Effects of fasting and aminophylline on norepinephrine-stimulated non-shivering thermogenesis

The vagus nerve in thermoregulation and energy metabolism

The vagus nerve may indirectly influence thermoregulation by modulation of energy balance: its afferent fibers convey signals that represent information on feeding state, resulting in either depression or stimulation of metabolic processes. A regulated metabolic depression can be detected in the background of fasting-induced hypometabolism and hypothermia. In its development (besides humoral signals) vagally transmitted neural signals of gastrointestinal and hepatoportal origin are important. These signals are related to hunger, to decrease of mechanical/chemical stimuli from the gut, to decline of blood glucose; they alter discharge rates of vagal afferents and activity of the nucleus of the solitary tract, eliciting inhibition of metabolic rate and enhancement of food intake. In this hunger-related metabolic inhibition the nucleus of the solitary tract is in interaction with hypothalamic nuclei, that contribute to neuropeptide changes characterized by high neuropeptide Y activity (with energy-conserving type of regulation) and depressed cholecystokinin and corticotropin releasing hormone activities (with depressed energy-expenditure). In postalimentary states the hypermetabolism and hyperthermia are due to opposite changes in metabolic regulation. Satiety-related stimulatory signals of abdominal origin, transmitted via hepatic vagal afferents to the nucleus of the solitary tract, contribute to enhancement of sympathetic activity and stress-responsiveness, leading to hypermetabolism and hyperthermia. Depressed neuropeptide Y release and enhanced cholecystokinin and corticotropin releasing hormone activities participate in the central regulatory changes, and in the high energy-expenditure. The biological role of these vagal functions is not directly the regulation of body temperature, rather the regulation of energy balance and energy content in the body.

Effects of fasting and food restriction on sympathetic activity in brown adipose tissue in mice

The activity of the sympathetic nervous system in mice that were either fed ad libitum, food restricted or fasted was estimated by measuring the accumulation of dopamine following the inhibition of dopamine beta-hydroxylase activity. Mice in each group were injected with the dopamine beta-hydroxylase inhibitor 1-cyclohexyl-2-mercaptoimidazole and were exposed to either 30 degrees C (warm) or 4 degrees C (cold). Mice were killed 1 h after the injection. Both heart and brown adipose tissue were then quickly removed and homogenized in ice-cold perchloric acid. Dopamine and noradrenaline were determined using high performance liquid chromatography. Regardless of whether mice were warm or cold exposed, both content and concentration of brown adipose tissue and dopamine were predictably higher in 1-cyclohexyl-2-mercaptoimidazole-injected mice than in non-injected animals. In mice fed ad libitum, post-injection content and concentration of dopamine in both brown adipose tissue and heart were higher in cold-exposed mice than in warm-exposed animals. In food-restricted and fasted mice, post-injection concentrations of dopamine in brown adipose tissue were higher in cold-exposed mice than in warm-exposed animals. In food-restricted and fasted mice there was no difference between warm- and cold-exposed animals with respect to post-injection contents and concentrations of dopamine in heart tissue. In fasted mice there was no difference between warm- and cold-exposed animals in post-injection content of dopamine in brown adipose tissue. This study provides further evidence that fasting, in contrast to food restriction, may blunt the tissue sympathetic nervous system response in brown adipose tissue of cold-exposed mice.

Potentiation of the thermogenic antiobesity effects of ephedrine by dietary methylxanthines: adenosine antagonism or phosphodiesterase inhibition?

Current concepts about the mechanisms underlying the therapeutic effects of dietary methylxanthines (caffeine, theophylline, and theobromine) favor their actions as antagonists of adenosine receptors, and attribute their other possible modes of action, namely those associated with translocation of intracellular calcium, inhibition of phosphodiesterase enzyme (PDE) activity, or the release of catecholamines, to high (near-toxic) doses. From studies measuring the respiration rate of brown adipose tissue (BAT), evidence is provided here that at concentrations compatible with therapeutic doses, the ability of methylxanthines (25 to 50 mumol/L) to potentiate the thermogenic effect of the sympathomimetic drug, ephedrine (0.25 mumol/L), particularly under conditions of caloric restriction, involves a minor contribution of adenosine antagonism, but could mainly be explained by the inhibition of PDE activity. In view of current interest in the pharmacological stimulation of metabolic rate to assist the management of obesity with low-calorie regimens, the targeting of PDE activity is therefore a rational approach in the search for drugs that could potentiate sympathomimetic stimulation of metabolic rate.

Short- and long-term effects of ovariectomy on food intake, body weight, carcass composition, and brown adipose tissue in rats

This experiment examined both the short-term and the long-term effects of ovariectomy on brown adipose tissue growth and function in rats to determine if reduced brown adipose tissue thermogenesis might contribute to the weight gain and adiposity. Brown adipose tissue function was assessed by measuring sympathetic nervous system activity (estimated by the rate of norepinephrine turnover) and mitochondrial proton conductance (estimated by specific GDP binding) in interscapular brown adipose tissue. Rats (n = 12 per group) were killed 0 (sham), 1, 2, 4, and 12 weeks after ovariectomy. During the first five weeks after ovariectomy rats overrate and rapidly gained weight. The weight gain was due to increases in all carcass components. Five weeks after ovariectomy food intake returned to control levels, and body weights stabilized 12-16% above sham-operated control weights for the duration of the experiment. Between week 4 and week 12 after surgery there was a redistribution of carcass composition, with decreases in carcass water and fat-free dry weights offset by a further increase in total carcass lipid. Brown adipose tissue pads were heavier in the 1-, 2-, and 4-week ovariectomy groups, but only in the 4-week group was the increase statistically significant. Brown adipose tissue protein, DNA, and norepinephrine content was unchanged 1-, 2-, 4-, or 12-weeks after ovariectomy. There was no difference in either the rate of norepinephrine turnover or specific mitochondrial GDP binding between sham-operated and 1-, 2-, 4-, or 12-week ovariectomized rats.(ABSTRACT TRUNCATED AT 250 WORDS)