Anesthesia and surgical interventions may induce cognitive impairments, such as postoperative delirium (POD) and postoperative cognitive dysfunction (POCD) [1]. These dysfunctions particularly affect the elderly population [2] and increase morbidity and mortality thus representing a major burden for the healthcare system [1]. The pathophysiology of cognitive dysfunctions is poorly understood, but several theories have been postulated [3,4]. Among these, the neuroinflammation hypothesis, which states that the perioperative stress induces an increased inflammatory activity, which in turn induces brain damage, is widely accepted [5]. However, due to the complex pathology of these diseases, effective treatment of POD/POCD remains difficult [1].

The α2 adrenoceptor (α2AR) agonist dexmedetomidine (DEX) is a promising therapeutic option to decrease cognitive dysfunctions and has gained an increased interest since its approval by the EMA in 2011: DEX was shown to reduce the occurrence of ICU delirium compared to midazolam [6] as well as propofol [7] and to decrease POD when administered perioperatively [8]. However, not all clinical studies are in agreement, as e.g. Deiner et al reported no difference in the occurrence of POD between the use of DEX and placebo when used intraoperatively [9] and Shehabi et al. found that DEX did not reduce the occurrence of intensive care unit (ICU) delirium [10]. Nonetheless, in experimental studies, DEX was shown to have anti-inflammatory as well as neuro- and organoprotective effects [11-13], which makes DEX a promising candidate for targeting (neuro-) inflammation-induced damages. Unfortunately, the underlying mechanism of these effects is not yet fully understood and thus, research in the field of DEX is still needed to determine its clinical use as well as the underlying mechanisms.

With this in mind, we recently published a study in the Journal of Neuroimmune Pharmacology [14], in which we examined the possible underlying effects of DEX in an experimental rat model of inflammation. In this study DEX was administered intraperitoneally (i.p.) 10 min prior to a i.p. injection of lipopolysaccharide (LPS) and the animals were sacrificed at different time points (6 h, 24 h, and 7 d) to address the temporal changes after induction of inflammation and treatment with DEX [14]. The study focused on a possible involvement of DEX on the cellular process of autophagy, the cells’ internal recycling and degradation system, and the cholinergic anti-inflammatory pathway, a communication route between the nervous and the immune system. The present commentary refers to this study and takes a deeper look at the recent literature on both pathways to depict the possible mechanism of action of DEX.

Autophagy is a constantly active process in every cell that is important for cell adaptation and survival as it serves the proteolytic degradation of old or damaged proteins and cell organelles. Autophagy can be subdivided into different types, with macroautophagy being the best-studied form of autophagy. In the autophagic pathway, an autophagosome forms around the material targeted for degradation. The autophagosome subsequently fuses with a lysosome to form an autolysosome, in which hydrolases degrade the material thereby recycling the material and generating new nutrients for the cell [15].

In our recent study, we demonstrated that DEX is also able to restore the autophagic flux and attenuate the increase of apoptosis markers upon LPS-induced inflammation in the spleen, as DEX was able to increase the expression of LC3-II and reduce the expression of p62/SQSTM1, while an LPS treatment increased the p62/SQSTM1 expression, and as DEX was also able to attenuate the LPS-induced increase in the apoptosis marker cleaved PARP [14]. Furthermore, an accumulation of SQSTM1/p62 also correlated with an increase in cleaved PARP in the spleen [14], thereby indicating that defects in autophagy are associated with apoptosis markers. Autophagy and apoptosis are closely intertwined pathways that can be activated by the same stimuli [14]. However, while a functioning autophagy helps the cell to adapt to and survive environmental changes, apoptosis leads to cell death. Still, a dysregulated, non-functioning autophagy can also lead to cell death, and thus a functioning autophagy, as was induced by DEX in our recent findings, is important for survival [16]. These new insights on DEX accompany our previous findings, in which we showed an inflammation-reducing effect of DEX in the brain of rats upon LPS-induced inflammation [12]. Apart from that, our recent study further demonstrated that a variety of microRNAs (miRNAs), small non-coding RNA molecules with autophagy modulating effects, were also changed upon the DEX treatment [14].

miRNAs regulate gene expression on a post-transcriptional level by inhibiting or activating translation and degrading mRNA strands [17,18]. miRNAs are implicated in the regulation of autophagy throughout the autophagic pathway [19]. Surprisingly, the effect of DEX on these miRNAs did not correlate with its effect on autophagic proteins but rather with apoptosis markers in our study [14], which underlines the ambiguous functions of miRNAs within various pathways [20]. Furthermore, this suggests that regulation of autophagy-associated miRNAs does not directly affect autophagy, but rather fine-tunes this process [21]. Nonetheless, more and more studies identified miRNAs involved in the DEX-mediated effects [22,23], including those that regulate autophagy [24,25]. A recent study also confirmed the modulatory role of DEX on miRNA profile within the rat brain [26], as well as other organs [27], and other studies demonstrated that DEX modulates circulating miRNAs after administration in patients [28]. Thus, additional investigations on the influence of DEX on miRNAs may provide a promising approach to understand its (neuro-) protective potential—especially as miRNAs can be used as circulating biomarkers to predict disease progression [29] and the efficacy of DEX [30].

Although in our study the miRNA profile changed in the brain and the spleen and we previously found DEX to have a reducing effect on pro-inflammatory cytokines in the brain, the changes in autophagy proteins were measured only in the spleen but not in the brain. This discrepancy between the inflammatory and the autophagic process in the brain might be explained by the experimental setup (the model and dose of LPS and DEX respectively), as most studies that observed DEX-induced autophagic changes in the brain investigated DEX in the context of an ischemic event and often applied a much higher DEX and LPS dose, respectively: Studies in the context of ischemia found that DEX protected the brain from an ischemic event by reducing excessive autophagy [31,32], whereas studies in the context of neuroinflammation found an activating effect on autophagy, but often used a higher DEX [33] and LPS dose [34]. In contrast, we used a moderate dose of DEX (5 µg/kg bodyweight) and induced a relatively mild damage (1 mg/kg bodyweight LPS). This is also reflected in the fact that no apoptotic proteins were increased in the brain either [14] and it further indicates that both, pathogen-and damage-associated molecular patterns (PAMPs/DAMPs) might be necessary to induce an autophagic response in the brain, as for example observed by von Haefen et al. [35]. Nonetheless, a rising number of evidences suggest that DEX mediates its effects via autophagy, as in addition to the spleen [14] and brain [33], DEX was also shown to induce autophagy in other organs, such as the heart [36] and kidneys [37], to exert its protective effects. Interestingly, DEX can both activate and inhibit autophagy, as shown in the context of ischemia [31,32]. This dual role of DEX on autophagy is not yet understood but holds a promising starting point for further research, especially in the field of neurodegeneration, because neurons as postmitotic cells cannot clear any accumulated cellular waste by cell division, but instead rely heavily on autophagy for this [38].

Furthermore, in the spleen, as the largest lymphoid organ, autophagy also plays a crucial role, as it is involved in immune cell regulation, ranging from the control of cytokine secretion to the development and survival of B and T cells [39]. In addition to the numerous red blood cells located in the spleen, this organ contains a vast variety of different immune cells with B and T cells being present throughout the spleen [40]. Thus, studies of autophagy in this organ can provide important insights into the autophagic activity of immune cells. An increase in autophagy might help the immune system to switch into its anti-inflammatory state to resolve an existing inflammation. The importance of a functioning autophagy within the spleen was also reflected in our study, as an impaired autophagic flux positively correlated with apoptosis markers in the spleen [14]. However, the autophagic activity has different regulatory roles depending on the immune cell type [39], and most studies, including our recent study, measured the DEX-induced changes in the whole spleen instead of the specific splenic cell types [11,14]. Further investigations that examine the cellular composition of the spleen are therefore needed to understand the cell-specific effect of DEX on autophagy.

In addition to autophagy, our recent study also found a DEX-mediated effect on components of the cholinergic anti-inflammatory pathway [14] – an important communication route in response to tissue injuries and pathogens [41], in which immune cells detect and react to damage- and/or pathogen-associated threats, thereby altering immune homeostasis. This alteration is detected by neurons of the vagus nerve and central root ganglia and communicated to the central nervous system (CNS), where the inflammation is contained centrally by releasing noradrenaline (NE), which in turn binds to the β2 adrenergic receptors of acetylcholine (ACh) producing T cells. This T cell subset releases ACh, which binds to the α7 nicotinic acetylcholine receptor (α7nAChR) of other immune cells, such as macrophages, and thus inhibits the cytokine production of these cells [41]. As afferent neurons innervate nearly all tissues and organs of the body, the cholinergic anti-inflammatory pathway functions as a crucial regulator of the immune response [41]. The cholinergic anti-inflammatory pathway can respond to a variety of stimuli, such as cytokines, DAMPs, and PAMPs [42].

In our recent study, DEX modulated the levels of ACh and acetylcholinesterase (AChE) in the brain and spleen, but unlike other studies, did not alter the expression of α7nAChR [14]. Others showed that DEX induces an increase of the α7nAChR expression [43-45]. The reason for this discrepancy in the results is unclear, but it is important to note that these studies have measured the α7nAChR expression in different organs and administered different doses of DEX as well as chose a different application route, making a straightforward comparison difficult. In addition to affecting the expression of α7nAChR, ACh, and AChE, DEX can also lead to an increase in vagal discharge [13,46]. The cholinergic anti-inflammatory pathway seems to be crucial for DEX-mediated protection, as disrupting this pathway by a blockage of the α7nAChR with antagonists or by vagotomy abolished the effects of DEX on inflammation resolution [13,47] or even exacerbated the outcome [43]. Interestingly, in addition to a vagotomy, a splenectomy can also abolish the protective effects of DEX, as was shown in kidney injury models [13,45]. The central role of the spleen was also highlighted in our recent study, as the effects on the components of the cholinergic pathway were most prominent in this organ [14].

It is worth noting that studies have indicated that the DEX-mediated effects cannot be reversed by sole blockage of the α2AR, but instead require a blockage of either the α7nAChR or the imidazole receptors [48]. Imidazole receptors are not very well studied yet but were initially simplified defined as nonadrenergic binding sites that recognize compounds with an imidazoline moiety [49]. These receptors can be subdivided into three subtypes, with I₁ and I₂ characterized by their binding affinity to clonidine and idazoxan respectively, and with I₃ less well studied. The imidazoline receptor family, in particular the I₂ subtype, has been described in neuroprotection [49] and these effects were also associated with autophagy [50,51]. Indeed, quite some time ago, Dahmani and colleagues already observed that the DEX-induced increase of hippocampal phosphorylated extracellular signal-regulated protein kinase 1 and 2 (pERK1 and 2) is not mediated by the α2AR, but rather by the I₁ receptor [52]. Other more recent studies also indicated that the protective and anti-inflammatory effects of DEX might be mediated by the I₂ receptor, as the effects of DEX in acute kidney injury and cognitive decline models were reversed by blockage of the I₂ receptor with idazoxan but not by blockage of the α2AR with atipamezole [48,53]. These findings are not surprising as DEX has an imidazoline ring structure. However, the role of imidazoline receptors in DEX-mediated protection is still controversial, as recent findings also found the opposite [54]. Furthermore, it is important to take a closer look at the different α2AR antagonists that were used in these studies, since atipamezole also inhibits imidazoline receptors [48]. Nonetheless, these findings represent an interesting new aspect of DEX-mediated protection. Future studies investigating the mechanisms of DEX and its effect on autophagy and the cholinergic anti-inflammatory reflex should keep this receptor family in mind.

In summary, both, the cholinergic anti-inflammatory pathway, and autophagy play a central role in the CNS and the immune system and our recent study as well as others have found both pathways to be influenced by DEX. Also, Yu and colleagues found an activation of both pathways upon DEX treatment in a septic myocardial dysfunction model and additionally showed that a blockage of the autophagic pathway also reverses the cardioprotective effects of DEX [36]. Indeed, similar to the function of the cholinergic anti-inflammatory pathway, the primary function of autophagy within the immune system is to mediate a balanced immune response with maximal protection from infection and yet minimal harm to the host organism [55]. All in all, DEX is a not yet fully understood drug that is currently being researched in various areas. Due to its modulatory effects on both the autophagic and cholinergic anti-inflammatory pathway this drug remains an interesting and promising therapeutic approach, worth to be explored further for its potential and as a starting point for the development of new drugs.