Ultrastructural changes in receptor terminals on prolonged action by local anesthetics

We have studied ultrastructural changes in the terminal plates of the bushy receptors in the frog urinary bladder after exposure for two hours to 0.05% novocaine solution and for one hour to 0.05 dicaine and trimecaine solution. During these periods a steady blockage of the receptor impulse activity develops. The local anesthetics essentially change the ultramicroscopical structure of the terminals. The reaction to the anesthetics investigated shows some common features and certain peculiarities. For each effect three types of change can be determined characterized by various degrees of rearrangement in the neurolemma, neuroplasma, and organelles. Each type of change is assumed to reflect a certain phase of the plate reactive response. Specific features of the reaction to novocaine include slight changes in mitochondria, accumulation of glycogen granules, and deformation and decrease in numbers of vesicles. Under the action of dicaine the mitochondria do not change and the number of vesicles increases without change in their shape, while under the action of trimecaine marked changes can be observed in mitochondria. Changes observed in the terminal plates are regarded as adaptive. The action of the local anesthetics on the receptors is not limited to the blockage of the sodium channels of the afferent fibers, while the biochemical processes occurring in the cytosol of the terminals also change. Their morphological manifestations reflect the ultrastructural changes observed.