Central neural alterations predominate in an insect model of nociceptive sensitization

Abstract

Many organisms respond to noxious stimuli with defensive maneuvers. This is noted in the hornworm, Manduca sexta, as a defensive strike response. After tissue damage, organisms typically display sensitized responses to both noxious or normally innocuous stimuli. To further understand this phenomenon, we used novel in situ and in vitro preparations based on paired extracellular nerve recordings and videography to identify central and peripheral nerves responsible for nociception and sensitization of the defensive behavior in M. sexta. In addition, we used the in vivo defensive strike response threshold assayed with von Frey filaments to examine the roles N-methyl-D-aspartate receptor (NMDAR) and hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels play in this nociceptive sensitization using the inhibitors MK-801 and AP5 (NMDAR), and ivabradine and ZD7288 (HCN). Using our new preparations, we found that afferent activity evoked by noxious pinch in these preparations was conveyed to central ganglia by axons in the anterior- and lateral-dorsal nerve branches, and that sensitization induced by tissue damage was mediated centrally. Furthermore, sensitization was blocked by all inhibitors tested except the inactive isomer L-AP5, and reversed by ivabradine both in vivo and in vitro. Our findings suggest that M. sexta’s sensitization occurs through central signal amplification. Due to the relatively natural sensitization method and conserved molecular actions, we suggest that M. sexta may be a valuable model for studying the electrophysiological properties of nociceptive sensitization and potentially related conditions such as allodynia and hyperalgesia in a comparative setting that offers unique experimental advantages.

Nociceptive Biology of Molluscs and Arthropods: Evolutionary Clues About Functions and Mechanisms Potentially Related to Pain

“…General sensitization after just a single noxious pinch of a proleg was later shown to be expressed as a marked decrease in strike threshold to mechanical stimulation, which persisted for at least 19 h ( McMackin et al, 2016 ). This robust sensitization survived dissection after in vivo pinch, which allowed neural correlates of the sensitization to be examined ( Tabuena et al, 2017 ). In contrast to nociceptive sensitization in Aplysia and mammals, neural sensitization did not include enhancement of primary afferent activity evoked by test stimulation of the previously pinched region, but it was expressed as increased evoked activity recorded from an interganglionic connective.…”

“…This shows a form of central sensitization, perhaps with some functional and mechanistic similarities to the central sensitization that contributes to pain states in mammals ( Woolf, 2011 ). Clues about the mechanisms of pinch-induced central sensitization came from blocking the induction of sensitization of strike responses (and central hyperactivity) by pre-treatment with NMDA receptor blockers, and reversal of sensitized strike responses by post-treatment with a blocker of cAMP-activated HCN channels ( Tabuena et al, 2017 ). This pattern is interesting because, like some forms of nociceptive sensitization in Aplysia and mammals, it suggests that NMDA receptor-dependent LTP is involved in the induction of sensitization, and ongoing generation of cAMP may be involved in the maintenance of sensitization (e.g., Bavencoffe et al, 2016 ).…”

“…Each group shows enhanced defensive responses after noxious stimulation, and the behavioral sensitization is often associated with enhanced function (hyperexcitability, hypersensitivity, synaptic potentiation, growth) in primary nociceptors. Possible exceptions are crustaceans, where nociceptors have not yet been identified, and perhaps Manduca larvae, where prominent sensitizing effects of pinch have been found in central neural activity but not primary afferent activity ( Tabuena et al, 2017 ). The neurophysiological alterations in nociceptors and their synaptic targets that have been identified in gastropods and insects involve signaling pathways known to contribute to the induction and maintenance of persistent pain in mammals.…”

Intensity coded octopaminergic modulation of aversive crawling behavior inDrosophila melanogasterlarvae

“…Additionally, it appears that a repetitive noxious stimulation of constant intensity can result in an increasing response strength of nociceptive neurons (centrally mediated sensitization) leading to even stronger escape behaviors after a second or a third noxious stimulation (Walters et al, 2001;Ji et al, 2003;Hu et al, 2017;Tabuena et al, 2017). Such an enhancer effect could be realized not only by increased sensitivity of nociceptors but also within downstream interneurons or parallel VUM neurons.…”

Local and generalized sensitization of thermally evoked defensive behavior in caterpillars

“…The degree of sensitization is also evaluated mechanically using von Frey filaments to estimate the threshold force for a strike response (McMackin et al, ). Such experiments, on both intact (Walters et al, ) and partially dissected (Tabuena, Solis, Geraldi, Moffatt, & Fuse, ) Manduca , have identified sensitization as a generalized central response with little evidence for site‐specific sensory enhancement. It is unclear if this difference from findings in Drosophila is species‐dependent or if it arises from differences in the experimental conditions.…”