Molecular basis of infrared detection by snakes

Snakes possess a unique sensory system for detecting infrared radiation, enabling them to generate a ‘thermal image’ of predators or prey. Infrared signals are initially received by the pit organ, a highly specialized facial structure that is innervated by nerve fibres of the somatosensory system. How this organ detects and transduces infrared signals into nerve impulses is not known. Here we use an unbiased transcriptional profiling approach to identify TRPA1 channels as infrared receptors on sensory nerve fibres that innervate the pit organ. TRPA1 orthologues from pit-bearing snakes (vipers, pythons and boas) are the most heat-sensitive vertebrate ion channels thus far identified, consistent with their role as primary transducers of infrared stimuli. Thus, snakes detect infrared signals through a mechanism involving radiant heating of the pit organ, rather than photochemical transduction. These findings illustrate the broad evolutionary tuning of transient receptor potential (TRP) channels as thermosensors in the vertebrate nervous system. Only four vertebrate species are known to possess the 'sixth sense' of infrared detection, which is used for both predatory and thermoregulatory purposes. These creatures include three distantly related species of snake (pit vipers, pythons and boas) and the vampire bats. The pit organ that mediates this sense has been extensively studied from anatomical and behavioural perspectives, but little is known about the signal transduction mechanism underlying infrared detection, or the molecules involved. Now Gracheva et al. show that pit-bearing snakes rely on exquisite heat detection by the ion channel TRPA1. This extends the sensory repertoire of TRPA1 family of proteins, which detect chemical irritants in mammals and thermal variations in insects. Snakes are notoriously apt at generating 'thermal images' of predators or prey. The underlying physiology has been unclear, although in snakes such as pythons, vipers and boas, infrared signals are initially received by the pit organ. Here it is shown that pit-bearing snakes rely on heat detection by the ion channel TRPA1. This extends the sensory repertoire of the TRPA1 family of proteins, which detect chemical irritants in mammals and thermal variations in insects.