Circulating free fatty acids, insulin, and glucose during chemical stimulation of hypothalamus in rats

Hypoglycemia-induced noradrenergic activation in the VMH is a result of decreased ambient glucose

During insulin-induced hypoglycemia, there is an increase in extracellular norepinephrine (NE) in the ventromedial hypothalamus (VMH). This brain area is known to play an important role in integrated hormonal and behavioral responses to systemic hypoglycemia. Selective glucoprivation restricted to the VMH is both necessary and sufficient to initiate secretion of counterregulatory hormones. The present study was designed to investigate whether increased release of NE in the VMH depends on detection of glucoprivation localized in this area. In awake, chronically catheterized male Sprague-Dawley rats, extracellular NE in the VMH was monitored using 1-mm microdialysis probes perfused with Krebs Ringer buffer (KRB) or KRB + 100 mM d-glucose (d-Glc). During insulin-induced hypoglycemia (glycemic nadir approximately 2.4 mM) extracellular NE was increased to >160% of baseline (P < 0.01) only in the KRB + insulin group. There was no increase in NE from baseline when glucose was added to the perfusate to maintain euglycemia at the periprobe environment. The sympathoadrenal response to hypoglycemia, present in the KRB + insulin group, was attenuated in the d-Glc + insulin group. The present results confirm that noradrenergic activation in the VMH during systemic hypoglycemia depends on detection of glucoprivation locally in this area. These data provide additional support for the importance of increased noradrenergic activity in the VMH in the counterregulatory hormonal responses to hypoglycemia.

Dissociation of hypothalamic noradrenergic activity and sympathoadrenal responses to recurrent hypoglycemia

This study evaluated whether attenuation of sympathoadrenal responses to recurrent hypoglycemia is mediated by diminished noradrenergic activity in the hypothalamus. Male Sprague-Dawley rats received either once daily insulin (1.0 units/kg) injections or an equal administration of saline for 3 days. Both groups received an administration of insulin on the fourth day, during which blood glucose and plasma catecholamines were determined, and extracellular norepinephrine (NE) in the ventromedial hypothalamus (VMH) or paraventricular hypothalamic nucleus (PVN) was monitored with microdialysis. The peak response of plasma epinephrine to insulin-induced hypoglycemia (nadir approximately 3.2 mmol/l) was significantly reduced during the fourth hypoglycemic episode (774 +/- 134 pg/ml) compared with the first episode (2,561 +/- 410 pg/ml, P < 0.001). Baseline levels of extracellular NE were elevated approximately 25% (P = 0.07) in the VMH and approximately 46% (P = 0.03) in the PVN after multiple hypoglycemic episodes. There was no difference in noradrenergic activity during the first or fourth hypoglycemic episode in either brain area. The reduced sympathoadrenal output after recurrent hypoglycemia is likely postsynaptic from hypothalamic NE release or is mediated via a collateral pathway.

Effects of food texture change on metabolic parameters: short- and long-term feeding patterns and body weight

A complete diet was prepared with cooked pieces of meat, beans, cream starch, and water and presented to the rats in two different textures: a blended purée and a rough mixture that required a lot of chewing. We hypothesized that this texture modification might change both anticipatory reflexes and feeding behavior. Feeding rate, meal size, intermeal intervals, and their correlation were monitored in response to each texture. The long-term (6 wk) effect on body weight was assessed. Periprandial plasma glucose, insulin, glucagon, and lipid concentrations were assayed. Whole and background metabolism, respiratory quotient, and locomotion were measured using a computerized calorimeter of original design. In the short term, rats preferred the mixture. However, after 3 wk, they ingested more purée than mixture and gained more body weight per gram of food ingested as purée. Insulin response declined earlier with the mixture. During meals, glycerol and free fatty acid increased earlier with purée, whereas in the postprandial period, glycerol increased earlier with mixture. The metabolic rate, however, was not significantly affected. We concluded that texture, an everyday manipulation performed on food for human consumption, affects not only palatability of ingestants but also their metabolic management in the short and long term.

Hypothalamic adrenergic receptor changes in the metabolic syndrome of genetically obese (ob/ob) mice

The genetically, seasonally, and diet-induced obese, glucose-intolerant states in rodents, including ob/ob mice, have each been associated with elevated hypothalamic levels of norepinephrine (NE). With the use of quantitative autoradiography on brain slices of 6-wk-old obese (ob/ob) and lean mice, the adrenergic receptor populations in several hypothalamic nuclei were examined. The binding of [(125)I]iodocyanopindolol to beta(1)- and beta(2)-adrenergic receptors in ob/ob mice was significantly increased in the paraventricular hypothalamic nucleus (PVN) by 30 and 38%, in the ventromedial hypothalamus (VMH) by 23 and 72%, and in the lateral hypothalamus (LH) by 10 and 15%, respectively, relative to lean controls. The binding of [(125)I]iodo-4-hydroxyphenyl-ethyl-aminomethyl-tetralone to alpha(1)-adrenergic receptors was also significantly increased in the PVN (26%), VMH (67%), and LH (21%) of ob/ob mice. In contrast, the binding of [(125)I]paraiodoclonidine to alpha(2)-adrenergic receptors in ob/ob mice was significantly decreased in the VMH (38%) and the dorsomedial hypothalamus (17%) relative to lean controls. This decrease was evident in the alpha(2A)- but not the alpha(2BC)-receptor subtype. Scatchard analysis confirmed this decreased density of alpha(2)-receptors in ob/ob mice. Together with earlier studies, these changes in hypothalamic adrenergic receptors support a role for increased hypothalamic NE activity in the development of the metabolic syndrome of ob/ob mice.

Chronic infusion of norepinephrine into the VMH of normal rats induces the obese glucose-intolerant state

Increases in ventromedial hypothalamic (VMH) norepinephrine (NE) levels and/or activities have been observed in a variety of animal models of the obese insulin-resistant condition. This study examined the metabolic effects of chronic NE infusion (25 nmol/h) into the unilateral VMH of normal rats. Within 4 days, VMH NE infusion significantly increased plasma insulin (140%), glucagon (45%), leptin (300%), triglyceride (100%), abdominal fat pad weight (50%), and white adipocyte lipogenic (100%) and lipolytic (100%) activities relative to vehicle-infused rats. Furthermore, isolated islet insulin secretory response to glucose (15 mM) within 4 days of such treatment was increased over twofold (P < 0.05). Among treated animals, fat stores continued to increase over time and plateaued at approximately 2 wk (3-fold increase), remaining elevated to the end of the study (5 wk). By week 4 of treatment, NE infusion induced glucose intolerance as evidenced by a 32% increase in plasma glucose total area under the glucose tolerance test curve (P < 0.01). Whole body fat oxidation rate measured after 5 wk of infusion was significantly increased among treated animals as evidenced by a reduced respiratory quotient (0.87 +/- 0.01) relative to controls (0. 90 +/- 0.01). VMH NE infusion induced hyperphagia (30%) only during the first week and did not affect body weight over the 5-wk period. Increases in VMH NE activity that are common among obese insulin-resistant animal models can cause the development of this obese glucose-intolerant (metabolic) syndrome.

Evidence for sexual differences in the preoptic area regulation of blood glucose in rats

The effect of noradrenaline (NA) injection (20 or 40 nmol) into the preoptic area (POA) on plasma glucose and insulin was studied in male and female rats. The rats were implanted with chronic jugular catheters for blood sampling and unilateral intracerebral cannulas placed just above the POA. Blood samples were taken before and at 5, 10, 15, 30 and 60 min after NA injection. As early as 5 min after NA injection, plasma glucose levels rose rapidly in both male and female rats, reaching a peak at 15 min poststimulus. NA injection into the POA caused a dose-dependent hyperglycemic response in both male and female rats, although the response was more intense and longer lasting in females than in males. However, NA injection into the POA induced an increase in plasma insulin concentration in male but not in female rats. In addition, the increase in plasma glucose induced by 40 nmol NA injection in males preceded that of insulin. Plasma levels of glucose after POA injection of NA were already significantly elevated (p < 0.01) within the first experimental interval (5 min), whereas a plasma insulin increase were first detected 15 min post injection. We conclude that, when administered locally into the POA, NA can activate the sympathetic outflow expressed by a neurally mediated hyperglycemia which is more intense in females than in males. These data demonstrate that the POA has a sexually differentiated function in the regulation of glycemia.

Intracarotid glucose selectively increases Fos-like immunoreactivity in paraventricular, ventromedial and dorsomedial nuclei neurons

Perfusion of the forebrain with glucose at concentrations which alter neither plasma insulin nor glucose levels leads to sympathetic activation in some rats. We used the expression of Fos-like immunoreactivity (FLI) as an index of neuronal activation to examine the anatomic substrate underlying this phenomenon. Male Sprague-Dawley rats were infused via the right internal carotid artery with glucose (4 mg/kg/min) or equiosmolar mannitol for 60 min. They were killed 3 h after infusion onset and their brains reacted for FLI. As compared to mannitol-infused controls, 105% and 117% more neurons in hypothalamic ventromedial nucleus (VMN) and parvocellular portion of the paraventricular nuclei (PVN) of glucose-infused rats showed FLI, respectively. Importantly, only about half the glucose-infused rats showed increased FLI cells in these areas when compared to controls. In these same animals, glucose also significantly activated cells in the dorsomedial n. There was little FLI expressed in the magnocellular neurons of the PVN. This selective glucose response was bilateral in keeping with the bilateral distribution of India ink to midline hypothalamic structures following unilateral carotid infusions. Retrograde transport of cholera toxin B from medullary and thoracic spinal cord sympathetic outflow areas showed labeling of about 10% of PVN neurons with FLI activated by intracarotid glucose. There was no double labeling of VMN neurons. This supports the presence of anatomic pathways by which a subpopulation of glucose responsive PVN neurons might activate the sympathetic outflow areas in the medulla and spinal cord. The apparent bimodal distribution of glucose-induced activation of VMN and PVN neurons is in keeping with a similar bimodal pattern of sympathetic activation which obesity-prone but not obesity-resistant rats show following glucose infusions. Taken together, these data support a role for glucose-sensitive VMN and parvocellular PVN neurons in the weight gain phenotype specific sympathetic activation to glucose.

Glucose homeostasis and sympathoadrenal activity in mercaptoacetate-treated rats

The effect of the fatty acid oxidation inhibitor, sodium mercaptoacetate (MA, 600 mumol/kg) on peripheral energy substrate metabolism was investigated in rats with permanent heart catheters. Rats were either fed, 48-h food deprived, or exercising for 30 min. Before and after intravenous MA injection, stress-free blood samples were taken for measurement of blood glucose, plasma free fatty acids (FFA), insulin, epinephrine (E), and norepinephrine (NE) concentrations. In fed animals, MA increased blood glucose, plasma FFA, and NE and decreased insulin concentrations. Plasma E levels did not change. In 48-h-deprived animals, MA elevated low baseline glucose concentrations to levels observed in MA-treated fed animals. Plasma insulin concentrations decreased to almost undetectable levels. Plasma catecholamines and FFA were increased compared to fed rats. In exercising rats, MA caused an exaggerated increase of blood glucose and a pronounced reduction of plasma insulin without affecting exercise-induced FFA and catecholamine responses. The data revealed that the mechanisms that regulate blood glucose concentrations during MA treatment are dependent on the nutritional state and ambient energy expenditure.

Sympathoadrenal responses to glucoprivation and lipoprivation in rats

The effects of glucoprivation and lipoprivation on sympathoadrenal outflow were investigated in rats with permanent intra-atrial catheters. Glucoprivation was induced by infusion of a hypoglycemic dose of insulin (3 U/kg) or by infusion of the glucose antimetabolite, 2-deoxy-D-glucose (2-DG, 200 mg/kg). Lipoprivation was induced by infusion of sodium mercaptoacetate (MA, 600 mumol/kg), which blocks beta oxidation of fatty acids. Stress-free blood samples for measurement of blood glucose, plasma nonesterified fatty acids (NEFA), and epinephrine (E) and norepinephrine (NE) concentrations were collected remotely before and after drug injection. Glucoprivation and lipoprivation differed significantly in their effects on the sympathoadrenal system. Both 2-DG- and insulin-induced glucoprivation appeared to increase adrenomedullary secretion selectively, leading to dramatically increased plasma E levels. Although plasma NE levels also rose during glucoprivation, other evidence suggests that this effect may be secondary to the rise in E. In contrast, MA-induced lipoprivation increased the outflow of NE from the sympathetic nerve endings without a significant effect on plasma E concentrations. Plasma E levels rose only late in the test, as blood glucose levels began to fall. Results indicate that glucoprivation and lipoprivation are distinct metabolic signals, each capable of selectively activating one branch of the sympathoadrenomedullary system and thereby facilitating the mobilization of metabolic fuels appropriate for the specific metabolic challenge.

Feeding- and chemical-related activity of ventromedial hypothalamic neurones in freely behaving rats

1. The activity of seventy-eight single neurones in the ventromedial hypothalamus (v.m.h.) was recorded in sixty-three freely behaving rats, and the effects of feeding, intraventricular (I.C.V.) administrations of noradrenaline, glucose, NaCl and ambient temperature on neuronal activity were analysed. If I.C.V. NaCl had an effect, intraperitoneal (I.P.) NaCl, mannitol and polyethylene glycol were also tested. 2. Neurones in the v.m.h. were classified into three groups according to diurnal variations and their relations to electroencephalogram (e.e.g.) and responses to feeding: diurnal-e.e.g. related (57/78, 73.1%); diurnal-e.e.g. independent (17/78, 21.8%); non-diurnal-e.e.g. independent (4/78, 5.1%). Of fifty-seven e.e.g.-related neurones, twenty-six decreased activity during feeding episodes. Of seventeen e.e.g.-independent neurones, eight increased activity gradually during feeding and sustained the increase after the feeding episode. The response magnitude of two e.e.g.-independent neurones depended on the kind of food available. 3. Of twenty-five e.e.g.-related neurones tested, twelve responded to I.C.V. noradrenaline, but not to I.C.V. glucose or NaCl. Neurones independent of e.e.g. responded variously to I.C.V. noradrenaline, glucose and NaCl. When I.C.V. NaCl had an effect, I.P. NaCl, mannitol and polyethylene glycol had the same effect. The activity of three neurones was increased by I.C.V. glucose and decreased by I.C.V. noradrenaline, but was not changed by I.C.V. NaCl. The activity of three was increased by I.C.V. glucose and decreased by I.C.V. NaCl and by I.C.V. noradrenaline. The activity of five was increased, and that of three was decreased by I.C.V. glucose, NaCl and noradrenaline. Collectively, fourteen of twenty-four tested neurones responded to I.C.V. glucose, twenty-six of forty-one tested neurones responded to I.C.V. noradrenaline and eleven of twenty-six tested neurones responded to I.C.V. NaCl. 4. Increase of ambient temperature changed the activity of five e.e.g.-independent neurones. Directions of these activity changes were the same as directions of responses to NaCl; two up, three down. 5. The results suggest two main neuronal groups in the v.m.h.: the e.e.g.-related group is involved in the processing of information about sleep-arousal. The e.e.g.-independent group contributes to the long-term processing of information concerned with the regulation of the internal environment such as glucose level, osmotic pressure, NaCl level, the trigger mechanism for feeding, ambient temperature, food preference, etc.