Acetylcholine in the lamprey spinal cord: Cellular, synaptic and network effects
Abstract
Acetylcholine (ACh) was found here to exert powerful modulation of locomotor network activity, cellular properties and synaptic strength in the spinal cord of lamprey (Ichthyomyzon unicuspis ). Addition of ACh (200 μM) significantly reduced the cycle period, burst proportion, and intersegmental phase lag of fictive swimming. Both nicotinic and muscarinic receptors were involved, as addition of either agonist produced similar changes. Furthermore cholinergic modulation of fictive swimming was endogenous, as addition of the cholinesterase inhibitor, eserine (20 μM) brought about similar changes in fictive swimming as ACh, and cholinergic antagonists reversed these changes beyond control levels, suggesting that fictive swimming is normally under cholinergic influence. During sharp electrode intracellular recording, individual neurons in the quiescent lamprey spinal cord responded to local ACh application (2-10 MM). Most neurons depolarized (47/69) with an associated decrease in input resistance (24/42) which persisted in the presence of tetrodotoxin (1-3 μM) and scopolamine (10 μM) but not mecamylamine (20 μM), indicating a direct nicotinic response. Postsynaptic potentials (psps), elicited either through paired intracellular recordings of the post-synaptic cell and presynaptic cell or axon, or stimulation of the hemicord were reduced by local application of muscarinic agonists. Reduction of compound psps and unitary excitatory psps by muscarinic agonists was not generally accompanied by changes in membrane potential, input resistance or response to glutamate in the postsynaptic cell, suggesting muscarinic effects are presynaptic. To assess contribution of motoneurons to cholinergic modulation, ventral roots were stimulated to antidromically fire the motoneurons. During antidromic motoneuron stimulation, potentials were observed in neurons that were depolarizing (18/64), hyperpolarizing (12/64), or both (13/64). A portion of the depolarization was due to passive current spread, possibly through gap junctions. In addition there was a chemical component that was sensitive to both glycinergic and nicotinic agents, suggesting the presence of a glycinergic cell that is excited to threshold by motoneurons. Endogenous cholinergic neuromodulation in the lamprey spinal cord could arise through motoneuron feedback, or also from non-motoneuron cholinergic cells. Retrograde labeling of motoneurons combined with choline acetyltransferase immunohistochemistry revealed cells positive for the cholinergic marker which were not motoneurons, a novel population of lamprey spinal neurons.
This paper has been withdrawn.