Bioactive peptides liberated from precursor proteins by hydrolysis, typically consist of 2–20 amino acid residues and exhibit various biological functions (Kong et al., 2023; Sheng et al., 2023; Wang et al., 2022; Zhang et al., 2022). Neuropeptides are active biological polypeptides secreted by neurones to transmit important regulatory signals in animals and are involved in numerous physiological processes (Qiu et al., 2022; Wang et al., 2015). The first isolated mollusc neuropeptide was FMRFamide, which was derived from the brain ganglion of the bivalve Macrocallista nimbosa (Price and Greenberg, 1977). FMRFamides have diverse physiological functions in various mollusc species. In addition to controlling hydric balance (Salzet et al., 1994), feeding (Dockray, 2004), and immune defence (Li et al., 2019), FMRFamide can also regulate synaptic plasticity in Aplysia sp. (Guan et al., 2003) and promote blood glucose regulation in Helix aspersa (Rőszer and Kiss-Tóth, 2014. It is involved in the cell apoptosis of Helix lucorum (Rőszer et al., 2006a), regulates the signals of the visual system of Octopus vulgaris (Di Cosmo and Di Cristo, 1998), and regulates the reproductive process of cephalopods (Di Cristo et al., 2003).

FMRFamide binds to G protein-coupled receptors (GPCR) or FMRFamide Na+-gated channels (FaNaC). FaNaC was first cloned from the nervous tissues of H. aspersa and was shown to react with FMRFamide and FLRFamides (Lingueglia et al., 1995). FaNaC, a member of the degenerin/epithelial sodium channel (ENaC/DEG) superfamily, is a non-voltage-gated sodium selective channel (Lingueglia et al., 2006; Perry et al., 2001). The ENaC/DEG family is a cluster of ion channels with two transmembrane domains and has various functions, such as pain sensation after tissue acidosis, neurotransmitters, and neuropeptide signalling (Axel et al., 2018; Benos and Stanton, 1999; Kellenberger and Schild, 2002; Stefan and Assmann, 2015). The structure and function of FaNaC in molluscs are similar to those of acid-sensitive ion channels (ASICs) in mammals (Kellenberger and Schild, 2002; Perry et al., 2001). Although FMRFamide directly gates FaNaC, it also modulates the proton-gating properties of ASICs (Askwith et al., 2000; Perry et al., 2001). ASICs are regulated by FMRFamide as well as the mammalian polypeptides neuropeptide FF (NPFF), neuropeptide AF (NPAF), and neuropeptide SF (NPSF) (Askwith et al., 2000; Lingueglia et al., 2006). So far, members of FaNaCs from 12 species have been cloned and characterised, including Helix aspersa (HaFaNaC) (Lingueglia et al., 1995), Aplysia kurodai (AkFaNaC) (Furukawa et al., 2006), Helisoma trivolvis (HtFaNaC) (Jeziorski et al., 2000), Lymnaea stagnalis (LsFaNaC) (Perry et al., 2001), Sepia pharonis (Cao et al., 2023) and those from 7 other species (Dandamudi et al., 2022). The structures of FMRFamide-bound and apo FaNaC from Aplysia californica (AcFaNaC) were determined using site-directed mutagenesis, electrophysiology, and molecular dynamics (MD) simulations (Liu et al., 2023). Their work provided vital insights into the in-depth structural and functional understanding of FaNaC.

The cuttlefish Sepiella japonica, belongs to the phylum Mollusca. It used to be one of the four traditional seafoods of the East China Sea (Wu et al., 2010) because of its great nutritional and medicinal value. However, since the mid-1970s, wild resources of this species have been severely damaged by overexploitation and deteriorating environmental conditions. However, it has significantly recovered over the years with the development of artificial breeding and proliferation in many coastal areas (Shi et al., 2015). Nevertheless, S. japonica displayed problems such as precocious puberty, small individuals, and low fecundity under artificial culture conditions. Therefore, exploring the potential physiological functions of FMRFamide and FaNaC in S. japonica (SjFaNaC) is necessary.

In this study, the full-length cDNA of SjFaNaC was cloned, its expression profile was detected during all developmental stages of males and females using quantitative Real-time PCR (qRT-PCR), and its tissue distribution was investigated using in situ hybridisation (ISH). Finally, the subcellular localisation of SjFaNaC was observed in HEK293 cells. These results provide basic information to further uncover the possible roles of SjFaNaC in multiple physiological processes in S. japonica.