the follicular phase of the estrous cycle on the expression of GnRH gene and GnRH-R gene in the hypothalamus and GnRH-R gene in the anterior pituitary gland was examined by Real time-PCR. Activation of micro opioid receptors decreased GnRH mRNA levels in the hypothalamus and led to complex changes in GnRH-R mRNA: an increase of GnRH-R mRNA in the preoptic area, no change in the anterior hypothalamus and decrease in the ventromedial hypothalamus and stalk/median eminence. In beta-endorphin treated ewes the levels of GnRH-R mRNA in the anterior pituitary gland also decreased significantly. These complex changes in the levels of GnRH mRNA and GnRH-R mRNA were reflected in the decrease of LH secretion. Blockade of micro opioid receptors affected neither GnRH mRNA and GnRH-R mRNA nor LH levels secretion. These results indicate that beta-endorphin displays a suppressive effect on the expression of the GnRH gene in the hypothalamus and GnRH-R gene in the anterior pituitary gland, but affects GnRH-R gene expression in a specific manner in the various parts of hypothalamus; altogether these events lead to the decrease in GnRH/LH secretion.