Vasopressin-deficient paraventricular magnocellular neurons of homozygous Brattleboro rats synthesize neuropeptide Y

Response of substances co-expressed in hypothalamic magnocellular neurons to osmotic challenges in normal and Brattleboro rats

The intention of this review is to emphasize the current knowledge about the extent and importance of the substances co-localized with magnocellular arginine vasopressin (AVP) and oxytocin (OXY) as potential candidates for the gradual clarification of their actual role in the regulation of hydromineral homeostasis. Maintenance of the body hydromineral balance depends on the coordinated action of principal biologically active compounds, AVP and OXY, synthesized in the hypothalamic supraoptic and paraventricular nuclei. However, on the regulation of water-salt balance, other substances, co-localized with the principal neuropetides, participate. These can be classified as (1) peptides co-localized with AVP or OXY with unambiguous osmotic function, including angiotensin II, apelin, corticotropin releasing hormone, and galanin and (2) peptides co-localized with AVP or OXY with an unknown role in osmotic regulation, including cholecystokinin, chromogranin/secretogranin, dynorphin, endothelin-1, enkephalin, ferritin protein, interleukin 6, kininogen, neurokinin B, neuropeptide Y, vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide, TAFA5 protein, thyrotropin releasing hormone, tyrosine hydroxylase, and urocortin. In this brief review, also the responses of these substances to different hyperosmotic and hypoosmotic challenges are pointed out. Based on the literature data published recently, the functional implication of the majority of co-localized substances is still better understood in non-osmotic than osmotic functional circuits. Brattleboro strain of rats that does not express functional vasopressin was also included in this review. These animals suffer from chronic hypernatremia and hyperosmolality, accompanied by sustained increase in OXY mRNA in PVN and SON and OXY levels in plasma. They represent an important model of animals with constantly sustained osmolality, which in the future, will be utilizable for revealing the physiological importance of biologically active substances co-expressed with AVP and OXY, involved in the regulation of plasma osmolality.

Galanin receptor antagonists decrease fat preference in Brattleboro rat

The Brattleboro rat eats spontaneously 46% of its diet per day in fat when given a choice of carbohydrate, protein and fat. An overexpression of galanin (GAL) has been also observed in the hypothalamic paraventricular nuclei (PVN). This associative correlation has led to a hypothesis of a functional relation between central galanin expression and the preference for a lipid diet. In the present experiments, the effects of two GAL receptor antagonists, C7 and galantide, on fat consumption and central overexpression of GAL were investigated. Both antagonists were injected into either the cerebral ventricles or directly above the PVN, and the diet consumption followed for the subsequent 24h. C7 decreased significantly fat consumption when injected into the ventricles or directly above the PVN. In contrast, galantide must be injected above the PVN to show the same effect. However, the two antagonists did not modify GAL mRNA expression in the PVN when they were injected 2h before sacrifice. These experiments confirm a functional link between the preferential consumption of fat and hypothalamic Galanin; different subtypes of the GAL receptor are probably involved, since both Galanin antagonists were differently efficient in decreasing spontaneous fat selection of the Brattleboro rat.

Neuropeptide Y in the magnocellular hypothalamic neurons of obese Zucker rats

The obesity syndrome in Zucker rats is associated with an elevated neuropeptide Y (NPY) content in the hypothalamus. It is recognized that the axons of NPY-ergic neurons arriving from the arcuate nucleus and the midbrain are the major source of NPY in this area. In magnocellular hypothalamic neurosecretory neurons (MCN) of normal rats NPY is expressed only in response to hyperosmotic stimulation. This study used immunohistochemistry with colchicine treatment aimed at MCN to compare NPY localization in obese (fa/fa) and in lean (Fa/Fa) Zucker rats. It was found that the obese (fa/fa), in contrast to the lean (Fa/Fa) Zucker rat, displays NPY immunoreactivity in numerous MCN of the paraventricular, supraoptic as well as accessory neurosecretory nuclei. This finding suggests local synthesis of NPY in the MCN of obese (fa/fa) rats and involvement of hydro-osmotic disorders in the Zucker syndrome of obesity.

NPY mRNA and peptide immunoreactivity in the arcuate nucleus are increased by osmotic stimuli: correlation with dehydration anorexia

The role of neuropeptide Y (NPY) in the central control of appetite and energy balance is now established, but its involvement in the control of drinking and fluid homeostasis is less well characterized. Central administration of NPY stimulates drinking in rats, an effect believed to be independent of its orexigenic effects. Recent studies have demonstrated increased preproneuropeptide Y (preproNPY) mRNA in the arcuate nucleus (ARC) of the rat following food deprivation (FD) or water deprivation (WD). Because WD also suppresses food intake, it was not clear whether the osmotic or the anorectic effects of this stimulus were responsible for increased ARC preproNPY mRNA. In an attempt to distinguish between these possibilities, the present study further examined the effects of hyperosmotic stimuli on preproNPY mRNA in the ARC. Salt loading (4 or 7 days) and WD (4 days) both increased the abundance of preproNPY mRNA in the ARC. These increases were proportional to the severity and duration of treatment and were related to the degree of anorexia and weight loss. In a separate study WD, FD, or combined food and water deprivation (4 days) all produced similar decreases in body weight, but WD produced a smaller increase in ARC preproNPY mRNA. All of these treatments resulted in the appearance of NPY-like immunoreactivity in ARC neuronal perikarya. Together these findings suggest that NPY neuron activity in the ARC may be regulated by decreases in food intake rather than changes in body weight per se or increased osmolarity and support other data implicating NPY in the central regulation of energy homeostasis.

Dynamics of 7B2 and galanin expression in solitary magnocellular hypothalamic vasopressin neurons of the homozygous Brattleboro rat

The homozygous Brattleboro rat (di/di), displaying a hypothalamic form of diabetes insipidus, synthesizes a vasopressin (VP) precursor with an abnormal C-terminus. The phenotypic expression of coexisting peptides in mutant magnocellular VP cells shows a differential pattern. 7B2 is one of the peptides which is not detectable, whereas there is a clear galanin expression. During postnatal life a small but increasing number of solitary post-mitotic VP neurons of the di/di rat undergoes a switch to a heterozygous phenotype. Here we report the presence of 7B2 and galanin in these heterozygous cells, which suggests that for the expression of 7B2, but not for that of galanin, the relative amount of mutant VP precursor must be diminished. Possible underlying mechanisms for this differential phenotypic expression of coexisting peptides are compartmentalization of precursor synthesis within the RER or a competition for sites involved in the translocation of the functionally reduced RER.

Neuropeptide Y messenger ribonucleic acid in the magnocellular hypothalamo-neurohypophysial system of the rat is increased during osmotic stimulation

In the magnocellular hypothalamo-neurohypophysial system of the rat, levels of neuropeptide Y (NPY) mRNA were investigated by in situ hybridization histochemistry both in normal and osmotically stimulated animals. As an osmotic stimulus, animals were given 340 mM NaCl to drink for 12 days. After saline treatment, there was a very marked increase in NPY mRNA in the hypothalamic magnocellular supraoptic (SON) and paraventricular (PVN) nuclei (7600% and 3200%, resp.). These data suggest that magnocellular SON and PVN neurons synthesize NPY in response to the physiological stimulus of increased plasma osmolality.

Mutant vasopressin precursor producing cells of the homozygous Brattleboro rat as a model for co-expression of neuropeptides

The homozygous Brattleboro rat (di/di) synthesizes a VP precursor with an abnormal C terminus, which is not transported from the rough endoplasmic reticulum to the Golgi apparatus. In addition, the phenotypic expression of co-existing peptides is differentially disturbed. Ang II and 7B2 are two of the peptides which are not detectable, whereas other peptides (e.g. galanin) are clearly expressed in mutant VP cells. During postnatal life a small but increasing number of solitary post-mitotic VP neurons of the di/di rat undergoes a switch to a heterozygous phenotype. At the same time Ang II and 7B2 show up again in these heterozygous cells, which suggests that for the expression of 7B2, but not for that of other peptides (e.g. galanin), a normal VP precursor is required. A possible underlying mechanism (i.e. the existence of several domains on the endoplasmic reticulum involved in the translocation of sets of neuropeptides) for this differential phenotypic expression of co-existing peptides is discussed.

Vasopressin and angiotensin II are absent but spontaneously reappear in solitary hypothalamic neurons of the homozygous Brattleboro rat

The homozygous Brattleboro rat (di/di) synthesizes a vasopressin (VP) precursor with a different C-terminus, which is not packaged in granules. In addition, the expression of a coexisting peptide, angiotensin II (Ang II), is disturbed. During postnatal life a small but increasing number of solitary post-mitotic hypothalamic neurons of the di/di rat undergoes a switch to a genuine heterozygous phenotype. Here we report the reappearance of Ang II in these heterozygous cells, which suggests that for the expression of Ang II a normal VP precursor is required. Based upon the present study and literature data it is proposed that at the level of the endoplasmic reticulum a compartmentalization of the synthesis of various peptide precursor occurs.

Intragranular co-storage of neuropeptide Y and arginine vasopressin in the paraventricular magnocellular neurons of the rat hypothalamus

Certain populations of arginine vasopressin (AVP) neurons in the magnocellular paraventricular nucleus became immunoreactive for neuropeptide Y (NPY) when rats were treated with colchicine or monosodium glutamate (MSG). The co-storage of these peptides was examined by employing a post-embedding electron-microscopic immunohistochemistry technique using gold-labeled antibodies to the two peptides. In colchicine-treated rats, the neuronal perikarya contained numerous secretory granules showing co-storage of the two peptides. The cells of the MSG-treated rats were characterized by having well-developed Golgi bodies with the granular structures also co-storing the two peptides, although the secretory granules in the perikarya were rather fewer than in the colchicine-treated rats. It is concluded that the destruction of the arcuate nucleus by MSG-treatment may potentiate the synthesis of NPY in AVP neurons, the synthesis of which is latent in intact animals.