A study on neuroinflammation and NMDA receptor function in STZ (ICV) induced memory impaired rats

Abstract

Present study was designed to investigate the status of neuroinflammation and NMDA receptor function in STZ (ICV) induced memory impaired rats. STZ produced significant increase in proinflammatory cytokines (TNF-α and IL-1β), ROS, nitrite and mRNA and protein expression of iNOS and nNOS indicating a state of neuroinflammation in rat brain which was significantly prevented by Memantine and Ibuprofen treatment. STZ also significantly altered NMDA subunits, NR2A and NR2B protein and mRNA expression which were restored by Memantine only. The results suggest that neuroinflammatory markers might be involved in memory impairment via modulating the NMDA receptor in STZ induced memory impaired rats.

Introduction

Neuroinflammation is a complex process of the cells within CNS including neurons and microglia with elevated levels of proinflammatory cytokines. The NMDA receptor (NMDAR) is involved in cellular mechanism for learning and memory function. Neuroinflammation with overexpression of cytokines is a characteristic of the brain pathology present in Alzheimer's disease (AD) (Mrak and Griffin, 2005). Animal studies have suggested the involvement of astrocytic glutamate receptor in glial cell signaling (Schipke et al., 2001, Wong, 2006). Activated microglia produces a variety of proinflammatory and neurotoxic factors, including cytokines, such as tumor necrosis factor (TNF-α); interleukin-1β (IL-1β); and free radicals, such as nitric oxide (NO) and superoxide (Banati et al., 1993, Minghetti and Levi, 1998). NO is also produced by activated astrocytes and has also become one of the major contributors in the formation of reactive nitrogen species. Reactive oxygen species (ROS) and NO which have been overexpressed during the neuroinflammatory process, are able to directly influence the induction of long term potentiation (LTP) (Min et al., 2009). Min et al. (2009) also suggested that proinflammatory cytokines and other molecules that are overexpressed during the neuroinflammatory process influence neurotransmitter release, receptor signal transduction mechanism and finally, the ability of the synapses to undergo long-term potentiation (LTP). NMDA receptors (NMDARs) are of major interest because they are involved in synaptogenesis, neuronal circuitry formation, synaptic plasticity, learning and memory, as well as in the molecular pathogenesis of neurological disorders (During et al., 2000).

NR2A and NR2B are likely to play a major role in synaptic plasticity modulating LTP and long-term depression (LTD) respectively (Monyer et al., 1994). Overactivity of glutamatergic NMDARs results in excitotoxicity which may participate in microglial activation in the hippocampus of brain (Chang et al., 2008). The triggering of LTP requires synaptic activation of post-synaptic Ca^2 + entry and NMDARs. Proinflammatory cytokines crucially influence several key molecular steps that are considered to be essential in the induction of NMDARs (Tan et al., 2007). Proinflammatory cytokines also dose dependently enhance NMDARs mediated currents and NMDA-induced intracellular Ca^2 + increase (Yoshiyama and Groat, 2007). Tweedie et al. (2007) reported that proinflammatory cytokines are involved in NMDAR activity in astrocytes. Further, Massey et al. (2004) had suggested that NR2A containing NMDARs are required for LTP whereas NR2B containing NMDARs are required for LTD in memory.

Memantine is an uncompetitive NMDAR antagonist and its mechanism of action is neuroprotective and potentially therapeutic in several neuropsychiatric diseases. Memantine can improve cognition and neuronal plasticity under pathological conditions might seem to be paradoxical (During et al., 2000). Ibuprofen is the most commonly used and most frequently prescribed NSAID (Bradbury, 2004). Its effects are due to the inhibitory actions on cyclooxygenases (COX), which are involved in the synthesis of prostaglandins and have an important role in the production of inflammation. Proinflammatory cytokines are thought to have a pivotal role during the physiological crosstalk occurring between neurons and astrocytes and the cytokine mediated modulation of glia–neuron communication (Tarkowski et al., 2003). It has been reported that intracerebroventricular (ICV) injection of STZ causes spatial memory impairment by disrupting glucose utilization through an insulin-dependent mechanism and neurotoxicity in myelin in the cerebral cortex and hippocampus (Shoham et al., 2003).

However, the effect of STZ (ICV) causing memory impairment on neuroinflammation and NMDA receptor subunits has not been explored. In this study we describe the alteration of the NMDAR subunits and involvement of neuroinflammation in STZ (ICV) induced memory impaired rats.