EFFERENT PROJECTION FROM THE PREOPTIC AREA FOR THE CONTROL OF NONSHIVERING THERMOGENESIS IN RATS

1. To investigate the characteristics of efferent projections from the preoptic area for the control of non-shivering thermogenesis, we test ed the effects of thermal or chemical stimulation, and transections of the preoptic area on the activity of interscapular brown adipose tiss ue in cold-acclimated and non-acclimated anaesthetized rats. 2. Electr ical stimulation of the ventromedial hypothalamic nucleus (VMH) elicit ed nonshivering thermogenesis in the brown adipose tissue (BAT); warmi ng the preoptic area to 41.5 degrees C completely suppressed the therm ogenic response. 3. Injections of D,L-homocysteic acid (DLH; 0.5 mM, 0 .3 mu l) into the preoptic area also significantly attenuated BAT ther mogenesis, whereas injections of control vehicle had no effect. 4. Tra nsections of the whole hypothalamus in the coronal plane at the level of the paraventricular nucleus induced rapid and large rises in BAT an d rectal temperatures. This response was not blocked by pretreatment w ith indomethacin. The high rectal and BAT temperatures were sustained more than 1 h, till the end of the experiment. Bilateral knife cuts th at included the medial forebrain bundle but not the paraventricular nu clei elicited similar rises in BAT and rectal temperatures. Medial kni fe cuts had no effect. 5. These results suggest that warm-sensitive ne urones in the preoptic area contribute a larger efferent signal for no n-shivering thermogenesis than do cold-sensitive neurones, and that th e preoptic area contributes a tonic inhibitory input to loci involved with non-shivering thermogenesis. This efferent inhibitory signal pass es via lateral, but not medial, hypothalamic pathways.