Organization of descending neurons in Drosophila melanogaster

Abstract

Neural processing in the brain controls behavior through descending neurons (DNs) - neurons which carry signals from the brain to the spinal cord (or thoracic ganglia in insects). Because DNs arise from multiple circuits in the brain, the numerical simplicity and availability of genetic tools make Drosophila a tractable model for understanding descending motor control. As a first step towards a comprehensive study of descending motor control, here we estimate the number and distribution of DNs in the Drosophila brain. We labeled DNs by backfilling them with dextran dye applied to the neck connective and estimated that there are ~1100 DNs distributed in 6 clusters in Drosophila. To assess the distribution of DNs by neurotransmitters, we labeled DNs in flies in which neurons expressing the major neurotransmitters were also labeled. We found DNs belonging to every neurotransmitter class we tested: acetylcholine, GABA, glutamate, serotonin, dopamine and octopamine. Both the major excitatory neurotransmitter (acetylcholine) and the major inhibitory neurotransmitter (GABA) are employed equally; this stands in contrast to vertebrate DNs which are predominantly excitatory. By comparing the distribution of DNs in Drosophila to those reported previously in other insects, we conclude that the organization of DNs in insects is highly conserved.

Figure 1. Drosophila has ~1,100 DNs distributed in 6 clusters.

(a) Representative confocal image stacks showing the distribution of DN clusters. Maximum projection of dextran label (white) over the range of specified depths, superimposed on a section of neuropil containing the anatomical landmarks (anti-nc82, purple). (b–d) Schematic illustrating the mean distribution of DNs in the 6 clusters. Each cluster is represented with a different color. Each dot represents approximately 10 DNs.

Figure 2. Pattern of neuropil labeling suggest distinct regions for sensory, associative, and motor processing.

(a) Density of labeling for different neuropil regions. (b) Projections of a confocal stack show dextran labeling (white) and neuropil labeling (magenta). Regions marked in yellow are expanded in panels (c–g) to show sparse innervation of different brain regions. (c) Sparse innervation of the lobula (d) Sparse innervation of the anterior optic tubercle. (e) Sparse innervation of the lateral horn. (f) Dense innervation of the posterior ventrolateral protocerebrum, where optic glomeruli are found. (g) Dense innervation of AMMC and surrounding neuropil. For clarity, only a single representative 1 μm slice is shown. The extent of the image stack is different for the images in (b–g).

Figure 3. Strategy for labeling DNs with a given neurotransmitter.

(a) Schematic illustrating that the subset of GFP+ neurons labeled by a Gal4 driver can be identified as DNs if they are also colabeled by retrograde labeling (yellow). (b–j) Projection of a confocal stack of a retrogradely labeled brain in which all cholinergic neurons are labeled using Cha-Gal4,UAS-GFP (green). Retrograde label is in red. Cholinergic DNs are colabled and appear yellow. (k–m) Close-up of the region in white square in (d,g,j) show the co-labeled neurons.

Figure 4. Schematics showing distribution of DNs by neurotransmitter.

(a) Distribution of cholinergic (red) and GABAergic DNs (green). Dots are in proportion to the fraction of DN of a given type. (b) Distribution of DNs which employ other (minor) neurotransmitters. Each dot is a single DN.

Figure 5. Schematics showing that the organization of DNs is conserved across insects.

(a) Distribution of DN clusters in hemimetabolous insects (cricket, cockroach18) relative to landmark neuropil regions (antennae lobe, central body, mushroom body calx). This is shown in comparison to the distribution of DN clusters in Drosophila (right), a holometabolous insect, as described in this study. The gnathal ganglion, which in holometabolous insects such as Drosophila is fused to the cerebral ganglia, is shown shaded in lavendar. (b) The approximate locations of insect DNs previously described in the literature is shown in the context of the clusters described in this study. The numbers correspond to rows listed in Table 3.