The adenosinergic system, not the cholinergic system, exerts an anti-inflammatory profile in lymphatic immune organs of fish naturally infected with Ichthyophthirius multifiliis

Studies have demonstrated that fish are able to mount protective immune responses against various parasites, including Ichthyophthirius multifiliis, an economically important protozoosis in freshwater aquaculture. Although the immune response against this parasite have been partly elucidated in the literature, the involvement of the cholinergic and adenosinergic systems, through the anti-inflammatory molecules acetylcholine (ACh) and adenosine (Ado), has not been studied. Thus, the aim of this study was to evaluate the involvement of the cholinergic and adenosinergic systems in the immune and inflammatory responses in the immune lymphatic organs of silver catfish, Rhamdia quelen, naturally infected with I. multifiliis. Acetylcholinesterase (AChE) activity increased, while adenosine deaminase (ADA) activity decreased in the head kidney and spleen of silver catfish naturally infected with I. multifiliis compared to the uninfected control group. Moreover, the levels of ACh in the head kidney and spleen decreased in infected animals, while the Ado levels increased compared to the uninfected control group. A negative and significant Pearson correlation was observed between AChE activity and ACh levels, as well as between ADA activity and Ado levels. Therefore, the cholinergic system exerts a pro-inflammatory profile due to upregulation of AChE activity and a consequent reduction in ACh levels, which contributes to inflammatory damage. Conversely, the adenosinergic system plays an anti-inflammatory profile via downregulation of ADA activity and consequent augmentation of Ado levels, which may contribute to restricting the inflammatory process and improving the immune response during ichthyophthiriasis.