The variation in action of excitatory amino acids in relation to distance of iontophoretic application to spinal motoneurones.

Intracellular recordings were made from lumbosacral motoneurones of barbiturate-anaesthetized cats. DL-homocysteate (DLH) and L-glutamate were iontophoresed extracellularly over a range of distances from the impaled motoneurone. Movement of the iontophoretic electrode unit was controlled by a micromanipulator which was advanced independently of that moving the intracellular electrode. Depolarizations to DLH were first detected at a greater distance from the impaled motoneurones (mean, 383 micron) than depolarizations to L-glutamate (mean, 165 micron). As the point of application approached the soma of the motoneurone, depolarizations developed more rapidly, were larger and the latent period of the L-glutamate depolarization became shorter. Dendritic 'hot-spots' of the depolarizing action of L-glutamate were not detected.