Neuropathic pain, a debilitating condition affecting millions worldwide, results from lesions or diseases that impair the somatosensory nervous system (Jensen et al., 2011). It can arise from various causes, including metabolic disorders, viral infections, autoimmune diseases, chemotherapy-induced neuropathies, and traumatic nerve injuries (Colloca et al., 2017). Neuropathic pain is characterized by several key symptoms, such as allodynia (pain caused by stimuli that are typically not painful), hyperalgesia (increased sensitivity to pain), and paresthesia (abnormal sensations). Despite its prevalence, neuropathic pain remains difficult to manage, as current treatments often provide limited relief and fail to address the underlying pathophysiology.

Global prevalence estimates of neuropathic pain range from 3 % to 17 %, as its causes are varied, with assessment often relying on chronic pain criteria (Cavalli et al., 2019). The causes of neuropathic pain are diverse, and many patients do not receive adequate treatment (Colloca et al., 2017; Torrance et al., 2013). This may be due to incorrect diagnoses or the ineffectiveness of prescribed medications, including those recommended by the International Association for the Study of Pain (IASP) for treating neuropathic pain (Martinez et al., 2014; Finnerup et al., 2015).

Approximately 70 % of the Earth's surface is covered by oceans, which account for 90 % of the biosphere. Peptides are essential bioactive natural products found in numerous marine species and have been extensively studied. Marine bioactive peptides exhibit a broad range of biological activities, including neurotoxic (Tu, 1974), cardiotonic (Norton et al., 1976), antiviral, and antitumor (Hossain et al., 1988), cardiac toxic (Bernheimer and Lai, 1982), and antimicrobial activities (Matsunaga et al., 1985). Marine antimicrobial peptides (AMPs) are commonly used to combat bacteria, fungi, protozoa, and viruses (Guryanova et al., 2023). In vitro studies have demonstrated that marine AMPs can effectively suppress the synthesis of pro-inflammatory mediators, including nitric oxide, tumor necrosis factor-α (TNF-α), and macrophage inflammatory proteins (MIP)-1 and MIP-2 (Lee et al., 2014). Furthermore, marine AMPs can reduce inflammation and treat wound infections in mouse models of skin injury (Huang et al., 2015a). These findings highlight the anti-inflammatory properties of marine AMPs.

Tilapia Piscidin 3 (TP3) is an antimicrobial peptide discovered in Nile tilapia (Oreochromis niloticus) (Peng et al., 2012). Previous studies have found that TP3 is upregulated after Streptococcus agalactiae infection, suggesting its crucial role in innate immunity (Peng et al., 2012; Lin and Chen, 2016). TP3 exhibits potent bactericidal activity against various bacterial species, including Vibrio vulnificus, Vibrio alginolyticus, Klebsiella pneumoniae, Acinetobacter baumannii, and methicillin-resistant Staphylococcus aureus (MRSA) (Huang et al., 2015a; Peng et al., 2012; Pan et al., 2015). Huang et al. demonstrated that TP3 enhanced the survival rate of mice infected with MRSA through its antimicrobial and anti-inflammatory properties (Huang et al., 2015a). Additionally, studies in zebrafish revealed that TP3 downregulates TLR4-mediated NF-κB signaling and reduces the inflammatory response (Su et al., 2018). Our previous studies on the marine AMP piscidin-1 have shown its anti-inflammatory and analgesic properties in rat models of neuropathic pain (Chen et al., 2015). Based on its known anti-inflammatory properties, we hypothesize that TP3 may reduce nociceptive sensitization in a rat model of neuropathic pain, potentially offering a novel analgesic strategy.

Previous studies on neuropathic pain have identified specific cyclic nucleotide-dependent signaling pathways. For instance, genetic deletion of cyclic adenosine monophosphate (cAMP)-producing adenylyl cyclase or cGMP-producing guanylyl cyclase improves neuropathic pain behavior in rodents (Kim et al., 2007; Schmidtko et al., 2008), while the use of cAMP and cGMP analogs induces pain hypersensitivity (Huang et al., 2012; Kallenborn-Gerhardt et al., 2014). Although TP3 has demonstrated anti-inflammatory effects in other models, its potential as an analgesic agent in neuropathic pain remains underexplored.

In this study, we aimed to use a chronic constriction injury (CCI) rat model to investigate the potential of TP3 in alleviating the heightened sensitivity of sensory neurons to painful stimuli through its anti-inflammatory mechanisms. Additionally, we explored the cyclic adenosine monophosphate (cAMP) pathway that may contribute to the effectiveness of TP3 in addressing neuropathic pain.