Feedback Action Of Gonadal Steroids Research Articles

Colocalisation of Dynorphin A and Neurokinin B Immunoreactivity in the Arcuate Nucleus and Median Eminence of the Sheep

Dynorphin A (DYN)-containing cells play a key role in conveying the negative feedback influence of progesterone upon pulsatile gonadotrophin-releasing hormone (GnRH) secretion in the ewe. A very high percentage of DYN cells in the arcuate nucleus express the progesterone receptor; another population of arcuate nucleus cells that also express steroid receptors in the sheep are those that express the tachykinin peptide, neurokinin B (NKB). Both DYN and NKB fibres have been shown to form close contacts with ovine GnRH cells. Therefore, the present study tested the hypothesis that neurones expressing NKB and DYN represent the same neuronal population in the arcuate nucleus. Confocal microscopic analysis of brain sections processed for dual immunofluorescence revealed that a large majority of DYN neurones in the arcuate nucleus were also immunoreactive for NKB. Likewise, a similar majority of NKB neurones in the arcuate nucleus were immunoreactive for DYN. By contrast, DYN cells in the preoptic area and anterior hypothalamus did not colocalise with NKB, nor did DYN cells in the paraventricular or supraoptic nuclei. Fibres that stained positively for both DYN and NKB were seen in the arcuate nucleus, where they formed close appositions with DYN/NKB-positive neurones, and in the external zone of the median eminence. Taken together with previous findings, these data suggest that a subpopulation of arcuate nucleus neurones coexpressing DYN and NKB mediate the negative feedback influence of progesterone on pulsatile GnRH secretion in the ewe and may also be involved in other feedback actions of gonadal steroids.

Stress and Reproduction: Central Mechanisms and Sex Differences in Non-rodent Species

Despite extensive research, the mechanisms by which stress affects reproduction are unknown. Activation of stress systems could potentially influence reproduction at any level of the hypothalamo-pituitary gonadal axis. Nonetheless, the predominant impact is on the secretion of gonadotrophin releasing hormone (GnRH) from the brain and the secretion of the gonadotrophins, luteinizing hormone (LH) and follicle stimulating hormone (FSH), from the gonadotrophs of the anterior pituitary gland. When stress is prolonged, it is likely that secretion of the gonadotrophins will be suppressed but the effects of acute stress or repeated acute stress are not clear. Different stressors activate different pathways for varying durations, and the actions of stress vary with sex and are influenced by the predominance of particular sex steroids in the circulation. The mechanisms by which stress influences reproduction are likely to involve complex interactions between a number of central and peripheral pathways and may be different in males and females. To understand these mechanisms, it is important to determine the stress pathways that are activated by particular stressors and to establish how these pathways affect the secretion and actions of GnRH. Furthermore, there is a need to know how stress influences the feedback actions of gonadal steroids and inhibin.

GABAergic neuronal activity and mRNA levels for both forms of glutamic acid decarboxylase (GAD 65 and GAD 67) are reduced in the diagonal band of Broca during the afternoon of proestrus

There is considerable evidence that GABAergic neurons play an important role in the regulation of gonadotropin-releasing hormone (GnRH) secretion, and that these neurons may mediate the feedback actions of gonadal steroids on GnRH neurons. The aim of the present study was to investigate whether endogenous changes in ovarian steroid secretion during the estrous cycle influenced GABAergic neuronal activity in the preoptic region of the hypothalamus, and in other steroid-sensitive brain regions. Intact, adult female rats were sacrificed at various times during the days of metestrus or proestrus. GABAergic neuronal activity was estimated by measuring the rate of accumulation of GABA in microdissected brain regions after pharmacological inhibition of GABA degradation. Concentrations of mRNA for both forms of glutamic acid decarboxylase (GAD 65 and GAD 67) were quantified in microdissected brain regions by a microlysate ribonuclease protection assay. In the diagonal band of Broca at the level of the organum vasculosum of the lamina terminalis (DBB(ovlt)), GABAergic neuronal activity was significantly reduced during the afternoon of proestrus compared with the morning of either proestrus or metestrus. In the lateral septal nucleus, GABAergic neuronal activity was significantly increased in the afternoon of proestrus compared with the morning. There were no significant effects of time of day or day of estrous cycle in the medial preoptic nucleus, median eminence, ventromedial nucleus, suprachiasmatic nucleus, medial septal nucleus, hippocampus (CAI region), or cingulate cortex. In the DBB(ovlt), mRNA levels for both GAD 65 and GAD 67 were significantly reduced in the afternoon of proestrus compared with the afternoon of metestrus. By contrast, there was no change in GAD 65 and GAD 67 mRNA levels in the cingulate cortex at any of the times examined. These results demonstrate that GABAergic neuronal activity, and mRNA levels for both GAD 65 and GAD 67 are reduced in the DBB(ovlt) during the afternoon of proestrus. These results support the hypothesis that decreased GABAergic neuronal activity in this region plays a major permissive role in the generation and maintenance of the estrogen-induced LH surge.

Homogeneity of intracellular electrophysiological properties in different neuronal subtypes in medial preoptic slices containing the sexually dimorphic nucleus of the rat.

The sexually dimorphic nucleus of the preoptic area (SDN-POA) is larger in male than in female rats, the male phenotype requiring the presence of circulating androgens perinatally. These experiments investigated the intracellular electrophysiology and morphology of SDN-POA neurons and compared these properties with those of other medial preoptic area (MPOA) neurons. Biocytin-injected cells in the SDN-POA either had one or two primary dendrites, or they had multipolar dendritic arrays; dendrites were aspiny or sparsely spiny and displayed limited branching. Neurons in other parts of the MPOA were similar morphologically. Regardless of morphology, neurons situated in either the SDN-POA or surrounding MPOA had low-threshold potentials and linear or nearly linear current-voltage relations. In most (73%) cells, stimulation of the dorsal preoptic region evoked a fast excitatory postsynaptic potential followed by a fast inhibitory postsynaptic potential (IPSP). Bicuculline blocked the fast IPSPs, which reversed near the Cl2 equilibrium potential (-71 +/- 5 mV), indicating their mediation by gamma-aminobutyric acid (GABA)A receptors. Neurons in the SDN-POA have electrophysiological properties similar to those of other medial preoptic cells. When compared with the hypothalamic paraventricular nucleus, the MPOA appears relatively homogeneous electrophysiologically. This is despite the morphological variability within this population of neurons and heterogeneities that are also apparent at other levels of analysis. Finally, GABA-mediated, inhibitory synaptic contacts are widespread among medial preoptic neurons, consistent with indications from earlier reports that GABA provides a link in the feedback actions of gonadal steroids on the release of gonadotropic hormones.

Regional specificity of testosterone regulation of proopiomelanocortin gene expression in the arcuate nucleus of the male rat brain.

Endogenous opioid peptides have been implicated as mediators in the negative feedback action of gonadal steroids on GnRH secretion. We have previously demonstrated that testosterone stimulates POMC gene expression in neurons of the arcuate nucleus. However, the wide distribution and variety of actions attributed to the numerous arcuate POMC neurons suggest that these cells may be heterogeneous in their responsiveness to steroid hormones. We tested the hypothesis that testosterone modulates a select population of POMC neurons within the arcuate nucleus of the adult male rat by comparing POMC mRNA signal levels throughout the arcuate nucleus of intact, castrated, and castrated testosterone-replaced adult males. Adult male rats were castrated and implanted (sc) with a Silastic capsule (30 mm) that was either empty (n = 6) or filled with crystalline testosterone (n = 5). Control sham-operated animals (n = 6) were left intact. In each animal the arcuate nucleus was divided into four equal rostral-caudal areas within which we measured POMC mRNA content in individual cells. We report that the effects of castration and testosterone replacement are observed in POMC neurons located in the most rostral region of the arcuate nucleus. After castration, POMC mRNA content was reduced in cells of the most rostral arcuate area (intact, 152 +/- 5 grains/cell; castrate, 119 +/- 2 grains/cell; P less than 0.0005), and replacement with physiological levels of testosterone prevented the decline in POMC mRNA levels so that they remained equivalent to that of the intact animal (castrated testosterone-replaced, 153 +/- 6 grains/cell). There was no significant difference in POMC mRNA signal between intact and castrated testosterone-replaced animals in the most rostral area. POMC neurons in the more caudal aspect of the arcuate (75% of the nucleus) were unaffected by the treatments; alternatively, it is possible that a real change in POMC message content in a subpopulation of cells was obscured by larger numbers of nonresponding cells within the same tissue sections. Based on these observations we conclude that there is a heterogeneous population of POMC neurons in the arcuate nucleus and that testosterone regulates POMC gene expression in a select group of these cells located in the rostral portion of the arcuate nucleus.

Simultaneous Measurement of Luteinizing Hormone (LH)-Releasing Hormone, LH, and Follicle-Stimulating Hormone Release in Intact and Short-Term Castrate Rats*

The temporal relationship between LHRH release and gonadotropin secretion as well as the effects of castration on LHRH release were investigated in conscious, freely moving male rats. LHRH release was measured in hypothalamic/median eminence perfusates, while levels of pituitary gonadotropins (LH, FSH) were determined in sequential blood samples obtained via atrial catheters. Twenty-four to 26 h before experiments, rats underwent sham surgery or castration. LHRH release in push-pull perfusates from both groups was pulsatile, and nearly all identified LH pulses (83.3%) were temporally associated with LHRH pulses. Of the fewer irregular FSH pulses that were observed, only 43.7% were temporally associated with LHRH pulses. Mean LHRH pulse amplitude and mean LHRH levels were not different in intact and castrate animals. The frequency of LHRH pulses was moderately increased in castrate rats (1.30 pulses/h) compared to that in intact animals (0.83 pulses/h), and this acceleration was accompanied by a significant increase in LH pulse frequency, pulse amplitude, and mean level. It was also noted that the number of silent LHRH pulses (those not associated with LH pulses) was dramatically reduced in castrate animals. Characteristics of gonadotropin release (pulse frequency, pulse amplitude, and mean level) were not significantly different in animals undergoing push-pull perfusion/bleeding procedures from those in rats not receiving push-pull cannula implants. We conclude from these studies that 1) LH pulses show a high concordance with LHRH pulses, providing evidence that the LHRH pulse generator operates as the neural determinant of LH pulses in male rats, 2) FSH secretion is not associated with LHRH release in an obvious and consistent manner, suggesting that LHRH/FSH relationships are not easily discerned in these animals or that a FSH-releasing factor distinct from the LHRH decapeptide may regulate FSH secretion, 3) a modest increase in LHRH pulse frequency occurs 24-30 h after castration, and 4) silent LHRH pulses occur with much greater regularity in intact than in castrate rats. The latter two observations suggest that both hypothalamic and intrapituitary sequelae of castration may be critically important in the development of postcastration increases in LH secretion and the negative feedback of gonadal steroids.

Influence of short days on diurnal patterns of serum gonadotrophins and prolactin concentrations in the male Djungarian hamster, Phodopus sungorus.

Exposure to short days for 8 weeks suppressed mean serum concentrations of FSH, LH and prolactin compared to hamsters kept in long days. Hamsters in short days exhibited a small afternoon rise in serum FSH, but serum LH and prolactin did not exhibit 24-h variations. In hamsters under long days, a late afternoon-early evening increase was evident for circulating prolactin but none was detected for the gonadotrophins. A fall in testes weights rapidly occurred by 14-28 days after transfer to short days. This was accompanied or preceded by a decrease in serum gonadotrophins and prolactin. Reductions in serum FSH and LH occurred in short days in blood samples taken at 09:00 h or 15:00 h. However, the nadir in serum prolactin was first achieved (at 09:00 h), at least 7 days before that at 15:00 h (i.e. Day 14 versus Day 21 of short photoperiod, respectively). The ability to secrete gonadotrophins was further tested in hamsters that had undergone gonadal regression. Castration of hamsters exposed to short days or injected with melatonin in the afternoon, a treatment known to mimic short day effects, induced a 3- to 5-fold increase in serum gonadotrophins. However, this rise in FSH and LH was significantly attenuated compared to the 10-fold response in controls in long days. The results indicate that gonadal involution induced by short days may be mediated by the decline in mean gonadotrophin secretion which, in turn, is regulated by responsiveness to steroids, as well as a mechanism independent of the negative feedback action of gonadal steroids.

Feedback effects of steroids and gonadotrophin control in adult rats with streptozotocin-induced diabetes mellitus.

The effects of long-and short-term streptozotocin-induced diabetes mellitus on the control of gonadotrophin secretion have been investigated in adult intact rats. A high dose of streptozotocin (80 mg/kg), administered intraperitoneally 3 days before experimentation, inhibited ovulation and reduced the pituitary luteinizing hormone response to luteinizing hormone releasing hormone in proestrous rats. A lower dose (40 mg/kg) did not inhibit ovulation but abolished the luteinizing hormone releasing hormone-priming effect on the pituitary which normally occurs on proestrus, prior to ovulation. Oestrous cyclicity was lost when diabetes was induced for 14 or 56 days, but there was no effect on pituitary responsiveness to luteinizing hormone-releasing hormone compared with control animals. Similar observations were made with rats placed on a food-restricted diet. In all experiments there was no difference between diabetic and control animals in the pituitary luteinizing hormone content, the hypothalamic content of luteinizing hormone-releasing hormone or the ovarian weights. Ovariectomized rats treated with streptozotocin (40 mg/kg) were used to investigate the effects of diabetes on steroid feedback mechanisms. There was an attenuated luteinizing hormone response to ovariectomy in diabetic compared with control animals, and an impaired positive feedback effect of progesterone in oestrogen-primed animals. The results show that streptozotocin-induced diabetes mellitus inhibits feedback action of gonadal steroids and this could account for both the loss of oestrous cyclicity and the reduced pituitary sensitivity to luteinizing hormone-releasing hormone.

Bio- and immunoactive luteinizing hormone in negative feedback action of gonadal steroids

To clarify the possible intervention of gonadal steroids on the secretion of bioactive LH from the pituitary, the levels of plasma LH were measured by in vitro bioassay as well as radioimmunoassay, and their changes in specific endocrine conditions were investigated. The ratio of bioactivity to immunoactivity (B/I ratio) of LH in patients with hypergonadotropic hypogonadism and in postmenopausal women was significantly greater than that in women during the follicular phase of the menstrual cycle, and estradiol levels in the former groups were significantly lower than those of the latter group. In cases of gonadotropin-secreting pituitary tumor with high LH and normal estradiol levels, B/I ratio was found at a level similar to that during the follicular phase. The administration of combined pills decreased the high B/I ratio of oophorectomized women to levels comparable to those in the normal follicular phase. Although bioactive LH responded in all groups to LH-RH with a similar pattern to that observed with immunoactive LH with a peak value 15 approximately 30 min. after the stimulation, followed by a gradual decrease thereafter, the B/I ratio of women during the follicular phase was the lowest at 15 min. and returned up to the initial value within 60 min. after the injection of LH-RH. In contrast, this drop of the B/I ratio did not occur in 7 out of 15 cases composed either of hypergonadotropic hypogonadism or postmenopausal women. The peak values of LH in patients without decrease in B/I ratios were higher than those of women showing decreased B/I ratios. In addition, circulating levels of gonadotropin alpha-subunit did not affect the B/I ratio. These results suggest that circulating levels of gonadal steroids regulate the secretion and synthesis of LH quantitatively as well as qualitatively.

Role of endogenous opiates in the expression of negative feedback actions of androgen and estrogen on pulsatile properties of luteinizing hormone secretion in man.

We have tested the participation of endogenous opiate pathways in the negative feedback actions of gonadal steroids on pulsatile properties of luteinizing (LH) hormone release in normal men. To this end, sex steroid hormones were infused intravenously at dosages that under steady state conditions selectively suppressed either the frequency or the amplitude of the pulsatile LH signal. The properties of pulsatile LH secretion were assessed quantitatively by computerized analysis of LH series derived from serial blood sampling over 12 h of observation. When the pure (nonaromatizable) androgen, 5-alpha-dihydrotestosterone, was infused continuously for 108 h at the blood production rate of testosterone, we were able to achieve selective inhibition of LH pulse frequency akin to that observed in experimental animals after low-dosage androgen replacement. Under these conditions, serum concentrations of testosterone and estradiol-17 beta did not change significantly, but serum 5 alpha-dihydrotestosterone concentrations increased approximately two- to threefold, with a corresponding increase in levels of its major metabolite, 5 alpha-androstan-3 alpha, 17 beta-diol. In separate experiments, the infusion of estradiol-17 beta at its blood production rate over a 4.5-d interval selectively suppressed LH pulse amplitude without influencing LH pulse frequency. Estrogen infusion increased serum estradiol-17 beta levels approximately twofold without significantly altering blood androgen concentrations. We then used these schedules of selective androgen or estrogen infusion to investigate the participation of endogenous opiates in the individual inhibitory feedback actions of pure androgen or estrogen on pulsatile LH release by administering a potent and specific opiate-receptor antagonist, naltrexone, during the infusions. Our observations indicate that, despite the continuous infusion of a dosage of 5 alpha-dihydrotestosterone that significantly suppresses LH pulse frequency, co-administration of an opiate-receptor antagonist effectively reinstates LH pulse frequency to control levels. Moreover, during the infusion of a suppressive dose of estradiol-17 beta, opiate receptor blockade significantly augments LH pulse frequency and increases LH peak amplitude to control levels. Thus, the present studies in normal men demonstrate for the first time that the selective inhibitory action of a pure androgen on LH pulse frequency is effectively antagonized by opiate-receptor blockade. This pivotal observation indicates that opiatergic and androgen-dependent mechanisms specifically and coordinately control the hypothalamic pulse generator for gonadotropin-releasing hormone (GnRH)

Regulation of alpha subunit synthesis by gonadal steroid feedback in the sheep anterior pituitary.

Cell-free translations of ovine pituitary RNA were used to examine the amount of alpha subunit synthesized in the presence and absence of physiological levels of gonadal steroids. Two animal groups were used: 1) no feedback-castrate ewes exhibiting high serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) and 2) negative feedback-castrate ewes treated with estradiol and progesterone which suppressed LH and FSH secretion. Cytoplasmic RNA was extracted from the pituitaries, translated in a wheat germ cell-free system, and alpha subunit identified by specific immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gels. Low amounts of alpha subunit (0.06% of total incorporated trichloroacetic acid-insoluble cpm) were seen in steroid-treated castrate ewes which also exhibited low serum LH and FSH concentrations. The amount of alpha translated by pituitary RNA from untreated castrates was significantly higher than those amounts from the steroid-treated castrates (p less than 0.02). The decreased amount of translated alpha subunit in steroid-treated castrates was probably not due to unique in vitro translation properties since the isotope incorporated/microgram of RNA and the amount of translated growth hormone and prolactin were similar for all RNAs. The results obtained from this animal model system suggest that the chronic negative feedback action of gonadal steroids on the gonadotropic hormones is effected, in part, via inhibition of synthesis of the alpha subunit of LH and FSH.

Counteraction of gonadal steroid inhibition of luteinizing hormone release by naloxone.

The effect of naloxone on the negative feedback action of gonadal steroids on luteinizing hormone (LH) release was studied in castrated male and female rats. The reduction by estradiol benzoate or testosterone propionate of elevated serum LH levels in rats ovariectomized for 4 weeks was reversed by a single injection of naloxone. Injection of estradiol benzoate, together with progesterone, similarly reduced serum LH levels in ovariectomized rats, and this inhibition was partially reversed by naloxone. A single injection of testosterone propionate decreased serum LH levels in male rats castrated 48 h earlier, and naloxone completely blocked the androgen-induced inhibition of LH secretion. Chronic administration of either testosterone propionate or morphine sulfate to castrated male rats prevented the postcastration rise of serum LH. The decrease of hypothalamic LHRH in castrated rats was completely blocked by testosterone propionate or morphine sulfate administration. These results are believed to indicate that hypothalamic opiates are involved in gonadal steroid feedback inhibition of LH release.

Blockade of progresterone-induced increase in hypothalamic luteinizing hormone-releasing hormone levels and serum gonadotropins by intrahypothalamic implantation of 6-hydroxydopamine

The present study was undertaken to identify the hypothalamic locus where norepinephrine (NE) nerve terminals communicate with leteinizing hormone-releasing hormone (LH-RH)-containing neurons involved in the stimulatory feedback action of gonadal steroids on LH and FSH release. Ovariectomized rats received estradiol benzoate (10 μg/rat s.c.) on day 0. Intracranial implants containing either 6-hydroxydopamine (6-OHDA), to destroy NE terminals, or cocoa butter (controls) were placed bilaterally in the suprachiasmatic nucleus (SCN), medial basal hypothalamus (MBH) or olfactory tubercle (OT) on day 1. Progesterone (P, 5 mg/rat s.c.) was administered at 10.00 h on day 2 to elicit increases in serum LH and FSH and the MBH LH-RH levels in the afternoon. Implantation of 6-OHDA in the SCN resulted in a marked depletion of NE in and around the region of the SCN in the preoptic-anterior hypothalamic area (POA-AH) without adversely affecting dopamine (DA) concentrations, and blocked the P-induced afternoon increase in the MBH LH-RH and serum gonadotropin levels. Similar reduction in the MBH NE concentrations occurred following placement of 6-OHDA in the MBH; however, these as well as implants in the OT were ineffective in suppressing the P-induced effects. These studies show that functional integrity of the SCN regions is critical in manifestation of the P-induced rise in the MBH LH-RH activity, and this region in the POA-AH, therefore, may be the primary locus of synaptic communication between NE terminals and LH-RH neurons.

Data on the sites of the stimulatory feedback action of gonadal steroids indispensable for follicle-stimulating hormone release in the rat.

The sites of the stimulatory feedback action of gonadal steroids on FSH release were examined in estrogen-primed ovariectomized rats. Estradiol benzoate (E2) injected 72 h after E2 priming induced a significant increase in serum FSH 6 and 30 h later. However, injections of estrone failed to induce a similar significant increase. Progesterone (P) injections also increased serum FSH 6 h later. Horizontal sections placed above the medial preoptic area prevented the increase in serum FSH after E2 injections but not after P injections. Furthermore, retrochiasmatic sections or bilateral lesions in the medial basal part of the suprachiasmatic area prevented the increase after E2 injections and P injections. E2, implanted into the bed nucleus of stria terminalis and the anterior hypothalamic area in E2-primed ovariectomized rats, induced a significant increase and decrease in serum FSH, respectively, whereas P implanted into the same areas failed to induce it. On the other hand, P implantations into the diagonal band of Broca and the lateral and the medial septum were effective in increasing and decreasing serum FSH, respectively, whereas E2 implantations into these areas were ineffective. Neither E2 nor P implanted into the medial amygdala, the medial preoptic area, or the hippocampus altered serum FSH levels. These results suggest that the stimulatory feedback actions of E2 and P on FSH release are exerted at different sites in the limbic structures.

Data on the sites of stimulatory feedback action of gonadal steroids indispensable for luteinizing hormone release in the rat.

The sites of the stimulatory feedback action of gonadal steroids on LH release were investigated in estrogen-primed ovariectomized rats. Either estradiol benzoate (E2) or estrone (E1) injections 72 h after E2 priming induced a significant increase in serum LH 30 h later, whereas injections of progesterone (P) did so 6 h later. Horizontal sections placed above the medial preoptic area prevented the increase after E2 injections but not after E1 and P injections. Retrochiasmatic sections or bilateral lesions placed in the medial-basal part of the suprachiasmatic area prevented any increases in serum LH induced by steroid injections. Intracerebral implantations of E2 and P in E2-primed ovariectomized rats induced a similar significant increase in serum LH when E2 was implanted into the bed nucleus of stria terminalis (BST), the lateral septum (l-SEPT) and the preoptic suprachiasmatic area (POSC), and when P was implanted in to the medial amygdala, the diagonal band of Broca (DBB), the POSC, the l-SEPT and the anterior hypothalamic area (AHA). These results suggest that the stimulatory feedback effect of E2 is exerted on the limbic structures, especially the BST and the l-SEPT, while the main sites of the stimulatory feedback action of P are located in the DBB, the POSC, and the AHA.

Induction of precocious sexual maturation in female rats by electrochemical stimulation of the brain

Precocious activation of the pituitary-gonadal axis was induced, in immature female rats, by electrochemical stimulation of the hypothalamus on day 23 of life. The interval between brain stimulation and the occurrence of vaginal opening, the degree of uterine hypertrophy, changes in ovarian steroid content and the incidence of first ovulation were assessed. Based on these criteria, 3 main types of response, each having a distinct time course, could be distinguished. In rats in which the induced increase in gonadal steroid content (estrogen and progestin) was most pronounced, the interval between brain stimulation and vaginal opening was shortest (about 72 h) and the incidence of ovulation was highest. In an intermediate group, vaginal opening occurred about 96 h after brain stimulation ( i.e. 24 h later), and the incidence of ovulation was less. In rats in which estrogen production appeared to lag and ovarian progestin content remained low, vaginal opening was delayed by another 24 h, and ovulation did not occur. The results suggest that brain stimulation, like the administration of pregnant mare serum or of estrogen, leads to a premature increase in ovarian steroidogenesis and that these ovarian hormones cause precocious sexual maturation through the activation of brain areas which are sensitive to the stimulatory feedback action of gonadal steroids and which control the cyclic (ovulatory) release of gonadotrophins.

The effects of hypothalamic lesions on the release of gonadotropins and prolactin in response to estrogen and progesterone treatment in female rats.

Plasma gonadotropins and prolactin were measured in rats with hypothalamic lesions. In ovariectomized females estrogen lowered plasma FSH and LH and elevated plasma prolactin. Lesions in the median eminence of ovariectomized rats which lowered plasma gonadotropins and elevated plasma prolactin prevented these effects of estrogen. The ability of progesterone to stimulate FSH and LH release in ovariectomized, estrogenprimed rats was also eliminated by these lesions. Lesions in the suprachiasmatic region which induced constant estrus in animals with intact ovaries failed to block the inhibitory effect of estrogen on gonadotropin release and the rise in prolactin evoked by estrogen still occurred. The stimulation of LH release by progesterone was blocked by these lesions, whereas the stimulation of FSH release was unimpaired. The results are discussed in terms of the sites of the feedback actions of gonadal steroids. (Endocrinology 91: 1404, 1972)