Expression of provasopressin gene during ontogeny in the hypothalamus of developing mice

Developmental patterning of peptide transcription in the central circadian clock in both sexes

Introduction

Neuropeptide signaling modulates the function of central clock neurons in the suprachiasmatic nucleus (SCN) during development and adulthood. Arginine vasopressin (AVP) and vasoactive intestinal peptide (VIP) are expressed early in SCN development, but the precise timing of transcriptional onset has been difficult to establish due to age-related changes in the rhythmic expression of each peptide.

Methods

To provide insight into spatial patterning of peptide transcription during SCN development, we used a transgenic approach to define the onset of Avp and Vip transcription. Avp-Cre or Vip-Cre males were crossed to Ai9+/+ females, producing offspring in which the fluorescent protein tdTomato (tdT) is expressed at the onset of Avp or Vip transcription. Spatial patterning of Avp-tdT and Vip-tdT expression was examined at critical developmental time points spanning mid-embryonic age to adulthood in both sexes.

Results

We find that Avp-tdT and Vip-tdT expression is initiated at different developmental time points in spatial subclusters of SCN neurons, with developmental patterning that differs by sex.

Conclusions

These data suggest that SCN neurons can be distinguished into further subtypes based on the developmental patterning of neuropeptide expression, which may contribute to regional and/or sex differences in cellular function in adulthood.

SOX2 Regulates Neuronal Differentiation of the Suprachiasmatic Nucleus

In mammals, the hypothalamic suprachiasmatic nucleus (SCN) functions as the central circadian pacemaker, orchestrating behavioral and physiological rhythms in alignment to the environmental light/dark cycle. The neurons that comprise the SCN are anatomically and functionally heterogeneous, but despite their physiological importance, little is known about the pathways that guide their specification and differentiation. Here, we report that the stem/progenitor cell transcription factor, Sex determining region Y-box 2 (Sox2), is required in the embryonic SCN to control the expression of SCN-enriched neuropeptides and transcription factors. Ablation of Sox2 in the developing SCN leads to downregulation of circadian neuropeptides as early as embryonic day (E) 15.5, followed by a decrease in the expression of two transcription factors involved in SCN development, Lhx1 and Six6, in neonates. Thymidine analog-retention assays revealed that Sox2 deficiency contributed to reduced survival of SCN neurons during the postnatal period of cell clearance, but did not affect progenitor cell proliferation or SCN specification. Our results identify SOX2 as an essential transcription factor for the proper differentiation and survival of neurons within the developing SCN.

Genesis of the Master Circadian Pacemaker in Mice

The suprachiasmatic nucleus (SCN) of the hypothalamus is the central circadian clock of mammals. It is responsible for communicating temporal information to peripheral oscillators via humoral and endocrine signaling, ultimately controlling overt rhythms such as sleep-wake cycles, body temperature, and locomotor activity. Given the heterogeneity and complexity of the SCN, its genesis is tightly regulated by countless intrinsic and extrinsic factors. Here, we provide a brief overview of the development of the SCN, with special emphasis on the murine system.

Birth elicits a conserved neuroendocrine response with implications for perinatal osmoregulation and neuronal cell death

Long-standing clinical findings report a dramatic surge of vasopressin in umbilical cord blood of the human neonate, but the neural underpinnings and function(s) of this phenomenon remain obscure. We studied neural activation in perinatal mice and rats, and found that birth triggers activation of the suprachiasmatic, supraoptic, and paraventricular nuclei of the hypothalamus. This was seen whether mice were born vaginally or via Cesarean section (C-section), and when birth timing was experimentally manipulated. Neuronal phenotyping showed that the activated neurons were predominantly vasopressinergic, and vasopressin mRNA increased fivefold in the hypothalamus during the 2–3 days before birth. Copeptin, a surrogate marker of vasopressin, was elevated 30-to 50-fold in plasma of perinatal mice, with higher levels after a vaginal than a C-section birth. We also found an acute decrease in plasma osmolality after a vaginal, but not C-section birth, suggesting that the difference in vasopressin release between birth modes is functionally meaningful. When vasopressin was administered centrally to newborns, we found an ~ 50% reduction in neuronal cell death in specific brain areas. Collectively, our results identify a conserved neuroendocrine response to birth that is sensitive to birth mode, and influences peripheral physiology and neurodevelopment.

The role of oxytocin and vasopressin in the pathophysiology of heart failure in pregnancy and in fetal and neonatal life

Pregnancy and early life create specific psychosomatic challenges for the mother and child, such as changes in hemodynamics, resetting of the water-electrolyte balance, hypoxia, pain, and stress, that all play an important role in the regulation of the release of oxytocin and vasopressin. Both of these hormones regulate the water-electrolyte balance and cardiovascular functions, maturation of the cardiovascular system, and cardiovascular responses to stress. These aspects may be of particular importance in a state of emergency, such as hypertension in the mother or severe heart failure in the child. In this review, we draw attention to a broad spectrum of actions exerted by oxytocin and vasopressin in the pregnant mother and the offspring during early life. To this end, we discuss the following topics: 1) regulation of the secretion of oxytocin and vasopressin and expression of their receptors in the pregnant mother and child, 2) direct and indirect effects of oxytocin and vasopressin on the cardiovascular system in the healthy mother and fetus, and 3) positive and negative consequences of altered secretion of oxytocin and vasopressin in the mother with cardiovascular pathology and in the progeny with heart failure. The present survey provides evidence that moderate stimulation of the oxytocin and vasopressin receptors plays a beneficial role in the healthy pregnant mother and fetus; however, under pathophysiological conditions the inappropriate action of these hormones exerts several negative effects on the cardiovascular system of the mother and progeny and may potentially contribute to the pathophysiology of heart failure in early life.

Circuit development in the master clock network of mammals

Daily rhythms are generated by the circadian timekeeping system, which is orchestrated by the master circadian clock in the suprachiasmatic nucleus (SCN) of mammals. Circadian timekeeping is endogenous and does not require exposure to external cues during development. Nevertheless, the circadian system is not fully formed at birth in many mammalian species and it is important to understand how SCN development can affect the function of the circadian system in adulthood. The purpose of the current review is to discuss the ontogeny of cellular and circuit function in the SCN, with a focus on work performed in model rodent species (i.e., mouse, rat, and hamster). Particular emphasis is placed on the spatial and temporal patterns of SCN development that may contribute to the function of the master clock during adulthood. Additional work aimed at decoding the mechanisms that guide circadian development is expected to provide a solid foundation upon which to better understand the sources and factors contributing to aberrant maturation of clock function.

Three-dimensional distribution of tyrosine hydroxylase, vasopressin and oxytocin neurones in the transparent postnatal mouse brain

Over the years, advances in immunohistochemistry techniques have been a critical step in detecting and mapping neuromodulatory substances in the central nervous system. The better quality and specificity of primary antibodies, new staining procedures and the spectacular development of imaging technologies have allowed such progress. Very recently, new methods permitting tissue transparency have been successfully used on brain tissues. In the present study, we combined whole-mount immunostaining for tyrosine hydroxylase (TH), oxytocin (OXT) and arginine vasopressin (AVP), with the iDISCO+ clearing method, light-sheet microscopy and semi-automated counting of three-dimensionally-labelled neurones to obtain a (3D) distribution of these neuronal populations in a 5-day postnatal (P5) mouse brain. Segmentation procedure and 3D reconstruction allowed us, with high resolution, to map TH staining of the various catecholaminergic cell groups and their ascending and descending fibre pathways. We show that TH pathways are present in the whole P5 mouse brain, similar to that observed in the adult rat brain. We also provide new information on the postnatal distribution of OXT and AVP immunoreactive cells in the mouse hypothalamus, and show that, compared to AVP neurones, OXT neurones in the supraoptic (SON) and paraventricular (PVN) nuclei are not yet mature in the early postnatal period. 3D semi-automatic quantitative analysis of the PVN reveals that OXT cell bodies are more numerous than AVP neurones, although their immunoreactive soma have a volume half smaller. More AVP nerve fibres compared to OXT were observed in the PVN and the retrochiasmatic area. In conclusion, the results of the present study demonstrate the utility and the potency of imaging large brain tissues with clearing procedures coupled to novel 3D imaging technologies to study, localise and quantify neurotransmitter substances involved in brain and neuroendocrine functions.

Developmental perspectives on oxytocin and vasopressin

The related neuropeptides oxytocin and vasopressin are involved in species-typical behavior, including social recognition behavior, maternal behavior, social bonding, communication, and aggression. A wealth of evidence from animal models demonstrates significant modulation of adult social behavior by both of these neuropeptides and their receptors. Over the last decade, there has been a flood of studies in humans also implicating a role for these neuropeptides in human social behavior. Despite popular assumptions that oxytocin is a molecule of social bonding in the infant brain, less mechanistic research emphasis has been placed on the potential role of these neuropeptides in the developmental emergence of the neural substrates of behavior. This review summarizes what is known and assumed about the developmental influence of these neuropeptides and outlines the important unanswered questions and testable hypotheses. There is tremendous translational need to understand the functions of these neuropeptides in mammalian experience-dependent development of the social brain. The activity of oxytocin and vasopressin during development should inform our understanding of individual, sex, and species differences in social behavior later in life.

Diversity of the hypothalamo-neurohypophysial system and its hormonal genes

The hypothalamic neurosecretory cells (NSCs) which produce and release neurohypophysial hormones are involved in controls of diverse physiological phenomena including homeostatic controls of unconscious functions and reproduction. The far and wide distribution of neurosecretory processes in the discrete brain loci and the neurohypophysis is appropriate for coordination of neural and endocrine events that are required for the functions of NSCs. The presence of dye couplings and intimate contacts among NSCs supports harmonious production and release of hormone to maintain the plasma level within a certain range which is adequate for a particular physiological condition. Neurosecretory cells integrate diverse input signals from internal and external sources that define this particular physiological condition, although reactions of NSCs vary among different species, and among different cell types. An input signal to NSC is received by specific receptors and transduced as unique intracellular signals, important for the various functions of neurohypophysial hormones. Orchestration of multiple intracellular signaling systems, activities of which are individually modulated by input signals, determines the rates of synthesis and release of hormone through regulation of gene expression. The first step of gene expression, i.e., transcription, is amenable for diverse reaction of NSCs, because the 5' upstream regions of genes encoding neurohypophysial hormones are highly variable.

Angiotensin II type 2 receptors have a major somatodendritic distribution in vasopressin-containing neurons in the mouse hypothalamic paraventricular nucleus

The hypothalamic paraventricular nucleus (PVN) and angiotensin II (AngII) play critical roles in cardiovascular and neurohumoral regulation ascribed in part to vasopressin (VP) release. The AngII actions in the PVN are mediated largely through angiotensin II type 1 (AT1) receptors. However, there is indirect evidence that the functionally elusive central angiotensin II type 2 (AT2) receptors are also mediators of AngII signaling in the PVN. We used electron microscopic dual immunolabeling of antisera recognizing the AT2 receptor and VP to test the hypothesis that mouse PVN neurons expressing VP are among the cellular sites where this receptor has a subcellular distribution conducive to local activation. Immunoreactivity for the AT2 receptor was detected in somatodendritic profiles, of which approximately 60% of the somata and approximately 28% of the dendrites also contained VP. In comparison with somata and dendrites, axons, axon terminals, and glia less frequently contained the AT2 receptor. Somatic labeling for the AT2 receptor was often seen in the cytoplasm near the Golgi lamellae and other endomembrane structures implicated in receptor trafficking. AT2 receptor immunoreactivity in dendrites was commonly localized to cytoplasmic endomembranes, but was occasionally observed on extra- or peri-synaptic portions of the plasma membrane apposed by astrocytic processes or by unlabeled axon terminals. The labeled dendritic plasmalemmal segments containing AT2 receptors received asymmetric excitatory-type or more rarely symmetric inhibitory-type contacts from unlabeled axon terminals containing dense core vesicles, many of which are known to store neuropeptides. These results provide the first ultrastructural evidence that AT2 receptors in PVN neurons expressing VP and other neuromodulators are strategically positioned for surface activation by AngII and/or intracellular trafficking.

Visually guided whole cell patch clamp of mouse supraoptic nucleus neurons in cultured and acute conditions

Recent advances in neuronal culturing techniques have supplied a new set of tools for studying neural tissue, providing effective means to study molecular aspects of regulatory elements in the supraoptic nucleus of the hypothalamus (SON). To combine molecular biology techniques with electrophysiological recording, we modified an organotypic culture protocol to permit transfection and whole cell patch-clamp recordings from SON cells. Neonatal mouse brain coronal sections containing the SON were dissected out, placed on a filter insert in culture medium, and incubated for at least 4 days to allow attachment to the insert. The SON was identifiable using gross anatomical landmarks, which remained intact throughout the culturing period. Immunohistochemical staining identified both vasopressinergic and oxytocinergic cells present in the cultures, typically appearing in well-defined clusters. Whole cell recordings from these cultures demonstrated that certain properties of the neonatal mouse SON were comparable to adult mouse magnocellular neurons. SON neurons in both neonatal cultures and acute adult slices showed similar sustained outward rectification above -60 mV and action potential broadening during evoked activity. Membrane potential, input resistance, and rapidly inactivating potassium current density (IA) were reduced in the cultures, whereas whole cell capacitance and spontaneous synaptic excitation were increased, perhaps reflecting developmental changes in cell physiology that warrant further study. The use of the outlined organotypic culturing procedures will allow the study of such electrophysiological properties of mouse SON using whole cell patch-clamp, in addition to various molecular, techniques that require longer incubation times.

Circadian dynamics of vasopressin in mouse selection lines: translation and release in the SCN

Arg8-vasopressin (AVP), a circadian clock-controlled gene product, is released from the hypothalamic suprachiasmatic nuclei (SCN) in mice in a circadian fashion. Previously reported differences in two mouse lines, initially selected for thermoregulatory nest-building behavior (building small nests (S-mice) or big nests (B-mice)) with different circadian organization of behavior and in number of SCN-AVP immunoreactive neurons, were further investigated. We confirmed and expanded the finding that S-mice exhibited constant high levels of SCN-AVP content with no apparent circadian rhythmicity, whereas B-mice had lower numbers of AVP positive cells which varied with time of day. We found that AVP mRNA expression levels at midnight and midday were similar in both lines, as established by in situ hybridization. When AVP transport and release were blocked by colchicine, SCN-AVP immunoreactivity was similar in both lines. This suggests that differences in SCN-AVP content depend on transport or release. Organotypic SCN cultures of B-mice showed more AVP release per neuron than cultures of S-mice. These results reveal that on a mechanistic level the mouse lines differed in transport and/or release of AVP in the SCN, rather than differential regulation of AVP gene transcription or number of AVP immunoreactive neurons.

Development of the mouse hypothalamo-neurohypophysial system in the munc18-1 null mutant that lacks regulated secretion

The hypothalamo-neurohypophysial system (HNS) is composed of hypothalamic magnocellular neurons and neural lobe pituicytes that accommodate the nerve terminals. Here we have investigated if the communication of the peptidergic neurons of the HNS with neighbouring cells plays a role in the development and assembly of the HNS. We employed munc18-1-deficient mice, which completely lack neurotransmitter secretion. Morphological and immunohistological analysis of the HNS in these mutant embryos during brain development showed that this peptidergic system was formed normally during early embryogenesis. However, the development arrested at embryonal day 14.5, the stage when terminal differentiation has to take place. The peptidergic neurons targeted axons in the correct direction, but few arrived at their final location and the neurons were not maintained in later stages. The pituicytes in the neural lobe of the pituitary were generated, but failed to organize normally. Our results indicate that peptide gene expression, axon outgrowth and migration are intrinsic developmental events in these peptidergic neurons, that are initiated in the munc18-1 null mutant. The further expansion and the integration of outgrowing axon terminals with neural lobe pituicytes requires munc18-1-dependent processes, probably exocytosis, at multiple levels. Firstly, to maintain and propagate neuronal outgrowth and guidance, and secondly, to control the cellular organization of the pituicytes. Thus, the communication between the outgrowing neurons and the pituicytes could serve to integrate these two cell types to constitute a functional peptidergic system.

Expression of neuropeptide FF, prolactin-releasing peptide, and the receptor UHR1/GPR10 genes during embryogenesis in the rat

Recently, several RF-amide peptides have been identified in mammals. These peptides have a similar C-terminal RF-motif and share some G-protein coupled receptors. Neuropeptide FF (NPFF) and prolactin-releasing peptide (PrRP) are expressed in the same brain areas in the adult rat and act both in prolactin release and cardiovascular regulation. Here, we characterized the embryonal expression from embryonal day 14 to postnatal day 0 of both peptide mRNAs and the mRNA distribution of UHR1/GPR10-like receptor by using in situ hybridization (ISH) and quantitative reverse transcriptase-polymerase chain reaction. NPFF mRNA was found in the spinal cord, caudal solitary tract nucleus, and surprisingly, in the medullary reticular formation. The only peripheral organs displaying NPFF mRNA expression were the lungs and the spleen. PrRP gene expression was seen in the caudal solitary tract nucleus, medullary reticular formation, pontine isthmus and liver, kidney, and testis. The receptor UHR1/GPR10 gene was expressed consistently in the medullary reticular formation and the adrenal gland but also transiently in several locations. All three genes showed weak but even ISH signal in the pituitary. These findings suggest different roles for the peptides during development and indicate that UHR1/GPR10-like receptor could also bind other ligands in addition to PrRP.

Magnocellular vasopressin system in ontogenesis: development and regulation

This review is devoted to the development, functional activity, and regulation of the magnocellular vasopressin (VP) system in ontogenesis. Magnocellular VP neurons originate in embryos from the neuroepithelium of the third ventricle and migrate first to the supraoptic nucleus and then to the paraventricular nucleus and accessory nuclei. The preproVP gene and synthesis are expressed simultaneously in the newly formed neurons either during migration or just after arrival in magnocellular nuclei. Still, a number of VP-immunoreactive neurons increase in immature mammals to prepuberty, which is explained by VP expression in the initially "silent" neurons, or by an increase of VP synthesis that makes a cell distinguishable by immunocytochemistry. An enzymatic processing of preproVP is slightly delayed compared to the onset of preproVP synthesis. Axons of magnocellular neurons reach the pituitary posterior lobe before or just after the neuron arrival in magnocellular nuclei. The mechanisms of VP release from the axon terminals are developed in immature animals over the perinatal period. The VP neurons begin to react to functional (osmotic) stimulation by increased synthesis of VP mRNA and VP in immature animals from the end of fetal life. A functional maturation of the VP system is under control by neural afferents, neuropeptides, and some hormones of endocrine glands. Namely, glucocorticoids, VP, catecholamines, glutamate, and opioids provide short-term or long-lasting effects on differentiating VP neurons. Most of the intercellular signals inhibit the specific phenotype expression of differentiating VP neurons: VP gene and synthesis in normal conditions, as well as TH gene and synthesis under functional stimulation.

Neuropathologic research strategies in holoprosencephaly

Hypotheses are presented to explain the pathogenesis of several clinical features of holoprosencephaly, and neuropathologic approaches to testing these hypotheses are suggested. The traditional morphologic classification of holoprosencephaly into alobar, semilobar, and lobar forms is grades of severity, and each occurs in all of the genetic mutations known. Of the four defective genes identified as primary in human holoprosencephaly, three exhibit a ventrodorsal gradient of expression (SHH, SIX3, and TGIF) and one a dorsoventral gradient (ZIC2). But, in addition to the vertical axis, genes expressed in the neural tube also may have rostrocaudal and mediolateral gradients in the other axes. These other gradients may be equally as important as the vertical. If the rostrocaudal gradient extends as far as the mesencephalic neuromere, it may interfere with the formation, migration, or apoptosis of the mesencephalic neural crest, which forms membranous bones of the face, orbits, nose, and parts of the eyes, and may explain the midfacial hypoplasia seen in many, but not all, children with holoprosencephaly. This rostrocaudal gradient also causes noncleavage of the caudate nucleus, thalamus, and hypothalamus and contributes to the formation of the dorsal cyst of holoprosencephaly, which is probably derived from an expanded suprapineal recess of the 3rd ventricle with secondary dilation of the telencephalic monoventricle and at times may produce a unique transfontanellar encephalocele. The extent of the mediolateral gradient may explain the severe disorganization of cerebral cortical architecture in medial parts of the forebrain and normal cortex in lateral parts, including the radial glial fibers. This preserved lateral cortex may explain why some children with holoprosencephaly have better intellectual function than expected and may also be important in the pathogenesis of epilepsy, by contrast with malformations such as lissencephaly, in which the entire cerebral cortex is involved. Epilepsy in some, but not all, cases also may be related to the sequential maturation of axonal terminals in relation to the neurons they innervate. Diabetes insipidus is a complication in a majority of patients; other neuroendocrinopathies occur less frequently. Secondary down-regulation of the OTP gene or of downstream genes such as BRN2 or SIM1 may result in failure of terminal differentiation of magnocellular neurons of the supraoptic and paraventricular hypothalamic nuclei. Disoriented radial glial fibers or abnormal ependyma may allow aberrant migration of neuroepithelial cells into the ventricle. A new classification of holoprosencephaly is needed to integrate morphologic and genetic criteria.

Defective development of secretory neurones in the hypothalamus of Arnt2-knockout mice

BACKGROUND

Within the basic region-helix-loop-helix (bHLH)-PAS family of transcription factors, Arnt and Arnt2 play unique roles; these two factors not only heterodimerize with themselves, but also with other members of this family and they act as transcription regulators which bind to specific DNA elements. Whereas Arnt is broadly expressed in various tissues, the expression of Arnt2 is known to be limited to the neural tissues.

RESULTS

To elucidate the function of Arnt2 in detail, we cloned the mouse Arnt2 gene and its gene structure was determined. We subsequently generated germ line Arnt2 mutant mice by gene targeting technology. Heterozygous Arnt2 mice were viable, but homozygous Arnt2 gene knockout mice died shortly after birth. Histological and immunological analyses revealed that the supraoptic nuclei (SON) and the paraventricular nuclei (PVN) are hypocellular. Moreover, secretory neurones identified by the expression of neurosecretory hormone such as arginine vasopressin, oxytocin, corticotrophin-releasing hormone and somatostatin are completely absent in SON and PVN in the mutant Arnt2 mice. Consistent with these observations, prospective SON and PVN neurones which express Brn2 appeared around E13.5 in the mantle zone, but no neurones which expressed the neurosecretory hormones were found in the SON and PVN regions.

CONCLUSIONS

These data show that the transcription factor Arnt2 controls the development of the secretory neurones at the later or final stages of differentiation rather than at the beginning stage. Strikingly similar observations have been reported with the Sim1 deficient mice. Taken together, our results demonstrate that Arnt2 is an indispensable transcription factor for the development of the hypothalamus, and suggest that Arnt2 is an obligatory partner molecule of Sim1 in the developmental process of the neuroendocrinological cell lineages.

The murine Otp homeobox gene plays an essential role in the specification of neuronal cell lineages in the developing hypothalamus

Hypothalamic nuclei, including the anterior periventricular (aPV), paraventricular (PVN), and supraoptic (SON) nuclei strongly express the homeobox gene Orthopedia (Otp) during embryogenesis. Targeted inactivation of Otp in the mouse results in the loss of these nuclei in the homozygous null neonates. The Otp null hypothalamus fails to secrete neuropeptides somatostatin, arginine vasopressin, oxytocin, corticotropin-releasing hormone, and thyrotropin-releasing hormone in an appropriate spatial and temporal fashion, and leads to the death of Otp null pups shortly after birth. Failure to produce these neuropeptide hormones is evident prior to E15.5, indicating a failure in terminal differentiation of the aPV/PVN/SON neurons. Absence of elevated apoptotic activity, but reduced cell proliferation together with the ectopic activation of Six3 expression in the presumptive PVN, indicates a critical role for Otp in terminal differentiation and maturation of these neuroendocrine cell lineages. Otp employs distinct regulatory mechanisms to modulate the expression of specific molecular markers in the developing hypothalamus. At early embryonic stages, expression of Sim2 is immediately downregulated as a result of the absence of Otp, indicating a potential role for Otp as an upstream regulator of Sim2. In contrast, the regulation of Brn4 which is also expressed in the SON and PVN is independent of Otp function. Hence no strong evidence links Otp and Brn4 in the same regulatory pathway. The involvement of Otp and Sim1 in specifying specific hypothalamic neurosecretory cell lineages is shown to operate via distinct signaling pathways that partially overlap with Brn2.

The role of Clock in the developmental expression of neuropeptides in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is the dominant circadian pacemaker in mammals. To understand better the ontogeny of mouse SCN and the role of the pacemaker in peptide expression, the authors examined the distribution of cells that were immunoreactive for vasopressin (AVP) or vasoactive intestinal polypeptide (VIP) in wild type and Clock mutant mice at two developmental stages. Clock homozygous mice failed to show the dramatic increase in the number of VIP-immunoreactive (VIP-ir) neurons from postnatal day 6 (P6) to P30 that was found in the SCN of wild type mice. The number of AVP-ir neurons was relatively constant in the postnatal SCN but was significantly reduced in Clock/Clock mice. The effects of the Clock mutation varied with position in the SCN for both peptides. Densitometry of immunolabeled brains indicated that the Clock mutation reduced AVP expression specifically in the SCN and not in other brain areas. The SCN did not significantly change shape or size with age or Clock genotype. Taken together, these results indicate that the neonatal mouse SCN has its full complement of cells, some of which are not yet mature in their neuropeptide content. Furthermore, the observation that the Clock mutation appears to act on a subset of AVP and VIP cells suggests heterogeneity within these cell classes in the SCN.

Development of sexually dimorphic vasotocinergic system in the bed nucleus of stria terminalis in chickens

The bed nucleus of stria terminalis (BnST) of the domestic fowl contains two groups of parvicellular vasotocinergic neurons that are sexually dimorphic. In adult cockerels, arginine vasotocin (AVT) synthesis is well expressed in the dorsolateral and ventromedial portions of the BnST, whereas in corresponding brain areas of hens, AVT synthesis is completely lacking. In the present study, in situ hybridization and immunocytochemical methods were used to compare the ontogeny of sexually dimorphic AVT gene expression in the BnST of male and female chickens from day 12 of embryonic development (E12) until the onset of sexual maturation. By E12, both parvicellular groups of AVT-immunoreactive (AVT-ir) perikarya in the developing BnST can be distinguished in some males, whereas in females their presence is questionable. A quantitative analysis, beginning at E14, showed that the parvicellular dorsolateral portion of the BnST of male embryos had more AVT perikarya compared with females. In contrast, no evident sex difference in distribution pattern and number of AVT mRNA containing neurons in this BnST portion was observable by in situ hybridization at E15. At E18, as well as on the first and second days posthatch (D1 and D2), no differences in the number of AVT synthesizing cells and intensity of immunoreactive staining in male versus female chickens were found. Between D2 and D7, the number of AVT-ir cells in the BnST declined rapidly in both sexes until it disappeared completely in females before D35. In males, another increase in sexually dimorphic AVT-ir cells and innervation of the lateral septum was associated with the onset of puberty and fully matched a pattern observed in adult fowls. These results demonstrate that the sexually dimorphic part of the AVT system undergoes sexual differentiation during early stages of ontogeny.

Development of neuroendocrine lineages requires the bHLH-PAS transcription factor SIM1

The bHLH-PAS transcription factor SIM1 is expressed during the development of the hypothalamic-pituitary axis in three hypothalamic nuclei: the paraventricular nucleus (PVN), the anterior periventricular nucleus (aPV), and the supraoptic nucleus (SON). To investigate Sim1 function in the hypothalamus, we produced mice carrying a null allele of Sim1 by gene targeting. Homozygous mutant mice die shortly after birth. Histological analysis shows that the PVN and the SON of these mice are hypocellular. At least five distinct types of secretory neurons, identified by the expression of oxytocin, vasopressin, thyrotropin-releasing hormone, corticotropin-releasing hormone, and somatostatin, are absent in the mutant PVN, aPV, and SON. Moreover, we show that SIM1 controls the development of these secretory neurons at the final stages of their differentiation. A subset of these neuronal lineages in the PVN/SON are also missing in mice bearing a mutation in the POU transcription factor BRN2. We provide evidence that, during development of the Sim1 mutant hypothalamus, the prospective PVN/SON region fails to express Brn2. Our results strongly indicate that SIM1 functions upstream to maintain Brn2 expression, which in turn directs the terminal differentiation of specific neuroendocrine lineages within the PVN/SON.

Hypothalamic synaptogenesis and its relationship with the maturation of hormonal secretion

1. Information obtained during the last decade has demonstrated that hypothalamic neurons release a wide variety of neuroactive substances, such as neurotransmitters, mostly monoamines and amino acids, and neuromodulators such as the peptides vasopressin (AVP) and oxytocin (OXT) and hypophysial releasing hormones. 2. Synapse formation between hypothalamic neurons was followed at different times within a given nucleus and among different nuclei during development of the mouse hypothalamus. 3. The amounts of various neurotransmitters and hormones were determined at various stages of development. 4. A correlation is presented of the biochemical and ultrastructural features and their functional implications during maturation.

Ontogeny of the vasopressin and oxytocin RNAs in the mouse hypothalamus

The single-copy genes encoding the vasopressin and oxytocin prepropeptides are closely linked in mouse genome, being separated by an intergenic region of only 3 kbp. These genes are expressed in anatomically defined hypothalamic neurons--in the adult rodent, vasopressin is synthesised in the paraventricular nucleus and the supraoptic nucleus, and in the dorsomedial region of the suprachiasmatic nucleus, whilst oxytocin is expressed in the supraoptic nucleus and paraventricular nucleus, but not in the suprachiasmatic nucleus. The molecular mechanisms that mediate the cell-specific and developmental expression patterns of the two transcription units within the vasopressin-oxytocin locus remain to be elucidated. As a first step in this process, we have used in situ hybridisation to study the expression of the RNAs encoded by the linked vasopressin and oxytocin genes during the development of the mouse hypothalamus. We have revealed a hierarchy of gene activation events, with vasopressin first being observed in presumptive supraoptic nucleus at day 13.5, and in the paraventricular at day 14.5. Oxytocin is seen first in the paraventricular at day 15.5; expression in the supraoptic nucleus is clearly seen at day 18.5. As early as day 15.5, the vasopressin and oxytocin RNAs are expressed in different groups of neurons.

Vasopressin and oxytocin gene expression in intact rats and under catecholamine deficiency during ontogenesis

The development of the hypothalamic vasopressin (VP) and oxytocin (OT) systems has been studied in rats from the 16th embryonic day (E16) until the 11th postnatal day (P11). The VP and OT mRNA-producing neurons were identified on cryostat sections by in situ hybridization using oligonucleotide probes labeled by [35S], [3H] or digoxigenin. Moreover, VP and OT gene expressions were evaluated either at E21 or at P11 following chronic depletion of catecholamines (CA). For this purpose, pregnant rats were daily injected with alpha-methyl-m(p)-tyrosine from gestational day 13 to 20 while neonates were daily injected with alpha-methyl-m(p)-tyrosine and neurotoxin 6-hydroxydopamine from postnatal day 2 to 10. No VP mRNA- or OT mRNA-expressing cells were observed in the hypothalamus of intact fetuses at E16, while 2 days later rather numerous VP and OT neurons occupied the anterior hypothalamus. One major bilateral group of VP and OT neurons was located in the supraoptic nucleus (SON). Less numerous labeled cells were found in the developing paraventricular nucleus (PVN). Some VP and OT neurons were also spread along the ventrolateral surface of the hypothalamus from the level of the median eminence, caudally, to the level of the optic nerves, rostrally. From E18 until birth, the OT neurons were localized in the dorsal portion of the SON, while its ventral portion was occupied by the VP neurons. The VP mRNA- and OT mRNA-expressing cells seemed to increase both in size and in number over the perinatal period. Frequent relatively long neuronal processes contained VP and OT mRNAs in fetuses and in newborns. When performed during the second half of the fetal life, the chronic depletion of CA did not cause any change in the VP and OT mRNA concentrations in the SON and PVN of fetuses. By contrast, similar treatment of neonates resulted in a significant increase of both mRNA levels in the SON. These data suggest that at least in the SON VP and OT gene expression might be under the inhibitory control of CA during the neonatal period.

Ontogenic development and distribution of mRNAs of chromogranin A and B, secretogranin II, p65 and synaptin/synaptophysin in rat brain

We have studied by in situ hybridization the mRNA levels of several constituents of transmitter storing vesicles during ontogenic development of rat brain. The following vesicle components were investigated: chromogranin A and B and secretogranin II, representing secretory peptides of large dense core vesicles, and the membrane proteins p65 and synaptin/synaptophysin which are found in both large and small synaptic vesicles but are concentrated in the latter ones. Several ontogenic patterns were observed: concomitant increases of most or all mRNAs in certain brain regions, e.g. in the thalamic nuclei at gestational day 18 or in the cortex at postnatal day 6. For some areas selective increases for the various chromogranin mRNAs occurred, thus throughout development the substantia nigra compacta contained only the chromogranin B mRNA, whereas the lateral and medial geniculate nuclei and the medial tuberal nucleus expressed only secretogranin II mRNA. In the paraventricular hypothalamic nucleus, secretogranin II mRNA declined at P1 and then increased again. In the intermediate cortex there was a rather selective appearance of a high level of chromogranin A mRNA already at gestational day 16. In general the mRNAs for the membrane components become detectable by in situ hybridization together with the chromogranin mRNA, however, in the claustrum a high level of the p65 mRNA is present already at gestational day 16 whereas the chromogranin mRNA only appears at day 20. In some nuclei there was also a differential expression of the membrane components with e.g. the synaptophysin mRNA being present without any concomitant appearance of p65. These results establish that the ontogenic development of the investigated components in many brain areas simply indicate the starting point of biosynthesis of both types of vesicles finally leading to functional synapses. In those cases where a selective dissociation in the biosynthesis of these components occurs, a functional relevance of one component for a certain stage of development might be postulated. Since these data define the time of onset of vesicle biosynthesis in the various brain regions, future studies on single components of these vesicles can be interpreted in the context of the present findings.

Ultrastructural evidence of sexual dimorphism in supraoptic neurons: a morphometric study

We have recently shown that in spite of the absence of receptors for gonadal steroids in the supraoptic nucleus (SON) of the rat hypothalamus, the volume of the nucleus and the size of its neurons are larger in males than in females, and that these differences between male and female rats are correlated with body weight and dependent on the vasopressinergic neurons. As supraoptic neurons and their organelles enlarge when they are engaged in active peptidergic secretion we have carried out a morphometric ultrastructural analysis to determine if cell structures involved in the synthesis and storage of neurosecretory material also display weight-dependent sex dimorphism. Groups of six male and six female rats aged 30, 60 and 180 days were used. Nucleoli, rough endoplasmic reticulum and neurosecretory granules were analysed and we estimated their volume or surface densities and the total volume of nucleoli and rough endoplasmic reticulum, and total surface area of rough endoplasmic reticulum. We found that, with the exception of neurosecretory granules, the densities of the organelles did not differ among the groups studied, but total values were higher in males. These differences were found to be weight-dependent. Since the organelles studied are regarded as reliable indicators of the neurosecretory activity of supraoptic neurons, our data fully support the view that the weight-dependent sexual dimorphism observed in this nucleus reflects greater synthetic activity of its vasopressinergic neurons associated with the need to maintain water balance in larger bodies.