Dopaminergic modulation of spinal neurons and synaptic potentials in the lamprey spinal cord

It has been shown previously that dopamine-immunoreactive cells and processes are present in the lamprey spinal cord and that dopamine modulates the cycle period of fictive swimming. The present study was undertaken to further characterize the effects of dopamine on the cellular properties of lamprey spinal neurons and on inhibitory and excitatory postsynaptic potentials to determine how dopaminergic modulation may affect the central pattern generator for locomotion. Dopamine reduced the late afterhyperpolarization (late AHP) following the action potential of motoneurons, and in three types of sensory neurons: dorsal cells, edge cells, and giant interneurons. The late AHP was not reduced in lateral interneurons or CC interneurons, both of which are part of the central motor pattern generating neural network. The reduction of the late AHP in motoneurons, edge cells, and giant interneurons resulted in an increase in firing frequency in response to depolarizing current injection. In the six cell classes examined, no changes were observed in the resting membrane potential, input resistance, rheobase, spike amplitude, or spike duration after application of dopamine. The durations of action potentials broadened by application of tetraethylammonium in motoneurons and of calcium action potentials in dorsal cells and giant interneurons were decreased after bath application of 10 microM dopamine. The durations of tetrodotoxin-resistant, N-methyl-D-aspartate-induced membrane potential oscillations in lamprey spinal motoneurons were increased after bath application of 1-100 microM dopamine, due perhaps to reduced calcium entry and thus reduced Ca(2+)-dependent K+ current responsible for the repolarization of the membrane potential during each oscillation. Polysynaptic inhibitory postsynaptic potentials (IPSPs) elicited in lamprey spinal motoneurons by stimulation of the contralateral half of the spinal cord were reduced by bath application of 10 microM dopamine. Polysynaptic excitatory postsynaptic potentials were not reduced by dopamine. Monosynaptic IPSPs in motoneurons elicited by stimulation of single contralateral inhibitory CC interneurons and single ipsilateral axons were reduced by bath application of dopamine (10 microM). Monosynaptic IPSPs in CC interneurons elicited by stimulation of ipsilateral lateral interneurons, however, showed no change after application of dopamine. The lack of dopaminergic effect on the late AHP of the locomotor network neurons, lateral interneurons and CC interneurons, and the selective reduction of IPSPs from CC interneurons suggest that synaptic modulation may play an important role in dopaminergic modulation of cycle period during fictive swimming in the lamprey.