Endogenous adrenergic control of reflexes evoked by mechanical stimulationof the heel in the decerebrated rabbit

In decerebrated rabbits, reflexes were evoked in medial gastrocnemius moton eurones by mechanical stimulation of the heel, using four pinch strengths f rom 183 (innocuous) to 4577 (noxious) mN. The alpha (2)-adrenoceptor antago nist idazoxan (1-156 mug intrathecal (i.th.)) significantly increased respo nses to pinch strengths of 607 mN and above. Subsequent administration of t he a, adrenoceptor selective antagonist prazosin (200 mug i.th.) decreased reflexes to 4577 mN pinches but had no other significant effects. The opioi d antagonist (-)-quadazocine (25 mug i.th.) caused no further changes in re flexes. Spinal section at Ll in the presence of this drug combination enhan ced gastrocnemius responses to 183 and 607 mN stimuli, had no effect on ref lexes to 1866 mN and significantly decreased responses to 4577 mN pinches. These data confirm that reflexes evoked by 'natural' stimulation of heel me chanoreceptors are subject to powerful tonic descending inhibition mediated by alpha (2)-adrenoceptors. For the highest strength stimulus, the results of alpha (2) blockade involved enabling of descending facilitation as well as reduction of descending inhibition. (C) 2001 Published by Elsevier Scie nce Ireland Ltd.