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

In decerebrated rabbits, reflexes were evoked in medial gastrocnemius motoneurones by mechanical stimulation of the heel, using four pinch strengths from 183 (innocuous) to 4577 (noxious) mN. The alpha (2)-adrenoceptor antagonist idazoxan (1-156 mug intrathecal (i.th.)) significantly increased responses to pinch strengths of 607 mN and above. Subsequent administration of the a, adrenoceptor selective antagonist prazosin (200 mug i.th.) decreased reflexes to 4577 mN pinches but had no other significant effects. The opioid antagonist (-)-quadazocine (25 mug i.th.) caused no further changes in reflexes. Spinal section at Ll in the presence of this drug combination enhanced gastrocnemius responses to 183 and 607 mN stimuli, had no effect on reflexes to 1866 mN and significantly decreased responses to 4577 mN pinches. These data confirm that reflexes evoked by 'natural' stimulation of heel mechanoreceptors 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 Science Ireland Ltd.