Modulation of the tonic stretch reflex by monoamines.

The effects of L-DOPA and 5-HTP on the tonic stretch reflex (TSR) in the decerebrate rat were studied. L-DOPA facilitated the TSR in a dose-dependent manner. The facilitation of the TSR was blocked by pimozide. A sensitive electromygraphic (EMG) technique capable of recording single motor unit discharges was used. The EMG results suggest that gamma motoneuron sensitivity was increased to a greater degree than alpha motoneuron sensitivity during the facilitation by L-DOPA. The L-DOPA-induced facilitation persisted in animals partly depleted of 5-hydroxytryptamine (5-HT) by reserpine, p-chlorophenylalanine or 5,6-dihydroxytryptamine. 5-HTP inhibited the TSR in a dose-dependent manner. It is concluded that DA not 5-HT is the amine which normally mediates facilitation of the TSR after L-DOPA and that gamma motoneuron activation is more likely to be involved to a greater degree than alpha motoneuron activation in the neural mechanisms of the facilitation.