Nociception, Networks, and the Neural Unknown

As someone who studies Drosophila larvae, I have certainly learned that behaviors are often unclear and hard to define. I have been attempting to match a population of cells of Drosophila larvae to what I believe to be a nociceptive behavior. As I stare into the screen of my recorded optogenetic experiments, there is a slur of pausing, squirming, rolling, turning, bending, and crawling. I can’t help but wonder, did I activate multiple circuits that include a slew of behaviors? Did I not activate enough neurons? Are my drivers effective? 

Cell expression and labeling can blur the lines, networks are messy, and genetic tools can sometimes completely miss the mark. Neurogenetic research is unpredictable. Nonlinear. Yet fascinating, with clever experiments and precise quantifications that can map out the complex nature and circuity of animal behavior. In their paper, “Nociceptive interneurons control modular motor pathways to promote escape behavior in Drosophila”, Dr. Burgos and others from Columbia University do just this, by using genetic tools to recognize the modular control of the sequential larval nociceptive escape behavior. 

In the wild, larva use a nociceptive escape behavior as a defensive response from a sting inflicted by parasitoid wasps, that can be recreated by thermal or chemical noxious stimuli on the side of their abdomen. The response to the noxious stimulus results in a C-shaped, followed a roll, or “corkscrew-like lateral turning”, then fast forward crawling movement. Class IV (cIV) dendritic arborization sensory neurons are well-known nociceptive neurons that are required for the escape behavior. However, the sequential nature of the behavior recognizes the complexity of this circuit. Burgos et al. discovered the interneurons, “Down-and-Back” (DnB), that were identified to be a part of the motor pathways linked to the larval nociceptive escape behavior.  

To study elements downstream of the DnB interneurons, electron microscopic (EM) reconstruction was utilized in the ventral nerve cord (VNC), where it was found that premotor neurons and nociceptive integrators had the most connections to the DnB neurons. Additionally, EM reconstruction has demonstrated that the nociceptive integrators also make connections with Basin cells, which have a role in the promotion of rolling behavior through Goro rolling command-like neurons. Burgos et al. then utilized GCaMP6s and Chrimson activation to measure calcium levels in DnB and Goro neurons to determine whether the neurons are in the same pathway. When DnB neurons were activated, it caused an increase in calcium levels in Goro axons, which confirmed that DnB and Goro neurons are functionally coupled. Further, Burgos et al. performed a series of activation and silencing experiments to test the modularity of the behavioral pathway. In their control group, which activated the DnBs, 61% of the larvae performed a bend and roll while 39% performed a bend and no roll. However, when DnBs were activated while Goro neurons were silenced, 12% of the larvae performed a bend and roll and 88% of larvae performed a bend and no roll. This clearly signified specific behaviors within the sequential nociceptive response that can be triggered by individual neuron populations.  

Ultimately, Dr. Burgos and others were able to elegantly separate and define a complex behavior to specific neurons. With this key experiment, they were able to prove the modularity of the behavior and further discover the network that corresponds to larval nociceptive behavior. Interestingly though, I would argue that there is still more to uncover. When quantifying their data, Burgos et al. consistently use reduced or increased percentages of behaviors to measure the effect of the neurons to the behaviors, as their manipulation of DnB and Goro neurons still do not prove to elicit all-or-none responses. This indicates that there could be some other unknown circuits that are also contributing to this behavior, which again, illustrates the complexity of studying behavior and neurogenetics. 

Nonetheless, as a researcher attempting to define behaviors, this paper has been insightful, serving as a model on how to organize my future experiments and acting as a reminder of how sometimes even the best and diverse array of experiments cannot give absolute results. There will always be various genetic and developmental influences at play, which demonstrates the modular and complicated networks at the roots of neurogenetics. However, we can begin to make sense of and map out these networks by researching them--neuron by neuron.