Perineural invasion (PNI) occurs when tumor cells infiltrate along nerves or within the outer sheath and spaces surrounding nerves (Bockman et al., 1994; Yang et al., 2020a). Perineural invasion is a notable characteristic of PDAC, present in over 80% of PDAC patients, even at early stages like pancreatic intraepithelial neoplasia (Ceyhan et al., 2009; Saloman et al., 2016). Perineural invasion is linked to the neural innervation of PDAC and correlates with pain experienced by these patients (Ceyhan et al., 2009; Bapat et al., 2011). Additionally, perineural invasion provides a pathway for tumor invasion into nearby tissues and enhances the local invasive capacity of the tumor (Ceyhan et al., 2009). The extent of perineural invasion in PDAC is significantly associated with postoperative prognosis, making it a predictive factor for survival. Controlling perineural invasion is therefore crucial in PDAC treatment.

Neurotrophic factors secreted by the nervous system play a critical role in PDAC, influencing the perineural invasion process through autocrine or paracrine mechanisms (Huang et al., 2018). Schwann cells and macrophages within the nerves can regulate tumor cell behavior and contribute to PNI exacerbation (Demir et al., 2014; Bakst et al., 2017). Furthermore, specific axon guidance molecules, such as Semaphorin 3D, Plexin D1 and serine, have been found to promote PNI in PDAC (Jurcak et al., 2019; Banh et al., 2020; White and Wang, 2021).

An increase in pancreatic neural plexus density is associated with PDAC development (Bockman et al., 1994). The sympathetic nervous system has a dual role in PDAC. On one hand, it has been reported that the sympathetic nervous system can directly influence the growth of PDAC through beta-adrenergic signaling by releasing norepinephrine (Guo et al., 2013; Kim-Fuchs et al., 2014) (Figure 1A). On the other hand, some evidence suggests that the sympathetic nervous system exhibits inhibitory effects on pancreatic cancer, either directly (Guillot et al., 2022) or indirectly via NK cells (Song et al., 2017).

Research has indicated that parasympathetic nerve signaling can slow down the progression of pancreatic tumors mediated by inflammation (De Couck et al., 2016). Parasympathetic nerve blockage promotes the growth of PDAC and shortens overall survival (Partecke et al., 2017), while the use of muscarinic agonists inhibits the occurrence of PDAC and prolongs overall survival (Renz et al., 2018a) (Figure 1B). However, besides its involvement in PDAC development through suppressing inflammatory responses, promoting immune regulation, and anti-tumor immune reactions, activation of the parasympathetic nervous system is also believed to be associated with poor prognosis in PDAC (Zhang et al., 2016a; Zhang et al., 2016b).

Thus, the roles of sympathetic and parasympathetic nerves in PDAC are complex and sometimes controversial, requiring further investigation.

Severe and persistent pain is a common accompanying symptom of PDAC and is frequently associated with the prognosis of patients in advanced stages of the disease (Laitinen et al., 2017). PDAC pain is a complex process involving various mechanisms such as local tumor infiltration, nerve compression, and inflammation (Mantyh et al., 2002; Wang et al., 2021).

While the mechanisms of PDAC pain are not yet fully understood, it is well established that neurogenic inflammation plays a significant role to induce pain (Wang et al., 2021). Neurogenic inflammation is an inflammatory response that influences the tumor microenvironment, involving vascular dilation and plasma protein extravasation due to peripheral release of substance P and calcitonin gene-related peptide, both of which are neuropeptides (Schmelz and Petersen, 2001) (Figure 1C). Pain-associated sensory neurons primarily release substance P and are involved in the transmission of pain signals (Pernow, 1953). Calcitonin gene-related peptide, belonging to the calcitonin peptide family, is mainly synthesized and secreted by sensory neurons within peripheral tissues. Studies have shown that calcitonin gene-related peptide usually co-localizes with substance P and both play a role in the transmission of sensory signals, particularly pain (Gibbins et al., 1985).

Therefore, neurogenic inflammation results in persistent neuropathic pain, which greatly influences the patient’s quality of life (Breivik et al., 2006; Grace et al., 2014). Tumor cells not only mechanically affect sensory nerve, they also release inflammatory factors that stimulate nerve endings and cause pain (Mantyh et al., 2002). Sensory neurons can also heighten nociceptive hypersensitivity by releasing specific proteins like substance P and calcitonin gene-related peptide. Additionally, sensory neurons can secrete chemokines that act on chemokine receptors, which enhances the sensitivity of nociceptive neurons and promoting neurogenic inflammation (White et al., 2009). Hirth et al. discovered a potential link between chemokines CXCL10 and CCL21 with PDAC pain, and pain relief can be achieved by neutralizing these chemokines (Hirth et al., 2020).

In addition to substance P, calcitonin gene-related peptide and chemokines, neurotransmitters play a crucial role in promoting the development and survival of PDAC cells, including Gamma-aminobutyric acid (GABA) and 5-hydroxytryptamine (5-HT), commonly known as serotonin (Figure 1C).

Suppression of the inhibitory neurotransmitter GABA has been reported to enhance the invasion and growth of tumor cells (Al-Wadei and Schuller, 2009; Banerjee et al., 2016). Studies have shown that supplementing with GABA can decrease cAMP levels and inhibit the release of the pro-inflammatory cytokine interleukin-6 (IL-6), thus preventing pancreatic inflammation from progressing to PDAC. GABA supplementation can also inhibit the abnormal signaling pathway caused by ethanol-induced inhibition of cAMP-mediated PKA signaling, suggesting it as a promising preventive approach for pancreatitis-related PDAC and PDAC caused by long-term alcohol consumption (Al-Wadei et al., 2013; Banerjee et al., 2016).

It is well-known that tumor cells primarily rely on glycolysis as their metabolic method under both aerobic and anaerobic conditions (Hsu and Sabatini, 2008). This phenomenon, known as the Warburg effect, is closely associated with the malignant progression of PDAC (Yang et al., 2020b). Research indicates that serotonin can modulate the Warburg effect by activating the PI₃K/Akt/mTOR pathway, thereby enhancing the survival capabilities of tumor cells (Jiang et al., 2017). Lyn, a kinase belonging to the Src family, has been found to play a promoting role in the serotonin-induced PI₃K/Akt/mTOR pathway. Knocking down Lyn leads to significant reductions in both serotonin levels and the extent of the Warburg effect (Jiang et al., 2017).

Furthermore, HTR2B, a vital subtype of the serotonin receptor (Nebigil et al., 2001), has a positive relationship with serotonin expression and can facilitate the development of PDAC (Jiang et al., 2017). Controlling the activity of HTR2B through knockdown methods may contribute to reduce disease progression and increase overall survival in PDAC patients (Jiang et al., 2017). The expression of HTR2B can independently predict the invasiveness of PDAC, suggesting that it could be a potential target for PDAC therapy. Lowering down the levels of serotonin through different approaches, such as targeting key factors involved in its production, shows potential in slowing down the progression of PDAC.

To sum up, there is a correlation between changes in the density of the pancreatic neural plexus and the development of PDAC (Ceyhan et al., 2009). However, more research is necessary to investigate the complex interaction of various neural networks in PDAC, in order to gain a better understanding of neuronal regulation of PDAC to intervene disease progression.

Sympathetic nerves are predominantly believed to promote tumor development, as seen in prostate and breast cancer (Bauman and McVary, 2013; Kamiya et al., 2019). This promotion may be attributed to the production of nerve growth factor (NGF) (Renz et al., 2018b) and potentially angiogenesis (Kim-Fuchs et al., 2014). In the case of PDAC, however, apart from the existing studies on sympathetic promotion of tumor growth, there have also been reports of sympathetic restriction of PDAC growth. This cancer-protective effect is primarily achieved by modifying the tumor microenvironment in PDAC. It has been demonstrated that PDAC tumor cells tended to grow after both surgical sympathectomy and peripheral chemical sympathectomy achieved by using the neurotoxin 6-hydroxydopamine (6-OHDA), respectively. This effect is likely caused by an increase in CD163 macrophage (M₂-type macrophage) cells (Guillot et al., 2022), which have the potential to drive tumor progression by suppressing T cell-mediated anti-tumor immunity (Etzerodt et al., 2012; Cheng et al., 2017).

Natural killer (NK) cells play a crucial role in tumor recognition and tumor cell elimination (Vivier et al., 2012; Chester et al., 2015). It has been demonstrated in a mouse model of PDAC that anti-tumor immunity was suppressed through peripheral chemical sympathectomy. This suppression was observed by inhibiting the expression of NKG2D and CCR5 in NK cells (Song et al., 2017). However, contradictory findings have been reported vitro studies, where norepinephrine has been shown to inhibit NK cell activity via the β-adrenergic receptor (Ben-Eliyahu et al., 2000). This suggests that sympathetic nerves may have a dual effect on NK cells. Although these pioneer works investigated the role of sympathetic nerves on NK cells in PDAC, further evidence is necessary to fully understand their relationship as microenvironment of PDAC.

To sum up, sympathetic nerves exhibit distinct mechanisms in different components of the PDAC environment and have demonstrated both pro- and anti-tumor effects in various studies. These differences may also be associated with α and β receptors, with β receptors consistently linked to tumor promotion in existing literature (Renz et al., 2018b). Conversely, the role of α receptors remains unknown and requires further investigation in subsequent studies.

The vagus nerve is the primary component of the parasympathetic nerves in pancreas and is involved in the production of inflammatory factors as well as the regulation of the immune systems. This is mainly achieved through the binding of the neurotransmitter acetylcholine to muscarinic-type receptors (M-receptors) or nicotinic-type receptors (N-receptors) (Borovikova et al., 2000; Huston et al., 2009). Numerous studies have demonstrated that the vagus nerve plays a protective role in various tumors (Gidron et al., 2005; Reijmen et al., 2018; Li et al., 2023). For instance, in pancreatic cancer, vagotomy, which involves the cutting of the vagus nerve, can lead to increased tumor growth, worsened survival, and higher levels of tumor-associated macrophages and TNF-α (Partecke et al., 2017). Further investigation has revealed that muscarinic receptors can inhibit tumorigenesis by down-regulating MAPK and PI₃K/AKT signaling through CHRM1 receptors in tumor cells. Additionally, activating muscarinic receptors has been found to increase levels of circulating TNFα and CD11b⁺ myeloid cells (Renz et al., 2018a). Nicotinic receptors also contribute to vagus nerve-mediated tumor suppression. Tracy’s study describes the vagus nerve’s role in suppressing inflammation as a “cholinergic anti-inflammatory pathway,” (Tracey, 2002) which relies on the nAChR (Wang et al., 2003). This pathway explains how the vagus nerve may reduce the production of inflammatory cytokines in PDAC. Acetylcholine released by the vagus nerve exerts its effect through the α7 nicotinic acetylcholine receptor (α7nAChR) found on macrophages, inhibiting the production and release of pro-inflammatory cytokines like TNF, IL-1β, IL-6, and IL-18 (Borovikova et al., 2000; Wang et al., 2003; Falvey, 2022).

There are clinical studies that suggest the vagus nerve can reduce the risk of death in metastatic PDAC (De Couck et al., 2016). Both muscarinic and nicotinic receptors play a role in inhibiting PDAC. Research has demonstrated that subdiaphragmatic vagotomy, the cutting of the vagus nerve below the diaphragm, leads to accelerated PDAC development. Normal cellular phenotypes can be restored by using the muscarinic agonist bethanechol. However, similar to sympathetic nerves, the vagus nerve also plays a dual role in PDAC by modulating the tumor microenvironment.

Research has indicated that when perineural invasion occurs in PDAC, there is an elevated level of acetylcholine, which promotes tumor growth primarily by affecting T cell responses. Acetylcholine impairs PDAC cells’ ability to recruit CD8⁺ T cells and reduces their anti-tumor immunity by inhibiting CCL5 through HDAC1-mediated mechanisms. Additionally, acetylcholine directly inhibits interferon production by CD8⁺ T cells in a dose-dependent manner through the nicotinic acetylcholine receptor (nAChR). This promotes Th2 polarization of T cells and a decrease in the Th1/Th2 ratio, which is essential balance in immune response, thereby facilitating tumor growth and lowering the survival rate (Yang et al., 2020a).

These existing studies suggest that the exact functions of acetylcholine are complex and sometimes controversial, which requires further investigation taking into account the specific context of acetylcholine receptors, components of the tumor microenvironment as well as the progression stages of the disease.

A significant influx of infiltrating tumor-associated macrophages is observed as early as the PanINs stage of the pancreatic cancer, persisting throughout PDAC (Clark et al., 2007). Initially, monocytes are recruited into the PDAC microenvironment and undergo differentiation into tumor-associated macrophages under the chemotactic influence of various factors secreted by tumor cells, Schwann cells, and other cells, resulting in the formation of a unique tumor microenvironment (Franklin et al., 2014; Bakst et al., 2017). Tumor cells not only secrete high levels of colony-stimulating factor 1 (CSF-1) to independently recruit macrophages (Zhu et al., 2014) but also release the chemokine CCL2 in conjunction with Schwann cells, facilitating the infiltration of inflammatory monocytes via the CCL2/CCR2 axis (Sanford et al., 2013; Bakst et al., 2017). Furthermore, the vascular endothelial growth factor (VEGF)/epidermal growth factor receptor (EGFR) signaling axis (Dineen et al., 2008) and the hypoxic tumor microenvironment are also implicated in macrophage recruitment in PDAC (Doedens et al., 2010).

As mentioned earlier, when VIP binds to VIP receptors on T cells, it sends inhibitory signals that suppress the anti-tumor activity of T cells and promote tumor cell growth. VIP receptor antagonists can inhibit this signaling, thereby preventing the reduction in anti-tumor activity of T cells and countering the immune-suppressive tumor microenvironment in PDAC (Ravindranathan et al., 2022). These antagonists can be used in combination with immune checkpoint inhibitors to enhance their therapeutic effects by mitigating the impact of the immune-suppressive tumor microenvironment (Zhang et al., 2022). Experimental studies have shown that VIP receptor antagonists alone can downregulate the expression levels of PD-1 and PD-L1 on immune cells (Li et al., 2016). Furthermore, the combination of VIP receptor antagonists with anti-PD-1 treatment significantly enhances the induction of tumor-specific T cell responses and provides protective immunity against tumor re-attack (Ravindranathan et al., 2022). Administration of anti-PD-1 alone leads to upregulation of CXCR4 expression in T cells, but this effect can be counteracted by co-administration of a VIP receptor antagonist with anti-PD-1 (Ravindranathan et al., 2022). Combination therapy has proven to be more effective than monotherapy. Although VIP receptor antagonists have shown efficacy in treating various types of tumors, their specific applications and characteristics are still under investigation and have not yet been formally implemented in clinical treatment (Moody et al., 1993; Zia et al., 1996; Ravindranathan et al., 2022).

CD74 acts as a co-factor for MHCII molecules, and its expression levels progressively increase with PDAC progression. CD74 can also increase the secretion of GDNF through the PI₃K/AKT/EGR-1 pathway (Zhang et al., 2021). Studies have shown a positive correlation between CD74 levels and GDNF. Since GDNF promotes the occurrence and development of pancreatic neuroplasticity invasion, targeting CD74 can lower down GDNF levels and inhibit PNI (Cavel et al., 2012). Milatuzumab, a novel immunotherapeutic drug targeting CD74, has shown promising results in clinical trials for cancer treatment (Kaufman et al., 2013). However, CD74 targeted therapy is more effective in hematological tumors compared to solid tumors (Burton et al., 2004; Govindan et al., 2013). Studies suggest that the efficacy of CD74 targeting in solid tumors can be improved through drug conjugation. For example, the Milatuzumab-SN-38 conjugate has increased the survival period of mice with solid tumor xenografts and enhanced targeting and anti-tumor toxicity (Govindan et al., 2013). Nonetheless, the specific therapeutic effects and characteristics of Milatuzumab conjugates are still under investigation and have not been widely used in clinical treatment.

Currently, therapeutic approaches that target the neuroimmune dialogue in PDAC are still in the clinical research stage, with imperfect techniques and outcomes. However, treatment methods based on the neuroimmune dialogue in PDAC show promising prospects and can be considered as future directions for PDAC treatment research.