A B S T R A C T 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 cafeteriafed 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

The decrease in resting oxygen consumption induced by starvation was found to occur not only in euthyroid rats but also in hypothyroid and even in hypothyroid animals treated with triiodothyronine. Furthermore, the effectiveness of triiodothyronine was decreased when given to hypothyroid animals.

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