SYNAPSIN-I EXPRESSION IN SPINAL-CORD NEURONS DURING CHICK-EMBRYO DEVELOPMENT

The cellular distribution of synapsin I in chick spinal cord has been examined during embryo development and in cultured neurons from differ ent developmental stages, Using immunocytochemical methods we have obs erved that synapsin I appears lightly detectable in spinal cord of emb ryonic day (E)5-E8 embryos when the motor neurons have already establi shed functional contacts with muscle fibers, and increases at E9. Unti l E8 synapsin I immunoreactivity appeared mainly localized in the gray matter of spinal cord; immunostaining of white matter becomes clearly evident only at E9. These observations indicate that synapsin I expre ssion and possibly its transport to the nerve terminals may be stimula ted by sequential signals, The cellular distribution of synapsin I obs erved in viva, is maintained in E8 and E9 spinal cord neuron cell cult ures, In fact, in E8 cultured neurons, synapsin I immunostaining is ob served only in the cell body, while in E9 cultured neurons both cell b ody and fibers are stained, The addition of muscle extracts to E8 cult ures induces synapsin I decoration of fibers similar to that observed in E9 cultured neurons, Indeed Western and Northern blot analysis and in situ hybridization demonstrate an increase of synapsin I and its mR NA in spinal cord neurons kept in the presence of muscle extracts, The se data suggest that synapsin I expression, as previously reported for other neuronal markers, can be modulated by soluble factors present i n target cells. (C) 1994 Wiley-Liss, Inc.