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 microg intrathecal (i.th.) significantly increased responses to pinch strengths of 607 mN and above. Subsequent administration of the alpha(1) adrenoceptor selective antagonist prazosin (200 microg i.th.) decreased reflexes to 4577 mN pinches but had no other significant effects. The opioid antagonist (-)-quadazocine (25 microg i.th.) caused no further changes in reflexes. Spinal section at L1 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.
Clarke
R W
RW
Division of Animal Physiology, School of Biosciences, Sutton Bonington Campus, LE12 5RD, Loughborough, UK. Robert.Clarke@Nottingham.ac.uk
Harris
J
J
Houghton
A K
AK
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001392:Azocines / Q000494:pharmacology; D001794:Blood Pressure; D003655:Decerebrate State / Q000503:physiopathology*; D005260:Female; D019329:Idazoxan / Q000494:pharmacology; D007278:Injections, Spinal; D008297:Male; D008465:Mechanoreceptors / Q000502:physiology; D009046:Motor Neurons / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation*; D009292:Narcotic Antagonists / Q000494:pharmacology; D009433:Neural Inhibition / Q000502:physiology; D009475:Neurons, Afferent / Q000502:physiology; D009638:Norepinephrine / Q000502:physiology*; D010812:Physical Stimulation; D011224:Prazosin / Q000494:pharmacology; D011817:Rabbits; D018341:Receptors, Adrenergic, alpha-2 / Q000502:physiology; D012018:Reflex / Q000187:drug effects / Q000502:physiology*; D013116:Spinal Cord / Q000166:cytology / Q000187:drug effects / Q000502:physiology
D000317:Adrenergic alpha-Antagonists; D001392:Azocines; D009292:Narcotic Antagonists; D018341:Receptors, Adrenergic, alpha-2; D009638:Norepinephrine; C035969:quadazocine; D011224:Prazosin; D019329:Idazoxan