Biogenic amines and the activity of the hypothalamo-pituitary-ovarian axis in ewes.

An important feature of communication between the central nervous system and the pituitary-ovarian axis is the pattern of pulsatile discharge of hypothalamic luteinizing hormone releasing hormone (LHRH). The discharge of LHRH is under control of noradrenergic, dopaminergic, and serotoninergic systems of the brain. These systems intervene between external and internal signals (e.g. photoperiod and gonadal steroids) and LHRH output. In the ewe, noradrenaline, dopamine and serotonin of the brain are implicated in the control of LHRH output, and hence luteinizing hormone (LH) release, during both the anoestrous and breeding seasons. These amines are involved in steroid-dependent and steroid-independent regulation of LHRH/LH discharge. An interplay of inhibitory versus excitatory influences of these amines on LHRH/LH release appears to govern the pattern of LHRH/LH output during the annual reproductive cycle and the ovulatory cycle. A concise overview of this topic will be provided.

Release of luteinizing hormone-releasing hormone, beta-endorphin and noradrenaline by the nucleus infundibularis/median eminence during periovulatory period in the sheep.

The relationships between the release of LHRH, beta-endorphin (beta-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. beta-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 x 10(4) pg/perfusate during proestrus, and rose from 1.0 x 10(4) to 6.6 x 10(4) 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 beta-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 beta-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.

Effect of stress on the course of oestrous cycle and the release of luteinizing hormone; the role of endorphin in these processes.

It has been postulated that stress induces discorrelations of the hypothalamo-pituitary and pituitary gonadal axis. In our experiments on the effect of stress on the reproductive physiology in rats and sheep we applied mild electrical footshocking of short or prolonged duration. Foot-shocking applied with some breaks during 9 h within one a day (15th day of the oestrous cycle) induced in ewes acceleration of the release of LH. Prolonged footshocking applied with some breaks during 3 days in cycling sheep caused disturbances in the circadian rhythm of the cortisol secretion, disturbances in the release of LH and led to the blockade of ovulation. Disturbances in the course of oestrous cycle occurred not only during the current cycle but also during two subsequent cycles. Rats exposed to relatively long-term stressful situation (24 h) during dioestrous displayed marked changes in the length of this phase in three subsequent post-stress oestrous cycles. To follow the neurohormonal background of the stress-induced disturbances in LH release and in the course of oestrous cycle in sheep the concentrations of beta-endorphin (beta-END) in the infundibular and paraventricular nuclei as well as in the pituitary gland under physiological and stress conditions were determined, while in rats the metabolism of brain serotonin was investigated. Footshocking in rats induced significant decrease in 5-HT concentrations in the fronto-parietal brain cortex, hippocampus, striatum, medial basal hypothalamus and the preoptic-anterior hypothalamic area. These results allow to suggest that the decline in brain 5-HT under stress conditions has some associations with the impairments in the course of oestrous cycle. Measurements of the beta-END in perfusates of medial basal hypothalamus (nucl. infundibularis) in sheep evidenced significant increase of this opioid under stress conditions and it was postulated that this increase might be the main cause of the stress-induced impairments in the course of oestrous cycle and inhibition of LH-release. In addition, it was found that beta-END suppressed the secretion of cortisol and attenuated some noxious consequences of general nature for organism.

Differential behavioural and hormonal responses to two different stressors (footshocking and immobilization) in sheep.

Two different stressors footshocking and immobilization applied for 3 days induced in sheep differential behavioural and hormonal responses in cortisol and prolactin secretion. Immobilization drastically disturbed rumen motoric activity (significantly attenuated its contractions), caused loss of appetite and severe general depression. Footshocking did not induce any of these symptoms. Immobilization induced 2-3 fold higher rise of plasma cortisol mesors on the days of stressing as compared to footshocking. On the poststressing days after footshocking as well as after immobilization plasma cortisol mesors fell to 50% of the prestimulatory values. The rise of plasma prolactin mesors in footshocked and in immobilized animals on the days of stressing was nearly of the same magnitude. However, a significant difference in the response of prolactin secretion between footshocked and immobilized animals occurred in the circadian rhythmicity of the hormone secretion. Footshocking induced circadian rhythmicity with characteristic acrophases, while immobilization did not induce the circadian pattern of prolactin secretion. An attempt of interpretation of the described phenomena has been undertaken.

Effect of oestradiol on the responses of regional brain serotonin to stress in the ovariectomized rat.

The effect of oestradiol on the response of regional brain 5-hydroxytryptamine (serotonin, 5-HT) to 24 hour electrical footshock was investigated in ovariectomized rats. The stress applied four weeks after ovariectomy induced significant decreases in 5-HT concentrations in the fronto-parietal brain cortex (Bc), hippocampus (Hipp), striatum (CP), medial-basal hypothalamus (MBH) and the preoptic-anterior hypothalamic area (PAHA). Pretreatment with oestradiol prevented the footshock-induced decreases in 5-HT in the Hipp, CP, MBH and the PAHA. It was concluded that oestradiol may prevent stress-induced response of regional brain 5-HT in ovariectomized rats. An interaction between oestradiol and brain 5-HT is proposed in the control of the course of the oestrous cycles in stressful situations.

Spontaneously prolonged dioestrus and serotonin in various regions of the rat brain.

5-Hydroxytryptamine (serotonin, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were investigated in various regions of the brain on the day of normal and spontaneously prolonged dioestrus in the rat. The 5-HT/5-HIAA concentration ratio and 5-HT content were found to be higher in the medial-basal hypothalamus (MBH) and the fronto-parietal cerebral cortex (Bc), and lower in the preoptic-anterior hypothalamic area (PAHA), when dioestrus was prolonged. No significant difference was observed with 5-HT and 5-HIAA concentrations in the hippocampus (Hipp). It was concluded that there are significant alterations in 5-HT activity in the MBH, PAHA and the Bc when dioestrus is spontaneously prolonged.

Biosynthesis, in vivo, of gonadotropin-releasing hormone in the hypothalamus of normal and ovariectomized female rats.

Normal female and ovariectomized rats were infused into the 3rd ventricle with [3H]glycine or [3H]alanine. Some rats were pretreated with cycloheximide. Hypothalamus, anterior pituitary and fragments of cortex were excised, homogenized, extracted and treated with specific antiserum to GnRH, bound to Sepharose. The radioactivity of immuno-absorbed products was counted either immediately of after extraction and thin-layer chromatography by using two different solvent systems. With the two systems, the location of the immuno-absorbed radioactivity always coincided with the spot of synthetic GnRH. Our results show that [3H]glycine was incorporated, as a function of time, into GnRH isolated from rat hypothalami. The amount was incorporated, as a function of time, into GnRH isolated from rat hypothalami. The amount of radioactivity incorporated into hypothalami from diestrous-I rats was similar to that of ovariectomized rats and twice as high as in late proestrous rats. Only minute amounts of radioactivity were incorporated into the immuno-absorbed product. Cycloheximide inhibited incorporation of [3H]glycine into the immuno-absorbed product to the same extent as its incorporation into the total protein from the hypothalamus. Our experimental results support the hypothesis of ribosomal mechanisms being involved in the biosynthesis of GnRH. They also suggest that the rate of accumulation of newly synthesized labeled GnRH is of the same order in the hypothalamus of ovariectomized rats as in diestrous-I rats.

Serotonin content of the anterior and medial-basal hypothalamus in the female rats throughout the course of estrous cycle.

Serotonin content in the anterior hypothalamic area (AHA) and medial basal (MBH) hypothalamus has been estimated (by the spectrofluorometric method) throughout the course of estrous cycle in the female rats. Serotonin content has been found to decrease significantly on the day of early and late proestrous in AHA and MBH, respectively. The involvement of serotonin in the process of regulation of gonadotropin releasing hormone(s) release on the level of AHA and MBH is discussed.

Activity of noradrenergic neurons of medial-basal hypothalamus of female rabbits in some stages of reproduction.

Activity of noradrenergic neurons of medial-basal hypothalamus of female rabbits in some stages of reproduction. Acta Physiol. Pol., 1978, 29 (2): 111--115. The differences were found in the activity of noradrenergic neurons of medial-basal hypothalmus (MBH) estimated by the analysis of metabolism of intraventricularly infused 3H-noradrenaline (3H-NA) in various stages of reproduction in female rabbits. The gonadal hormones and especially 17beta-estradiol seem to be involved in this activity. The rapid disappearance of 3H-NA from MBH, found immediately after mating, implicates an involvement of noradrenaline in the process of Gn-RHs release form the rabbit hypothalamus.

Noradrenaline turnover rate in the mediobasal and anterior hypothalamus of the rabbit.

Noradrenaline turnover rate in the mediobasal and anterior hypothalamus of the rabbit. Acta Physiol. Pol., 1977, 28 (1): 39-43. The rate of noradrenaline (NA) turnover in mediobasal hypothalamus (MBH) and anterior hypothalamus (AH) of the rabbit was estimated by steady-state isotopic method with a tritiated noradrenaline (3H-NA) as a tracer. The disappearance rate of 3H-NA both in MBH and in AH was found to be biphasic; the first rapid phase of the NA half-life of about 30 min, followed by the second phase of slower decay of the half-life of 2.4 h and 10 h for MBH and AH respectively. The results suggest an existence of more than one metabolic pool of endogenous noradrenaline in MBH and AH and indicate regional difference in the metabolism of NA stores in the hypothalamus.