Altered GABA_A Receptor Density and Unaltered Blood–Brain Barrier Transport in a Kainate Model of Epilepsy: An In Vivo Study Using ¹¹C-Flumazenil and PET

Abstract

The aim of the present study was to investigate if flumazenil blood-brain barrier transport and binding to the benzodiazepine site on the g-aminobutyric acid A (GABA A ) receptor complex is altered in an experimental model of epilepsy and subsequently to study if changes in P-glycoprotein (P-gp)-mediated efflux of flumazenil at the blood-brain barrier may confound interpretation of 11 C-flumazenil PET in epilepsy. Methods: The transport of flumazenil across the blood-brain barrier and the binding to the benzodiazepine site on the GABA A receptors in 5 different brain regions was studied and compared between controls and kainate-treated rats, a model of temporal lobe epilepsy, with and without tariquidar pretreatment. In total, 29 rats underwent 2 consecutive 11 C-flumazenil PET scans, each one lasting 30 min. The tracer was mixed with different amounts of isotopically unmodified flumazenil (4, 20, 100, or 400 mg) to cover a wide range of receptor occupancies during the scan. Before the second scan, the rats were pretreated with a 3 or 15 mg/kg dose of the P-gp inhibitor tariquidar. The second scan was then obtained according to the same protocol as the first scan. Results: GABA A receptor density, B max , was estimated as 44 6 2 ngÁmL 21 in the hippocampus and as 33 6 2 ngÁmL 21 in the cerebellum, with intermediate values in the occipital cortex, parietal cortex, and caudate putamen. B max was decreased by 12% in kainate-treated rats, compared with controls. The radiotracer equilibrium dissociation constant, K D , was similar in both rat groups and all brain regions and was estimated as 5.9 6 0.9 ngÁmL 21 . There was no difference in flumazenil transport across the blood-brain barrier between control and kainate-treated rats, and the effect of tariquidar treatment was similar in both rat groups. Tariquidar treatment also decreased flumazenil transport out of the brain by 73%, increased the volume of distribution in the brain by 24%, and did not influence B max or K D , compared with baseline . Conclusion: B max was decreased in kainate-treated rats, compared with controls, but no alteration in the bloodbrain barrier transport of flumazenil was observed. P-gp inhibition by tariquidar treatment increased brain concentrations of flumazenil in both groups, but B max estimates were not influenced, suggesting that 11 C-flumazenil scanning is not confounded by alterations in P-gp function.

Altered GABAA receptor density and unaltered blood–brain barrier [11C]flumazenil transport in drug-resistant epilepsy patients with mesial temporal sclerosis

“…In addition, BP ND was not affected significantly by P-gp inhibition, which is also in line with animal studies showing that tariquidar had no effect on [ 11 C]flumazenil binding to the GABA A receptor in both naïve and kainate-treated rats. 21 …”

In Vivo Measurement of Hippocampal GABAA/cBZR Density with [18F]-Flumazenil PET for the Study of Disease Progression in an Animal Model of Temporal Lobe Epilepsy

“…This reduction in GABAergic neurotransmission may be caused by any or all of the following: loss of GABAergic interneurons, loss of GABA A receptors, or changes to GABA A receptor subunits, leading to alterations in receptor properties [2], [3], [4] and needs to be further investigated. Previous animal and human studies have shown decreased GABA A /central benzodiazepine receptor (GABA A /cBZR) density in structures important to seizure generation in the mesial temporal lobe [5], [6], [7], [8], [9].…”

Development of 18F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

“…For example, Syvänen [253] determined the GABA A receptor density, B max , in rat brain using four doses (between 4 μg and 400 μg) of As long as the tracer kinetics can be considered linear (which is usually a valid assumption) a sufficiently comprehensive compartmental model (with a sufficient number of compartments) will be able to describe any given system. Increasing the number of compartments suffi ciently, one can even model diffusive processes (which inherently imply the presence of concentration gradients).…”

“…For example, Syvänen [253] determined the GABA A receptor density, B max , in rat brain using four doses (between 4 μg and 400 μg) of reported, while K D remained unchanged [253] .…”