ARIA IS CONCENTRATED IN NERVE-TERMINALS AT NEUROMUSCULAR-JUNCTIONS AND AT OTHER SYNAPSES

Skeletal muscle ACh receptors (AChRs) accumulate at neuromuscular junc tions (nmjs) at least partly because of the selective induction of ACh R subunit genes in subsynaptic myotube nuclei by the motor nerve termi nal. Additionally, mammalian AChRs undergo a postnatal change in subun it composition from embryonic (alpha(2) beta gamma delta) to adult (al pha(2) beta epsilon delta) forms, a switch that also depends on innerv ation. ARIA, a protein purified from chicken brains based on its abili ty to induce AChR synthesis in primary chick muscle cells, is a strong candidate for being the molecule responsible for these early developm ental events. ARIA mRNA has been detected in embryonic motor neurons d uring synapse formation, and the gene continues to be expressed postna tally. In this report, we provide evidence that ARIA-like immunoreacti vity is concentrated in rat motor nerve terminals from early postnatal ages, and that it can be detected in motor neurons in E18 embryos. AR IA is also detectable in axons within colchicine-treated sciatic nerve s, suggesting that the protein in the nerve terminal has been transpor ted from the cell body. ARIA mRNA is present in, but not restricted to , cholinergic neurons. Likewise, we report here that ARIA-like immunor eactivity is present in some noncholinergic central synapses. We also present evidence that isoforms of ARIA are differentially distributed among functionally distinct classes of neurons.