A descending inhibitory mechanism of nociception mediated by an evolutionarily conserved neuropeptide system in Drosophila.

Nociception is a neural process that animals have developed to avoid potentially tissue-damaging stimuli. While nociception is triggered in the peripheral nervous system, its modulation by the central nervous system is a critical process in mammals, whose dysfunction has been extensively implicated in chronic pain pathogenesis. The peripheral mechanisms of nociception are largely conserved across the animal kingdom. However, it is unclear whether the brain-mediated modulation is also conserved in non-mammalian species. Here, we show that Drosophila has a descending inhibitory mechanism of nociception from the brain, mediated by the neuropeptide Drosulfakinin (DSK), a homolog of cholecystokinin (CCK) that plays an important role in the descending control of nociception in mammals. We found that mutants lacking dsk or its receptors are hypersensitive to noxious heat. Through a combination of genetic, behavioral, histological, and Ca2+ imaging analyses, we subsequently revealed neurons involved in DSK-mediated nociceptive regulation at a single-cell resolution and identified a DSKergic descending neuronal pathway that inhibits nociception. This study provides the first evidence for a descending modulatory mechanism of nociception from the brain in a non-mammalian species that is mediated by the evolutionarily conserved CCK system, raising the possibility that the descending inhibition is an ancient mechanism to regulate nociception.

Avoiding harm is fundamental for the survival of animals. Nerve cells called nociceptors can detect potential damage, such as extreme temperatures, sharp objects and certain chemicals. In humans, this detection ­ known as nociception ­ leads to signals travelling from nociceptors through the spinal cord to the brain, which perceives them as pain. Mammals such as humans and rodents can inhibit nociception by sending signals from the brain to the spinal cord to dampen pain. This top-down dampening process is believed to play a crucial role in regulating pain in mammals, and it has been implicated in the development of chronic pain. It was not known whether non-mammalian animals shared this inhibitory pathway. However, previous work had shown that fruit fly produce a molecule called Drosulfakinin, which is similar to the chemical that mammals use in the top-down signalling pathway which controls pain. To determine the role of Drosulfakinin in controlling fly nociception, Oikawa et al. manipulated its activity ­ and the activity of related genes ­ in specific neurons in the fruit fly nervous system. Without Drosulfakinin, fly larvae were more sensitive to heat exposure, suggesting that this molecule is required to inhibit nociception. Further experiments showed that Drosulfakinin is present only in the brain of fly larvae and activation of its signaling lowers the activity of neurons that transmit nociceptive signals in the insect equivalent of the spinal cord. This confirms that insect brains can dampen nociception via a top-down pathway, using a similar molecule to mammals. The findings provide an important foundation for pain studies using non-mammalian animals. The ability to manipulate nociception using genetic techniques in flies offers a powerful tool to understand the top-down process of controlling pain. This result also raises the possibility that this shared top-down inhibition mechanism may have developed over 550 million years ago, which could lead to further research into how nociception and pain regulation systems evolved.

Design and Synthesis of Orexin 1 Receptor-Selective Agonists.

Orexins are a family of neuropeptides that regulate various physiological events, such as sleep/wakefulness as well as emotional and feeding behavior, and that act on two G-protein-coupled receptors, i.e., orexin 1 (OX1R) and orexin 2 receptors (OX2R). Since the discovery that dysfunction of the orexin/OX2R system causes the sleep disorder narcolepsy, several OX2R-selective and OX1/2R dual agonists have been disclosed. However, an OX1R-selective agonist has not yet been reported, despite the importance of the biological function of OX1R. Herein, we report the discovery of a potent OX1R-selective agonist, (R,E)-3-(4-methoxy-3-(N-(8-(2-(3-methoxyphenyl)-N-methylacetamido)-5,6,7,8-tetrahydronaphthalen-2-yl)sulfamoyl)phenyl)-N-(pyridin-4-yl)acrylamide [(R)-YNT-3708; EC50 = 7.48 nM for OX1R; OX2R/OX1R EC50 ratio = 22.5]. The OX1R-selective agonist (R)-YNT-3708 exhibited antinociceptive and reinforcing effects through the activation of OX1R in mice.

Synthesis of unnatural morphinan compounds to induce itch-like behaviors in mice: Towards the development of MRGPRX2 selective ligands.

Mas-related G protein-coupled receptor X2 (MRGPRX2) mediates the itch response in neurons and is involved in atopic dermatitis (AD)-associated inflammation and itch. Potent and MRGPRX2-selective ligands are essential to an understanding of the detailed function of the receptor and to develop new therapeutic agents for its related diseases. (+)-TAN-67 (1), the enantiomer of the δ-opioid receptor (DOR) selective ligand (-)-TAN-67 (1), has been reported to activate MRGPRX2, although (+)-1 also interacts with DOR, which prevents investigators from interrogating the function of MRGPRX2. Here, we have succeeded in developing a novel unnatural morphinan compound (+)-2a by a transformation based on the structure of (+)-1, which removes the DOR binding affinity. (+)-2a activated both human MRGPRX2 and the mouse orthologue Mrgprb2 in in vitro experiments and induced itch-like behaviors in mice to the same extent as (+)-1. The (+)-2a-induced itch response in mice was suppressed by administration of the tripeptide QWF, an MRGPRX2/Mrgprb2 antagonist, or the antipruritic drug nalfurafine. Together, (+)-2a serves as a useful tool to elucidate the itch-related function/action of MRGPRX2 and its mouse orthologue Mrgprb2.