Suppression by spinal alpha-2 agonists of motor and autonomic responses evoked by low- and high-intensity thermal stimuli.

In halothane (0.9%)-anesthetized rats, the immersion of the tail in 52.5 degrees C or 60 degrees C water for 15 sec resulted in a short-latencied tail jerk and a subsequent tachycardia and elevation in blood pressure. The i.t. administration of the alpha-2 agonists, dexmedetomidine (DMET), clonidine (CLON) and ST-91, resulted in a dose-dependent increase in the somatomotor reflex latency and a reduction in the magnitude of the increase in the magnitude of the cardiovascular response, with the ordering of activity on all endpoints at all temperatures being: DMET greater than CLON greater than or equal to ST-91. Increasing the stimulus intensity from 52.5 to 60 degrees C resulted in right shifts in the dose-response curves for all agents with the magnitude of shift being: 1) greatest on the blood pressure and least on the tail flick, and 2) greatest for ST-91 and CLON and least for DMET. Thus, the dose ratio (ED50 at 60 degrees C/ED50 at 52.5 degrees C) for these three agents on the tail flick and blood pressure response was, respectively, 1.1 and 1.8 for DMET, 1.1 and 4.3 for CLON and 4.8 and 9.7 for ST-91 (mean +/- S.E.). On resting blood pressure, i.t. DMET and CLON produced a dose-dependent hypotension and bradycardia. ST-91 had no consistent dose-dependent effect.(ABSTRACT TRUNCATED AT 250 WORDS)