Regulation of forward and backward locomotion through intersegmental feedback circuits in Drosophila larvae

Abstract

Animal locomotion requires spatiotemporally coordinated contraction of muscles throughout the body. Here, we investigate how contractions of antagonistic groups of muscles are intersegmentally coordinated during bidirectional crawling of Drosophila larvae. We identify two pairs of higher-order premotor excitatory interneurons present in each abdominal neuromere that intersegmentally provide feedback to the adjacent neuromere during motor propagation. The two feedback neuron pairs are differentially active during either forward or backward locomotion but commonly target a group of premotor interneurons that together provide excitatory inputs to transverse muscles and inhibitory inputs to the antagonistic longitudinal muscles. Inhibition of either feedback neuron pair compromises contraction of transverse muscles in a direction-specific manner. Our results suggest that the intersegmental feedback neurons coordinate contraction of synergistic muscles by acting as delay circuits representing the phase lag between segments. The identified circuit architecture also shows how bidirectional motor networks could be economically embedded in the nervous system.

Electrophysiological validation of premotor interneurons monosynaptically connected to the aCC motoneuron in theDrosophilalarval CNS

“…The connectome identifies inhibitory neurons making direct synaptic connections to motoneurons, including aCC (Clark et al, 2018, Kohsaka, 2019. We found that expression of NaChBac in all GABAergic neurons (Gad1-T2A-Gal4), as well as all cholinergic neurons (ChAT-BAC-Gal4), is lethal.…”

“…w -; +; Gad1-T2A-Gal4 / TM6b which expresses in all GABAergic neurons. Three different lines designed to target A23a: R78F07-Gal4 (Zarin et al 2019), R78F07-AD; R49C08-DBD split Gal4 and R41G07-AD; R78F07-DBD split Gal4 (also termed SS04495-Gal4) (Kohsaka et al, 2019). Three different lines designed to target A31k: R87H09-Gal4 (Zarin et al 2019), R20A03-AD; R87H09-DBD split Gal4 and R20A03-AD (Zarin, unpublished); R93B07-DBD split Gal4 (also termed SS04399-Gal4) (Kohsaka et al, 2019).…”

“…The connectome identifies A23a as a GABAergic interneuron directly presynaptic to aCC, involved in locomotion and activated in both forward and backward peristaltic waves (Kohsaka et al, 2019). We tested three different lines reported to drive Gal4 in the A23a interneuron: R78F07-Gal4 (Zarin et al, 2019); R78F07-AD; R49C08-DBD split Gal4 (Zarin, unpublished); and R41G07-AD 78F07-DBD split Gal4, also termed SS04495-Gal4 (Kohsaka et al, 2019).…”

“…The eventual goal of this endeavour is to identify and characterise all neurons in the larval CNS and, moreover, to trace their wired synaptic connectivity. Once identified, specific neurons are amenable to genetic manipulation using a range of driver lines, including specific split-GAL4 lines (Kohsaka et al, 2014;Fushiki et al, 2016;Hasegawa et al, 2016;Schneider-Mizell et al, 2016;Kohsaka et al, 2019). Thus, a large dataset of putative neuronal connections is now available and researchers are making increasing use of this information and derived driver lines to investigate the physiology of the larval CNS.…”

“…However, there is a danger of 'running before walking' when utilizing this impressive dataset. This is because connectivity between neurons has rarely been directly verified, but inferred from EM and validated through behavioural analysis and/or functional imaging using GCaMP to establish correlated activity (Kohsaka et al, 2019;Zarin et al, 2019). A caveat when using behavioural responses, and to a lesser extent functional imaging, is that these methods can lack the required resolution necessary to establish unambiguously whether or not two cells are monosynaptically connected; as opposed to being connected through intermediate neurons (i.e.…”