A MOLECULAR MECHANISM FOR SYNAPSE ELIMINATION - NOVEL INHIBITION OF LOCALLY GENERATED THROMBIN DELAYS SYNAPSE LOSS IN NEONATAL MOUSE MUSCLE

Activity-dependent, polyneuronal synapse elimination (ADPSE) is a prog rammed, regressive event in the development of the nervous system and readily studied at the neuromuscular junction, where it is complete 15 -20 days after birth. Local excess, or imbalanced, protease activity i s one of several possible underlying mechanisms. In this regard, throm bin mediates activity-dependent synapse loss in an in vitro model of A DPSE. To test the involvement of thrombin in vivo, we locally applied the leech thrombin-specific inhibitor, hirudin. We monitored neuromusc ular behavior, correlated with acetylcholinesterase and silver nitrate histochemistry at endplates, for changes in the timecourse of in vivo synapse elimination and assayed both thrombin activity and prothrombi n expression in developing muscle. Hirudin retarded elimination, witho ut altering motor performance, uniquely at Postnatal Day 5 (P5) and ma ximally at Po. Reverse transcription-polymerase chain reaction (PCR) s howed that neonatal muscle was a source of local prothrombin, with pea k expression during the first week after birth. A specific chromogenic assay revealed that local thrombin, activated from muscle-derived pro thrombin, peaked during maximal synapse remodeling. (C) 1996 Academic Press, Inc.