Comparison between two rat sympathetic pathways activated in cold defense

In cold defense and fever, activity increases in sympathetic nerves supplying both tail vessels and interscapular brown adipose tissue (iBAT). These mediate cutaneous vasoconstrictor and thermogenic responses, respectively, and both depend upon neurons in the rostral medullary raphé. To examine the commonality of brain circuits driving these two outflows, sympathetic nerve activity (SNA) was recorded simultaneously from sympathetic fibers in the ventral tail artery (tail SNA) and the nerve to iBAT (iBAT SNA) in urethane-anesthetized rats. From a warm baseline, cold-defense responses were evoked by intermittently circulating cold water through a water jacket around the animal's shaved trunk. Repeated episodes of trunk skin cooling decreased core (rectal) temperature. The threshold skin temperature to activate iBAT SNA was 37.3 +/- 0.5 degrees C (n = 7), significantly lower than that to activate tail SNA (40.1 +/- 0.4 degrees C; P < 0.01, n = 7). A fall in core temperature always strongly activated tail SNA (threshold 38.3 +/- 0.2 degrees C, n = 7), but its effect on iBAT SNA was absent (2 of 7 rats) or weak (threshold 36.9 +/- 0.1 degrees C, n = 5). The relative sensitivity to core vs. skin cooling (K-ratio) was significantly greater for tail SNA than for iBAT SNA. Spectral analysis of paired recordings showed significant coherence between tail SNA and iBAT SNA only at 1.0 +/- 0.1 Hz. The coherence was due entirely to the modulation of both signals by the ventilatory cycle because it disappeared when the coherence spectrum was partialized with respect to airway pressure. These findings indicate that independent central pathways drive cutaneous vasoconstrictor and thermogenic sympathetic pathways during cold defense.