Beta-endorphin-, adrenocorticotrophic hormone- and neuropeptide y-containing projection fibers from the arcuate hypothalamic nucleus make synaptic contacts on to nucleus preopticus medianus neurons projecting to the paraventricular hypothalamic nucleus in the rat

The median preoptic nucleus: A major regulator of fluid, temperature, sleep, and cardiovascular homeostasis

Located in the midline lamina terminalis of the anterior wall of the third ventricle, the median preoptic nucleus is a thin elongated nucleus stretching around the rostral border of the anterior commissure. Its neuronal elements, composed of various types of excitatory glutamatergic and inhibitory GABAergic neurons, receive afferent neural signals from (1) neighboring subfornical organ and organum vasculosum of the lamina terminalis related to plasma osmolality and hormone concentrations, e.g., angiotensin II; (2) from peripheral sensors such as arterial baroreceptors and cutaneous thermosensors. Different sets of these MnPO glutamatergic and GABAergic neurons relay output signals to hypothalamic, midbrain, and medullary regions that drive homeostatic effector responses. Included in the effector responses are (1) thirst, antidiuretic hormone secretion and renal sodium excretion that subserve osmoregulation and body fluid homeostasis; (2) vasoconstriction or dilatation of skin blood vessels, and shivering and brown adipose tissue thermogenesis for core temperature homeostasis; (3) inhibition of hypothalamic and midbrain nuclei that stimulate wakefulness and arousal, thereby promoting both REM and non-REM sleep; and (4) activation of sympathetic pathways that drive vasoconstriction and heart rate to maintain arterial pressure and the perfusion of vital organs. The small size of MnPO belies its massive homeostatic significance.

Hypothalamic Integration of the Endocrine Signaling Related to Food Intake

Hypothalamic integration of gastrointestinal and adipose tissue-derived hormones serves as a key element of neuroendocrine control of food intake. Leptin, adiponectin, oleoylethanolamide, cholecystokinin, and ghrelin, to name a few, are in a constant "cross talk" with the feeding-related brain circuits that encompass hypothalamic populations synthesizing anorexigens (melanocortins, CART, oxytocin) and orexigens (Agouti-related protein, neuropeptide Y, orexins). While this integrated neuroendocrine circuit successfully ensures that enough energy is acquired, it does not seem to be equally efficient in preventing excessive energy intake, especially in the obesogenic environment in which highly caloric and palatable food is constantly available. The current review presents an overview of intricate mechanisms underlying hypothalamic integration of energy balance-related peripheral endocrine input. We discuss vulnerabilities and maladaptive neuroregulatory processes, including changes in hypothalamic neuronal plasticity that propel overeating despite negative consequences.

The median preoptic nucleus: front and centre for the regulation of body fluid, sodium, temperature, sleep and cardiovascular homeostasis

Located in the midline anterior wall of the third cerebral ventricle (i.e. the lamina terminalis), the median preoptic nucleus (MnPO) receives a unique set of afferent neural inputs from fore-, mid- and hindbrain. These afferent connections enable it to receive neural signals related to several important aspects of homeostasis. Included in these afferent projections are (i) neural inputs from two adjacent circumventricular organs, the subfornical organ and organum vasculosum laminae terminalis, that respond to hypertonicity, circulating angiotensin II or other humoural factors, (ii) signals from cutaneous warm and cold receptors that are relayed to MnPO, respectively, via different subnuclei in the lateral parabrachial nucleus and (iii) input from the medulla associated with baroreceptor and vagal afferents. These afferent signals reach appropriate neurones within the MnPO that enable relevant neural outputs, both excitatory and inhibitory, to be activated or inhibited. The efferent neural pathways that proceed from the MnPO terminate on (i) neuroendocrine cells in the hypothalamic supraoptic and paraventricular nuclei to regulate vasopressin release, while polysynaptic pathways from MnPO to cortical sites may drive thirst and water intake, (ii) thermoregulatory pathways to the dorsomedial hypothalamic nucleus and medullary raphé to regulate shivering, brown adipose tissue and skin vasoconstriction, (iii) parvocellular neurones in the hypothalamic paraventricular nucleus that drive autonomic pathways influencing cardiovascular function. As well, (iv) other efferent pathways from the MnPO to sites in the ventrolateral pre-optic nucleus, perifornical region of the lateral hypothalamic area and midbrain influence sleep mechanisms.

Renal responses produced by microinjection of the kappa opioid receptor agonist, U50-488H, into sites within the rat lamina terminalis

Activation of central kappa opioid receptors (KOR) has been demonstrated to produce marked free water diuresis with a concurrent increase in renal sympathetic nerve activity (RSNA). This study investigated the cardiovascular (CV) and renal effects evoked by central activation of KOR in two lamina terminalis sites, the median preoptic area (MPA) and anterolateral division of the bed nuclei of the stria terminalis (BST). Rats anesthetized with urethane alpha-chloralose were instrumented to record mean arterial pressure, heart rate, RSNA, and urine output (V). Rats were infused with isotonic saline (25 μL/min) and urine samples were collected during two 10-min control periods and six consecutive 10-min experimental periods following microinjection of vehicle, U50-448H (U50, KOR agonist) alone or norbinaltorphimine (nor-BNI, KOR antagonist) plus U50. Microinjection of U50 into the BST increased V (peak at 30 min, 84.8 ± 12.9 μL/min) as compared to its respective control, vehicle, or nor-BNI plus U50. This diuretic effect occurred without any significant changes in CV parameters, RSNA, or urinary sodium excretion. In contrast, U50 injection into the MPA significantly increased RSNA (peak at 20 mins: 129 ± 9.9) without increasing the other parameters. This study demonstrated novel sites through which activation of KOR selectively increases V and RSNA. The ability of U50 to increase V without affecting sodium excretion and RSNA raises the possibility that LT neurons could be an important substrate through which drugs targeting KOR could selectively facilitate water excretion in sodium-retaining diseases such as congestive heart failure.

Fetal alcohol exposure increases susceptibility to carcinogenesis and promotes tumor progression in prostate gland

The idea that exposure to adverse environmental conditions and lifestyle choices during pregnancy can result in fetal programming that underlies disease susceptibility in adulthood is now widely accepted. Fetal alcohol exposed offspring displays many behavioral and physiological abnormalities including neuroendocrine-immune functions, which often carry over into their adult life. Since the neuroendocrine-immune system plays an important role in controlling tumor surveillance, fetal alcohol exposed offspring can be vulnerable to develop cancer. Animal studies have recently showed increased cancer growth and progression in various tissues of fetal alcohol exposed offspring. I will detail in this chapter the recent evidence for increased prostate carcinogenesis in fetal alcohol exposed rats. I will also provide evidence for a role of excessive estrogenization during prostatic development in the increased incidence of prostatic carcinoma in these animals. Furthermore, I will discuss the additional possibility of the involvement of impaired stress regulation and resulting immune incompetence in the increased prostatic neoplasia in the fetal alcohol exposed offspring.

Beta-Endorphin Neuron Regulates Stress Response and Innate Immunity to Prevent Breast Cancer Growth and Progression

Body and mind interact extensively with each other to control health. Emerging evidence suggests that chronic neurobehavioral stress can promote various tumor growth and progression. The biological reaction to stress involves a chemical cascade initiated within the central nervous system and extends to the periphery, encompassing the immune, endocrine, and autonomic systems. Activation of sympathetic nervous system, such as what happens in the "fight or flight" response, downregulates tumor-suppressive genes, inhibits immune function, and promotes tumor growth. On the other hand, an optimistic attitude or psychological intervention helps cancer patients to survive longer via increase in β-endorphin neuronal suppression of stress hormone levels and sympathetic outflows and activation of parasympathetic control of tumor suppressor gene and innate immune cells to destroy and clear tumor cells.

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

Control of metabolism by nutrient-regulated nuclear receptors acting in the brain

Today, we are witnessing a rising incidence of obesity worldwide. This increase is due to a sedentary life style, an increased caloric intake and a decrease in physical activity. Obesity contributes to the appearance of type 2 diabetes, dyslipidemia and cardiovascular complications due to atherosclerosis, and nephropathy. Therefore, the development of new therapeutic strategies may become a necessity. Given the metabolism controlling properties of nuclear receptors in peripheral organs (such as liver, adipose tissues, pancreas) and their implication in various processes underlying metabolic diseases, they constitute interesting therapeutic targets for obesity, dyslipidemia, cardiovascular disease and type 2 diabetes. The recent identification of the central nervous system as a player in the control of peripheral metabolism opens new avenues to our understanding of the pathophysiology of obesity and type 2 diabetes and potential novel ways to treat these diseases. While the metabolic functions of nuclear receptors in peripheral organs have been extensively investigated, little is known about their functions in the brain, in particular with respect to brain control of energy homeostasis. This review provides an overview of the relationships between nuclear receptors in the brain, mainly at the hypothalamic level, and the central regulation of energy homeostasis. In this context, we will particularly focus on the role of PPARα, PPARγ, LXR and Rev-erbα.

Regulation of cancer progression by β-endorphin neuron

It is becoming increasingly clear that stressful life events can affect cancer growth and metastasis by modulating nervous, endocrine, and immune systems. The purpose of this review is to briefly describe the process by which stress may potentiate carcinogenesis and how reducing body stress may prevent cancer growth and progression. The opioid peptide β-endorphin plays a critical role in bringing the stress axis to a state of homeostasis. We have recently shown that enhancement of endogenous levels of β-endorphin in the hypothalamus via β-endorphin neuron transplantation suppresses stress response, promotes immune function, and reduces the incidence of cancer in rat models of prostate and breast cancers. The cancer-preventive effect of β-endorphin is mediated through the suppression of sympathetic neuronal function, which results in increased peripheral natural killer cell and macrophage activities, elevated levels of anti-inflammatory cytokines, and reduced levels of inflammatory cytokines. β-endorphin inhibition of tumor progression also involves alteration in the tumor microenvironment, possibly because of suppression of catecholamine and inflammatory cytokine production, which are known to alter DNA repair, cell-matrix attachments, angiogenic process, and epithelial-mesenchymal transition. Thus, β-endorphin cell therapy may offer some therapeutic value in cancer prevention.

Transplantation of β-endorphin neurons into the hypothalamus promotes immune function and restricts the growth and metastasis of mammary carcinoma

Neurobehavioral stress has been shown to promote tumor growth and progression and dampen the immune system. In this study, we investigated whether inhibiting stress hormone production could inhibit the development of mammary carcinoma and metastasis in a rat model of breast carcinogenesis. To enhance β-endorphin (BEP), the endogenous opioid polypeptide that boosts immune activity and decreases stress, we generated BEP neurons by in vitro differentiation from fetal neuronal stem cells and transplanted them into the hypothalami of rats subjected to breast carcinogenesis. BEP-transplanted rats displayed a reduction in mammary tumor incidence, growth, malignancy rate, and metastasis compared with cortical cells-transplanted rats. BEP neuron transplants also reduced inflammation and epithelial to mesenchymal transition in the tumor tissues. In addition, BEP neuron transplants increased peripheral natural killer (NK) cell and macrophage activities, elevated plasma levels of antiinflammatory cytokines, and reduced plasma levels of inflammatory cytokines. Antimetastatic effects along with stimulation of NK cells and macrophages could be reversed by treatment with the opiate antagonist naloxone, the β-receptor agonist metaproterenol, or the nicotine acetylcholine receptor antagonist methyllycaconitine. Together, our findings establish a protective role for BEP against the growth and metastasis of mammary tumor cells by altering autonomic nervous system activities that enhance innate immune function.

Developmental programming of energy balance and its hypothalamic regulation

Developmental programming is an important physiological process that allows different phenotypes to originate from a single genotype. Through plasticity in early life, the developing organism can adopt a phenotype (within the limits of its genetic background) that is best suited to its expected environment. In humans, together with the relative irreversibility of the phenomenon, the low predictive value of the fetal environment for later conditions in affluent countries makes it a potential contributor to the obesity epidemic of recent decades. Here, we review the current evidence for developmental programming of energy balance. For a proper understanding of the subject, knowledge about energy balance is indispensable. Therefore, we first present an overview of the major hypothalamic routes through which energy balance is regulated and their ontogeny. With this background, we then turn to the available evidence for programming of energy balance by the early nutritional environment, in both man and rodent models. A wealth of studies suggest that energy balance can indeed be permanently affected by the early-life environment. However, the direction of the effects of programming appears to vary considerably, both between and within different animal models. Because of these inconsistencies, a comprehensive picture is still elusive. More standardization between studies seems essential to reach veritable conclusions about the role of developmental programming in adult energy balance and obesity.

Hypothalamic nutrient sensing in the control of energy homeostasis

The hypothalamus is a center of convergence and integration of multiple nutrient-related signals. It can sense changes in circulating adiposity hormones, gastric hormones and nutrients, and receives neuroanatomical projections from other nutrient sensors, mainly within the brainstem. The hypothalamus also integrates these signals with various cognitive forebrain-descending information and reward/motivation-related signals coming from the midbrain-dopamine system, to coordinate neuroendocrine, behavioral and metabolic effectors of energy balance. Some of the key nutrient-sensing hypothalamic neurons have been identified in the arcuate, the ventro-medial and the lateral nuclei of the hypothalamus, and the molecular mechanisms underlying intracellular integration of nutrient-related signals in these neurons are currently under intensive investigation. However, little is known about the neural pathways downstream from hypothalamic nutrient sensors, and how they drive effectors of energy homeostasis under physiological conditions. This manuscript will review recent progress from molecular, genetic and neurophysiological studies that identify and characterize the critical intracellular signalling pathways and neurocircuits involved in determining hypothalamic nutrient detection, and link these circuits to behavioral and metabolic effectors of energy balance. We will provide a critical analysis of current data to identify ongoing challenges for future research in this field.

Dorsomedial hypothalamus mediates autonomic, neuroendocrine, and locomotor responses evoked from the medial preoptic area

Previous studies suggest that sympathetic responses evoked from the preoptic area in anesthetized rats require activation of neurons in the dorsomedial hypothalamus. Disinhibition of neurons in the dorsomedial hypothalamus in conscious rats produces physiological and behavioral changes resembling those evoked by microinjection of muscimol, a GABA(A) receptor agonist and neuronal inhibitor, into the medial preoptic area. We tested the hypothesis that all of these effects evoked from the medial preoptic area are mediated through neurons in the dorsomedial hypothalamus by assessing the effect of bilateral microinjection of muscimol into the DMH on these changes. After injection of vehicle into the dorsomedial hypothalamus, injection of muscimol into the medial preoptic area elicited marked increases in heart rate, arterial pressure, body temperature, plasma ACTH, and locomotor activity and also increased c-Fos expression in the hypothalamic paraventricular nucleus, a region known to control the release of ACTH from the adenohypophysis. Prior bilateral microinjection of muscimol into the dorsomedial hypothalamus produced a modest depression of baseline heart rate and body temperature but completely abolished all changes evoked from the medial preoptic area. Microinjection of muscimol just anterior to the dorsomedial hypothalamus had no effect on autonomic and neuroendocrine changes evoked from the medial preoptic area. Thus, activity of neurons in the dorsomedial hypothalamus mediates a diverse array of physiological and behavioral responses elicited from the medial preoptic area, suggesting that the latter region represents an important source of inhibitory tone to key neurons in the dorsomedial hypothalamus.

Central adiponectin functions to inhibit arginine vasopressin release in conscious rats

The adipocyte-derived hormone adiponectin plays an important role in modulating energy homeostasis through peripheral tissues and the central nervous system. Several studies have reported that adiponectin exists in cerebrospinal fluid and that adiponectin receptors are expressed in the hypothalamus, including the paraventricular nucleus (PVN), which plays a key role in controlling pituitary hormone secretion. Furthermore, it has been reported that magnocellular arginine vasopressin (AVP) neurones within the PVN express adiponectin receptors. These findings suggest a central role of adiponectin in the modulation of neuroendocrinological functions. In the present study, we investigated the effect of centrally-administered adiponectin on AVP release in conscious rats. Intracerebroventricular (i.c.v.) administration of adiponectin significantly reduced the basal plasma AVP concentration in a dose-dependent manner, with a maximal effect being obtained 10 min after administration. The plasma AVP increase in response to either hyperosmolar or hypovolaemic stimulation was also significantly attenuated by an i.c.v. injection of adiponectin. Treatment with AMP-activated protein kinase (AMPK) inhibitor compound C (100 nmol, i.c.v.) partially reversed the inhibitory effects of adiponectin on AVP release. These findings suggest that central adiponectin plays an inhibitory role in the osmoregulation and baroregulation of AVP release, that the AMPK pathway is at least partly involved in the action of adiponectin, and further suggest a novel physiological or pathophysiological role for central adiponectin in water balance via inhibition of AVP release.

Role of beta-endorphin, corticotropin-releasing hormone, and autonomic nervous system in mediation of the effect of chronic ethanol on natural killer cell cytolytic activity

BACKGROUND

We have recently shown that alcohol feeding suppresses natural killer (NK) cell cytolytic activity partly by decreasing the function of hypothalamic beta-endorphin (beta-EP) neurons. The neuronal mechanism by which hypothalamic beta-EP communicates with the spleen to regulate the action of ethanol on NK cells is not known. In the present study, we evaluated the roles of beta-EP neurons, corticotropin releasing hormone (CRH) neurons, and the autonomic nervous system (ANS) in regulation of the ethanol effect on splenic NK cell cytolytic function.

METHODS

Male rats were fed an ethanol-containing liquid diet or control diets. These rats were used to determine the hormone release from the paraventricular nuclei (PVN) of the hypothalamus or used to determine the splenic NK cell cytolytic function after PVN administration of CRH or intraperitoneal (i.p.) administration of a ganglionic blocker chlorisondamine. The release of hormones from the PVN was measured using the push-pull perfusion method. Splenic cytolytic activity was determined using the 4-hour (51)Cr release assay against YAC-1 lymphoma target cells.

RESULTS

Alcohol feeding decreased the amount of beta-EP but increased the amount of CRH in the push-pull perfusate (PPP) samples collected from the PVN. When exogenous beta-EP was perfused into the PVN, it suppressed the release of endogenous CRH found in PPP samples of the PVN. Conversely, perfusion of an opiate antagonist naltrexone into the PVN increased the levels of endogenous CRH in PPP samples of the PVN. In addition, administration of exogenous beta-EP in the PVN stimulated the cytolytic function of NK cells, an action that was antagonized by CRH as well as by ethanol. Corticotropin-releasing hormone and ethanol alone also had an inhibitory action on NK cells. Finally, the ganglionic blocker used prevented the effect that ethanol, beta-EP, and CRH had on NK cells. These data suggest that ethanol inhibits the function of NK cells partly by suppressing the influence of the beta-EP-CRH-ANS signal to the spleen.

β-Endorphin involvement in the regulatory response to body sodium overload

The present study was performed to examine the role of the endogenous beta-endorphinergic system on blood pressure regulation, sympathetic and brain activity during body sodium overload. Beta-endorphin knockout (beta end-/-), heterozygous (beta end+/-) and wild-type (beta end+/+) mice were submitted for two weeks to either a normal- or a high-sodium diet (NSD and HSD, respectively), and systolic blood pressure (SBP), urinary catecholamines (as an index of sympathetic nervous system activity), and the brain pattern of Fos-like immunoreactivity (as a marker of neuronal activation) were evaluated in each group. HSD caused a significant increase in SBP in beta end-/- mutant mice compared with beta end+/+ mice kept in the same experimental conditions (P < 0.01), but no statistical differences were observed between beta end+/- and beta end+/+ on a HSD. Moreover, when animals from the three genetic lines were fed with a NSD no changes in SBP were evidenced. With regard to brain activity, beta end-/- mice maintained on a HSD showed a significant increase in Fos-like immunoreactive neurons in the median preoptic nucleus (P < 0.01) compared with beta end+/- and beta end+/+ animals. Additionally, beta end-/- mice had higher levels of urinary epinephrine excretion (P < 0.05) on a HSD in comparison to beta end+/+ and beta end+/- animals in the same experimental conditions. No differences, however, were registered in norepinephrine and dopamine urinary excretion in animals from the three genetic lines after two weeks on either a HSD or a NSD. In summary, our results indicate that the beta-endorphinergic system may play a part in the compensatory response to sodium overload, since the absence of beta-endorphin causes an increase in systolic blood pressure, and increases median preoptic nucleus neural activity and urinary epinephrine excretion.

Peptidergic and catecholaminergic synaptic contacts onto nucleus preopticus medianus neurons projecting to the subfornical organ in the rat

The nucleus preopticus medianus (POMe) is known to be a key site in regulation of cardiovascular and body fluid homeostasis. To clarify the regulation mechanism to the POMe, the innervation pattern of synapses made by axon terminals immunoreactive to beta-endorphin, neuropeptide Y and tyrosine hydroxylase onto POMe neurons projecting to the subfornical organ (SFO) was investigated in the rat. After injection of a retrograde tracer, wheat germ agglutinin-conjugated horseradish peroxidase-colloidal gold complex, into the SFO, many neurons were retrogradely labeled in the POMe, more frequently in its dorsal part. Electron microscopy of the POMe revealed that beta-endorphin- and tyrosine hydroxylase-immunoreactive axon terminals formed predominantly axo-somatic synapses, and neuropeptide Y-immunoreactive axon terminals formed more axo-dendritic than axo-somatic synapses with retrogradely labeled neurons. The present localization patterns of POMe neurons retrogradely labeled from the SFO and the type of synapses of axon terminals immunoreactive to three neurochemical markers on these neurons were compared to those of POMe neurons retrogradely labeled from the paraventricular hypothalamic nucleus demonstrated in our previous report.

Progesterone-receptive dopaminergic and neuropeptide Y neurons project from the arcuate nucleus to gonadotropin-releasing hormone-rich regions of the ovine preoptic area

Progesterone inhibits gonadotropin-releasing hormone (GnRH) secretion in sheep through an interneuronal system located in the mediobasal hypothalamus. This study focused on known inhibitors of GnRH secretion in sheep, dopamine and neuropeptide Y (NPY). As the distributions of tyrosine hydroxylase (TH)- and NPY-immunoreactive neurons overlap with progesterone receptors (PR) in the arcuate nucleus, we hypothesized that, if these neurons mediate, at least partially, the inhibitory feedback signal of progesterone, then they should co-express PRs. Fluorogold (FG), a retrograde tracer, was injected into the preoptic area of ovariectomized ewes pretreated with estrogen and progesterone. When the FG injection site encompassed at least 80 GnRH neurons, sections from the arcuate nucleus were processed using dual immunocytochemistry for PR and either TH or NPY. We found that 30% of PR-immunoreactive, 12% of TH-containing and 21% of NPY-synthesizing neurons project toward this GnRH-rich region. Of the PR/TH dual-labeled cells, which represent 21% of PR and 31% of TH cells, respectively, 22% displayed FG labeling. Of the PR/NPY neurons, which account for 19% of PR and 67% of NPY neurons, respectively, 26% were FG fluorescent. This study suggests that subsets of arcuate nucleus dopaminergic and NPY neurons may transduce, at least in part, the progesterone-mediated inhibition of GnRH secretion.

Reduced sodium appetite and increased oxytocin gene expression in mutant mice lacking β-endorphin

Central opioid and oxytocinergic systems have been involved in the regulatory control of sodium appetite. In addition, previous studies support the existence of a functional interaction between opioid peptides and oxytocinergic pathways, and suggest that beta-endorphin neurons would modulate the activity of central oxytocinergic pathways, its pituitary secretion and sodium appetite. To investigate the role of this opioid peptide in the control of oxytocin (OT) synthesis and sodium appetite regulation we used mice with gene dosage-dependent variations in brain beta-endorphin content, expressing either 100%, 50%, or 0% of normal beta-endorphin content. Our results show that beta-endorphin knockout (KO) and heterozygous (HT) mutant mice consume approximately a 50% less 2% NaCl solution compared with wild type mice (WT), after furosemide and low sodium diet treatment. These data suggest that beta-endorphin may facilitate induced sodium appetite, giving new evidence about the role of beta-endorphin on sodium appetite behavior. Our data also indicate that OT mRNA levels evaluated by in situ hybridization significantly increased within the hypothalamic paraventricular nucleus of WT animals after induced sodium ingestion, giving support to former evidence indicating an inhibitory role for central OT in the control of sodium appetite. Moreover, beta-endorphin mutated mice have similar higher levels of OT mRNA expression after the different conditions analyzed: basal, control or experimental, compared with WT mice. Both control HT and KO mice showed higher OT mRNA expression levels than control WT group and these levels did not change after induced sodium intake. Taken together, our data suggest that the reduced sodium ingestion observed in beta-endorphin deficient mice could be due to a higher expression of the OT gene. This conclusion would support the hypothesis that OT inhibits sodium intake and provides new evidence about beta-endorphin modulation of OT synthesis and sodium appetite.

Centrally administered tuberoinfundibular peptide of 39 residues inhibits arginine vasopressin release in conscious rats

Tuberoinfundibular peptide of 39 residues (TIP39) is a recently discovered neuropeptide identified on the basis of its ability to activate the PTH2 receptor, and it is thought to be the brain PTH2 receptor's endogenous ligand. The PTH2 receptor is highly expressed in the hypothalamus, suggesting a role in the modulation of neuroendocrinological functions. PTHrP, which also belongs to the PTH-related peptides family, stimulates arginine vasopressin (AVP) release. In the present study, therefore, we investigated the effect of centrally administered TIP39 on AVP release in conscious rats. Intracerebroventricular administration of TIP39 (10-500 pmol/rat) significantly suppressed the plasma AVP concentration in dehydrated rats, and the maximum effect was obtained 5 min after administration (dehydration with 100 pmol/rat TIP39, 4.32 +/- 1.17 pg/ml; vs. control, 8.21 +/- 0.70 pg/ml). The plasma AVP increase in response to either hyperosmolality [ip injection of hypertonic saline (HS), 600 mosmol/kg] or hypovolemia [ip injection of polyethylene glycol (PEG)] was also significantly attenuated by an intracerebroventricular injection of TIP39 (HS with 100 pmol/rat TIP39, 2.65 +/- 0.52 pg/ml; vs. HS alone, 4.69 +/- 0.80 pg/ml; PEG with 100 pmol/rat TIP39, 4.10 +/- 0.79 pg/ml; vs. PEG alone, 6.19 +/- 0.34 pg/ml). Treatment with naloxone [1.5 mg/rat, sc injection], a nonselective opioid receptor antagonist, significantly reversed the inhibitory effects of TIP39 on AVP release. These results suggest that central TIP39 plays an inhibitory role in the osmoregulation and baroregulation of AVP release and that intrinsic opioid systems are involved in its mechanism.

Pancreatic polypeptide receptors: affinity, sodium sensitivity and stability of agonist binding

Cloned rat, human and guinea-pig Y4 pancreatic polypeptide (PP) receptors expressed in Chinese hamster ovary (CHO) cells, as well as the rabbit Y4-like PP receptor, show a selective sensitivity to Na+ over K+ ion in PP attachment, but little sensitivity to Na+ in dissociation of bound PP peptides. Agonist binding to Y4 receptors of intact CHO cells also shows much greater sensitivity to Na+ over K+, and a tenacious attachment of the bound agonist. Binding sensitivity to K+ is greatly enhanced upon receptor solubilization. Pancreatic polypeptide sites also show large sensitivity to modulators of Na+ transport such as N5-substituted amilorides and to RFamides, as different from Y1 or Y2 receptors. Thus, PP binding is modulated by cation-induced changes in site environment (with selectivity for Na+) and ultimately results in a blocking attachment. This would support receptor operation in the presence of ion gradients, as well as prolonged agonist-delimited signaling activity (which can include partial antagonism). Also, this could point to an evolutionary adaptation enabling small numbers of PP receptors to perform extensive metabolic tasks in response to low agonist signals.

Tyrosine hydroxylase-immunoreactive projections from the caudal ventrolateral medulla to the subfornical organ in the rat

The subfornical organ (SFO) is known to be innervated by noradrenergic fibers. One possible origin of these fibers, which carry peripheral baroreceptor information to enhance the activity of SFO neurons, is the nucleus tractus solitarius (NTS). To investigate possible sites of origin of the catecholaminergic projections to the SFO, a retrograde tracing method was combined with immunohistochemistry in the rat. Stereotaxical injection of a retrograde tracer, wheat germ agglutinin-conjugated horseradish peroxidase--colloidal gold complex, into the SFO from the dorsal aspect revealed retrogradely labeled neurons in several catecholaminergic cell groups. A substantial number of retrogradely labeled neurons showing tyrosine hydroxylase (TH) immunoreactivity were found in the NTS and ventrolateral medulla (VLM) at levels caudal to the obex and in the locus coeruleus, while retrogradely labeled neurons without TH immunoreactivity were found in the VLM at levels rostral to the obex and in the nucleus prepositus hypoglossi. When the tracer was injected into the structures dorsal to the SFO, including the triangular septal nucleus, the frequency of retrogradely labeled neurons in the NTS and VLM at the caudal level was very low. These findings indicate the existence of catecholaminergic projections from the VLM (probably A1) to the SFO, in addition to the noradrenergic projections from the NTS previously reported.