MULTIPLE ACTIONS OF IONTOPHORETICALLY APPLIED SEROTONIN ON MOTOR-NEURONS IN THE TURTLE SPINAL-CORD IN-VITRO

The effects of focal activation of serotonergic receptors in motorneur ones were investigated in a slice preparation of the turtle spinal cor d. The test response to glutamate evoked from a dendrite by iontophore sis was attenuated by serotonin or 8-hydroxy-dipropyl-aminotetralin (8 -OH-DPAT) applied from an independent pipette within 100 mu m of the g lutamate pipette. This effect was not associated with a conductance ch ange and did not affect glutamate responses evoked more than 100 mu m from the serotonin pipette. The effect of serotonin was not reproduced by H+ ions. Plateau potentials were rarely facilitated by serotonin w hen applied in the dendritic field. Plateau potentials were readily fa cilitated by serotonin applied near the soma. This effect was preceded by attenuation of the slow after-hyperpolarization following an actio n potential. Applied near the soma. serotonin inhibited the response t o a depolarizing current pulse of moderate strength and enhanced the r esponse to a current pulse of high amplitude. These effects were assoc iated with a conductance increase. We conclude that serotonin has spat ially diverse effects on motorneurones.