Date of Award
Doctor of Philosophy (PhD)
Nociception, the detection and avoidance of harmful cues, is a crucial system in all organisms. Animals use nociceptive systems to escape from substances that decrease survival, and can also modulate the threshold for avoidance behaviors to weigh the attractive features of an environment against its harmful features. To allow regulation, the nociception system of mammals incorporates multiple feedback and feedforward loops in its central and peripheral pathways. The nociception system of the roundworm Caenorhabditis elegans shares many features of the mammalian circuit. Both neural circuits feature a direct path from sensory neurons to motor neurons that is connected by a single class of interneuron, bypassing the higher processing centers. Both neural circuits also feature higher processing pathways that receive information from sensory neurons and provide further input onto the direct pathway. While the anatomical wiring of the C. elegans nervous system has been known for decades, how sensory neurons access different downstream paths in the circuit is less clear. One possible route of differential access of sensory input to downstream neurons is through different dynamics of activation. The temporal dimension of neural circuits cannot be deduced by anatomical wiring, but must be measured directly. In my thesis, I have characterized and manipulated the dynamic properties of a classical nociceptor in C. elegans, the polymodal sensory neuron ASH, and asked how these properties instruct downstream circuits and behavior. I thus first elucidated ASH calcium activation dynamics using simple step responses and using a newly developed systems identification approach for C. elegans. Using both long pulses and rapidly fluctuating “white noise” sequences of different nociceptive stimuli, I deduced their ASH activation profiles and linear temporal filters describing how the neuron summates the history of stimulus encounter. This analysis demonstrated that ASH calcium responses to natural stimuli include both linear features and multiple nonlinear components. Mutations in G protein-coupled sensory signaling disrupt both fast linear filtering and sustained responses to nociceptive stimuli. Mutations in a voltage gated calcium channel alter the temporal qualities of the ASH response in a pattern suggesting a role of this channel in sensory adaptation. In the course of these studies, I discovered several additional classes of sensory neurons that respond to nociceptive stimuli with robust calcium responses, even though past studies did not demonstrate a role for these neurons in nociceptive behavior. To gain experimental control over the dynamic activity that initiates nociceptive signaling, I ectopically expressed the pheromone receptors SRG-34 and SRG-36 in ASH and activated this system with their endogenous ligand, the ascaroside C3. ASH does not normally detect C3, but when it expresses either of these receptors it generates robust calcium responses to C3. These calcium signals have distinct temporal dynamics: SRG-34 mediated calcium signals are fast rising and fast adapting, while SRG-36 mediated calcium signals increase slowly during stimulation with little adaptation. Expression of SRG-34 or SRG-36 in ASH caused animals to avoid C3. Remarkably, time-aligned histograms of C3-induced avoidance behavior during stimulus onset, presence, and removal closely followed the dynamics of ASH calcium activity at these same time points, with a fast onset and adaptation for SRG-34 and a slow, sustained avoidance of SRG-36. ASH can directly activated the backward command motor neuron AVA or indirectly activate AVA through other neuronal pathways, including the intermediate interneuron AIB. Selectively silencing the AIB interneuron with the a chemical genetics system using the histamine-gated chloride channel resulted in complete loss of nociceptive avoidance behaviors induced by slow-ramping SRG-36 receptor in ASH, but had less of an effect on SRG-34 avoidance. Selectively silencing the AVA backward command interneuron reduced reversals, but spared or increased other avoidance behaviors for both SRG-34 and SRG-36. These results indicate that downstream interneurons are engaged in different ways, and to different degrees, depending on the mechanism of ASH activation. I next monitored the activity of AIB and AVA neurons in freely-moving ASH:srg-34 or ASH:srg-36 animals responding to C3. In ASH:srg-34 animals, AIB and AVA begin increasing activity upon C3 onset. In ASH:srg-36 worms, AIB increased activity before AVA. Together with my AIB silencing results, these observations suggest that AIB accumulates signals from ASH over time to promote AVA activity. Using a coherent type-1 feed forward loop with a calcium slope-determined AND or OR logic, I modeled features of AIB contribution to nociceptive behaviors in response to different ASH temporal dynamics. These findings suggest that feedforward excitation loops, a motif seen in C. elegans and mammalian nervous systems, can result in behaviorally-salient consequences in response to different sensory neuron calcium dynamics.
Xu, Yifan, "Neural Circuit Dependence of Acute and Subacute Nociception in C. Elegans" (2015). Student Theses and Dissertations. 411.