title: 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.
abstract: The aim of the present study was to investigate if flumazenil blood-brain barrier transport and binding to the benzodiazepine site on the γ-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 μg) 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 ± 2 ng x mL(-1) in the hippocampus and as 33 ± 2 ng x mL(-1) 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 ± 0.9 ng x mL(-1). 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.
  
  
  CONCLUSIONS
  B(max) was decreased in kainate-treated rats, compared with controls, but no alteration in the blood-brain 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.
authors.affiliation: Division of Pharmacology, LACDR, Leiden University, Leiden, The Netherlands. s.syvanen@lacdr.leidenuniv.nl
authors.forename: Stina
authors.initials: S
authors.lastname: Syvänen
authors.forename: Maaike
authors.initials: M
authors.lastname: Labots
authors.forename: Yoshihiko
authors.initials: Y
authors.lastname: Tagawa
authors.forename: Jonas
authors.initials: J
authors.lastname: Eriksson
authors.forename: Albert D
authors.initials: AD
authors.lastname: Windhorst
authors.forename: Adriaan A
authors.initials: AA
authors.lastname: Lammertsma
authors.forename: Elizabeth C
authors.initials: EC
authors.lastname: de Lange
authors.forename: Rob A
authors.initials: RA
authors.lastname: Voskuyl
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D001812:Blood-Brain Barrier / Q000187:drug effects* / Q000378:metabolism*; D002250:Carbon Radioisotopes; D004827:Epilepsy / Q000000981:diagnostic imaging / Q000378:metabolism*; D005442:Flumazenil / Q000378:metabolism*; D007608:Kainic Acid / Q000494:pharmacology*; D008297:Male; D049268:Positron-Emission Tomography; D011804:Quinolines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000378:metabolism*
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002250:Carbon Radioisotopes; D011804:Quinolines; D011963:Receptors, GABA-A; D005442:Flumazenil; C402343:tariquidar; D007608:Kainic Acid