Reduced rearing temperature augments responses in sympathetic outflow to brown adipose tissue

Abstract

Sympathetic outflow to brown adipose tissue (BAT) contributes to both thermoregulation and energy expenditure in rats through regulation of BAT thermogenesis. Acute cold exposure in mature animals augments BAT thermogenesis; however, the enhanced BAT thermogenic response returns to normal shortly after cessation of the cold exposure. In this study, we sought to determine whether cold exposure in early neonatal life could induce enhanced responses in the sympathetic outflow to BAT and whether this altered sympathetic regulation would be sustained after the cold stimulus was removed. BAT sympathetic nerve activity (SNA) was recorded in urethane-chloralose-anesthetized, artificially ventilated rats that were raised from birth in either 18 or 30 degrees C environments and then, at 8 weeks of age, were maintained in 23 degrees C for at least 4 weeks. An acute hypothermic stimulus, disinhibition of a brainstem thermogenic network in the raphe pallidus, or electrical stimulation in this raphe site produced increases in BAT SNA that were twice as great in rats reared at 18 degrees C as in those reared at 30 degrees C. The norepinephrine content of the interscapular BAT (IBAT) and the number of sympathetic ganglion cells projecting to interscapular BAT were 70% greater in the 18 degrees C-reared rats. We conclude that neonatal exposure to a cold environment induces a permanent developmental alteration in the capacity for sympathetic stimulation of BAT thermogenesis that may be mediated, in part, by a greater number of sympathetic ganglion cells innervating BAT in cold-reared animals.

Fig. 1.

Comparison of the effects of acute hypothermia on sympathetic nerve activity to brown adipose tissue (BAT SNA) in animals raised at 18 and 30°C. A, Arterial pressure (AP), heart rate (HR), BAT SNA, and the averaged power spectrum of BAT SNA (BAT SNA PWR) in a rat raised at 18°C under normothermic, control conditions (left panel) and after core temperature had been lowered to 33.8°C (right panel). Note differences in scale factorfor BAT SNA PWR. B, Increase in BAT SNA when body temperature was lowered to 35.2°C in an animal raised at 30°C. Note differences in scale factor for BAT SNA PWR between panels A and B. C, Reciprocal relationship between falling core temperature and reflex rise in BAT SNA during the hypothermia. Top trace: logistic curve fit for data in A from an animal raised at 18°C indicated a reflex gain (slope of the linear portion of the curve) of 1321 BAT SNA PWR units/°C and an operating point (core temperature at the center of the linear portion of the curve) of 34.2°C. Bottom trace: curve fitting for data inB from an animal raised at 30°C indicated a reflex gain of 229 BAT SNA PWR units/°C and an operating point of 35.7°C. Horizontal calibration represents 1 sec for the top three traces in A and B, and the vertical calibration represents 50μV for the BAT SNAtraces in panels A andB.

Fig. 2.

Comparison of the effects of disinhibition of raphe pallidus (RPa) neurons on the sympathetic nerve activity to brown adipose tissue (BAT SNA) in animals raised at 18 and 30°C. A, Arterial pressure (AP), heart rate (HR), BAT SNA, and the averaged power spectrum of BAT SNA (BAT SNA PWR) in a rat raised at 18°C, under normothermic, control conditions (left) and 6 min after bicuculline was microinjected (60 nl, 500 μm) into the RPa (right). Note differences in the scale for BAT SNA PWR. B, Same traces as in A in an animal raised at 30°C under normothermic, control conditions (right) and 7 min after a bicuculline microinjection into RPa (left). Note differences in scale factor for BAT SNA PWR between Aand B. Horizontal calibration represents 1 sec for thetop three traces in A andB, and vertical calibration represents 50 μV for the BAT SNA traces in A and B.

Fig. 3.

Comparison of averaged excitatory potentials evoked on a sympathetic nerve to BAT SNA by electrical stimulation in raphe pallidus (RPa) in an animal raised at 18°C (heavy trace) and an animal raised at 30°C (light trace). Traces are peristimulus averages of the responses in BAT SNA to 10 stimuli consisting of paired pulses, 6 msec interval, 100 μA, 0.4 Hz. Calibration: 50 μV; 100 msec.

Fig. 4.

Locations of electrical stimulation and bicuculline microinjection sites in the rostral raphe pallidus (RPa). A, Histological section through the rostral RPa containing fast green dye (arrow) deposited from the tip of a bicuculline microinjection pipette.B, Representative bicuculline microinjection sites from nine animals raised at 18°C (●) and seven animals raised at 30°C (▵), plotted on an atlas (Paxinos and Watson, 1986) drawing at interaural −2.30 mm. Microinjection sites in remaining animals were omitted for clarity. Pr, prepositus hypoglossal nulceus;RMg, nucleus raphe magnus; Sol, nucleus of the solitary tract; LPGi, lateral paragigantocellular nucleus;7, facial nucleus; py, pyramidal tract.

Fig. 5.

Comparison of the retrograde labeling of middle cervical ganglion cells in an animal raised at 18°C (A) and an animal raised at 30°C (B) after fast blue dye deposits in the ipsilateral IBAT. Histological sections have been illuminated to reveal fast blue fluorescence in ganglion cells innervating IBAT.Insets at bottom right show low-power images of these sections. The section from the middle cervical ganglion of the animal raised at 18°C contained 303 retrogradely labeled neurons (A), whereas that from the same site in an animal raised at 30°C contained 117 neurons with fast blue fluorescence (B). Scale bar: high-power images, 300 μm; low-power images, 2400 μm.