Electrophysiological actions of luteinizing hormone-releasing hormone: intracellular studies in the rat hippocampal slice preparation

GnRH protective effects against amyloid β-induced cognitive decline: A potential role of the 17β-estradiol

INTRODUCTION

The 17β-estradiol (E2) enhances hippocampal dendritic spine synapses, facilitates learning processes, and exerts neuroprotection. Brain estrogen decline has been reported in Alzheimer's disease. The role of GnRH in modulating steroid biosynthesis convinced us to examine whether hippocampal GnRH administration could enhance the local E2 levels and overcome the development of cognition decline in amyloid β (Aβ) neurotoxicity. To explore if GnRH acts through regulating E2 synthesis, letrozole, an aromatase inhibitor, has been applied in combination with GnRH.

METHODS

Female rats received an intracerebroventricular injection of Aβ. The GnRH and, or letrozole were injected into the CA1 for 14 consecutive days. Working memory, novel object recognition memory, and anxiety-like behavior were evaluated. Serum and hippocampal E2 levels were measured. Hippocampal mRNA expression of GnRH (GnRH-R) and E2 (ERα and ERβ) receptors was assessed. GnRH effect on the excitability of pyramidal cells was studied by in vivo single-unit recording.

RESULTS

GnRH increased hippocampal E2 levels, evoked an increase in the spontaneous firing of pyramidal neurons, and caused mRNA overexpression of hippocampal GnRH receptors. GnRH prevented the adverse effects of Aβ on working memory, NOR index, and anxiogenic behavior. Letrozole did not reverse GnRH modulatory effects on hippocampal E2 levels and neuroprotection.

CONCLUSION

GnRH prevented the Aβ-induced memory deficit, which may be mediated through hippocampal E2 levels enhancement. The electrophysiological analysis revealed the enhanced neuronal excitability in the CA1 region. All these data suggest that GnRH might be a promising candidate that reduces anxiety and improves memory indices in the context of Aβ neurotoxicity.

Seizure burden fluctuates with the female reproductive cycle in a mouse model of chronic temporal lobe epilepsy

Women with catamenial epilepsy often experience increased seizure burden near the time of ovulation (periovulatory) or menstruation (perimenstrual). To date, a rodent model of chronic temporal lobe epilepsy (TLE) that exhibits similar endogenous fluctuations in seizures has not been identified. Here, we investigated whether seizure burden changes with the estrous cycle in the intrahippocampal kainic acid (IHKA) mouse model of TLE. Adult female IHKA mice and saline-injected controls were implanted with EEG electrodes in the ipsilateral hippocampus. At one and two months post-injection, 24/7 video-EEG recordings were collected and estrous cycle stage was assessed daily. Seizures were detected using a custom convolutional neural network machine learning process. Seizure burden was compared within each mouse between diestrus and combined proestrus and estrus days (pro/estrus) at two months post-injection. IHKA mice showed higher seizure burden on pro/estrus compared with diestrus, characterized by increased time in seizures and longer seizure duration. When all IHKA mice were included, no group differences were observed in seizure frequency or EEG power. However, increased baseline seizure burden on diestrus was correlated with larger cycle-associated differences, and when analyses were restricted to mice that showed the severe epilepsy typical of the IHKA model, increased seizure frequency on pro/estrus was also revealed. Controls showed no differences in EEG parameters with cycle stage. These results suggest that the stages of proestrus and estrus are associated with higher seizure burden in IHKA mice. The IHKA model may thus recapitulate at least some aspects of reproductive cycle-associated seizure clustering.

Multiple functions of non-hypophysiotropic gonadotropin releasing hormone neurons in vertebrates

Gonadotropin releasing hormone (GnRH) is a hypophysiotropic hormone that is generally thought to be important for reproduction. This hormone is produced by hypothalamic GnRH neurons and stimulates the secretion of gonadotropins. On the other hand, vertebrates also have non-hypophysiotropic GnRH peptides, which are produced by extrahypothalamic GnRH neurons. They are mainly located in the terminal nerve, midbrain tegmentum, trigeminal nerve, and spinal cord (sympathetic preganglionic nerves). In vertebrates, there are typically three gnrh paralogues (gnrh1, gnrh2, gnrh3). GnRH-expression in the non-hypophysiotropic neurons (gnrh1 or gnrh3 in the terminal nerve and the trigeminal nerve, gnrh2 in the midbrain tegmentum) occurs from the early developmental stages. Recent studies have suggested that non-hypophysiotropic GnRH neurons play various functional roles. Here, we summarize their anatomical/physiological properties and discuss their possible functions, focusing on studies in vertebrates. GnRH neurons in the terminal nerve show different spontaneous firing properties during the developmental stages. These neurons in adulthood show regular pacemaker firing, and it has been suggested that these neurons show neuromodulatory function related to the regulation of behavioral motivation, etc. In addition to their recognized role in neuromodulation in adult, in juvenile fish, these neurons, which show more frequent burst firing than in adults, are suggested to have novel functions. GnRH neurons in the midbrain tegmentum show regular pacemaker firing similar to that of the adult terminal nerve and are suggested to be involved in modulations of feeding (teleosts) or nutrition-related sexual behaviors (musk shrew). GnRH neurons in the trigeminal nerve are suggested to be involved in nociception and chemosensory avoidance, although the literature on their electrophysiological properties is limited. Sympathetic preganglionic cells in the spinal cord were first reported as peptidergic modulatory neurons releasing GnRH with a putative function in coordinating interaction between vasomotor and exocrine outflow in the sympathetic nervous system. The functional role of non-hypophysiotropic GnRH neurons may thus be in the global modulation of neural circuits in a manner dependent on internal conditions or the external environment.

Long-Term Estrogen Receptor Beta Agonist Treatment Modifies the Hippocampal Transcriptome in Middle-Aged Ovariectomized Rats

Estradiol (E2) robustly activates transcription of a broad array of genes in the hippocampal formation of middle-aged ovariectomized rats via estrogen receptors (ERα, ERβ, and G protein-coupled ER). Selective ERβ agonists also influence hippocampal functions, although their downstream molecular targets and mechanisms are not known. In this study, we explored the effects of long-term treatment with ERβ agonist diarylpropionitrile (DPN, 0.05 mg/kg/day, sc.) on the hippocampal transcriptome in ovariectomized, middle-aged (13 month) rats. Isolated hippocampal formations were analyzed by Affymetrix oligonucleotide microarray and quantitative real-time PCR. Four hundred ninety-seven genes fulfilled the absolute fold change higher than 2 (FC > 2) selection criterion. Among them 370 genes were activated. Pathway analysis identified terms including glutamatergic and cholinergic synapse, RNA transport, endocytosis, thyroid hormone signaling, RNA degradation, retrograde endocannabinoid signaling, and mRNA surveillance. PCR studies showed transcriptional regulation of 58 genes encoding growth factors (Igf2, Igfb2, Igf1r, Fgf1, Mdk, Ntf3, Bdnf), transcription factors (Otx2, Msx1), potassium channels (Kcne2), neuropeptides (Cck, Pdyn), peptide receptors (Crhr2, Oprm1, Gnrhr, Galr2, Sstr1, Sstr3), neurotransmitter receptors (Htr1a, Htr2c, Htr2a, Gria2, Gria3, Grm5, Gabra1, Chrm5, Adrb1), and vesicular neurotransmitter transporters (Slc32a1, Slc17a7). Protein-protein interaction analysis revealed networking of clusters associated with the regulation of growth/troph factor signaling, transcription, translation, neurotransmitter and neurohormone signaling mechanisms and potassium channels. Collectively, the results reveal the contribution of ERβ-mediated processes to the regulation of transcription, translation, neurogenesis, neuromodulation, and neuroprotection in the hippocampal formation of ovariectomized, middle-aged rats and elucidate regulatory channels responsible for DPN-altered functional patterns. These findings support the notion that selective activation of ERβ may be a viable approach for treating the neural symptoms of E2 deficiency in menopause.

Early manipulation of juvenile hormone has sexually dimorphic effects on mature adult behavior in Drosophila melanogaster

Hormones are critical for the development, maturation, and maintenance of physiological systems; therefore, understanding their involvement during maturation of the brain is important for the elucidation of mechanisms by which adults become behaviorally competent. Changes in exogenous and endogenous factors encountered during sexual maturation can have long lasting effects in mature adults. In this study, we investigated the role of the gonadotropic hormone, juvenile hormone (JH), in the modulation of adult behaviors in Drosophila. Here we utilized methoprene (a synthetic JH analog) and precocene (a JH synthesis inhibitor) to manipulate levels of JH in sexually immature male and female Drosophila with or without decreased synthesis of neuronal dopamine (DA). Locomotion and courtship behavior were assayed once the animals had grown to sexual maturity. The results demonstrate a sexually dimorphic role for JH in the modulation of these centrally controlled behaviors in mature animals that is dependent on the age of the animals assayed, and present DA as a candidate neuronal factor that differentially interacts with JH depending on the sex of the animal. The data also suggest that JH modulates these behaviors through an indirect mechanism. Since gonadotropic hormones and DA interact in mammals to affect brain development and later function, our results suggest that this mechanism for the development of adult behavioral competence may be evolutionarily conserved.

Effects of gonadotropin releasing hormone and estradiol on c-fos expression in the rat hippocampus

The effects of ovariectomy, estradiol, and gonadotropin releasing hormone (GnRH) on the protein synthetic activity of hippocampal neurons were studied with immunohistochemistry for the proto-oncogene c-fos. Ovariectomy caused a reduction in the number of c-fos-positive neurons to 16% in area CA(1) and to 25% in area CA(3) when compared to that in the intact control animal. The dentate gyrus was only slightly affected. The decline in the number of c-fos-immunoreactive neurons in the Ammon's horn was partially reversed by a single intravenous injection of estradiol which resulted in the expression of c-fos in 71% of the neurons in area CA(1) and 74% in CA(3) when compared to the numbers of positive cells in the control animals. Similarly, intracerebroventricular injections of 10 muM GnRH caused an increase in the number of c-fos-positive cells to 68% in area CA(1) and to 52% in area CA(3) compared to that in the control animals. The induction of c-fos synthesis after estradiol and GnRH was transient and reached a maximum after 1 to 2 h before it declined to pretreatment levels after 8 h. The results suggest that both estradiol and GnRH exert specific effects on protein synthesis in certain neurons of the hippocampus and that these effects of the hormones are, at least in part, mediated by c-fos.

Gonadotrophin-releasing hormone (GnRH) exerts stimulatory effects on GnRH neurons in intact adult male and female mice

There is substantial evidence for a role of the neuropeptide gonadotrophin-releasing hormone (GnRH) in the regulation of GnRH neurone secretion but how this is achieved is not understood. We examined here the effects of GnRH on the electrical excitability and intracellular calcium concentration ([Ca2+](i)) of GnRH neurones in intact adult male and female mice. Perforated-patch electrophysiological recordings from GnRH-green fluorescent protein-tagged GnRH neurones revealed that 3 nm-3 mum GnRH evoked gradual approximately 3 mV depolarisations in membrane potential from up to 50% of GnRH neurones in male and female mice. The depolarising effect of GnRH was observed on approximately 50% of GnRH neurones throughout the oestrous cycle. However, at pro-oestrus alone, GnRH was also found to transiently hyperpolarise approximately 30% of GnRH neurones. Both hyperpolarising and depolarising responses were maintained in the presence of tetrodotoxin. Calcium imaging studies undertaken in transgenic GnRH-pericam mice showed that GnRH suppressed [Ca2+](i) in approximately 50% of GnRH neurones in dioestrous and oestrous mice. At pro-oestrus, 25% of GnRH neurones exhibited a suppressive [Ca2+](i) response to GnRH, whereas 17% were stimulated. These results demonstrate that nm to mum concentrations of GnRH exert depolarising actions on approximately 50% of GnRH neurones in males and females throughout the oestrous cycle. This is associated with a reduction in [Ca2+](i). At pro-oestrus, however, a further population of GnRH neurones exhibit a hyperpolarising response to GnRH. Taken together, these studies indicate that GnRH acts predominantly as a neuromodulator at the level of the GnRH cell bodies to exert a predominant excitatory influence upon GnRH neurones in intact adult male and female mice.

A novel animal model to study hot flashes: no effect of gonadotropin-releasing hormone

OBJECTIVE

Menopausal hot flashes compromise the quality of life for most women. The physiological mechanisms underlying hot flashes remain poorly understood, and the absence of an animal model to investigate hot flashes hinders investigations in this field.

METHODS

We first developed the sheep as a model to study peripheral skin temperature changes using fever-inducing lipopolysaccharide (LPS; 200 microg/kg) administered to ovary-intact ewes. Because a strong correlation between luteinizing hormone pulses and hot flashes has previously been reported, we then determined whether intravenous gonadotropin-releasing hormone (GnRH; 1 mg), a dose sufficient to elevate cerebrospinal fluid-GnRH concentrations, could modulate ear skin temperature in both ovariectomized and low-estrogen-replaced ovariectomized ewes.

RESULTS

Some ewes responded to LPS in heart rate and abdominal temperature, but there was no significant effect on either parameter or cheek temperature for the group. In contrast, LPS injection caused a significant (P < 0.001) change in skin temperature at the ear. Ear temperature showed no significant change in response to GnRH relative to control injections in both ovariectomized and low estrogen ewes.

CONCLUSIONS

We developed a model animal system in the ewe that can accurately detect small changes in peripheral skin temperature. This system has the potential to be extremely useful in future studies investigating the pathology of hot flashes and holds several advantages over previous model systems developed for this research. GnRH per se does not seem to be involved in thermoregulatory events.

Gonadotropin-releasing hormone (GnRH) activates the m-current in GnRH neurons: an autoregulatory negative feedback mechanism?

GnRH autoregulates GnRH neurons through an ultrashort feedback loop. One potential mechanism is the regulation of K(+) channel activity through the GnRH receptor. Whereas GnRH inhibits the activity of the M-current in peripheral neurons, there is no direct evidence that the M-current is involved in the autoregulatory pathway of GnRH or if the M-current is expressed in GnRH neurons. The M-current is a noninactivating, subthreshold K(+) current that inhibits cell excitability and is ubiquitously expressed in the central nervous system. We found that GnRH neurons expressed the neuronal M-current subunits, KCNQ2, -3, and -5 in addition to GnRH receptor (GnRH R1). Therefore, using whole-cell patch clamp recording and single-cell RT-PCR, we explored the effects of mammalian GnRH peptide on enhanced green fluorescent protein-tagged GnRH neurons acutely dispersed as well as in slice preparations. GnRH (100nm) inhibited GnRH neuronal excitability by hyperpolarizing the membrane. In the presence of CdCl(2), BaCl(2), and tetrodotoxin, GnRH activated an outward current in a dose-dependent manner (EC(50) 11 nm) in 30% of GnRH neurons. In voltage clamp, the selective M-channel blocker, XE-991, inhibited a K(+) current in GnRH neurons. XE-991 also antagonized the outward K(+) current induced by GnRH. Moreover, the GnRH effects on the M-current were blocked by the GnRH R1 antagonist antide. Therefore, these findings indicate that GnRH activates the M-current in a subpopulation of GnRH neurons via GnRH R1. This ultrashort circuit is one potential mechanism by which GnRH could modulate its own neuronal excitability through an autoreceptor.

Gonadotropin-releasing hormone regulates spine density via its regulatory role in hippocampal estrogen synthesis

Spine density in the hippocampus changes during the estrus cycle and is dependent on the activity of local aromatase, the final enzyme in estrogen synthesis. In view of the abundant gonadotropin-releasing hormone receptor (GnRH-R) messenger RNA expression in the hippocampus and the direct effect of GnRH on estradiol (E2) synthesis in gonadal cells, we asked whether GnRH serves as a regulator of hippocampal E2 synthesis. In hippocampal cultures, E2 synthesis, spine synapse density, and immunoreactivity of spinophilin, a reliable spine marker, are consistently up-regulated in a dose-dependent manner at low doses of GnRH but decrease at higher doses. GnRH is ineffective in the presence of GnRH antagonists or aromatase inhibitors. Conversely, GnRH-R expression increases after inhibition of hippocampal aromatase. As we found estrus cyclicity of spine density in the hippocampus but not in the neocortex and GnRH-R expression to be fivefold higher in the hippocampus compared with the neocortex, our data strongly suggest that estrus cycle–dependent synaptogenesis in the female hippocampus results from cyclic release of GnRH.

The influence of gonadal hormones on neuronal excitability, seizures, and epilepsy in the female

It is clear from both clinical observations of women, and research in laboratory animals, that gonadal hormones exert a profound influence on neuronal excitability, seizures, and epilepsy. These studies have led to a focus on two of the primary ovarian steroid hormones, estrogen and progesterone, to clarify how gonadal hormones influence seizures in women with epilepsy. The prevailing view is that estrogen is proconvulsant, whereas progesterone is anticonvulsant. However, estrogen and progesterone may not be the only reproductive hormones to consider in evaluating excitability, seizures, or epilepsy in the female. It seems unlikely that estrogen and progesterone would exert single, uniform actions given our current understanding of their complex pharmacological and physiological relationships. Their modulatory effects are likely to depend on endocrine state, relative concentration, metabolism, and many other factors. Despite the challenges these issues raise to future research, some recent advances have helped clarify past confusion in the literature. In addition, testable hypotheses have developed for complex clinical problems such as "catamenial epilepsy." Clinical and animal research, designed with the relevant endocrinological and neurobiological issues in mind, will help advance this field in the future.

Gonadotropins: potential targets for preventive and therapeutic interventions in Alzheimer’s disease

Increased prevalence of Alzheimer’s disease (AD) in women has led to an interest in the role of hormonal changes in the neurodegenerative process. In particular, research has been directed towards investigating the effect of changes in sex hormone levels following reproductive senescence. Clinical trials of hormone-replacement therapy for the prevention of AD are proving contentious, and considerably more research is necessary before the benefit of the hormone replacement strategy can be ascertained. However, evidence is now emerging to support the notion that increased gonadotropin levels may confer an increased risk of AD. Gonadotropins have been implicated in the metabolism of β-amyloid, a key protein that is central to the pathogenesis of AD. Gonadotropin reduction represents a promising new target for therapeutic intervention in AD and, potentially, dementia in general. In this review, the authors discuss the therapeutic and preventive potential of gonadotropin-reducing agents in the management of AD.

Three gonadotropin-releasing hormone neuronal groups with special reference to teleosts

Gonadotropin-releasing hormone (GnRH) is a decapeptide, which has been isolated from the hypothalamus as a releasing hormone of gonadotropins from the pituitary. However, subsequent morphological studies have demonstrated the presence of multiple GnRH neuronal groups outside the hypothalamus and preoptic area. In most vertebrate lineages studied to date, GnRH neuronal groups are present along the terminal nerve and in the midbrain tegmentum, in addition to a population in the preoptico-hypothalamic areas. The presence of GnRH fibers in extrahypothalamic areas has also been demonstrated, indicating a significance for GnRH neurons in functions other than those that are purely hypophysiotropic. Among vertebrate lineages, GnRH neurons have been most extensively studied in teleost fish through morphological, electrophysiological, behavioral and molecular approaches. To date, studies on differential roles of GnRH neuronal groups have been mostly restricted to teleosts. In the present review, the anatomy and functions of each GnRH neuronal group are reconsidered, based mainly on knowledge from teleosts. Recent findings in teleosts indicate that the preoptico-hypothalamic GnRH neurons are hypophysiotropic and that GnRH neurons of the terminal nerve and midbrain tegmentum regulate neural activities in various regions, including extrahypothalamic areas. The latter populations presumably serve as neuromodulatory systems to control aspects of neural functions such as reproductive behavior. Similar functional differentiation may be generalized to other vertebrate lineages as well.

Mast cells in the rat brain synthesize gonadotropin-releasing hormone

Mast cells occur in the brain and their number changes with reproductive status. While it has been suggested that brain mast cells contain the mammalian hypothalamic form of gonadotropin-releasing hormone (GnRH-I), it is not known whether mast cells synthesize GnRH-I de novo. In the present study, mast cells in the rat thalamus were immunoreactive to antisera generated against GnRH-I and the GnRH-I associated peptide (GAP); mast cell identity was confirmed by the presence of heparin, a molecule specific to mast cells, or serotonin. To test whether mast cells synthesize GnRH-I mRNA, in situ hybridization was performed using a GnRH-I cRNA probe, and the signal was identified as being within mast cells by the binding of avidin to heparin. GnRH-I mRNA was also found, using RT-PCR, in mast cells isolated from the peritoneal cavity. Given the function of GnRH-I in the regulation of reproduction, changes in the population of brain GnRH-I mast cells were investigated. While housing males with sexually receptive females for 2 h or 5 days resulted in a significant increase in the number of brain mast cells, the proportion of mast cells positive for GnRH-I was similar to that in males housed with a familiar male. These findings represent the first report showing that mast cells synthesize GnRH-I and that the mast cell increase seen in a reproductive context is the result of a parallel increase in GnRH-I positive and non-GnRH-I positive mast cells.

An agonist-induced switch in G protein coupling of the gonadotropin-releasing hormone receptor regulates pulsatile neuropeptide secretion

The pulsatile secretion of gonadotropin-releasing hormone (GnRH) from normal and immortalized hypothalamic GnRH neurons is highly calcium-dependent and is stimulated by cAMP. It is also influenced by agonist activation of the endogenous GnRH receptor (GnRH-R), which couples to G(q/11) as indicated by release of membrane-bound alpha(q/11) subunits and increased inositol phosphate/Ca(2+) signaling. Conversely, GnRH antagonists increase membrane-associated alpha(q/11) subunits and abolish pulsatile GnRH secretion. GnRH also stimulates cAMP production but at high concentrations has a pertussis toxin-sensitive inhibitory effect, indicative of receptor coupling to G(i). Coupling of the agonist-activated GnRH-R to both G(s) and G(i) proteins was demonstrated by the ability of nanomolar GnRH concentrations to reduce membrane-associated alpha(s) and alpha(i3) levels and of higher concentrations to diminish alpha(i3) levels. Conversely, alpha(i3) was increased during GnRH antagonist and pertussis toxin treatment, with concomitant loss of pulsatile GnRH secretion. In cholera toxin-treated GnRH neurons, decreases in alpha(s) immunoreactivity and increases in cAMP production paralleled the responses to nanomolar GnRH concentrations. Treatment with cholera toxin and 8-bromo-cAMP amplified episodic GnRH pulses but did not affect their frequency. These findings suggest that an agonist concentration-dependent switch in coupling of the GnRH-R between specific G proteins modulates neuronal Ca(2+) signaling via G(s)-cAMP stimulatory and G(i)-cAMP inhibitory mechanisms. Activation of G(i) may also inhibit GnRH neuronal function and episodic secretion by regulating membrane ion currents. This autocrine mechanism could serve as a timer to determine the frequency of pulsatile GnRH release by regulating Ca(2+)- and cAMP-dependent signaling and GnRH neuronal firing.

Gonadotropin-releasing hormone neuronal system of the freshwater snails Helisoma trivolvis and Lymnaea stagnalis: possible involvement in reproduction

Peptides of the gonadotropin-releasing hormone (GnRH) family are present in neural and nonneural tissues throughout the chordate phylum. Although GnRH peptides have been implicated in nonreproductive functions, their primary function is to control reproduction by regulating sexual behaviors and inducing gonadotropin hormone release from the pituitary. Evidence suggesting the presence of a similar peptide in the central nervous system (CNS) of the gastropod mollusc Helisoma trivolvis has recently been provided. In the present study, we examined the tissue distribution of the peptide and found that it is likely restricted to the nervous system. The neuronal system containing the endogenous GnRH-like peptide is described further and is shown, in part, to innervate the male reproductive tract. Immunostaining in the closely related snail, Lymnaea stagnalis, showed a conservation in the locations of some immunoreactive neurons. Notably, staining occurred in and adjacent to the lateral lobes of both snails. Because these lobes contain neurons involved in the stimulation of egg laying and GnRH staining occurred in additional areas in the Helisoma CNS that are involved in reproduction, we suggest that the endogenous GnRH-like peptide plays a role in regulating reproduction in freshwater snails.

Activation of gonadotropin-releasing hormone receptors induces a long-term enhancement of excitatory postsynaptic currents mediated by ionotropic glutamate receptors in the rat hippocampus

Whole-cell patch-clamp recordings were made from CA1 pyramidal neurons of the rat hippocampus to study the modulation of gonadotropin-releasing hormone (GnRH) on synaptic transmission mediated by ionotropic glutamate receptors. Leuprolide (10(-9)-10(-7) M), a specific GnRH analog, concentration-dependently elicited a long-lasting potentiation of excitatory postsynaptic currents (EPSCs) mediated by ionotropic glutamate receptors. GnRH receptor-induced synaptic potentiation was blocked by 1 microM [Acetyl-3,4-dehydro-Pro1,D-p-F-Phe2,D-Trp3,6]-LHRH, a specific GnRH receptor antagonist. Furthermore, GnRH receptor-induced synaptic potentiation was associated with the stimulation of protein kinase C (PKC), being considerably attenuated by a potent PKC inhibitor (30 microM H-7). The results suggest a long-term enhanced modulation of GnRH on synaptic transmission mediated by ionotropic glutamate receptors, possibly via the actions of PKC in the hippocampus that is an important integrative system in the regulation of reproductive processes.

Multiple gonadotropin-releasing hormone (GnRH)-immunoreactive systems in the brain of the dwarf gourami, Colisa lalia: immunohistochemistry and radioimmunoassay

The present study characterizes gonadotropin-releasing hormone (GnRH) neuronal groups that are located in several different brain regions by investigating GnRH molecular species and projection patterns in an anabantid fish, Colisa lalia. First, we examined the molecular species of GnRHs in extracts of the brain and the pituitary by reverse-phase high-performance liquid chromatography followed by radioimmunoassays. We found salmon GnRH (sGnRH), chicken GnRH-II (cGnRH-II), and an unfamiliar GnRH-like substance. Next, to examine the distribution of each GnRH molecule in different GnRH neuronal groups, we performed immunohistochemistry using four kinds of antisera and an antibody. Furthermore, we performed brain lesioning experiments of terminal nerve (TN) cells, the most conspicuous GnRH-immunoreactive cells in Colisa lalia. Comparisons of immunoreactive structures between TN-lesioned fish and untreated fish elucidated the projection area of each neuronal group. Three major neuronal groups were observed. TN-GnRH cells, which are located in the transitional area between the olfactory bulb and the telencephalon, showed strong sGnRH and weaker cGnRH-II immunoreactivity. TN-GnRH cells projected to wide areas of the central nervous system from the olfactory bulb to the spinal cord. The second group, located in the preoptic area, showed only sGnRH immunoreactivity and projected only to the pituitary. The third one, located in the midbrain tegmentum, exhibited strong cGnRH-II and weaker sGnRH immunoreactivity. This cell group projected mainly to brain regions posterior to the hypothalamus and the spinal cord. These different projection patterns suggest functional differentiation of each GnRH neuronal group.

Effects of a biologically active LHRH fragment on CA1 hippocampal neurons

The actions of a behaviorally active luteinizing hormone-releasing hormone fragment, Ac-LHRH(5-10), on CA1 pyramidal cells were studied utilizing conventional intracellular recordings from the in vitro rat hippocampal slice preparation. The behaviorally active fragment (10(-7) M) and the natural decapeptide [LHRH(1-10), 10(-8) M] had similar actions on CA1 neurons: a long-duration depolarization associated with increased input resistance, a reduction in the slow afterhyperpolarization (AHP), and a decrease in accommodation. In contrast, a biologically inactive LHRH fragment [LHRH(1-6) at 10(-7) M] had no effect on electrical properties of CA1 neurons. These data suggest that Ac-LHRH(5-10), like LHRH(1-10), may have a modulatory action on hippocampal neurons.

The Kallmann's syndrome variant (KSV) model of the schizophrenias

The KSV model of the schizophrenias proposes that up to 70% of schizophrenics have a pathogenic allele, or abnormal expression, of the KALIG-1 gene which is located at Xp22.3. This gene encodes a nerve-cell adhesion molecule (N-CAM) like protein, and is deleted in 66% of patients with Kallmann's syndrome, anosmia with secondary hypogonadism. Although superficially distinct, the schizophrenias and Kallmann's syndrome show numerous parallel trait defects which occur with a similar sex distribution. These defects are usually more profound in Kallmann's syndrome. Occasionally, Kallmann's patients exhibit additional defects, such as ichthyosis, which are due to the further deletion or translocation of adjacent genes. Since schizophrenics exhibit virtually all known trait defects in Kallmann's except these, it suggests that the aberrant genes are defective, but not deleted in schizophrenia. It also appears that compensatory mechanisms, involving serine proteases, are active in schizophrenia, which largely preserve fertility, but at the expense of an increased vulnerability to develop a psychosis by an episodic disruption of the blood-CSF barrier. Consequently, schizophrenia is rare in Kallmann's patients, while most schizophrenics are capable of reproduction.

LHRH neurons in the medial septal-diagonal band-preoptic area do not project directly to the hippocampus: a double-labeling immunohistochemical study

While neurons containing immunoreactive luteinizing hormone-releasing hormone (LHRH) are scattered primarily in the medial septal-diagonal band of Broca-medial preoptic area (mS-dbB-PO) complex, autoradiographic studies have demonstrated dense concentrations of LHRH receptors in the hippocampus. The route by which LHRH is transported to its hippocampal receptors is unknown. The present study was designed to test the hypothesis that LHRH-containing neurons in the mS-dbB-PO complex project to hippocampal sites containing LHRH receptors, thereby serving as a source of innervation to these receptors. Large (0.10 microliters) or small (0.02 microliters) volumes of the retrograde tracer wheat germ agglutinin (WGA) were injected unilaterally into four separate hippocampal locations in six ovariectomized female rats. In an additional five females, a 0.15 microliter volume of the retrograde tracer fluorogold (FG) was similarly injected. After a five day survival period, the animals were sacrificed. Vibratome sections of the brain were stained for both WGA and LHRH with a dual immunohistochemical technique. Since FG is a fluorescent chromagen, brains of animals injected with FG only required processing for LHRH immunofluorescence. As a positive control, some sections containing retrogradely labeled cells filled with either WGA or FG were processed for choline acetyltransferase (CHAT) immunoreactivity. The WGA and FG injections covered targeted hippocampal sites and neurons containing retrogradely transported WGA or FG were found in abundance in the mS-dbB-PO complex. In accord with previous reports, many CHAT-positive and fewer LHRH-positive neurons were found in this complex. Approximately 5-10% of the CHAT-positive neurons also contained WGA or FG; however, no neurons were found to co-localize LHRH and either of the retrograde tracers. The results indicate that LHRH neurons in the mS-dbB-PO complex do not project directly to hippocampal sites containing LHRH receptors.

Specific binding of 125I-LHRH agonist to hippocampal membranes: fluctuations during the estrous cycle

Specific binding of 125I-[D-Ala6-CH3-Leu7-Pro9,NHET]LHRH, a LHRH agonist, to hippocampal membranes prepared from ovariectomized female rates was examined. One high affinity binding site was observed with a Kd of 0.12 +/- 0.01 nM and an apparent Bmax of 13.0 +/- 3.8 fmol/mg. Luteinizing hormone-releasing hormone and the behaviorally active Ac-LHRH(5-10) were able to compete for the agonist binding site. Native LHRH had an apparent Ki of 1.73 nM, while AC-LHRH(5-10) was 30 times less potent. Competition studies examined over the rat estrous cycle revealed an eighteenfold decrease in apparent affinity during diestrus I and estrus compared with ovariectomized animals. Tissue from animals in proestrus had a Ki of 5.0 nM. Specific binding studies indicate that receptor concentration is highest in proestrus (6.11 +/- 0.90 fmol/mg) and significantly lower during estrus (2.4 +/- 0.29 fmol/mg). These data suggest that at least one fragment of native LHRH can interact with neuronal LHRH receptors and that these receptors, like those in the pituitary, can be modulated by circulating steroid hormones.

Effects of lordosis-relevant neuropeptides on midbrain periaqueductal gray neuronal activity in vitro

Certain neuropeptides can facilitate lordosis by acting on midbrain periaqueductal gray (PAG) in estrogen-primed female rats. Here, we investigated responses of individual PAG neurons in vitro, to five neuropeptides: substance P (SP), luteinizing hormone-releasing hormone (LHRH), prolactin (PRL), oxytocin (OT), and thyrotropin-releasing hormone (TRH). Substance P, OT, and TRH excited spontaneous activity of PAG neurons through neurotransmitter-like actions in a dose-dependent manner, whereas LHRH and PRL virtually never affected PAG neurons this way. Oxytocin acted through oxytocin receptors located on the recorded PAG neurons, since excitatory actions of OT were 1) not abolished by synaptic blockade, 2) mimicked by the OT-specific agonist [Thr4, Gly7]OT but not by arginine vasopressin, and 3) blocked by the OT-specific antagonist [d(CH2)5,Tyr(Me)2,Orn8]vasotocin. Although LHRH had no neurotransmitter-like action on spontaneous activity of PAG neurons, it, as well as SP, could modulate responses of some dorsal PAG neurons to GABAA and GABAB agonists or norepinephrine. Neuromodulatory actions of LHRH and SP could help facilitate lordosis through PAG neurons.

Gonadotropin-releasing hormone (GnRH) cells of the terminal nerve as a model neuromodulator system

Modulation of ionic channel properties by neurotransmitters and hormones is called neuromodulation and may be the basis for many long-lasting changes in animal behavior, e.g. changes in the arousal or motivational states. Gonadotropin-releasing hormone (GnRH), originally identified as a hypophysiotropic hormone, is now believed to act also as a neuromodulator. From studies of electrical activities and morphology of terminal nerve cells (major source of GnRH) of a fish brain, a general hypothesis regarding modulator neurons is proposed; modulator neurons have endogenous oscillatory activities which vary according to the animal's hormonal or environmental conditions. These modulator neurons, in turn, regulate neuronal excitabilities in a wide variety of brain regions simultaneously via multiple axonal branches.

Ultrastructural characterization of gonadotropin-releasing hormone (GnRH)-immunoreactive terminal nerve cells in the dwarf gourami

Recently we have been studying gonadotropin-releasing hormone (GnRH) cells of the terminal nerve (TN) in the dwarf gourami, which may serve as a good model system for the study of neuromodulator functions. Here we report on the ultrastructural characterization of TN-GnRH cells using postembedding immunoelectron microscopy. The GnRH immunoreactivities could be demonstrated on dense-cored vesicles (DCVs) in cell bodies, fibers and varicosities. However, we could find no evidence of GnRH-immunoreactive synapses which are characterized by active zones. This may suggest that GnRH is released non-synaptically from DCV-containing fiber varicosities and that it exerts its modulatory action on GnRH receptors located on nearby as well as distant target neurons.

A sequence related to the human gonadoliberin precursor near the N-termini of HIV and SIV gag polyproteins

A highly conserved sequence near the N-terminus of all human (HIV) and simian (SIV) immunodeficiency virus gag polyproteins appears to be a precursor for a viral mimic of the amidated C-terminus of human gonadoliberin. The gag polyproteins are known to be myristylated; processing of the amidation site would yield myristylated 23-residue peptides whose C-terminal sequences mimic gonadoliberin and presumably behave as ligands for the gonadoliberin receptor. This paper describes the discovery of conserved gonadoliberin-precursor-related sequences in HIV and SIV gag polyproteins and in the p-17 core proteins derived from them. Arguments are presented that the conserved precursor structure requires post-translational processing to a peptide amide derivative which is a ligand for the gonadoliberin receptor. A model has been developed for entry of the viral genomic RNA into host cells through the gonadoliberin receptor and experiments are suggested to confirm or refute the model. This proposed mechanism for entry of HIV genomic RNA into host cells, if it proves to be substantially correct, suggests several new approaches to prevention and treatment of acquired immunodeficiency syndrome (AIDS).

Electrophysiological evidence for a rapid membrane action of the gonadal steroid, 17 beta-estradiol, on CA1 pyramidal neurons of the rat hippocampus

The rapid electrophysiological effects of 17 beta-estradiol on CA1 pyramidal neurons (n = 86) were investigated utilizing intracellular recording from the rat hippocampal slice preparation. Bath application of 17 beta-estradiol, but not 17 alpha-estradiol, caused a reversible depolarization and increased input resistance with a latency of less than 1 min in 19.8% of CA1 neurons tested. There was no significant difference in the percentage of estradiol-responsive cells between male and female rats. Estradiol-responsive cells were identified from prepubertal female rats, as well as females in all stages of the estrous cycle. 17 beta-estradiol had no effect on the slow afterhyperpolarization or accommodative properties of CA1 neurons. In 2 out of 4 cells tested, the specific antiestrogen, tamoxifen, blocked the excitatory response to 17 beta-estradiol.

Involvement of metallo-endopeptidase in degradation of luteinizing hormone-releasing hormone by neuronal and glial cells cultured from rat fetal brain

Luteinizing hormone-releasing hormone (LHRH) was degraded by neuronal and glial cells cultured from fetal rat brain. The degradation of LHRH by neuronal cells was strongly inhibited by a metal chelator. Captopril only inhibited by generation of fragment (1-3) from fragment (1-5). In the presence of captopril, fragment (1-5) accumulated in the highest amount among the N-terminal fragments identified. The initial cleavage of LHRH, as determined by following the loss of the LHRH peak, was strongly inhibited by thiol-blocking reagents, as well as metal chelators. The results with glial cells were almost the same as those seen with neuronal cells. Thus, we propose that a thiol-dependent membrane-bound metallo-endopeptidase plays a major role in the initial stage of degradation of LHRH at the Tyr5-Gly6 bond in both neurons and glia. Angiotensin-converting enzyme is involved in the secondary process of the LHRH degradation in both cells.