Localization of Drosophila neurons that contain choline acetyltransferase messenger RNA: an in situ hybridization study.

In situ hybridization with radiolabeled complementary RNA (cRNA) probes was used to determine the location of the messenger RNA (mRNA) encoding choline acetyltransferase (ChAT) in Drosophila nervous system. Areas in the cell-rich cortical regions of the cerebrum and optic lobes hybridized with substantial concentrations of the probe. This contrasted with the cell-sparse neuropil areas where no significant concentrations of probe were observed. Although most of the cortical regions were substantially labeled, there were regions within all of the areas where labeling was sparse or nonexistent. For example in the lamina, even though the monopolar cell layer appeared to be heavily labeled, there were some neuronal profiles that were not associated with the probe. Moreover, the epithelial glia that form an arch of cell profiles subjacent to the monopolar cells were not labeled, nor were amacrine neurons in the apex of the lamina near the external optic chiasma. The highest concentration of probe (approximately 140 grains/400 microns2) was observed in the laminar monopolar cell region and the cerebral cortical rind. The next most heavily labeled region (approximately 90 grains/400 microns2) occurred over cortical cells of the medulla-lobula. In the peripheral nervous system, label over the antennal sensory neurons amounted to about 75 grains/400 microns2, and the retinular cell layer of the compound eye exhibited about 60 grains/400 microns2. The control probe did not hybridize in significant quantities in either cellular or noncellular regions. This study presents evidence that large numbers of Drosophila cortical and primary sensory neurons contain the messenger RNA necessary for the production of ChAT, the acetylcholine-synthesizing enzyme. Further, our findings provide baseline information for use in ontogenetic studies of cholinergic neurons in Drosophila, and they also provide normative data for studying the effects of mutant alleles at the Cha or Ace loci upon the transcription of ChAT messenger RNA.