Formation of the “setting” level of body temperature regulation during endotoxin fever

The experiments on rats and rabbits using modern physiological, biochemical research methods and the pharmacological approach established that in the body, the action of bacterial endotoxin, accompanied by fever, leads to a significant decrease in blood plasma and in CSF of the arginine content. In rabbits after 30 min intravenous administration of carbon-labeled arginine hydrochloride (25 μCi/kg) at the endotoxin fever peak (after the 60 min injection of endotoxine E. coli), the radioactivity level in the blood plasma decreases and significantly increases in the cerebrospinal fluid and the hypothalamus tissue. It was revealed that although the content and speed of norepinephrine turnover in the hypothalamus after the introduction of L-arginine hydrochloride (100 μg) into the ventricles of the rats does not change in comparison with control animals, however, the chemoreactive properties of the thermoregulatory structures of the brain have changed, which manifests itself in the change in the expression and duration of thermoregulatory effects of the central action of norepinephrine and acetylcholine. It was established that the administration of L-arginine hydrochloride into the brain ventricles at a dose of 100 μg per animal or in the blood flow at a dose of 20 mg/kg caused the pronounced antipyretic effect. It was found that L-arginine hydrochloride (100 μg), after it has been introduced into the ventricles of the brain, increases the impulse activity of heat-sensitive neurons of the medial preoptic region of the anterior hypothalamus in rabbits due to a brain temperature growth when the animal’s body is overheated. Apparently, CSF arginine can be considered as an important factor in the changes in the excitability thresholds of cold and heat-sensitive neurons in the hypothalamus and in the formation of the “setpoint” of body temperature regulation during endotoxin fever.

Norepinephrine Turnover in the Left Ventricle of Subtotally Nephrectomized Rats

Increased activity of the sympathetic nervous system (SNS) has been proposed as a risk factor for increased cardiovascular mortality in patients with chronic kidney disease (CKD). Information on the activity of cardiac sympathetic innervation is non-homogeneous and incomplete. The aim of our study was to evaluate the tonic effect of SNS on heart rate, norepinephrine turnover and direct and indirect effects of norepinephrine in left ventricles of subtotally nephrectomized rats (SNX) in comparison with sham-operated animals (SHAM). Renal failure was verified by measuring serum creatinine and urea levels. SNX rats developed increased heart rates and blood pressure (BP). The increase in heart rate was not caused by sympathetic overactivity as the negative chronotropic effect of metipranolol did not differ between the SNX and SHAM animals. The positive inotropic effects of norepinephrine and tyramine on papillary muscle were not significantly different. Norepinephrine turnover was measured after the administration of tyrosine hydroxylase inhibitor, pargyline, tyramine, desipramine, and KCl induced depolarization. The absolute amount of released norepinephrine was comparable in both groups despite a significantly decreased norepinephrine concentration in the cardiac tissue of the SNX rats. We conclude that CKD associated with renal denervation in rats led to adaptive changes characterized by an increased reuptake and intracellular norepinephrine turnover which maintained normal reactivity of the heart to sympathetic stimulation.

Separate and shared sympathetic outflow to white and brown fat coordinately regulates thermoregulation and beige adipocyte recruitment

White adipose tissue (WAT) and brown adipose tissue (BAT) are innervated and regulated by the sympathetic nervous system (SNS). It is not clear, however, whether there are shared or separate central SNS outflows to WAT and BAT that regulate their function. We injected two isogenic strains of pseudorabies virus, a retrograde transneuronal viral tract tracer, with unique fluorescent reporters into interscapular BAT (IBAT) and inguinal WAT (IWAT) of the same Siberian hamsters to define SNS pathways to both. To test the functional importance of SNS coordinated control of BAT and WAT, we exposed hamsters with denervated SNS nerves to IBAT to 4°C for 16–24 h and measured core and fat temperatures and norepinephrine turnover (NETO) and uncoupling protein 1 (UCP1) expression in fat tissues. Overall, there were more SNS neurons innervating IBAT than IWAT across the neuroaxis. However, there was a greater percentage of singly labeled IWAT neurons in midbrain reticular nuclei than singly labeled IBAT neurons. The hindbrain had ~30–40% of doubly labeled neurons while the forebrain had ~25% suggesting shared SNS circuitry to BAT and WAT across the brain. The raphe nucleus, a key region in thermoregulation, had ~40% doubly labeled neurons. Hamsters with IBAT SNS denervation maintained core body temperature during acute cold challenge and had increased beige adipocyte formation in IWAT. They also had increased IWAT NETO, temperature, and UCP1 expression compared with intact hamsters. These data provide strong neuroanatomical and functional evidence of WAT and BAT SNS cross talk for thermoregulation and beige adipocyte formation.

Energetic responses to cold temperatures in rats lacking forebrain-caudal brain stem connections

Hypothalamic neurons are regarded as essential for integrating thermal afferent information from skin and core and issuing commands to autonomic and behavioral effectors that maintain core temperature (Tc) during cold exposure and for the control of energy expenditure more generally. Caudal brain stem neurons are necessary elements of the hypothalamic effector pathway and also are directly driven by skin and brain cooling. To assess whether caudal brain stem processing of thermal afferent signals is sufficient to drive endemic effectors for thermogenesis, heart rate (HR), Tc, and activity responses of chronic decerebrate (CD) and control rats adapted to 23°C were compared during cold exposure (4, 8, or 12°C) for 6 h. Other CDs and controls were exposed to 4 or 23°C for 2 h, and tissues were processed for norepinephrine turnover (NETO), a neurochemical measure of sympathetic drive. Controls maintained Tc for all temperatures. CDs maintained Tc for the 8 and 12°C exposures, but Tc declined 2°C during the 4°C exposure. Cold exposure elevated HR in CDs and controls alike. Tachycardia magnitude correlated with decreases in environmental temperature for controls, but not CDs. Cold increased NETO in brown adipose tissue, heart, and some white adipose tissue pads in CDs and controls compared with their respective room temperature controls. These data demonstrate that, in neural isolation from the hypothalamus, cold exposure drives caudal brain stem neuronal activity and engages local effectors that trigger sympathetic energetic and cardiac responses that are comparable in many, but not in all, respects to those seen in neurologically intact rats.

Melanocortin-4 receptor mRNA expressed in sympathetic outflow neurons to brown adipose tissue: neuroanatomical and functional evidence

A precise understanding of neural circuits controlling lipid mobilization and thermogenesis remains to be determined. We have been studying the sympathetic nervous system (SNS) contributions to white adipose tissue (WAT) lipolysis largely in Siberian hamsters. Central melanocortins are implicated in the control of the sympathetic outflow to WAT, and, moreover, the melanocortin 4 receptors (MC4-R) appear to be principally involved. We previously found that acute third ventricular melanotan II (MTII; an MC3/4-R agonist) injections increase sympathetic drive (norepinephrine turnover) to interscapular brown adipose tissue (IBAT) and IBAT temperature. Here we tested whether MC4-R mRNA is expressed in IBAT SNS outflow neurons using in situ hybridization for the former and injections of the transneuronal viral retrograde tract tracer, pseudorabies virus (PRV) into IBAT, for the latter. Significant numbers of double-labeled cells for PRV and MC4-R mRNA were found across the neuroaxis (mean of all brain sites ∼60%), including the hypothalamic paraventricular nucleus (PVH; ∼80%). Acute parenchymal MTII microinjections into the PVH of awake, freely-moving hamsters, using doses below those able to increase IBAT temperature when injected into the third ventricle, increased IBAT temperature for as long as 4 h, as measured by temperature transponders implanted below the tissue. Collectively, these data add significant support to the view that central melanocortins are important in controlling IBAT thermogenesis via the SNS innervation of this tissue, likely through the MC4-Rs.

Effects of central or peripheral leptin administration on norepinephrine turnover in defined fat depots

Leptin preserves lean tissue but decreases adipose tissue by increasing lipolysis and/or inhibiting lipogenesis. The sympathetic nervous system (SNS) is a primary regulator of lipolysis, but it is not known if leptin increases norepinephrine turnover (NETO) in white adipose tissue. In this study, we examined the effect of leptin administered either as a chronic physiological dose (40 μg/day for 4 days from ip miniosmotic pumps) or as an acute injection in the third ventricle (1.5 μg injected two times daily for 2 days) on NETO and the size of brown and white fat depots in male Sprague Dawley rats. NETO was determined from the decline in tissue norepinephrine (NE) during 4 h following administration of the NE synthesis inhibitor α-methyl-para-tryrosine. The centrally injected leptin-treated animals demonstrated more dramatic reductions in food intake, body weight, and fat pad size and an increase in NETO compared with the peripherally infused animals. Neither route of leptin administration caused a uniform increase in NETO across all fat pads tested, and in both treatment conditions leptin decreased the size of certain fat pads independent of an increase in NETO. Similar discrepancies in white fat NETO were found for rats pair fed to leptin-treated animals. These results demonstrate that leptin acting either centrally or peripherally selectively increases sympathetic outflow to white fat depots and that a leptin-induced change in fat pad weight does not require an increase in NETO.