Effect of Diet and Cold Exposure on Norepinephrine Turnover in Brown Adipose Tissue of the Rat

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Abstract
Brown adipose tissue (BAT) is an important site of adaptive changes in thermogenesis in the rat. The sympathetic nervous system, which richly supplies BAT, is thought to play an important role in the regulation of BAT thermogenesis because catecholamines stimulate and beta adrenergic blocking agents inhibit oxygen consumption in this tissue. The present studies were carried out to assess directly sympathetic activity in BAT in response to cold exposure and to changes in dietary intake, both of which alter heat production in the rat. Sympathetic activity was determined from the rate of norepinephrine (NE) turnover in interscapular brown adipose tissue (IBAT) after preliminary experiments validated the use of NE turnover techniques in IBAT. Acute exposure to 4°C increased NE turnover in IBAT 4- to 12-fold compared with ambient temperature controls, depending upon the interval over which the turnover measurement was made, while in the heart NE turnover doubled in response to the same cold stimulus. In animals exposed to cold continuously for 10 d before study, NE turnover measurements in IBAT and in the heart were elevated comparably to those obtained during acute exposure. Alterations in feeding were also associated with changes in NE turnover in IBAT. Fasting for 2 d decreased NE turnover in IBAT (-35% from 29.2±4.2 ng NE/h to 18.9±5.9) and in heart (-52%). In animals fed a “cafeteria” diet, a model of voluntary overfeeding in the rat, NE turnover was increased in both IBAT (+108% from 24.8±4.5 ng NE/h to 51.7±6.8) and heart (+66%). Because ganglionic blockade exerted a greater effect on NE turnover in IBAT in cafeteria-fed rats than in controls, the increase in NE turnover in IBAT with this overfeeding regimen reflects enhanced central sympathetic outflow. Thus NE turnover techniques can be satisfactorily applied to the assessment of sympathetic nervous system activity in IBAT. The experiments reported here demonstrate changes in sympathetic activity in IBAT that parallel known adaptive changes in heat production in the rat. These studies, therefore, support the concept that the increased thermogenesis of chronic cold exposure and of cafeteria feeding occur by similar mechanisms and imply an important role for the sympathetic nervous system, mediated in part through BAT, in the regulation of energy balance in the rat.