Luteinizing Hormone-releasing Hormone Release Research Articles

Role of the Vomeronasal Organ in the Male-Induced Enhancement of Sexual Receptivity in Female Rats

The role of the vomeronasal organ (VNO) in the male-induced enhancement of sexual receptivity in ovariectomized estrogen-primed rats was investigated. Removal of the VNO significantly reduced the enhancement of sexual receptivity following mating, as compared with the sham-operated controls. The sham-operated females exhibited a surge of luteinizing hormone (LH) following mating; however, LH release induced by pairing with males was less manifested in the VNO-removed females. This suggests that in female rats, VNO removal impairs the male-induced release of LH-releasing hormone (LHRH). Since LHRH enhances sexual receptivity in ovariectomized estrogen-primed rats, the present results suggest that the increase in sexual receptivity in female rats following mating probably results from a VNO-mediated LHRH release.

Norepinephrine-Stimulated in Vitro Release of Luteinizing Hormone-Releasing Hormone (LHRH) from Median Eminence Tissue is Facilitated by Inhibition of LHRH-Degrading Activity in Hens1

We and others have previously reported the existence of hypothalamic and anterior pituitary (AP) enzymes that degrade luteinizing hormone (LH)-releasing hormone (LHRH). We have further characterized these LHRH-degrading activities (LHRH-DA) and in addition assessed the role of LHRH-DA in LHRH release from median eminence (ME) tissue in vitro. Major LHRH-DA components were separated and their molecular weights were estimated by gel filtration chromatography. The role of LHRH-DA in LHRH release was determined by release studies from isolated ME, in the presence and absence of N-tosyl L-phenylalanine chloromethyl ketone (TPCK) and/or norepinephrine (NEpi). Degradation and in vitro release studies were performed by using LHRH analogs with amino acid substitutions at their 5-6 bond. Biological activity of these analogs was assessed by measuring in vitro LH release from dispersed anterior pituitary cells. LHRH-DA was determined by high-performance liquid chromatography; LH and LHRH were measured by radioimmunoassay. Separation of LHRH-DA by gel filtration chromatography yielded two major enzymatic activities: a Tyr5-Gly6 cleaving endopeptidase and a post-proline cleaving enzyme. Although LHRH-DA from AP and ME produced identical degradation fragments, the former had 3-fold greater specific activity than the latter. LHRH moieties with a Tyr5-Gly6 bond substitution were more resistant to enzymatic degradation and had greater biological activity than LHRH moieties with a Tyr5-Gly6 bond. TPCK decreased LHRH-DA and increased NEpi-stimulated in vitro release of LHRH from isolated ME.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of Glucose on Thyrotrophin‐Releasing Hormone, Growth Hormone‐Releasing Hormone, Somatostatin and Luteinizing Hormone‐Releasing Hormone Release from Rat Hypothalamus in vitro

Abstract Increasing concentrations of D-glucose (1 to 25 mM) inhibited somatostatin, thyrotrophin-releasing hormone (TRH) and growth hormone-releasing hormone (GHRH) release from incubated adult rat hypothalami in a stereospecific manner. In contrast, the effects of D- and L-glucose on luteinizing hormone-releasing hormone release were virtually identical. Increasing concentrations of D-glucose also inhibited somatostatin release following depolarization with high K(+), but had no obvious effect on depolarization-induced TRH or GHRH release when compared with L-glucose. In conclusion, D-glucose exerts a potent, dose-related modulatory action on the release of rat hypothalamic TRH and GHRH as well as somatostatin in vitro. Further studies are required to establish any physiological relevance of glucose in the modulation of these hypothalamic neuropeptides.

Progesterone increases messenger ribonucleic acid (mRNA) encoding luteinizing hormone releasing hormone (LHRH) level in the hypothalamus of ovariectomized estradiol-primed prepubertal rats

In order to investigate the mechanism underlying ovarian steroid action on gene expression of hypothalamic luteinizing hormone releasing hormone (LHRH), changes in LHRH mRNA level were determined by RNA-blot hybridization assay. Twenty-eight-day-old female rats were ovariectomized (OVX) and implanted with Silastic capsule containing either 17β-estradiol (E) or vehicle (V). Two days later (day 30), OVX + E-primed rats were given s.c. progesterone (P, 1 mg) 6 h prior to decapitation. Four experimental groups were studied: (1) intact, (2) OVX + V, (3) OVX + E, and (4) OVX + E + P-treated rats. Poly(A) RNA fractions from hypothalami (40–50/group) were isolated, blotted onto nitrocellulose paper and hybridized with 32P-end-labeled LHRH oligonucleotides (29 mer) which are complementary to rat LHRH mRNA. The hypothalamic LHRH mRNA signal markedly attenuated 2 days following ovariectomy. E replacement to OVX rats slightly increased LHRH mRNA level, which is lower than that of the intact group. However, a single injection of P to OVX + E-treated rats notably augmented the LHRH mRNA level over that observed in the intact group. In addition, LHRH content and release in vitro were examined to correlate with changes in LHRH gene expression. Ovariectomy and the replacement of E and/or P resulted in a similar fashion of changes in LHRH release and content as compared to alteration of LHRH mRNA level. This study clearly demonstrates that P increases LHRH mRNA level in the hypothalamus of OVX + E-primed immature rats. Moreover, the changes in LHRH mRNA level by ovarian steroids appear to be in parallel with alteration of LHRH content and release in vitro.

In situ hybridization for LHRH mRNA following estrogen treatment

Elucidation of mechanisms governing the synthesis and release of the decapeptide luteinizing hormone-releasing hormone (LHRH) in the female are central for understanding how the brain influences the ovulatory surge of luteinizing hormone and facilitates female rat mating behavior. Previous immunocytochemical results suggested that estrogen treatment might increase LHRH forebrain content, but measures of peptide content often are confounded by alterations in synthesis versus peptide processing versus release. Therefore, we have used in situ hybridization with an oligomer for the LHRH coding region to detect LHRH gene expressing cells and to study LHRH message levels in estrogen treated compared to control ovariectomized rats. Neurons expressing the gene for LHRH are in the medial preoptic area and amongst the fibers of the diagonal bands of Broca. When the numbers of neurons with detectable LHRH message levels were multiplied by the average numbers of grains per cell, to achieve a metric proportional to LHRH message content, we saw a significant stimulatory effect of seven days of estrogen treatment.

A rapid, Ca 2+-independent, onset of prostaglandin E 2 stimulation of the release of luteinizing hormone releasing hormone from copper-treated median eminence explants

Prostaglandin E 2 (PGE 2) is known to stimulate the release of luteinizing hormone releasing hormone (LHRH) from explants of the median eminence area (MEA). We have previously shown that a short preincubation of the MEA with copper markedly amplifies PGE 2 stimulation of LHRH release and that the Ca 2+-cAMP pathway is involved in this release process. In this study, we defined the kinetics of the onset of PGE 2 action and examined the Ca 2+ requirement for the onset and manifestation of PGE 2 action. MEA explants from immature female rats were incubated with copper (150 μM copper-histidine complex); thereafter, expiants were exposed to 10 μM PGE 2 for periods of 2−7 min with or without Ca 2+ (Ca 2+-free buffer containing 1 mM EGTA). A 2 min PGE 2 exposure was as effective as a 7 min PGE 2 exposure in stimulating LHRH release; the latter was manifested during the period between 2–7 min. When MEA were exposed to PGE 2 for 5 min, maximal rate of release was attained within this 5 min period. When MEA were exposed to PGE 2 for 2 min and then to forskolin (100 μM) for 5 min, there was no further increase in the rate of LHRH release (although exposure to forskolin alone maximally stimulated LHRH release). In the presence of EGTA (no Ca 2+), PGE 2 stimulation of LHRH release was abolished and this effect of EGTA was totally reversed by the addition of Ca 2+ 2 min after PGE 2 exposure and partially reversed (40%) by the addition of Ca 2+ 5 min after PGE 2 exposure in the presence of EGTA. In summary, (i) a 2 min exposure of copper-treated MEA expiants to PGE 2 suffices to maximally stimulate LHRH release presumably by maximally activating adenylate cyclase; (ii) maximal rates of LHRH release are attained within 3–5 min after PGE 2 exposure; (iii) there is a differential Ca 2+ requirement for the onset and for the manifestation of PGE 2 action: the initial 2 min interaction does not require extracellular Ca 2+; the subsequent events leading to LHRH release require Ca 2+ and the Ca 2+ requiring element(s) is short-lived (3–5 min).

Failure of in vitro ethanol to inhibit LHRH release from the hypothalamus

The reproductive alterations induced by ethanol (ETOH) in the male rodent have been intensively investigated. Although gonadal effects are well characterized, the impact of ETOH on the hypothalamic peptide luteinizing hormone-releasing hormone (LHRH) has been less well defined. The releasability of hypothalamic LHRH in the presence of ETOH has not been directly studied. We report here that ETOH in concentrations of 50 mg% to 400 mg% failed to inhibit LHRH release in vitro.

Release of luteinizing hormone-releasing hormone: interrelations between eicosanoids and catecholamines

The in vitro release of luteinizing hormone-releasing hormone (LHRH), prostaglandin (PG) E 2 and leukotriene (LT) C 4 from male rat median eminences (ME), was estimated by radioimmunoassay (RIA) in the presence of the catecholamines (CA), norepinephrine (NE) and dopamine (DA). NE increased the release of PGE 2 in the presence and in the absence of the Ca 2+ ionophore A23187 (5 × 10 −6 M), but it did not modify the A23187-induced release of LTC 4 from endogenous precursors or radiolabelled arachidonic acid. DA also stimulated the A23187-induced release of PGE 2 but inhibited that of LTC 4. However, while NE increased both the basal and the A23187-induced release of LHRH, DA increased the basal release of LHRH and inhibited the A23187-induced LHRH release. Exogenous LTC 4 cancelled the inhibitory effect of DA on LHRH release. Blockade of dopaminergic receptors with haloperidol suppressed the effects of DA on PGE 2, LTC 4 and LHRH release. Neither eicosanoid affected the K +-evoked [ 3H]DA release, whereas only PGE 2 inhibited the K +-evoked [ 3H]NE release. We conclude that LTC 4 does not interact with the noradrenergic pathway and that the stimulatory effect of both catecholamines on LHRH release involves PGE 2, but the inhibitory effect of DA is associated with reduced LTC 4 production.

In-vivo effect of ethanol on release of LH-releasing hormone and LH in rats.

The effect of exposure to ethanol on hypothalamic LH-releasing hormone (LHRH) release in vivo was investigated in rats both acutely (i.p. injection) and after 3 days of administration, utilizing a permanent gastric cannula. In both designs, the animals were castrated before being given ethanol and, in both experiments, ethanol successfully lowered the post-castration LH rise compared with control castrated animals. In both the acutely and chronically treated groups, basal LHRH release was not impaired, despite the documented decrease in LH levels. Finally, stimulated LHRH release was investigated with depolarizing concentrations of potassium and, again, no change was noted between the hypothalamic release of this decapeptide in the ethanol-exposed compared with the ethanol-naive animals. Thus, ethanol failed to inhibit basal or stimulated LHRH secretion in the acutely and chronically treated animal. This lack of effect on LHRH occurred despite a concomitant lowering of serum concentrations of LH.

Functional and morphological changes in mediobasal hypothalamus of streptozocin-induced diabetic rats. In vitro study of LHRH release.

To investigate the role of the mediobasal hypothalamus (MBH) in diabetic gonadal axis disorders, the MBHs of adult male streptozocin-induced diabetic (STZ-D) rats were examined after incubation in basal conditions or in K+-enriched medium and compared with those of controls. Diabetes lasted 1 mo. Both luteinizing-hormone-releasing hormone (LHRH) release and MBH morphology were studied. After incubation in basal conditions, the LHRH release was unchanged. By light microscopy, the dilated-axon cross sections were more numerous (P less than .01) in the basal arcuate nucleus and in the median eminence. By electron microscopy, the ratio of exocytoses to neurosecretory granules observed in the median eminence axon cross sections was smaller (P less than .05). The total LHRH immunoreactivity, the number of labeled axons, and the amount of positive material in the axons were reduced (P less than .05). After incubation in K+-enriched medium, the LHRH release was markedly reduced (P less than .01). The number and area of dilated-axon cross sections, possibly because of the relation between exocytosis and physiological dilation, were less augmented (P less than .01). Whereas the number of exocytoses and the ratio of exocytoses to neurosecretory granules were not decreased, the total LHRH immunoreactivity and the number of labeled axons were reduced (P less than .05). The releasable LHRH pool therefore seems to be exhausted in control MBH because of long-term stimulation and reduced in the MBH of STZ-D rats because of diabetes. In conclusion, STZ-D causes functional and anatomical MBH lesions that should be pathogenetically relevant for the disorders of the gonadal axis documented in this animal model.

Serotonin stimulation of the period of in vitro LHRH release is estradiol dependent

The neurotransmitter serotonin (5-HT) has well known effects on the release of luteinizing hormone (LH) which may be estrogen and photoperiod dependent. An in vitro superfusion system was used to assess the potential role of 5-HT and photoperiod in modulating the release of luteinizing hormone releasing hormone (LHRH) from the mediobasal-suprachiasmatic-preoptic area (MBH-POA-SCN) in the ovariectomized (OVX) and OVX estradiol (E 2) implanted rat model of LH release. Regularly cycling female Holtzman strain rats, maintained on a photoperiod of 0500–1900 or 0100–1500 hr light were OVX, and silastic capsules containing estradiol-17 beta (150 μg/ml) or sesame oil implanted SC. Two days later the rats were killed at 1000 hr and the MBH-POA-SCN isolated. Tissue was superfused in vitro with Krebs-Ringer-Phosphate buffer (KRP) for 2 hours followed by 3 hours of KRP with intermittent superfusion of 5-HT at increasing concentrations: 1 × 10 −10 M (1st hour), 1 × 10 −8 (2nd hour), 1 x 10 −6 M (3rd hour). Superfusate was collected every 10 min for the entire 5 hour incubation period and LHRH measured by RIA with the following results. In the OVX rat 5-HT significantly ( p 0 ̌ .05 ) increased the period of LHRH pulses and decreased the release of LHRH compared to control superfusions, while in the OVX E 2 rat 5-HT significantly ( p 0 ̌ .05 ) decreased the period of LHRH release and increased the release of LHRH. In another series of experiments the tissue was perfused for 2 hours with KRP, 2 hours with 1 x 10 −8 M 5-HT, and 2 hours with KRP. 5-HT again significantly increased ( p 0 ̌ .05 ) the period of LHRH pulses from tissue derived from the OVX rat and decreased the period ( p 0 ̌ .05 ) of LHRH pulses in tissue from the OVX E 2-treated rat. However, in the OVX-treated rat LHRH release was also increased ( p 0 ̌ .05 ) during the first hour of 5-HT superfusion. Photoperiod had no significant effect on the release of LHRH from OVX- and OVX E 2-treated rats. However, the effect of 5-HT in modulating the frequency of LHRH release may be related to the presence of E 2 in the rat and could serve as one mechanism in vivo to coordinate the release of LHRH with endocrine cues.

Immunocytochemical and physiological evidence of a synapse between dopamine- and Luteinizing Hormone Releasing Hormone-containing neurons in the ewe median eminence.

Immunocytochemical labeling revealed that the arcuate nucleus (ARN) of the ewe's hypothalamus contains numerous tyrosine hydroxylase (TH)-positive neurons, that appear to lack dopamine-beta-hydroxylase (DBH)-like immunoreactivity. Axons of these presumed dopaminergic neurons converge in the median eminence (ME) with Luteinizing Hormone Releasing Hormone (LHRH)-containing axons originating mostly from neurons situated in the medial preoptic area. Electron microscopic double labeling revealed synaptic contacts between TH-positive presynaptic profiles and LHRH-containing postsynaptic elements. Samples of ME, ARN, paraarcuate and lateral hypothalamus were dissected and incubated to assess LHRH release and tissue content. Only ME-LHRH release was significantly reduced in the presence of dopamine (DA). All other regions released equal amounts with and without DA. Thus, a presynaptic dopaminergic inhibition of LHRH-containing axons at the level of the ME might contribute to the regulation of LHRH release into the portal vessels.

Hypothalamic prostaglandin E 2 receptors coupled to an adenylyl cyclase

We show that the effect of prostaglandin (PG) E 2 on luteinizing hormone-releasing hormone (LHRH) release involves a receptor-mediated process coupled to an adenylyl cyclase system. The adenylyl cyclase activity in rat hypothalamus synaptic membrane preparations was stimulated by PGE 2 and this stimulation was directly related to the presence of guanine nucleotide (GTP). PGE 2 specifically bound to P 2 membranes from rat and porcine hypothalami with similar characteristics. Computer-fitted saturation curves provided evidence for two binding components which may be two states of the same receptor (R H and R L). Experiments with Gpp(NH)p, a non-metabolizable analogue of GTP, suggested the interconversion of R H and R L. These results may reflect different states of the ternary complex (hormone-receptor-guanine binding protein). Magnesium (Mg 2+) can modify the R H and R L binding parameters, but seems to act directly on the PGE 2 receptor site.

Arcuate area of the female rat maintained in vitro exhibits increased afternoon electrical activity.

The arcuate area of the rat hypothalamus has previously been shown to have increased neuronal activity in vivo corresponding to the estradiol-induced luteinizing hormone rise in the afternoon. The present experiments investigate the arcuate area to determine whether afternoon increases in activity occur in vitro and are dependent on estradiol pretreatment. Light-dark entrained pubertal rats were ovariectomized and implanted with estradiol- or cholesterol-filled silastic capsules. Two days later they were sacrificed at 10.00 h and coronal sections containing the arcuate area were maintained in oxygenated artificial cerebrospinal fluid at 34 degrees C. With estrogen pretreatment, units recorded from 11.00 to 15.00 h had a slow discharge with hourly mean rates ranging from 0.7 to 0.86 spikes/s. However, in the next hour the mean firing rate doubled (p less than 0.01) and an increase in units encountered per electrode penetration was observed. In tissue from animals sacrificed 3 h later, an increase in firing rate was also noted after 15.00 h, suggesting that a diurnal mechanism rather than a technical artifact was responsible for the timing of the activity increase. In control experiments with cholesterol capsules, electrical activity did not have a diurnal pattern. In summary, the arcuate area of the rat isolated in vitro was observed to display an increase in neuronal activity during the afternoon that was dependent on estrogen pretreatment and correlated to the time of day that luteinizing hormone rises are seen in intact animals. Although the arcuate area is involved in numerous functions, this electrical activity may represent interneurons controlling the release of luteinizing hormone-releasing hormone from axonal endings in the median eminence.

Spontaneous changes in LHRH release during the rat estrous cycle, as measured with repetitive push-pull perfusions of the pituitary gland in the same female rats.

In the present study, we examined the in vivo release profiles of luteinizing hormone-releasing hormone (LHRH) during the rat estrous cycle by utilizing repetitive push-pull perfusions of the anterior pituitary in the same freely behaving animal. Throughout the estrous cycle, LHRH levels were higher than those reported from previous work using a push-pull cannula placed in the mediobasal hypothalamus. A significant increase in LHRH input to the anterior pituitary was obtained in proestrus (Pro), though the magnitude of increment was different among animals. This LHRH increment in Pro was attributable to a significant increase in the amplitude of LHRH pulses. Interestingly, no changes were observed in the frequency of LHRH pulses (about 1 pulse/50 min) throughout the estrous cycle. The data demonstrate that the anterior pituitary of female rats receives an increased amount of LHRH during Pro due to changes in the amplitude of the LHRH signal without changes in the frequency.

Alterations in Luteinizing Hormone-Releasing Hormone Release from the Mediobasal Hypothalamus of Ovariectomized, Steroid-Primed Middle-Aged Rats as Measured by Push-Pull Perfusion

The spontaneous luteinizing hormone (LH) surge on proestrus as well as the steroid-induced LH surge and hypersecretion of LH following ovariectomy are attenuated in middle-aged female rats when compared to their young counterparts. It is generally assumed that the lower titers of serum LH measured in aging animals result, in part, from age-related alterations in luteinizing hormone-releasing hormone (LHRH) neurosecretion, yet no direct measurements of LHRH release from the hypothalamus of aging females are currently available. The present study utilized the push-pull perfusion technique to characterize and compare in vivo LHRH output from the mediobasal hypothalamus of ovariectomized middle-aged and young females during a steroid-induced LH surge. Twelve-minute perfusates were collected for a period of 6 h from middle-aged and young animals outfitted with push-pull cannulae resting in the mediobasal hypothalamus in close proximity to the median eminence. The overall pattern of LHRH release differed in the two age groups. Mean levels of LHRH measured in brain perfusates from middle-aged females were significantly lower than mean levels measured in young animals. Moreover, mean levels of LHRH detected in perfusates during the 1- and 2-hour intervals prior to and the 1-hour interval after the time of maximal LHRH output were lower in middle-aged animals, and a larger percentage of samples from middle-aged females contained levels of LHRH below the detectability of the assay. The mean amplitude of identified LHRH pulses and maximal LHRH output, however, did not differ in the two age groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic inhibition of hypothalamic-pituitary-ovarian axis and body weight gain by brain-directed delivery of estradiol-17 beta in female rats.

The effect of preferential delivery of estradiol (E2) into the brain on both the hypothalamic-pituitary-ovarian axis and weight gain was studied in female rats. When E2 was coupled to a lipoidal dihydropyridine-pyridinium carrier, the resulting carrier E2 complex (CE), upon a single intravenous administration to cycling female rats, caused a dose-dependent inhibition of ovulation which lasted 3 times longer than with uncoupled E2. The dose of CE that delayed ovulation for 4 days was one twentieth the amount of E2 needed to produce the same effect. Studies in ovariectomized (OVEX) rats indicated that the prolonged ovulation-blocking action of CE appeared to be related to a sustained storage and release of E2 in the brain, which in turn suppressed the release of hypothalamic luteinizing hormone-releasing hormone (LHRH) and pituitary luteinizing hormone (LH). Upon single intravenous administration in pubertal female rats, CE caused a dose-dependent reduction of body weight gain for a minimum period of 28 days. The inhibitory action of CE on body weight gain was more potent and longer lasting than that of E2 in pubertal rats. When administered in OVEX rats, CE produced a loss of body weight that lasted significantly longer than that produced by uncoupled E2 in these rats. These results suggest that the biological action of E2 can be potentiated by this novel chemical delivery system.

Examination of a possible mechanism by which anisomycin suppresses pulsatile luteinizing hormone release in ovariectomized rats.

An initial study was performed to ascertain the effects of anisomycin, a reversible inhibitor of protein synthesis, on pulsatile luteinizing hormone (LH) release in adult, ovarietomized (OVX) rats. For this experiment, rats OVX 3-4 weeks earlier were fitted with indwelling atrial cannulae. On the next day (approximately 13.00 h), the rats received a subcutaneous injection of either 100 mg/kg body weight (BW) anisomycin or its saline vehicle. Administration of anisomycin significantly suppressed mean plasma LH levels, mean trough values, and both LH pulse frequency (saline: 6.3 pulses/3 h vs. anisomycin: 2.7 pulses/3 h) and amplitude. To determine whether anisomycin affected anterior pituitary LH responses to LH-releasing hormone (LHRH), a second experiment was performed in which saline- and anisomycin-treated OVX rats were given an intravenous injection of 10 ng/100 g BW LHRH 1.5 h later (14.30 h). Rats then were sacrificed and the anterior pituitary and brain removed. Whereas preinjection plasma LH levels were significantly lower in anisomycin-treated rats, they were significantly higher in anisomycin-treated rats 20 min after LHRH. Consequently, mean maximal increments and percent increments were significantly higher in anisomycin-treated rats. AP LH content and content of LHRH in the medial preoptic and suprachiasmatic nuclei were not influenced by anisomycin treatment. However, median eminence (ME) LHRH concentrations in anisomycin-treated rats were almost double the LHRH levels measured in control rats. A third study was conducted to assess the effects of anisomycin on basal and potassium (K+)-stimulated LHRH release from superfused ME explants.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the physiological role and mechanism of action of neuropeptide Y in the regulation of luteinizing hormone secretion in the rat

It has been recently shown that intraventricular or systemic injection of neuropeptide Y (NPY) can produce a decrease in plasma luteinizing hormone (LH) levels in castrated rats of both sexes. In order to evaluate the physiological role of NPY in the regulation of LH secretion in the female rat, we proceeded to immunoneutralization experiments using specific antibodies to NPY. Injection of 0.5 ml antiserum to NPY produce a 20-fold increase of LH plasma levels, whereas injection of preimmune serum did not modify the plasma concentrations of LH. To investigate the possibility that catecholamines or serotonin might be involved in the effect of NPY in LH secretion, castrated rats were treated with α-methylparatyrosine (α-MPT), an inhibitor of catecholamine biosynthesis, or received an i.c.v. injection of the neurotoxin 5–7-dihydroxytryptamine (5,7-DHT) prior to the intraventricular injections of NPY. The pretreatment with α-MPT could not prevent the decrease of plasma LH induced by NPY injection whereas the pretreatment with 5,7-DHT reversed the effect of NPY injection. The anatomical connection between LH-releasing hormone (LHRH) and NPY neuronal systems were also investigated using double immunostaining technique. It appeared that NPY endings are in apposition to LHRH cell bodies in the preoptic area in proximity to the organum vasculosum of the lamina terminalis (OVLT). These studies strongly suggest that NPY is of physiological importance in the regulation of LH secretion in the rat and that the action of NPY could be exerted by at least two different ways: (1) a stimulatory effect of NPY on serotoninergic systems which in turn inhibit LHRH release and (2) a direct stimulatory effect of the peptide on LHRH neuronal system.

Luteinizing hormone releasing hormone (LHRH) neuroterminals mapped using the push-pull perfusion method in the rhesus monkey

In vivo luteinizing hormone releasing hormone (LHRH) release was measured in conscious, ovariectomized rhesus monkeys using a push-pull cannula inserted into the stalk-median eminence, and the relationship between sampling location and LHRH release was examined. Within an individual animal in which multiple experiments were conducted with different cannula placements, LHRH pulse frequency was consistent. In contrast, LHRH pulse amplitude and mean LHRH release varied with cannula tip location in a pattern which reflected the anatomical distribution of LHRH-immunoreactive fibers described for the rhesus monkey. These results suggest that our push-pull perfusion method is reliable for the in vivo measurement of LHRH and perhaps other neuropeptides and/or neurotransmitters, as well.