The neurobiological mechanisms underlying DHK

The neurobiological mechanisms underlying DHK-induced NOR learning and memory impairments remain unclear. Accumulated evidence shows that astrocyte-neuron lactate shuttle is of crucial important in synaptic plasticity and memory formation, and regulation of neuronal gene expression related to synaptic plasticity and memory formation [[28], [29], [30]]. Astrocytic glutamate uptake can trigger glucose uptake and processing via glycolysis, which results in astrocytic lactate production and release [31]. GLT1 maintains low glutamate concentrations in the synaptic cleft and ensures a high signal-to-noise ratio in synaptic transmission [6,32]. Inhibition of GLT1 impairs the temporal contingency that is required for Hebbian learning [15]. Mutant mice lacking GLT-1 shows impaired long-term potentiation in the hippocampal CA1 region [14]. Possibly, blockade of astrocytic glutamate uptake disrupts neuron–glia metabolic coupling, synaptic plasticity and its underlying molecules, which might underlies DHK-induced NOR learning and memory impairments.
Introduction Epilepsy is a neurological disorder with a lifetime prevalence of 2–5% (excluding febrile seizures) that affects ∼67 million people worldwide (1). It has been suggested that epilepsy is the result of an imbalance between excitatory and inhibitory neurotransmission 2, 3. Glutamate is the principal excitatory neurotransmitter in the central nervous system (CNS) and there is evidence of increased glutamate levels during seizures 2, 4, 5. Glutamate transporters are surface proteins located on neurons and glial akt inhibitor that remove glutamate from the extracellular space, returning concentrations to basal levels. To date, at least five glutamate transporter subtypes have been identified and cloned: GLAST (EAAT1), GLT-1 (EAAT2), EAAC-1 (EAAT3), EAAT-4 and EAAT-5. Of these, the EAAT-3 protein in particular is expressed in neurons of the dentate gyrus (DG) and hippocampus 6, 7. Different models of acquired epilepsy and spontaneous seizures have demonstrated important changes in glutamate transporter expression, as also seen in human temporal lobe epilepsy (TLE) 8, 9, 10, 11. GABA, the principal inhibitory neurotransmitter, is also thought to play an important role in epilepsy. Four GABA transporter subtypes have been identified in mammals: GAT-1, GAT-2, GAT-3 and GAT-4 (12). GAT-1 is expressed in the pyramidal and granule cell layers of the hippocampal formation and is considered therapeutically significant given the antiepileptic effects elicited by its inhibition 13, 14. Alterations in GAT-1 protein and mRNA expression have been reported in kainic acid, picrotoxin and FeCl models of epilepsy as well as in TLE models and patients 15, 16, 17, 18. Epileptic seizures can be induced in neonatal rats by administering monosodium glutamate (MSG) (19). In this model, increased glutamate levels have been associated with convulsive behavior and EEG hippocampal discharges induced by the first three subcutaneous injections of MSG. Despite abundant evidence of alterations to glutamate and GABA transporter expression in different models of epilepsy, no such findings have been reported to date using this model of early neonatal glutamate exposure. We also believe it is important to study EAAT-3 and GAT-1 transporters in this model due to the fact that EAAT-3 transporters are found at both asymmetrical and symmetrical synapses in the dendrites and soma of granular and pyramidal cells 20, 21, 22. In cooperation with the cysteine/glutamate antiporter Xc-, EAAT-3 helps protects the neuronal HT22 cells (an immortalized hippocampal cell line) from oxidative glutamate toxicity (23). Moreover, EAAT-3 can attenuate the activation of NMDA receptors when they are co-expressed in Xenopus oocytes (24), and this transporter is coupled to Kv4.2 dephosphorylation through the activation of extrasynaptic NMDA receptors (25), important elements in the MSG model (26). Finally, it is noteworthy that there is a decrease in several GABAergic markers in the adult cerebral cortex and hippocampus in the same model of epilepsy (27). To study the expression of the EAAT-3 and GAT-1 transporters in the hippocampus, we examined rats at postnatal days 14 and 60 following repeated MSG treatment because the main molecular and biochemical effects of early glutamate exposure occur by postnatal day 14, some persisting into adulthood 20, 21, 22. Immunofluorescent staining was then used to identify and count EAAT-3 and GAT-1 immunolabelled cells in the dentate gyrus and CA1 region of the hippocampus.