Nonshivering thermogenesis

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.

Administration of muscimol into the posterior hypothalamus reduces hyperthermia induced by hippocampal neostigmine injection

The firing rate of the sympathetic nerves innervating interscapular brown adipose tissue (IBAT), IBAT and colonic temperatures (T(IBAT) and T(C)) and oxygen (O(2)) consumption were monitored in urethane-anesthetized male Sprague-Dawley rats. These variables were measured for 40 min before (baseline values) and 40 min after an injection of neostigmine (5 x 10(-7) mol in 1 microl of saline) into the hippocampus and a bilateral administration of a GABA(a)-agonist, muscimol (28 ng in 0.5 microl of saline, per side) into the posterior hypothalamus. The same variables were recorded in other rats, but the muscimol was replaced by saline. Control animals were used with muscimol or saline alone. The results show an increase of sympathetic firing rate, T(IBAT), T(C) and O(2) consumption after neostigmine injection. Muscimol significantly reduces this enhancement. The findings suggest that hippocampus controls the sympathetic and thermogenic activation induced by neostigmine through an influence on GABAergic tone of the posterior hypothalamus.

Seasonal thermogenic acclimation of diurnally and nocturnally active desert spiny mice

Diurnally active golden spiny mice (Acomys russatus) and nocturnal common spiny mice (Acomys cahirinus) coexist in hot rocky deserts of Israel. Diurnal and nocturnal activities expose these species to different climatic conditions. Nonshivering thermogenesis (NST) capacity of individuals of both species immediately upon removal from the field exhibited seasonal changes, with no significant interspecific difference. Colony-reared mice of either species transferred in the laboratory from long to short photoperiod increased NST capacity, though to a lesser extent than observed in the seasonal acclimatization. The underlying biochemical mechanisms of short photoperiod acclimation differed between the species. In both Cytochrome-c oxidase (Cox) activity was higher in short as compared to long photoperiod. In short-photoperiod-acclimated A. cahirinus uncoupling protein (UCP) content in brown adipose tissue (BAT) was significantly higher than in long photoperiod, while in A. russatus there was no significant change. In A. russatus there was a significant increase in lipoprotein lipase (LPL) activity in BAT in short-photoperiod-acclimated individuals, while in A. cahirinus LPL activity was high under both acclimations. The low LPL activity in brown adipose tissue of desert-adapted A. russatus may facilitate lipid uptake in white adipose tissue, an advantage in desert conditions where food is scarce and irregularly distributed in space and time.

Suppression of PGE(2) fever at near term: reduced thermogenesis but not enhanced vasopressin antipyresis

Fevers are known to be suppressed near term in the mother, but the mechanism responsible for this phenomenon is not understood. We tested the hypothesis that the suppression of fever at term is a result of enhanced vasopressin-induced antipyresis. Effects of intracerebroventricular prostaglandin E(2) (PGE(2)) were examined in rats at gestational days 16-17 and 19-20 (near term) and days 1-2 postpartum. PGE(2) (50 ng) elevated body and interscapular brown adipose tissue (iBAT) temperatures and increased sympathetic nerve activity to the iBAT. PGE(2)-induced changes in iBAT temperature and nerve activity, as well as in rectal temperature, were reduced or eliminated near term, and responses were recovered in the postpartum period. Blood pressure and heart rate changes induced by central PGE(2) were also decreased at near term. Coinfusion of Manning compound, a V(1) vasopressin receptor antagonist, with PGE(2) throughout the peripartum period did not reverse the suppressed iBAT temperature and nerve activity or body temperature responses to PGE(2). Microdialysis experiments revealed unchanged terminal release of vasopressin in the ventral septal area after PGE(2) infusion in either pregnant or parturient rats. These results suggest that fever reduction at near term is not associated with enhanced vasopressin antipyresis, but may be a result of reduced sympathetic tone and in particular a reduced sympathetic drive to the iBAT. This finding may reflect a generalized reduction in autonomic output around the time of parturition.

CGRP microinjection into the ventromedial or dorsomedial hypothalamic nucleus activates heat production

Unilateral microinjection of calcitonin gene-related peptide (CGRP, 1.6 pmol; 0.2 microl) into the ventromedial hypothalamus (VMH) and dorsomedial hypothalamus (DMH) immediately increased oxygen consumption (VO2), heart rate (HR), colonic temperature (Tco), and temperature of interscapular brown adipose tissue (TIBAT) in urethane-anesthetized rats, whereas vehicle saline injection into the VMH and CGRP injection into other hypothalamic regions such as the preoptic area, lateral hypothalamic area, paraventricular nucleus, and bed nucleus of the stria terminalis had no effect. The effects of CGRP injection into the VMH were dose-dependent over the range of 0.016-1.6 pmol. CGRP administration to the lateral ventricle (LV) required 16-320 pmol to elicit similar degrees of responses that were observed after the injection into the VMH. The increase in TIBAT was always higher than that in Tco after CGRP injection. Injection of [Cys(ACM)2,7]hCGRPalpha, a selective CGRP2 receptor agonist, did not induce any thermogenic effects. Human CGRP8-37, a proposed CGRP1 receptor antagonist, by itself induced heat production responses with no signs of inhibition of CGRP-induced responses. Thus, the receptor subtype of the thermogenic effect of CGRP could not be determined by the available pharmacological tools. The present results show that centrally administrated CGRP induces heat production in the BAT specifically through the VMH or DMH.

Norepinephrine Injection into the Paraventricular Nucleus Induces a Reduced Modification of Eating Behavior and Thermogenesis in Brattleboro Rats

Intake of carbohydrates, lipids, proteins and total calories, temperature of interscapular brown adipose tissue, and oxygen consumption were monitored in vasopressin-containing and vasopressin-deficient rats. These variables were measured after a 20 nmol norepinephrine (NE) or saline injection into the paraventricular nucleus (PVN) of the hypothalamus. NE increased the intake of carbohydrates, lipids and total calories, decreased brown adipose tissue temperature and oxygen consumption in vasopressin-containing rats. NE reduced the intake of carbohydrates, while it increased the consumption of lipids in vasopressin-deficient rats. These findings indicate that vasopressin is involved in the modifications of eating behavioral and thermogenesis induced by NE injection into the hypothalamic PVN.

Cold exposure regulates the norepinephrine uptake transporter in rat brown adipose tissue

The neuronal uptake of norepinephrine (NE) in sympathetically innervated tissues is mediated by a high-affinity NE uptake transporter (NET). Rat interscapular brown adipose tissue (ISBAT) is densely innervated by the sympathetic nervous system for the control of cold- and diet-induced thermogenesis. To determine if cold exposure regulates the NET, kinetic parameters for [3H]NE uptake and [3H]nisoxetine (Nis) binding were determined in ISBAT from 7-day cold-exposed (CE) and control rats. Uptake of [3H]NE in ISBAT slices was of high affinity (1.6 microM). After 7 days of cold exposure the affinity for [3H]NE uptake was not altered; however, the uptake capacity was decreased (38%) in ISBAT slices from CE rats. Kinetic parameters for [3H]Nis binding demonstrated a single high-affinity site in ISBAT from CE and control rats with similar affinity. The density of [3H]Nis sites in ISBAT was decreased (38%) following cold exposure. A time course (2 h-7 days) for cold exposure demonstrated downregulation of [3H]Nis binding density by day 3, which remained through day 7. The affinity for [3H]Nis binding was transiently decreased at 2 h of cold exposure. Similarly, ISBAT NE content was decreased at 2 h of cold exposure. Pair feeding CE rats to food intake of controls normalized plasma NE content; however, [3H]Nis binding density in ISBAT remained decreased in pair-fed rats. These results demonstrate that the ISBAT NET is downregulated following cold exposure. Reductions in ISBAT NE content precede alterations in NET density; however, plasma NE content is not related to regulation of the NET.

Sympatho-adrenal involvement in methamphetamine-induced hyperthermia through skeletal muscle hypermetabolism

We investigated the involvement of the sympatho-adrenal axis in the hyperthermia induced by methamphetamine by using a biotelemetric system. The intraperitoneal injection of methamphetamine (1 mg/kg) induced hyperthermia preceded by an increase in oxygen consumption in freely moving rats. The hyperthermic effect of methamphetamine was completely blocked by chemical sympathectomy with 6-hydroxydopamine (50 mg/kg, i.p.). Adrenalectomy, but not adrenal demedullation, prevented the hyperthermia. In adrenalectomized rats, dexamethasone supplementation (0.5 mg/kg, s.c.) restored the methamphetamine-induced hyperthermia. Furthermore, dantrolene (1 or 2 mg/kg, i.v.), which blocks Ca2+ release from the sarcoplasmic reticulum in skeletal muscle, attenuated the hyperthermia. These results suggest that methamphetamine stimulates norepinephrine release from sympathetic nerve terminals, which then enhances thermogenesis in skeletal muscle under the permissive action of glucocorticoids.

Procaine injection into the paraventricular nucleus reduces sympathetic and thermogenic activation induced by frontal cortex stimulation in the rat

These experiments were designed to test the effect of procaine injection into the paraventricular nucleus on the sympathetic and thermogenic changes induced by frontal cortex stimulation. Oxygen consumption, firing rate of the sympathetic nerves to interscapular brown adipose tissue (IBAT), along with IBAT and colonic temperatures (T(IBAT) and T(C)) were monitored in fasted male Sprague-Dawley rats before and 25 min after an electrical stimulation of the frontal cortex. The same variables were monitored in rats with administration of procaine into the paraventricular nucleus. The results show that cortical stimulation raises oxygen consumption, sympathetic neuron firing rates, T(IBAT), and T(C). This increase is reduced by procaine injection. These findings suggest that the paraventricular nucleus plays a key role in the sympathetic and thermogenic changes induced by cortical stimulation.

Sucrose rich diet modifies thermogenic response to injection of muscimol into the posterior hypothalamus in the rat

The firing rate of the nerves innervating interscapular brown adipose tissue, temperatures of colon and interscapular brown adipose tissue, heart rate and oxygen consumption were monitored in urethane-anaesthetized male Sprague-Dawley rats fed with a sucrose rich diet. These variables were measured for 40 min before (baseline values) and 40 min after a 56 ng muscimol injection into the posterior hypothalamus. The same variables were monitored in other rats fed with a laboratory standard diet. Saline was injected into the posterior hypothalamus of control rats fed with sucrose or standard diet. Muscimol injection induced a decrease in firing rate, interscapular brown adipose tissue and colonic temperatures and oxygen consumption. This reduction was more evident in the rat fed with a sucrose rich diet than animals fed with standard diet. The kind of diet did not modify the decrease in heart rate induced by muscimol. These findings suggest that a sucrose rich diet modifies GABA-ergic responses to muscimol injection into the posterior hypothalamus.

Aspartic and glutamic acids increase in the frontal cortex during prostaglandin E1 hyperthermia

The aim of the present experiment was to evaluate the role played by aspartic acid and glutamic acid of frontal cerebral cortex during the hyperthermia induced by prostaglandin E1. Two groups of six Sprague Dawley male rats were anaesthetized with ethyl-urethane. The frontal cortical concentrations of aspartic and glutamic acids, the firing rate of the sympathetic nerves to the interscapular brown adipose tissue, the colonic and interscapular brown adipose tissue temperatures were monitored both before and after an intracerebroventricular injection of prostaglandin E1 (500 ng) or saline. Aspartic and glutamic acids were collected using a microdialysis probe placed in the frontal cortex. Concentrations of aspartic and glutamic acids were measured by high-pressure liquid chromatography with fluorescence detector. Prostaglandin E1 induced an increase in the concentrations of aspartic and glutamic acids, in the firing rate of sympathetic nerves and in the colonic and interscapular brown adipose tissue temperatures. The findings of the present experiment indicate that an intracerebroventricular injection of prostaglandin E1 causes release of aspartic and glutamic acids in the frontal cortex.

Acute lesions of the ventromedial hypothalamus reduce sympathetic activation and thermogenic changes induced by PGE1

The aim of this experiment was to evaluate the effects of an intracerebroventricular (icv) injection of prostaglandin E1 (PGE1) on the sympathetic activation and the thermogenic changes in rats with acute lesions of the ventromedial hypothalamus (VMH). Four groups of six Sprague-Dawley male rats were anesthetized with ethyl-urethane. The firing rate of the sympathetic nerves innervating the interscapular brown adipose tissue (IBAT) and the colonic and IBAT temperatures were monitored both before and after one of the following treatments: 1) VMH lesion plus icv injection of PGE1 (500 ng); 2) VMH lesion plus icv injection of saline: 3) sham lesion plus icv injection of PGE1; and 4) sham lesion plus icv injection of saline. PGE1 induced an increase in the firing rate of IBAT nerves and the colonic and IBAT temperatures. These effects were reduced by VMH lesion. The findings indicate that acute lesions of the VMH reduce the effects of PGE1 and seem to suggest a possible role played by the VMH in the control of the sympathetic activation and the thermogenic changes during PGE1 hyperthermia.

Role of hypothalamic neuropeptide Y neurons in the defective thermogenic response to acute cold exposure in fatty Zucker rats

The fatty Zucker rat has impaired heat production and fails to mount an adequate thermogenic response to cold exposure, partly because of decreased sympathetic drive to thermogenesis in brown adipose tissue. Neuropeptide Y, synthesized in neurons of the hypothalamic arcuate nucleus and released in the paraventricular nucleus, stimulates feeding and inhibits brown adipose tissue activity. The neuropeptide Y neurons are overactive in fatty Zucker rats and are thought to contribute to hyperphagia, reduced energy expenditure and obesity. We have examined the relationship between thermogenic activity in brown adipose tissue (measured as uncoupling protein messenger RNA levels) and hypothalamic neuropeptide Y and neuropeptide Y messenger RNA levels in response to cold exposure (4 degrees C) for 2.5 and 18 h, in fatty and lean Zucker rats. In lean Zucker rats, cold exposure at 4 degrees C for 2.5 and 18 h significantly increased uncoupling protein messenger RNA levels by 3.5-fold (P<0.01) and 3.3-fold (P<0.01), respectively, compared with warm-maintained controls. Exposure to cold for 18 h also increased neuropeptide Y concentrations in the paraventricular nucleus (P<0.01) and ventromedial nucleus (P<0.001) in lean rats, with no change in neuropeptide Y messenger RNA after either 2.5 or 18 h. By contrast, fatty Zucker rats showed no significant changes in uncoupling protein messenger RNA (P>0.05) at either duration of cold exposure. There were also no significant changes in neuropeptide Y levels in any region nor in neuropeptide Y messenger RNA, with cold exposure for either period (P>0.05). In lean rats, cold exposure therefore stimulates brown fat uncoupling protein messenger RNA and also increases neuropeptide Y concentrations in its hypothalamic sites of release. We suggest that increased brown fat thermogenic capacity induced by cold in lean rats may be mediated, at least in part, by decreased neuropeptide Y release in the paraventricular nucleus, resulting in its accumulation in this site. Defective thermogenic responses in fatty rats may result from central dysregulation of brown adipose tissue due to sustained and non-suppressible overactivity of hypothalamic neuropeptide Y neurons.

Lesions of the ventromedial hypothalamus reduce postingestional thermogenesis

The aim of this experiment was to evaluate the effects of ventromedial hypothalamus lesions on the thermogenic changes that follow food intake. Four groups of six Sprague-Dawley male rats were used. Under anesthesia with pentobarbital, the animals in the first and second groups received lesions at the ventromedial hypothalamus, and animals in the third and fourth groups received sham lesions. Body weight and food intake were monitored daily until the experimental procedure began. Twenty days after lesion, oxygen consumption, firing rate of sympathetic nerves to interscapular brown adipose tissue (IBAT), and IBAT temperature were monitored for 45 min both before and after 5 g food intake in 24 h fasted rats from the first and third groups. The same variables were measured in the animals of the second and fourth groups 50 days after receiving the lesions. Lesion placements were histologically verified. The results showed that lesions produced hyperphagia and obesity. Firing rate of nerves to IBAT, IBAT temperature, and oxygen consumption increased after food intake in sham-lesioned rats. This increase was significantly reduced by the lesion at both the 20- and 50-day time points. These findings indicate that the ventromedial hypothalamus controls postingestional activation of sympathetic discharge to IBAT. The reduction of postingestional thermogenesis could be involved in the development of obesity induced by lesion of the ventromedial hypothalamus.

Lateral hypothalamic lesion induces sympathetic stimulation and hyperthermia by activating synthesis of cerebral prostaglandins

This experiment tests the effect of intracerebroventricular (icv) injection of lysine acetylsalicylate on the sympathetic and thermogenic changes induced by lesion of the lateral hypothalamus (LH). The firing rate of the nerves innervating interscapular brown adipose tissue (IBAT), along with IBAT and colonic temperatures (TIBAT and Tc) were monitored in urethane-anaesthetized male Sprague-Dawley rats lesioned in the LH. These variables were measured before and after an icv injection of 1mg lysine acetylsalicylate. The same variables were also monitored in: a) lesioned rats with icv administration of saline; b) sham-lesioned animals with icv injection of lysine acetylsalicylate; c) sham-lesioned rats with icv injection of saline. In an additional experiment, the same variables were monitored after an icv injection of lysine acetylsalicylate or saline in rats with LH lesion performed 48 h before the icv injection. The results show that lysine acetylsalicylate injection reduces the increases in firing rate, TIBAT and Tc induced by LH lesion. These findings suggest that cerebral prostaglandin synthesis plays a key role in the sympathetic and thermogenic changes following LH lesion.

NG methyl-L-arginine increases the hyperthermic effects of prostaglandin E1

The sympathetic firing rate of the nerves innervating interscapular brown adipose tissue (IBAT), IBAT and colonic temperatures (TIBAT and TC) were monitored in urethane-anaesthetized male Sprague-Dawley rats. These variables were measured for a period of 40 min before (baseline values) and 40 min after a 2 mg NG-methyl-L-arginine (NMA) injection plus an intracerebroventricular administration of 500 ng prostaglandin E1 (PGE1) into a lateral cerebral ventricle. No drug was injected in control rats. The results show that NMA enhances the increases in firing rate, TIBAT and TC induced by PGE1. These findings indicate that an inhibitor of nitric oxide synthesis, such as NMA, increases the sympathetic and thermogenic responses to injection of PGE1.

Bombesin-induced hypothermia in rats tested at normal ambient temperatures: contribution of the sympathetic nervous system

Rats infused centrally with bombesin become hypothermic at normal ambient temperatures when acutely deprived of food, but not while allowed unrestrained access to food. Ad lib-fed rats, tested at normal ambient temperatures, become hypothermic after receiving intracerebroventricular (ICV) bombesin when they have ventromedial hypothalamic lesions or when administered insulin or 2-deoxy-D-glucose peripherally. All of these conditions have been linked to reductions of sympathetic nervous system activity to brown adipose tissue (BAT), a major thermogenic mechanism of many homeotherms. A between group design was used to examine the effects of ICV bombesin infusions on the response to peripheral injections of a) the sympathetic ganglionic blocker chlorisondamine (2.5 mg/kg, IP) in ad lib-fed rats, b) the nonspecific beta-agonist isoproterenol (30 mg/kg, IP) in food-deprived rats, and c) the combination of isoproterenol and chlorisondamine in ad lib-fed rats. Ad lib-fed rats receiving ICV bombesin (100 ng/5 microliters), in combination with peripheral chlorisondamine injection, became hypothermic 60 min postbombesin administration (-2.84 +/- 0.33 degrees C), while ad lib-fed rats receiving ICV bombesin infusion and peripheral injections of saline did not (-0.08 +/- 0.37 degrees C). Isoproterenol blocked hyperthermia in ad lib-fed rats injected with chlorisondamine and ICV bombesin. Food-derived rats receiving ICV bombesin infusion and peripheral saline injection exhibited hypothermia 60 min postbombesin administration (-2.51 +/- 0.29 degrees C). Peripheral injections of isoproterenol prevented bombesin-induced hypothermia in food-deprived rats. These data suggest that bombesin induces hypothermia at normal ambient temperatures when the sympathetic nervous system drive to BAT cannot be (or is not) activated.

Injection of muscimol in the posterior hypothalamus reduces the PGE1-hyperthermia in the rat

The firing rate of the nerves innervating interscapular brown adipose tissue (IBAT), IBAT and colonic temperatures (TIBAT and Tc), heart rate, and oxygen (O2) consumption were monitored in urethane-anesthetized male Sprague-Dawley rats. These variables were measured for 40 min before (baseline values) and 40 min after a 56 ng muscimol injection in the posterior hypothalamus and an intracerebroventricular administration of 500 ng prostaglandin E1 (PGE1). The same variables were monitored in other rats with muscimol injection or PGE1 administration alone. No drug was injected in control rats. The results show that muscimol injection reduces the increases in firing rate, TIBAT, Tc, heart rate, O2 consumption induced by PGE1. These findings suggest that GABAergic tone in the posterior hypothalamus is important in the control of thermogenic changes induced by PGE1.

Nitric oxide reduces body temperature and sympathetic input to brown adipose tissue during PGE1-hyperthermia

The firing rate of the nerves innervating interscapular brown adipose tissue (IBAT) and IBAT and colonic temperatures (TIBAT and TC were monitored in urethane-anaesthetized male Sprague-Dawley rats. These variables were measured for 40 min before (baseline values) and 40 min after a 4 micromoles L-arginine (L-arg) or 400 nmoles nitroprusside (NP) injection in a lateral cerebral ventricle and an intracerebroventricular administration of 500 ng prostaglandin E1 (PGE1). The same variables were monitored in other rats with L-arg or NP or PGE1 administration alone. No drug was injected in control rats. The results show that L-arg or NP injection reduces the increases in firing rate, TIBAT, Tc induced by PGE1. These findings suggest that nitric oxide is important in the control of thermogenic changes during the PGE1 hyperthermia.

Non-shivering thermogenesis during prostaglandin E1 fever in rats: role of the cerebral cortex

We have tested the hypothesis that there is a role for the cerebral cortex in the control of non-shivering thermogenesis during fever induced by prostaglandin E1 (PGE1). While under urethan anesthesia, the firing rate of nerves innervating interscapular brown adipose tissue (IBAT), IBAT and colonic temperatures (TIBAT and Tc) and oxygen (O2) consumption were monitored during the fever from PGE1 injection (400 and 800 ng) in a lateral cerebral ventricle in controls and in functionally decorticated Sprague-Dawley rats. Rats were functionally decorticated by applying 3.3 M KCl solution on the frontal cortex which causes cortical spreading depression (CSD). Pyrogen injections caused dose-related increases in firing rate, TIBAT, Tc and O2 consumption and CSD reduced these enhancements. Our findings indicate that the cerebral cortex could be involved in the control of non-shivering thermogenesis during PGE1-induced febrile response.

Nitric oxide reduces the thermogenic changes induced by lateral hypothalamic lesion

The experiment described here tests the effect of intracerebroventricular (icv) injection of nitric oxide (NO) precursors, such as L-arginine (L-arg) and nitroprusside (NP), on the thermogenic changes induced by lesion of the lateral hypothalamus (LH). The firing rate of the nerves innervating interscapular brown adipose tissue (IBAT), along with IBAT and colonic temperatures (TIBAT and TC) were monitored in urethane-anaesthetized male Sprague-Dawley rats lesioned in the LH. These variables were measured before and after an icv injection of 4 mumol L-arg or 400 nmol NP. The same variables were also monitored in: a) lesioned rats with icv administration of saline; b) sham-lesioned animals with icv injection of L-arg or NP; c) sham-lesioned rats with icv injection of saline. The results show that L-arg or NP injection reduces the increases in firing rate. TIBAT and TC induced by LH lesion. These findings suggest that NO plays a key role in the thermogenic changes following LH lesion.

Comparison of DNA synthesis in white and brown adipose tissue in rats with ventromedial hypothalamic lesions

Ventromedial hypothalamic (VMH) lesions cause excessive fat accumulation in white adipose tissue (WAT), and brown adipose tissue (BAT); however, little information is available on whether or not cell proliferation occurs in WAT and BAT after VMH lesioning. In this study, we determined the DNA content and thymidine incorporation in unilateral parametrial WAT and interscapular BAT 0, 1, 3, and 7 days after VMH lesioning, and examined the mechanism of increased DNA content in WAT. In rats with VMH lesions, the weight of WAT and BAT had increased significantly at 7 days, and the DNA content and thymidine incorporation of WAT had increased significantly at 3 days and continued to increase for up to 7 days, while those of BAT did not increase for as long as 7 days after VMH lesioning. Restricted food intake according to the pair-feeding method partially inhibited the increased DNA content in WAT. The increased DNA content in WAT was mostly restored but not completely by the administration of anti-insulin antibody, and by administration of propranolol, a beta-adrenergic blocker. The results demonstrated that VMH lesions induced DNA synthesis in WAT early after VMH lesioning, but did not induce DNA synthesis in BAT, and suggested that either hyperinsulinemia or a beta-adrenergic receptor mechanism or both may be responsible for the increased DNA content in WAT.

The effects of prostaglandin E2 injected into the paraventricular nucleus of the hypothalamus on brown adipose tissue thermogenesis in spontaneously hypertensive rats

We examined brown adipose tissue (BAT) thermogenic responses to prostaglandin E2 (PGE2) injected into the paraventricular nucleus of the hypothalamus in the spontaneously hypertensive rat (SHR). 250 ng of PGE2 produced smaller increases in BAT and core temperatures in SHRs compared to their normotensive controls, the WKYs. The results of the present study suggest that the ability of SHRs to mount a febrile response may be compromised.

Widespread increases in regional hypothalamic neuropeptide Y levels in acute cold-exposed rats

Neuropeptide Y injected into the hypothalamus or third ventricle stimulates feeding and inhibits the sympathetic activation of brown adipose tissue. To clarify the involvement of hypothalamic neuropeptide Y in cold-induced thermogenesis, groups of rats exposed to 4 degrees for 2.5 or 18 h were compared with warm-adapted rats (22 degrees C). Neuropeptide Y was measured in eight selected hypothalamic regions, including those known to be involved in the regulation of energy expenditure. Activation of brown adipose tissue was confirmed by significant six- to nine-fold increases in brown adipose tissue uncoupling protein messenger RNA. Compared with warm-adapted controls, neuropeptide Y levels were significantly raised by 80-170% in several hypothalamic regions of rats exposed to cold for 2.5 h, namely the medial preoptic area, paraventricular nucleus, ventromedial nucleus, dorsomedial nucleus and lateral hypothalamic area. Neuropeptide Y levels in 18-h cold-exposed rats were similarly elevated in these regions and were also significantly increased in the anterior hypothalamic area (75%). By contrast, neuropeptide Y levels in the arcuate nucleus, the main hypothalamic site of synthesis, were not increased by cold exposure, being significantly reduced by 21% after 2.5 h exposure and comparable with controls after 18 h. As neuropeptide Y injection inhibits brown adipose tissue activation, we suggest that the rapid and dramatic increases in neuropeptide Y levels in specific hypothalamic regions occur because cold exposure might inhibit the release of neuropeptide Y and so cause accumulation of neuropeptide Y in these sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Cold tolerance and metabolic heat production in male C57BL/6J mice at different times of day

Nine-month-old male C57BL/6J mice were subjected to three-hour cold stress tests (partial restraint at 6 degrees C) at 9:00 a.m. or at 1:00 p.m. Tests were repeated three times at two-week intervals at the same time of day. Body temperature was measured by colonic thermoprobe, and metabolic heat production was measured by indirect calorimetry during each test. All mice showed habituation to repeated cold exposures (an improvement of cold tolerance across tests) due to an increase in metabolic heat production. The levels of metabolic heat production were similar during morning and afternoon testing; however, mice tested in the afternoon had consistently poorer cold tolerance, which indicated increased heat loss. Increased heat loss in mice of similar body weight and presumably similar body composition, suggests that there is less effective cold-induced skin vasoconstriction during the afternoon. We hypothesize that the compromised skin vasomotor response during the afternoon cold exposure results from competing effects of vasodilation due to local autoregulation stimulated by a circadian reduction of cardiac output during the sleep phase, and vasoconstriction due to the cold stress.

Effects of ovariectomy on thermogenesis in brown adipose tissue and liver in Syrian hamsters

Weight gain in ovariectomized Syrian hamsters occurs without increased food intake, which suggests that metabolic efficiency may be enhanced through a reduction in energy expenditure. We examined the effect of ovariectomy on metabolic activity in brown adipose tissue and liver. Four groups of hamsters (n = 13, each) were killed 0, 2, 4, or 16 weeks following ovariectomy. Ovariectomized hamsters rapidly gained weight without overeating. Body weights stabilized after 8 weeks and remained 12-17% above sham-operated control weights for the duration of the experiment. Weight gain in the hamsters ovariectomized for 16 weeks was characterized by significant increases in retroperitoneal white adipose tissue weight and carcass lipid content. Similar trends were seen in 2-week and 4-week ovariectomized animals. There were no differences in interscapular brown adipose tissue weight, protein content, DNA content, or norepinephrine (NE) content among sham-operated and 2-, 4-, or 16-week ovariectomized hamsters, indicating that ovariectomy had no effect on brown adipose tissue growth. Similarly, there was no difference in either sympathetic nervous system activity (estimated by the rate of NE turnover) or mitochondrial GDP binding among the four groups of hamsters. In contrast, hepatic cytochrome P-450 activity was significantly reduced 2, 4, and 16 weeks after ovariectomy. These results suggest that reduced thermogenic activity in liver, but not in brown adipose tissue, could contribute to the weight gain in Syrian hamsters after ovariectomy.

Histochemical and biochemical detection of alcohol dehydrogenase in rat brown adipose tissue

Alcohol dehydrogenase (ADH) was studied histochemically in rat brown adipose tissue (BAT), white adipose tissue (WAT) and liver, and its activity was also measured in these tissues. A positive ADH reaction was seen histochemically in the liver and BAT, but not in the WAT, and similarly ADH activity was detected biochemically in the liver and BAT, but there was no measurable activity in the WAT. ADH activity was 80% higher in the liver than in the BAT. It is suggested that the occurrence of an ethanol metabolizing enzyme in brown adipose tissue may be connected with the previously reported ethanol-induced increase in the oxidative capacity of BAT, or, at least, that BAT is able to use ethanol as a substrate for thermogenesis.

Time-course variations of effective proton conductance and GDP binding in brown adipose tissue mitochondria of rats during prolonged cold exposure

1. Time-course variations of the thermogenic pathway in rat brown adipose tissue (BAT) mitochondria were examined. 2. Several parameters of mitochondrial energization, protonmotive force and its components pH gradient and membrane potential were investigated. The specific binding of GDP was compared with the effective proton conductance (CmH+) of the membrane. 3. Ten-days cold exposure led to maximal GDP binding and GDP-dependent CmH+. 4. The subsequent relative decrease in GDP binding observed during prolonged cold exposure (40 days) was functional and led to a lower GDP-dependent CmH+. CmH+ showed greater variation than GDP binding. 5. The CmH+ decrease was not due to a masking of active sites of the uncoupling protein. 6. Basal GDP-independent CmH+ was not modified. 7. Results are discussed with reference to the significance of biochemical measures and to the physiological regulation of BAT thermogenesis.

Unmasking of GDP binding sites on hamster brown adipose tissue mitochondria and uncoupling protein

1. A rapid unmasking of GDP binding sites on brown adipose tissue (BAT) mitochondria was observed when hamsters acclimatized to 28 degrees C were exposed to a temperature of 4 degrees C for 2 hr. 2. No rapid unmasking of GDP binding sites was observed when hamsters housed at 22 degrees C were briefly exposed to 4 degrees C. 3. The amount of GDP bound to BAT mitochondria from hamsters increased during 2 weeks of exposure to 4 degrees C, but did not change between 2 weeks and 30 days of cold exposure. 4. Incubation of mitochondria with 10 mM Mg2+ prior to the GDP binding assay increased the subsequent GDP binding to BAT mitochondria from hamsters housed at 28, 22 or 4 degrees C, albeit to different degrees. 5. The amount of GDP bound to uncoupling proteins isolated from untreated and Mg(2+)-treated mitochondria of hamsters and rats was measured. Scatchard analyses of the binding of GDP to purified uncoupling protein indicate that increases in the number of binding sites due to Mg2+ treatment of mitochondria do not change the affinity of the protein for GDP.

Activation of brown adipose tissue thermogenesis by chemical stimulation of the posterior hypothalamus

The posterior hypothalamus (PH) is involved in the generation of behavioral thermoregulatory responses, but the importance of the PH in the control of autonomic thermoregulatory responses such as heat production in brown adipose tissue (BAT) is not well defined. In the present study, selective stimulation of PH neurons by local application of the excitatory amino acid glutamate (250 nl of 1 M solution, unilaterally) caused a sharp, transient increase in interscapular BAT (IBAT) and core temperature in urethane-anesthetized rats. This effect was blocked by pretreatment with the sympathetic ganglionic blocker, chlorisondamine chloride (2 mg/kg) or the beta-adrenergic receptor blocker, propranolol (2 mg/kg), implicating the involvement of the sympathetic system. The effect of intra-PH injection of glutamate on IBAT and core temperatures could be mimicked by injection of the gamma-aminobutyric acid (GABA) receptor antagonist, bicucullin methiodide (BMI, 50 ng), into the same PH site. This effect of BMI could be blocked by co-injection of the GABA(A) receptor agonist, muscimol (25 ng). Further, BMI co-injection potentiated the effect of intra-PH injection of glutamate on IBAT and core temperatures. Conversely, muscimol co-injection prevented the stimulatory effect of intra-PH injection of glutamate. Taken together, the results indicate that direct chemical stimulation of neurons in the PH can activate an autonomic mechanism controlling heat production in BAT. Further, they suggest the neural mechanism in the PH mediating this effect is tonically inhibited by GABA, as blockade of GABAergic function in the PH produces an effect similar to that observed after direct stimulation of PH neurons with glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)

Urinary catecholamines in hyperthermia-related deaths

A group of five hyperthermia-related deaths is presented in which urinary noradrenaline (NA) concentrations were elevated (172.1 +/- 119.4 ng/ml) compared with a control group of rapid violent deaths (43.7 +/- 26.1 ng/ml). Urinary adrenaline (A) concentrations were not elevated in the hyperthermia cases, nor were there any significant differences in urinary dopamine (DA) concentrations between the two groups. All except one of the hyperthermia victims were under the influence of ethanol. It is suggested that a combination of heat stress and ethanol consumption was responsible for the elevated urinary NA in the hyperthermia cases, reflecting increased sympathetic nervous system activity. A combination of high urinary NA with low A seems to be characteristic of hyperthermia fatalities, in contrast to hypothermia deaths, where both NA and A are usually elevated.

Injection of prostaglandin E2 into the anterior hypothalamic preoptic area activates brown adipose tissue thermogenesis in the rat

E series prostaglandins (PGE) are known to elicit potent hyperthermia when injected into the anterior hypothalamic preoptic area (POAH) in rats, but the effector mechanisms mediating the rise in temperature are not well defined. In the present study, microinjection of PGE2 into the POAH dose-dependently increased non-shivering thermogenesis in brown adipose tissue (BAT) in urethananesthetized rats, bringing about a marked and sustained rise in interscapular BAT (IBAT) and core temperatures. The effect of intra-POAH PGE2 injection on IBAT and core temperatures could be blocked by systemic pretreatment with the sympathetic ganglionic blocker chlorisondamine chloride or the beta-adrenergic receptor blocker propranolol, thus implicating the involvement of the sympathetic system. Furthermore, the increase in IBAT and core temperatures induced by intra-POAH PGE2 could be blocked by prior injection of the local anesthetic procaine or the GABA receptor agonist muscimol into the ipsilateral ventromedial hypothalamic nucleus (VMH). Taken together, the results suggest that PGE2 increases body temperature by acting in the POAH to stimulate heat production in BAT via a sympathetic efferent mechanism located in the VMH.

Thermal biology of the laboratory rat

The purpose of this paper is to thoroughly review the literature and present a data base of the basic thermoregulatory parameters of the laboratory rat. This review surveys the pertinent papers dealing with various aspects of the thermal biology of the laboratory rat, including: metabolism, thermoneutrality, core and brain temperature, thermal tolerance, thermal conductance and insulation, thermoregulatory effectors (i.e., thermogenesis, peripheral vasomotor tone, evaporation, and behavior), thermal acclimation, growth and reproduction, ontogeny, aging, motor activity and exercise, circadian rhythm and sleep, gender differences, and other parameters. It is shown that many facets of the thermoregulatory system of the laboratory rat are typical to that of most homeothermic species. However, is several instances the rat exhibits unique thermoregulatory responses which are not comparable to other species.

Influence of two beta-adrenoceptor antagonists, propranolol and pindolol, on cold adaptation in the rat

1. Adult male rats were treated with propranolol (2.0 mg kg-1 day-1 i.p.), pindolol (0.2 mg kg-1 day-1 i.p.) or 0.9% NaCl day-1 i.p. and exposed to +4 degrees C for 42 days, or treated with 0.9% NaCl day-1 i.p. and kept at +23 degrees C for 42 days. They were weighed once a week, when a 24 h urine sample was also collected and colon temperature measured. 2. Urinary noradrenaline (NA), adrenaline (Ad) and dopamine were analysed by high-performance liquid chromatography with an electrochemical detector. After the acclimatization period the interscapular brown adipose tissue was excised and weighed and the activity of the oxidative enzymes succinate dehydrogenase and cytochrome oxidase measured. 3. The pindolol-treated and propranolol-treated rats gained weight during the cold-acclimatization period. The amount of interscapular brown adipose tissue increased in the cold, but the increase was lowest in the pindolol-treated group. No changes were seen in the other brown adipose tissue parameters in cold-exposed animals. The excretion of catecholamines followed the same pattern in all three cold-exposed groups, with an initial rise in noradrenaline and adrenaline excretion and a slight rise in dopamine excretion. 4. The results suggest possible connections between beta-adrenoceptor antagonists, weight gain and cold acclimatization. Pindolol had a slight inhibitory effect on cold-induced brown adipose tissue hypertrophy in rats.

Experimental obesity: a homeostatic failure due to defective nutrient stimulation of the sympathetic nervous system

The basic hypothesis of this review is that studies on models of experimental obesity can provide insight into the control systems regulating body nutrient stores in humans. In this homeostatic or feedback approach to analysis of the nutrient control system, we have examined the afferent feedback signals, the central controller, and the efferent control elements regulating the controlled system of nutrient intake, storage, and oxidation. The mechanisms involved in the beginning and ending of single meals must clearly be related to the long-term changes in fat stores, although this relationship is far from clear. Changes in total nutrient storage in adipose tissue can arise as a consequence of changes in the quantity of nutrients ingested in one form or another or a decrease in the utilization of the ingested nutrients. A change in energy intake can be effected by increased size of individual meals, increased number of meals in a 24-hour period, or a combination of these events. Similarly, a decrease in utilization of these nutrients can develop through changes in resting metabolic energy expenditure which are associated with one of more of the biological cycles such as protein metabolism, triglyceride for glycogen synthesis and breakdown, or maintenance of ionic gradients for Na+ + K+ across cell walls. In addition, differences in energy expenditure related to the thermogenesis of eating or to the level of physical activity may account for differences in nutrient utilization.

Phorbol esters, protein kinase C, and thyroxine 5'-deiodinase in brown adipocytes

Protein kinase C activity has been identified in the rat brown adipocyte. About 60% of this activity is found in the cytosolic fraction under basal conditions, and 12-O-tetradecanoylphorbol 13-acetate (TPA) causes a rapid shift from the cytosol to the particulate fraction. Norepinephrine and phenylephrine cause a similar redistribution that can be blocked by prazosin but not by alprenolol. alpha 1-Adrenergic agonists cause three- to fivefold stimulation of type 2 iodothyronine 5'-deiodinase activity in brown adipocytes. TPA has no effect on basal deiodinase activity and reduces the response of the enzyme to alpha 1-adrenergic agonists. These results suggest that the translocation of protein kinase C from cytosol to particulate fraction is not sufficient to increase deiodinase activity but can modulate the alpha 1-adrenergic agonist-mediated responses in these cells.

Metabolic integrity and stability of isolated rat brown adipocytes

Using rats previously anesthetized with pentobarbital and with subtle modifications to existing procedures, rat brown adipocytes of good yield, viability, and stability were consistently obtained. The metabolic integrity of the cells, indicated by an 8-11 fold norepinephrine-dependent stimulation of respiration, was reproducible between cell preparations and maintained even after 3-5 hours of cell storage at room temperature. The ATP content of freshly isolated and stored cells per mg DNA (using a modified DNA assay) closely reflected the in vivo content determined from freeze clamped tissue.

Changes in thyroid hormones and thyroxine glucuronidation in hamsters compared with rats following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin

In rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds, serum thyroxine (T4) is depressed. Since hamsters are relatively insensitive to TCDD-induced lethality, the effects of TCDD on several parameters of thyroid status were measured in hamsters as a comparison with the more sensitive rat. At 7 days after ip injection of TCDD, there was a dose-dependent increase in serum 3,5,3'-triiodothyronine (T3) and T4 in hamsters to a maximum level 200% of control; the ED50 was approximately 10 micrograms/kg. Hamsters receiving 100 micrograms/kg lost up to 4% of their body weight but began to recover after about 3 weeks. Serum T4 in these animals was elevated compared to pair-fed and ad libitum controls throughout the 53-day experiment, although it also began to recover after Day 21. This was in direct contrast to the marked reduction of T4 in rats exposed to lower doses of TCDD. T3 was significantly higher in TCDD-treated hamsters than in pair-fed controls on Days 2-7, and TSH was also elevated on Days 2-21. Reverse T3, like T4, was increased by TCDD in hamsters whereas it was decreased in rats. Hepatic microsomal UDP-glucuronosyltransferase (GT) activity was measured using T4 as substrate (T4-GT). On a whole liver basis, T4-GT was induced by TCDD by the same proportion in both rats and hamsters (170-180% of controls) although absolute activities in rats were 3- to 4-fold higher than in hamsters. This similarity in T4-GT inducibility by TCDD suggests that there are likely mechanisms in addition to T4-GT induction which account for the species-specific alterations in T4. Thus, while the response of thyroid hormones to TCDD differed qualitatively, effective doses in hamsters were higher than in rats, suggesting that these changes, although secondary, may correlate more directly with toxicity than does enzyme induction (whose ED50s are similar in both species). An understanding of the mechanism of this species difference may be helpful in unravelling the primary mechanisms of TCDD toxicity.

Unchanged nonshivering thermogenic capacity of dystrophic mice

The rate of oxygen consumption measured at 32.5 degrees C of lightly anesthetized 129/ReJ dy/dy mice was greater than that of dy/+ or +/+ control mice. However, the norepinephrine-stimulated rates of oxygen consumption of dystrophic and normal mice were similar. Brown adipose tissue cellularity (DNA content) of dystrophic mice was unchanged, and the tissue protein and succinate dehydrogenase contents were slightly reduced. The mitochondrial concentration of the uncoupling protein, thermogenin, and purine nucleotide binding to mitochondria isolated from brown fat of normal or dystrophic mice, were similar. These results indicate that the nonshivering thermogenic capacity of dystrophic mice is not significantly altered.

Long-term effects of lateral hypothalamic lesions on brown adipose tissue in rats

A classic feature of animals with lateral hypothalamic (LH) lesions is their regulation of body weight at sub-normal levels. The present studies were done to determine whether this is associated with enhanced thermogenic activity of their brown adipose tissue (BAT). Three groups of young chow-fed male Holtzman rats were formed: (1) animals receiving bilateral radiofrequency heat lesions of the dorsal LH and then permitted free access to chow (LH rats); (2) non-lesioned animals that were pair-fed (PF) to the lesioned rats during a 2 week post-operative recovery period (Phase 1); (3) non-lesioned, ad lib fed (NORM) controls. After Phase 1, each group was divided and permitted free access to chow alone or an additional selection of palatable, novel food items (a "cafeteria" diet) for 2-3 weeks (Phase 2) to stimulate diet-induced thermogenesis in BAT. Finally, half of each sub-group was exposed to 4 degrees C for 15 hr to stimulate nonshivering thermogenesis in BAT. During Phase 1 LHs and PFs ate 50% less than NORMs. This resulted in a weight deficit of 16% for LHs and 12% for PFs. After the additional period of feeding palatable foods (Phase 2) LHs collectively weighed 14% less than NORMs whereas previously PFs had a weight deficit of only 4%. They gained less weight than NORMs or PFs despite a similar energy intake. LHs had small deposits of gonadal white adipose tissue [both total amount and expressed per metabolic body mass (kg 0.75)]. The weight of interscapular BAT was less in the LHs but its concentration of protein (mg/g) was higher.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cold stress and exercise on fat loss in females

A Latin Square design has been used to test the responses of 24 relatively fit young women to 200 minute bouts of exercise performed over 5 day trials under each of three different ambient conditions: 15 degrees C (warm-warm; (WW)); -20 degrees C while inhaling, from a facemask, air heated to 18 degrees C (cold-warm; (CW)); and -20 degrees C (cold-cold; (CC)). In both of the cold environments, special clothing and boots were provided (insulation 0.47 degree C X watt-1 X m-2 and 0.62 degree C X watt-1 X m-2; (4 and 3 CLO units)). All three trials led to a small (0.6-0.7 degree C) rise of rectal temperature, but in the two cold environments mean body temperatures fell by over 1.0 degree C. A large increase of serum ketones occurred under all conditions, and the exercise respiratory quotient suggested some increase of fat utilization, WW (0.85) through CW (0.84) to CC (0.83). A fat loss of about 0.5 kg over the five days was confirmed by hydrostatic weighing and measurement of skinfold thicknesses. This was much less than the change previously observed in men, and moreover, it seemed to be independent of ambient conditions. Possible reasons why cold did not increase fat loss in these women include: a lower relative intensity of exercise; a greater stability of fat stores in women; avoidance of caffeine; a possible translocation of subcutaneous fat to deep fat depots; and a greater desire to "lose weight" irrespective of environmental conditions.

Short photoperiod stimulates brown adipose tissue growth and thermogenesis but not norepinephrine turnover in Syrian hamsters

This experiment examined the effect of short photoperiod on food intake, body weight, carcass composition, and brown adipose tissue (BAT) activity in female Syrian hamsters (Mesocricetus auratus). BAT function was assessed by measuring sympathetic nervous system (SNS) activity in BAT (estimated by the rate of norepinephrine (NE) turnover), BAT mitochondrial content (estimated by cytochrome c oxidase activity), and BAT mitochondrial proton conductance (estimated by guanosine-5'-diphosphate (GDP) binding to isolated BAT mitochondria). Food intake and body weight were both increased in short photoperiod-housed hamsters (8-hr light, 16-hr dark; LD 8:16) when compared to those of the long photoperiod-housed controls (LD 16:8). The weight gain was entirely due to an increase in carcass lipid. Interscapular BAT (IBAT) pads from short photoperiod-housed hamsters were 53% heavier and contained comparably more protein and DNA. Short photoperiod produced a 118% increase in BAT cytochrome c oxidase activity and a 41% increase in specific mitochondrial GDP binding. Whether expressed per mg wet tissue or per pad, neither the endogenous concentration of NE nor its rate of turnover were changed by short photoperiod exposure. These results demonstrate a dissociation of BAT thermogenesis from SNS activity in BAT from short photoperiod-housed Syrian hamsters.

Mitochondrial thermogenin content is unchanged during atrophy of BAT of fasting mice

The objectives of this study were to evaluate the rate at which brown adipose tissue (BAT) from mice atrophies when its thermogenic activity is suppressed during fasting or exposure to a thermoneutral environment (33 degrees C) and whether such atrophy is accompanied by loss from BAT mitochondria of "thermogenin," the GDP binding protein associated with the calorigenic proton conductance pathway. Atrophy of mouse BAT was characterized by rapid loss of protein but unchanged tissue DNA content. The rate of protein loss varied from 2 to 6 mg protein/day depending on the environmental and feeding status of the mice. In synchrony with tissue protein loss, there was a marked reduction in the tissue content of mitochondrial proteins and of thermogenin, measured by immunoassay. However, the concentration of thermogenin in isolated mitochondria was unchanged by fasting or exposure of the mice to 33 degrees C for 48 h. By contrast, marked reduction in [3H]GDP binding to isolated mitochondria were observed after exposure of the mice to 33 degrees C. Mice acclimated at 4 but not those acclimated at 21 degrees C showed reduction in GDP binding to isolated mitochondria during fasting. These results clearly indicate that changes in purine nucleotide binding to isolated mitochondria can occur in the absence of changes in the mitochondrial concentration of thermogenin. Thus rapid decrease in BAT thermogenic capacity (e.g., during fasting or 33 degrees C exposure) appears dependent on extensive loss of tissue protein, probably whole mitochondria, rather than rapid and selective removal of thermogenin from the mitochondria.

Impaired diet-induced thermogenesis in brown adipose tissue from rats made obese with parasagittal hypothalamic knife-cuts

Two experiments were performed to determine if bilateral parasagittal hypothalamic knife-cuts (KCs), which produce long-term overeating and obesity, after biochemical indices of brown adipose tissue (BAT) reactivity to thermogenic stimuli. In the first study, responses to environmental cold were tested. Four weeks after surgery, KC rats had gained 4-5 times more weight than controls and were obese (increased Lee Obesity Index and weight of gonadal white fat). Before being sacrificed, groups of KC and control rats were exposed to 4 degrees C for 21 hr or remained at 28 degrees C. Interscapular BAT weighed 300% more in KC rats, due largely to increased white fat content. Functional indices of BAT thermogenic capacity (protein content, DNA content, cytochrome oxidase activity and mitochondrial guanosine diphosphate (GDP) binding) were normal at 28 degrees C. Exposure to 4 degrees C produced greatly enhanced responses but these were equivalent for both groups. This suggested an intact capacity for non-shivering thermogenesis in obese KC rats. In the second study, the same BAT responses were examined in other rats fed a palatable "cafeteria" diet (CAFE). One week after surgery, KC and control rats were subdivided into groups that received chow alone or chow plus four different palatable foods daily. Before sacrificing 4-5 weeks later, KC rats had gained 3-4 times more weight than controls and were obese. Interscapular BAT weighed 200-300% more in KC rats. CAFE feeding produced larger increments in all variables for KC vs. control rats. Most importantly, GDP binding was reduced in both KC groups, and significantly more so after CAFE feeding.(ABSTRACT TRUNCATED AT 250 WORDS)

Defective trophic response of brown adipose tissue of myopathic hamsters

Hamsters with muscular dystrophy (BIO 14.6) have a smaller than normal amount of brown adipose tissue. Two stimuli that promote growth of brown adipose tissue in normal hamsters, short photoperiod and eating a high-fat diet, are here shown to be without effect on brown adipose tissue of myopathic hamsters. Cold-induced growth of brown adipose tissue occurs normally [Am. J. Physiol. 239 (Cell Physiol. 8): C18-C22, 1980]. There is a normal rate of turnover of norepinephrine in brown adipose tissue of the myopathic hamster but a failure of the tissue to hypertrophy in response to norepinephrine is unlikely since norepinephrine does not appear to mediate the trophic response [Am. J. Physiol. 247 (Endocrinol. Metab. 10): E793-E799, 1984]. Denervation results in a marked reduction in size (protein content) of brown adipose tissue of normal hamsters but has very little effect on the size of brown adipose tissue of myopathic hamsters. A central, possibly hypothalamic, defect in the myopathic hamster is postulated to underlie its abnormal control of brown adipose tissue hypertrophy.

Trophic response of hamster brown adipose tissue: roles of norepinephrine and pineal gland

The objective was to find out whether the reduced amount of brown adipose tissue in myopathic hamsters [Am. J. Physiol. 239 (Cell Physiol. 8): C18-C22, 1980] was secondary to a refractoriness to the trophic influence of norepinephrine. However, no evidence for any trophic influence of norepinephrine on brown adipose tissue of either normal or myopathic hamsters could be detected under experimental conditions that have demonstrated such an influence in rats. A mediator other than norepinephrine, melatonin, secreted by the pineal gland, is known to mediate the control of brown adipose tissue growth induced in hamsters by short photoperiod. Further studies of intact or pinealectomized hamsters showed that the pineal gland was not required for either cold- or diet-induced growth of brown adipose tissue. It is concluded that the defect in control of brown adipose tissue size in the hamster with muscular dystrophy is not due either to abnormal control by norepinephrine or to the pineal gland since neither of these appears to participate in the normal regulation of brown adipose tissue size in relation to environmental temperature or to diet.

The development of obesity in animals: the role of genetic susceptibility

There are a number of animals in which obesity is genetically determined. In some the inheritance is polygenic while in others it is by a single-gene mutation. In the most widely studied single-gene mutants--the obese (ob/ob) mouse, the diabetic-obese (db/db) mouse and the Zucker (fa/fa) rat-obesity is very substantial and is initiated before the animals are weaned. Although hyperphagia is a feature of all the major obese mutants, it is not a prerequisite for the development of obesity. The initiation of the disorder during the suckling period takes place on a normal energy intake, and excess rates of energy deposition will still continue after weaning if the obese mutants are pair-fed to the ad libitum energy intake of lean siblings. The ability to become obese without hyperphagia indicates that one or more components of energy expenditure must be reduced in the obese mutants. Studies on the ob/ob mouse have demonstrated that a reduction in thermogenesis in BAT is the main way by which this is achieved. The reduction in energy expenditure in BAT is due primarily to a low activity of the sympathetic innervation to the tissue. Once hyperphagia is established, apparently as a secondary feature of the obese syndrome, the development of obesity is accelerated, the obese mutants having an impairment in the dietary stimulation of BAT thermogenesis. Studies on different types of obese animal suggest that an inability to respond to dietary stimuli is a general feature of obesity. The final syndrome presented by genetically obese animals is of considerable metabolic and endocrinological complexity. However, it is now possible to begin to integrate some of the endocrinological abnormalities within the energy balance framework, centred on BAT, that has been developed over the past few years. This is particularly evident in the case of adrenal function in the Zucker rat; adrenalectomy of this mutant has important effects on energy balance and in the normalization of the thermogenic activity of BAT. The sensitivity of BAT to insulin is also emerging as a possible factor of importance in the modulation of thermogenesis in obese animals.(ABSTRACT TRUNCATED AT 400 WORDS)