Inhibitory effects of L-glutamate on central processes of crustacean leg motoneurons.
In crustaceans, glutamatergic excitation at the neuromuscular synapse has been extensively studied. Fewer reports exist of the central and possibly inhibitory actions of glutamate on neurons. The present study analyses the response of intracellularly identified motoneurons, which innervate the proximal leg muscles, to local glutamate pressure applications in the neuropil, in an in vitro thoracic preparation of the crayfish Procambarus clarkii. L-Glutamate application always inhibited motoneuron activity, with a decrease in input resistance. The resulting depolarization or hyperpolarization could usually be reversed within 10 mV of the resting potential. The response persisted in neurons pharmacologically isolated with Cd2+ or tetrodotoxin. The reversal potential of the response to glutamate was displaced in a low-chloride solution. Similar responses were obtained with GABA. Application of GABA blocked the glutamate response in a competitive manner. Both responses were suppressed by beta-guanidino-propionic acid, a competitive antagonist for GABA receptors. This indicates that glutamate activates a chloride-GABA receptor-channel. Micromolar concentrations of picrotoxin reduced both the L-glutamate and the GABA inhibitory responses, thereby unmasking a smaller, picrotoxin-resistant effect of glutamate (but not of GABA), which was excitatory and sensitive to 6,7-dinitroquinoxaline-2,3-dione (DNQX). These results suggest dual and opposite roles for motoneuron glutamatergic connections--a peripheral (well known) net excitatory one and a central net inhibitory one. Direct inhibition of motoneurons by L-glutamatergic neurons is to be expected.
Pearlstein
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E
CNRS, Laboratoire Neurobiologie et Mouvements, Marseille, France.
Marchand
A R
AR
Clarac
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F
D000818:Animals; D003400:Astacoidea; D001921:Brain / Q000187:drug effects* / Q000502:physiology; D005260:Female; D018755:GABA Agonists / Q000494:pharmacology; D018698:Glutamic Acid / Q000494:pharmacology*; D006146:Guanidines / Q000494:pharmacology; D006614:Hindlimb / Q000294:innervation*; D008297:Male; D009046:Motor Neurons / Q000187:drug effects / Q000502:physiology*; D009433:Neural Inhibition; D011422:Propionates / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D018755:GABA Agonists; D006146:Guanidines; D011422:Propionates; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; C005775:guanidinopropionic acid