Effects of ovariectomy and estradiol replacement on the binding of 125I-neurotensin in rat suprachiasmatic nucleus

Diverse roles of neurotensin agonists in the central nervous system

Neurotensin (NT) is a tridecapeptide that is found in the central nervous system (CNS) and the gastrointestinal tract. NT behaves as a neurotransmitter in the brain and as a hormone in the gut. Additionally, NT acts as a neuromodulator to several neurotransmitter systems including dopaminergic, sertonergic, GABAergic, glutamatergic, and cholinergic systems. Due to its association with such a wide variety of neurotransmitters, NT has been implicated in the pathophysiology of several CNS disorders such as schizophrenia, drug abuse, Parkinson's disease (PD), pain, central control of blood pressure, eating disorders, as well as, cancer and inflammation. The present review will focus on the role that NT and its analogs play in schizophrenia, endocrine function, pain, psychostimulant abuse, and PD.

The regulatory role of neurotensin on the hypothalamic-anterior pituitary axons: emphasis on the control of thyroid-related functions

Neurotensin (NT) is a 13 amino acid neurohormone and/or neuromodulator, located in the synaptic vesicles and released from the neuronal terminals in a calcium-dependent manner. This peptide is present among mammalian and nonmammalian species, mainly in the central nervous system and the gastrointestinal tract. Due to its neuroendocrine activity, NT has been related to the pathophysiology of a series of disorders, such as schizophrenia, drug-abuse, Parkinson's disease, cancer, stroke, eating disorders and other neurodegenerative conditions. Moreover, NT participates in the physiology of pain-induction, central blood pressure control and inflammation. NT also plays an important interactive role in all components of the hypothalamic-anterior pituitary circuit, which is mediated by an endocrine, paracrine or/and autocrine manner, towards most of the anatomical regions that define this circuit. A considerable amount of data implicates NT in thyroid-related regulation through this circuit, the exact mechanisms of which should be further investigated for the potential development of more targeted approaches towards the treatment of thyroid-related endocrine diseases. The aim of this study was to provide an up-to-date review of the literature concerning the regulatory role of NT on the hypothalamic-anterior pituitary axons, with an emphasis on the control of thyroid-related functions.

Neurotensin and neuroendocrine regulation

More than two decades of research indicate that the peptide neurotensin (NT) and its cognate receptors participate to a remarkable extent in the regulation of mammalian neuroendocrine systems, potentially at multiple levels in a given system. NT-synthesizing neurons appear to exert a direct or indirect stimulatory influence on neurosecretory cells that synthesize gonadotropin-releasing hormone, dopamine (DA), somatostatin, and corticotropin-releasing hormone (CRH). In addition, context-specific synthesis of NT occurs in hypothalamic neurosecretory cells located in the arcuate nucleus and parvocellular paraventricular nucleus, including distinct subsets of cells which release DA, CRH, or growth hormone-releasing hormone into the hypophysial portal circulation. At the level of the anterior pituitary, NT stimulates secretion of prolactin and occurs in subsets of gonadotropes and thyrotropes. Moreover, circulating hormones influence NT synthesis in the hypothalamus and anterior pituitary, raising the possibility that NT mediates certain feedback effects of the hormones on neuroendocrine cells. Gonadal steroids alter NT levels in the preoptic area, arcuate nucleus, and anterior pituitary; adrenal steroids alter NT levels in the hypothalamic periventricular nucleus and arcuate nucleus; and thyroid hormones alter NT levels in the hypothalamus and anterior pituitary. Finally, clarification of the specific neuroendocrine roles subserved by NT should be greatly facilitated by the use of newly developed agonists and antagonists of the peptide.

Activation of galanin pathways across puberty in the male rat: assessment of regional densities of galanin binding sites

Galanin-like immunoreactivity and galanin messenger RNA levels increase across puberty in neurons of gonadal steroid-dependent brain nuclei. We hypothesized that this activation and the associated increase in endogenous galanin release would result in changes across puberty in both galanin binding density and the level of receptor occupancy. Here we have assessed the density of galanin binding sites in several brain regions of prepubertal and adult male rats with or without GTP to induce dissociation of endogenous galanin from its binding sites. The developmental changes in the level of receptor occupancy were used as an indirect measure of changes in neuropeptide release from galanin expressing neurons. In standard binding conditions (buffer preincubation), 125I-labeled galanin binding showed a generalized decline in adult brains (34-68%) compared with prepubertal levels in most regions of the telencephalon and diencephalon. Following preincubation with 10(-5) M GTP, galanin binding showed a dramatic increase in most regions of the adult (152-504%) and several regions of the prepubertal brain (132-245%) over their standard binding levels. However, this increase was greatest in adult animals. Finally, although preincubation of brain slices with GTP eliminated most of the apparent age-related differences observed in standard binding conditions, several brain regions of the adult brain continued to show a significant reduction (38-76%) in 125I-labeled galanin binding compared with prepubertal animals. Only one region, the lateral preoptic area, exhibited enhanced 125I-labeled galanin binding in adult (160%) compared with prepubertal brain after GTP preincubation.(ABSTRACT TRUNCATED AT 250 WORDS)

VIP binding sites in adult rat hypothalamus: nuclear distribution and daily variations

Vasoactive intestinal peptide (VIP) neurons are present in the suprachiasmatic nucleus (SCN) and project to various hypothalamic nuclei. Several results suggest that hypothalamic VIP is involved in the mediation of synchronizing information. [125I]Vasoactive intestinal peptide binding has been reported to be especially high in the hypothalamus. We show here by radioautography by dipping that [125I]VIP binding is high and homogeneously distributed in the SCN. The correspondence between VIP binding and terminals densities supports the hypothesis that VIP acts through synapses in the SCN. Moderate and fairly homogeneous VIP binding is also found in the paraventricular nucleus (PaVN), whereas VIP terminals are reported to be restricted to the sub-paraventricular area. This suggests that VIP could diffuse and thus function as a paracrine neuromodulator in the PaVN. [125I]Vasoactive intestinal peptide binding is stable along the nycthemer in the suprachiasmatic, peri-, and paraventricular nuclei as well as in all other areas investigated.

Sexual differences and seasonal variations in vasoactive intestinal peptide immunoreactivity in the suprachiasmatic nucleus of jerboa (Jaculus orientalis)

The sexual differences and the seasonal variation in the vasoactive intestinal peptide (VIP) content of the suprachiasmatic nucleus (SCN) of a desert rodent, the jerboa (Jaculus orientalis) were studied using immunocytochemical techniques. During the period of sexual activity (spring-early summer), the VIP immunoreactivity in the SCN was higher in females than in males. In the period of sexual quiescence (autumn), both males and females exhibited an even more intense VIP immunoreaction in the SCN as compared to spring. However, during this period, the sex-related differences in the VIP content of the SCN observed in spring were no longer detectable. The direct possible influence of sex hormones on the VIP content in the SCN, and the physiological significance of the seasonal variation observed in the VIP immunoreactivity in the SCN in this species remains to be determined.

Neurokinin A in the anterior pituitary of female rats: effects of ovariectomy and estradiol

The effect of acute and chronic ovariectomy and the substitutive treatment with 17-beta estradiol and/or progesterone on anterior pituitary levels of neurokinin A (NKA) was studied in female rats. Acute ovariectomy did not result in significant changes of NKA in the anterior pituitary gland as compared with the levels in diestrous intact rats, but a single injection of 5 micrograms of estradiol in ovariectomized rats significantly decreased NKA levels in the anterior pituitary gland. Progesterone was without effect and did not modify the decrease of NKA in the anterior pituitary gland induced by estradiol. In rats examined 11 to 17 days after ovariectomy, NKA in the anterior pituitary gland was significantly higher than in diestrous intact rats. In the hypothalamus, ovariectomy resulted in decreased levels of NKA in the median eminence-arcuate nucleus. Estradiol significantly reduced NKA stores in the anterior pituitary gland but increased them in the whole hypothalamus and in the median eminence-arcuate nucleus. Thus, estradiol seems to be a powerful regulator of NKA stores in the adenohypophysis and also in the hypothalamus.

Association of neurotensin receptors with VIP-containing neurons and serotonin-containing axons in the suprachiasmatic nucleus of the rat

The aim of the present study was to identify cellular elements bearing high-affinity neurotensin (NT) binding sites in the suprachiasmatic nucleus (SCN) of the rat hypothalamus. Because the distribution of these binding sites had previously been reported to conform to that of both vasoactive intestinal peptide (VIP)-containing nerve cell bodies and serotonin (5-HT)-containing axons, the following experimental approaches were used: (1) the overlap between autoradiographically labeled NT binding sites and immunocytochemically labeled VIP neurons was examined in adjacent 5-microns-thick sections taken across the entire rostrocaudal extent of the SCN; and (2) the density of NT binding sites was examined by quantitative autoradiography following cytotoxic lesioning of 5-HT afferents. Double-labeling studies demonstrated precise overlap between 125I-NT binding and VIP immunostaining throughout the SCN. Moreover, at high magnification intensely VIP-immunoreactive neurons were found in direct register with 125I-NT-labeled cells visualized in adjacent sections. Densitometric autoradiographic studies demonstrated a significant reduction in specific 125I-NT binding within the SCN following intracerebroventricular injection of the neurotoxin, 5,7-dihydroxytryptamine. Taken together, these results indicate that within the SCN, NT receptors are present both presynaptically on serotonin axons and postsynaptically on the perikarya and dendrites of VIP-containing neurons.

A selective sexually dimorphic response in the median eminence neuropeptide Y

Recent studies show that neuropeptide Y (NPY) is widely distributed in the hypothalamus and that it stimulates luteinizing hormone-releasing hormone (LHRH) release from the medial basal hypothalamus (MBH) of male and female rats. The neuroendocrine factors that regulate NPY neurosecretion in two sexes are not well understood. We have previously observed that orchidectomy (orch) in male rats decreased and testosterone (T) replacement increased NPY levels selectively in the median eminence (ME), arcuate nucleus (ARC) and ventromedial nucleus (VMN) and the KCl-evoked in vitro release of NPY from the MBH was likewise decreased after orch and restored by testosterone replacement. Now we report that these NPY responses are different in female rats following removal of the ovaries. Ovariectomy (ovx) decreased NPY concentrations in the ARC and VMN and not in the ME. Estrogen replacement restored NPY concentrations in the ARC and VMN, however, in the ME NPY concentration decreased significantly. Further, the KCl-evoked NPY release in vitro from either the MBH or ME of intact and ovx rats was similar. In an additional concurrent comparative study in male and female rats, we observed that of the 6 microdissected hypothalamic sites, NPY levels were approximately 50% less in the ME of intact females than in those of intact male rats. Again, in male rats there was a significant decrease in NPY levels in the ME, ARC and VMN after orch, whereas in female rats, NPY levels in the ARC and VMN, but not in the ME decreased after ovx.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation by estradiol of GABAA and GABAB binding sites in the diencephalon of the rat: an autoradiographic study

Using in vitro quantitative autoradiography we studied the in vivo effects of estradiol on GABAA and GABAB receptors in the rat brain. In all the areas studied (suprachiasmatic nucleus, medial preoptic area, striatum, frontal cortex), estradiol failed to significantly affect the GABAA receptor density. Chronic treatment with estradiol led however in the suprachiasmatic nucleus and the striatum to a decrease in the density of GABAB receptors. GABAB receptor regulation by estradiol was found to be area-specific within the hypothalamus since it was not observed in the medial preoptic area. The down regulation of GABAB receptors in the suprachiasmatic nucleus induced by estradiol treatment might thus explain the inhibitory effect of the steroid on the GABA control of serotonin metabolism we recently reported.

Hyperprolactinemia-induced modifications in vasoactive intestinal peptide binding site densities in the rat central nervous system and pituitary gland: evidence for an interaction between estradiol-17 beta and prolactin effects

Hyperprolactinemia was induced in ovariectomized rats by implanting estradiol-17 beta pellets, grafting extrapituitaries, or by a combination of both treatments. Subsequently, the effect of increasing plasma prolactin levels on both central and pituitary receptors for vasoactive intestinal peptide (VIP), a neuropeptide known to stimulate prolactin release was investigated. The results obtained by quantitative autoradiography show that the density of VIP binding sites is modified in restricted areas of the central nervous system (striatum, several cortical, thalamic and limbic structures) and in the pituitary in hyperprolactinemic animals. The present results suggest that changes in plasma prolactin levels may control VIP receptor site density in both brain and pituitary. Moreover, direct effects of estradiol-17 beta and possible interactions between estradiol-17 beta and prolactin are observed on both brain and pituitary VIP binding sites.