Suppression of transmission at foreign synapses in adult newt muscle involves reduction in quantal content

Abstract

IN the search for the principles underlying the orderly development of the nervous system, it is interesting to know whether vertebrate neurones or muscle fibres can distinguish synaptic contacts of the axons which normally innervate them from the input of incorrect (or foreign) nerves. This possibility has been considered by asking whether foreign synapses induced on a denervated muscle fibre are displaced or suppressed when the muscle is reinnervated by its normal nerve. Observations of oculomotor reflexes in goldfish after experimental cross innervation of extraocular muscles led to the proposal that correct reinnervation suppressed foreign synapses1. Attempts to reproduce these results in the same system2, however, led to the opposite conclusion—that foreign transmission continued unabated after reinnervation by the correct nerve. A similar persistence of foreign synaptic function was found in reinnervated perch gill muscle3, frog fast muscle4, mammalian skeletal muscle5,6 and sympathetic ganglion cells7. Adult salamander muscle is the one other system in which suppression of incorrect synapses has been indicated; by the criterion of muscle fibre contraction monitored during foreign innervation and correct reinnervation, Cass et al.8 concluded that return of the correct nerve caused inactivation of foreign synaptic input. The present investigation examines this phenomenon at the level of transmitter release from individual synapses, and provides evidence that transmission by foreign nerve terminals declines in quantal content when adult newt muscle is reinnervated by its correct nerve.