With a rapidly growing elderly population worldwide, a focus on finding efficient preventive and therapeutic strategies in the management of AD should prove timely and topical to researchers. The available evidence suggests that culinary herbs, such as Melissa officinalis L. (Lamiaceae), can potentially provide a natural treatment for AD (Howes et al. 2003; Gürbüz et al. 2019). In the relevant scientific literature, there also exist other studies indicating various clinical, cholinesterase inhibitory and neurobiological activities of different Lamiaceae plants (Perry et al. 2003; Orhan et al. 2008). All these as well as previous promising yet limited work on the bioactivity of traditionally consumed O. onites L. have enabled us in designing and executing the current study, in which the previously underdetermined neurobiological activity of the essential oil from this herb was investigated thoroughly. Putative neuroprotective and cognitive-ameliorating effects of OOEO on Sco-induced learning and memory impairments in rats were examined by behavioral tests, biochemical assessment, and computational simulations.
Here, we evaluated the cognitive effects of OOEO on memory and learning as well as the possible mechanisms underlying these effects in a rat model of Sco-induced amnesia. Initially, we tested whether OOEO treatment caused changes in behavioral parameters, such as locomotory activity and learning and memory abilities, in rats. Our results suggest that OOEO administered at a dose of 0.25 ml/kg enhanced memory functions previously impaired by Sco, corrected the negative impact of Sco on memory acquisition without affecting the locomotor activity, and also improved Sco-induced amnesia. It has been shown that administration of Sco at 0.056 mg/kg and more increases locomotor activity (as a non-cognitive effect) in rats (Gholamreza et al. 2002; Chintoh et al. 2003). In our experiments, we used a Sco concentration of 1 mg/kg which is ~18-fold higher than the minimal effective dose; however, no change was observed in the locomotor activities in different experimental groups. Therefore, OOEO (and also Gal) treatment is likely to suppress the non-cognitive effects of Sco. Memory impairment caused by Sco administered intraperitoneally to rats at a dose of 1 mg/kg was assessed based on the NOR test, which provides a rapid performance assessment for rodents (Van Momtazi-Borojeni et al. 2017). Rats treated with Sco were unable distinguish between the novel and familiar objects, which ultimately showed us that recognition was significantly deficient in this group of animals. Upon OOEO treatment, however, Sco-induced deterioration of cognitive abilities in rats was found to be abolished, and improvements in memory performance were noted. The results of the MWM test demonstrated that the administration of Sco significantly reduced the discrimination index, escape rate and time spent in the target quadrant and increased latency to find the platform, all of which are indicators of learning skills and memory in rodents. OOEO administration, however, was found to ameliorate Sco-induced impairments on the discrimination index, latency to find the platform, escape rate and time spent in the target quadrant. Collectively, these results suggest that OOEO may prevent Sco-induced learning and memory impairments in rats.
Due to the important role of the cholinergic system in learning and memory, maintaining acetylcholine levels are critical for brain functions (Blake et al. 2014). The action of AChE breaks down the naturally occurring neurotransmitter acetylcholine into acetate and choline at synaptic sites, which terminates or suspends cholinergic transmission (Ballard et al. 2005). Accordingly, an agent promoting the activity of AChE should ideally impair memory via lowering acetylcholine levels as Sco achieved in the amnestic group. On the other hand, in the present study, the standard drug Gal and OOEO significantly reduced the activity of AChE and allowed the rats to retain the memory of the tasks they learned as observed in our rich array of behavioral experiments. This AChE inhibition could be attributed to one or more components of OOEO. Indeed, AChE from electric eel has previously been shown to be inhibited by carvacrol (IC50: 0.063 mg/ml) and its derivative thymohydroquinone (IC50: 0.04 mg/ml) in an in-vitro setting (Jukic et al. 2007). Considering that impairments in the human brain cholinergic pathways are among the underlying causes of AD and that the use of AChE inhibitors that increase acetylcholine levels in the CNS constitutes the essential remedy for the disease (Terry and Buccafusco 2003), the inhibition of AChE in vivo by OOEO suggests that this essential oil may represent a novel candidate therapeutic agent for AD.
The CNS is particularly susceptible to pro-oxidants because of its unique architecture and operation, exemplified by a high oxygen consumption concomitant with a high unsaturated lipid content and low antioxidant protection (Kowalczyk et al. 2020). Therefore, it is no surprise that the overproduction of reactive oxygen species (ROS) in the CNS is associated with several neurodegenerative diseases including AD. The most commonly affected brain regions are believed to be the hippocampus, substantia nigra, and striatum (Phaniendra et al. 2015). Our study confirms that oxidative stress is involved in Sco-induced dementia in rats as revealed by an increase in the concentration of MDA, a secondary oxidation product of polyunsaturated fatty acids, and a decrease in the concentration of GSH, a major cellular antioxidant buffer molecule, in both hippocampus and frontal cortex. In contrary, the standard drug Gal and OOEO were able to significantly improve this oxidative stress profile in the single brain structures. ROS can also induce neuronal death in multiple forms including apoptosis (Fricker et al. 2018). Animal studies have shown that the activation and execution of apoptosis in neurons in the cortex and hippocampus impairs learning and memory (Kuhn et al. 2005; Sun et al. 2009). While encounter with Sco increased the bax/bcl-2 ratio and caspase levels in the frontal cortex and hippocampus of amnestic rats, Gal and OOEO treatments were effective in decreasing the production of apoptotic protein factors in both regions. Taken together, it is plausible to assume that the antioxidant and pro-survival properties of OOEO at a cellular scale may contribute to the reversal of Sco-induced memory deficits in rats.
Neuroinflammation can be defined as an inflammatory response within the CNS, which is usually caused by one of a variety of pathological insults including trauma, ischemia, infection, and toxins. The production of pro-inflammatory molecules (e.g., cytokines, chemokines, NO, and ROS) by innate immune cells, such as microglia/macrophages and astrocytes, in the course of neuroinflammation may ultimately result in synaptic dysfunction, impaired neurogenesis, and neuronal death (Leng and Edison, 2021). A major contribution to neuronal death is ROS generation by the induction or activation of inflammation-related enzyme systems including COX-2, iNOS, and MPO. For instance, MPO has been shown to be aberrantly expressed in astrocytes in human AD brain (Maki et al. 2009). The involvement of MPO in disease pathogenesis is further supported by the presence of 3-chlorotyrosine, a specific biomarker of MPO-catalyzed oxidation, in the hippocampus from patients with AD (Green et al. 2009). In addition, activated macrophages/microglia are known to induce the expression of other downstream pro-inflammatory enzymes such as COX-2 and iNOS (Choi et al. 2009). It has been demonstrated that COX-2 levels are elevated in AD brain (Pasinetti and Aisen 1998; Yasojima et al. 1999; Earley et al. 2000) and that they correlate with the clinical progression of the disease (Ho et al. 2001). Similarly, iNOS expression has been found in the hippocampal region of the brain affected by AD and linked to neuronal degeneration in the disease (Sunhee et al. 1999). Our computational predictions illustrate that carvacrol can be housed in the inhibitor-binding pockets of MPO, COX-2 and iNOS. In fact, COX-2 from sheep has previously been shown to be inhibited by carvacrol in an in-vitro setting (Landa et al. 2009). Furthermore, the potency of COX-2 inhibition by carvacrol (IC50: 0.8 µM) has been found to be comparable to that by an NSAID. It is therefore plausible to assume that the in vivo observed anti-inflammatory action of oregano (Taleb et al. 2018) and other traditionally used carvacrol-rich plant drugs (Sosa et al. 2005) could be partly due to the modulation of pro-inflammatory enzyme activities.
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