Desensitization of glutamate receptors on innervated and denervated locust muscle fibres.

Abstract

Depolarizations to L-glutamate, applied locally by microionophoresis to the extrajunctional membrane of locust extensor tibiae muscle fibers and measured either in current clamp or voltage clamp, increased in amplitude for equivalent doses of glutamate following chronic denervation of the muscle. A 2-pulse method was used to examine recovery from desensitization of junctional and extrajunctional receptors. A response ratio, i.e., the amplitude of response to the 2nd (test) of a pair of glutamate pulses over the response to the 1st (control), was determined as a function of the time interval between the pulses. The response ratio for extrajunctional depolarizations of innervated fibers increased exponentially with pulse interval, with a time constant of 15.6 .+-. 4.7 s (n = 11). Recovery of extrajunctional receptor populations from desensitization was accelerated after denervation. The recovery kinetics for responses from fibers 6-22 days after denervation were generally described by 2 exponential terms, with time constants in the range 0.5-10 s which were inversely related to the glutamate sensitivity of the extrajunctional membrane. For junctional receptors on both innervated and denervated fibers the recovery kinetics were described by a single exponential with a time constant of 0.2-1 s. The increased extrajunctional glutamate sensitivity which occurs after denervation results from the appearance of glutamate receptors with properties similar to those found at the post-junctional membrane on locust muscle fibers.