Analysis of the effects of p-methoxy-phenylethylamine on spinal cord neurones.

1 Para-methoxyphenylethylamine (PMPEA) was applied microiontophoretically onto interneurones and motoneurones in the spinal cords of acute spinal cats anaesthetized with alpha-chloralose. Its effects were compared with those of noradrenaline (NA) and 5-hydroxytryptamine (5-HT). 2 PMPEA had effects on interneurones which were similar to those of NA and/or 5-HT; its action was predominantly depressant, and it rarely affected interneurones which could not be influenced by NA or 5-HT. 3 The actions of PMPEA on interneurones excited by electrical stimulation of leg nerves showed that the population of interneurones influenced by the drug coincides with the population affected by NA and 5-HT and by intravenously administered PMPEA. 4 Renshaw cells, which are depolarized by intravenous PMPEA, were hyperpolarized by micoiontophoretically applied PMPEA. 5 Alpha motoneurones, which are depolarized by intravenous PMPEA, were hyperpolarized by micoiontophoretically applied PMPEA. Antidromic firing of the cells could be blocked by PMPEA. 6 The differences between the effects of intravenous infusion and the iontophoretic application of PMPEA upon motoneurones is most easily explained by inhibition of interneurones and a concomitant disinhibition of motoneurones. A similar mechanism may also account for the different effects seen with intravenous and iontophoretic application of PMPEA on Renshaw cells.