Arcuate Nucleus-median Eminence Region Research Articles

Inhibition of Metastin (Kisspeptin-54)-GPR54 Signaling in the Arcuate Nucleus-Median Eminence Region during Lactation in Rats

Follicular development and ovulation are suppressed during lactation in various mammalian species, mainly due to the suppression of pulsatile GnRH/LH secretion. Metastin (kisspeptin-54), a KiSS-1 gene product, is an endogenous ligand for GPR54, a G-protein-coupled receptor, and suggested to play a critical role in regulating the gonadal axis. The present study therefore aims to determine whether metastin (kisspeptin-54)-GPR54 signaling in discrete brain areas is inhibited by the suckling stimulus that causes suppression of LH secretion in lactating rats. Quantitative RT-PCR revealed that the KiSS-1 mRNA level was significantly lower in the arcuate nucleus (ARC)-median eminence region in lactating ovariectomized (OVX) and estrogen-treated OVX rats than in nonlactating controls. KiSS-1 mRNA in the anteroventral periventricular nucleus was kept at a low level in both lactating and nonlactating rats despite estrogen treatment. GPR54 mRNA levels were significantly lower in lactating than nonlactating rats in the anteroventral periventricular nucleus, but the levels in lactating mothers of the preoptic area and ARC-median eminence were comparable with nonlactating controls. Although KiSS-1 mRNA-expressing cells or metastin (kisspeptin-54) immunoreactivities were densely located in the ARC of nonlactating controls, few were found in the ARC of lactating OVX animals. Various doses of metastin (kisspeptin-54) (0.02, 0.2, and 2 nmol) injected into the third ventricle caused a significant increase in LH secretion in both lactating and nonlactating OVX rats, suggesting that lactating rats are responsive to metastin (kisspeptin-54) stimulus. Thus, the present study demonstrated that KiSS-1 mRNA/metastin (kisspeptin-54) expression is inhibited in the ARC by the suckling stimulus, suggesting that the inhibition is most probably involved in suppressing LH secretion in lactating rats.

Distribution of Corticotropin‐Releasing Factor Binding Protein‐Immunoreactivity in the Rat Hypothalamus: Association With Corticotropin‐Releasing Factor‐, Urocortin 1‐ and Vimentin‐Immunoreactive Fibres

Corticotropin-releasing factor binding protein (CRF-BP) is a 37-kDa protein with high affinity binding sites for both corticotropin-releasing factor (CRF) and urocortin 1. Previous studies have examined the distribution of CRF-BP mRNA and peptide within the central nervous system. Due to the predominant cortical localisation, very little is known about CRF-BP in subcortical structures including the hypothalamus. The present study employed immunohistochemistry to characterise the distribution of CRF-BP-like-immunoreactive (-ir) cells and fibres in the rat hypothalamus. Bipolar and multipolar CRF-BP-ir neurones were scattered throughout the rostro-caudal extent of the hypothalamus. Distinct clusters of CRF-BP-ir neurones were identified in the anterior and posterior parvocellular and dorsal cap subdivisions of the paraventricular nucleus (PVN), as well as in the dorsal hypothalamic area, dorsomedial hypothalamic nucleus (DMN), ventral premammillary nucleus and zona incerta. CRF-BP-ir fibres extending from the third ventricle were found in the mediobasal hypothalamus and within the arcuate nucleus-median eminence region. Double immunostaining together with confocal microscopy demonstrated that the CRF-BP-immunostained fibres within the mediobasal hypothalamus coincided with vimentin immunostaining indicating that CRF-BP-ir is present within tanycytes. To define the relationship between CRF-BP-ir cells and endogenous ligands for CRF-BP, double immunohistochemistry was performed to examine possible sites within the hypothalamus where CRF- or urocortin 1-ir fibres innervate regions that contain CRF-BP-ir cell bodies. CRF-BP-ir cell bodies typically coincided with dense CRF-ir, but not urocortin 1-ir fibre innervation. CRF-ir fibre innervation was moderate to high within the anterior and posterior parvocellular subdivisions of the PVN, the dorsal cap of the PVN, DMN and the zona incerta; all regions that contained CRF-BP-ir cell populations. These studies demonstrate that, within the hypothalamus, CRF-BP-ir cells and fibres are concentrated within a circuitry known to be involved in mediating neuroendocrine and autonomic responses to stress.

Effect of season on neuropeptide Y and galanin within the hypothalamus of the ewe in relation to plasma luteinizing hormone concentrations and the breeding season: an immunohistochemical analysis.

Within the hypothalamus, neurones that express neuropeptide Y (NPY) and galanin have been implicated in the regulation of gonadotropin-releasing hormone (GnRH) and gonadotropin secretion. We aimed to determine the extent to which the expression of these two neuronal systems is linked to the seasonal reproductive cycle, and the effect of chronic oestrogen treatment. Immunohistochemical analysis was used to examine changes between the breeding season and anestrus in ovariectomized (OVX) ewes with or without oestrogen treatment (s.c. implants for 2 weeks). Serial blood sampling established plasma luteinizing hormone (LH) profiles, and the ewes were subsequently killed and the brains perfused for immunohistochemistry. In OVX ewes, the amplitude of LH pulses was greater in the nonbreeding season than in the breeding season. Oestrogen treatment caused a marked reduction in plasma LH concentrations during anestrus, but not in the breeding season. The number of cells in the arcuate nucleus/median eminence region (ARC-ME) that stained for NPY was lower in ewes killed in anestrus (September) than in ewes killed in the breeding season (May), but there was no seasonal change in the number of galanin-stained cells. Within season, oestrogen treatment did not affect NPY- or galanin-cell number. There was no effect of season or oestrogen on the area of varicose fibres/terminals for either peptide in the ARC-ME, but galanin immunostaining was more intense during the breeding season. We conclude that the amount of NPY in cell bodies of the ARC-ME is lower in ewes in the nonbreeding season; this could reflect a steroid-independent effect of photoperiod. We also conclude that the long-term negative-feedback effect of oestrogen on GnRH/LH secretion does not appear to be mediated by NPY- or galanin-containing neurones in the ewe.

Neuroglial Responses to Elevated Glutamate in the Medial Basal Hypothalamus of the Infant Mouse

Elevated plasma glutamate can cause selective loss of neurons in the brains of infant mice. The arcuate nucleus-median eminence region exhibits the greatest sensitivity to glutamate while it undergoes developmental maturation during early postnatal life. To investigate glutamate-induced cellular responses, groups of nursing 7-d-old mice (n = 31-93) were given single subcutaneous injections of 0.1-0.5 mg monosodium glutamate (MSG)/g body wt or an equivalent volume (30-50 microL) of water vehicle (n = 93). Injection of 0.2 mg MSG/g body wt produced a 16-fold rise in plasma glutamate after 15 min (2.10 vs. 0. 122 mmol/L control) and was the lowest harmful dose tested. It not only induced injury of small bilateral groups of medial basal hypothalamic neurons at 5 h postinjection, but also enhanced their expression of the N-methyl-D-aspartate (NMDA)R1 glutamate receptor subunit. Higher dosages of 0.3-0.5 mg MSG/g body wt yielded dose-related increases in NMDAR1 staining intensity and larger numbers of damaged neurons within the ventromedial arcuate nucleus. Administration of the live-cell nuclear stain bis-benzimide (0.95 micromol/L) indicated that MSG accessed the entire brain (n = 20) and methylene blue (1.0 g/L) permeated extracellular spaces by 15 min postinjection (n = 19), before cell death was evident (0.75 mmol/L propidium iodide) from co-injected MSG. Immunostaining, which mimicked that for glial fibrillary acidic protein, suggested that glutamate was retained in tanycytes. We conclude that elevated plasma glutamate induces glutamate receptor expression during selective injury of ventromedial arcuate neurons and propose that by sequestering glutamate, tanycytes may amplify local concentrations and promote neuronal damage in infant mice.

Short communication

Orexin A and orexin B are a newly described family of orexigenic hypothalamic neuropeptides. The distribution of orexin immunoreactive fibers overlaps with the luteinizing hormone-releasing hormone (LHRH) neuronal system in the septo-preoptic area and the arcuate nucleus-median eminence region. To test the hypothesis that orexins may be involved in the regulation of pituitary LH secretion by influencing LHRH release, the effects of orexin A and orexin B on LH secretion were evaluated in ovariectomized (ovx) and ovarian steroid-treated ovx rats. Intracerebroventricular injection of orexin A or orexin B rapidly stimulated LH secretion in a dose- (0.3 and 3.0 nmol) and time-related fashion in estradiol benzoate- (EB) and progesterone (P)-pretreated ovx rats. Peak plasma LH concentration at 10 min were significantly higher in orexin A- than in orexin B-injected rats. On the other hand, both orexins (3 nmol) inhibited LH release in unprimed ovx rats. This ovarian steroid-dependent bimodal LH response is reminiscent of that of other orexigenic neuropeptides, such as neuropeptide Y. These findings show that orexins belong to a group of hypothalamic messengers that neurochemically bridge the regulatory networks that controls reproduction and energy balance.

Monosynaptic pathway between the arcuate nucleus expressing glial type II iodothyronine 5'-deiodinase mRNA and the median eminence-projective TRH cells of the rat paraventricular nucleus.

Recent evidence suggests that the thyroid regulation of thyrotropin-releasing hormone (TRH)-containing neurons in the paraventricular nucleus of the hypothalamus involves the activation of other hypothalamic neural circuits. For example, the arcuate nucleus and not the paraventricular nucleus contains the highest enzyme activity of 5'-deiodinase type II, an enzyme that is pivotal for the local synthesis of T3. This experiment was undertaken to demonstrate whether a monosynaptic pathway exists between the arcuate nucleus and those TRH cells of the paraventricular nucleus that are neuroendocrine, i.e. project to the external layer of the median eminence. A specific cRNA probe derived from the coding region of deiodinase type II was used for the in situ hybridization histochemistry which was combined with immunocytochemistry for a specific marker of glial cells, glial fibrillary acidic protein (GFAP). The hybridization signals were present within the hypothalamus in the arcuate nucleus-median eminence region and in the periventricular area. The periventricular labeling was localized to the ependymal layer of the third ventricle and no hybridization product was detected in the paraventricular nucleus and other hypothalamic nuclei adjacent to the third ventricle. Within the median eminence, numerous cells containing the hybridization product were located in the internal layer adjacent to the floor of the third ventricle and in the external layer adjacent to the surface of the brain. In the dorso- and ventromedial regions of the arcuate nucleus, deiodinase type II mRNA-containing cells were also detected. Numerous type II deiodinase mRNA-containing cells in the median eminence and arcuate nucleus were also found to be immunopositive for GFAP. The abundance of arcuate cells expressing the hybridization product was lower than those in the periventricular region or in the median eminence. The anterograde tracer, Phaseolus vulgaris leucoagglutinin, was injected into the medial parts of the arcuate nucleus where the in situ hybridization experiment detected deiodinase type II mRNA. Simultaneously with the anterograde tracing, the retrograde tracer, Fluoro-Gold, was injected into either the median eminence or the general circulation. Light and electron microscopic double and triple immunolabeling experiments on vibratome sections of colchicine-pretreated animals revealed that arcuate fibers innervate TRH cells within the parvicellular region of the paraventricular nucleus. Populations of these TRH cells receiving afferents from the arcuate nucleus were also retrogradely labelled from either the median eminence or the general circulation indicating their direct role in the regulation of thyrotropin secretion from the anterior pituitary. The majority of arcuate nucleus efferents on TRH cells were found to establish symmetrical synaptic connections. The present results provided direct evidence of a monosynaptic pathway between the hypothalamic site of local thyroid hormone production, the arcuate nucleus, and neuroendocrine TRH cells in the paraventricular nucleus. This signalling modality may play an important role in thyroid feedback on TRH cells. Since the arcuate nucleus is involved in the regulation of central mechanisms controlling diverse homeostatic functions, including reproduction and feeding, the pathway described in this study may also carry integrated signals related to reproduction and ingestion to TRH-producing cells.

Hypothalamic electrical activity that relates to the pulsatile release of luteinizing hormone exhibits diurnal variation in ovariectomized rats

The multiunit activity (MUA) was recorded in the arcuate nucleus-median eminence region in freely moving ovariectomized rats maintained under controlled lighting conditions (lights on 05.00–19.00 h) over a period of 24 h. The MUA of each rat had been confirmed to exhibit characteristic increases (volleys) in association with pulses of luteinizing hormone. In 6 MUAs examined, the frequency of MUA volleys was significantly greater in the dark period than in the light period, suggesting a diurnal variation in the release of luteinizing hormone-releasing hormone in the ovariectomized rat.

Naloxone Increases the Frequency of the Electrical Activity of Luteinizing Hormone-Releasing Hormone Pulse Generator in Long-Term Ovariectomized Rats

Characteristic increases in neuronal activity accompanied by the initiation of each luteinizing hormone (LH) pulse have been successfully recorded in freely moving ovariectomized rats by means of multiunit activity (MUA) recording techniques. In the present study, the effect of the opioid receptor antagonist naloxone on this neuronal activity was examined in rats ovariectomized for 6-10 weeks. Electrodes were chronically implanted into the medial basal hypothalamus (n = 78) or the medial preoptic area (n = 29). During the MUA recording, blood samples were taken through an indwelling atrial cannula at 3- or 6-min intervals to determine the serum LH concentration. Naloxone was administered intravenously for 1 h by either intermittent injection every 6 min (0.05 mg/kg, 10 times) or continuous infusion (0.1, 0.2 and 0.5 mg/kg/h). Explosive rises in the MUA (volleys) associated with the initiation of LH pulse were recorded in 7 animals in which the tips of electrodes were located in the arcuate nucleus-median eminence region. In 5 animals studied, the mean (+/- SE) duration and interval of the MUA volleys were 2.1 +/- 0.1 and 22.1 +/- 0.9 min, respectively. Naloxone given every 6 min significantly increased the duration to 2.6 +/- 0.1 min, and decreased the interval to 9.5 +/- 0.9 min. This naloxone treatment disturbed the clear pulsatility of LH secretion observed in the pretreatment control period. Analysis of the effects of naloxone infused at 3 different doses revealed that the facilitatory effects of naloxone on the duration and frequency of the MUA volleys were dose-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Estrogen decreases rat hypothalamic proopiomelanocortin messenger ribonucleic acid levels.

Ovariectomy and estrogen (E) or E plus progesterone treatment has previously been shown to alter both hypothalamic content and portal plasma levels of beta-endorphin. To determine if these changes were accompanied by changes in beta-endorphin synthesis, we used a RNA dot blot method to quantify proopiomelanocortin (POMC) mRNA levels in the arcuate nucleus-median eminence region of rats. Animals were bilaterally ovariectomized, implanted with Silastic capsules containing E or oil, and killed 1 or 3 days after implantation. Total nucleic acid was isolated from dissections of the arcuate-median eminence by proteinase-K/sodium dodecyl sulfate/phenol extraction, and POMC mRNA was quantified by dot blot analysis. Although 1 day of E treatment had no effect on hypothalamic POMC mRNA levels, 3 days of E treatment caused a significant reduction of approximately 40% of POMC mRNA levels relative to oil controls in two replicate experiments. These results suggest that the decreases in hypothalamic POMC peptide levels after E administration reported previously may be due to a decrease in POMC peptide biosynthesis resulting from a decrease in hypothalamic POMC mRNA.

Immunocytochemical localization of TRH and autoradiographic determination of 3H-TRH-binding sites in the arcuate nucleus-median eminence of the rat.

Immunocytochemical and autoradiographic localization of thyrotropin-releasing hormone (TRH)- and 3H-TRH-binding sites was studied in the arcuate nucleus-median eminence region of the rat. TRH-like immunoreactivity was found in dense granular vesicles (90-140 nm in diameter) in TRH-like immunoreactive nerve fibers and terminals. In the median eminence, the immunoreactive terminals were observed to be in direct contact with the perivascular basal lamina of the portal vessel and to form synaptoid contacts with tanycytes. In the arcuate nucleus, the immunoreactive terminals were often found to form axosomatic and axo-axonic, and/or axo-dendritic synapses. The uptake of tritiated TRH into the nerve fibers and terminals of the median eminence was also observed by autoradiography and the distribution and localization of silver grains in them were analyzed quantitatively by circle analysis. Thirty minutes after intraventricular infusion of 3H-TRH, radioactive labeling occurred in type-2 and 3-nerve fibers and terminals containing dense granular vesicles in the median eminence. It is therefore suggested that the neurons labeled after 3H-TRH infusion possess certain functions as physiological recognition sites or receptors for TRH.

Immediate, preferential prolactin release after discrete brain lesions in male rats

Adult male rats under pentobarbital anesthesia received electrolytic lesions in various sites within the preoptic-hypothalamic complex. Anodic direct current (500 μA, 30 s) was delivered through platinum electrodes. Blood samples were taken before and after the lesions, at 30-min intervals. LH, TSH, and prolactin were measured by RIA procedures in the same plasma sample. LH and TSH mean values did not show any significant increase after discrete lesions in several brain regions. On the contrary, prolactin increased significantly 30 min after a lesion in a region encompassing the dorsal preoptic area and bed nucleus of the stria terminalis. Lesions placed in other preoptic-hypothalamic regions (excluding the arcuate nucleus-median eminence region) did not result in prolactin release. A common effective region for the control of the preferential release of pituitary prolactin in the male rat seemed to include neural elements within the dorsal preoptic area and/or the bed nucleus of the stria terminalis which probably projected caudally and ventrally toward the arcuate nucleus-median eminence region. Whether this contribution is locally generated within the dorsal preoptic area-bed nucleus stria terminalis region and/or is mediated by incoming fibers remains to be established. Results are discussed in terms of possible local effects of the induced lesion, i.e., activation vs. suppression.

The localization of projection neurons in the rat hypothalamic paraventricular nucleus following vascular and neurohypophysial injections of HRP

Projection neurons in the rat hypothalamic paraventricular nucleus (pv) were identified using the retrograde transport of horseradish peroxidase (HRP) following injections of this tracer into the neurohypophysis or jugular vein. The objective was to determine if parvocellular pv neurons might be labelled by either injection in addition to the well known magnocellular contingent. Injections confined to the neurohypophysis resulted in the labelling of only magnocellular pv neurons, while intrajugular injections which produced spread of the HRP to the arcuate nucleus-median eminence region resulted in the labelling of both parvocellular and magnocellular pv elements. Thus parvocellular pv neurons may project to the arcuate nucleus and/or the external layer of the median eminance but not to the neurohypophysis.

Distribution of LHRH in the rat and mouse brain with special reference to the tanycytes

The distribution of luteinizing hormone-releasing hormone (LHRH) was studied in the rat and mouse brain by means of light and electron microscopic immunohistochemistry using the peroxidase-antiperoxidase method. An immunoreactive product to LHRH antiserum was found near the blood vessels of the vascular organ of the lamina terminalis. In the arcuate nucleus-median eminence region, an immunoreactive material occurred bilaterally in the hypothalamic tissue around the tuberoinfundibular sulci. Electron microscopy revealed that immunoreactive fibers observed light microscopically contain numerous granules 100--130 nm in diameter. No immunoreactive product was located in the tanycytes of the median eminence, the perikarya of hypothalamic neurons, and the parenchyma of several circumventricular organs (subfornical organ, subcommissural organ, pineal organ, area postrema).

Hypothalamic areas involved in prostaglandin (PG)-induced gonadotropin release. I: effects of PGE2 and PGF2alpha implants on luteinizing hormone release.

Ovariectomized rats had a cannula inserted unilaterally within various hypothalamic areas. Several days later they were primed with a sc dose of 10 microng of estradiol benzoate (Eb). Two days after priming they were etherized and an initial blood sample was drawn from the external jugular vein. An inner cannula containing PGE2 or PGF2alpha at its tip was inserted into the previously implanted outer cannula. Blood samples were drawn at 20, 40, 60, and 120 min following the implantation. PGE2 induced a 4-5-fold increase in plasma LH 40 to 60 min following its implantation in the arcuate nucleus-median eminence region (ARH-ME). Levels were already significantly elevated at 20 min. When PGE2 was placed slightly more dorsally, close to the ventromedial nucleus (VMH), LH titers rose to comparable levels but only after a delay of 120 min. PGE2 implanted in the caudal portion of the ARH-ME or dorsally in the VMH-dorsomedial nuclei, barely increased plasma LH, whereas its placement in the anterior portion of the ARH-ME clearly elevated LH titers. PGE2 implants located more than 1 mm lateral from the midline or outside the hypothalamus were ineffective. When PGE2 was placed in the preoptic area (POA) or anterior ventral portion of the anterior hypothalamic area (AHA), plasma LH levels rose strikingly, the first significant increase being observed at 20 min. PGE2 implants located in the vicinity of the paraventricular nucleus-dorsal portion of AHA were much less effective. PGF2alpha implanted in the ARH-ME or POA induced a small increase in plasma LH and the implantation of empty cannulae in the same areas was ineffective. Intrapituitary implants of PGE2 failed to alter plasma LH significantly. The results indicate that PGE2 acts at the ARH-ME region to induce LH release and that an even more effective site of action seems to be located in the POA-AHA. Since these are areas which contain LHRH, the results support the view that PGs can activated LHRH-secreting neurons in these regions.

Hypothalamic areas involved in prostaglandin (PG)-induced gonadotropin release. II: effect of PGE2 and PGF2alpha implants on follicle stimulating hormone release.

The effect of unilateral intrahypothalamic implants of PGE2 or PGF2alpha on FSH release in ovariectomized, estrogen-treated rats was studied. Both PGs induced an equally small increase in plasma FSH following their implantation in the body of the arcuate nucleus-median eminence region (BARH-ME). Levels were maximally elevated 120 min post-implantation. PGE2 implanted in the caudal portion of the ARH-ME (TARH-post-ME) also induced a similar increase in plasma FSH, whereas its placement in the region of the ventromedial nucleus (VMH)-dorsomedial nucleus (DMH) did not significantly alter FSH titers. By contrast, PGE2 placed in the post-chiasmatic region (HARH-ME) clearly elevated plasma FSH as early as 40 min after implantation. Implants of PGF2alpha in this area were ineffective. PGE2 implants located more than 1 mm lateral from the midline or outside the hypothalamus were also ineffective. When PGE2 was placed in the preoptic area (POA) or anterior ventral portion of the anterior hypothalamic area (AHA), plasma FSH rose markedly showing the first significant increase between 40 and 60 min after implantation. PGE2 implants located in the dorsal portion of the AHA-paraventricular nucleus (DAHA-PVH) were also clearly more effective than implants in BARH-ME, but less effective than implants in the POA. PGF2 implanted in the POA evoked a smaller increase in plasma FSH than that induced by PGE2 implants in this region. Implantation of empty cannulae in the ARH-ME region or POA failed to increase plasma FSH levels significantly. Intrapituitary implants of PGE2 did not alter plasma FSH. The results indicate that PGE2 acts mainly on the POA-AHA region to induce FSH release and that PGF2alpha, although less effective, also stimulates FSH release in this area. The ARH-ME appears to be a less important site. The greater effectiveness in the POA-AHA region than in the ARH-ME may indicate that PGE2 is more effective in stimulating cell bodies of LHRH neurons than their axons. Although most of the sites at which PGE2 released FSH coincide with the distribution of LHRH, it is noteworthy that a relatively enhanced release of FSH took place from certain sites in the dorsal AHA-PVH and from sites in the post-ME, which suggests that PGE2 may act on neurons which secrete a specific FSH-RF as well as on neurons which release LHRH.

Prostaglandin E 2 (PGE 2)-induced growth hormone (GH) release: Effect of intrahypothalamic and intrapituitary implants

Overiectomized rats were unilaterally implanted with a 23-gauge stainless steel cannula in different hypothalamic areas or in the pituitary gland and subsequently were treated with estrogen (sc, 10 μg estradiol benzoate, Eb). Two days after the estrogen injection, an inner cannula containing PGE 2 or PHF 2α at its tip was inserted into the cannula. Other animals were implanted with empty inner cannula. Plasma GH concentrations were measured by RIA in blood samples drawn from the jugular vein while the animals were lightly etherized before (−2) and at 20, 40, 60 and 120 min following the implantation. Plasma GH levels in control animals bearing an empty cannula in the body of the arcuate nucleus-median eminence region (BARH-ME) were signifantly depressed by the ether stress. The implantation of PGF 2α in this area was completely ineffective in preventing ether stress-induced decline in plasma GH. By contrast, PGE 2 implanted in BARH-ME or the post-chiasmatic region of the hypothalamus (HARH-ME) elevated plasma GH 20 min following its implantation and partially prevented the subsequent decrease in GH levels induced by ether stress. PGE 2 implants located in several other hypothalamic areas failed to induce GH release or to prevent the decline in GH levels induced by ether stress. However, PGE 2 implanted in the pituitary gland elicited a marked increase in plasma GH at 20 min and completely prevented the subsequent ether stress-induced decline in GH levels. The results suggest that PGE 2 can act at both hypothalamic (ARH-ME) and pituitary levels to stimulate GH release. At the hypothalamus, PGE 2 may inhibit GH-inhibiting factor (GIF) release or induce release of GH releasing factor (GHRF).

Effects of luteinizing hormone on multiple-unit activity in the rat hippocampus.

In estrogen-primed ovariectomized rats under urethane anesthesia, multiple-unit electrical activity (MUA) in the subiculum of the ventral hippocampus responded to intravenous injection of luteinizing hormone (LH) with an increased level of activity in about half of the cases; the others showed no change. Similar responses were observed in the arcuate nucleus-median eminence region of the hypothalamus, although some decreases in MUA were also observed there. The positive hippocampal response was not induced by FSH, TSH, or boiled LH. It is suggested that LH receptors may exist in the subiculum and that the effect is transmitted to the hypothalamus by the medial corticohypothalamic tract.

Hypothalamic sites of action for methallibure (ICI 33828) inhibition of gonadotropin secretion.

Small quantities of the compound methallibure (ICI 33828) were surgically implanted in the anterior pituitary or hypothalamus of female guinea pigs on day 12 of the estrous cycle. Eight days later the ovaries were examined for the presence of new CL and the exact location of each methallibure implant was determined. Relatively large implants of methallibure within or near the hypothalamus prevented ovulation in 13 of 14 cases while similar implants within the anterior pituitary prevented ovulation in only two of eight animals. Relatively small implants of methallibure within or near the hypothalamus blocked ovulation in 26 of 47 cases while ovulation occurred in all of 21 animals with control implants. Ovulation could be blocked for the 8 day experimental period with as little as 20 μg methallibure. Those animals in which ovulation was blocked had methallibure implants in many different sites. The most consistent inhibition of ovulation occurred when methallibure was implanted in the arcuate nucleus-median eminence region or in the pre-optic area. Ovulation was also blocked in a less consistent manner by implants located in the dorsomedial hypothalamus or in the lateral hypothalamus and the ventral part of the medial forebrain bundle. Methallibure implants which blocked ovulation were never able to prevent growth, preovulatory luteinization and persistent estrogen secretion by the ovarian follicles. Similarly, intrahypothalamic methallibure did not consistently reduce feed consumption beyond that caused by the surgical procedures of intracranial implantation.

Induction of ovulation by electrochemical stimulation in androgenized and spontaneously constant-estrous rats.

Comparison of thresholds and responsiveness of androgen-sterilized rats spontaneously constant-estrous rats and of cyclic rats to electrical and electrochemical stimulation applied directly to the brain is presented. Electrochemical stimulation of the medial preoptic area in androgen-sterilized rats was highly effective in causing ovulation. Ovulation was also induced by electrochemical stimulation of the arcuate nucleus and median eminence in androgen-sterilized rats. Electrical stimulation of the medial preoptic area and the arcuate nucleus-median eminence region also caused ovulation in a limited number of androgenized rats although its effectiveness was less than that of electrochemical stimulation. In spontaneously constant-estrous rats electrochemical stimulation of the above regions induced ovulation but the dc threshold in the preoptic area was considerably higher than that of either controls or androgen-sterilized rats. The androgen-sterilized rats were resistant to progesterone a hormonal stimulus which induced ovulation in the spontaneously constant-estrous rat. None of the androgenized rats ovulated even when the progesterone dosage was raised to twice the level effective in the spontaneously constant-estrous group.