Preoptic-suprachiasmatic Region Research Articles

Dissociation of prolactin and LH release responses after stimulation within the preoptic-suprachiasmatic region in male rats.

The relative efficiency of unilateral electrical stimuli applied within the preoptic-suprachiasmatic region (POA-Sch) to induce the release of prolactin and/or LH was studied in male rats under pentobarbital anesthesia. It was observed that unilateral stimulation with 10 Hz, 1.0 msec width pulses applied in the ventral half of the POA-Sch resulted primarily in the release of prolactin rather than LH. Fifty Hz stimuli applied at this same locus resulted also in LH release. It is concluded that the probability to dissociate the induced release of prolactin and LH from the anterior pituitary in male rats after stimulation from a given locus within the ventral POA-Sch is significantly affected by the characteristics of the applied stimulus.

Changes in Feeding Activity, Plasma Luteinizing Hormone, and Testes Weight in Ring Doves Following Hypothalamic Injections of Prolactin

Abstract Microinjections of ovine prolactin were administered unilaterally to the ventromedial hypothalamic nucleus and the preoptic-suprachiasmatic region in adult male ring doves in an attempt to determine the site(s) at which intracranial injections of prolactin act to alter feeding behaviour and gonadotropin secretion in this species. Food intake and body weight were measured daily during a 6-day pretreatment period and the 5-day treatment period that immediately followed. During the treatment period, birds received twice daily injections (0.5 mul) of either 2.5 ng ovine prolactin or saline vehicle. An additional group of birds with cannulae in the ventromedial nucleus were given twice daily injections of 25 ng ovine prolactin. Although food consumption was unaffected by low dose prolactin treatment, birds given 25 ng prolactin injections into the ventromedial nucleus showed a significant augmentation in food intake. Injections of 25 ng prolactin into the preoptic area also increased feeding; however, the magnitude of this hyperphagic response, as expressed relative to pretreatment levels, was less than that observed following prolactin injection into the ventromedial nucleus. No differences were observed between prolactin-treated and vehicle-treated birds in either cannulation group when testes weights and plasma luteinizing hormone concentrations were compared at the end of the treatment period. However, the possibility that prolactin influenced changes in luteinizing hormone and testes weight relative to baseline values could not be assessed due to constraints imposed by the experimental paradigm used. These results suggest that prolactin-sensitive neurons in the ventromedial hypothalamic region and the preoptic area are potential sites of prolactin action in promoting hyperphagia in ring doves. However, the role of these sites in mediating prolactin-induced suppression of gonadotropin secretion in this species remains to be clarified.

Thyrotropin-releasing hormone (TRH)-immunoreactive structures in the brain of the domestic mallard.

The distribution of immunoreactive thyrotropin-releasing hormone (TRH) in the central nervous system of the domestic mallard was studied by means of the peroxidase-antiperoxidase technique. After colchicine pretreatment, the highest number of TRH-immunoreactive perikarya was found in the parvocellular subdivision of the paraventricular nucleus and in the preoptic region; a smaller number of immunostained perikarya was observed in the lateral hypothalamic area and in the posterior medical hypothalamic nucleus. TRH-immunoreactive nerve fibers were detected throughout the hypothalamus, forming a dense network in the periventricular area, paraventricular nucleus, preoptic-suprachiasmatic region, and baso-lateral hypothalamic area. TRH-containing nerve fibers and terminals occurred in the organon vasculosum of the lamina terminalis and in the external zone of the median eminence in juxtaposition with hypophyseal protal vessels. Scattered fibers were also seen in the internal zone of the median eminence and in the rostral portion of the neural lobe. Numerous TRH-immunoreactive fibers were detected in extrahypothalamic brain regions: the highest number of immunoreactive nerve fibers was found in the lateral septum, nucleus accumbens, olfactory tubercle, and parolfactory lobe. Moderate numbers of fibers were located in the basal forebrain, dorsomedial thalamic nuclei, hippocampus, interpeduncular nucleus, and the central gray of the mesencephalon. The present findings suggest that TRH may be involved in hypophysiotropic regulatory mechanisms and, in addition, may also act as neuromodulator or neurotransmitter in other regions of the avian brain.

Androgens stimulate preoptic area Na +,K +-ATPase activity in male rats

This paper describes the effects of castration and testosterone replacement on the Na +,K +-ATPase activity levels of the cerebral cortex (CC), preoptic-suprachiasmatic region (POSC) and mediobasal hypothalamus (MBH) in male rats. Na +,K +-ATPase activity was estimated as the ouabain-sensitive fraction of ADP and AMP generation rate, measured by high-pressure liquid chromatography (HPLC) with UV detection, from a standard incubation mixture containing 3 mM ATP. Orchidectomy, performed 4 weeks before sacrifice, decreased ATPase activity of MBH. Testosterone propionate treatment (50 μg/day × 2 days) to castrated animals resulted in a 4-fold increase in enzyme activity in the POSC, an effect that might be related to the behavioral effects of androgens. None of the treatments seemed to influence the enzyme activity of the cerebral cortex.

Regional changes of brain Na +, K +-transporting adenosine triphosphatase related to ovarian function

Synaptosomal Na +,K +-transporting ATPase activity of the mediobasal hypothalamus (MBH), the medial preoptic-suprachiasmatic (POSC) region and cerebral cortex was measured in rats at different stages of the estrous cycle and after ovariectomy and estradiol replacement. Enzyme activity of the MBH and POSC showed cyclic changes. In both regions it increased shortly before the proestrus surge of LH. However, the two areas responded to castration and estrogen treatment in an opposite fashion. No changes were detected in enzyme activity in the cerebral cortex. These findings are consistent with previous reports on cyclic changes in electrical activity and suggest that Na +, K +-ATPase activity could be a useful indicator of neural activity for the study of neuroendocrine interactions.

The preoptic-suprachiasmatic nuclei though morphologically heterogeneous are equally affected by streptozotocin diabetes

Pituitary and gonadal disorders consistent with abnormal LHRH and LH secretion occur in streptozotocin-diabetic rats. A key role in the synthesis and regulation of LHRH and in the phasic LH release is played by the preoptic-suprachiasmatic region which is mainly formed by the medial preoptic area, the sexually dimorphic nucleus of the medial preoptic area, and the suprachiasmatic nucleus. Therefore we have studied this region by morphology and morphometry in normal and streptozotocin-diabetic rats. In normal animals, the neurons of the above mentioned nuclei were morphologically and morphometrically dissimilar. Independent of their localization, reduced cytoplasmic and nuclear areas were observed in the neurons of diabetic animals. These lesions are consistent with hypotrophied neurons. Consequently, diabetes may impair both synthesis and regulation of LHRH and may therefore account for pituitary disorders, testicular atrophy, and lacking preovulatory LH peaks. The structural differences of the neurons of the three nuclei in normal animals underline their different physiological role. Yet, the similarity of the changes found in all three nuclei suggests a generalized hypofunction of the whole preoptic-suprachiasmatic region under diabetic condition.

Neural projections of the preoptic-suprachiasmatic region to the median eminence in the rat: A quantitative autoradiographic study

The possibility of a preferential distribution of projections from the preoptic-suprachiasmatic region (POA-Sch) to the median eminence in the male rat was studied using autoradiographic and computer procedures. A cocktail of [ 3H]leucine-[ 3H]proline was injected into the POA-Sch. Animals were killed after either 1, 4, or 24 h of incorporation time (IT). The number of silver grains per 25-μm square was counted throughout the median eminence. Squares with silver grain densities at or above the 75 percentile of the population for any given IT group were labeled as “concentrating areas”. The distribution of these concentrating areas in median eminence tissue was analyzed utilizing univariate procedures. Visual inspection of the distribution of silver grain densities in a rostrocaudal fashion indicated the likelihood of uneven distribution; however, these differences were not statistically significant. In the dorsoventral plane, results indicated that the highest silver grain density was at 50 to 75 μm from the floor of the 3rd ventricle. Such distribution was apparent both ipsi- and contralateral to the injection site, but was greater on the ipsilateral side.

Retinofugal and retinopetal projections in the cichlid fish Astronotus ocellatus.

The retinofugal and retinopetal projections of the cichlid fish Astronotus ocellatus were studied by applying cobaltous-lysine to the optic nerve. Retinal axons terminate bilaterally in a preoptic-suprachiasmatic region between the base of the third ventricle and the anterior genu of the horizontal commissure and among periventricular cells along the sides of the ventricle. Other retinal axons innervate the tuberal region of the hypothalamus anterior to the infundibulum. Targets innervated in the pretectum include nucleus lateralis geniculatus and dorsal, medial, and ventral pretectal nuclei. Three other targets (nucleus opticus dorsolateralis, nucleus opticus commissurae posterior, nucleus opticus ventrolateralis) are innervated by fibers that leave the medial edge of the dorsal optic tract. Two other targets (basal optic nucleus and accessory optic nucleus) are innervated by fibers from the ventral optic tract. These retinal projections are similar to those previously reported for goldfish in an experiment that used the cobaltous-lysine method (Springer and Gaffney, J. Comp. Neurol. 203:401-424, '81). Retinotectal optic axons were found in a superficial lamina just above the stratum opticum, in the stratum opticum, in three layers of the stratum fibrosum et griseum superficiale, in a lamina just beneath the stratum fibrosum et griseum superficiale, and in the stratum album centrale just above the stratum periventriculare. This result is similar to that previously reported for goldfish; however, the spatial relationships between the various retinorecipient laminae differ for goldfish and Astronotus ocellatus. Efferents to the retina originate in two nuclei and both project contralaterally. The first is the nucleus olfactoretinalis, which is located ventrally between the olfactory lobe and telencephalon. It consists of about 400 cells, of which, approximately 200 cells project to the retina. The second retinopetal nucleus, nucleus thalamoretinalis, is a diffuse group of about 200 cells that project to the retina.

Changes in beta-endorphin content in discrete areas of the hypothalmus throughout proestrus and diestrus of the rat

The aim of the present study is to investigate changes in β-endorphin content in the hypothalamus during different stages of the estrous cycle. Groups of 9 to 10 Sprague-Dawley rats were sacrificed every two hours on proestrus from 8.00 to 18.00 h and groups of 7 to 8 rats were sacrificed on diestrus at 8.00, 12.00, 14.00 and 18.00 h. Preoptic suprachiasmatic region, posterior hypothalamus, arcuate nucleus and median eminence were dissected and assayed for β-endorphin. A significant increase in β-endorphin content was detected in the arcuate nucleus during proestrus (9.00 h: 1.76 ± .31; 14.00 h: 4.10 ± .85 μg/g tissue wet weight). Levels did not change during diestrus (1.18 ± .06 μg/g). The increase caused significant differences in β-endorphin values between both days at 12.00, 14.00 and 18.00 h, while the concentrations at 8.00 h were similar. The opposite pattern was observed in the median eminence with significantly higher proestrous β-endorphin levels at 8.00 h (11.24 ± 3.1 vs 3.52 ± .64 μg/g) and nonsignificant differences for the rest of the day. No significant change in β-endorphin concentration was seen in the preoptic suprachiasmatic region over the day of proestrus (1.35 ± .09 μg/g). Diestrous β-endorphin concentrations in this region were higher during the morning (2.60 ± .65 μg/g) and lower at 18.00 h (0.94 ± .12 μg/g) when compared to proestrous values. This pattern was caused by a 50% increase in β-endorphin during the afternoon of diestrus. No changes were observed in the posterior hypothalamus on either day with comparable levels of β-endorphin except at 18.00 h, when values were significantly higher on proestrus (1.66 ± .30 vs 0.83 ± .06 μg/g).

Prolactin and LH release induced by kainic acid administration within the preoptic-suprachiasmatic region in behaving male rats.

Adult male rats were chronically implanted with a multiple metal cannula assembly to allow for the administration of kainic acid within the POA-SCH region. Kainic acid was applied in its solid form at two dose levels: 2 micrograms and 4 micrograms. A chronic heart cannula implanted through the external jugular vein allowed for sequential blood sampling. Animals were subjected to two different regimes, with 2-3 days difference, one in which blood sampling took place without kainic acid administration, and a second one in which the drug was administered after a first, control sample was obtained. Administration of kainic acid at both dose levels induced motor hyperactivity that lasted for more than 1 h in the group that received the higher dose. Prolactin concentration in plasma rose by 30 min and declined thereafter in the "low" dose group, or remained high for at least 90 min in the "high" dose group. LH levels increased significantly only after administration of the "high" dose. These results are indicative of the presence of facilitatory elements for the release of prolactin (and LH) which originate within the POA-SCH region, probably involving glutamatergic receptors. Stimulation of these elements, rather than passing fibers, are probably responsible for the observed hormonal results.

Cathodal versus anodal stimulation within the preoptic-suprachiasmatic region in male rats: Effects on the release of prolactin and luteinizing hormone

The relative efficiency for the release of luteinizing hormone (LH) and prolactin after anodal and cathodal stimulation applied within the preoptic-suprachiasmatic region (POA-SCH) was studied in adult male rats under pentobarbital anesthesia. Single monopolar pulses 0.5 ms duration, 100 or 200 μA intensity, were applied within the dorsal [vertical (V) 7.3] or ventral (V 7.8 to 7.9) aspects of the POA-SCH at a mean rate of 10 Hz in 30-s trains every other 30 s for a period of 30 min. Blood samples were obtained at 30-min intervals starting immediately before onset of stimulation. Although there was no obvious effect on plasma LH titers, prolactin concentrations tended to be higher when cathodal stimulation was applied at V 7.9 and when anodal pulses were applied at V 7.3. Stimulation with paired twin pulses (1.0 ms duration, 20.0 ms intrapair interval) resulted in clearer hormonal effects, namely, a significant increase in plasma prolactin concentration at 30 min only when cathodal stimuli were applied to the ventral aspect of the POA-SCH. Plasma LH concentrations were higher also at 30 min in the same experimental group, although the increase was not statistically significant. These results indicate the effectiveness of cathodal stimuli to induce hormonal release when applied within the ventral half of the POA-SCH in male rats. Present results suggest the existence of predominantly facilitatory neural elements for the release of prolactin (and LH) in this brain region.

Immediate prolactin release after pericommissural deafferentation in behaving male rats

Adult male rats received a chronic midline implant of Halász-type knife placed at the level of the anterior commissure. A chronically implanted heart cannula allowed for sequential blood sampling in unrestrained conditions. Anterior knife rotation of 180° induced a significant increase of plasma prolactin concentration in 30 min. Plasma LH levels remained within control levels. It is suggested that neural projections dorsally entering the preoptic-suprachiasmatic region are significantly involved in the control of prolactin release in male rats.

Prolactin release induced by paired-pulsed stimulation within the preoptic-suprachiasmatic region in male rats: Response to varying interpulse intervals

Adult male rats under pentobarbital anesthesia received bilateral electrical stimuli within the preoptic-suprachiasmatic region. Stimulation consisted of paired identical pulses with the following characteristics: monophasic, 100 or 200 μA, 0.5-ms width, 30-s trains on/off applied during 30 min. Several interpulse intervals were tested in different groups of animals: 1.0, 2.5, 5.0, 20.0, and 50.0 ms. Twin pulses were delivered at a mean frequency of 10 Hz through bipolar concentric electrodes. Blood samples were obtained every 30 min starting before the onset of stimulation for a total period of 90 min. Luteinizing hormone (LH) and prolactin concentrations were determined by radioimmunoassay. The LH concentrations at 30 min after stimulation with interpulse intervals of 20.0 and 50.0 ms were significantly higher than control values. Prolactin concentration was higher than in controls at 30 min for all stimulated groups. Prolactin responses did not show significant differences among the various interpulse intervals tested. These results are discussed in terms of the possible differences in the overall neural strategies controlling the release of each hormone.

Efferent connections of the rat suprachiasmatic nucleus

The efferent projections of the suprachiasmatic nucleus were studied in rats using three different light-microscopic tracing techniques. These techniques included, (1) degeneration subsequent to lesions of the nucleus, (2) autoradiography of axoplasmically-transported molecules following injections of [ 3H]leucine into the nucleus, and (3) autoradiography of molecules which had been transneuronally-transported following injections of [ 3H]fucose and [ 3H]proline into the eye. Analysis of this material demonstrated that the suprachiasmatic nucleus has sparse but widespread efferent projections that extend ipsilaterally and sometimes bilaterally throughout much of the length of the periventricular system. With some exceptions, these projections tend to be confined within 0.4 mm of the midline. Four major efferent pathways were identified. (1) A ventro-caudal projection to the retrochiasmatic area, arcuate nucleus, median eminence, the ventro-medial aspect of the dorsomedial and ventromedial hypothalamic nuclei, baso-lateral hypothalamus, prelateral mammillary nucleus and lateral interpeduncular zone. A dorsal branch of this pathway followed the periventricular system into the central grey. (2) A lateral pathway to the anterior hypothalamus, a region dorsal to the supraoptic nucleus and the supraoptic nucleus. Some lateral projections may reach the anterior amygdaloid area. (3) A dorsal pathway to the paraventricular nucleus, a rostral branch to the fimbrial and triangular septal nuclei and a caudal branch via the paraventricular thalamus to the habenula, and further caudally to the mesencephalic central grey. These dorsal projections were not labeled by transneuronal transport. (4) A sparse rostral pathway through the preoptic-suprachiasmatic region. These results agree with and extend the results of other investigators. In addition, the diffuse and extensive pattern of projections of the suprachiasmatic nucleus as demonstrated here is consistent with the suggestion that the nucleus is a major component of a system of circadian pacemakers whose destruction disrupts or abolishes the circadian rhythms of a wide range of different behaviors.

Neural control of prolactin release in male rats: A possible inhibitory pericommissural component

Adult male rats under pentobarbital anesthesia received different types of “roof” knife cuts at the level of the anterior commissure. The cutting blade was 1.5 mm long and L-shape. Plasma prolactin concentrations were monitored every 30 min starting immediately before the lesion for a total period of 90 min. Mean prolactin values did not show significant changes in the following groups: no knife rotation, posterior rotation at a depth of 7.5 mm (subcommissural), and posterior rotation at a depth of 6.5 mm (supracommissural). On the other hand, anterior knife rotation at 6.5 mm (supracommissural) resulted in an immediate, significant increase in the plasma prolactin concentration 30 min after the lesion. Hormone concentrations were still high at 60 min. The results suggest the existence of significant projection components passing through and/or making synaptic contact at the level of the preoptic-suprachiasmatic region with elements involved in the (inhibitory) control of prolactin release in the male rat. These projections may take origin in the amygdala and/or hippocampus.

Intracellular transport in neurons.

Intracellular transport in neurons.

Effects of discrete lesions of preoptic and suprachiasmatic structures in the female rat. Alterations in the feedback regulation of gonadotropin secretion.

The purpose of this study was to determine which specific structures within the medial preoptic-anterior hypothalamic area are necessary to maintain cyclic ovulation in the rat, and to define the deifcit(s) in the feedback regulation of gonadotropin secretion associated with lesions that result in anovulation. Large (approximately 1.1 mm dia.) or small (approximately 0.7 mm dia.) electrolytic lesions were placed in several loci within preoptic, anterior hyopthalamic and suprachiasmatic areas in regularly cycling adult female rats. Large lesions which included the suprachiasmatic nuclei (SCN) induced an anovulatory condition characterized by persistent vaginal cornification and polyfollicular ovaries (persistent estrus). Large or small lesions which included the medial preoptic nucleus (MPN), a small periventricular column of cells located immediately caudal to the organum vasculosum of the lamina terminalis (OVLT), also induced persistent estrus. Lesions placed elsewhere within the medial preoptic-anterior hypothalamic area never induced persistent estrus but were frequently associated with repeated periods of prolonged diestrus separated by brief periods of vaginal cornification. These prolonged diestrous intervals appeared to be related to spontaneous luteal activation following ovulation rather than impaired folliculogenesis. LH and FSH surges induced by sequential administration of estradiol benzoate and progesterone (P) were completely abolished only by lesions which included the MPN. Small lesions involving only the MPN and OVLT or the MPN and caudally adjacent loci in the suprachiasmatic region were as effective in this respect as larger lesions encompassing most of the preoptic-suprachiasmatic region from the diagonal band of Broca to the rostral pole of the SCN. On the other hand, P-induced gonadotropin surges were never completely blocked by SCN lesions, although the magnitude of the surge was highly variable and frequently attenuated compared to controls. It is concluded that both the MPN and SCN are required for the long-term maintenance of spontaneous cyclic ovulation in the rat. However, the characteristically dissimilar deficits in P-induced gonadotropin release associated with lesions of one or the other of these structures indicate that these nuclei may play different roles in the regulation of gonadotropin surges. It is suggested that neural elements indispensable for phasic gonadotropin release are located within and/or immediately adjacent to the MPN. The SCN may influence phasic gonadotropin release indirectly, by regulating circadian rhythms which govern the responsiveness of other neural elements to hormonal stimuli.

Regional Differences in Response to Electrical Stimulation within the Medial Preoptic-Suprachiasmatic Region on Blood Luteinizing Hormone Levels in Ovariectomized and Ovariectomized, Estrogen-Primed Rats*

The effect of electrical stimulation of the medial preoptic-suprachiasmatic nucleus (MPOA-SCN) region of the forebrain on blood LH levels was studied in ovariectomized, pentobarbital-anesthetized rats. Animals were either not primed with estrogen or previously given 5 micrograms estradiol benzoate (EB)/100 g BW . day for the 2 days before stimulation. Rats were bled continuously (30, 40, or 50 microliter whole blood/5--6 min) through indwelling right atrial cannulae for 1.5 h before stimulation, 1.5 h during stimulation, and up to 1 h afterwards. Whole blood was analyzed for LH by RIA. In ovariectomized, unprimed rats, electrical stimulation of the ventral MPOA consistently increased LH release. In contrast, stimulation of the peri-SCN region (immediately caudal to the MPOA, and lateral and dorsal to, but not within, the SCN), uniformly inhibited pulsatile LH secretion. Activation of the SCN elevated blood LH levels in most unprimed rats tested, but suppression of episodic LH release occasionally occurred. Pretreatment with estrogen resulted in increased LH secretion in response to stimulation of each of these three regions. Estrogen prolonged the LH increase occurring during MPOA stimulation, completely reversed the inhibitory LH response to peri-SCN stimulation, and either reversed any possible inhibitory response to SCN stimulation or greatly increased the magnitude and duration of the increase in blood LH levels produced in the unprimed rat during activation of this nucleus. In summary, the present study indicates that well defined areas in the MPOA-SCN region can have strikingly different effects on LH secretion in the absence of ovarian estrogen and that this steroid is critically important in determining the direction, magnitude, and duration of the LH response to a localized brain stimulus.

Sex differences in prolactin release after stimulation of the preoptic-suprachiasmatic region at selected pulse frequencies in castrated rats

Long-term adult castrated rats of either sex were used. The preopticsuprachiasmatic (POA-SCH) region was stimulated bilaterally with 200-μA monophasic pulses of 0.5-ms width, in trains of 30 s on :30 s off, and at 10 or 50 Hz during 30 min under pentobarbital anesthesia. Sequential blood samples were taken starting immediately before stimulus onset and thereafter every 30 min during a 90-min period. Plasma luteinizing hormone (LH) and prolactin were measured by radioimmunoassay in the same sample. LH concentrations in control samples in both sexes were significantly reduced compared to values obtained in trunk blood after decapitation of unanesthetized rats. Stimulation at 10 Hz induced a significant rise in LH values in both sexes, with similar response profiles. Stimuli of 50 Hz delivered only in females resulted in LH values comparable to those obtained after stimuli applied at 10 Hz. Maximum average responses had values similar to those obtained in decapitated animals. In male rats, 10-Hz stimulation resulted in a significant prolactin increase. A similar stimulation in females was not effective in inducing significant prolactin changes, but stimuli delivered at 50 Hz resulted in a significant, brief increase, with a response profile different from that obtained in males. Current results indicate (a) the efficiency of “low”-frequency stimulation of the POA-SCH region to induce the release of LH and/or prolactin in castrated animals; (b) the similarity in the LH response profiles in both sexes; (c) the existence of sex differences in the prolactin release response to electrical stimulation of the POA-SCH region; and (d) the possibility of affecting differentially LH and prolactin release control mechanisms based on the temporal pattern of electrical stimuli.

Effects of estradiol-17beta on the induction of gonadotropin release by electrical stimulation of the hypothalamus in rhesus monkeys.

Serum LH and FSH were measured at 60, 30, and 0 min before, at 5, 15, and 30 min during, and at 10, 45, and 90 min after bilateral electrical stimulation (ES) of various hypothalamic regions in 12 unanesthetized ovariectomized rhesus monkeys. ES of the arcuate-ventromedial nuclei (medial basal hypothalamus; MBH) induced a prompt increase in serum LH that persisted throughout stimulation and returned to basal levels within 90 min thereafter. FSH was also released, but the release was slower and less dramatic than that of LH. Sham stimulation (0muA) caused no change in serum gonadotropins. The amount of LH released after MBH-ES depended upon current strength (1.0 mA greater than 0,5 or 0.7 mA). Three sequential 30-min MBH-ES trials at 90-min intervals induced comparable LH responses and 3 h of continuous MBH-ES maintained elevated serum LH levels throughout the stimulation period, suggesting that these stimulation period, suggesting that these stimulation parameters did not completely deplete pituitary stores of releasable LH. The character of the LH response was similar in individual monkeys through 3 to 24 trials during 4 to 18 months. Comparisons were made of the effects of estradiol-17beta (E2) treatment at different doses and for different intervals of time before MBH-ES. ES-induced LH release was not affected by low levels (25 and 55 pg/ml) ofE2 for 48 h, but was reduced by higher E2 concentrations (100 or 230 pg/ml). E2 concentrations of 100 pg/ml had no effect at 24 h, but reduced MBH-ES-activated LH release at 48 to 96 h; the degree of depression was time-related (48 h less than 72 h less than 96 h). ES of the preoptic-suprachiasmatic region (rostral hypothalamus; RH) in non-E2-treated monkeys also released LH, but this increase was less than after MBH-ES. FSH release was not measurable after RH-ES. In contrast to the depressed LH response to MBH-ES after 48 h of E2 (100 pg/ml), the response to RH-ES was not inhibited by this E2 regimen. These data suggest that ES of an area extending caudally from the rostral hypothalamus to the arcurate-median eminence region will evoke LH release in rhesus monkeys. This electrically induced gonadotropin release was affected by administration of physiological levels of E2 but the nature of effect depended on the specific region stimulated: distinct inhibition of the gonadotropic response to MBH-ES and slight facilitation of the response to RH-ES.