Decrease in transmitter output and synaptic ultrastructure at lobster neuromuscular terminals with decentralization

Abstract

The effects of decentralization on the physiology and ultrastructure of neuromuscular terminals were examined by transecting the single excitor axon to the distal accessory flexor muscle in the walking legs of lobsters (Homarus americanus). Decentralization caused a reduction in the amplitude of the excitatory junctional potential without altering the resting potential or input resistance of the muscle fiber thereby suggesting a reduction in transmitter release. Confirmation was obtained by recording of synaptic currents at focal sites which showed failure of transmission and a reduced amplitude on decentralized fibers compared to their intact counterparts on the contralateral leg. The mean quantal content of synaptic transmission decreased approximately 2-7-fold at these decentralized sites compared to their intact counterparts. The ultrastructure of these identified sites was examined with serial section electron microscopy. There are few if any qualitative changes in synaptic ultrastructure between decentralized and control terminals. However, quantitatively there were changes in synaptic ultrastructure which were progressive in nature depending on the severity of the reaction to decentralization. Thus terminals showing a moderate decline in quantal content were characterized by a reduction in the number of presynaptic dense bars and synapses. Terminals showing a severe drop in transmitter release showed in addition to the above changes, a reduction in the size of synapses and terminals. These results show a progression in the loss of the structural parameters controlling transmitter release. Finally synaptic vesicles and mitochondria did not reveal any consistent or marked change with decentralization.