Here we demonstrate a novel assay to study cold nociception in Drosophila larvae. This assay utilizes a custom-built Peltier probe capable of applying a focal noxious cold stimulus and results in quantifiable cold-specific behaviors. This technique will allow further cellular and molecular dissection of cold nociception.
How organisms sense and respond to noxious temperatures is still poorly understood. Further, the mechanisms underlying sensitization of the sensory machinery, such as in patients experiencing peripheral neuropathy or injury-induced sensitization, are not well characterized. The genetically tractable Drosophila model has been used to study the cells and genes required for noxious heat detection, which has yielded multiple conserved genes of interest. Little is known however about the cells and receptors important for noxious cold sensing. Although, Drosophila does not survive prolonged exposure to cold temperatures (≤10 ºC), and will avoid cool, preferring warmer temperatures in behavioral preference assays, how they sense and possibly avoid noxious cold stimuli has only recently been investigated.
Here we describe and characterize the first noxious cold (≤10 ºC) behavioral assay in Drosophila. Using this tool and assay, we show an investigator how to qualitatively and quantitatively assess cold nociceptive behaviors. This can be done under normal/healthy culture conditions, or presumably in the context of disease, injury or sensitization. Further, this assay can be applied to larvae selected for desired genotypes, which might impact thermosensation, pain, or nociceptive sensitization. Given that pain is a highly conserved process, using this assay to further study thermal nociception will likely glean important understanding of pain processes in other species, including vertebrates.
Drosophila has proven to be highly useful for the identification of novel conserved genes and neuronal circuits that underlie complex behaviors. Flies provide a sophisticated genetic toolkit and a simplified nervous system that allow for precise genetic and neuronal manipulation1,2,3,4 to dissect the cellular and molecular bases of nociception5,6,7. Larvae are particularly useful for these analyses, given that behavioral assays for gentle touch8,9,10, noxious heat11,12,13 and mechanical sensation of noxious stimuli4,11 have already been established, and the transparent larval cuticle allows for live or fixed imaging of the epidermis and underlying sensory neurons. Recently, an assay for noxious cold has also been developed7, which we describe in more detail here.
Using a fine, conical-tipped cold probe, we show that Drosophila larvae exhibit a set of cold-specific reactive behaviors, distinct from behaviors observed during normal locomotion, following gentle touch, or after harsh mechanical or high temperature stimuli7,8,11. The cold-specific behaviors include a robust full-body contraction (CT), a 45-90º raise of the posterior segments (PR) and a simultaneous raise of the anterior and posterior segments into a U-Shape (US). The prevalence of these behaviors increases with decreasing temperatures but each peaks at slightly different cold temperatures. Recent work suggests that CT responses are mediated by different peripheral sensory neurons than those that respond to noxious heat or harsh mechanical stimuli7.
Much like vertebrate nociceptors, Drosophila multiple dendritic (md) peripheral sensory neurons have complex dendritic structures that arborize over the epidermis1. md neurons are present in every larval body segment, projecting their axons to the ventral nerve cord14. md sensory neurons are separated into four different classes (I-IV) based on dendritic morphology and have varying sensory functions4,9,10,15,16,17. While class IV neurons are required for larval lateral body roll responses to high temperatures or harsh mechanical stimuli4, class III neurons are required for gentle touch responses9,10 and are not only activated by cold, but also are required for the cold-evoked behavioral responses7. Both class III and class IV neurons utilize discrete transient receptor potential (TRP) channels to facilitate behavioral responses to noxious7,11,18 and non-noxious stimuli9,10,17,19. Further, larval nociception is sensitized following injury, at the cellular20 and behavioral levels12,21.
The assay described here allows for the quantification of either normal, or potentially altered behavioral responses to cold temperatures ranging from noxious cold (≤ 10 ºC), innocuous cool (11-17 ºC), to ambient temperatures (18-22 ºC). The cold temperatures used in this assay are capable of directly activating class III sensory neurons, eliciting robust, reproducible calcium increases and cold-evoked behavioral responses, which can be qualitatively and quantitatively analyzed7. This assay can be applied to larvae of virtually any genotype as well as to larvae exposed to diverse environmental conditions (altered nutrition, injury, pharmacological agents) to determine both genetic and environmental factors that impact cold nociception, nociceptive sensitization or nociceptive plasticity. Given that thermosensation is ubiquitous across many species, this assay provides a valuable tool for the study of nociception and may uncover novel gene targets or neuronal interactions that will improve our understanding of vertebrate nociception.
The custom-built cold probe (see cold probe, Table of Materials) utilizes a closed loop temperature controlled Peltier device, which cools the aluminum shaft and conical tip through thermal conduction. A thermistor is embedded inside the aluminum conical tip reports the real-time temperature on the control unit. A heat sink and fan are attached to the thermoelectric module to regulate the Peltier effect's heat load (Qc) so the desired temperature range of (22-0 °C) can be achieved (see Thermal Control Unit, Table of Materials). The noxious cold stimulus of the cold probe tip is applied by hand to the dorsal midline, to segment(s) equidistant from anterior and posterior ends (roughly segment A4, see Figure 1A) of the larva. In response to cold stimuli, larvae generally produce one of three cold-evoked behaviors within a 10 s cutoff: a full body contraction (CT), a 45-90º raise of anterior and posterior segments into a U-Shape (US), or a raise of the posterior segments (PR) (described in Results). None of these behaviors are performed during normal peristaltic locomotion or foraging behavior. These behaviors are also distinct from gentle touch responses and the aversive rolling response to high temperature or noxious mechanical stimuli.
The assay described here can be used to qualitatively and quantitatively assess nociception or nociceptive sensitization in larvae of various genetic backgrounds, environmental influences, and/or damage-induced conditions. Since this assay allows for focal application of a cold stimulus, with this tool one can assess the function of a subset of peripheral sensory neurons specifically in responding to cold temperatures. Interestingly, these cold-evoked behaviors seem to utilize different classes of sensory neurons th…
The authors have nothing to disclose.
We thank Sarah Wu and Camille Graham for developing early phases of the cold probe assay, the Bloomington Drosophila Stock Center for fly stocks, and Galko lab members for critically reading the manuscript. This work was supported by NIH NRSA (NIH F31NS083306) to HNT, and by NIH R01NS069828, R21NS087360 and a University of Texas MD Anderson Clark Fellowship in Basic Research to MJG.
Cold Probe | Pro-Dev Engineering | Custom-built on demand | Part numbers and construction details can be provided on request |
Thermal Control Unit | TE Technology | Custom Built enclosure | Part numbers and construction details can be provided on request |
Zeiss Stemi 2000 microscope | Zeiss | NT55-605 | |
Fiber-Lite MI-150 High Intensity Illuminator | Dolan-Jenner Industries. | A20500 | |
Schott Dual Gooseneck 23 inch Fiber Optic Light Guide | Schott North America, Inc. | Schott A08575 | |
Forceps | FST | FS-1670 | Used to sort and handle larvae. Be sure to smooth and blunt forceps tips slightly to lower the risk of accidently puncturing or injuring the larvae |
Paintbrush | Dick Blick Art Materials | 06762-1002 | Used to sort and handle larvae. It is helpful if the paintbrush is damp during use. |
35 X 10 mm Polystyrene Petri Dish | Falcon | 351008 | |
60 X 10 mm Polystyrene Petri Dish | Falcon | 351007 | |
Piece of black vinyl (at least 2 x 2 inches) | Used to provide contrast and orient larvae to the cold probe | ||
Fisherbrand Scoopula Spatula | Fisher Scientific | 14-357Q | Used to move food |
Kimtech Science Kimwipes | Fisher Scientific | 06-666A | Used to dry the larvae and cold probe if there is excess moisture |