Ethylmethylhydroxypyridine Succinate Induces Anti-inflammatory Polarization of Microglia in the Brain of Aging Rat

The SUCNR1 succinate receptor (GPR91) is constitutively expressed by the main populations of immunocytes. Succinate, as an immunometabolite (metabokine), performs SUCNR1-dependent modulation of the morphofunctional state of immune cells and is an important regulator of innate and adaptive immunity, as well as tissue homeostasis. Contradictory interpretations of the role of succinate signaling in the functioning of macrophages, the lack of studies of the effect of SUCNR1 on the morphofunctional transformation of microglia, the problem of low permeability of the blood-brain barrier (BBB) for any promising analogues of succinate have predetermined the conduct of this study. The aim of this study was to evaluate the effect of ethylmethylhydroxypyridine succinate on the polarization of microglia in chronic inflammation in the aging brain. The work was performed on white mongrel male rats aged 3 (young), 6 (middle-aged), and 18 months (old). The drug Mexidol (2-ethyl-6-methyl-3-hydroxypyridine succinate) was used as a succinate derivative that overcomes BBB. Mexidol was administered intraperitoneally at a dose of 100 mg/kg daily for 3, 7, and 14 days. Surface markers of the pro- (M1) and anti-inflammatory (M2) phenotype of microglia (CD86 and CD206, respectively), Iba1 cytoplasmic marker of microglia, as well as IL-1β and TNF-α pro-inflammatory cytokines (expressed by M1 microglia), TGF-β1 immunosuppressive cytokine and BDNF neurotrophin (expressed by M2 microglia) were detected in the samples of the cerebral cortex by immunoblotting. It was shown that the expression of cellular markers of pro- (CD86) and especially anti-inflammatory (CD206) microglia decreased in old rats; however, the level of the marker Iba1 that was expressed independently of the phenotype remained unchanged in all the studied groups, which indicated the maintenance of quantitatively equivalent populations of microglia in different age groups. At the same time, the levels of TNF-α and IL-1β increased, and the content of TGF-β1 and BDNF was significantly lower in old rats compared with young and middle-aged ones. In general, the data obtained indicated the dominance of the pro-inflammatory status of microglia in the aging brain. A 14-day course of Mexidol caused a slight decrease in the level of CD86 in old rats and a significant increase of 45% in the content of CD206 up to the level of CD206 in 6-month-old rats. The increase in the CD206 expression was associated with an increase in the levels of TGF-β1 and BDNF by 60% and 35%, respectively, which indicates the involvement of succinate/SUCNR1 signaling in the anti-inflammatory polarization of microglia in the aging brain. The obtained data develop ideas about the cerebral effects of succinate/SUCNR1 signaling and reveal a new component of the mechanism of the neuroprotective action of Mexidol.