Abstracts

RATIONALE: The objective of this study is to elucidate GABA-A receptor modulation in an unique acquired absence epilepsy model using patch-clamp techniques. Block of cholesterol synthesis in neonatal rats leads to life-long seizures with an associated EEG pattern of spike-waves (SW). This model for inhibition of synthesis of cholesterol provides a model of [dsquote]acquired spike-wave epilepsy[dsquote] (ASWE). Accumulating lines of evidence indicate that abnormality of GABA-A receptors on thalamic reticular (nRt) plays an important role in thalamocortical mechanisms underlying genesis of absence epilepsy.
METHODS: Experiments were performed using Long-Evans Hooded and Sprague-Dawley rats, ages 14-20 days, previously treated with a cholesterol synthesis inhibitor, U18666A (10 mg/kg, i.p.) at postnatal days 1,5,9,13. The neurons were enzyme-mechanically dissociated from the nRt. The patch-clamp whole-cell recording techniques were applied to record GABA-induced currents in single dissociated nRt neurons. GABA-A receptor agonists and modulators were applied through a multibarrel application system.
RESULTS: Bath-applied GABA induced an inward current at a holding potential of -45 mV. Co-application of GABA with diazepam enhanced GABA-induced current in control, but not in ASWE model, nRt neurons. However, pentobarbital potentiated GABA responses of nRt neurons in both control and ASWE model rats. Under whole-cell recording conditions, 100 [mu]M ZnCl was co-applied with 100 [mu]M GABA to recorded neuron. In nRt neurons of the ASWE model, ZnCl suppressed GABA peak current by 59%, while in control nRt neurons, ZnCl only suppressed GABA peak current by 6%. We compared benzodiazepine modulation between these two species. Long-Evans Hooded ASWE model rats exhibited total loss of benzodiazepine modulating ability. In contrast, Sprague-Dawley ASWE model rats showed a diazepam potentiation in a concentration-dependent manner. However, the diazepam potentiation in Sprague-Dawley rats was significantly lower in ASWE model animals compared to controls.
CONCLUSIONS: GABA modulation in nRT neurons is abnormal in the acquired spike-wave epilepsy model. Neurons in the model demonstrate selective loss of benzodiazepine regulation and high sensitivity to Zn²⁺. These findings are more prominent in Long-Evans than in Sprague-Dawley rats, corresponding to propensity for showing EEG spike-waves. Regulation of GABA-A receptor function by benzodiazepine and Zn²⁺ is mainly related to the gamma-2 subunit, so our results raise the possibility of abnormal gamma-2 subunit expression of GABA-A receptors in the ASWE model.
[Supported by: The Sandra Solheim Aiken and Maslah Saul MD and James and Carrie Anderson Fund, the Barrow Neurological Institute Womens[ssquote] Board, and the Bioengineer Seed Foundation of Arizona State University.]