Electrographic seizures are significantly reduced by in vivo inhibition of neuronal uptake of extracellular glutamine in rat hippocampus

Rats were given unilateral kainate injection into hippocampal CA3 region, and the effect of chronic electrographic seizures on extracellular glutamine (GLN(ECF)) was examined in those with low and steady levels of extracellular glutamate (GLU(ECF)). GLN(ECF), collected by microdialysis in awake rats for 5h, decreased to 62 +/- 4.4% of the initial concentration (n = 6). This change correlated with the frequency and magnitude of seizure activity, and occurred in the ipsilateral but not in contralateral hippocampus, nor in kainate-injected rats that did not undergo seizure (n = 6). Hippocampal intracellular GLN did not differ between the Seizure and No-Seizure Groups. These results suggested an intriguing possibility that seizure-induced decrease of GLN(ECF) reflects not decreased GLN efflux into the extracellular fluid, but increased uptake into neurons. To examine this possibility, neuronal uptake of GLN(ECF) was inhibited in vivo by intrahippocampal perfusion of 2-(methylamino)isobutyrate, a competitive and reversible inhibitor of the sodium-coupled neutral amino acid transporter (SNAT) subtypes 1 and 2, as demonstrated by 1.8 +/- 0.17 fold elevation of GLN(ECF) (n = 7). The frequency of electrographic seizures during uptake inhibition was reduced to 35 +/- 7% (n = 7) of the frequency in pre-perfusion period, and returned to 88 +/- 9% in the post-perfusion period. These novel in vivo results strongly suggest that, in this well-established animal model of temporal-lobe epilepsy, the observed seizure-induced decrease of GLN(ECF) reflects its increased uptake into neurons to sustain enhanced glutamatergic epileptiform activity, thereby demonstrating a possible new target for anti-seizure therapies. (C) 2013 Elsevier B.V. All rights reserved.