Duration and frequency of multiunit electrical activity associated with the hypothalamic gonadotropin releasing hormone pulse generator in the rhesus monkey: differential effects of morphine

Ketamine-induced general anesthesia is compatible with gonadotropin-releasing hormone pulse generator activity in gonadectomized rats: prospects for detailed electrophysiological studies in vivo

Previous studies on the mediobasal hypothalamus (MBH) of rats, rhesus monkeys and goats have detected episodic increases in multiunit activity (MUA volleys) which immediately precede a pulse of luteinizing hormone (LH). These volleys are considered to reflect the activity of the gonadotropin-releasing hormone (GnRH) pulse generator. Our understanding of this system would be greatly enhanced if it were possible to study the electrophysiological aspects of this system at the single cell level; such an approach requires anesthesia of the animals (as in the classic studies on the oxytocin and vasopressin systems). Although it is widely held that anesthetic agents disrupt the processes regulating LH release, little is known about their specific effects on the dynamics of this system. Using on-line electrophysiological techniques in addition to subsequent radioimmunoassay for LH, we have found that anesthesia by ketamine is compatible with the continued production of MUA volleys and LH pulses in gonadectomized rats. In contrast to the hypothermia induced by the LH pulse-suppressing anesthetic sodium pentobarbitone, a small rise in core temperature occurs following ketamine. The present findings offer the prospect of detailed electrophysiological analysis of GnRH pulse generator activity in rats maintained under general anesthesia.

Estradiol and the inhibition of hypothalamic gonadotropin-releasing hormone pulse generator activity in the rhesus monkey

In mammals, gonadal function is controlled by a hypothalamic signal generator that directs the pulsatile release of gonadotropin-releasing hormone (GnRH) and the consequent pulsatile secretion of luteinizing hormone. In female rhesus monkeys, the electrophysiological correlates of GnRH pulse generator activity are abrupt, rhythmic increases in hypothalamic multiunit activity (MUA volleys), which represent the simultaneous increase in firing rate of individual neurons. MUA volleys are arrested by estradiol, either spontaneously at midcycle or after the administration of the steroid. Multiunit recordings, however, provide only a measure of total neuronal activity, leaving the behavior of the individual cells obscure. This study was conducted to determine the mode of action of estradiol at the level of single neurons associated with the GnRH pulse generator. Twenty-three such single units were identified by cluster analysis of multiunit recordings obtained from a total of six electrodes implanted in the mediobasal hypothalamus of three ovariectomized rhesus monkeys, and their activity was monitored before and after estradiol administration. The bursting of all 23 units was arrested within 4 h of estradiol administration although their baseline activity was maintained. The bursts of most units reappeared at the same time as the MUA volleys, the recovery of some was delayed, and one remained inhibited for the duration of the study (43 days). The results indicate that estradiol does not desynchronize the bursting of single units associated with the GnRH pulse generator but that it inhibits this phenomenon. The site and mechanism of action of estradiol in this regard remain to be determined.

Aetiological approach of female reproductive physiology in lactational amenorrhoea

There is a wide range of duration of post-partum amenorrhoea and resumption of ovulation between individuals, within an individual or between populations. Several extraneous variables, such as parity, mother's age, sex of the breast-fed baby, socioeconomic status and cultural level of the family, can be controlled; then the remaining variables will probably explain a part of the total variability in post-partum amenorrhoea duration but say nothing about the physiological process. In attempting to question physiological aspects of the return of fertility several observational studies have tended to favour one of the different factors which are supposed to play a major role in the regulation and have compared different levels of it, such as body composition of the mother (Frisch & McArthur, 1974), breast-feeding pattern (Jones, 1989) or the life style of the women. Life style can be related to women's physical activity in normal life (Ellison, 1991), the difference between urban and rural life (Carael, 1981) or the environment (Laurenson et al., 1985). Prolactin as a possible mediator of the central regulation has been carefully considered (Lunn, Austin & Whitehead, 1984; Howie et al., 1982). These studies were mainly observational rather than experimental, supplementing mothers during the lactating period or during the pregnancy. If this information is added to what is known of other animal species (Loudon, 1987) or animal experimentation (Plant et al., 1989; Williams et al., 1990a; Williams et al., 1990b), the combination of several of the main factors believed to have a major role in the human species can be clarified and the aetiology of the resumption of fertility in nursing women investigated.

The female reproductive axis and its modifications during the post-partum period

The female reproductive axis in mammals is a highly complex dynamic system which goes through different transient or absorbent states during the course of a life-time. Little is known about the mechanisms controlling this system during fetal life and at birth, although it has been shown in numerous species, including primates, that the whole machinery is already functioning (Brooks et al., 1990; Plant, 1986). After a delay ranging from a few days to a few weeks, according to the species, the reproductive axis becomes quiescent and activity apparently resumes only at the time of puberty. Here again understanding of the phenomenon is still limited (Ojeda, 1991).

Estrogenic effects of the new opioid antagonist naltrexone-estrone azine on pituitary luteinizing hormone secretion in ovariectomized rats

The effect of the new opioid antagonist naltrexone-estrone azine (EH-NX) on pituitary luteinizing hormone (LH) secretion in the ovariectomized rat was studied. EH-NX is a hybrid between the steroid component estrone and the opioid antagonist naltrexone (NX). It is a potent and long-acting opioid antagonist in vitro and in vivo, but its effect upon in vivo LH secretion has not been tested before. The aims of the study were to investigate whether, unlike naltrexone, EH-NX can stimulate LH secretion without the need of additional estrogen pretreatment and whether EH-NX has peripheral estrogenic effects upon the uterine weight, when administered chronically to long-term ovariectomized rats. Female rats were injected subcutaneously with EH-NX 21 days after ovariectomy. The effects of EH-NX injections on LH secretion were compared to the effects of NX and estrone hydrazone (EH) alone, or in combination, with or without estradiol-benzoate (EB) pretreatment. Inhibition of LH secretion and uterine proliferation were observed in rats treated chronically with EH-NX in dosages of 0.250 mg/kg bw and higher. These effects were similar to those caused by EH and EB. In short-term OVX rats EH-NX appeared to act faster than EH. In contrast to NX, no stimulatory effect on LH secretion was seen with EH-NX in EB primed OVX rats. These results surprisingly demonstrate that EH-NX behaves like an estrogen and not like an opioid antagonist. The unexpected pharmacological profile of this new drug may open up doors for several medical applications.

Neuroendocrine rhythms

Hormones are secreted with circhoral, circadian and seasonal periodicities. Circhoral pulsatility is a temporal code, many chronic and acute changes in neuroendocrine status being mediated by changes in the frequency of circhoral release. The identity of the neuronal circuits controlling circhoral release is not known. Circadian release of hormones occurs with a precise temporal order entrained to the light-dark cycle, synchronized to the activity/rest rhythm and generated by circadian oscillators, of which the suprachiasmatic nuclei are the most important. Seasonal rhythms are driven either by an endogenous circannual clock mechanism or by a process of photoperiodic time measurement which is dependent upon the duration of the nocturnal peak of the pineal hormone melatonin.

Duration of phasic electrical activity of the hypothalamic gonadotropin-releasing hormone pulse generator and dynamics of luteinizing hormone pulses in the rhesus monkey

The secretion of luteinizing hormone (LH) by the pituitary gland is a pulsatile phenomenon. In the rhesus monkey, each pulse of LH in the peripheral circulation is associated with a characteristic increase in multiunit electrical activity (MUA) recorded from the medial basal hypothalamus. These "volleys" of electrical activity initiate the release of gonadotropin-releasing hormone (GnRH) into the pituitary portal circulation from the terminals of neurosecretory cells. Their duration varies from 1-3 min in normal, adult intact females to 10-25 min in long-term ovariectomized monkeys. A variety of pharmacological interventions also modify volley duration. The purpose of this investigation was to determine the physiological significance of alterations in volley duration. The dynamics of LH pulses in ovariectomized animals were observed in a number of experimental circumstances in which MUA volley duration was reduced from a maximum of 23 min to a minimum of 4 min without significantly altering their frequency. The magnitude of each LH pulse was assessed by calculating the area under the curve delineated by the time course of LH above baseline. In eight experiments, a linear regression of these values on volley duration failed to reveal a significant correlation between MUA volley duration and the magnitude of LH pulses. These results suggest that all of the GnRH secreted per pulse is released at the onset of each MUA volley, the remainder of the increase in electrical activity having no further action on GnRH secretion, although effects on other systems cannot be excluded.