Thermoregulatory control of sympathetic fibres supplying the rat's tail.

We investigated the thermoregulatory responses of sympathetic fibres supplying the tail in urethane-anaesthetised rats. When skin and rectal temperatures were kept above 39 degrees C, tail sympathetic fibre activity was low or absent. When the trunk skin was cooled episodically by 2-7 degrees C by a water jacket, tail sympathetic activity increased in a graded fashion below a threshold skin temperature of 37.8 +/- 0.6 degrees C, whether or not core (rectal) temperature changed. Repeated cooling episodes lowered body core temperature by 1.3-3.1 degrees C, and this independently activated tail sympathetic fibre activity, in a graded fashion, below a threshold rectal temperature of 38.4 +/- 0.2 degrees C. Tail blood flow showed corresponding graded vasoconstrictor responses to skin and core cooling, albeit over a limited range. Tail sympathetic activity was more sensitive to core than to trunk skin cooling by a factor that varied widely (24-fold) between animals. Combined skin and core cooling gave additive or facilitatory responses near threshold but occlusive interactions with stronger stimuli. Unilateral warming of the preoptic area reversibly inhibited tail sympathetic activity. This was true for activity generated by either skin or core cooling. Single tail sympathetic units behaved homogeneously. Their sensitivity to trunk skin cooling was 0.3 +/- 0.08 spikes s(-1) degrees C(-1) and to core cooling was 2.2 +/- 0.5 spikes s(-1) degrees C(-1). Their maximum sustained firing rate in the cold was 1.82 +/- 0.35 spikes s(-1).