Neuroendocrine plasticity in GnRH release during rat estrous cycle: correlation with molecular markers of synaptic remodeling

The Regulatory Effect of Insulin-Like Growth Factor-2 on Hypothalamic Gonadotropin-Releasing Hormone Neurons during the Pubertal Period

BACKGROUND

The insulin-like growth factor (IGF) system plays a vital role in regulating gonadotropin-releasing hormone (GnRH), whether the IGF2 can act on the GnRH neurons during the pubertal period is unclear.

METHODS

Central precocious puberty (CPP) rats were induced by danazol, and when the rats met the first diestrus, they were euthanized and tissues were collected. GT1-7 cells were cultured and treated with 0, 1, 10 ng/mL IGF2 for 4 hours and the changes in GnRH were measured. Mice were injected intracerebroventricularly with IGF2 (15 ng/g, 5 μL) or with the same dose of phosphate buffered saline (PBS), after eight hours, they were euthanized and tissues collected.

RESULTS

CPP rats had increased expression of IGF2 and GnRH mRNA and their respective proteins in the preoptic area (POA) of the hypothalamus. Treatment of GT1-7 cells with 10 ng/mL of IGF2 increased GnRH mRNA and protein expression, and GnRH concentration in the culture medium. Injection of IGF2 protein into the lateral ventricle of mice increased the expression of GnRH mRNA and protein in the POA.

CONCLUSIONS

IGF2 may upregulate the synthesis of GnRH during the pubertal period, and may also take part in the pathology of CPP.

Estradiol Regulates Polysialylated Form of the Neural Cell Adhesion Molecule Expression and Connectivity of O-LM Interneurons in the Hippocampus of Adult Female Mice

The estrous cycle is caused by the changing concentration of ovarian hormones, particularly 17β-estradiol, a hormone whose effect on excitatory circuits has been extensively reported. However, fewer studies have tried to elucidate how this cycle, or this hormone, affects the plasticity of inhibitory networks and the structure of interneurons. Among these cells, somatostatin-expressing O-LM neurons of the hippocampus are especially interesting. They have a role in the modulation of theta oscillations, and they receive direct input from the entorhinal cortex, which place them in the center of hippocampal function. In this study, we report that the expression of polysialylated form of the neural cell adhesion molecule (PSA-NCAM) in the hippocampus, a molecule involved in the plasticity of somatostatin-expressing interneurons in the adult brain, fluctuated through the different stages of the estrous cycle. Likewise, these stages and the expression of PSA-NCAM affected the density of dendritic spines of O-LM cells. We also describe that 17β-estradiol replacement of adult ovariectomized female mice caused an increase in the perisomatic inhibitory puncta in O-LM interneurons as well as an increase in their axonal bouton density. Interestingly, this treatment also induced a decrease in their dendritic spine density, specifically in O-LM interneurons lacking PSA-NCAM expression. Finally, using an ex vivo real-time assay with entorhinal-hippocampal organotypic cultures, we show that this hormone decreased the dynamics in spinogenesis, altogether highlighting the modulatory effect that 17β-estradiol has on inhibitory circuits.

PSA mimetic 5-nonyloxytryptamine protects cerebellar neurons against glutamate induced excitotoxicity: An in vitro perspective

PSA-NCAM is a molecule of therapeutic interest for its key role in promoting neuritogenesis and synaptic plasticity. The current study was aimed to investigate the neuroregenerative potential of 5-nonyloxytryptamine (5-NOT) as PSA mimetic compound against glutamate induced excitotoxicity. 2D and 3D cultures of cerebellar neurons challenged with glutamate were used to ascertain the effect of 5-NOT on neurite outgrowth, migration and expression of neuronal plasticity markers. Glutamate excitotoxicity is one of the major underlying pathological factor responsible for neurodegeneration in various neurological disorders such as trauma, stroke, ischemia, epilepsy and neurodegenerative diseases.5-NOT treatment was observed to promote axonal growth and defasiculation in glutamate challenged neurons as well as promoted the migration of cerebellar neurons in both wound scratched area and cerebellar explant cultures. Further, 5-NOT treatment upregulated the expression of synaptic plasticity and cell survival pathway proteins which showed reduced expression after glutamate induced excitotoxicity. Thus, this preliminary data reveals thatPSA-mimetic,5-NOT may prove to be a potential neuroprotective candidate for neurodegenerative diseases.

The effects of neuromedin S on the hypothalamic-pituitary-testicular axis in male pigs in vitro

Evidence has shown that neuromedin S (NMS) and its receptor (NMU2R) are expressed in the hypothalamus, pituitary, and testis of pigs. To determine the potential mechanisms of NMS, we systematically investigated the direct effects of NMS on the hypothalamic-pituitary-testicular (HPT) axis of male pigs in vitro. We initially confirmed that NMU2R distributed in isolated hypothalamic cells, anterior pituitary cells and Leydig cells using immunocytochemistry. Subsequently we investigated the direct effects of NMS on hormone secretion from cells (anterior pituitary cells and Leydig cells) treated with different doses of NMS. The results showed that NMS increase the release of LH and FSH from anterior pituitary cells and testosterone from Leydig cells. NMS up-regulated the expression of NMU2R and GnRH mRNAs in hypothalamic cells, NMU2R, LH and FSH mRNAs in anterior pituitary cells, and NMU2R, STAR, P450 and 3β-HSD mRNAs and the expression of PCNA and Cyclin B1 protein in Leydig cells; moreover, it down-regulated the expression of GnIH mRNA in hypothalamic cells. Using immunofluorescence staining and confocal microscopy, we also demonstrated the colocalization of NMU2R and AR or GnIH in Leydig cells. These data in vitro indicated that NMS may regulate the release and/or synthesis of LH, FSH and testosterone at different levels of the reproductive axis through NMU2R, which provided novel evidence of the potential roles of NMS in regulation of pig reproduction.

Distribution and regulation of gonadotropin-releasing hormone, kisspeptin, RF-amide related peptide-3, and dynorphin in the bovine hypothalamus

Recent work has led to the hypothesis that kisspeptin/neurokinin B/dynorphin (KNDy) neurons in the arcuate nucleus (ARC) play a key role in gonadotropin-releasing hormone (GnRH) pulse generation and gonadal steroid feedback, with kisspeptin driving GnRH release and neurokinin B and dynorphin acting as pulse start and stop signals, respectively. A separate cell group, expressing RFamide-related peptide-3 (RFRP-3) has been shown to be a primary inhibitor of GnRH release. Very little is known regarding these cell groups in the bovine. In this study, we examined the relative immunoreactivity of kisspeptin, dynorphin, and RFRP-3 and their possible connectivity to GnRH neurons in the hypothalami of periestrus and diestrus bovine. While GnRH and RFRP-3 immunoreactivity were unchanged, kisspeptin and dynorphin immunoreactivity levels varied in relation to plasma progesterone concentrations and estrous status. Animals with higher plasma progesterone concentrations in diestrus had lower kisspeptin and increased dynorphin immunoreactivity in the ARC. The percentage of GnRH cells with kisspeptin or RFRP-3 fibers in close apposition did not differ between estrous stages. However, the proportions of GnRH cells with kisspeptin or RFRP-3 contacts (∼49.8% and ∼31.3%, respectively) suggest direct communication between kisspeptin and RFRP-3 cells to GnRH cells in the bovine. The data produced in this work support roles for kisspeptin and dynorphin, within the KNDy neural network, in controlling GnRH release over the ovarian cycle and conveying progesterone-negative feedback onto GnRH neurons in the bovine.

Second generation anti-epileptic drugs adversely affect reproductive functions in young non-epileptic female rats

Reproductive endocrine disturbances are a major health concern in women with epilepsy due to their long term use of antiepileptic drugs (AEDs). Second generation AEDs such as topiramate (TPM) and gabapentin are frequently used for the treatment of epilepsy as well as migraine, bipolar disorder etc. Despite the widespread clinical complications, however the definitive mechanism(s) mediating the side effects of TPM and gabapentin remain obscure. The present study was aimed to evaluate the long term effects of TPM and gabapentin on reproductive functions in young female Wistar rats. Estrous cyclicity, ovarian histology as well as estradiol, LH, leptin and insulin hormones level were studied to elucidate the long-term effect of these AEDs monotherapy on reproductive functions in non-epileptic animals. Further to explore the effects on gonadotropin releasing hormone (GnRH) neuroendocrine plasticity, the expression of GnRH, gamma-amino butyric acid (GABA), glutamic acid decarboxylase (GAD), glial fibrilliary acidic protein (GFAP) and polysialylated form of neural cell adhesion molecule (PSA-NCAM) was studied in median eminence (ME) region of these animals by immunohistochemistry, Western blot hybridization and RT-PCR. Our results demonstrate that TPM and gabapentin treatment for 8 weeks cause reproductive dysfunction as ascertained by disturbed hormonal levels and estrous cyclicity as well as alterations in GABAergic system and GnRH neuronal-glial plasticity. Our findings suggest that treatment with TPM and gabapentin disrupts the complete hypothalamo-hypophyseal-gonadal axis (HPG) through GnRH pulse generator in hypothalamus.

Intermittent fasting dietary restriction regimen negatively influences reproduction in young rats: a study of hypothalamo-hypophysial-gonadal axis

Nutritional infertility is very common in societies where women fail to eat enough to match their energy expenditure and such females often present as clinical cases of anorexia nervosa. The cellular and molecular mechanisms that link energy balance and central regulation of reproduction are still not well understood. Peripheral hormones such as estradiol, testosterone and leptin, as well as neuropeptides like kisspeptin and neuropeptides Y (NPY) play a potential role in regulation of reproduction and energy balance with their primary target converging on the hypothalamic median eminence-arcuate region. The present study was aimed to explore the effects of negative energy state resulting from intermittent fasting dietary restriction (IF-DR) regimen on complete hypothalamo-hypophysial-gonadal axis in Wistar strain young female and male rats. Significant changes in body weight, blood glucose, estrous cyclicity and serum estradiol, testosterone and LH level indicated the negative role of IF-DR regimen on reproduction in these young animals. Further, it was elucidated whether serum level of metabolic hormone, leptin plays a mechanistic role in suppressing hypothalamo-hypophysial-gonadal (HPG) axis via energy regulators, kisspeptin and NPY in rats on IF-DR regimen. We also studied the effect of IF-DR regimen on structural remodeling of GnRH axon terminals in median eminence region of hypothalamus along with the glial cell marker, GFAP and neuronal plasticity marker, PSA-NCAM using immunostaining, Western blotting and RT-PCR. Together these data suggest that IF-DR regimen negatively influences reproduction in young animals due to its adverse effects on complete hypothalamus-hypophysial-gonadal axis and may explain underlying mechanism(s) to understand the clinical basis of nutritional infertility.

Effects of photoperiod and food restriction on the reproductive physiology of female California mice

Many temperate-zone animals use changes in photoperiod to time breeding. Shorter term cues, like food availability, are integrated with photoperiod to adjust reproductive timing under unexpected conditions. Many mice of the genus Peromyscus breed in the summer. California mice (Peromyscus californicus), however, can breed year round, but tend to begin breeding in the winter. Glial cells may be involved in transduction of environmental signals that regulate gonadotrophin releasing hormone I (GnRH) activity. We examined the effects of diet and photoperiod on reproduction in female California mice. Mice placed on either short days (8L:16D) or long days (16L:8D) were food restricted (80% of normal intake) or fed ad libitum. Short day-food restricted mice showed significant regression of the reproductive system. GnRH-immunoreactivity was increased in the tuberal hypothalamus of long day-food restricted mice. This may be associated with the sparing effect long days have when mice are food restricted. The number of GFAP-immunoreactive fibers in proximity to GnRH nerve terminals correlated negatively with uterine size in ad libitum but not food restricted mice, suggesting diet may alter glial regulation of the reproductive axis. There was a trend towards food restriction increasing uterine expression of c-fos mRNA, an estrogen dependent gene. Similar to other seasonally breeding rodents, short days render the reproductive system of female California mice more susceptible to effects of food restriction. This may be vestigial, or it may have evolved to mitigate consequences of unexpectedly poor winter food supplies.

[Structural plasticity of the adult central nervous system: insights from the neuroendocrine hypothalamus]

Accumulating evidence renders the dogma obsolete according to which the structural organization of the brain would remain essentially stable in adulthood, changing only in response to a need for compensatory processes during increasing age and degeneration. It has indeed become clear from investigations on various models that the adult nervous system can adapt to physiological demands by altering reversibly its synaptic circuits. This potential for structural and functional modifications results not only from the plastic properties of neurons but also from the inherent capacity of the glial cellular components to undergo remodeling as well. This is currently known for astrocytes, the major glial cells in brain which are well-recognized as dynamic partners in the mechanisms of synaptic transmission, and for the tanycytes and pituicytes which contribute to the regulation of neurosecretory processes in neurohemal regions of the hypothalamus. Studies on the neuroendocrine hypothalamus, whose role is central in homeostatic regulations, have gained good insights into the spectacular neuronal-glial rearrangements that may subserve functional plasticity in the adult brain. Following pioneering works on the morphological reorganizations taking place in the hypothalamo-neurohypophyseal system under certain physiological conditions such as dehydration and lactation, studies on the gonadotropic system that orchestrates reproductive functions have re-emphasized the dynamic interplay between neurons and glia in brain structural plasticity processes. This review summarizes the major contributions provided by these researches in the field and also addresses the question of the morphological rearrangements that occur on a 24-h basis in the central component of the circadian clock responsible for the temporal aspects of endocrine regulations. Taken together, the reviewed data highlight the close cooperation between neurons and glia in developing strategies for functional adaptation of the brain to the changing conditions of the internal and external environment.

Expression of kv4.3 voltage-gated potassium channels in rat gonadotrophin-releasing hormone (GnRH) neurons during the estrous cycle

Regular and timely electrical activity of gonadotrophin-releasing hormone (GnRH) neurons accompanies the pulsatile release of GnRH that plays a central role in regulating fertility. Although transient outward A-type currents (I(A)) have been electrophysiologically identified in GnRH neurons, the molecular identity of the channels that underlie these currents are unknown. Several families of voltage-gated potassium channels can underlie I(A). However, the biophysical properties of I(A) described in previous electrophysiological studies are strongly characteristic of members of the Kv4 family of voltage-gated channels. We, therefore, sought to determine the presence of Kv4 channels in GnRH neurons. We used reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis to determine whether Kv4 messenger RNA (mRNA) and protein are present in the rat medial preoptic area (MPOA) and median eminence (ME). We used double-label immunohistochemistry to determine whether Kv4 colocalized with GnRH cell bodies in the MPOA and GnRH axons in the ME. Kv4.3 channels co-localized with GnRH in the MPOA but not in the ME. Neither Kv4.2 nor Kv4.1 co-localized with GnRH in either the MPOA or the ME. The electrical activity of GnRH neurons changes dramatically during the estrous cycle. We, therefore, studied the change in Kv4.3 expression in GnRH neurons during the estrous cycle. In the estrus phase, 58.05% of GnRH neurons expressed Kv4.3 compared to 74.48% in diestrus-proestrus rats (P < .05). Our data suggest that Kv4.3 is the major molecular component of I(A) in GnRH neurons, and furthermore that the expression of Kv4.3 changes significantly during the rat estrous cycle.

Estrogens regulate posttranslational modification of neural cell adhesion molecule during the estrogen-induced gonadotropin surge

Estrogen-induced synaptic plasticity (EISP) in the periventricular area (PVA) of the hypothalamus is necessary for the preovulatory gonadotropin surge. Because in situ enzymatic desialization of hypothalamic polysialylated (PSA) neural cell adhesion molecule (NCAM) blocked EISP, we examined the presence and amount of NCAM isotopes, PSA-NCAM, and sialylation enzymes in microdissected mouse hypothalamus tissues from proestrous afternoon [peak of estrogens and nadir of arcuate nucleus (AN) synapses] and metestrous morning (nadir of estrogens and highest AN synapses). Immunohistochemistry confirmed immunoreactive (ir) PSA-NCAM staining in the perineural spaces of the PVA. The extent of staining was cycle dependent, with more dense and complete profiles of individual neurons limned by the ir-PSA-NCAM staining on proestrus and less on metestrus. Western blots showed that high levels of ir-PSA-NCAM on proestrus are accompanied by diminished ir-NCAM-140 and -180 but not ir-NCAM-120 and the reverse on metestrus (P < 0.05). To evaluate the increase of sialylated NCAM at the expense of desialylated protein, expression of the responsible polysialyltransferase enzymes polysialyltransferase (ST8Sia IV) and sialyltransferase (ST8Sia II) mRNA levels were measured using RT-PCR. Both polysialyltransferase and sialyltransferase mRNA are more abundant on proestrus than metestrus (P < 0.05), indicating that these enzymes are regulated by estrogens. These results support estrogen-regulated formation and extrusion of hydrophilic PSA-NCAM into perineural spaces in the PVA as part of the mechanism of EISP.

Age-related impairments in neuronal plasticity markers and astrocytic GFAP and their reversal by late-onset short term dietary restriction

Recent studies on the effects of dietary restriction (DR) in rodents and primates have shown that even late-onset short-term regimens can bring about comparable beneficial changes seen in animals subjected to life-long DR. We studied the effect of aging on the expression of neural cell adhesion molecule (NCAM), its polysialylated form PSA-NCAM and astrocytic marker glial fibrillary acidic protein (GFAP) by immunohistofluorescent staining and immunoblotting in 1, 3, 6, 18 and 24 months old male wistar rats. Maximum expression of NCAM and PSA-NCAM was observed in sub-granular zone (SGZ) or granular cell layer (GCL) of hippocampus, arcuate region and paraventricular area of hypothalamus and piriform cortex layer II from 1 and 3 months old rats, thereafter, gradual downregulation was observed in 6, 18 and 24 months old rats. Progressive increase in astrocytic GFAP expression was noticed in these regions of brain with age. We further addressed whether DR initiated in late adulthood in 24 months old rats confers beneficial effects and can reverse changes in expression of NCAM, PSA-NCAM and GFAP. These results suggest that even late-onset short term DR regimen in old rats can have beneficial effects on neuroplasticity.

The role of glia in the hypothalamus: implications for gonadal steroid feedback and reproductive neuroendocrine output

Neuron-to-glia, glia-to-neuron, and glia-to-glia communication are implicated in the modulation of neuronal activity and synaptic transmission relevant to reproduction. Glial cells play an important role in neuroendocrine regulation and participate in the sexual differentiation of neuronal connectivity of brain regions involved in the control of reproductive neuroendocrine output. During puberty, modifications in the morphology and chemistry of astrocytes and tanycytes in the hypothalamus and median eminence influence the maturation of the neuronal circuits controlling the secretion of GnRH. During adult reproductive life, the glial cells participate in the transient remodeling of neuronal connectivity in the preoptic area, the arcuate nucleus, the median eminence, and other brain regions involved in the control of reproduction. Gonadal hormones regulate glial plasticity by direct and indirect effects and regulate various other endocrine signals, local soluble factors and adhesion molecules that also affect glial function and glia-to-neuron communication. The glial cells, therefore, are central to the coordination of endocrine and local inputs that bring about neural plasticity and adapt reproductive capacity to homeostatic signals.

Cross-talk between estrogen and leptin signaling in the hypothalamus

Obesity, characterized by enhanced food intake (hyperphagia) and reduced energy expenditure that results in the accumulation of body fat, is a major risk factor for various diseases, including diabetes, cardiovascular disease, and cancer. In the United States, more than half of adults are overweight, and this number continues to increase. The adipocyte-secreted hormone leptin and its downstream signaling mediators play crucial roles in the regulation of energy balance. Leptin decreases feeding while increasing energy expenditure and permitting energy-intensive neuroendocrine processes, such as reproduction. Thus, leptin also modulates the neuroendocrine reproductive axis. The gonadal steroid hormone estrogen plays a central role in the regulation of reproduction and also contributes to the regulation of energy balance. Estrogen deficiency promotes feeding and weight gain, and estrogen facilitates, and to some extent mimics, some actions of leptin. In this review, we examine the functions of estrogen and leptin in the brain, with a focus on mechanisms by which leptin and estrogen cooperate in the regulation of energy homeostasis.

Potential of PSA-NCAM in neuron-glial plasticity in the adult hypothalamus: role of noradrenergic and GABAergic neurotransmitters

The present study was designed to establish the dynamic regulation of polysialylated form of neural cell adhesion molecule (PSA-NCAM) expression by neurotransmitters controlling gonadotropin releasing hormone (GnRH) secretion. The expression of PSA-NCAM and glial fibrillary acidic protein (GFAP) on GnRH cell bodies and axon terminals in the medial preoptic area (mPOA) and median eminence-arcuate (ME-ARC) region of hypothalamus was studied in the proestrous phase of cycling rats treated with alpha-adrenergic receptor blocker phenoxybenzamine (PBZ) and gamma-aminobutyric acid (GABA) by using dual immunohistofluorescent staining and Western blot hybridization. To further elucidate whether activity mediated regulation of PSA-NCAM in GnRH neuron is via regulation of PSA biosynthesis by polysialytransferase (PST) enzyme, the expression of PST-1 enzyme was studied by using fluorescent in situ hybridization (FISH). Both GnRH and PSA-NCAM immunostaining was much higher in the mPOA and ME-ARC region from proestrous phase rats, whereas, PBZ and GABA treatments significantly reduced their expression, GFAP-ir and its content were increased in the PBZ and GABA treated proestrous rats. Taken together, our observations add to the growing evidence that PSA-NCAM plays permissive role for neuronal-glial remodeling and further suggest a functional link between activity dependent structural remodeling in GnRH neurons. Further, enhanced mRNA expression of PST suggests that the biosynthesis of PSA on NCAM is regulated at the transcriptional level.

Transcriptional regulation of polysialylated neural cell adhesion molecule expression by NMDA receptor activation in retinoic acid-differentiated SH-SY5Y neuroblastoma cultures

NMDA receptors exhibit a dichotomy of signaling with excessive stimulation leading to neuronal damage that occurs during neurodegenerative disorders, whereas the normal burst of activity results in plastic responses with the expression of molecular substrates of long-term plasticity, growth and survival. Control of polysialylated neural cell adhesion molecule (PSA-NCAM) expression by NMDA receptor activation has been described in several systems, suggesting a functional link between these two proteins. The coordinated induction of several different transcription factors initiated by NMDA receptor stimulation may be a key mechanism in the orchestration of specific target gene expression that underlies various aspects of CNS function, including plastic responses. We report here the transcriptional regulation of PSA-NCAM expression by subtoxic dose of NMDA in retinoic acid-differentiated SH-SY5Y cell cultures. SH-SY5Y cell cultures differentiated with retinoic acid (10 microM) were exposed to NMDA (100 microM) or to antagonist MK-801 (200 nM) prior to treatment with NMDA and cells were harvested after 24 h of treatment to study the expression of PSA-NCAM, nuclear factor kappaB (NF-kappaB) and activator protein-1 (AP-1) by Western blotting and dual immunocytofluorescence and expression of polysialyltransferase (PST) mRNA by fluorescent in situ hybridization (FISH). We observed the induction of transcription factors NF-kappaB and AP-1 along with PSA-NCAM expression in response to NMDA receptor activation. Also, PSA-NCAM regulation in response to NMDA receptor activity was shown to be transcriptionally controlled, as seen by temporal and spatial changes observed in the expression of PST mRNA in NMDA-treated SH-SY5Y cell cultures. This raises the interesting possibility that NF-kappaB and AP-1 expression is involved in propagating the signals of NMDA receptor activity that leads to downstream strengthening of long-term plasticity changes in differentiated SH-SY5Y neuroblastoma cell cultures. Thus understanding the regulation of PSA-NCAM expression by NMDA receptor-mediated activity may represent a fundamental prerequisite for the development of therapies in order to maintain neuronal plasticity throughout life and functional recovery after brain damage.

Involvement of Anteroventral Periventricular Metastin/Kisspeptin Neurons in Estrogen Positive Feedback Action on Luteinizing Hormone Release in Female Rats

Metastin/kisspeptin, the KiSS-1 gene product, has been identified as an endogenous ligand of GPR54 that reportedly regulates GnRH/LH surges and estrous cyclicity in female rats. The aim of the present study was to determine if metastin/kisspeptin neurons are a target of estrogen positive feedback to induce GnRH/LH surges. We demonstrated that preoptic area (POA) infusion of the anti-rat metastin/kisspeptin monoclonal antibody blocked the estrogen-induced LH surge, indicating that endogenous metastin/kisspeptin released around the POA mediates the estrogen positive feedback effect on GnRH/LH release. Metastin/kisspeptin neurons in the anteroventral periventricular nucleus (AVPV) may be responsible for mediating the feedback effect because the percentage of c-Fos-expressing KiSS-1 mRNA-positive cells to total KiSS-1 mRNA-positive cells was significantly higher in the afternoon than in the morning in the anteroventral periventricular nucleus (AVPV) of high estradiol (E(2))-treated females. The percentage of c-Fos-expressing metastin/kisspeptin neurons was not different between the afternoon and morning in the arcuate nucleus (ARC). Most of the KiSS-1 mRNA expressing cells contain ERalpha immunoreactivity in the AVPV and ARC. In addition, AVPV KiSS-1 mRNA expressions were highest in the proestrous afternoon and lowest in the diestrus 1 in females and were increased by estrogen treatment in ovariectomized animals. On the other hand, the ARC KiSS-1 mRNA expressions were highest at diestrus 2 and lowest at proestrous afternoon and were increased by ovariectomy and decreased by high estrogen treatment. Males lacking the surge mode of GnRH/LH release showed no obvious cluster of metastin/kisspeptin-immunoreactive neurons in the AVPV when compared with high E(2)-treated females, which showed a much greater density of these neurons. Taken together, the present study demonstrates that the AVPV metastin/kisspeptin neurons are a target of estrogen positive feedback to induce GnRH/LH surges in female rats.

PSA-NCAM in mammalian structural plasticity and neurogenesis

Polysialic acid (PSA) is a linear homopolymer of alpha2-8-N acetylneuraminic acid whose major carrier in vertebrates is the neural cell adhesion molecule (NCAM). PSA serves as a potent negative regulator of cell interactions via its unusual biophysical properties. PSA on NCAM is developmentally regulated thus playing a prominent role in different forms of neural plasticity spanning from embryonic to adult nervous system, including axonal growth, outgrowth and fasciculation, cell migration, synaptic plasticity, activity-induced plasticity, neuronal-glial plasticity, embryonic and adult neurogenesis. The cellular distribution, developmental changes and possible function(s) of PSA-NCAM in the central nervous system of mammals here are reviewed, along with recent findings and theories about the relationships between NCAM protein and PSA as well as the role of different polysialyltransferases. Particular attention is focused on postnatal/adult neurogenesis, an issue which has been deeply investigated in the last decade as an example of persisting structural plasticity with potential implications for brain repair strategies. Adult neurogenic sites, although harbouring all subsequent steps of cell differentiation, from stem cell division to cell replacement, do not faithfully recapitulate development. After birth, they undergo morphological and molecular modifications allowing structural plasticity to adapt to the non-permissive environment of the mature nervous tissue, that are paralled by changes in the expression of PSA-NCAM. The use of PSA-NCAM as a marker for exploring differences in structural plasticity and neurogenesis among mammalian species is also discussed.

Relationship between contents of adrenomedullin and distributions of neutral endopeptidase in blood and tissues of rats in septic shock

Adrenomedullin (ADM), a multifunction peptide with important roles in regulating cardiovascular homeostasis, has the vasodilatory properties and is of particular interest in the pathophysiology of sepsis. ADM levels in plasma and tissues are regulated by the proteolysis of neutral endopeptidase (NEP), the major enzyme of ADM degradation. We observed the NEP activity in the plasma, the activity and distribution of NEP and its mRNA expression in the tissues of rats in septic shock to study the possible role of NEP in elevating tissue ADM concentration during sepsis. ADM level increases progressively during sepsis except in the jejunum. Rats in early phase of shock (ES) showed diverse changes in tissue NEP activity. Plasma NEP activity, tissue NEP activity and its protein and mRNA expression in the left ventricle, aorta, jejunum and lung in the late phase of shock (LS) rats were lower than those in ES and the control, but no statistical change of NEP activity in the kidney was observed. The level of ADM was inversely correlated with NEP activity in the plasma, ventricle and aorta and positively correlated with NEP activity in the jejunum. Thus, in sepsis, the local concentration and action of ADM in tissues may be differentially regulated by NEP.

Gonadotropin‐Releasing Hormone: Gene Evolution, Expression, and Regulation

The gonadotropin-releasing hormone (GnRH) gene is a superb example of the diverse regulation that is required to maintain the function of an evolutionarily conserved and fundamental gene. Because reproductive capacity is critical to the survival of the species, physiological homeostasis dictates optimal conditions for reproductive success, and any perturbation from this balance may affect GnRH expression. These disturbances may include alterations in signals dictated by stress, nutritional imbalance, body weight, and neurological problems; therefore, changes in other neuroendocrine systems may directly influence the hypothalamic-pituitary-gonadal axis through direct regulation of GnRH. Thus, to maintain optimal reproductive capacity, the regulation of the GnRH gene is tightly constrained by a number of diverse signaling pathways and neuromodulators. In this review, we summarize what is currently known of GnRH gene structure, the location and function of the two isoforms of the GnRH gene, some of the many hormones and neuromodulators found to affect GnRH expression, and the molecular mechanisms responsible for the regulation of the GnRH gene. We also discuss the latest models used to study the transcriptional regulation of the GnRH gene, from cell models to evolving in vivo technologies. Although we have come a long way in the last two decades toward uncovering the intricacies behind the control of the GnRH neuron, there remain vast distances to cover before direct therapeutic manipulation of the GnRH gene to control reproductive competence is possible.

Changes in production and metabolism of brain natriuretic peptide in rats with myocardial necrosis

In this study, we employed rat model of acute myocardial necrosis induced by isoproterenol (ISO) to study the possible roles of corin, the protease uniquely distributing in myocardium to convert pro-brain natriuretic peptide (proBNP) to BNP, and neutral endopeptidase (NEP), the major enzyme to degrade BNP, in changing the levels of BNP. In rats with isoproterenol alone, the myocardium necrosis occurred and the cardiac function was inhibited; the BNP contents in plasma and myocardium were upregulated, so did the myocardial corin mRNA level; the NEP activity in plasma and myocardium were downregulated. Omapatrilat (OMA) treatment relieved myocardial lesions and improved cardiac function. In the plasma and myocardium, omapatrilat treatment increased BNP contents, reduced NEP activity; in myocardium, mRNA level of proBNP and corin decreased, but NEP mRNA expression increased. Our study confirmed that omapatrilat treated myocardial necrosis effectively and suggested that increased BNP in rats with myocardial necrosis could depend on increased production and conversion as well as decreased degradation.

Neuroanatomical organization of gonadotropin-releasing hormone neurons during the oestrus cycle in the ewe

BACKGROUND

During the preovulatory surge of gonadotropin-releasing hormone (GnRH), a very large amount of the peptide is released in the hypothalamo-hypophyseal portal blood for 24-36H00. To study whether this release is linked to a modification of the morphological organization of the GnRH-containing neurons, i.e. morphological plasticity, we conducted experiments in intact ewes at 4 different times of the oestrous cycle (before the expected LH surge, during the LH surge, and on day 8 and day 15 of the subsequent luteal phase). The cycle stage was verified by determination of progesterone and LH concentrations in the peripheral blood samples collected prior to euthanasia.

RESULTS

The distribution of GnRH-containing neurons throughout the preoptic area around the vascular organ of the lamina terminalis was studied following visualisation using immunohistochemistry. No difference was observed in the staining intensity for GnRH between the different groups. Clusters of GnRH-containing neurons (defined as 2 or more neurons being observed in close contact) were more numerous during the late follicular phase (43 +/- 7) than during the luteal phase (25 +/- 6), and the percentage of clusters was higher during the beginning of the follicular phase than during the luteal phase. There was no difference in the number of labelled neurons in each group.

CONCLUSIONS

These results indicate that the morphological organization of the GnRH-containing neurons in ewes is modified during the follicular phase. This transitory re-organization may contribute to the putative synchronization of these neurons during the surge. The molecular signal inducing this plasticity has not yet been identified, but oestradiol might play an important role, since in sheep it is the only signal which initiates the GnRH preovulatory surge.

Relationship between the Contents of Adrenomedullin and Distributions of Neutral Endopeptidase in Blood and Tissues of Spontaneously Hypertensive Rats

Adrenomedullin (ADM) is a multifunctional peptide with important roles in the cardiovascular system, especially in the adjustment of cardiovascular and renal homeostasis. ADM is present in plasma, organs and tissues, and its activity increases during hypertension. It remains unknown whether the clearance of this peptide is altered during hypertension. Neutral endopeptidase (NEP) is the major enzyme in ADM's degradation. We observed the activity and distribution of NEP and the expression of its mRNA in the plasma, cardiac ventricle, aorta, jejunum and kidney of spontaneously hypertensive rats (SHRs) in order to study the possible role of NEP in elevating tissue ADM concentrations during hypertension. ADM and NEP were diffuse in all tissues studied. The level of tissue ADM was generally higher in SHR tissues than in control tissues, except in the renal medulla, and its mRNA expression was higher in all tissues. Plasma NEP activity, general NEP activity and the expression of NEP mRNA in the left ventricle, aorta and jejunum in SHRs was lower than that of controls, and the level of ADM was inversely correlated with NEP activity. NEP activity and mRNA and protein expression in SHR kidneys were higher than in control kidneys; moreover, the ADM content was positively correlated with NEP activity in the renal cortex. NEP activity in the lung of SHRs did not differ from that of controls. Thus, in SHRs, the local concentration and action of ADM in the tissues may be differentially regulated by NEP.