Drosophila glutamate receptor RNA expression in embryonic and larval muscle fibers

Glutamate is the excitatory transmitter at neuromuscular synapses in Drosophila, and electrophysiological studies indicate that the receptors for glutamate are concentrated in muscle fibers at synaptic sites. Acetylcholine is the excitatory transmitter at vertebrate neuromuscular synapses, and previous studies have shown that accumulation of acetylcholine receptors (AChRs) at synaptic sites is controlled both by transcriptional and post-translational mechanisms. The transcriptional pathway culminates in selective expression of AChR subunit genes in nuclei near the synaptic site, causing AChR mRNA to accumulate in the synaptic region of the muscle fiber. We used a cDNA encoding a subunit of the Drosophila muscle glutamate receptor (DGluR-II) to determine the temporal and spatial expression pattern of the DGluR-II gene during embryogenesis and in larval muscle. We show that DGluR-II mRNA is first expressed at stage 12 of embryogenesis and that expression is detected in developing dorsal, lateral, and ventral somatic muscles within the next 2 hr. By stage 16 DGluR-II mRNA is expressed in all somatic muscles and in pharyngeal muscles. In third instar larvae DGluR-II mRNA is expressed in all body-wall muscle fibers. DGluR-II mRNA, however, is expressed throughout the larval muscle fibers and is not concentrated within muscle fibers at neuromuscular synapses. These results indicate that although the DGluR-II gene is expressed in somatic muscle cells it is not selectively expressed in nuclei near the synaptic site.