Two sides of the same coin no longer: genetic separation of nociceptive sensitization responses

Impairment of proprioceptive movement and mechanical nociception in Drosophila melanogaster larvae lacking Ppk30, a Drosophila member of the Degenerin/Epithelial Sodium Channel family

The mechanosensory neurons of Drosophila larvae are demonstrably activated by diverse mechanical stimuli, but the mechanisms underlying this function are not completely understood. Here we report a genetic, immunohistochemical, and electrophysiological analysis of the Ppk30 ion channel, a member of the Drosophila pickpocket (ppk) family, counterpart of the mammalian Degenerin/Epithelial Na⁺ Channel family. Ppk30 mutant larvae displayed deficits in proprioceptive movement and mechanical nociception, which are detected by class IV sensory (mdIV) neurons. The same neurons also detect heat nociception, which was not impaired in ppk30 mutant larvae. Similarly, Ppk30 mutation did not alter gentle touch mechanosensation, a distinct mechanosensation detected by other neurons, suggesting that Ppk30 has a functional role in mechanosensation in mdIV neurons. Consistently, Ppk30 was expressed in class IV neurons, but was not detectable in other larval skin sensory neurons. Mutant phenotypes were rescued by expressing Ppk30 in mdIV neurons. Electrophysiological analysis of heterologous cells expressing Ppk30 did not detect mechanosensitive channel activities, but did detect acid-induced currents. These data show that Ppk30 has a role in mechanosensation, but not in thermosensation, in class IV neurons, and possibly has other functions related to acid response.

Drosophila Insulin receptor regulates the persistence of injury-induced nociceptive sensitization

Diabetes-associated nociceptive hypersensitivity affects diabetic patients with hard-to-treat chronic pain. Because multiple tissues are affected by systemic alterations in insulin signaling, the functional locus of insulin signaling in diabetes-associated hypersensitivity remains obscure. Here, we used Drosophila nociception/nociceptive sensitization assays to investigate the role of Insulin receptor (Insulin-like receptor, InR) in nociceptive hypersensitivity. InR mutant larvae exhibited mostly normal baseline thermal nociception (absence of injury) and normal acute thermal hypersensitivity following UV-induced injury. However, their acute thermal hypersensitivity persists and fails to return to baseline, unlike in controls. Remarkably, injury-induced persistent hypersensitivity is also observed in larvae that exhibit either type 1 or type 2 diabetes. Cell type-specific genetic analysis indicates that InR function is required in multidendritic sensory neurons including nociceptive class IV neurons. In these same nociceptive sensory neurons, only modest changes in dendritic morphology were observed in the InRRNAi -expressing and diabetic larvae. At the cellular level, InR-deficient nociceptive sensory neurons show elevated calcium responses after injury. Sensory neuron-specific expression of InR rescues the persistent thermal hypersensitivity of InR mutants and constitutive activation of InR in sensory neurons ameliorates the hypersensitivity observed with a type 2-like diabetic state. Our results suggest that a sensory neuron-specific function of InR regulates the persistence of injury-associated hypersensitivity. It is likely that this new system will be an informative genetically tractable model of diabetes-associated hypersensitivity.

Central neural alterations predominate in an insect model of nociceptive sensitization

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 that 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. J. Comp. Neurol. 525:1176-1191, 2017. © 2016 Wiley Periodicals, Inc.

Use of von Frey filaments to assess nociceptive sensitization in the hornworm, Manduca sexta

BACKGROUND

The hornworm Manduca sexta exhibits a defensive strike to noxious assaults, a response that is robust and is easily observed by experimenters. Von Frey filaments and methods typical for studying nociception in other animals were used to assess the strike response in M. sexta.

NEW METHODS

A series of von Frey filaments was applied to the body wall in ascending order and the data generated were used to determine the strike threshold by (i) the up-and-down method, (ii) the first response method, and (iii) the simplified up-and-down order method (SUDO). The effect of a noxious pinch on strike threshold was assessed.

COMPARISON WITH EXISTING METHODS

To our knowledge none of these methods has been used on M. sexta previously, making the use of the up-and-down and SUDO methods the first in an invertebrate. The use of the first response method has been used in other invertebrates, and the method appears equally suited to M. sexta.

RESULTS

All three methods were successful in monitoring the threshold sensitivity to touch, which was lowered (sensitized) by tissue damage induced with a pinch. Sensitization lasted 19h.

CONCLUSIONS

All three methods of assessing nociception were successfully applied to quantify the defensive strike response in M. sexta, although the SUDO method required empirical assessment of which filament to start the test sequence with. The results revealed both short- and long-term sensitization. These methods should prove to be useful for quantifying sensitization in M. sexta.

Wengen, the sole tumour necrosis factor receptor in Drosophila, collaborates with moesin to control photoreceptor axon targeting during development

Photoreceptor neurons (R cells) in the Drosophila eye define a map of visual space by connecting to targets in distinct layers of the optic lobe, with R1-6 cells connecting to the lamina (the first optic ganglion) and R7 and R8 cells connecting to the medulla (the second optic ganglion). Here, we show that Wengen (Wgn) directly binds Moesin (Moe) through a cytosolic membrane proximal domain and this interaction is important for mediating two distinct aspects of axonal targeting. First, we show that loss of wgn or moe function disrupts cell autonomous R8 axon targeting. Second, we report that wgn or moe mutants show defects in R2–R5 targeting that result from disruption of non-cell autonomous effects, which are secondary to the cell autonomous R8 phenotype. Thus, these studies reveal that the Wgn-Moe signaling cascade plays a key role in photoreceptor target field innervations through cell autonomous and non-cell autonomous mechanisms.

Hedgehog signaling regulates nociceptive sensitization

BACKGROUND

Nociceptive sensitization is a tissue damage response whereby sensory neurons near damaged tissue enhance their responsiveness to external stimuli. This sensitization manifests as allodynia (aversive withdrawal to previously nonnoxious stimuli) and/or hyperalgesia (exaggerated responsiveness to noxious stimuli). Although some factors mediating nociceptive sensitization are known, inadequacies of current analgesic drugs have prompted a search for additional targets.

RESULTS

Here we use a Drosophila model of thermal nociceptive sensitization to show that Hedgehog (Hh) signaling is required for both thermal allodynia and hyperalgesia following ultraviolet irradiation (UV)-induced tissue damage. Sensitization does not appear to result from developmental changes in the differentiation or arborization of nociceptive sensory neurons. Genetic analysis shows that Hh signaling acts in parallel to tumor necrosis factor (TNF) signaling to mediate allodynia and that distinct transient receptor potential (TRP) channels mediate allodynia and hyperalgesia downstream of these pathways. We also demonstrate a role for Hh in analgesic signaling in mammals. Intrathecal or peripheral administration of cyclopamine (CP), a specific inhibitor of Sonic Hedgehog signaling, blocked the development of analgesic tolerance to morphine (MS) or morphine antinociception in standard assays of inflammatory pain in rats and synergistically augmented and sustained morphine analgesia in assays of neuropathic pain.

CONCLUSIONS

We demonstrate a novel physiological role for Hh signaling, which has not previously been implicated in nociception. Our results also identify new potential therapeutic targets for pain treatment.