Modulatory action of noradrenergic system on spinal motoneurons in humans

PREVIOUS findings in animals demonstrated that the noradrenergic coeruleosp inal system exerts a tonic facilitation on spinal reflexes and that activat ion of alpha(2)-autoinhibitory receptors can be responsible for a dis-facil itation of the spinal activity. To investigate this issue further, we exami ned whether this system is also involved in descending facilitatory control of spinal motoneurons in healthy humans. The H-reflex technique was utiliz ed to assay the motoneuronal excitability. The ratio between the maximal re flex response (H) and maximal direct response (M) was determined in each su bject and was calculated at 10min intervals before and after i.v. administr ation of the alpha(2)-agonist clonidine (0.5 mu g/kg). In all subjects a ma rked decrease of the H/M ratio, due to depression of the H response, occurr ed 10min following the clonidine injection and reached its maximum within 3 0min. No significant changes of blood pressure values were provoked by drug injections. These results suggest that an autoinhibitory action may be ind uced by alpha(2)-receptor activation of locus coeruleus neurons in humans, and that this device may serve as a mechanism for a myotonolytic action on spinal motoneurons. (C) 1999 Lippincott Williams & Wilkins.