Changes in GABA_A receptor properties in amygdala kindled animals: In vivo studies using [¹¹C]flumazenil and positron emission tomography

Abstract

SUMMARYPurpose: The purpose of the present investigation was to quantify alterations in GABA A receptor density in vivo in rats subjected to amygdala kindling. Methods: The GABA A receptor density was quantified by conducting a [ 11 C]flumazenil (FMZ) positron emission tomography (PET) study according to the full saturation method, in which each animal received a single injection of FMZ to fully saturate the GABA A receptors. Subsequently, the concentration-time curves of FMZ in blood [using highpressure liquid chromatography with UV detector (HPLC-UV) or high-performance liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS)] and brain (with PET-scanning) were analyzed by population modeling using a pharmacokinetic model, containing expressions to describe the time course of FMZ in blood and brain. Results: The GABA A receptor density (B max ) in kindled rats was decreased by 36% compared with controls. This is consistent with a reduction of 28% in electroencephalography (EEG) effect of midazolam in the same animal model, suggesting that a reduced number of GABA A receptors underlies the decreased efficacy of midazolam. Furthermore, receptor affinity (K D ) was not changed, but the total volume of distribution in the brain (V Br ), is increased to 178% of control after kindling, which might indicate an alteration in the transport of FMZ across the blood-brain barrier. Conclusions: Both the GABA A receptor density (B max ), and possibly also the blood-brain barrier transport of FMZ (V Br ) are altered after kindling. Furthermore, this study indicates the feasibility of conducting PET studies for quantifying moderate changes in GABA A receptor density in a rat model of epilepsy in vivo.

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

“…Results suggested that receptor density, B max , was lower in kainate-treated animals than in controls. This result is in line with previous preclinical and clinical studies using 11 C-flumazenil (17,22) and suggests a reduction in GABA A receptor binding capacity in the epileptic brain, compared with the naïve brain. A previous study in kindled rats, another rat model for temporal lobe epilepsy (23,24), showed that B max was reduced by 36%, compared with controls (17).…”

“…(27), and 54.5 ngÁmL 21 in rat brain homogenates (28). The K D estimate, 5.9 6 0.9 ngÁmL 21 , obtained in the present study is also in line with previously published studies (17,18,(25)(26)(27)(28). On the basis of time-activity data obtained in a previous study when 11 C-flumazenil was administered without the addition of unlabeled flumazenil (18), the doses used in the present study resulted in approximate occupancy levels of 30%-40% for 4-mg, 60%-70% for 20-mg, 70%-80% for 100-mg, and around 90% for 400-mg dose groups.…”

“…This analysis was performed in the same laboratory and with the same equipment (i.e., using identical conditions) as described in 2 previous reports (17,18). The limit of quantification was 0.5 ngÁmL 21 .…”

“…Flumazenil concentrations in blood were analyzed using a previously published method comprising high-performance liquid chromatography coupled to tandem mass spectrometry (17). This analysis was performed in the same laboratory and with the same equipment (i.e., using identical conditions) as described in 2 previous reports (17,18).…”

A Functional Role for Microglia in Epilepsy

“…Metabolic neuroimaging with positron emission tomography (PET) is the most widely used diagnostic, (for review (Mirrione and Tsirka, 2011). A handful of recent studies in animal models have also begun to utilize specific radioligands studying GABA, serotonin, dopamine, and the cannabinoid systems (Werhahn et al, 2006, Goffin et al, 2008a, Liefaard et al, 2009, Liew et al, 2009, Assem-Hilger et al, 2010. There are also a few pilot studies in the literature examining the acetylcholine (Mohamed et al, 2005) and opioid systems (Theodore et al, 1992) in epilepsy patients.…”