Activation of thermogenesis of brown fat in rats by baclofen.

Injection of baclofen (0.5-5 micrograms) into the ventromedial hypothalamus (VMH) of anaesthetized rats produced marked increases in the temperature (over 2 degrees C) and thermogenic activity of brown adipose tissue (BAT). These effects were abolished by ganglionic or beta-adrenergic blockade, or denervation of the tissue, but unaffected by hypophysectomy, adrenalectomy or vagotomy. Injections into the hypothalamus close to, but outside the ventromedial hypothalamus did not affect brown adipose tissue. Intravenous administration of baclofen produced similar increases in the temperature of brown adipose tissue, but at larger doses (50-1000 micrograms), and a subcutaneous injection stimulated the metabolic rate in conscious rats. Chronic treatment with baclofen suppressed weight gain and stimulated activity of brown adipose tissue without affecting food intake. These effects of baclofen, which were not mimicked by injections of gamma-aminobutyric acid (GABA), indicate a novel action of baclofen in the ventromedial hypothalamus, leading to marked increases in metabolic rate and body temperature by stimulating sympathetic outflow to brown fat.