Glutamate sensitivity and distribution of receptors along normal and denervated locust muscle fibres.

1. Factors influencing the glutamate sensitivity of extrajunctional regions of innervated and denervated locust muscle fibres have been investigated. Properties of the two types of extrajunctional glutamate receptors, D- and H-receptors, have been studied in regions of high and low sensitivity. 2. The low level of extrajunctional sensitivity which is normally present in innervated fibres was 20-30 times higher at the muscle-tendon junction than at other sites; increased sensitivity extended about 20-40 micron from the tendon. After denervation or localized damage the entire extrajunctional sensitivity was increased approximately 100 times above control levels. 3. Applying L-glutamate (which activates D- and H-receptors) and DL-ibotenate (which activates H-receptors) from multibarrelled micropipettes showed that increased extrajunctional sensitivity resulted from an increase in D-receptors while H-receptors were apparently unchanged. 4. Coulomb dose vs. response relationships for the action of glutamate on D-receptors were similar when obtained at the muscle-tendon junction and nerve-muscle junction of innervated fibres or at extrajunctional regions in denervated fibres. 5. Time course of onset and percentage desensitization of D- and H-receptors in innervated fibres were similar. The percentage desensitization of D-receptors in extrajunctional regions of high sensitivity was greatly reduced. 6. It is suggested that D- and H-receptors are independent and that the trigger for increased receptor sensitivity acts specifically on D-receptors. In all respects so far studied, the D-receptors resemble extrajunctional ACh-receptors in vertebrate muscle.