Neuroinflammation is the one of the emerging
Neuroinflammation is the one of the emerging causal factors contributing to these cognitive deficits. Therefore, we further tested the hypothesis that neuroinflammation is a key component contributing to the progression of spatial memory deficits during HH. Spatial memory deficits were observed after 7 days of HH exposure, while the proinflammatory cytokines, IL-6 and IL-1β, were induced from 1 day of HH exposure onwards. This observation is in agreement with previous reports, leading to the conclusion that the initial increase in proinflammatory cytokines contributes to the secondary progression of the inflammatory reaction, which results in subsequent cognitive deficits (Pfau and Russo, 2016, Wadhwa et al., 2017). It is also reported that COX-1 induces cognitive deficits via mediating susceptibility to systemic inflammation (Griffin et al., 2013). IL-1β overexpression in the hippocampus has been shown to impair spatial memory and Morris water maze task completion, with an increase in hippocampal neuroinflammation (Hein et al., 2010). COX-1 mediates chronic inflammatory effects and memory impairment via PGE2 elevation in an IL-1β overexpression model (Matousek et al.,2010). Positive correlation between COX product PGE2 and latency to reach the platform validates this in our case, as HH group animals had higher expression of PGE2 in hippocampus and took more time to reach the platform, also showed significantly higher expression of IL1β in contrast to CC group animals. Several lines of evidence indicate that neuroinflammation correlates well with neuronal damage in the hippocampus (Qin et al., 2007). Results obtained in the present investigation show an increase in TUNEL-positive foci and pyknotic neurons, occurring in a temporally similar trend to neuroinflammation in the hippocampus. Our observations suggest that augmented proinflammatory cytokine production may contribute to a series of events that link HH to neuronal injury. Microglia are the major resident immune cells in the alpha-Endorphin and they continuously survey the healthy brain in ramified morphology. In response to stress, there are functional and morphological changes in microglia as they are activated (Kettenmann et al., 2011). Morphological analysis of microglia in the hippocampus revealed a spatiotemporal relationship between the progression of spatial memory loss and de-ramified microglial morphology after HH exposure. Astrocyte activation was observed after 3 and 7 days of HH exposure, while the increase in proinflammatory cytokines accompanied microglial activation after 1 day. This suggests that activated microglia secrete proinflammatory cytokines, which in turn, activate astrocytes. In vitro studies have shown that PGE2, secreted from microglia, increases the proliferation of astrocytes (Zhang et al., 2015). Normal astrocytes provide trophic support for neurons and promote the formation and function of synapses. However, once they are activated (A1 astrocytes), they gain neurotoxic functions and cause neuronal cell death in the hippocampus (Liddelow et al., 2017). Our results show a similar series of events in the hippocampus after HH exposure, where initial microglial and astrocyte activation was followed by neuronal injury, as indicated by TUNEL- and CV-positive staining results. COX-mediated neuroinflammation and neurotoxicity depend on the specific cell types involved, whether there is a primary or a secondary response, and the stress model used. The distinct cellular distributions of COX isoforms have implicated them in immune system to brain signaling after HH exposure. Our study showed early expression events for both COX-1 and COX-2 in the activated endothelium. In subsequent days of HH exposure, the level of PGE2 in the hippocampal region of the brain parenchyma was primarily due to COX-1 activity in microglia and COX-2 activity in neurons. Microglial COX-1 and neuronal COX-2 expression accompanied with PGE2 production reported to appear upon inflammatory stimuli and conditions where neurons are directly challenged respectively (Choi et al., 2009). Analysis of immunoreactive localization of COX-1 and COX-2 isoforms in different brain cell types revealed distinct mechanistic roles for the two enzymes. COX-1 was primarily present in microglia and its expression was enhanced after HH exposure. Since COX-1 expression in microglial and ECs increases as early as day 1, it may also increase the level of eicosanoids, thus playing a critical role in inflammation in the hippocampus. Therefore, COX-1 expressing, activated microglia in HH-exposed animals may cause an increase in PGE2 and cytokine production in the hippocampus, thereby affecting the inflammatory milieu and increasing neuronal vulnerability. On the other hand, COX-2, which is expressed in pyramidal neurons in the hippocampus, may contribute to increased PG synthesis in response to HH-induced neuronal insult (Bartels and Leenders, 2010). In addition, the increase in COX-2 expression in activated glial cells highlights the role of COX-2 derived PGE2 in this cell type during HH exposure. Astrocyte activation accompanied by increased glutamate levels and augmented cytokine production causes a positive feed forward loop, leading to COX-2 induction in neurons. Several studies have described the role of COX-1 and COX-2 in neuroinflammation-associated cognitive deficits (Furuyashiki and Narumiya, 2011). While COX-2 is considered to be an inducible and proinflammatory isoform (Kaizaki et al., 2013), COX-1 has traditionally been considered to be a constitutive isoform. However, COX-1 has recently been implicated in neuroinflammation (Johansson et al., 2013) and neuronal death (Griffin et al., 2013). Therefore, both isoforms may be involved in the progression of neuroinflammation and its associated cognition maladies. In this study, we used a pharmacological approach to elucidate the mechanisms by which COX-1 and COX-2 contribute to neuroinflammation and neurodegeneration during HH.