PRECISION OF REINNERVATION AND SYNAPTIC REMODELING OBSERVED IN NEUROMUSCULAR-JUNCTIONS OF LIVING FROGS

Repeated in vivo observations were used to study regenerated nerve ter minals in neuromuscular junctions of the adult frog Rana pipiens. Sart orius junctions in living animals were stained with the fluorescent vi tal dye RH414 and viewed with video fluorescence microscopy. Each junc tion was observed in the intact muscle and then again 7, 10, and 13 we eks after nerve crush. At 13 weeks, junctions were determined to be mo no- or polyneuronally innervated using intracellular recording. Betwee n 7 and 13 weeks, most identified junctions were reinnervated less pre cisely and completely than described previously. Although some of the original synaptic gutters were reoccupied by regenerated terminal bran ches, other gutters were only partially occupied, and many appeared ab andoned. Junctions showing precise recapitulation of original terminal arborizations comprised a small number of the total examined, as did those where reinnervation was very imprecise. Striking differences in the precision of reinnervation were found within the muscle such that distal terminals regenerated more precisely and completely than did pr oximal terminals. Terminals in reinnervated muscles were more dynamic than terminals in unoperated muscles over equivalent times. In singly innervated junctions, terminal growth was favored over regression. In doubly innervated junctions, regressive events were more common. Impre cise reinnervation is explained in terms of multisite innervation of m uscle fibers and the activity dependence of synaptic stability. We hyp othesize that when axons reinnervate the second or third junctions on a fiber, they do so less precisely, because the activity restored by r einnervation of the first junction renders later sites less attractive or less stable.