Multiple actions of iontophoretically applied serotonin on motorneurones in the turtle spinal cord in vitro.

The effects of focal activation of serotonergic receptors in motorneurones were investigated in a slice preparation of the turtle spinal cord. The test response to glutamate evoked from a dendrite by iontophoresis was attenuated by serotonin or 8-hydroxy-dipropyl-aminotetralin (8-OH-DPAT) applied from an independent pipette within 100 microns of the glutamate pipette. This effect was not associated with a conductance change and did not affect glutamate responses evoked more than 100 microns from the serotonin pipette. The effect of serotonin was not reproduced by H+ ions. Plateau potentials were rarely facilitated by serotonin when applied in the dendritic field. Plateau potentials were readily facilitated by serotonin applied near the soma. This effect was preceded by attenuation of the slow after-hyperpolarization following an action potential. Applied near the soma, serotonin inhibited the response to a depolarizing current pulse of moderate strength and enhanced the response to a current pulse of high amplitude. These effects were associated with a conductance increase. We conclude that serotonin has spatially diverse effects on motorneurones.