Neuropeptide Y suppressed the T-3-induced rise in brown fat mitochondrial respiration in hypothyroid rats

It has been postulated that a cold-induced decrease in hypothalamic neurope ptide Y (NPY) might disinhibit the sympathetic outflow that activates brown adipose tissue (BAT) thermogenesis. The present work, has assessed the int erscapular BAT NPY and norepinephrine (NE) concentrations 7 days after surg ical excision of the sympathetic nerves (Sx) of each BAT lobe in rats expos ed to 4 degreesC or 24 degreesC during 24 h. In addition, the effects of NP Y treatment on BAT oxygen (O-2) consumption in normal and T-3-treated hypot hyroid rats was determined. Sprague-Dawley rats weighing 220-240 g were use d. BAT was removed and homogenized in 0.1 M HCL BAT NPY content at 24 degre esC was 109 +/- 19 pg/mg protein. Sx decreased this value significantly (P < 0.001) and cold increased it by two-fold (P < 0.001). Neither cold nor Sx altered hypothalamic NPY. BAT O-2 consumption was depressed by NPY treatme nt (P < 0.001) and increased by NE (P < 0.001). In euthyroid Sx rats, NPY d ecreased O-2 consumption (P < 0.001). NPY failed to alter O-2 consumption i n hypothyroid rats and sharply decreased it in T3-treated rats. The results show that NPY blocked the effects of T-3 on BAT O-2 consumption, and that cold-induced activation of the thermogenic process did not produce measurab le changes in hypothalamic NPY.