Physiological activation of the hypothalamo-neurohypophyseal system (HNS) by dehydration results is a massive release of vasopressin (VP) from the posterior pituitary. This is accompanied by a functional remodeling of the HNS. In this study we used cDNA arrays in an attempt to identify genes that exhibit differential expression in the hypothalamus following dehydration. Our study revealed nine candidate genes, including interleukin-6 (IL-6) as a putative novel secretory product of HNS worthy of further analysis. In situ hybridization and immunocytochemistry confirmed that IL-6 is robustly expressed in the supraoptic (SON) and the paraventricular (PVN) nuclei of the hypothalamus. By doublestaining immunofluorescence we showed that IL-6 is largely co-localized with VP in the SON and PVN. In situ hybridization, immunocytochemistry, and Western blotting all revealed IL-6 up-regulation in the SON and PVN following dehydration, thus validating the array data. The same dehydration stimulus resulted in an increase in IL-6 immunoreactivity in the axons of the internal zone of the median eminence and a marked reduction in IL-6-like material in the posterior pituitary gland. We thus suggest that IL-6 takes the same secretory pathway as VP and is secreted from the posterior pituitary following a physiological stimulus.The hypothalamo-neurohypophyseal system (HNS) 1 consists of large magnocellular neurons (MCNs) of the supraoptic (SON) and paraventricular (PVN) hypothalamic nuclei that have axons terminating on blood capillaries of the posterior pituitary (1), along with associated glia, blood vessels, interneurons, and afferent terminals. The HNS is the source of the neuropeptide hormone vasopressin (VP), which has a crucial role in osmoregulation (2). Following the onset of an osmotic stimulus such as dehydration (fluid deprivation), mammals respond to plasma hyperosmolality by reducing the renal excretion of water. Tubular reabsorption of water is controlled by circulating levels of VP. A rise in plasma osmolality is detected by osmoreceptor mechanisms in the circumventricular organs (3-6). Subsequent angiotensinergic (7,8) and glutamatergic (9 -11) excitation of hypothalamic neurons leads to a massive release of stored VP into the general circulation. Through an interaction with V2-type receptors located in the kidney, VP increases the permeability of the collecting ducts to water, promoting water conservation by decreasing the amount of water lost in urine.Physiological activation of the MCNs by dehydration results in a massive release of stored hormone from posterior pituitary terminals, and a concomitant functional remodeling of the HNS, characterized by activity-dependent secretory, electrophysiological, biosynthetic, and gene expression plasticity (12)(13)(14). At the morphological level, this plasticity is manifested as synaptic remodeling, increased direct neuronal membrane apposition and dendritic bundling in the SON, and by changes in the organization of neurovascular contacts in the neurohypophysis (12). For ex...

The hypothalamo-neurohypophyseal system (HNS) is the specialised brain neurosecretory apparatus responsible for the production of a peptide hormone, vasopressin, that maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of hormone from the HNS, and this is accompanied by a plethora of changes in morphology, electrical properties and biosynthetic and secretory activity, all of which are thought to facilitate hormone production and delivery, and hence the survival of the organism. We have adopted a functional genomic strategy to understand the activity dependent plasticity of the HNS in terms of the co-ordinated action of cellular and genetic networks. Firstly, using microarray gene-profiling technologies, we are elucidating which genes are expressed in the HNS, and how the pattern of expression changes following physiological challenge. The next step is to use transgenic rats to probe the functions of these genes in the context of the physiological integrity of the whole organism.

The magnocellular neurons (MCNs) of the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the hypothalamus are the principal site of biosynthesis of prepropeptide precursor of the antidiuretic hormone vasopressin (VP). This precursor is processed during anterograde axonal transportation to terminals in the posterior pituitary gland, where biologically active VP is stored until release into the general circulation in response to physiological activation of the SON by osmotic cues. By binding to V2-type receptors located in the kidney, VP decreases the amount of water lost in urine. Osmotic activation of the SON is accompanied by a dramatic morphological and functional remodeling. We have sought to understand the mechanistic basis of this plasticity in terms of the differential expression of genes. To identify such genes, we adopted an unbiased global approach based on suppressive subtractive hybridization-polymerase chain reaction (SSH-PCR) Using this method, we generated libraries of clones putatively differentially expressed in control vs. dehydrated SON. To rapidly screen these libraries, 1,152 clones were subjected to microarray analysis, resulting in the identification of 459 differentially expressed transcripts. cDNA clones corresponding to 56 of these RNAs were sequenced, revealing many of them to be novel expressed sequence tags (ESTs). Four transcripts were shown by in situ hybridization (ISH) to be significantly up- or downregulated in the SON after dehydration. These genes may represent novel effectors or mediators of SON physiological remodeling.