Today, a large number of studies are underway to establish and standardize laboratory methods for using saliva as a diagnostic fluid. The main reason for these efforts is the simplicity and non-invasiveness of saliva sampling, which allows easy, low-cost, painless and frequent sampling of patients. On the other hand, it is thought that saliva is a reflector of the body and will be a diagnostic fluid in the future and therefore will be included in the health maintenance program in the diagnosis, monitoring and screening of diseases [23]. According to the benefits of saliva, in this study, serum and salivary levels of cholinesterase activity in healthy individuals and patients with MS were evaluated and the results showed that serum cholinesterase activity was reduced in stimulated and unstimulated saliva.
In MS, brain tissue becomes inflamed [24]. Inflammation activates the "cholinergic anti-inflammatory pathway" that stimulates the vagus nerve and releases acetylcholine (Ach) from its end [10]. Acetylcholine can inhibit inflammation [11], and affects macrophages via the nicotinic receptor α7, which consequences in the blocking of the NF-κB nuclear transport signaling pathway, leading to a reduce in the production of proinflammatory cytokines such as TNF-α, IL-1β, IL-18 and IL6). Thus, the cholinergic system can improve inflammation in patients by receiving information from different parts of the body, rapidly through the vagus nerve and through nicotinic alpha 7 receptors [10, 13].
Cholinesterase activity decreases in a variety of chronic and severe inflammatory diseases [12, 14]. Because cholinesterase is produced in the liver, it is also reduced in patients with liver disease and malnutrition [10, 25]. Cholinesterase activity decreases in many inflammations disease such as burns [25], gingivitis and periodontitis [26], cancer [27], heart failure [28], brain injury [29], and ischemic stroke [30] that are in agreement with the results of our study. There is a study that shows an increase in the concentration of cholinesterase in the blood and lymphocytes of patients with MS [15], which contradicts the results of our study. Perhaps the reason for the difference is that we examined the activity of the enzyme while Polachini et al. measured its concentration.
The amount of cholinesterase is so strongly affected by inflammation that it decreases in the acute phase of inflammation and increases rapidly when the inflammation improves [25]. It has been shown that in severe systemic inflammation, the level of cholinesterase activity in the blood decreases [13]. Since serum cholinesterase levels show changes in the body's overall ability to hydrolyze Ach, a decrease in serum cholinesterase levels is related to a decreasing regulation of cholinesterase activity as a compensatory response to increase acetylcholine anti-inflammatory activity [10] which can justify our results in reducing cholinesterase activity in saliva and serum.
In this study, ROC test was applied to evaluate the diagnostic significance of cholinesterase activity in serum and saliva, to diagnose MS group. The cut-off point of cholinesterase activity in serum and saliva was significant for separating patients from healthy individuals and also a positive correlation was detected between serum and salivary values. Therefore, it can be said that serum and salivary cholinesterase activity can have diagnostic value. Therefore, measuring cholinesterase activity can be helpful in diagnosing MS.