On the mechanism of the positive feedback action of estradiol on luteinizing hormone secretion in the rhesus monkey.

In women and rhesus monkeys, both the negative and positive feedback actions of estradiol (E2) on gonadotropin secretion (inhibition followed by a surge) can be exerted directly at the level of the pituitary gland. We have tested the hypothesis that the positive feedback action of E2 represents but an "escape" from its negative feedback inhibition of gonadotropin secretion consequent to a desensitization of the gonadotropes occasioned by sustained exposure to elevated concentrations of the steroid. We have attempted to replicate such a desensitization by blocking the negative feedback action of E2 by the administration of a potent estrogen receptor antagonist devoid of any agonistic properties (ZM 182,780) to rhesus monkeys in the midfollicular phase of the menstrual cycle (n = 14). The estrogen antagonist, administered at a dose that in separate experiments completely blocked both the negative and the positive feedback effect of exogenous E2 on pituitary LH secretion, failed to produce a surge-like increase in serum LH concentrations. The present results do not support the hypothesis that the LH surge is the consequence of the removal of the negative feedback action of E2. Evidence is presented that ZM 182,780, in contrast to its inhibition of E2-induced LH surges, cannot block the inhibition of hypothalamic GnRH pulse generator activity by E2.

Mitigation of the somnolence of insulin-induced hypoglycemia by a vasopressin V1 receptor antagonist in the rhesus monkey.

Insulin-induced hypoglycemia causes somnolence in rhesus monkeys, a phenomenon usually considered an aspecific consequence of neuroglycopenia. Previous observations from our laboratory have raised the possibility that arginine vasopressin (AVP) may also play a role in this decrease in wakefulness. In the present study we tested this hypothesis by inducing hypoglycemia (approximately 40 mg/dl) in ovariectomized rhesus monkeys by intravenous administration of insulin in the presence of continuous intracerebroventricular infusions of the V1 receptor antagonist [deamino-Pen1, O-Me-Tyr2,Arg8]-vasopressin (180 micrograms/60 microliters per h) or of its vehicle alone (artificial cerebrospinal fluid, 60 microliters/h). Wakefulness was assessed by a scoring system by observers blinded to the experimental protocol. The AVP antagonist significantly attenuated the decrease in wakefulness observed in response to insulin-induced hypoglycemia (p < .03) without increasing blood glucose levels. These and previous findings suggest that the somnolence induced by a moderate degree of hypoglycemia may not entirely be the direct consequence of neuroglycopenia and that AVP may, directly or indirectly, be involved.

On the mechanism of lactational anovulation in the rhesus monkey.

The relative roles of infant suckling and of maternal prolactin (PRL) secretion in lactational anovulation were studied in ovary-intact and ovariectomized rhesus monkeys nursing young that had been removed from their natural mothers. Hypothalamic gonadotropin-releasing hormone (GnRH) pulse generator activity was monitored electrophysiologically in freely behaving animals by radiotelemetry. Serum luteinizing hormone, PRL, estradiol, and progesterone were also measured. Suckling inhibited GnRH pulse generator activity and ovarian cyclicity in all ovary-intact females but had no such effect on the pulse generator in long-term ovariectomized animals. When PRL secretion was suppressed by daily bromocriptine administration, GnRH pulse generator activity remained significantly inhibited and ovulation was prevented in four monkeys (6 trials), whereas in two females (6 trials) a rapid increase in pulse generator frequency and the resumption of ovarian cyclicity were observed although suckling activity was maintained. One monkey displayed both response types. Although these results indicate that suckling per se is able to restrain GnRH pulse generator activity in the absence of PRL, they also suggest that the relative importance of these determinants is variable depending on factors that remain to be determined. The present study also confirms the permissive role of the ovary in the lactational suppression of GnRH pulse generator activity.

On the role of gonadotropin-releasing hormone (GnRH) in the operation of the GnRH pulse generator in the rhesus monkey.

The pulsatile secretion of luteinizing hormone (LH), occasioned by the pulsatile release of gonadotropin-releasing hormone (GnRH), is closely associated with concurrent increases in multiunit electrical activity in the mediobasal hypothalamus (MUA volleys), the electrophysiological correlates of GnRH pulse generator activity. These volleys represent a highly synchronized increase in firing frequency of individual neurons. The origin of these rhythmic oscillations in unit activity and the mechanisms responsible for their synchronization are unknown. The purpose of the present study was to examine the role, if any, of GnRH in the functioning of the GnRH pulse generator in rhesus monkeys. Ovariectomized animals bearing recording electrodes chronically implanted in the mediobasal hypothalamus and fitted with intracerebroventricular (ICV) cannulae in the lateral ventricle and with indwelling cardiac catheters were studied. LH was measured in venous blood withdrawn from the cardiac catheters every 10 min while hypothalamic electrical activity was monitored continuously. In Experiment 1, following a 3- to 4-hour control period, GnRH was infused ICV at a rate of 300 ng/kg body weight (BW)/h over 4-5 h. In Experiment 2, antide, a long-acting GnRH antagonist, was injected ICV in a dose of 105 microg/kg BW after a control period of 3-4 h. Additional control experiments were performed in each animal using vehicle alone. Neither GnRH nor antide affected the frequency of MUA volleys and attendant LH pulses despite significant alterations in LH secretion. These results suggest that, in the rhesus monkey, GnRH may not be involved in the operation of the GnRH pulse generator.

Doctoral education in the biomedical sciences: back to the future?

The contraction of employment opportunities for scientists and engineers has initiated debate about the nature and aims of graduate education. The course of study leading to the PhD degree is seen as being too long, too focused on a small, circumscribed scientific area, and too akin to technical training rather than education. There is a clamor for reform, though much disagreement about what the reform should be. The author places this debate in a historical context, revealing that the arguments and recommendations articulated today were first voiced in the late nineteenth century, when graduate education began in the United States, and have continued to be restated throughout this century. There is no quick fix of the problem. Two long-term remedies have received considerable advocacy, but both are flawed. The first is to reduce the production of PhDs to address the perceived oversupply, but even if such a reduction were possible, the ability of graduate education to predict future employment opportunities is imprecise. The second is to broaden the traditional PhD program to increase the employability of graduates in wider spheres of activity, including some that do not require scientific research, such as business, journalism, law, and secondary school teaching. The author asserts that the broad nonscientific educational background that some would like to see included in the graduate curriculum is principally the responsibility of the student and should be acquired mainly during the undergraduate years. In the long term, the paucity of employment opportunities for recent PhDs and postdoctoral research fellows can realistically be addressed only by increasing the rigor of graduate education, which should not be expected to serve as a job-training program. The PhD is an academic, not a professional, degree and can be no more than an entree to further study or to a number of possible career paths. The responsibility of the graduate school is to ensure that its graduates have a well-rounded scientific education, have each pursued a challenging problem in depth, and possess the intellect, resourcefulness, and drive to succeed in any endeavor that requires keen problem-solving abilities.

The insulin hypoglycemia-induced inhibition of gonadotropin-releasing hormone pulse generator activity in the rhesus monkey: roles of vasopressin and corticotropin-releasing factor.

Insulin-induced hypoglycemia (IIH) profoundly inhibits the activity of the hypothalamic GnRH pulse generator. The aim of this study was to determine the role of vasopressin and CRF in this response. Ovariectomized rhesus monkeys with chronically implanted recording electrodes in the mediobasal hypothalamus and with intracerebroventricular (icv) cannulas in the lateral ventricle were placed in primate chairs, and blood samples were taken every 10 min. Pulse generator activity was monitored electrophysiologically by detecting characteristic increases in hypothalamic multi-unit activity (MUA volleys) and by attendant LH pulses measured in peripheral blood. Arginine vasopressin (AVP) infused via the i.c.v. cannula (50 micrograms/60 microliters.h) in eight monkeys failed to decrease pulse generator activity, as measured by the frequency of MUA volleys, but decreased mean serum LH concentrations (P < 0.001) while increasing serum cortisol levels (P < 0.02). Central administration of an AVP antagonist ([deamino-Pen1, O-Me-Tyr2-Arg8] vasopressin) in four monkeys at a rate (180 micrograms/60 microliters.h) that had previously been found to block the aforementioned effects of coadministered AVP failed to prevent the IIH-induced inhibition of GnRH pulse generator activity and LH secretion in the same animals. On the other hand, a CRF antagonist, [D-Phe12,Nle21,38,C alpha MeLeu37] rat CRF-(12-41), infused i.c.v. at a rate of 500 micrograms/60 microliters.h in four monkeys, delayed the inhibition of pulse generator frequency in response to IIH. These results suggest that AVP does not mediate the hypoglycemia-induced inhibition of GnRH pulse generator frequency in the rhesus monkey, but that CRF may be involved in this response.

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.

Electrophysiological approaches to gonadotrophin releasing hormone pulse generator activity in the rhesus monkey.

Precipitous increases in multi-unit electrical activity (MUA volleys) can be recorded from the mediobasal hypothalamus of the rhesus monkey that are invariably synchronous with the initiation of luteinizing hormone (LH) pulses and thus serve as markers of gonadotrophin releasing hormone (GnRH) pulse generator activity. Continuous radiotelemetric monitoring of this activity throughout the menstrual cycle confirmed a lower pulse generator frequency during the luteal phase than in the follicular phase. Unexpectedly, however, just before the initiation of the LH surge an abrupt reduction in frequency was observed, followed, on occasion, by a total arrest of GnRH pulse generator activity for a day or two. This monitoring technique also revealed a reduction of pulse generator frequency at night. This appeared to be an acute phenomenon observable in the first MUA volleys after the lights are turned off, as is the increase in frequency when the lights are turned on in the morning suggesting a direct action of light, a conclusion that was verified using lighting periods other than the customary ones. This action of light, however, is superimposed upon a diurnal rhythm, as shown by a reduction in pulse generator frequency during the subjective night when the monkeys are kept in constant light or darkness. The duration of the MUA volley in the intact monkey is 1-3 min, compared with some 20 min in the ovariectomized female. While oestradiol reduces the volley duration in the ovariectomized female to that of intact monkeys within 6 h, it takes some 6 weeks following ovariectomy for the duration characteristic of castrates to be re-established.(ABSTRACT TRUNCATED AT 250 WORDS)

Ambient light modifies gonadotropin-releasing hormone pulse generator frequency in the rhesus monkey.

In the course of previous studies using continuous monitoring of the electrophysiological correlates of GnRH pulse generator activity, characterized by episodic increases in hypothalamic multiunit electrical activity (MUA volley), it was noticed that the nocturnal slowing of pulse generator frequency was an acute phenomenon observable in the first MUA volleys after the lights were turned off, as was the increase in frequency when the lights were turned on in the morning. This suggested that the reduction in pulse generator frequency at night may not be the consequence of an intrinsic diurnal rhythm, but an effect of light per se. Indeed, as reported herein, such an effect was observed when the lights were turned on or off at times other than the normal illumination period (normal light schedule, lights on from 0700-1900 h). That this was not simply a response to arousal was shown by awakening the animals with loud recorded noises in total darkness at the same unaccustomed times without a resulting change in frequency. This suggests that the effect of light is specific, perhaps mediated by the retino-hypothalamic tract. This direct action of light, however, is superimposed upon a diurnal rhythm, as shown by a reduction in pulse generator frequency during the subjective night when the monkeys were kept in constant light or constant darkness.

Single unit components of the hypothalamic multiunit electrical activity associated with the central signal generator that directs the pulsatile secretion of gonadotropic hormones.

Vertebrate reproduction is dependent on the operation of a central signal generator that directs the episodic release of gonadotropin-releasing hormone, a neuropeptide that stimulates secretion of the pituitary gonadotropic hormones and, thereby, controls gonadal function. The electrophysiological correlates of this pulse generator are characterized by abrupt increases in hypothalamic multiunit electrical activity (MUA volleys) invariably associated with the initiation of secretory episodes of luteinizing hormone. Using cluster analysis, we extracted single units from the multiunit signals recorded from the mediobasal hypothalamus of four intact and four ovariectomized rhesus monkeys. Of the 40 individual units identified in this manner, 24 increased their frequency with the MUA volleys. The onset and termination of these single-unit bursts occurred coincidently with those of the MUA volleys in both intact and ovariectomized animals, indicating that the longer duration of the MUA volleys characteristic of the gonadectomized animals was due not to the sequential activation of different units but to the longer bursts of the individual cells. Four other units showed decreases in firing rate during the MUA volleys, while the frequency of the remainder did not change. All the examined units were active during the intervals between the volleys of electrical activity. The results indicate that the MUA volleys associated with the activity of the gonadotropin-releasing hormone pulse generator represent the simultaneous increase in firing rate of some individual hypothalamic neurons and the decrease in the frequency of others.

Ovarian control of gonadotropin hormone-releasing hormone pulse generator activity in the rhesus monkey: duration of the associated hypothalamic signal.

The activity of the GnRH pulse generator in the rhesus monkey is associated with abrupt increases in multiunit electrical activity (MUA) volleys recorded from the mediobasal hypothalamus that precede each pulse of LH in the peripheral circulation. In long-term ovariectomized animals the duration of these MUA volleys is 10-25 min and consists of a brief initial 'overshoot' followed by a plateau phase that ends in a rapid decline to baseline activity. In intact monkeys, however, the MUA volley lasts only 1-3 min, a duration equivalent to the overshoot in ovariectomized animals. In addition, the maximal frequency of neuronal activity during each MUA volley is reduced in normal animals when compared to castrates. As shown in earlier studies, estradiol given to ovariectomized monkeys causes a reduction in the duration of MUA volleys to that characteristic of intact animals within 3-5 h. In contrast to this acute effect of estradiol, the increase in MUA volley duration following ovariectomy is a gradual phenomenon, 4-6 weeks being required to achieve the MUA volley duration observed in long-term ovariectomized monkeys. A similar slow time course was observed for the increase in maximal neuronal frequency during each MUA volley. This protracted effect of ovariectomy on MUA volley duration and firing rate may be the consequence of hypothalamic remodelling but this consideration must be tempered by the observation that estradiol reverses these phenomena within hours.

Effects of naloxone on estrogen-induced changes in hypothalamic gonadotropin-releasing hormone pulse generator activity in the rhesus monkey.

In the ovariectomized rhesus monkey, estradiol (E2) markedly reduces the frequency of the GnRH pulse generator as monitored by LH pulse frequency and the concurrent changes in hypothalamic electrical activity, an action mimicked by morphine. In addition, the duration of the increments in multiunit electrical activity (MUA volleys) that precede each LH pulse is decreased by estrogen administration, an action also shared by morphine. The role of endogenous opioids in these actions of E2 was investigated in 8 ovariectomized animals restrained in primate chairs. They were fitted with indwelling cardiac catheters and with bilateral arrays of recording electrodes chronically implanted in the mediobasal hypothalamus. Physiological serum E2 levels achieved by subcutaneous implantation of E2-containing Silastic capsules increased MUA volley interval from 50.8 +/- (SEM) 1.6 min in the control period to 81.1 +/- 6.2 min following E2. Mean MUA volley duration decreased from 21.9 +/- 1.0 to 13.0 +/- 0.7 min. The placement of empty Silastic capsules had no effect on MUA volley duration or interval. Naloxone administration (2.5 mg bolus followed by a 1 mg/h infusion lasting 4-8 h) completely (n = 4) or partially (n = 2) blocked the effects of E2 on MUA volley interval in 6 of the 8 monkeys, and was without effect in the remainder. In contrast, however, naloxone had little or no effect on the action of E2 on MUA volley duration, (13.0 +/- 0.7 vs. 14.0 +/- 0.9 min). These findings suggest that the inhibitory action of E2 on GnRH pulse generator frequency, like that of all other gonadal steroids studied to date, may be mediated by endogenous opioids.(ABSTRACT TRUNCATED AT 250 WORDS)