Research reportCarbamylated erythropoietin improves recognition memory by modulating microglia in a rat model of pain
Introduction
Pain is commonly connected to cognitive processing and may lead to learning and memory impairments [6], [68]. Hippocampus is among the most important parts of the brain, which plays a key role in learning and memory formation [61]. Moreover, several studies have indicated a close association between hippocampal cell death and memory impairments. In addition, it has been found that conditions leading to hippocampal neural cell death also cause recognition and non-recognition memory deficit [31], [47]. Furthermore, brain’s non-neuronal cell activation including microglia can interfere with hippocampal cell death, ultimately leading to impaired memory [4].
Microglia are a type of glial cells located throughout the brain and spinal cord, which also plays a direct role in induction of pain [12], [49]. Accordingly, they respond to Central Nervous System (CNS) pathological changes by releasing pro inflammatory and anti-inflammatory cytokines [10]. Disturbance in balance between CNS pro inflammatory and anti-inflammatory cytokines due to a change in microglial activity may also interfere with CNS cell death pathways.[2], [66]. Additionally, minocycline is commonly used to suppress microglial activation and enhances spatial memory without any known direct action on astrocytes and neurons [39], [62], [63].
Pro inflammatory cytokines, such as interlukine-1β (IL-1β), interlukine-6 (IL-6) and tumor necrosis factor-(TNF-a) are directly related with pain generation. Among these cytokines, IL-1β is mainly produced by activated microglia, astrocytes and neurons and is supposed to has a dual role in memory formation [18], [50]. Whereas, several studies suggest beneficial effects of IL-1β in normal memory processing [40], [55], peripheral manipulation strongly increases hippocampal IL-1β levels, which eventually results in several negative functional consequences in hippocampus dependent memory process [54]. In addition, IL-1β can also induce cell death among different parts of CNS [22]. In this regard, a study by Lee et al. indicated that subcutaneous formalin injection increase the level of brain IL-1β, resulting in hippocampal cells apoptosis and memory impairment [32].
According to the importance of microglial activity and its produced cytokines that may interfere with hippocampal cell death pathways, little is currently known about the roles and interactions of microglial dependent IL-1β in different kinds of hippocampal cell death and recognition memory impairment related to pain following CFA induced inflammatory pain, which was investigated in the first step of the present study.
Moreover, Erythropoietin (EPO), as an immune modulator factor, is well understood for its hematopoietic features [46]. Recent studies have demonstrated neuroprotective effects of EPO through binding to its receptor. EPO can regulate neurogenesis and also prevent cells from apoptosis [71], [74]. Neuroprotective effects of EPO reported for a types of neurodegenerative diseases, such as Parkinson [59] and Alzheimer’s disease [33]. Also Hassouna et al. reported that EPO administration leads to increase of the number of oligodendrocytes and pyramidal neurons in the hippocampus of healthy young mice [23]. Protective effects of Erythropoietin on memory impairments have also been indicated in some studies [26], [33]. However, administrating higher dosage of erythropoietin has some serious adverse effects such as erythrocytosis, heart failure, and even seizure [52]. Carbamylated erythropoietin (Cepo-Fc) is a synthetized compound of erythropoietin, that is usually used in variety of neurodegenerative diseases including Alzheimer [3], Traumatic Brain Injury (TBI) [69], Parkinson [57] and periventricular leukomalacia [36]. Although Cepo-Fc manifests neurogenesis effects of erythropoietin, it lacks some of hematopoietic adverse activities [34], [51]. Cepo-Fc prevents learning and memory impairment through reducing hippocampal cell death [28]. However, no studies have directly considered memory protective effects of Cepo-Fc through modulating hippocampal microglial activity and releasing IL-1β.
Thus, considering the neuroprotective effects of Cepo-Fc as well as possible deteriorating effects of microglial activity and IL-1β on hippocampal cell death, in the second step, we aimed to investigate possible neuroprotective effects of Cepo-Fc on the pain related recognition memory impairment due to modulating microglial and IL-1β level following a persistent peripheral inflammatory pain.
Section snippets
Animals
Adult male Wistar rats (90 days old; weight 200–230 g in the beginning of study) were used in all experiments of this study, obtained from Laboratory Animal Center, Shahid Beheshti University of Medical Sciences. Animals were housed in Plexiglass cages in groups of two per cage at room temperature (25 ± 2 ◦C) under standard 12–12-hour light– dark cycle (lights on at 07:00) with free access to standard food and water. The experimental protocols were approved by the ethics committee of Shahid
Cepo-Fc reduced CFA induced thermal hyperalgesia
Thermal hyperalgesia has significantly increased on days 7 and 21 of CFA group compared to day 0 (P < 0.001 for day 7, and P < 0.01 for day 21). However, the increased thermal hyperalgesia significantly decreased on the 21st day compared with day 7 in the CFA group (P < 0.05). As shown in Fig. 2, administration of Cepo-Fc in CFA+Cepo group significantly decreased thermal hyperalgesia on days 7 and 21 compared to the same days of CFA group (P < 0.05). There were no significant differences in
Discussion
The main goal of the current study was to investigate a possible mechanism for Cepo-Fc memory protection during the persistent peripheral inflammatory pain. The results indicate that Cepo-Fc improved recognition memory impairment during CFA induced peripheral pain. Based on the obtained results, it seems that memory protective activity of Cepo-Fc could be due to the mediation of hippocampal cell death, probably by reducing hippocampal microglial expression and IL-1β production.
CFA induced pain
Conclusion
In conclusion, this study showed that Cepo-Fc prevents hyperalgesia and recognition memory impairment following the CFA induced inflammatory pain. In addition, the protective effects of Cepo-Fc in preventing the pain related recognition memory deficit are associated with the decreased hippocampal cell death markers (apoptosis and necroptosis). Our findings indicate that neuroprotective effects of Cepo-Fc may possibly be due to inhibiting the increased hippocampal microglial expression and IL-1β
CRediT authorship contribution statement
Nasser Rahmani: Conceptualization, Methodology, Formal analysis, Investigation, Writing – original draft, Jalal Zaringhalam: Supervision, Methodology, Resources, Project administration, Writing – review & editing, Mola Mohammadi: Writing – review & editing, Homa Manaheji: Resources, Nader Maghsoudi: Methodology, Hermann Katinger: Material providing, Mansoureh Baniasadi: Investigation.
Conflict of interest
The authors declare that there are no conflicts of interest.
Acknowledgment
This article has been extracted from the MSc thesis written by Mr. Nasser Rahmani" in School of Medicine, Shahid Beheshti University of Medical Sciences (No. 16991) and supported by Cognitive Sciences and Technologies Council (No. 5887).
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