Molecular basis of infrared detection by snakes

Abstract

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.

Figure 1. Anatomy of the pit organ and comparison of gene expression in snake sensory ganglia

a Rattlesnake head showing location of nostril and loreal pit organ (black and red arrows, respectively)(from Wikimedia Commons). b Schematic of pit organ structure showing innervation of pit membrane suspended within hollow cavity. (c – d) Number of mRNA-Seq reads from snake ganglia that align to the chicken proteome. TRPA1 and TRPV1 are highlighted, as are other TRP channels. Blue line indicates expected number of sequencing reads for genes with similar expression levels in the two samples based on total number of aligned reads from each. Signals < 20 reads are within statistical noise and therefore scored as non-expressed sequences. Rattlesnake refers to C. atrox and non-pit refers to a combination of Texas Rat (Elaphe obsolete lindheimerii) and Western Coachwhip (Masticophis flagellum testaceus) snakes.

Figure 2. Expression of TRPA1 and TRPV1 in rattlesnake sensory ganglia

a In situ hybridization showing expression of TRPA1 or TRPV1 in tissue sections from rattlesnake TG or DRG, as indicated. Scale bar = 20 µm. b Quantification of neuronal cell size (diameter) determined from histological sections of rattlesnake TG (n = 70 cells from 5 independent sections). c Quantitative analysis of cells within TG or DRG that express TRPA1 or TRPV1 transcripts (mean ± s.d.; n = 448 neurons from 11 independent sections for TRPA1 and 151 neurons from 5 independent sections for TRPV1).

Figure 3. Functional analysis of snake TRPA1 channels

a HEK293 cells expressing cloned rattlesnake or rat snake TRPA1 channels were analyzed for heat- or mustard oil (AITC; 200 µM; 24°C)-evoked responses using calcium imaging; color bar indicates relative change in fluorescence ratio, with purple and white denoting lowest and highest cytoplasmic calcium, respectively (n ≥ 105 neurons per species). b Relative heat response profiles of rattlesnake and rat snake channels expressed in oocytes (response at each temperature was normalized to maximal response at 45°C (V_h = +80 mV; n ≥ 6). c Arrhenius plots show thermal thresholds and Q₁₀ values for baseline and evoked responses of rattlesnake and rat snake TRPA1 channels, as indicated (temperature ramp of 1°C/sec).

Figure 4. Analysis of TRPA1 from python and boa

a – b Transcriptome profiling of ancient snakes. Number of mRNA-Seq reads from python and boa ganglia that align to chicken proteome, as described in Fig. 1. c Phylogenetic tree of TRPA1 channel protein sequences with bootstrap values from 100 trials. Red denotes heat-sensitive channels with lower thermal threshold compared to rat snake (orange). Blue indicates non-heat sensitive channels according to this study. d Relative heat response profiles for python and boa TRPA1 as measured in oocytes (recorded and normalized as described in Fig 3b).

Figure 5. Functional analysis of snake sensory neurons

a Expression of TRPA1 or TRPV1 transcripts in python and rat snake TG (scale bar = 40 µm). b Thermal sensitivity of python and rat snake TG neurons as measured by calcium imaging. Temperature ramps (24 to 46°C) were applied by continuous perfusion to assess thresholds (color scale as in Fig. 3a). Corresponding temperature-response profiles are shown at right (n = 5 and 26 neurons, respectively). Thresholds (28.0 ± 0.7 and 36.2 ± 0.6; P < 0.0001) were determined from average of 43 and 89 neurons from python and rat snake, respectively (10 independent fields each). c Patch-clamp recordings from python neurons showing robust heat- and AITC-evoked currents that were suppressed by cold (left) and blocked by ruthenium red (RR, 10 µM) (center)(n > 45). A minority of neurons was insensitive to heat and AITC (right)(n > 8).