Neuropharmacological analysis of synaptic transmission in the Lorenzinian ampulla of the skate Raja clavata |
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| Authors: | G Akoev G N Andrianov T Szabo B Bromm |
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| Institution: | (1) Pavlov Institute of Physiology of the Academy of Sciences of the USSR, 199034 Leningrad, USSR;(2) Departement de Neurophysiologie Sensorielle, Laboratoire de Physiologie Nerveuse, F-91198 Gif-sur-Yvette Cedex, France;(3) Present address: Institute of Physiology, University Hospital Eppendorf, W-2000, 20 Hamburg, Germany |
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| Abstract: | Summary Dissected ampullae of Lorenzini of the skate (Raja clavata) were studied with the aim of determining the synaptic transmitter between electroreceptor cell and afferent fibre. Resting activity and stimulus-evoked activity in response to electrical pulses were recorded in single afferent units at constant perfusion with normal and test solutions containing different putative neurotransmitters. Presynaptic transmitter release was blocked by Mg2+ (up to 50 mM) to investigate the effects of the test substances upon the postsynaptic membrane. l-Glutamate (l-GLU) and l-aspartate (l-ASP), both at concentrations between 10-7 and 10-3
M, enlarged strongly resting and stimulus-evoked discharge frequency in the afferent fibre. If transmission was blocked by high Mg2+, resting discharge frequency could be restored by l-GLU or l-ASP. The glutamate agonists quisqualate (10-8–105
M) and N-methyl-D-aspartate (10-5–10-3
M) enlarged spontaneous activity in the afferent fiber. The same was found for kainic acid (10-9–10-5
M). Taurine at concentrations between 10-5 and 10-3
M caused a concentration-dependent decrease in afferent activity. The same was found for gammaaminobutyric acid (GABA; 10-5–10-4
M), and for the catecholamines adrenaline and noradrenaline, both in concentrations between 10-5 and 10-3
M. Serotonine (10-5–10-3
M) and dopamine (10-5-10-3
M) had no effect on resting or evoked activity in the Lorenzinian ampulla afferents. Acetylcholine (ACh; 10-4
M) enlarged discharge frequency in those units with initial rates lower than 22–25 Hz, but diminished discharge frequency in fibres with initial activity higher than 25 Hz. When synaptic transmission was blocked by high Mg2+ solution, perfusion with additional ACh did not restore resting activity in the afferent fibre. The results suggest that the most probable transmitter in the afferent synapse of the ampullae of Lorenzini is l-GLU or l-ASP, or a substance of similar nature.Abbreviations
ACh
acetylcholine
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GABA
gamma aminobutyric acid
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KA
kainic acid
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l-ASP
l-aspartate
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l-GLU
l-glutamate
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NMDA
N-methyl-D-aspartate
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Q
quisqualate
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n.s.
normal solution |
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| Keywords: | Lorenzinian ampulla Discharge frequency Electroreception Neurotransmitter Primary synapse Amino acids l-glutamate" target="_blank">l-glutamate l-aspartate" target="_blank">l-aspartate |
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