Effects of noradrenaline and 5-hydroxytryptamine on spinal Ia afferent terminations.

When administered microelectrophoretically, noradrenaline (NA) and 5-hydroxytryptamine (5-HT) increased the thresholds of the terminal portions of extensor muscle Ia afferents stimulated extracellularly near lumbar motoneurons of anesthetized cats. This effect, and the concomitant increase in the electrical resistance of the extracellular medium near the orifices of multibarrel micropipettes, could be reversibly altered by ouabain. The results suggest further evidence is required of a direct effect of these amines at transmitter-related receptors on Ia terminations since the observed increase in threshold may be indirect, resulting from the sodium-dependent uptake of the administered amines by neurons and glia.