Discovery of a new cryptic Achalinus Peters, 1869 (Serpentes, Xenodermidae) species from Hunan Province, China.

A new species, Achalinusshenisp. nov., from central Hunan Province is described, based on the results of molecular systematics and morphological characters according to five specimens. Our molecular phylogeny inferred from the mitochondrial CO1 gene fragment revealed that this new species is most closely related to A.yunkaiensis, but a considerable amount of genetic divergence exists between them (p-distance ranging from 5.8% to 6.4%) and much distinct genetic divergence exists compared with other known Achalinus species (p-distance ranging from 10.4% to 15.8%), supporting its validity. Morphologically, it can be distinguished from its congeners by: (1) dorsal scales strongly keeled, 23 rows throughout the body, the outmost row smooth and significantly enlarged; (2) tail relatively short, TaL/TL 0.183 ~ 0.224; (3) the suture between internasals subequal to the suture between prefrontals; (4) loreal one, subrectangular, LorH/LorL 0.53 ~ 0.57; (5) ventrals 161-170, anal entire, subcaudals 55-61, not paired; (6) the length of supraocular equal to or longer than the length of upper anterior temporal; and (7) vertebral line inconspicuous and subcaudal streak absent. Currently, 27 species of Achalinus are known in the world, amongst which 20 species are distributed in China. Moreover, a key to species of the genus Achalinus is provided in this study.

Cascading effects of Pb on the environmental and symbiotic microbiota and tadpoles' physiology based on field data and laboratory validation.

Heavy metal pollution poses a serious threat to ecosystems. Currently, there is a lack of field data that would enable us to gain a systematic understanding of the influences of heavy metals on aquatic ecosystems, especially the interactions between environments and animals. We studied the relationships between the variations in heavy metal concentrations (10 species including Pb in sediments and surface water), the community structure of environmental and symbiotic microbiota, and the gut traits of Bufo gargarizans tadpoles across 16 sampling sites on the Chengdu Plain through rigorous statistical analysis and laboratory validation. The results show that heavy metal concentrations, especially the Pb concentration of the sediment, are linked to the variations in sediment and tadpoles' gut microbiomes but not to water microbiota. For the sediment microbiota, Pb causes a trade-off between the proportions of Burkholderiales and Verrucomicrobiae and affects the methane, sulfide, and nitrate metabolisms. For tadpoles, a high sediment Pb content leads to a low abundance of gut aerobic bacteria and a large relative gut weight under both field and laboratory conditions. In addition, Pb promotes the growth of B. gargarizans tadpoles under laboratory conditions. These effects seem to be beneficial to tadpoles. However, a high Pb content leads to a low abundance of probiotic bacteria (e.g., Verrucomicrobiae, Eubacteriaceae, and Cetobacterium) and a high abundance of pathogenic bacteria in the gut and environment, suggesting potential health risks posed by Pb. Interestingly, there is a causal relationship between Pb-induced variations in sediment and symbiotic microbiotas, and the latter is further linked to the variation in relative gut weight of tadpoles. This suggests a cascading effect of Pb on the ecosystem. In conclusion, our results indicate that among the heavy metals, the Pb in sediment is a critical factor affecting the aquatic ecosystem through an environment-gut-physiology pathway mediated by microbiota.

A wide hybrid zone mediated by precipitation contributed to confused geographical structure of Scutiger boulengeri.

Confused geographical structure of a population and mitonuclear discordance are shaped by a combination of rapid changes in population demographics and shifts in ecology. In this study, we generated a time-calibrated phylogeny of Scutiger boulengeri, an endemic Xizang alpine toad occurring in mountain streams on the Qinghai-Xizang (Tibet) Plateau (QTP). Based on three mitochondrial DNA (mtDNA) genes, eight clades were assigned to three deeply divergent lineages. Analysis of nuclear DNA (nuDNA) genes revealed three distinct clusters without geographic structure, indicating significantly high rates of gene flow. Coalescent theory framework analysis (approximate Bayesian computation model DIYABC and Migrate-N) suggested that divergence of the main intraspecific clusters was the result of hybridization after secondary contact in the Holocene around 0.59 million years ago (Ma). The ratio of mtDNA F ST (fixation index) to nuDNA F ST was 2.3, thus failing to show male-biased dispersal. Geographic cline analysis showed that a wide hybrid zone was initially established in southwestern China, without significant reproductive isolation but with strong introgression in S. boulengeri, suggesting high hybrid fitness. Furthermore, mtDNA genes exhibited isolation by distance (IBD) while nuDNA genes exhibited significant isolation by environment (IBE). Results suggested that mitonuclear discordance may have initially been caused by geographic isolation, followed by precipitation-mediated hybridization, producing a wide hybrid zone and geographic structure confusion of nuDNA genes in S. boulengeri. This study indicated that complicated historical processes may have led to specific genetic patterns, with a specific climate factor facilitating gene flow in the system.

A new species of Achalinus Peters, 1869 (Squamata, Xenodermidae) from Hunan Province, China.

A new species, Achalinushunanensissp. nov., is described from middle and western Hunan Province based on the results of molecular systematics and morphological characters. It diverges from known congeners by a significant genetic divergence (p-distance 3.2%-16.9% based on CO1 mitochondrial gene), and it can be distinguished from all known congeners by the following morphological characters: (1) all dorsal scales strongly keeled, 23 rows throughout the body, the outmost one strongly keeled and enlarged; (2) tail relatively short, TaL/TL 0.221 ~ 0.225; (3) maxillary teeth 23; (4) the suture between internasals 2 × as long as that between prefrontals; (5) loreal one, subrectangular, LorH/LorL 0.62 ~ 0.70; (6) supralabials 6, the 4th and 5th touch the eye; (7) the two anterior temporals in contact with eye; (8) ventrals 163-165, subcaudals 69-72, not paired. This raises the number of known species of Achalinus to 24.

A new Asian lazy toad of the genus Scutiger Theobald, 1868 (Anura, Megophryidae) from southern Tibet, China.

In this study, a new species named Scutigerluozhaensissp. nov. is described from Luozha, southern Tibet, China. Genetic analysis based on two mitochondrial genes 16S rRNA and COI and the nuclear gene RAG1 revealed that the new species belongs to an independent phylogenetic clade close to S.gongshanensis and S.nyingchiensis and shares no RAG1 haplotype with other species. Morphological comparisons based on examined specimens and literatures indicated that it can be diagnosed from congeners by the following combination of characters: (1) body moderate, male body length 47.0-67.2 mm (n = 13), female body length 49.8-66.2 mm (n = 8); (2) maxillary teeth and budding absent; (3) numerous tiny dense nuptial spines present on dorsal surface of fingers I, II and inner surface of finger III of males in breeding condition with similar size; (4) spine patches on belly of males in breeding condition absent; (5) spines on inner surface of forearm and upper arm of males in breeding condition absent; (6) small patches of black spines present near armpit of males in breeding condition absent; (7) adult males without vocal sac; (8) some large warts and tubercles on dorsum gathered into short skin ridges with several spines present on top; (9) space between upper eyelids wider than upper eyelids; (10) spots or irregular cross bands on limbs absent; (11) webbing between toes rudimentary; (12) coloration of dorsal body olive brown to bronze.

Ossification patterns of the carpus and tarsus in salamanders and impacts of preaxial dominance on the fin-to-limb transition.

Early limb skeletogenesis in salamanders is characterized by preaxial elements, digits I and II forming earlier than their postaxial counterparts (digits III to V), a phenomenon known as preaxial dominance, whereas in amniotes and anurans, these developmental sequences are reversed. This pattern characterizes the late skeletogenesis of digits and zeugopodium of anamniote tetrapods but remains unknown in carpals/tarsals. To correct this gap in knowledge, we investigate the ossification patterns of the carpals/tarsals in six salamander families/clades based on micro-computed tomography scans. We found that preaxial dominance is seen in the distal carpals/tarsals of several salamander clades and diverse early tetrapods, such as temnospondyls and amniotes. This distribution suggests that preaxial dominance is a primitive developmental pattern in tetrapods. Our results demonstrate that the distal carpals/tarsals are developmentally and evolutionarily independent in the autopodium, and preaxial dominance facilitates stabilization of the number of distal carpals/tarsals during fin-to-limb transition and digit reduction in early tetrapods.

Multi-omics provide mechanistic insight into the Pb-induced changes in tadpole fitness-related traits and environmental water quality.

Water pollution from lead/Pb2+ poses a significant threat to aquatic ecosystems, and its repercussions on aquatic animals have received considerable attention. Although Pb2+ has been found to affect numerous aspects of animals, including individual fitness, metabolic status, and symbiotic microbiota, few studies have focused on the associations between Pb2+-induced variations in fitness, metabolome, symbiotic microbiome, and environmental parameters in the same system, limiting a comprehensive understanding of ecotoxicological mechanisms from a holistic perspective. Moreover, most ecotoxicological studies neglected the potential contributions of anions to the consequences generated by inorganic lead compounds. We investigated the effects of Pb(NO3)2 at environmentally relevant concentrations on the Rana omeimontis tadpoles and the water quality around them, using blank and NaNO3-treated groups as control. Results showed that Pb(NO3)2 not only induced a rise in water nitrite level, but exposure to this chemical also impaired tadpole fitness-related traits (e.g., growth and development). The impacts on tadpoles were most likely a combination of Pb2+ and NO3-. Tissue metabolomics revealed that Pb(NO3)2 exposure influenced animal substrate (i.e., carbohydrate, lipid, and amino acid) and prostaglandin metabolism. Pb(NO3)2 produced profound shifts in gut microbiota, with increased Proteobacteria impairing Firmicutes, resulting in higher aerobic and possibly pathogenic bacteria. NaNO3 also influenced tadpole metabolome and gut microbiome, in a manner different to that of Pb(NO3)2. The presence of NO3- seemed to counteract some changes caused by Pb2+, particularly on the microbiota. Piecewise structural equation model and correlation analyses demonstrated connections between tissue metabolome and gut microbiome, and the variations in tadpole phenotypic traits and water quality were linked to changes in tissue metabolome and gut microbiome. These findings emphasized the important roles of gut microbiome in mediating the effects of toxin on aquatic ecosystem. Moreover, it is suggested to consider the influences of anions in the risk assessment of heavy metal pollutions.

Citrus aurantium 'Changshan-huyou'-An ethnopharmacological and phytochemical review.

Citrus fruits are composed of oil cells layer, white membrane layer, pulp and seeds. The cultivar Citrus aurantium 'Changshan-huyou' (CACH) is a hybridization of Citrus grandis Osbeck and C. sinensis Osbeck. It is a rutaceae plant, and mainly grows in Changshan, Zhejiang, China. With the exploration of its high traditional values, it has been paid more and more attention by the scientific community in recent years. At present, one hundred and two chemical constituents have been identified from the pulp and peel of CACH, including volatile oils, terpenoids, phenols, limonins, sugars, etc., As the representative active component of CACH, phenols have been widely investigated. Studies have shown that CACH shows a variety of significant pharmacological activities, such as anti-inflammatory, antioxidant, hepatoprotective activity, respiratory system protection and intestinal regulation activity. This review mainly introduces the chemical constituents and pharmacological activities of CACH, and discusses its future research and development directions. It will provide theoretical basis for further research of its pharmacodynamic substances, functional mechanism and rational utilization.

Temperature and Diet Acclimation Modify the Acute Thermal Performance of the Largest Extant Amphibian.

The Chinese giant salamander (Andrias davidianus), one of the largest extant amphibian species, has dramatically declined in the wild. As an ectotherm, it may be further threatened by climate change. Therefore, understanding the thermal physiology of this species should be the priority to formulate related conservation strategies. In this study, the plasticity in metabolic rate and thermal tolerance limits of A. davidianus larvae were studied. Specifically, the larvae were acclimated to three temperature levels (7 °C, cold stress; 15 °C, optimum; and 25 °C, heat stress) and two diet items (red worm or fish fray) for 20 days. Our results indicated that cold-acclimated larvae showed increased metabolic capacity, while warm-acclimated larvae showed a decrease in metabolic capacity. These results suggested the existence of thermal compensation. Moreover, the thermal tolerance windows of cold-acclimated and warm-acclimated larvae shifted to cooler and hotter ranges, respectively. Metabolic capacity is not affected by diet but fish-fed larvae showed superiority in both cold and heat tolerance, potentially due to the input of greater nutrient loads. Overall, our results suggested a plastic thermal tolerance of A. davidianus in response to temperature and diet variations. These results are meaningful in guiding the conservation of this species.

Multidimensional amphibian diversity and community structure along a 2 600 m elevational gradient on the eastern margin of the Qinghai-Tibetan Plateau.

Mountain systems harbor an evolutionarily unique and exceptionally rich biodiversity, especially for amphibians. However, the associated elevational gradients and underlying mechanisms of amphibian diversity in most mountain systems remain poorly understood. Here, we explored amphibian phylogenetic and functional diversity along a 2 600 m elevational gradient on Mount Emei on the eastern margin of the Qinghai-Tibetan Plateau in southwestern China. We also assessed the relative importance of spatial (area) and environmental factors (temperature, precipitation, solar radiation, normalized difference vegetation index, and potential evapotranspiration) in shaping amphibian distribution and community structure. Results showed that the phylogenetic and functional diversities were unimodal with elevation, while the standardized effect size of phylogenetic and functional diversity increased linearly with elevation. Phylogenetic net relatedness, nearest taxon index, and functional net relatedness index all showed a positive to negative trend with elevation, indicating a shift from clustering to overdispersion and suggesting a potential change in key processes from environmental filtering to competitive exclusion. Overall, our results illustrate the importance of deterministic processes in structuring amphibian communities in subtropical mountains, with the dominant role potentially switching with elevation. This study provides insights into the underlying assembly mechanisms of mountain amphibians, integrating multidimensional diversity.

Rapid genetic divergence and mitonuclear discordance in the Taliang knobby newt ( Liangshantriton taliangensis, Salamandridae, Caudata) and their driving forces.

The Hengduan Mountains Region (HMR) is the largest "evolutionary frontier" of the northern temperate zone, and the origin and maintenance of species in this area is a research hotspot. Exploring species-specific responses to historical and contemporary environmental changes will improve our understanding of the role of this region in maintaining biodiversity. In this study, mitochondrial and microsatellite diversities were used to assess the contributions of paleogeological events, Pleistocene climatic oscillations, and contemporary landscape characteristics to the rapid intraspecific diversification of Liangshantriton taliangensis, a vulnerable amphibian species endemic to several sky-island mountains in the southeastern HMR. Divergence date estimations suggested that the East Asian monsoon, local uplifting events (Xigeda Formation strata), and Early-Middle Pleistocene transition (EMPT) promoted rapid divergence of L. taliangensis during the Pleistocene, yielding eight mitochondrial lineages and six nuclear genetic lineages. Moreover, population genetic structures were mainly fixed through isolation by resistance. Multiple in situ refugia were identified by ecological niche models and high genetic diversity, which played crucial roles in the persistence and divergence of L. taliangensis during glacial-interglacial cycles. Dramatic climatic fluctuations further promoted recurrent isolation and admixing of populations in scattered glacial refugia. The apparent mitonuclear discordance was likely the result of introgression by secondary contact and/or female-biased dispersal. Postglacial expansion generated two major secondary contact zones (Ganluo (GL) and Chuhongjue (CHJ)). Identification of conservation management units and dispersal corridors offers important recommendations for the conservation of this species.

Exploring cryptic biodiversity in a world heritage site: a new pitviper (Squamata, Viperidae, Crotalinae) from Jiuzhaigou, Aba, Sichuan, China.

This study presents a comprehensive morphological comparison along with molecular phylogeny of the genus Gloydius based on five mitochondrial genes (12S, 16S, COI, cytb, and ND4). The specimens collected from Jiuzhaigou National Nature Reserve are shown to be a new species, Gloydiuslateralis sp. nov. Zhang, Shi, Jiang & Shi based on a combination of morphological and molecular accounts. G.lateralis sp. nov. differs from other congeneric species by a series of diagnostic morphological characteristics and forms a strongly supported monophyletic group. The new species is phylogenetically closely related to G.swild, another recently described species from Heishui, Aba, Sichuan.

Systematics, distribution and morphology of the newt parasitic water mites of the subgenus Lurchibates Goldschmidt amp; Fu, 2011 (Acari, Hydrachnidia, Hygrobatidae, Hygrobates Koch, 1837), including the description of four new species and a key to all so far known species.

Four new water mite species of the genus Hygrobates, subgenus Lurchibates (Acari, Hydrachnidia, Hygrobatidae) are described. Hygrobates (Lurchibates) macrochela sp. nov. Goldschmidt, Nishikawa Shimano is described in male and female; Hygrobates (Lurchibates) malosimilis sp. nov. Goldschmidt, Nishikawa Shimano is described in male, both new water mite species were collected from newts of the species Pachytriton inexpectatus Nishikawa, Jiang, Matsui Mo, 2011; Hygrobates (Lurchibates) incognitus sp. nov. Goldschmidt, Nishikawa Shimano is described in female collected from newts of the species Paramesotriton guangxiensis (Huang, Tang and Tang, 1983); Hygrobates (Lurchibates) fragmentarius sp. nov. Goldschmidt, Nishikawa Shimano is described in a single severely fragmented specimen (sex could not be determined) collected from a newt of the species Paramesotriton yunwuensis Wu, Jiang, and Hanken, 2010. Principal differences between the subgenus Lurchibates and the nominal subgenus Hygrobates s. str. are illustrated and discussed as well as the sexual dimorphism of Lurchibates. Morphometric analyses confirmed the morphological differences of ten out of the eleven so far described species (H. (L.) fragmentarius could not be included in the analysis); two morphological groups became evident mainly based upon the shape of the anterior coxae. These analyses as well provide an idea of possible phylogenetic relationships among the species. A key to all currently known species of the subgenus Lurchibates is given. So far the subgenus is restricted to SE-Asia, a map showing the distribution of all species is presented.

Spatiotemporal patterns of anuran functional diversity in temperate montane forests.

Functional diversity is an integrative approach to better understand biodiversity across space and time. In the present study, we investigated the spatiotemporal patterns (i.e., elevation and season) and environmental determinants of anuran functional diversity on Tianping Mountain, northwest Hunan, China. Specifically, 10 transects were established from low (300 m a.s.l.) to high (1 492 m a.s.l.) elevations, and anuran communities were sampled in spring, early summer, midsummer, and autumn in 2017. Four functional diversity indices were computed for each transect in each season using ecomorphological functional traits. Our results demonstrated that these indices had contrasting responses to increasing elevations. However, they did not differ significantly among seasons in terms of temporal patterns. Interestingly, the unique spatiotemporal functional diversity patterns were impacted by distinct environmental variables, such as leaf litter cover, water temperature, number of trees, and water conductivity.

Fatter or stronger: Resource allocation strategy and the underlying metabolic mechanisms in amphibian tadpoles.

The allocation of resources between storage and somatic growth is an essential physiological phenomenon in animals. Allocation mechanisms have broad theoretical and applied implications. The real-time resource allocation patterns in animals remain to be elucidated, and there is limited understanding of the metabolic mechanisms. We investigated the resource allocation strategy of Rana omeimontis tadpoles. Their ontogenetic fat accumulation began when body weight increased to 30-50 mg, at which time storage had a high priority in resource allocation. Beyond this weight range, somatic growth accelerated but storage investment was maintained, resulting in a positive correlation between body fat index and body weight at the population level. This pattern could be explained by assuming a positive relationship between storage abundance and growth investment, and this was supported by the prioritized increment of body fat to body weight when tadpoles were provided with increased food. At the metabolic level, hepatic fat accumulation was accompanied by upregulated utilization of fat storage, and the tadpoles presented lipid-based energy metabolism. Activating the mobilization of hepatic fat storage promoted somatic growth. In short, the liver is like a reservoir with valves that regulate energy flow for downstream developmental processes. These results provide novel mechanistic insights into resource allocation.

Phylogeographic study of the Bufo gargarizans species complex, with emphasis on Northeast Asia.

We conduct a phylogeographic and population genetic study of the Asiatic toad (Bufo gargarizans) to understand its evolutionary history, and the influence of geology and climate. A total of 292 individuals from 94 locations were genotyped for two mitochondrial loci (cytb, ND2) and five nuclear introns (Sox9-2, Rho-3, CCNB2-3, UCH-2, and DBI-2), and we performed a suite of phylogenetic, population genetic, and divergence dating analyses. The phylogenetic trees constructed using mitochondrial loci inferred B. gargarizans being divided into two major groups: China mainland and Northeast Asia (Northeast China, Russia, and Korean Peninsula). As with previous studies of this species, we recover population genetic structure not tied to geographic region. Additionally, we discover a new genetic clade restricted to Northeast Asia that points towards the Korean Peninsula being a glacial refugium during the Pleistocene. The weak phylogeographic pattern of B. gargarizans is likely the result of multiple biological, anthropogenic, and historical factors - robust dispersal abilities as a consequence of physiological adaptations, human translocation, geologic activity, and glacial cycles of the Pleistocene. We highlight the complex geologic and climatic history of Northeast Asia and encourage further research to understand its impact on the biodiversity in the region.

Comparative osteology of the hynobiid complex Liua-Protohynobius-Pseudohynobius (Amphibia, Urodela): â . Cranial anatomy of Pseudohynobius.

Hynobiidae are a clade of salamanders that diverged early within the crown radiation and that retain a considerable number of features plesiomorphic for the group. Their evolutionary history is informed by a fossil record that extends to the Middle Jurassic Bathonian time. Our understanding of the evolution within the total group of Hynobiidae has benefited considerably from recent discoveries of stem hynobiids but is constrained by inadequate anatomical knowledge of some extant forms. Pseudohynobius is a derived hynobiid clade consisting of five to seven extant species living endemic to southwestern China. Although this clade has been recognized for over 37 years, osteological details of these extant hynobiids remain elusive, which undoubtedly has contributed to taxonomic controversies over the hynobiid complex Liua-Protohynobius-Pseudohynobius. Here we provide a bone-by-bone study of the cranium in the five extant species of Pseudohynobius (Ps. flavomaculatus, Ps. guizhouensis, Ps. jinfo, Ps. kuankuoshuiensis and Ps. shuichengensis) based on x-ray computer tomography data for 18 specimens. Our results indicate that the cranium in each of these species has a combination of differences in morphology, proportions and articulation patterns in both dermal and endochondral bones. Our study establishes a range of intraspecific differences that will serve as organizing hypotheses for future studies as more extensive collections of these species become available. Morphological features in the cranium for terrestrial ecological adaptation in Hynobiidae are summarized. Based on the results, we also discuss the evolution and development of several potential synapomorphies of Hynobiidae, including features of the orbitosphenoid and articular.

Efficacy and Safety Profile of Montgomery T-Tube Implantation in Patients with Tracheal Stenosis.

Background: Tracheal stenosis is able to lead to airway obstruction. Objective: To evaluate the efficacy and safety profile of Montgomery T-tube implantation in patients with tracheal stenosis. Methods: Fifty-two patients with tracheal stenosis diagnosed between 2016 and 2019 were included in this retrospective cohort study. The patients were divided into observation group (n = 25 cases) and control group (n = 27). The therapeutic effect, arterial blood gas analysis, arterial oxygen partial pressure (PaO2), arterial carbon dioxide partial pressure (PaCO2), shortness of breath score, airway diameter change, dyspnea score, quality of life, and safety were compared between the two groups before and after treatment. Results: The therapeutic effect of the observation group gained better results than that of the control group (84.00% vs. 62.96%). One week after operation, the pH value, SaO2, PaCO2, shortness of breath score, airway diameter change, dyspnea score, life quality, and incidence of postoperative complications in the observation group exerted better results as compared to the control group. Conclusion: The implantation of Montgomery T-tube has effective function in terms of improving the symptoms of dyspnea and the life quality of patients with safety profile in patients harboring tracheal stenosis.

Spatial and temporal patterns of amphibian species richness on Tianping Mountain, Hunan Province, China.

Exploring species richness patterns across space and time can help in understanding species distribution and in formulating conservation strategies. Among taxa, amphibians are of utmost importance as they are highly sensitive to environmental changes due to their unique life histories (Zhong et al., 2018). Here, we investigated the spatial and temporal patterns of amphibian species richness on Tianping Mountain in China. Specifically, we established 10 transects at low to high elevations, and sampled amphibians in April, June, August, and October 2017. Our results demonstrated that amphibian species composition and richness varied significantly at both spatial and temporal scales and were associated with gradients of environmental change in microhabitats on Tianping Mountain.

[Involvement of intrathecal activation of MrgC receptor in pathological pain and morphine tolerance].

Rodent MrgC receptor (Mas-related G-protein-coupled receptor subtype C) shares 65% sequence homology and similarities in terms of expression pattern and binding profile with human Mas-related gene X receptor 1 (hMrgX1). Therefore, researchers generally explore the role of hMrgX1 by studying the function of MrgC receptor. Murine MrgC receptor is uniquely expressed in small-diameter neurons of dorsal root ganglia (DRG) and trigeminal ganglia (TG), which is closely related to the transmission process of pain. This review summarizes the analgesic effects of intrathecal activation of MrgC receptors in pathological pain and morphine tolerance.