1. To investigate the characteristics of efferent projections from the preoptic area for the control of non-shivering thermogenesis, we tested the effects of thermal or chemical stimulation, and transections of the preoptic area on the activity of interscapular brown adipose tissue in cold-acclimated and non-acclimated anaesthetized rats. 2. Electrical stimulation of the ventromedial hypothalamic nucleus (VMH) elicited nonshivering thermogenesis in the brown adipose tissue (BAT); warming the preoptic area to 41.5°C completely suppressed the thermogenic response. 3. Injections of D,L-homocysteic acid (DLH; 0.5 mM, 0.3 μl) into the preoptic area also significantly attenuated BAT thermogenesis, whereas injections of control vehicle had no effect. 4. Transections of the whole hypothalamus in the coronal plane at the level of the paraventricular nucleus induced rapid and large rises in BAT and rectal temperatures. This response was not blocked by pretreatment with indomethacin. The high rectal and BAT temperatures were sustained more than 1 h, till the end of the experiment. Bilateral knife cuts that included the medial forebrain bundle but not the paraventricular nuclei elicited similar rises in BAT and rectal temperatures. Medial knife cuts had no effect. 5. These results suggest that warm-sensitive neurones in the preoptic area contribute a larger efferent signal for non-shivering thermogenesis than do cold-sensitive neurones, and that the preoptic area contributes a tonic inhibitory; input to loci involved with non-shivering thermogenesis. This efferent inhibitory signal passes via lateral, but not medial, hypothalamic pathways.
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