Ascending neurons convey behavioral state to integrative sensory and action selection centers in the brain

Abstract

Knowledge of one's own behavioral state---whether one is walking, grooming, or resting---is critical for contextualizing sensory cues including interpreting visual motion and tracking odor sources. Additionally, awareness of one's own posture is important to avoid initiating destabilizing or physically impossible actions. Ascending neurons (ANs), interneurons in the vertebrate spinal cord or insect ventral nerve cord (VNC) that project to the brain, may provide such high-fidelity behavioral state signals. However, little is known about what ANs encode and where they convey signals in any brain. To address this gap, we performed a large-scale functional screen of AN movement encoding, brain targeting, and motor system patterning in the adult fly, Drosophila melanogaster. Using a new library of AN sparse driver lines, we measured the functional properties of 247 genetically-identifiable ANs by performing two-photon microscopy recordings of neural activity in tethered, behaving flies. Quantitative, deep network-based neural and behavioral analyses revealed that ANs nearly exclusively encode high-level behaviors---primarily walking as well as resting and grooming---rather than low-level joint or limb movements. ANs that convey self-motion---resting, walking, and responses to gust-like puff stimuli---project to the brain's anterior ventrolateral protocerebrum (AVLP), a multimodal, integrative sensory hub, while those that encode discrete actions---eye grooming, turning, and proboscis extension---project to the brain's gnathal ganglion (GNG), a locus for action selection. The structure and polarity of AN projections within the VNC are predictive of their functional encoding and imply that ANs participate in motor computations while also relaying state signals to the brain. Illustrative of this are ANs that temporally integrate proboscis extensions over tens-of-seconds, likely through recurrent interconnectivity. Thus, in line with long-held theoretical predictions, ascending populations convey high-level behavioral state signals almost exclusively to brain regions implicated in sensory feature contextualization and action selection.

Descending neuron population dynamics during odor-evoked and spontaneous limb-dependent behaviors

“…To further reveal the identity of these DNs, we examined data from our functional screen of sparse Gal4 and split Gal4 driver lines [49]. In this dataset, we observed similar asymmetric activity during antennal grooming in R65D11-GAL4.…”

“…Recordings of DNx01s in the R65D11-GAL4 driver line were performed as described in [49]. This differed only slightly from DN population recordings in the following ways.…”

“…This resulted in a slower two-photon image acquisition rate (4.3 fps) which was matched by a slower behavior recording frequency (30 Hz). For additional details and a description of the stimulation system see [49]. Note that images of the connective in Figure 5a and Figure 5e appear to have different heights due to a difference in z-step size during image acquisition.…”

“…Raw, non-denoised traces were used for analysis. For more details concerning data post processing see [49].…”

“…Outliers in 3D pose data were corrected as described in [49]. Briefly, we detected outliers based on changes in leg segment length and chose the pair of cameras with minimal reprojection error for triangulation.…”