Alterations of GABA-mediated synaptic transmission in human epilepsy

Although animal models consistently indicate that gamma-amino-butyric acid (GABA) synaptic function (GABA levels, synthesis, uptake and/or receptors) is decreased in seizure states, there is little evidence to date in support of such a hypothesis for human epilepsy. This chapter presents the results of an in-depth study of the activity of the GABA-synthesizing enzyme L-glutamic acid decarboxylase (GAD) in brain tissue removed during neurosurgical resection for intractable epilepsy. The tissue studied is unique in that identified (by stereo EEG) foci were excised (rather than large blocks of tissue containing mixtures of foci and nonepileptic material) and compared with nonepileptic (stereo EEG and morphological definitions) tissue from the same patients. In patients in which there was no indication of a tumor, GAD activity in the foci was low in more than 50% of the patients examined. Furthermore, when the population distribution of GAD was compared in epileptic versus nonepileptic tissue fragments from all patients, the peak distribution of epileptic tissue fragments occurred at much lower GAD activities than for the nonepileptic fragments (0-20 versus 41-80 nmol CO2/mg protein X hr, respectively). A small subgroup of epileptic fragments occurred with a normal GAD distribution, indicating that the presence of an epileptic focus was not invariably associated with low GAD activity. When the low levels of GABA "A" binding sites in these epileptic tissue fragments are taken into consideration in combination with the low GAD levels, then it can be estimated that 60 to 70% of the present patient population had deficient GABAergic transmission in epileptic foci as compared to nonepileptic brain tissue from the same patients. It follows that the GABA hypothesis of human epilepsy is not an exclusive or unitary hypothesis, and some patients appear to have normally functioning GABA synapses (as assessed biochemically) in epileptogenic areas. Thus, other neurotransmitter and neurohumoral systems certainly play a role in the epileptic process.