Summary
Rats were given unilateral kainate injection into hippocampal CA3 region,
and the effect of chronic electrographic seizures on extracellular glutamine
(GLNECF) was examined in those with low and steady levels of
extracellular glutamate (GLUECF). GLNECF, collected by
microdialysis in awake rats for 5 h, 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 GLNECF
reflects not decreased GLN efflux into the extracellular fluid, but increased
uptake into neurons. To examine this possibility, neuronal uptake of
GLNECF 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 GLNECF
(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 GLNECF reflects its increased uptake into
neurons to sustain enhanced glutamatergic epileptiform activity, thereby
demonstrating a possible new target for anti-seizure therapies.