Abstracts

The GABA[sub]A[/sub]-receptor plays an important role in epileptogenesis. A key question is whether downregulation or changes in receptor properties are involved in reduced efficacy of antiepileptic drugs, in particular in pharmacoresistance. Recently, a full saturation approach was developed, in which the whole range of receptor occupancies, obtained from a single PET-experiment per animal, was used to calculate both GABA[sub]A[/sub]-receptor density ([italic]B[sub]max[/sub][/italic]) and affinity ([italic]K[sub]D[/sub][/italic]). This method was used to examine the influence of kindling on [italic]B[sub]max[/sub][/italic] and [italic]K[sub]D[/sub][/italic]., Sprague Dawley rats were amygdala kindled up to 5 consecutive stage V seizures (n=12). Implanted but unstimulated rats served as controls (n=22). The PET experiment was performed 10-14 days after the last seizure. After injection of an excess amount of flumazenil (FMZ) to fully saturate the receptors, the concentration-time curves of FMZ were measured in blood with HPLC-UV and in brain with PET. Next, [italic]B[sub]max[/sub][/italic] and [italic]K[sub]D[/sub][/italic] were estimated using population pharmacokinetic (PK) modelling. The model consists of a blood, a tissue and 2 brain ([ldquo]free[rdquo] and [ldquo]bound[rdquo]) compartments. The total FMZ concentration in brain, as measured by PET, reflects the sum of [ldquo]free[rdquo] and [ldquo]bound[rdquo] concentrations. In the model, exchange between blood, tissue and [ldquo]brain free[rdquo] compartments is described by first order rate constants, and specific binding depends on both the concentration of free ligand and the concentration of receptors available for binding. Population PK modelling allows for simultaneous analysis of data from all animals, taking interindividual parameter variability into account., In control rats, all model parameters, including specific binding of FMZ in the brain, as characterized by [italic]B[sub]max[/sub][/italic] (14.5 [plusmn] 3.7 ng/ml) and [italic]K[sub]D[/sub][/italic] (4.68 [plusmn] 1.5 ng/ml), could be estimated adequately. Kindling did not affect [italic]K[sub]D[/sub][/italic], but [italic]B[sub]max[/sub][/italic] decreased to 64 [plusmn] 16% of control. This finding corresponds with a reduction in maximal effect of the allosteric modulator midazolam to 72% of control, as reported by Cleton [italic]et al[/italic]. In addition, the volume of distribution of the brain was increased to 180 [plusmn] 27% of control, indicating that transport from the brain was decreased., This study shows that the full saturation technique, using a single injection of FMZ, allows detection of moderate reductions in GABA[sub]A[/sub]-receptor density and changes in brain PK. The decreased B[sub][italic]max[/italic][/sub] may underlie the reduced efficacy of midazolam in kindled rats. In addition, the study shows the feasibility to conduct (longitudinal) studies on epileptogenesis and pharmacoresistance development in chronic epilepsy models., (Supported by Christelijke Vereniging voor de Verpleging van Lijders aan Epilepsie and National Epilepsy Fund, #02-06.)