Organization of Mammalian Neuroendocrine System

Development of neuroendocrine neurons in the mammalian hypothalamus

The neuroendocrine system consists of a heterogeneous collection of (mostly) neuropeptidergic neurons found in four hypothalamic nuclei and sharing the ability to secrete neurohormones (all of them neuropeptides except dopamine) into the bloodstream. There are, however, abundant hypothalamic non-neuroendocrine neuropeptidergic neurons developing in parallel with the neuroendocrine system, so that both cannot be entirely disentangled. This heterogeneity results from the workings of a network of transcription factors many of which are already known. Olig2 and Fezf2 expressed in the progenitors, acting through mantle-expressed Otp and Sim1, Sim2 and Pou3f2 (Brn2), regulate production of magnocellular and anterior parvocellular neurons. Nkx2-1, Rax, Ascl1, Neurog3 and Dbx1 expressed in the progenitors, acting through mantle-expressed Isl1, Dlx1, Gsx1, Bsx, Hmx2/3, Ikzf1, Nr5a2 (LH-1) and Nr5a1 (SF-1) are responsible for tuberal parvocellular (arcuate nucleus) and other neuropeptidergic neurons. The existence of multiple progenitor domains whose progeny undergoes intricate tangential migrations as one source of complexity in the neuropeptidergic hypothalamus is the focus of much attention. How neurosecretory cells target axons to the medial eminence and posterior hypophysis is gradually becoming clear and exciting progress has been made on the mechanisms underlying neurovascular interface formation. While rat neuroanatomy and targeted mutations in mice have yielded fundamental knowledge about the neuroendocrine system in mammals, experiments on chick and zebrafish are providing key information about cellular and molecular mechanisms. Looking forward, data from every source will be necessary to unravel the ways in which the environment affects neuroendocrine development with consequences for adult health and disease.

Expression of Robo/Slit and Semaphorin/Plexin/Neuropilin family members in the developing hypothalamic paraventricular and supraoptic nuclei

The hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON) contain neuroendocrine cells that modulate pituitary secretion to maintain homeostasis. These two nuclei have a common developmental origin but they eventually form at locations distant from each other. Little is known about the molecular cues that direct the segregation of PVN and SON. As a means to identify potential factors, we have documented expression patterns of genes with known guidance roles in neural migration. Here, we focus on two groups of ligand/receptor families classified to mediate chemo-repulsion of neurons and their axons: the Slit/Robo and the Semaphorin/Plexin/Neuropilin families. Their dynamic expression patterns within and around the common PVN/SON progenitor as well as the mature PVN and SON may provide a framework for understanding the formation of these two important nuclei.

Actions of neuropeptide W in paraventricular hypothalamus: implications for the control of stress hormone secretion

Neuropeptide W (NPW) is produced in neurons located in hypothalamus and brain stem, and its receptors are present in the hypothalamus, in particular in the paraventricular nucleus (PVN). Intracerebroventricular (ICV) administration of NPW activated, in a dose-related fashion, the hypothalamic-pituitary-adrenal axis, as determined by plasma corticosterone levels in conscious rats but, at those same doses, did not stimulate the release of oxytocin or vasopressin into the peripheral circulation or alter blood pressure or heart rate. The ability of ICV-administered NPW to stimulate the hypothalamic-pituitary-adrenal axis in conscious male rats was blocked by intravenous pretreatment with a corticotropin-releasing hormone antagonist. This suggested an action of NPW in the parvocellular division of the PVN. Indeed, in hypothalamic slice preparations (whole cell patch recording), bath application of NPW depolarized and increased the spike frequency of the majority of electrophysiologically identified putative neuroendocrine PVN neurons. Effects on membrane potential were maintained in the presence of TTX, suggesting them to be direct postsynaptic actions on these neuroendocrine cells. Our data suggest that endogenous NPW, produced in brain, may play a physiologically relevant role in the neuroendocrine response to stress.

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.

Metabolic mapping of the brain in pregnant, parturient and lactating rats using fos immunohistochemistry

The present study was designed to investigate Fos-positive neurons of the female rat brain at various reproductive states in order to analyze the metabolic map connected with pregnancy, parturition and lactation. The number of Fos-positive neurons in each brain nucleus was analyzed with a quantitative immunohistochemical method in virgin, pregnant, parturient, lactating and arrested lactating rats. In parturient rats, a significant number of Fos-positive neurons was observed as compared to virgin or pregnant females in the following brain regions; the bed nucleus of the stria terminalis (BST), lateral septal nucleus (LS), medial preoptic area (MPA), periventricular hypothalamic nucleus (Pe), parvocellular paraventricular hypothalamic nucleus (PaPVN), magnocellular paraventricular hypothalamic nucleus (MaPVN), supraoptic nucleus (SON), paraventricular thalamic nucleus (PV), anterior hypothalamic area (AHA), lateral hypothalamic area (LH), amygdaloid nucleus (AM), supramammillary nucleus (SuM), substantia nigra (SN), central grey (CG), microcellular tegmental nucleus (MiTg), subparafascicular thalamic nucleus (SPF), posterior hypothalamic area (PH), dorsal raphe nucleus (DR), locus coeruleus (LC), dorsal parabrachial nucleus (DPB), nucleus of solitary tract (Sol), and ventrolateral medulla (VLM). Significant differences were found in the number of Fos-positive neurons between parturient and lactating females, although localization of Fos-positive neurons in lactating females was quite similar to parturient ones. Between parturient and lactating rats: (1) In the MPA, PaPVN, AHA, arcuate hypothalamic nucleus (Arc), ventromedial hypothalamic nucleus (VMH), MLT, and Ge, the number of Fos-positive neurons of lactating females were significantly higher than those of parturient ones; (2) In the LS, Pe, PV, LH, AM, SuM, CG, MiTg, SPF, PH, DR, LC, and VLM, there was no significant differences in the number of Fos-positive neurons; (3) In the BST, MaPVN, SON, SN, DPB and Sol, the number of Fos-positive neurons of lactating rats were significantly lower than those of parturient ones. These different patterns of Fos expression among many brain regions may be owing to the functional differences in each region. Fos expression in lactating rats was apparently induced by suckling stimulation because the removal of their litters immediately after parturition completely eliminated expression of Fos protein in each nucleus. These results suggest that the localization of Fos-positive neurons in a number of neural populations throughout the brain may be revealing the neural circuits in response to parturition or lactation.

Development and survival of the endocrine hypothalamus and posterior pituitary gland requires the neuronal POU domain factor Brn-2

Neurons comprising the endocrine hypothalamus are disposed in several nuclei that develop in tandem with their ultimate target the pituitary gland, and arise from a primordium in which three related class III POU domain factors, Brn-2, Brn-4, and Brn-1, are initially coexpressed. Subsequently, these factors exhibit stratified patterns of ontogenic expression, correlating with the appearance of distinct neuropeptides that define three major endocrine hypothalamic cell types. Strikingly, deletion of the Brn-2 genomic locus results in loss of endocrine hypothalamic nuclei and the posterior pituitary gland. Lack of Brn-2 does not affect initial hypothalamic developmental events, but instead results in a failure of differentiation to mature neurosecretory neurons of the paraventricular and supraoptic nuclei, characterized by an inability to activate genes encoding regulatory neuropeptides or to make correct axonal projections, with subsequent loss of these neurons. Thus, both neuronal and endocrine components of the hypothalamic-pituitary axis are critically dependent on the action of specific POU domain factors at a penultimate step in the sequential events that underlie the appearance of mature cellular phenotypes.