Influence of cold and warm acclimation on neuronal responses in the lower brain stem.

Neurons in two lower brain stem areas, the nucleus raphe magnus and the subcoeruleus region, have been shown to be part of the thermoafferent system. It is concluded from microcut experiments in unanaesthetized guinea pigs that inhibition of shivering caused by nucleus raphe magnus stimulation is mediated partly by ascending and partly by descending efferents of the nucleus raphe magnus. Electrical stimulation of the subcoeruleus area caused excitatory metabolic responses. Interruption of the ascending efferents of the subcoeruleus area did not prevent the metabolic activation. It is concluded that the excitatory responses are partly mediated by descending efferents of the subcoeruleus area. The descending pathways project mainly to motoneurone pools and to dorsal horn cells. In cold-acclimated guinea pigs, the average maximum activity of bell-shaped subcoeruleus cold-responsive units was reduced significantly in comparison with cold-responsive neurons in animals acclimated to normal room temperature. Furthermore, peak activity of warm-responsive units in the nucleus raphe magnus was larger in cold-acclimated animals than in animals acclimated to normal room temperature. These neuronal changes may contribute via descending lower loops and via ascending upper loops to long-term slope reduction of metabolic cold defence and shivering threshold displacements.