Chemosensory Physiology and Chemical Nociception: TRPA1, TRPV1, TRPM5
Mechanisms underlying complex physiological phenotypes: resistance to chemicals that cause pain and inflammation involve adaptation of the sensory system and inflammation signaling pathway
Sensory systems are integral to an animal’s behavioral repertoire (i.e., searching for food, habitats and mates while avoiding predators). Survival depends on the ability to detect, interpret and respond appropriately to external stimuli, particularly painful (tissue damaging) stimuli. This is critical when foraging or hunting for food – animals must distinguish beneficial from potentially harmful food items. Chemosensation or “chemical sense” is the process by which sensory systems distinguish beneficial from harmful chemicals. Transient Receptor Potential (TRP) channels are a group of membrane proteins that transduce both innocuous and nociceptive (painful) chemical, thermal and mechanical stimuli. TRPA1 (ankyrin family) and TRPV1 (vanilloid family) are particularly interesting because they are expressed in the trigeminal ganglia (TG) of sensory neurons that innervate the eyes, nose, mouth and upper airways, and in the dorsal root ganglia (DRG) of sensory neurons that innervate the lower airways. These polymodal receptors initiate pain signals in response to a number of noxious chemicals, extreme temperatures, changes in pH and tissue damage. Activation of TRPA1 and TRPV1 is also associated with initiation of inflammatory and immune responses. However, TRPA1 and TRPV1 expression patterns, molecular mechanisms of polymodal activation, and role in inflammatory and immune responses are not completely understood.
Darkling beetles (Eleodes longicollis) and their predators, scorpion mice (Onychomys torridus) provide an unprecedented opportunity to examine the molecular physiology of TRPA1 and TRPV1 pain signaling, and to ask how changes in TRP channel structure, function and expression impact predatory behavior and species interactions. Darkling beetles, when threatened, stand on their head and spray a chemical cocktail in the face of their predator. One chemical, benzoquinone, activates TRPA1 while the acidic pH activates TRPV1 causing irritation of the eyes, nasal and oral membranes in sensitive animals. Inhalation of the noxious spray can also cause irritation of the lower airways. Scorpion mice (carnivores) prey on darkling beetles, while opportunistic insectivores (Cactus mice, Peromyscus eremicus) avoid the beetles. Scorpion mice show little response to beetle sprays. Hind-paw injections suggest that scorpion mice TRPA1 and TRPV1 channels are less sensitive to noxious chemicals (formalin, capsaicin) compared to house mice.
Left: darkling beetle stands on head when threatened by predator (photo by Matt Rowe). Right: single frame from high speed video of darkling beetle discharging benzoquinone spray during scorpion mouse attack (Ann Prum, Coneflower Studios).
To characterize the molecular and physiological mechanisms that underlie reduced sensitivity to noxious chemicals, we are integrating genomics/transcriptomics, calcium imaging, physiology and behavior.Preliminary data from our RNA-seq transcriptome analyses of TG and DRG from scorpion mice revealed that TRPM5 was significantly downregulated in the TG of mice exposed to darkling beetle spray. TRPM5 is expressed in taste cells where it regulates bitter taste. Downregulation of TRPM5 could prevent scorpion mice from becoming aversively conditioned to darkling beetles. In addition, we are examining the expression of genes related to inflammation that may be involved in resistance to beetle sprays. Preliminary data revealed that a number of genes related to inflammation were down regulated in the DRG of mice exposed to beetle spray.