Vasopressin and glucoprivic-feeding behavior: a new perspective on an 'old' peptide

Oxytocin and Vasopressin Systems in Obesity and Metabolic Health: Mechanisms and Perspectives

PURPOSE OF REVIEW

The neurohypophysial endocrine system is identified here as a potential target for therapeutic interventions toward improving obesity-related metabolic dysfunction, given its coinciding pleiotropic effects on psychological, neurological and metabolic systems that are disrupted in obesity.

RECENT FINDINGS

Copeptin, the C-terminal portion of the precursor of arginine-vasopressin, is positively associated with body mass index and risk of type 2 diabetes. Plasma oxytocin is decreased in obesity and several other conditions of abnormal glucose homeostasis. Recent data also show non-classical tissues, such as myocytes, hepatocytes and β-cells, exhibit responses to oxytocin and vasopressin receptor binding that may contribute to alterations in metabolic function. The modulation of anorexigenic and orexigenic pathways appears to be the dominant mechanism underlying the effects of oxytocin and vasopressin on body weight regulation; however, there are apparent limitations associated with their use in direct pharmacological applications. A clearer picture of their wider physiological effects is needed before either system can be considered for therapeutic use.

Oxytocin and vasopressin neurones in principal and accessory hypothalamic magnocellular structures express Fos-immunoreactivity in response to acute glucose deprivation

Neurohypophyseal secretion of oxytocin and vasopressin is elevated in response to decreased systemic glucose availability. In these studies, dual-label immunocytochemistry was used to identify hypothalamic neuropeptidergic magnocellular neurones that are transcriptionally activated in response to glucose substrate imbalance. Two h after i.p. injection of the glucose antimetabolite, 2-deoxy-D-glucose (2DG), or the vehicle, saline, groups of adult male rats were anaesthetized by i.p. injection with sodium pentobarbital and killed by transcardial perfusion. Sections (25 microm) through anterior and tuberal levels of the hypothalamus were processed for nuclear Fos- and cytoplasmic neuropeptide immunoreactivity (-ir). A high proportion of oxytocin-ir neurones in the supraoptic, paraventricular, and adjunct structures, including the anterior commissural, periventricular magnocellular, posterior perifornical, recurrent supraoptic, medial forebrain, and circular nuclei, were colabelled for nuclear Fos-ir following administration of 2DG. Large numbers of vasopressin neurones in the supraoptic, circular, posterior perifornical, and medial forebrain nuclei, and posterior magnocellular division and posterior subnucleus of the paraventricular nucleus were also immunostained for Fos in rats injected with the antimetabolite. These results show that decreased glucose metabolism is a stimulus for activation of the Fos stimulus-transcription cascade within oxytocin-and vasopressin-immunopositive neurones in several hypothalamic loci, findings that reflect activation of the Fos-stimulus transcription cascade within large proportions of these cell populations during this metabolic challenge. These data suggest that both peripheral hormonal and central modulatory functions of these neuropeptidergic neurones may be influenced by cellular glucose availability.

Exercise in food-restricted rats produces 2DG feeding and metabolic abnormalities similar to anorexia nervosa

Anorexia nervosa (AN) is associated with a paradoxical reduction in hunger ratings following 2-deoxy-D-glucose (2DG) induced glucose insufficiency. Because of the relationship between exercise and AN, there is interest in the weight-loss phenomenon produced by exercise in food restricted rats. This investigation determined if the weight-loss phenomenon is associated with a paradoxical suppression of food intake following 2DG and if the effect is related to reductions in prevailing glucose and insulin levels. Weight-matched, normal-weight exercised and normal-weight unexercised rats served as controls. As predicted, 2DG reduced food intake in animals subjected to the phenomenon (1.5 h/day food access and 22.5 h/day running wheel access). This effect was related to reductions in plasma glucose and insulin under the conditions that prevailed at the time of injection. Since these changes also occurred in weight-matched controls, they were attributed to the general effects of weight loss. A situational specificity for the "anorexia" of the weight-loss syndrome was also demonstrated. Finally, the strengths and weaknesses of the phenomenon as a model of AN were considered.

Effects of insulin and 2-deoxy-D-glucose administration on CNS IGF-II content

In order to determine whether effects of insulin on central nervous system (CNS) insulin-like growth factor II (IGF-II) content are direct or whether they are due to insulin-induced glucoprivation, short-term (2 h) time course studies were conducted utilizing the glucoprivic agent 2-deoxy-D-glucose (2-DG). Insulin (2 U/kg b.wt.), 2-DG (500 mg/kg b.wt.) or vehicle were administered to Sprague-Dawley rats (350-450 kg) and groups of animals were killed at time 0 and 15, 30, 45, 60, and 120 min following injection. Specific hypothalamic and hindbrain regions obtained by microdissection, were analyzed for IGF-II content by RIA. Insulin and 2-DG exerted similar effects on IGF-II content in the dorsomedial hypothalamus and the paraventricular nucleus. These data suggest that IGF-II was primarily regulated by glucoprivation in these regions. Only 2-DG altered ventromedial hypothalamic IGF-II content, and opposite responses to the two agents were observed in the arcuate nucleus. This uncoupling of IGF-II response suggests that differences in peripheral vs. central signals for IGF-II secretion may be involved. The vagal complex was responsive only to insulin injection indicating a specific response that may be tied to sensory vagal function.

Beta-endorphin and dynorphin abnormalities in rats subjected to exercise and restricted feeding: relationship to anorexia nervosa?

Exercise and the endogenous opioids have been linked to anorexia nervosa. This investigation determined the effects of the weight-loss syndrome induced by voluntary exercise (22.5 h/day) in food-restricted rats (1.5 h/day food access) on the endogenous opioids. The animals were tested under resting-fed and 2-deoxy-D-glucose (2DG) stimulated conditions. Weight-matched, freely fed exercised and ad libitum fed unexercised groups served as controls. Specific opioid abnormalities were found in the syndrome. These included a basal elevation in plasma beta-endorphin, which was abnormally suppressed by 2DG, and 2DG-induced elevations in arcuate hypothalamic beta-endorphin content and supraoptic hypothalamic dynorphin-A content. None of these changes occurred in controls. Finally, it was found that short-term moderate exercise itself chronically reduced adenohypophysial beta-endorphin content and elevated supraoptic dynorphin-A content. The relationship of the syndrome's hyperendorphinism to the hypothalamo-pituitary-adrenal axis and the auto-addiction hypothesis of anorexia nervosa was considered, as was the significance of the supraoptic dynorphin-A abnormality to the hypothalamo-neurohypophysial system. The differential sensitivity of the supraoptic dynorphin-A system compared to the arcuate hypothalamic beta-endorphin system to moderate exercise was also discussed.

Fluoxetine induces vasopressin and oxytocin abnormalities in food-restricted rats given voluntary exercise: relationship to anorexia nervosa

Anorexia nervosa is associated with vasopressin, oxytocin and serotonin abnormalities. Because of the relationship between exercise and anorexia nervosa, we explored the weight-loss syndrome produced by wheel running in food-deprived rats. Its effects on regional vasopressin and oxytocin concentrations were determined under basal conditions and following systemic fluoxetine. Weight-matched, exercised and unexercised rats served as controls. Fluoxetine caused abnormalities in suprachiasmatic vasopressin and dynorphin A content and in thymus oxytocin content that did not occur in weight-matched or exercised controls. No syndrome-specific anomalies occurred in the hypothalamo-neurohypophysial system or dorsal vagal complex (DVC). However, weight reduction and fluoxetine increased circulating vasopressin; moderate exercise caused fluoxetine-induced elevations in posterior pituitary vasopressin and oxytocin; and, unlike the other groups, fluoxetine increased DVC oxytocin in freely fed unexercised rats. It was concluded that syndrome-specific vasopressin and oxytocin abnormalities occur that are not secondary to weight loss or moderate exercise; that weight loss or fluoxetine increases circulating vasopressin; that moderate exercise alters neurohypophysial vasopressin and oxytocin content; and that weight loss or exercise inhibits a fluoxetine-stimulated increase in DVC oxytocin. Finally, it was argued that the fluoxetine abnormalities indicate possible serotonin dysfunction in the syndrome.

Effect of food deprivation and refeeding on the concentration of vasopressin and oxytocin in discrete hypothalamic sites

Recent evidence has implicated hypothalamic peptides, such as arginine vasopressin (AVP) and oxytocin (OT) in the control of feeding behavior. In this study, we investigated the impact of food deprivation (48 h) and subsequent refeeding (6 h) on the concentration of AVP and OT in discrete hypothalamic areas, as well as in the neurohypophysis. We also estimated in these rats certain peripheral measures, including hydroelectrolytic parameters, plasma and urine AVP, and plasma corticosterone. The results of this study revealed that food deprivation for 48 h produced little change in OT concentration in the various hypothalamic nuclei studied, including the paraventricular and supraoptic nuclei, with the exception of the median eminence (ME), where a significant decline (-36%; p < 0.05) was detected. This effect was not significantly reversed by 6 h of refeeding. With respect to AVP concentration, food deprivation caused a reliable decline exclusively in the parvocellular subdivision of the paraventricular nucleus (pPVN; -45%; p < 0.01) and in the supraoptic nucleus (SON; -45%; p < 0.01). No change in AVP was detected in the ME or in most other hypothalamic nuclei examined. Refeeding for 6 h actually potentiated the effect of food deprivation, decreasing further from baseline the content of AVP in the pPVN and SON. The only other hypothalamic area to exhibit a change in AVP content was the ventromedial nucleus, where AVP level increased (p < 0.001) after deprivation and declined to normal after 6 h of refeeding. The content of AVP and OT in the neurohypophysis was unaffected by food deprivation and subsequent refeeding.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin deficiency and the modulation of consummatory behavior

This study examined patterns of consummatory behavior in normal and vasopressin-deficient rats under ad lib and food-restricted conditions. Differing patterns of intake of food, water, sucrose, and saccharin were found in the two groups. During the ad lib period, the normal rats gained more weight and ate significantly more food than vasopressin-deficient rats. The vasopressin-deficient animals consumed significantly more sucrose than normal animals during this period. No significant differences in body weight, food intake, sucrose intake, or saccharin intake were noted during the food-restricted period between the two groups. During the food-restricted condition the normal animals showed a cyclic pattern of food, sucrose, and saccharin intake. This cyclic pattern of intake was absent in vasopressin-deficient animals. A vasopressin-serotonin interaction is hypothesized as a possible mechanism producing the differences in the two strains of animals. In addition, the data support the position that food consumption is regulated primarily by caloric and/or nutritional factors.

Mapping study of noradrenergic stimulation of vasopressin release

The precise role of hypothalamic norepinephrine (NE) in the control of vasopressin (AVP) release has remained unclear, due to reports of both inhibitory and excitatory effects of NE and only a few studies with direct hypothalamic manipulations. The present study utilized a chronically implanted swivel brain cannula to investigate, in undisturbed and freely behaving rats, the impact of acute hypothalamic infusions of monoamines on circulating AVP levels. The first study examined and compared the responsiveness of six hypothalamic sites to NE infusion through the swivel cannula. Results indicated that the excitatory effect of central noradrenergic stimulation on serum AVP is highly site specific, localized to the paraventricular (PVN) and supraoptic (SON) nuclei. These two nuclei appeared to be equally responsive to NE infusion, yielding a threefold rise in serum AVP over baseline levels. In contrast, NE in the dorsomedial nucleus produced a significantly smaller increase in AVP, and no response was observed in the ventromedial nucleus, posterior hypothalamus, or perifornical lateral hypothalamus. Further tests conducted in the PVN showed this nucleus to respond in a dose-dependent manner to NE infusion. In contrast, under similar test conditions, dopamine caused only a small increase in AVP at a relatively high dose, while a PVN injection of serotonin produced no response. These results support the existence of an excitatory noradrenergic system controlling AVP release and specifically demonstrate that this function of NE is localized to the PVN and SON, in contrast to other hypothalamic areas, and is mimicked to some extent by dopamine but not by serotonin.

Ingestive behaviors of the rat deficient in vasopressin synthesis (Brattleboro strain). Effect of chronic treatment by dDAVP

Spontaneous manipulator and locomotor activities, food and fluid intake have been recorded from rats suffering from a genetic lack of central vasopressin (VP) synthesis (Brattleboro strain, DI), their heterozygous litter mates (HZ) or Long Evans (LE) rats. The daily patterns of activities did not differ, except for their drinking behavior. This was mainly associated with food intake during the dark period with LE rats but was distributed equally during light and dark periods with DI rats. HZ rats showed a behavioral heterogeneity, some of them following the daily pattern of LE rats, and others, that of DI rats. The daily feeding pattern was identical in the three genotypes but the selection between two isocaloric contrasted diets was different. When they were fed ad lib, HZ and DI rats consumed less carbohydrate than LE rats, the protein intake being unchanged. On the contrary, when the DI rats were only fed during the dark period, they ate more carbohydrate than LE rats. The peripheral infusion of a V2 AVP agonist (dDAVP) restored a normal hydric balance in DI rats but failed to modify the diet selection. These data show that in the rats, the lack of central VP synthesis disturbs both the selection of diets and the efficiency of the satiety signals. These disturbances were unchanged by the peripheral VP treatment which suggested the direct involvement of the central release of the neuropeptide.

Hypothalamic paraventricular nucleus: interaction between alpha 2-noradrenergic system and circulating hormones and nutrients in relation to energy balance

Extensive evidence suggests that norepinephrine (NE) in the brain is active in the control of eating behavior. Central injection studies demonstrate a stimulatory effect of NE on food intake, a response which is mediated by alpha 2-noradrenergic receptors located in the medial hypothalamus, in particular the paraventricular nucleus (PVN). Activation of these PVN receptors stimulates ingestion specifically of carbohydrate-rich foods, and this response is believed to reflect the role of endogenous NE in controlling natural appetite for this macronutrient. This alpha 2-noradrenergic system in the PVN appears to be physiologically activated at the onset of the animals' active cycle, when there is a natural peak in preference for carbohydrate. At this time, the adrenal hormone corticosterone, which is known to play a major role in carbohydrate metabolism, is found to interact positively with NE in the potentiation of carbohydrate ingestion. Circulating glucose also influences the activity of PVN alpha 2-noradrenergic receptors at this time, and, moreover, alpha-noradrenergic stimulation of the PVN produces an increase in circulating levels of both corticosterone and glucose. This and other evidence has led to the hypothesis that NE in the PVN, through the activation of glucocorticoid- and glucose-sensitive alpha 2-receptor sites, is physiologically active in energy homeostasis, most particularly at the onset of the animal's active cycle. Specifically, this neurotransmitter in the PVN evokes a state of energy conservation. This state involves adjustments in carbohydrate ingestion as well as metabolism, that allow animals to maintain energy reserves by anticipating or responding to a depletion.

Radioimmunoassay of desglycinamide-arginine vasopressin and its application in a pharmacokinetic study in the rat

The purpose of this investigation was to develop a sensitive and selective radioimmunoassay for Desglycinamide-Arginine Vasopressin (DGAVP). DGAVP was extracted from rat plasma after protein precipitation, using Sep-Pak C18 cartridges and 50 mM glycine buffer/methanol (10:90) solution. Extraction recovery was 73 +/- 14% (mean +/- S.D.; n = 11) and good linearity was achieved in the concentration range of 0.25-128 pg/tube. Instantaneous tracer addition resulted in a detection limit of 250 fg/tube, whereas 24 hours preincubation and delayed tracer addition resulted in a detection limit of 100 fg/tube. Intra-assay variation ranged between 7.4% and 10.0% depending on the peptide concentration and inter-assay variation was 13.2%. Using this procedure, plasma pharmacokinetics of DGAVP in the rat were determined after IV administration. DGAVP plasma concentration showed a rapid distribution phase (t1/2 = 1.0 +/- 0.2 min) and a somewhat slower elimination phase (t1/2 = 7.2 +/- 2.1 min). High clearance values (CLss = 97 +/- 30 ml.min-1) suggest rapid metabolism by amino- and carboxy-peptidases.

Effects of naloxone and cholecystokinin on food and water intake in vasopressin-deficient rats (Brattleboro strain)

Opioid peptides and cholecystokinin (CCK) have been shown to play a role in regulation of feeding behavior. Another neuropeptide that has recently been suggested to be involved in feeding is vasopressin. We explored possible interactions between opiates, CCK and vasopressin in feeding regulation by studying feeding suppression produced by naloxone and CCK in Brattleboro (DI) rats, which are homozygous for diabetes insipidus and lack the ability to synthesize vasopressin. Ten DI and 15 age-matched Long Evans (LE) rats were food deprived for 14 hours on two different days and then injected with naloxone (2.5 mg/kg) on one day or saline on the other. Thirty minutes later the food was returned and food and water consumption were measured after 1, 3 and 4 hr. Naloxone suppressed the food consumption of both DI and LE rats but the suppression was greater for the DI rats. This result was specific to feeding as water consumption was suppressed in LE more than in DI rats. Two weeks later, the same rats were food deprived for 6 hours on two different days and then injected with CCK-8 (2.5 micrograms/kg) on one day and with saline on the other. Food was returned one minute after the injection and food and water consumption were measured 30 and 60 minutes later. Food intake was reduced equally for both DI and LE rats. Water intake was not reduced. The results suggest that the suppression of feeding by CCK does not require an intact vasopressinergic system. The greater feeding suppression by naloxone in DI rats may suggest that opiates are interacting with vasopressin in producing their effects on food intake.