EVIDENCE FOR COMPARTMENTAL IDENTITY IN THE DEVELOPMENT OF THE RAT LATERAL GASTROCNEMIUS-MUSCLE

In adult rats, each neuromuscular compartment of the lateral gastrocne mius muscle (LG) is exclusively innervated by a primary branch of the LG nerve. In neonates, however, a small percentage of LG cells receive s inputs from more than one primary nerve branch; these inputs are kno wn as cross-compartmental. Cross-compartmental inputs are normally los t from the medial compartment of LG (LGm) by the 8th postnatal day. To investigate the mechanisms involved in the elimination of cross-compa rtmental inputs, muscle fibers in the LGm compartment were denervated by cutting the LGm nerve branch in 1-4 day old rat pups and in adult r ats. We then assessed the degree of cross-compartmental innervation wi thin the ''denervated'' compartment using intracellular recordings fro m neonatal muscle fibers or immunohistochemical staining for nerve cel l adhesion molecule (N-CAM) and neurofilament protein in adult muscles . Following LGm axotomy in neonates, cross-compartmental innervation i s more extensive than in controls and is present as late as 20 days af ter birth. Thus, in the absence of ''native'' LGm axons, neonatal cros s-compartmental inputs proliferate by axonal sprouting and the formati on of new synapses on vacant LGm fibers. In contrast, axotomized adult s do not form new cross-compartmental inputs over the same time period . The differential response of neonates and adults to muscle nerve bra nch denervation is evidence for the existence of some form of compartm ent-specific recognition. We propose that compartmental identity eithe r arises or becomes relatively more potent during ontogeny and normall y acts selectively to eliminate foreign axons and deter the formation of new cross-compartmental inputs. (C) 1993 Wiley-Liss, Inc.