Disinhibition reduces extracellular glutamine and elevates extracellular glutamate in rat hippocampus in vivo

- Disinhibition was induced in rat hippocampus in vivo by GABAAR or GABABR blockade.
- Unilateral blockade induced bilateral epileptiform discharges or Na+ spikes.
- GABABR-mediated disinhibition caused bilateral elevation of extracellular glutamate.
- Extracellular, but not intracellular, glutamine decreased significantly.
- Extracellular glutamine is a major source of neurotransmitter glutamate in vivo.

SummaryDisinhibition was induced in the hippocampal CA1/CA3 region of normal adult rats by unilateral perfusion of the GABAAR antagonist, 4-[6-imino-3-(4-methoxyphenyl)pyridazin-1-yl] butanoic acid hydrobromide (gabazine), or a GABABR antagonist, p-(3-aminopropyl)-p-diethoxymethyl-phosphinic acid (CGP 35348), through a microdialysis probe. Effects of disinhibition on EEG recordings and the concentrations of extracellular glutamate (GLUECF), the major excitatory neurotransmitter, and of extracellular glutamine (GLNECF), its precursor, were examined bilaterally in freely behaving rats. Unilateral perfusion of 10 μM gabazine in artificial CSF of normal electrolyte composition for 34 min induced epileptiform discharges which represent synchronized glutamatergic population bursts, not only in the gabazine-perfused ipsilateral hippocampus, but also in the aCSF-perfused contralateral hippocampus. The concentration of GLUECF remained unchanged, but the concentration of its precursor, GLNECF, decreased to 73 ± 4% (n = 5) of the baseline during frequent epileptiform discharges, not only in the ipsilateral, but also in the contralateral hippocampus, where the change can be attributed to recurrent epileptiform discharges per se, with recovery to 95% of baseline when epileptiform discharges diminished.The blockade of GABABR, by CGP 35348 perfusion in the ipsilateral hippocampus for 30 min, induced bilateral Na+ spikes in extracellular recording. These can reasonably be attributed to somatic and dendritic action potentials and are indicative of synchronized excitatory activity. This disinhibition induced, in both hippocampi, (a) transient 1.6-2.4-fold elevation of GLUECF which correlated with the number of Na+ spike cluster events and (b) concomitant reduction of GLNECF to ∼70%.Intracellular GLN concentration was measured in the hippocampal CA1/CA3 region sampled by microdialysis in separate groups of rats by snap-freezing the brain after 25 min of gabazine perfusion or 20 min of CGP perfusion when extracellular GLN (GLNECF) was 60-70% of the pre-perfusion level. These intracellular GLN concentrations in the disinhibited hippocampi showed no statistically significant difference from the untreated control. This result strongly suggests that the observed decrease of GLNECF is not due to reduced glutamine synthesis or decrease in the rate of efflux of GLN to ECF. This strengthens the likelihood that reduced GLNECF reflects increased GLN uptake into neurons to sustain enhanced GLU flux during excitatory population bursts in disinhibited hippocampus. The results are consistent with the emerging concept that neuronal uptake of GLNECF plays a major role in sustaining epileptiform activities in the kainate-induced model of temporal-lobe epilepsy.