@ San Francisco State University
The Physiology of Pain & Tissue Damage
PAIN
Nociception
VIDEO: High school students Teodora Sutilovic & Ciara Kosai created a video on the up-down method assessing injury-induced sensitization in Manduca sexta.
In vivo assay
The in vivo Up-Down method was modified from a rodent assay to assess thresholds to respond to nociceptive cues.
In vitro assay
The in vitro electrophysiology prep records spike frequency after touching the attached body wall
NOCICEPTIVE SENSITIZATION
Nociceptors are sensory neurons that are activated by injury or inflammation, and this activation is perceived as pain in humans. With persistent insults, the nociceptors become sensitized, often resulting in chronic pain. This is manifest as long-term hyperexcitability and/or spontaneous electrical activity in the nociceptors, and is noted in both vertebrates and invertebrates. This is a form of nociceptive memory (non-associative learning) that appears to utilize mechanisms similar to those used in inducing other forms of learning and memory. Our lab is interested in the conservation of nociceptive signaling mechanisms between invertebrates and vertebrates and across categories of learning and memory. We use the defensive strike response in the hornworm Manduca sexta as a gauge of nociception in insects. In particular, we are looking at the mechanisms resulting in injury-induced sensitization and analgesia, using both our in vivo strike bioassay and our in vitro semi-isolated electrophysiological preparation.
THE DEFENSIVE STRIKE RESPONSE
M. sexta shows a defensive strike behavior in response to trivial mechanical stimuli, and this strike behavior is modified after injury by mechanical, chemical or thermal means. The modifications include increased strike number for a given stimulus, or reduced threshold to initiate the first strike. We are interested in the signaling mechanisms underlying this injury-induced sensitization of the defensive strike response in M. sexta, as well as possible mechanisms of analgesia.
CURRENT RESEARCH
FIG 1 Change in threshold to strike after receiving a noxious pinch is maintained for almost 24 hr.
FIG 2 Long-term, but not short term, sensitization depends on protein synthesis.
We have shown that larvae become sensitized after the pinch, such that lower threshold stimuli elicit the strike response for almost 24 hours (Fig. 1 and McMackin et al., 2016).
Our semi-isolated electrophysiology preparation has revealed that sensitization arises centrally, although habituation of the dorsal afferent is also modified after noxious stimulation (pinch to the body wall). Central sensitization appears to depend on NMDA receptors and cyclic-gated nucleotide channels (Tabuena et al., 2017), and the neural circuitry inducing sensitization is under investigation.