DOPAMINERGIC MODULATION OF SPINAL NEURONS AND SYNAPTIC POTENTIALS IN THE
LAMPREY SPINAL CORD.
Kemnitz, Christopher P.
Department of Biology, Marquette University, Milwaukee Wisconsin,
APStracts 3:0240N, 1996.
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 in
order 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 following 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
following bath application of 10 śM dopamine. The durations of tetrodotoxin-
resistant, N-methyl-D-aspartate-induced membrane potential oscillations in
lamprey spinal motoneurons were increased following bath application of 1-100
śM dopamine, due perhaps to reduced calcium entry and thus reduced Ca++-
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 śM dopamine. Polysynaptic EPSPs 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 śM). Monosynaptic IPSPs in CC interneurons
elicited by stimulation of ipsilateral lateral interneurons, however, showed
no change following 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.
Received 1 February 1996; accepted in final form 12 September 1996.
APS Manuscript Number J81-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 November 1996