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The effects of lesions of the suprachiasmatic nuclei (SCN), anterior hypothalamic area (AHA) above the SCN and medial basal hypothalmus (MBH) on oestrous cycles in Merino ewes were observed. Ten animals with lesions of the SCN area continued to have oestrous cycles and ovulations throughout the whole breeding season; in six of these ewes the cycles and ovulations did not stop but were extended into the quiescent non-breeding period. Lesions placed above the SCN in the AHA (three animals) did not affect the oestrous cycles. Together with earlier experiments involving anterior hypothalamic deafferentation rostral and caudal to the SCN, it is suggested that the SCN has a role in transmitting inhibitory action from the medial preoptic area and suppressing the liberation of gonadotrophin-releasing hormone(s) (GnRH) during the non-breeding anoestrous period.Injury to the MBH was followed by cessation of oestrous cycles and of ovulation, a decrease in the number and size of gonadotrophic cells, atrophy of the ovaries and a lowered basal concentration of LH in plasma. This was probably the result of injury to the neurones that produce or transport GnRH to the median eminence.

Abstract. It has previously been observed that sheep subjected to repeated and prolonged stress stimuli showed biphasic cortisol responses. On the first and second day of stimulation an elevation was observed, while on the subsequent days and on the day after the stimulation a marked suppression of plasma cortisol level and a disappearance of its circadian rhythm was noted. It was hypothesized that these changes in the secretion of the hormone were caused by the alteration of catecholaminergic systems in the CNS. To verify this suggestion chemical lesions of the catecholergic systems of the diencephalon were carried out by the infusion of 6-hydroxydopamine (6-OHDA) into the third cerebral ventricle and animals treated in this fashion were subjected to repeated and prolonged electrical mild footshocking (applied during 3 days). The pretreated animals lost the circadian rhythm in cortisol secretion on the days before as well as during and after the electrical stimulation. The animals pretreated with 6-OHDA showed a significant rise of the plasma cortisol level during stimulation. This rise, as the highest daily concentration, occurring within about 1 h after the beginning of footshocking, was significantly accelerated in time with respect to the physiological acrophase, occurring in the early morning hours at the end of prestimulatory days.On the other hand, the pretreated animals did not show the decrease of plasma cortisol levels on the day after the stimulation, observed in normal non-pretreated ones. It is suggested that the absence of the suppression of plasma cortisol concentration in the animals pretreated with 6-OHDA on the day after the stimulation may be due to the blockade of the ventral noradrenergic bundle innervating the medial basal hypothalamus, while disappearance of the circadian rhythm of cortisol secretion was due to the disturbance in the function of retinohypothalamic projection.

Abstract. 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-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.

Domański & Kochman (1968) showed that sheep were more responsive to infusion of purified hypothalamic extract into the adenohypophysis in late anoestrus than in mid-anoestrus. Natural (Schally, Redding, Matsuo & Arimura, 1972) and synthetic (Matsuo, Arimura, Nair & Schally, 1971) LH-RH have been used to induce ovulation in many species, including sheep (Reeves et al., 1972). Administration of a single intravenous injection of 150 μg synthetic LH-RH to seasonally anoestrous ewes induces LH release in all injected animals, but the LH peak is much smaller than that observed at oestrus and luteal function, assessed by peripheral progesterone levels, is absent in most animals (Crighton, Foster, Haresign & Scott, 1975). When the LH-RH was given as five injections the LH peak resembled that at natural ovulation but plasma progesterone levels were still low. Shareha, Ward & Birchall (1976) gave much smaller doses of LH-RH (25 μg/day in continuous infusions for 2 days) to anoestrous ewes but the results were again unsatisfactory.

In this paper the problem of the hypothalamic centres involved in the control of gonadotrophin secretion will be presented, based on investigations performed on rats and sheep. The data obtained for the rat were chosen because the control of gonadotrophin secretion in this animal is best known, and therefore this animal may be considered as a model to be compared with the others. On the other hand, the sheep, retaining seasonality of reproductive processes, represents the most natural model of reproduction among farm animals.

Electrical stimulation of the anterior hypothalamus (regions of the supraoptic and anterior hypothalamic nuclei) or of the ventro-medial hypothalamus (infundibular or ventro-medial nuclei) during the last month of anoestrus in ewes induced ovulation within 72 hr., while similar stimulation of the same centres during the breeding season on the 12th day of the oestrous cycle did not advance ovulation. The role of the hypothalamic centres in the control of gonadotrophin secretion and ovulation is discussed in the light of these results.