Alleviation of thermal nociception depends on heat-sensitive neurons and a TRP channel in the brain

Acute avoidance of dangerous temperatures is critical for animals to prevent or minimize injury. Therefore, surface receptors have evolved to endow neurons with the capacity to detect noxious heat so that animals can initiate escape behaviors. Animals including humans have evolved intrinsic pain-suppressing systems to attenuate nociception under some circumstances. Here, using Drosophila melanogaster, we uncovered a new mechanism through which thermal nociception is suppressed. We identified a single descending neuron in each brain hemisphere, which is the center for suppression of thermal nociception. These Epi neurons, for Epione-the goddess of soothing of pain-express a nociception-suppressing neuropeptide Allatostatin C (AstC), which is related to a mammalian anti-nociceptive peptide, somatostatin. Epi neurons are direct sensors for noxious heat, and when activated they release AstC, which diminishes nociception. We found that Epi neurons also express the heat-activated TRP channel, Painless (Pain), and thermal activation of Epi neurons and the subsequent suppression of thermal nociception depend on Pain. Thus, while TRP channels are well known to sense noxious temperatures to promote avoidance behavior, this work reveals the first role for a TRP channel for detecting noxious temperatures for the purpose of suppressing rather than enhancing nociception behavior in response to hot thermal stimuli.

Allatostatins C, double C and triple C, the result of a local gene triplication in an ancestral arthropod

Allatostatin C is the arthropod homolog of vertebrate somatostatin. The gene went through a local gene triplification leading to the existence of three genes coding such peptides, allatostatins C, CC and CCC. All three genes are still present in several chelicerates, such as the horseshoe crab Limulus polyphemus, several spiders and the scorpion Mesobuthus martensii, the myriapod Strigamia maritima, as well as at least two insect species, Locusta migratoria and Athalia rosae, a sawfly. All three peptides have well conserved primary structures and peptides can easily be classified as either allatostatin C, CC or CCC. In most insect species only two of the genes have been preserved. In many species, these are CC and CCC, but in Diptera, Lepidoptera and Coleoptera it are allatostatins C and CC that are still present. In some arthropod species two or even all three genes can still be found closely associated in the genome and are present on the same scaffold showing that a local amplification was at the origin of these genes.