The medio-basal hypothalamus as a dynamic and plastic reproduction-related kisspeptin-gnrh-pituitary center in fish.

Kisspeptin regulates reproductive events, including puberty and ovulation, primarily via GnRH neurons. Prolonged treatment of prepubertal striped bass females with kisspeptin (Kiss) 1 or Kiss2 peptides failed to enhance puberty but suggested a gnrh-independent pituitary control pathway. Kiss2 inhibited, but Kiss1 stimulated, FShß expression and gonadal development, although hypophysiotropic gnrh1 and gnrh receptor expression remained unchanged. In situ hybridization and immunohistochemistry on brains and pituitaries revealed a differential plasticity between the 2 kisspeptin neurons. The differences were most pronounced at the prespawning phase in 2 regions along the path of gnrh1 axons: the nucleus lateralis tuberis (NLT) and the neurohypophysis. Kiss1 neurons appeared in the NLT and innervated the neurohypophysis of prespawning males and females, reaching Lh gonadotropes in the proximal pars distalis. Males, at all reproductive stages, had Kiss2 innervations in the NLT and the neurohypophysis, forming large axonal bundles in the former and intermingling with gnrh1 axons. Unlike in males, only preovulatory females had massive NLT-neurohypophysis staining of kiss2. Kiss2 neurons showed a distinct appearance in the NLT pars ventralis-equivalent region only in spawning zebrafish, indicating that this phenomenon is widespread. These results underscore the NLT as important nuclei for kisspeptin action in 2 facets: 1) kisspeptin-gnrh interaction, both kisspeptins are involved in the regulation of gnrh release, in a stage- and sex-dependent manner, especially at the prespawning phase; and 2) gnrh-independent effect of Kiss peptides on the pituitary, which together with the plastic nature of their neuronal projections to the pituitary implies that a direct gonadotropic regulation is plausible.

Development of the medial hypothalamus: forming a functional hypothalamic-neurohypophyseal interface.

The medial hypothalamus is composed of nuclei of the tuberal hypothalamus, the paraventricular nucleus of the anterior hypothalamus, and the neurohypophysis. Its arrangement, around the third ventricle of the brain, above the adenohypophysis, and in direct contact with the vasculature, means that it serves as an interface with circulating systems, providing a key conduit through which the brain can sample, and control, peripheral body systems. Through these interfaces, and interactions with other parts of the brain, the medial hypothalamus centrally governs diverse homeostatic processes, including energy and fluid balance, stress responses, growth, and reproductive behaviors. Here, we summarize recent studies that reveal how the diverse cell types within the medial hypothalamus are assembled in an integrated manner to enable its later function. In particular, we discuss how the temporally protracted operation of signaling pathways and transcription factors governs the appearance and regionalization of the hypothalamic primordium from the prosencephalic territory, the specification and differentiation of progenitors into neurons in organized nuclei, and the establishment of interfaces. Through analyses of mouse, chick, and zebrafish, a picture emerges of an evolutionarily conserved and highly coordinated developmental program. Early indications suggest that deregulation of this program may underlie complex human pathological conditions and dysfunctional behaviors, including stress and eating disorders.

Lack of functional GABAB receptors alters Kiss1 , Gnrh1 and Gad1 mRNA expression in the medial basal hypothalamus at postnatal day 4.

BACKGROUND/AIMS: Adult mice lacking functional GABAB receptors (GABAB1KO) show altered Gnrh1 and Gad1 expressions in the preoptic area-anterior hypothalamus (POA-AH) and females display disruption of cyclicity and fertility. Here we addressed whether sexual differentiation of the brain and the proper wiring of the GnRH and kisspeptin systems were already disturbed in postnatal day 4 (PND4) GABAB1KO mice. METHODS: PND4 wild-type (WT) and GABAB1KO mice of both sexes were sacrificed; tissues were collected to determine mRNA expression (qPCR), amino acids (HPLC), and hormones (RIA and/or IHC). RESULTS: GnRH neuron number (IHC) did not differ among groups in olfactory bulbs or OVLT-POA. Gnrh1 mRNA (qPCR) in POA-AH was similar among groups. Gnrh1 mRNA in medial basal hypothalamus (MBH) was similar in WTs but was increased in GABAB1KO females compared to GABAB1KO males. Hypothalamic GnRH (RIA) was sexually different in WTs (males > females), but this sex difference was lost in GABAB1KOs; the same pattern was observed when analyzing only the MBH, but not in the POA-AH. Arcuate nucleus Kiss1 mRNA (micropunch-qPCR) was higher in WT females than in WT males and GABAB1KO females. Gad1 mRNA in MBH was increased in GABAB1KO females compared to GABAB1KO males. Serum LH and gonadal estradiol content were also increased in GABAB1KOs. CONCLUSION: We demonstrate that GABABRs participate in the sexual differentiation of the ARC/MBH, because sex differences in several reproductive genes, such as Gad1, Kiss1 and Gnrh1, are critically disturbed in GABAB1KO mice at PND4, probably altering the organization and development of neural circuits governing the reproductive axis.

Rax is a selector gene for mediobasal hypothalamic cell types.

The brain plays a central role in controlling energy, glucose, and lipid homeostasis, with specialized neurons within nuclei of the mediobasal hypothalamus, namely the arcuate (ARC) and ventromedial (VMH), tasked with proper signal integration. Exactly how the exquisite cytoarchitecture and underlying circuitry becomes established within these nuclei remains largely unknown, in part because hypothalamic developmental programs are just beginning to be elucidated. Here, we demonstrate that the Retina and anterior neural fold homeobox (Rax) gene plays a key role in establishing ARC and VMH nuclei in mice. First, we show that Rax is expressed in ARC and VMH progenitors throughout development, consistent with genetic fate mapping studies demonstrating that Rax+ lineages give rise to VMH neurons. Second, the conditional ablation of Rax in a subset of VMH progenitors using a Shh::Cre driver leads to a fate switch from a VMH neuronal phenotype to a hypothalamic but non-VMH identity, suggesting that Rax is a selector gene for VMH cellular fates. Finally, the broader elimination of Rax throughout ARC/VMH progenitors using Six3::Cre leads to a severe loss of both VMH and ARC cellular phenotypes, demonstrating a role for Rax in both VMH and ARC fate specification. Combined, our study illustrates that Rax is required in ARC/VMH progenitors to specify neuronal phenotypes within this hypothalamic brain region. Rax thus provides a molecular entry point for further study of the ontology and establishment of hypothalamic feeding circuits.

Molecular profiling of postnatal development of the hypothalamus in female and male rats.

Reproductive function is highly dynamic during postnatal developmental. Here, we performed molecular profiling of gene expression patterns in the hypothalamus of developing male and female rats to identify which genes are sexually dimorphic, to gain insight into a more complex network of hypothalamic genes, and to ascertain dynamic changes in their relationships with one another and with sex steroid hormones during development. Using a low-density PCR platform, we quantified mRNA levels in the preoptic area (POA) and medial basal hypothalamus (MBH), and assayed circulating estradiol, testosterone, and progesterone at six ages from birth through adulthood. Numerous genes underwent developmental change, particularly postnatal increases, decreases, or peaks/plateaus at puberty. Surprisingly, there were few sex differences; only Esr1, Kiss1, and Tac2 were dimorphic (higher in females). Cluster analysis of gene expression revealed sexually dimorphic correlations in the POA but not the MBH from P30 (Postnatal Day 30) to P60. Hormone measurements showed few sex differences in developmental profiles of estradiol; higher levels of progesterone in females only after P30; and a developmental pattern of testosterone with a nadir at P30 followed by a dramatic increase through P60 (males). Furthermore, bionetwork analysis revealed that hypothalamic gene expression profiles and their relationships to hormones undergo dynamic developmental changes that differ considerably from adults. These data underscore the importance of developmental stage in considering the effects of hormones on the regulation of neuroendocrine genes in the hypothalamus. Moreover, the finding that few neuroendocrine genes are sexually dimorphic highlights the need to consider postnatal development from a network approach that allows assessment of interactions and patterns of expression.

Delayed developmental changes in neonatal vocalizations correlates with variations in ventral medial hypothalamus and central amygdala development in the rodent infant: effects of prenatal cocaine.

While variations in neonatal distress vocalizations have long been shown to reflect the integrity of nervous system development following a wide range of prenatal and perinatal insults, a paucity of research has explored the neurobiological basis of these variations. To address this, virgin Sprague-Dawley rats were bred and divided into three groups: [1] untreated, [2] chronic-cocaine treated (30 mg/kg/day, gestation days (GDs) 1-20); or [3] chronic saline treated (2 mg/kg/day, GDs 1-20). Pregnant dams were injected with Bromodeoxyuridine (10 mg/kg) on GDs 13-15 to label proliferating cells in limbic regions of interest. Ultrasonic vocalizations (USVs) were recorded on postnatal days (PNDs) 1, 14, and 21, from one male and female pup per litter. Variations in acoustic properties of USVs following cocaine-exposure were age and sex-dependent including measures of total number, total duration and amplitude of USVs, and percent of USVs with at least one harmonic. Following USV testing brains were stained with standard fluorescent immunohistochemistry protocols and examined for variations in neuronal development and if variations were associated with acoustic characteristics. Limbic region developmental differences following cocaine-exposure were sex- and age-dependent with variations in the ventral medial hypothalamus and central amygdala correlating with variations in vocalizations on PND 14 and 21. Results suggest maturation of the ventral medial hypothalamus and central amygdala may provide the basis for variations in the sound and production of USVs. As vocalizations may serve as a neurobehavioral marker for nervous system integrity, understanding the neurobiological basis of neonatal vocalizations may provide the basis for early intervention strategies in high-risk infant populations.

Gene expression in neuroendocrine cells during the critical period for sexual differentiation of the brain.

Following transcription of the SRY gene on the Y chromosome of genetic males, a cascade of genomic and biochemical events causes the developing brain to be influenced by two testosterone metabolites, the potent androgen dihydrotestosterone and the aromatization product estradiol (E2). These steroid hormones binding to their cognate nuclear receptors produce differential gene expression profiles between male and female brains, and as a result, male-typical sex behaviors appear in adulthood and female-typical sex behaviors are suppressed. Although anatomical and cellular substrates underlying sexually dimorphic brain and behavior have been identified, still very little information is available about the molecular mechanisms involved. Microarray technology is a powerful technique that can be a used to assess the changes in thousands of gene transcripts simultaneously. Thus such high-throughput screening may be a useful initial step in the identification of estrogen-responsive genes involved in the sexual differentiation of brain.

Gonadotrophin-releasing hormone and GnRH-associated peptide neurobiology from the rearing period until puberty in the female sheep.

Using morphological criteria we describe the postnatal ontogeny of gonadotrophin-releasing hormone (GnRH) and GnRH-associated peptide (GAP) of the GnRH prohormone in the ovine preoptic area (POA)-hypothalamus. The immunohistochemical GnRH- and GAP-neuroanatomy was investigated in infantile (12-week-old weanlings), prepubertal (15-, 22-week-old juveniles), peripubertal (30-, 52-week-old during maturational quiescence) and pubertal (63-week-old) females born after the summer solstice, thus undergoing delayed puberty. The changes observed in the medial POA included a postinfantile increase in the number of multipolar GnRH-immunoreactive (ir) perikarya until late prepuberty (p < 0.05). Whereas in peripubertal lambs were observed both an increase in the number of GnRH-ir and GAP-ir cell clusters in the medial POA and an increase in the mediobasal hypothalamus (MBH) subpopulation of GnRH-ir and GAP-ir neurons (p < 0.05). The GnRH-ir perikarya, nerve fibers and terminals showed a progressive increase in numbers until peripuberty. Conversely, the acute postinfantile alterations of decrease and then increase in the population of GAP-ir nerve elements were observed in the POA-hypothalamus until late prepuberty (p < 0.05). The preoptico-hypothalamic ir GAP and GnRH were of comparable abundance in both peripubertal groups and scarce during puberty. In conclusion, the infantile/prepubertal transition period changes GnRH posttranslational biosynthesis and GnRH-GAP-neuroanatomy from the infantile to the pubertal pattern, which allows the inauguration of the breeding season. The postinfantile MBH distribution of GnRH-containing neurons and increase in the POA population of GnRH-GAP neurons and their somatic appositions seem important for maturation. The nature of GnRH-GAP neurobiology during inauguration of puberty seems morphotic and in female sheep is seasonal.

Ontogeny of the projections from the anteroventral periventricular nucleus of the hypothalamus in the female rat.

Neurons in the anteroventral periventricular nucleus of the hypothalamus (AVPV) mediate a variety of autonomic functions. In adults they primarily innervate neuroendocrine nuclei in the periventricular zone of the hypothalamus, including the paraventricular and arcuate nuclei (PVH, ARH). Ascending projections from the AVPV also provide inputs to the ventrolateral septum (LSv) and the principal division of the bed nuclei of the stria terminalis (BSTp). Consistent with a role in regulating preovulatory luteinizing hormone secretion, rostral projections from the AVPV contact gonadotropin-releasing hormone (GnRH) neurons surrounding the vascular organ of the lamina terminalis (OVLT). To study the development of these pathways, we placed implants of the lipophilic tracers DiI and CMDiI into the AVPV of female rats ranging in age from embryonic day 19 (E19) through adulthood. The earliest projections targeted a population of GnRH neurons, with apparent contacts from labeled fibers observed as early as E19. These connections appeared to be fully developed before birth, as similar numbers of appositions from AVPV projections onto the GnRH-immunoreactive cells were observed at all ages examined. Caudal projections were delayed relative to projections to the OVLT. Labeled AVPV fibers reached the PVH during the first postnatal week, and fibers targeting the BSTp and LSv were not observed until the second and third postnatal weeks, respectively. Labeled AVPV fibers were not seen in the ARH of animals at any age. Our results demonstrate that projections from the AVPV develop with both spatial and temporal specificity, innervating each target with a unique developmental profile.

Cross-fostering to diabetic rat dams affects early development of mediobasal hypothalamic nuclei regulating food intake, body weight, and metabolism.

Exposure to maternal gestational diabetes (GD) "programs" offspring for obesity in childhood and later life. Recent clinical data suggest that neonatal ingestion of breast milk from diabetic mothers might be crucially involved. Mediobasal hypothalamic nuclei such as the ventromedial nucleus (VMN), the paraventricular nucleus (PVN) and the arcuate nucleus (ARC) play a key role in the central nervous system regulation of food intake and body weight. In the ARC, orexigenic neuropeptides such as neuropeptide Y (NPY), galanin (GAL), and agouti-related peptide (AGRP) and anorexigenic neuropeptides such as proopiomelanocortin (POMC) and alpha-melanocyte-stimulating hormone (MSH) are expressed. We investigated the effects of neonatal exposure to milk from GD rat dams on the development of hypothalamic nuclei in weanling rats. Offspring of control (CO) rat dams cross-fostered to GD rat dams (CO-GD) developed early postnatal growth delay. On d 21 of life, CO-GD rats showed structural and functional hypothalamic "malprogramming." The ARC of CO-GD rats showed increased immunopositivity of both NPY and AGRP under basal conditions, despite normal levels of glucose, leptin, and insulin. Conversely, CO-GD rats showed decreased immunopositivity of both POMC and MSH and decreased density of immunopositive neurons, compared with offspring of control rat dams cross-fostered to control rat dams. No morphometric alterations were found in the VMN, whereas CO-GD rats showed an increased total number of neurons in the PVN. In summary, neonatal exposure to maternal diabetes through the intake of dam's milk in rats leads to a complex malprogramming of hypothalamic orexigenic and anorexigenic circuits that are critically involved in the lifelong regulation of food intake, body weight, and metabolism.

Neurons possessing enzymes of dopamine synthesis in the mediobasal hypothalamus of rats. Topographic relations and axonal projections to the median eminence in ontogenesis.

We evaluated the topographic relations between tyrosine hydroxylase (TH)- and/or aromatic L-amino acid decarboxylase (AADC)-immunoreactive neurons in the arcuate nucleus (AN), as well as between TH- and/or AADC-immunoreactive axons in the median eminence (ME) in rats at the 21st embryonic day, 9th postnatal day, and in adulthood. The double-immunofluorescent technique in combination with confocal microscopy was used. Occasional bienzymatic neurons but numerous monoenzymatic TH- or AADC-immunoreactive neurons were observed in fetuses. There was almost no overlap in the distribution of monoenzymatic neurons, and therefore few appositions were observed in between. In postnatal animals, numerous bienzymatic neurons appeared in addition to monoenzymatic neurons. They were distributed throughout the AN resulting in the increased frequency of appositions. Furthermore, specialized-like contacts between monoenzymatic TH- and AADC-immunoreactive neurons appeared. The quantification of the fibers in the ME showed that there were large specific areas of the monoenzymatic TH-immunoreactive fibers and bienzymatic fibers in fetuses, followed by the gradual reduction of the former and the increase of the latter to adulthood. The specific area of the monoenzymatic AADC-immunoreactive fibers in fetuses was rather low, and thereafter increased progressively to adulthood. The fibers of all the types were in apposition in the ME at each studied age. Close topographic relations between the neurons containing individual complementary enzymes of dopamine synthesis at the level of cell bodies and axons suggest functional interaction in between.

Immunohistochemical analyses of thyroid-specific enhancer-binding protein in the fetal and adult rat hypothalami and pituitary glands.

Thyroid-specific enhancer-binding protein (T/EBP), also known as NKX2.1 or TTF-1, regulates the expression of thyroid- and lung-specific genes. The t/ebp/Nkx2.1-null mutant mouse was stillborn but lacked the thyroid gland, pituitary gland, ventral region of the forebrain and normal lungs. These data demonstrated that T/EBP/NKX2.1 plays an important role not only in tissue-specific gene expressions in adults but also in genesis of these organs during development. Although the expression of t/ebp/Nkx2.1 in the brain has been reported, its function in the brain remains unknown. The present study was designed to determine the localization of T/EBP/NKX2.1 in the hypothalamus and pituitary gland of fetal and adult rats by immunohistochemistry as the first step toward understanding the function of T/EBP/NKX2.1 in the rat brain. In the fetal rat hypothalamus, T/EBP/NKX2.1 was localized widely in the ventral hypothalamic areas. In the adult rat brain, T/EBP/NKX2.1 was localized in the ventromedial hypothalamic nucleus, medial tuberal nucleus, arcuate nucleus and mammillary body. No T/EBP/NKX2.1 immunoreactivity was observed in the anterior or intermediate lobe of the pituitary gland in either fetal or adult rats. On the other hand, immunoreactive T/EBP/NKX2.1 was found in the posterior lobe of the pituitary gland. This paper presents results of detailed analyses of the distributions of T/EBP/NKX2.1 protein in the fetal and adult rat hypothalami and pituitary glands, and these results should provide important information for understanding the function of T/EBP/NKX2.1 in the brain.

Regional distribution of benzodiazepine binding sites in the human newborn and infant hypothalamus. A quantitative autoradiographic study.

Using in vitro quantitative autoradiography and [3H]flunitrazepam we examined the rostrocaudal distribution of benzodiazepine binding sites in the human neonate/infant hypothalamus. The autoradiographic analysis shows the presence of a heterogeneous distribution throughout the rostrocaudal extent of this brain structure. High [3H]flunitrazepam binding corresponds primarily to the diagonal band of Broca and the preoptic region. The labelling in the preoptic region showed a rostrocaudal increase, contrasting in that with the other hypothalamic structures. Intermediate densities were present in the septohypothalamic, suprachiasmatic, periventricular and paraventricular nuclei as well as in the mammillary complex. Low binding was observed in the other hypothalamic structures. The benzodiazepine binding sites analyzed belong mostly to type II receptors. In an attempt to unravel possible differences related to age, we compared the autoradiographic distribution in three postnatal age ranges. The topographical distribution of these binding sites was almost identical in each period analyzed. We found, however, that benzodiazepine binding is generally low in the neonatal period and a tendency in increasing densities is observed during development. Taken together, these results provide evidence for a large distribution of benzodiazepine binding sites in neonate/infant hypothalamus, suggesting their implication in the development of this brain structure and the maintenance of its various functions.

A second look at the barriers of the medial basal hypothalamus.

The cell bodies of hypothalamic secretory neurons are localized in areas protected by the blood-brain barrier (BBB), whereas their axon terminals are localized in the median eminence, which lacks a BBB. This implies a complex barrier system, allowing neurons of the central nervous system to secrete into the blood stream without making the BBB leaky. In the present study, three experimental protocols were applied to clarify certain relevant aspects of the barriers operating in the medial basal hypothalamus of the rat. We established that the milieu of the arcuate nucleus is exposed to both the ventricular and the subarachnoidal cerebrospinal fluid (CSF). The median eminence milieu, the perivascular space of the portal vessels, and the subarachnoid space appear to be in open communication; also, beta2-tanycytes establish an efficient barrier between the median eminence milieu and the ventricular CSF. Similarly, beta1-tanycytes establish a lateral barrier, separating the intercellular space of the median eminence from that of the arcuate nucleus. We also found that the glucose transporter I (GLUT I), a BBB marker, is localized throughout the whole plasma membrane of beta1-tanycytes, but is missing from beta2-tanycytes. Expression of GLUT I by tanycytes progressively develops during the first postnatal weeks; while the degree of damage of the arcuate nucleus by administration of monosodium glutamate, at different postnatal intervals, parallels that of the GLUT I immunoreactivity of beta1-tanycytes. An explanation is offered for the selective destruction of the arcuate neurons by the parenteral administration of monosodium glutamate to infant rats.

The development of POMC gene expression in the medial basal hypothalamus of prepubertal rats.

Alterations in brain opioid gene expression may underlie the dramatic change in the latency to display parental behavior in juvenile rats. Male and female juvenile rats (18-25 days of age) exhibit parental behavior either immediately or within 1-2 days after coming in contact with foster pups. By 30 days of age, however, their response latencies increase to adult levels of 5-10 days. Given the established involvement of the endogenous opioid system in adult maternal and juvenile parental behaviors, the objective of the present report was to determine possible changes in proopiomelanocortin (POMC) gene expression in the medial basal hypothalamus (MBH) during this early developmental window. We compared POMC gene expression in the MBH of male and female juvenile rats from 21 to 33 days of age by in situ hybridization histochemistry. A significant increase in the number of POMC cells in males and females was detected at 30 days of age in the central portion of the arcuate nucleus. This increase in POMC mRNA may contribute to the shift in parental behavior that occurs in male and female juvenile rats.

Developmental expression of calretinin in the medial basal hypothalamus and amygdala from male and female rats.

Developmental expression of calretinin in the medial basal hypothalamic (MBH) and amygdala region was examined by Western analysis. Males displayed significantly higher calretinin levels compared to females in the MBH (but not the amygdala) on gestational day 19 and 20. These data imply that hormonal factors may regulate developmental MBH calretinin expression. In turn, sexually dimorphic brain structures might be influenced by calretinin levels that can alter sexually dimorphic patterns of steroidogenesis, cellular migration or programmed cell loss mechanism(s) during neuronal development by modulating intracellular calcium concentrations.

GnRH perikarya in medial basal hypothalamus of pubertal female rhesus macaque are ensheathed with glia.

A previous study in our laboratory revealed that the cell bodies of gonadotropin releasing hormone (GnRH) neurons in the preoptic area (POA) of early to midpubertal female rhesus monkeys were extensively invested with thick glial processes. Because the medial basal hypothalamus (MBH) plays a critical role in the control of pulsatile and cyclic gonadotropin release in the primate, we have now focused on the ultrastructural milieu of GnRH neurons of this region in the same sample of monkeys. The ensheathment of the perikarya of GnRH neurons in the MBH with such glial processes was more pronounced than in the POA. Whereas the mean proportion of the cell membrane covered by these glia was 57% in the POA, it was 72% in the MBH. In addition, the cell bodies of GnRH neurons in the MBH of the pubertal monkey (unlike those in the POA) were less well innervated than were those in the adult cycling monkey, further highlighting differences between these brain regions. Differences in the anatomical milieu of the MBH between immature monkeys, in which GnRH release is still relatively quiescent, and adult cycling monkeys are consistent with the hypothesis that GnRH neurons within the MBH are under particular constraint in the immature animals. The functional significance of these observations must, however, await further studies.

Heterogeneous development of voltage dependency of NMDA receptor-mediated response in central nervous system.

Developmental changes of voltage dependency of NMDA response were investigated on nucleus solitarii (NTS) ventromedial hypothalamic (VMH) and visual cortical neurons acutely dissociated from rats aged between 19 days in gestation (E19) and 21 days after birth (P21) using a whole cell patch clamp recording. Sensitivity of NMDA response to extracellular Mg2+ developed by P3 in the NTS, by P6 in the VMH and by P6 in the VMH and by P12 in the VMH and by P12 in the cortex, Protein kinase C modulators could not modulate the voltage dependency of NMDA response in immature neurons. These finding suggest that a regional heterogeneity of developmental change of the sensitivity of NMDA response to extracellular Mg2+ exists. This regional difference in development of NMDA response might be related to the order of vital necessities after birth.

The LHRH pulse generator: a mediobasal hypothalamic location.

The location and mechanism of LHRH pulse generator are discussed based on our series of experiments. Suckling stimulus is a novel stimulus that inhibits LH pulses without any cooperation from ovarian steroids, unlike other stimuli such as stress, photoperiod etc. It is directly involved in suppressing the activity of the LHRH pulse generator. The information from teats suckled by pups or babies is conveyed dorsally to the mediobasal hypothalamus (MBH), where the LHRH pulse generator may be located. Experiments using various types of deafferentation and fetal brain tissue transplantation confirmed that the LHRH pulse generator is located in the MBH and suggested that LHRH pulse generator consists of nonLHRH neurons. Endogenous excitatory amino acid is one of the possible neurotransmitters that regulate LHRH release at the nerve terminal in ME.