CENTRAL CHOLINERGIC TRANSMISSION AT MAUTHNER FIBER SYNAPSES
DAY, JOHN WEST
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The hatchetfish Mauthner fiber is presynaptic to 8-14 large axons in the medulla; the large ("giant") central synapses formed by these fibers have been thoroughly studied. Experiments were undertaken to: (1) show that transmission is cholinergic at teleost Mauthner fiber synapses, and (2) to compare transmission at the hatchetfish giant synapse with that at peripheral cholinergic synapses.;Autoradiography using radiolabeled (alpha)-bungarotoxin and histochemistry using acetylcholinesterase strains were the two anatomical techniques employed to demonstrate cholinergic transmission. The autoradiography showed dramatic labeling of all identified hatchetfish giant synapses. There was no labeling of synapses if the (alpha)-bungarotoxin was introduced together with d-tubocurarine. The acetylcholinesterase histochemistry showed large deposits of stain at all identified Mauthner fiber synapses in hatchetfish, goldfish, and killifish. Stain deposition was not diminished by butyrylcholinesterase blockers, but was eliminated by poisoning acetylcholinesterase.;Miniature postsynaptic potentials (MPSPs) from individual giant synapses were averaged: the rise time was 70 (mu)s, and the fall was biphasic with decay constants for the two phases of 280, and 800 (mu)s, (in 25% of the records a third, very slow tail was seen which had a decay constant of 4.2 ms). This shape was identical to that of the postsynaptic currents at this synapse, and indicates that the mechanisms responsible for the biphasic decay operate on individual quanta.;The anatomical results are good evidence that the teleost Mauthner fiber is nicotinic cholinergic. The biphasic decay of MPSPs indicates that transmitter-receptor kinetics at this synapse differ from the kinetics at peripheral nicotinic cholinergic synapses where single exponential decays are seen. These data add to the understanding of the Mauthner fiber, and hold promise for elucidating mechanisms of central nicotinic cholinergic transmission.