Effect of Amphetamine on Spinal Reflexes

Abstract

L-DOPA (L-3,4-dihydroxyphenylalanine), precursor of catecholamines (adrenaline, noradrenaline and dopamine)¹, and DL-5-hydroxytryptophan (5-HTP)², precursor of 5-hydroxytryptamine, have recently been found to influence spinal reflexes. These drugs applied intravenously cause a pronounced increase of the flexor reflex evoked by pinching the skin. An electrophysiological investigation revealed that these drugs depress the short latency transmission of single volleys from the flexor reflex afferents (FRA, that is, Group II and III muscle afferents, cutaneous and high threshold joint afferents) to motoneurones, primary afferents and ascending pathways³. The depression of this short-latency FRA pathway accounts for the disinhibition of the long-latency pathway from the FRA to flexor motoneurones and Ia primary afferents^4,5. It has been suggested that the effect of L-DOPA and at least in part of 5-HTP is accounted for by increased synthesis and release of monoamines at the monoaminergic synaptic terminals in the spinal cord.