CONTROL OF LAMPREY LOCOMOTOR NEURONS BY COLOCALIZED MONOAMINE TRANSMITTERS

NEURONS in the central nervous system (CNS) often store more than one neurotransmitter(1,2), but as yet the functional significance of this type of coexistence is poorly understood. 5-Hydroxytryptamine (5-HT) m odulates calcium-dependent K+ channels (K-Ca) responsible for the post spike afterhyperpolarization in different regions of the CNS3,4. In la mprey, 5-HT neurons control apamine-sensitive K-Ca channels in spinal locomotor network interneurons(4-6), thereby in addition regulating th e duration of locomotor bursts(7,8). We report here that these spinal 5-HT neurons also contain dopamine. Like 5-HT, dopamine causes a reduc tion of the afterhyperpolarization, but in this case it is due to a re duction of calcium entry during the action potential, which results in a reduced activation of Kc,. 5-HT and dopamine are both released from these midline neurons, and both reduce the afterhyperpolarization thr ough two distinctly different, but complementary cellular mechanisms. The net effect of dopamine (10-100 mu M) on the locomotor network is s imilar to that of 5-HT, and the effects of dopamine and 5-HT are addit ive at the network level.