Seebacher F and Murray SA (2007), Transient receptor potential ion channels contr...

XB-ART-35592

PLoS One 2007 Mar 14;23:e281. doi: 10.1371/journal.pone.0000281.

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Transient receptor potential ion channels control thermoregulatory behaviour in reptiles.

Seebacher F , Murray SA .

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Biological functions are governed by thermodynamics, and animals regulate their body temperature to optimise cellular performance and to avoid harmful extremes. The capacity to sense environmental and internal temperatures is a prerequisite for the evolution of thermoregulation. However, the mechanisms that enable ectothermic vertebrates to sense heat remain unknown. The recently discovered thermal characteristics of transient receptor potential ion channels (TRP) render these proteins suitable to act as temperature sensors. Here we test the hypothesis that TRPs are present in reptiles and function to control thermoregulatory behaviour. We show that the hot-sensing TRPV1 is expressed in a crocodile (Crocodylus porosus), an agamid (Amphibolurus muricatus) and a scincid (Pseudemoia entrecasteauxii) lizard, as well as in the quail and zebrafinch (Coturnix chinensis and Poephila guttata). The TRPV1 genes from all reptiles form a unique clade that is delineated from the mammalian and the ancestral Xenopus sequences by an insertion of two amino acids. TRPV1 and the cool-sensing TRPM8 are expressed in liver, muscle (transversospinalis complex), and heart tissues of the crocodile, and have the potential to act as internal thermometer and as external temperatures sensors. Inhibition of TRPV1 and TRPM8 in C. porosus abolishes the typically reptilian shuttling behaviour between cooling and heating environments, and leads to significantly altered body temperature patterns. Our results provide the proximate mechanism of thermal selection in terrestrial ectotherms, which heralds a fundamental change in interpretation, because TRPs provide the mechanism for a tissue-specific input into the animals' thermoregulatory response.

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Species referenced: Xenopus
Genes referenced: trpm8 trpv1

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	Figure 1. Initial 100 amino acids of the alignment of partial TRPV1 genes, showing a shared indel in the reptilian clade that is absent from the common ancestor, Xenopus tropicalis. Fragments were â¼650 bp (Crocodylus porosus, Poephila guttata, Coturnix chinensis) and â¼350 bp (Pseudomoia entrecasteauxii and Amphibolurus muricatus) (a). Phylogenetic relationships of partial TRPV1 and 2 genes based on an alignment of amino acids and analysed using Bayesian Inference (BI). The numbers at nodes indicate posterior probabilities and ML bootstrap values (BI/ML) for clades where these exceed 50%. The new sequences from this work are in red. The reptilian clade is shown in yellow. The full alignment is available from the authors.
	Figure 2. Phylogenetic relationships of TRPM8 genes based on an alignment of amino acids, analysed using BI. The numbers at nodes indicate posterior probabilities and ML bootstrap values (BI/ML) for clades, where it exceeds 50%. The new sequence from this work is in red, and it was â¼500 bp. The full alignment is available from the authors.
	Figure 3. TRPV1 mRNA is expressed in dorsal surface muscle, liver, and heart tissues. Relative expression is significantly greater in heart than in muscle or liver.
	Figure 4. Before treatment, both control (blue lines) and experimental (red lines) animals performed characteristic shuttling behaviour between heating and cooling environments, resulting in periodic oscillations in body temperature (a). After administration of capsazepine, the body temperature of experimental animals ceased to oscillate periodically, while body temperature patterns of control animals remained unchanged (b). Both (a) and (b) show representative examples of data from the same pair of animals. Body temperature time series were transformed into a frequency domain, and the periodogram of the control and experimental animals before treatment shows regular body temperature cycles at 43 and 86 min cycleâ1 (c). After treatment, these regular cycles disappear when TRPV1 and TRPM8 were blocked, but there is no change in control animals (d). Means from all animalsÂ±s.e. are shown in c and d.
	Figure 5. The total power density, i.e. the intensity of periodic body temperature cycles, is significantly less when TRPV1 and TRPM8 were blocked, compared to the other treatments which do not differ significantly from each other (CTLâ=âcontrol animals, EXPâ=âexperimental animals, DMSOâ=âDMSO treatment, CAPâ=âcapsazepine treatment; means from all animalsÂ±s.e. are shown).

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