Control of lamprey locomotor neurons by colocalized monoamine transmitters.

Neurons in the central nervous system (CNS) often store more than one neurotransmitter, but as yet the functional significance of this type of coexistence is poorly understood. 5-Hydroxytryptamine (5-HT) modulates calcium-dependent K+ channels (KCa) responsible for the postspike afterhyperpolarization in different regions of the CNS. In lamprey, 5-HT neurons control apamine-sensitive KCa channels in spinal locomotor network interneurons, thereby in addition regulating the duration of locomotor bursts. We report here that these spinal 5-HT neurons also contain dopamine. Like 5-HT, dopamine causes a reduction of the afterhyperpolarization, but in this case it is due to a reduction of calcium entry during the action potential, which results in a reduced activation of KCa. 5-HT and dopamine are both released from these midline neurons, and both reduce the afterhyperpolarization through two distinctly different, but complementary cellular mechanisms. The net effect of dopamine (10-100 microM) on the locomotor network is similar to that of 5-HT, and the effects of dopamine and 5-HT are additive at the network level.