Circumventricular organs: receptors and mediators of direct peptide hormone action on brain

Insulin Signalling: The Inside Story

Insulin signalling begins with binding to its cell surface insulin receptor (IR), which is a tyrosine kinase. The insulin receptor kinase (IRK) is subsequently autophosphorylated and activated to tyrosine phosphorylate key cellular substrates that are essential for entraining the insulin response. Although IRK activation begins at the cell surface, it is maintained and augmented following internalization into the endosomal system (ENS). The peroxovanadium compounds (pVs) were discovered to activate the IRK in the absence of insulin and lead to a full insulin response. Thus, IRK activation is both necessary and sufficient for insulin signalling. Furthermore, this could be shown to occur with activation of only the endosomal IRK. The mechanism of pV action was shown to be the inhibition of IRK-associated phosphotyrosine phosphatases (PTPs). Our studies showed that the duration and intensity of insulin signalling are modulated within ENS by the recruitment of cellular substrates to ENS; intra-endosomal acidification, which promotes dissociation of insulin from the IRK; an endosomal acidic insulinase, which degrades intra-endosomal insulin; and IRK-associated PTPs, which dephosphorylate and, hence, deactivate the IRK. Therefore, the internalization of IRKs is central to insulin signalling and its regulation.

Brainstem metabotropic glutamate receptors reduce food intake and activate dorsal pontine and medullar structures after peripheral bacterial lipopolysaccharide administration

During infection-induced inflammation food intake is reduced. Vagal and brainstem pathways are important both in feeding regulation and immune-to-brain communication. Glutamate is released by vagal afferent terminals in the nucleus of the solitary tract and by its neurons projecting to the parabrachial nuclei. We therefore studied the role of brainstem glutamate receptors in spontaneous food intake of healthy animals and during sickness-associated hypophagia after peripheral administration of bacterial lipopolysaccharides or interleukin-1beta. Brainstem group I and II metabotropic, but not ionotropic, glutamate receptor antagonism increased food intake both in saline- and lipopolysaccharide-treated rats. In these animals, expression of the cellular activation marker c-Fos in the lateral parabrachial nuclei and lipopolysaccharide-induced activation of the nucleus of the solitary tract rostral to the area postrema were suppressed. Group I metabotropic glutamate receptors did not colocalize with c-Fos or neurons regulating gastric function in these structures. Group I metabotropic glutamate receptors were, however, found on raphé magnus neurons that were part of the brainstem circuit innervating the stomach and on trigeminal and hypoglossal motor neurons. In conclusion, our findings show that brainstem metabotropic glutamate receptors reduce food intake and activate the lateral parabrachial nuclei as well as the rostral nucleus of the solitary tract after peripheral bacterial lipopolysaccharide administration. They also provide insight into potential group I metabotropic glutamate receptor-dependent brainstem circuits mediating these effects.

[Targeting the brain through the nose. Effects of intranasally administered insulin]

The assumption that the human brain is an insulin-independent organ was disproved with the discovery of insulin receptors in the central nervous system in the year 1978. Evidence has been provided for a high density of insulin receptors in brain regions responsible for cognitive memory processes (hippocampus) and for the regulation of appetite (hypothalamus). Accordingly, in animal studies an increased insulin level in the central nervous system leads to an improvement of hippocampal memory function and a decrease of food intake. Similar results were obtained in humans using the method of intranasal administration of insulin. Intranasal insulin reaches the brain and the cerebrospinal fluid via the olfactory epithelium and olfactory nerve fiber bundles leading through the lamina cribrosa to the olfactory bulb. Thus, this method renders the investigation of specific insulin effects in humans possible. The therapeutic potential of an intranasal insulin administration for the treatment of diseases for which an imbalance of the central nervous insulin metabolism is discussed (e.g. Alzheimer's disease, diabetes mellitus and obesity) can only be estimated with the help of further clinical studies.

Effect of pioglitazone on arterial baroreflex sensitivity and sympathetic nerve activity in patients with acute myocardial infarction and type 2 diabetes mellitus

Pioglitazone has been shown to reduce the occurrence of fatal and nonfatal myocardial infarction (MI) in type 2 diabetes mellitus (DM). However, the mechanisms of such favorable effects remain speculative. The aim of this study was to investigate the effect of pioglitazone on arterial baroreflex sensitivity (BRS) and muscle sympathetic nerve activity (MSNA) in 30 DM patients with recent MI. Patients were randomly assigned to those taking pioglitazone (n = 15) and those not taking pioglitazone (n = 15) at 4 weeks after the onset of MI. BRS, MSNA, calculated homeostasis model assessment of insulin resistance index (HOMA-IR), and plasma adiponectin were measured at baseline and after 12 weeks. Pioglitazone increased plasma adiponectin (from 6.9 ± 3.3 μg/dL to 12.2 ± 7.1 μg/dL) and reduced HOMA-IR (from 4.0 ± 2.2 to 2.1 ± 0.9). In the pioglitazone group, MSNA decreased significantly (from 37 ± 7 bursts/min to 25 ± 8 bursts/min) and BRS increased significantly (from 6.7 ± 3.0 to 9.9 ± 3.2 ms/mm Hg) after 12 weeks. Furthermore, a significant relationship was found between the change in MSNA and HOMA-IR (r = 0.6, P = 0.042). Thus, pioglitazone decreased the sympathetic nerve traffic through the improvement of insulin resistance in DM patients with recent MI, which indicate that the sympathoinhibitory effects of pioglitazone may, at least in part, have contributed to the beneficial effects of pioglitazone.

Pregnancy and the endocrine regulation of the baroreceptor reflex

The purpose of this review is to delineate the general features of endocrine regulation of the baroreceptor reflex, as well as specific contributions during pregnancy. In contrast to the programmed changes in baroreflex function that occur in situations initiated by central command (e.g., exercise or stress), the complex endocrine milieu often associated with physiological and pathophysiological states can influence the central baroreflex neuronal circuitry via multiple sites and mechanisms, thereby producing varied changes in baroreflex function. During pregnancy, baroreflex gain is markedly attenuated, and at least two hormonal mechanisms contribute, each at different brain sites: increased levels of the neurosteroid 3alpha-hydroxy-dihydroprogesterone (3alpha-OH-DHP), acting in the rostral ventrolateral medulla (RVLM), and reduced actions of insulin in the forebrain. 3alpha-OH-DHP appears to potentiate baroreflex-independent GABAergic inhibition of premotor neurons in the RVLM, which decreases the range of sympathetic nerve activity that can be elicited by changes in arterial pressure. In contrast, reductions in the levels or actions of insulin in the brain blunt baroreflex efferent responses to increments or decrements in arterial pressure. Although plasma levels of angiotensin II are increased in pregnancy, this is not responsible for the reduction in baroreflex gain, although it may contribute to the increased level of sympathetic nerve activity in this condition. How these different hormonal effects are integrated within the brain, as well as possible interactions with additional potential neuromodulators that influence baroreflex function during pregnancy and other physiological and pathophysiological states, remains to be clearly delineated.

Windows of the brain: Towards a developmental biology of circumventricular and other neurohemal organs

We review the anatomical and functional features of circumventricular organs in vertebrates and their homologous neurohemal organs in invertebrates. Focusing on cyclostomes (lamprey) and urochordates (ascidians), we discuss the evolutionary origin of these organs as a function of their cell type specification and morphogenesis.

Corticotropin-releasing factor (CRF)-induced behaviors are modulated by intravenous administration of teneurin C-terminal associated peptide-1 (TCAP-1)

The teneurin C-terminal associated peptides (TCAP) are a recently discovered family of bioactive peptides that can attenuate aspects of the behavioral stress responses of rats. Because TCAP has some structural similarity to the corticotropin-releasing factor (CRF) family of peptides, and modulates elements of the stress response, TCAP may act to modulate CRF actions in vivo. This hypothesis was tested by investigating anxiety-related behaviors in male rats following repeated intravenous (IV) TCAP-1 administration with either an acute intracerebroventricular (ICV) or IV CRF challenge. TCAP-1 alone did not affect behavioral responses significantly, however did significantly affect CRF-regulated behaviors depending on CRF's mode of injection. In both the elevated plus-maze and the open field tests, TCAP-1 had an anxiolytic effect on ICV CRF responses as indicated by decreased stretched-attend postures in the elevated plus maze (p<0.05), and increased center time and center entries in the open field (p<0.05). However, prior TCAP-1 treatment has an anxiogenic effect on the IV CRF-induced behaviors (decreased center entries and total distance in the open field (p<0.05)). TCAP-1's actions are not mediated through acute changes in glucocorticoid levels and may occur via a central action in the brain. A fluorescently (FITC)-labeled TCAP-1 analog was IV-administered to investigate whether IV TCAP-1 has the potential to regulate central mechanisms by crossing the blood-brain barrier. FITC-TCAP-1 was detected in blood vessels and fibers in the brain indicating that uptake into the brain is a possible route for its interaction with CRF and its receptors. Thus, TCAP may modulate CRF-associated behaviors by a direct action in the CNS.

Changes in blood pressure and plasma catecholamine levels during prolonged hyperinsulinemia

Hyperinsulinemia has been shown to induce activation of the sympathetic nervous system and vasodilatation. Whether these effects result in changes in blood pressure (BP) is discussed controversially. We measured BP and plasma catecholamine levels in 30 healthy men during a 60-minute baseline phase and 360-minute period of insulin infusion. In a double-blind, between-subject comparison, insulin was infused at a low rate (1.5 mU insulin/kg per minute) in one half of the subjects and at a high rate (15 mU/kg per minute) in the other half. Throughout the experiments, blood glucose levels were held constantly within the normal range by a simultaneous infusion of glucose. Serum insulin levels increased to a plateau of 543 +/- 34 pmol/L during low rate and to 24,029 +/- 1,595 pmol/L during high rate of insulin infusion. Compared with baseline, insulin infusion of either rate significantly increased systolic BP, BP amplitude, and heart rate (all P < .05). In comparison with the low rate of insulin infusion, the high rate provoked a more pronounced increase in heart rate (P < .02) and systolic BP (P < .05) but tended to decrease diastolic BP (P < .08) summing up to a distinctly more increased BP amplitude (P < .05). Plasma norepinephrine as well as epinephrine levels did not significantly change during the low-rate insulin infusion but significantly increased during high-rate insulin infusion (both P < .05). By showing a dose-dependent increasing influence of insulin on systolic BP and circulating catecholamine levels, the present study provides experimental evidence for the notion that hyperinsulinemia contributes to the development of hypertension.

Influence of nervous blockade on insulin-mediated glucose uptake in the human forearm

This study was undertaken to investigate if a nonpharmacologic increase in forearm blood flow (FBF) could increase forearm glucose uptake (FGU) during hyperinsulinemia. In 10 young volunteers, FBF and the arterial-venous glucose difference were measured in both arms during a 2-hour euglycemic hyperinsulinemic clamp procedure when 1 of the arms was subjected to axillary plexus nervous blockade with local anesthesia. FBF was measured in both arms by venous occlusion plethysmography. Nervous blockade, increasing FBF by more than 3-fold, did not improve insulin-mediated FGU. On the contrary, a tendency towards a reduced FGU compared with the control arm was seen (P =.07). Furthermore, while insulin increased FBF to a similar degree in both arms (+ 3.0 and 4.4 mL/min/100 mL tissue, P <.01 for both arms), nervous blockade abolished the rapid increase in glucose extraction seen in the control arm when insulin infusion was initiated. The present study showed that an increase in FBF induced by nervous blockade did not increase insulin-mediated FGU. On the contrary, a tendency towards a reduction was seen. Furthermore, insulin induced vasodilation in the blocked arm, but delayed the ability of insulin to promote glucose extraction, suggesting that the well-documented increase in skeletal muscle sympathetic nerve activity seen during acute hyperinsulinemia has metabolic rather than hemodynamic consequences.

Role of nitric oxide in insulin-induced hypothermia in rats

Hypothermia is a well-known phenomenon which accompanies hypoglycemia in mammals. The present study was designed to test the hypothesis that nitric oxide (NO) plays a role in insulin-induced hypothermia. The body temperature (Tb) of awake, unrestrained rats was measured before and after systemic infusion of insulin (2U x kg(-1) x h(-1)), and intracerebroventricular administration of NG-nitro-(L)-arginine methyl ester (L-NAME, a nonselective NO synthase inhibitor, 200 microg/1 microl). We observed a significant reduction in body temperature after insulin infusion. L-NAME alone caused no significant change in body temperature. When the two treatments were combined, no change in Tb was observed. The data indicate that NO plays a key role in insulin-induced hypothermia.

Coronary hyperreactivity to adrenergic stimulation and increased nocturnal vagal tone trigger coronary vasospasm

The relationship between autonomic nervous system activity (ANA) and coronary vasoreactivity during transient myocardial ischemia was determined in patients with vasospastic angina (VA). ANA was measured by power spectral analysis of heart rate variability and humoral factors following intravenous infusion of insulin in 24 patients with VA and 6 control patients. Nine (38%) of the VA patients had significant ST segment depression (STD), and 4 of these patients had symptomatic STD. The frequency of anginal episodes in the 9 patients with VA and STD was significantly greater than that in the 15 VA patients without STD (3.4 +/- 3.1 vs 0.5 +/- 0.8 episodes/week, p < 0.05). The increase in the LF/HF ratio 30 min after insulin injection in patients with STD was significantly greater than that in patients without STD (34 +/- 31% vs 4 +/- 34%, p < 0.05). All of the patients with VA and STD had significant coronary vasospasm in response to the infusion of < or = 20 micrograms of acetylcholine, higher levels of nocturnal parasympathetic activity, and greater norepinephrine production in response to insulin stimulation than the VA patients without STD. These findings suggest that increased vagal tone and hyperreactivity to adrenergic stimulation may trigger vasospasm in patients with VA.

Blockade of angiotensin II receptors inhibits the increase in blood pressure induced by insulin

To elucidate whether hyperinsulinemia increases blood pressure by increasing sympathetic outflow via the activation of the central angiotensin system, insulin was infused into urethane-anesthetized rats intravenously (i.v.) or intracerebroventricularly (i.c.v.) under euglycemic conditions. Infusion (i.v.) of insulin elicited pressor effects in a dose-dependent manner (13, 20, and 40 mU/min). Although depressor responses to i.v. injections of hexamethonium were significantly greater in insulin-infused than in saline-infused rats, i.v. captopril and d(CH2)5Tyr(Me)-arginine vasopressin did not show any differences between the groups. Infusions (i.c.v.) of insulin (8 mU/10 microl) also induced cardiovascular acceleration and augmented the depressor response to i.v. hexamethonium in insulin-infused rats. The i.c.v. pretreatment with the angiotensin II antagonist losartan inhibited the pressor responses to both the i.c.v. and i.v. infusion of insulin. These results suggest that the increase in blood pressure induced by euglycemic hyperinsulinemia is elicited by sympathetic activation and that hyperinsulinemia stimulates the angiotensin system in the brain to increase sympathetic nerve activity.

Adrenocorticotropin widens the focus of attention in humans. A nonliner electroencephalographic analysis

OBJECTIVE

This study examined the effects of ACTH 4-10, a fragment of adrenocorticotropin (ACTH) with known central nervous system (CNS) activity, on the dimensional complexity of the ongoing electroencephalographic (EEG) activity. Stressful stimuli cause ACTH to be released from the pituitary, and as a neuropeptide ACTH may concurrently exert adaptive influences on the brain's processing of these stimuli. Previous studies have indicated an impairing influence of ACTH on selective attention.

METHODS

Dimensional complexity of the EEG, which indexes the brain's way of stimulus processing, was evaluated while subjects performed tasks with different attention demands. Sixteen healthy men (23 to 33 years) were tested once after placebo and another time after administration of ACTH 4-10 (1.25 mg intravenously (i.v.), 30 minutes before testing). The EEG was recorded while subjects were presented with a dichotic listening task (consisting of the concurrent presentation of tone pips to the left and right ear). Subjects either a) listened to pips in both ears (divided attention), or b) listened selectively to pips in one ear (selective attention), or c) ignored all pips.

RESULTS

Dimensional complexity of the EEG was higher during divided than selective attention. ACTH significantly increased the EEG complexity during selective attention, in particular over the midfrontal cortex (Fz, Cz).

CONCLUSIONS

The effects support the view of a de-focusing action of ACTH during selective attention that could serve to improve the organism's adaptation to stress stimuli.

Desensitization of the neuronal insulin receptor: a new approach in the etiopathogenesis of late-onset sporadic dementia of the Alzheimer type (SDAT)?

Even in its incipient stage, late-onset sporadic dementia of the Alzheimer type (SDAT) is characterized by an abnormal reduction in brain glucose consumption and energy formation. Gathering evidence indicates that cerebral glucose metabolism is controlled by brain insulin/insulin receptors. This led us to hypothesize that the abnormal reduction in glucose utilization found in Alzheimer brains is preceded by a desensitization of cerebral insulin receptors which might be due to enhanced levels of stress factors such as cortisol and catecholamines. The hypothesis is supported by clinical findings of an abnormal response to the oral glucose tolerance test in AD patients. Furthermore, experimental desensitization of the cerebral insulin receptor resulted in both cognitive deficits and metabolic abnormalities in cerebral oxidative glucose metabolism resembling those described in incipient late-onset SDAT. Glucose is the major source of energy in the CNS, and any impairment in cerebral glucose oxidation can be expected to result in deficits in both acetylcholine synthesis and ATP formation, which might contribute to altered APP processing and enhanced susceptibility to neurotoxicity.

The cellular and physiological actions of insulin in the central nervous system

Insulin is a peptide hormone involved in the regulation of glucose homeostasis. Its synthesis and function in the peripheral tissues have been extensively studied and well understood. In contrast, demonstration of insulin in the brain has raised questions concerning its origin and physiological significance. In spite of extensive studies, the source of insulin present in the brain has not yet been conclusively identified. Evidence exists in support of both peripheral and central origins of this hormone in the brain. Recognized physiological effects of insulin in the central nervous system (CNS) include regulation of food intake, control of glucose uptake and trophic actions on neuronal and glial cells. These actions of insulin are mediated by insulin receptor resembling closely that in peripheral tissues and coupled with tyrosine kinase signal transduction pathway. In this review we will discuss theories concerning the origin of insulin in the CNS. In addition, we will present current information on both cellular and physiological effects of this hormone in the brain.

A new twist in the brain-gut axis

Gastrointestinal functions are precisely regulated by hormonal and neural negative feedback loops. In addition to the classic hormonal and vago-vagal reflex mechanisms, these studies indicate that there are direct actions of gut hormones on the dorsal vagal complex. The current data demonstrate that pancreatic polypeptide is released into the circulation by vagal-cholinergic dependent mechanisms. It travels to the brainstem in the circulation, transverses the blood-brain barrier through "leaky" regions of this barrier in the area postrema and nucleus of the tractus solitarius and binds to specific receptors in the dorsal vagal complex. By binding to these sites, pancreatic polypeptide can directly inhibit vagal input to the pancreas and other gastrointestinal organs. These observations provide an anatomic basis to explain why pancreatic polypeptide is a more potent inhibitor of the action of central stimulants of pancreatic secretion than it is of the response to peripheral secretagogues. They also establish a novel mechanism by which gut peptides can influence brain function directly.

Abnormal sympathetic overactivity evoked by insulin in the skeletal muscle of patients with essential hypertension

The reason why hyperinsulinemia is associated with essential hypertension is not known. To test the hypothesis of a pathophysiologic link mediated by the sympathetic nervous system, we measured the changes in forearm norepinephrine release, by using the forearm perfusion technique in conjunction with the infusion of tritiated NE, in patients with essential hypertension and in normal subjects receiving insulin intravenously (1 mU/kg per min) while maintaining euglycemia. Hyperinsulinemia (50-60 microU/ml in the deep forearm vein) evoked a significant increase in forearm NE release in both groups of subjects. However, the response of hypertensives was threefold greater compared to that of normotensives (2.28 +/- 45 ng.liter-1.min-1 in hypertensives and 0.80 +/- 0.27 ng.liter-1 in normals; P less than 0.01). Forearm glucose uptake rose to 5.1 +/- .7 mg.liter-1.min-1 in response to insulin in hypertensives and to 7.9 +/- 1.3 mg.liter-1.min-1 in normotensives (P less than 0.05). To clarify whether insulin action was due to a direct effect on muscle NE metabolism, in another set of experiments insulin was infused locally into the brachial artery to expose only the forearm tissues to the same insulin levels as in the systemic studies. During local hyperinsulinemia, forearm NE release remained virtually unchanged both in hypertensive and in normal subjects. Furthermore, forearm glucose disposal was activated to a similar extent in both groups (5.0 +/- 0.6 and 5.2 +/- 1.1 mg.liter-1.min-1 in hypertensives and in normals, respectively). These data demonstrate that: (a) insulin evokes an abnormal muscle sympathetic overactivity in essential hypertension which is mediated by mechanisms involving the central nervous system; and (b) insulin resistance associated with hypertension is demonstrable in the skeletal muscle tissue only with systemic insulin administration which produces muscle sympathetic overactivity. The data fit the hypothesis that the sympathetic system mediates the pathophysiologic link between hyperinsulinemia and essential hypertension.

Insulin in the cerebrospinal fluid

The presence, distribution and specific localization of insulin and its receptors in the central nervous system (CNS) have been described in numerous reports. Insulin in the CNS appears to be similar to pancreatic insulin by biochemical and immunological criteria. While the presence of insulin in the cerebrospinal fluid (CSF)--an essential neurohumoral transport system--has been widely reported, the available information is fragmented and therefore it is difficult to determine the significance of insulin in the CSF and to establish future research directions. This paper presents an integrative view of the studies concerning insulin in the CSF of various species including the human. Evidence suggests that insulin in the CSF and brain may be the result of local synthesis in the CNS, and uptake from the peripheral blood through the blood-brain barrier and circumventricular organs. The passage of insulin from the peripheral blood through the blood-brain barrier may be mediated by a specific transport system coupled to insulin receptors in cerebral microvessels. The transfer of insulin from the peripheral blood through the circumventricular organs is not specific and may depend on simple diffusion. Slow access of insulin to brain interstitial fluid adjacent to the blood-brain barrier and circumventricular organs may be followed by selective transport to other brain sites and into the ventricular-subarachnoideal CSF. It has been hypothesized that the choroid plexuses, which constitute the blood-CSF interface, might be a nonspecific pathway for rapid insulin transport into the CSF. Insulin may also pass from the CSF into the peripheral blood via absorption into the arachnoid villi. This evidence indicates that insulin may be transported in both directions between the CSF-brain and the peripheral blood. Evidence also suggests that the presence of insulin in the CSF is of pivotal importance for its neurophysiological or neuropathophysiological significance.

Hyperinsulinemia produces both sympathetic neural activation and vasodilation in normal humans

Hyperinsulinemia may contribute to hypertension by increasing sympathetic activity and vascular resistance. We sought to determine if insulin increases central sympathetic neural outflow and vascular resistance in humans. We recorded muscle sympathetic nerve activity (MSNA; microneurography, peroneal nerve), forearm blood flow (plethysmography), heart rate, and blood pressure in 14 normotensive males during 1-h infusions of low (38 mU/m2/min) and high (76 mU/m2/min) doses of insulin while holding blood glucose constant. Plasma insulin rose from 8 +/- 1 microU/ml during control, to 72 +/- 8 and 144 +/- 13 microU/ml during the low and high insulin doses, respectively, and fell to 15 +/- 6 microU/ml 1 h after insulin infusion was stopped. MSNA, which averaged 21.5 +/- 1.5 bursts/min in control, increased significantly (P less than 0.001) during both the low and high doses of insulin (+/- 5.4 and +/- 9.3 bursts/min, respectively) and further increased during 1-h recovery (+15.2 bursts/min). Plasma norepinephrine levels (119 +/- 19 pg/ml during control) rose during both low (258 +/- 25; P less than 0.02) and high (285 +/- 95; P less than 0.01) doses of insulin and recovery (316 +/- 23; P less than 0.01). Plasma epinephrine levels did not change during insulin infusion. Despite the increased MSNA and plasma norepinephrine, there were significant (P less than 0.001) increases in forearm blood flow and decreases in forearm vascular resistance during both doses of insulin. Systolic pressure did not change significantly during infusion of insulin and diastolic pressure fell approximately 4-5 mmHg (P less than 0.01). This study suggests that acute increases in plasma insulin within the physiological range elevate sympathetic neural outflow but produce forearm vasodilation and do not elevate arterial pressure in normal humans.

Focal metabolic effects of angiotensin and captopril on subregions of the rat subfornical organ

Angiotensin infusion increased glucose metabolism in 4 of 7 subdivisions of the rat subfornical organ, the effect being stronger in ventromedial compared to dorsolateral zones across the rostrocaudal axis. [Sar1-Leu8]Angiotensin II attenuated metabolic responses to intravenous angiotensin in all subfornical organ subregions. Brattleboro rats, having high circulating levels of angiotensin, displayed greater rates of glucose metabolism than Long-Evans rats in all subregions, differences that were eliminated by captopril, an inhibitor of angiotensin converting enzyme. The studies reveal focal subfornical organ zones where in vivo metabolic activity corresponds to cytoarchitectonic evidence for topographical processing within this angiotensin-sensitive structure.

Differential rates of glucose metabolism across subregions of the subfornical organ in Brattleboro rats

We applied [14C]deoxyglucose autoradiography and imaging techniques to determine rates of glucose metabolism in distinct subdivisions of the subfornical organ (SFO) of conscious Brattleboro rats. Seven anatomically-defineD SFO subregions were discerned having metabolic activities that differed from one another by as much as 29% in water-sated Brattleboro rats. The highest metabolic activity was found in the ventromedial zone of central and caudal subregions where previous studies identified the greatest densities of neurons, capillaries, putative angiotensin receptors, and angiotensin-immunoreactive fibers. Homozygous Brattleboro rats had rates of glucose metabolism that were 39-68% greater than those in corresponding SFO subregions of Long-Evans rats; these differences were accentuated by about 50% following 18 h of water deprivation. Exogenous treatment of Brattleboro rats with vasopressin uniformly normalized subregional glucose metabolism in the SFO. In Sprague-Dawley rats, water deprivation over 120 h provoked greater increases in metabolism of ventromedial than of dorsolateral SFO zones in amounts similar to the differences between Long-Evans and Brattleboro rats. The findings identify focal areas of high metabolic activity within subregions of the SFO where central responses are likely initiated to defend against homeostatic disturbances. The data represent further evidence for the probability that angiotensin II, as both hormone and neurotransmitter, is a metabolic stimulant of its target cells in the nervous system.

Intra-ventromedial hypothalamic injection of insulin suppresses brown fat thermogenesis in the anaesthetized rat

Insulin can affect metabolic functions such as glucose production and fat mobilization through action in the ventromedial nucleus of the hypothalamus (VMH), and the VMH has been implicated in the regulation of heat generation in brown adipose tissue (BAT) in the rat. To study the role of insulin in modulating VMH mechanisms concerned with BAT thermogenic activity we evaluated the effect of intra-VMH microinjection of insulin on BAT (T(bat)) and core (T(core)) temperatures and BAT thermogenic activity (T(bat)-T(core)) in anaesthetized rats. Intra-VMH insulin (10 ng, 100 ng and 1 microgram) enhanced the decreases in T(bat) and T(core) resulting from exposure of the anaesthetized rats to mild cold, as well as diminished BAT thermogenic activity in a dose-dependent manner. This effect could be partially reversed by systemic treatment with norepinephrine (400 micrograms/kg). Intra-VMH injection of insulin analogs having reduced binding affinity to insulin receptors and diminished biological activity--i.e., acetyl3 insulin, succinyl3 insulin and TNB3 insulin--was much less effective at enhancing the decrease in T(bat) and T(core) or at inhibiting BAT thermogenic activity. These results demonstrate that insulin can modulate BAT thermogenesis in a specific manner through action in the VMH.

Intrahypothalamic injection of insulin decreases firing rate of sympathetic nerves

Injection of picomolar quantities of insulin into the ventromedial hypothalamus of rats significantly reduced the firing rate of sympathetic nerves that supply interscapular brown adipose tissue. The minimal firing rate was reached in 2 min, and the effect was gone within 4 min. The effect of insulin was dose-related and did not occur when comparable volumes of physiological saline were injected into the ventromedial hypothalamus. Destruction of neurons in the ventromedial hypothalamus by injection of kainic acid abolished the inhibitory effects of insulin. These data suggest that insulin may play a role in modulating the sympathetic firing rate to thermogenically important tissues.

Dependence of food intake on acute and chronic ventricular administration of insulin

Several lines of evidences indicate that insulin affords short- and long-term neuroendocrine signals to modulate ingestive behavior. To further study a possible role of insulin in the control of food intake, male Wistar rats were subjected to various intra-third cerebro-ventricular applications of saline and insulin. Infusion of 2.0 mIU/rat of insulin at 1100 and 1900 decreased food intake in a 23.5 hr test period. Infusion of 0.5 mIU/rat of insulin between 1100 and 1200 decreased nighttime food intake during the 1st and 2nd days. Infusion of 2.0 mIU/rat/24 hr of insulin from osmotic minipumps decreased nighttime food intake throughout the active pump period and the effect persisted into the post-pump period. The results support the notion that insulin is involved in the regulation of food intake in the rat.

Blood-brain barrier and peptides

The brain is both the source and the recipient of peptide signals. The question is: Do endogenous, blood-borne peptide molecules influence brain function? Brain regions with the tight capillaries of the blood-brain barrier (BBB) extract low but measurable amounts of labeled peptide molecules from an intracarotid bolus injection. In the rat, the extraction fractions of beta-casomorphin-5, DesGlyNH2-arginine-vasopressin, arginine-vasopressin, lysine-vasopressin, oxytocin, gonadoliberin, substance P, and beta-endorphin, studied in this laboratory, range from 0.5% (substance P) to 2.4% (arginine-vasopressin). Extraction varies little among the 15 examined brain regions. As shown for arginine-vasopressin, the extracted peptides may be bound in part to specific binding sites located on the luminal membrane of the tight endothelial cells. Transport of peptide molecules across the BBB cannot be ruled out, but it is unlikely that endogenous peptides pass the BBB in physiologically significant amounts. In contrast, in brain regions with leaky capillaries, e.g., selected circumventricular organs including the pineal gland, neurohypophysis, and choroid plexus, the peptide fraction extracted approaches that of water. Within the circumventricular organs, the peptide molecules actually reach the cellular elements of the tissue. However, no studies definitively show that peptides reach neurons in the deeper layers of the brain. On the other hand, blood-borne peptides influence the BBB permeability by altering the transport of essential substances. The effect may be mediated by specific peptide binding sites located at the luminal membrane of the endothelium. It is possible that the effect of peptides on the BBB is necessary for proper brain function.(ABSTRACT TRUNCATED AT 250 WORDS)

NH2-terminal specificity and axonal localization of adrenocorticotropin binding sites in rat median eminence

Adrenocorticotropin binding sites in the rat median eminence have been localized in vivo. These binding sites occur in the basalar zone, which is rich in axonal endings. Using competitive binding and quantitative light-microscope radioautography, we found that the median-eminence binding site, in contradistinction to the adrenal receptor, binds specifically the residue 4-10 region of the adrenocorticotropin molecule. Using quantitative electron-microscope radioautography and median-eminence deafferentation, we localized the binding sites to axon terminals in this region. In time-delayed uptake studies using light-microscope radioautography, we failed to observe concentration of radiolabel in neurons of the medial basal hypothalamus after the direct injection of radioiodinated adrenocorticotropin(1-24) into the median eminence.