Differences between rats and rabbits in NMDA receptor-mediated calcium signalling in hippocampal neurones

Repeated exposure of adult rats to Aroclor 1254 induces neuronal injury and impairs the neurochemical manifestations of the NMDA receptor-mediated intracellular signaling in the hippocampus

Aroclor 1254 is a mixture of polychlorinated biphenyls (PCBs), a class of environmental toxins which cause a wide spectrum of neurotoxic effects. Learning and memory deficits are the profound effects of PCBs which may be related to hippocampal dysfunction. To get insight into the underlying neurochemical mechanisms, we employed the microdialysis technique to investigate the effect of repeated exposure of adult male Wistar rats to Aroclor 1254 (10mg/kg b.w., daily, ig., for 14days), on the neurochemical parameters of NMDA receptor-mediated glutamatergic signaling in the hippocampus in vivo assessed using the microdialysis technique. The results demonstrated that exposure to Aroclor 1254, which was associated with substantial neuronal damage and loss in the hippocampus, markedly decreased the NMDA-induced extracellular accumulation of newly loaded (45)CaCl(2), cGMP and glutamate, and reduced the basal content of the NO precursor, arginine, indicating inhibition of the NMDA/NO/cGMP pathway. Aroclor 1254 exposure also decreased the basal microdialysate content of glutamate and glutamine, which may cause inadequate supply of the neurotransmitter glutamate, while the level of two other neuroactive amino acids, aspartate or taurine was not affected by the exposure. The results underscore neuronal lesion and inhibition of NMDA receptor-mediated glutamatergic signaling in hippocampus as a potential major contributor to the cognitive deficits associated with exposure to PCB.

The role of ASIC1a in neuroprotection elicited by quercetin in focal cerebral ischemia

One of the major instigators to neuronal cell death and brain damage following cerebral ischemia is calcium dysregulation. The intracellular calcium overload resulting from glutamate excitotoxicity is considered a major determinant for neuronal loss during cerebral ischemia. Moreover, ASIC1a activation due to acidosis also promotes intracellular calcium overload during ischemic insult. Interestingly, ASIC1a was found to be inhibited by some flavonoids which carry an anti-inflammatory property particularly quercetin, which could be exploited in hypoxic conditions like cerebral ischemia. This encourages us to investigate the neuroprotective effect of quercetin besides its possible downstream signaling mechanism in focal cerebral ischemia. The treatment of quercetin 30min before ischemia and 4h after reperfusion shows significant protection from ischemic injury as noticed by reduction in cerebral infarct volume and neurobehavioral deficit. In addition to earlier calcium dependent rise in the levels of nitrite and MDA exhibited marked reduction (P<0.01) in their levels when given quercetin pretreatment in ischemic brain regions. The quercetin treatment also reduced the spectrin break down products (SBDP) caused by ischemic activation of calcium dependent protease calpain. In ex-vivo study, it was also observed that quercetin inhibited the acid mediated intracellular calcium levels in rat brain synaptoneurosomes. These studies suggest the neuroprotective role of quercetin in focal cerebral ischemia by regulation of ASIC1a.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.