Abstracts

An intensely investigated pathogenic mechanism of temporal lobe epilepsy (TLE) is altered GABA[sub]A[/sub] receptor-mediated inhibition in hippocampal dentate granule cells (DGCs)., To study tonic and synaptic inhibition in DGCs in TLE, whole-cell voltage clamp recordings of GABA[sub]A[/sub] receptor currents were made from DGCs in hippocampal slices from epileptic and age-matched control rats, after blocking ionotropic glutamatergic receptors. Tonic conductance was measured by applying previouslsy used methods of analyzing background noise (I[sub]rms[/sub]) in traces of spontaneous IPSCs and synaptic currents were studied by recording of mIPSCs. Immunohistochemical, western blot and postembedding electron microscopic studies evaluated the expression and targeting of [delta] and [alpha]4 subunits., 1 [mu]M GABA increased I[sub]rms[/sub] and slope GABA conductance in control DGCs, but not in TLE. Allopregnanolone (10 and 30 nM) enhanced I[sub]rms[/sub] in DGCs in control, but not in TLE animals. Penicillin (300 [mu]M) diminished I[sub]rms[/sub] in control, but not in TLE animals. A saturating concentration of GABA (1 mM) evoked a slowly-desensitizing and faster decaying current. Slowly decaying component of the peak whole-cell current was a smaller proportion of total current in TLE animals compared to controls. Thus, tonic inhibition was reduced in DGCs in TLE. Immunohistochemistry and western blot revealed that the expression of [delta] subunit was diminished, and expression of [alpha]4 subunit was increased in dentate gyrus of epileptic animals. Double-labelling for [alpha]4 subunit and synaptic marker GAD[sub]65[/sub], revealed that a higher fraction of immunoreactivity to [alpha]4 subunit co-localized with GAD[sub]65[/sub] in TLE rats compared to controls. This suggested that [alpha]4 subunit-containing receptors were targeted to synapses in TLE animals. Pharmacological properties of synaptic currents were consistent with synaptic expression of [alpha]4 subunit. Furosemide (100 [mu]M) inhibited decay time constants of mIPSCs in TLE, but not in controls. Diazepam (30 nM), zolpidem (100 nM) and allopregnanolone (30 nM) prolonged decay time constants of mIPSCs in control but not in TLE animals. The [alpha]4 subunit-selective inverse agonist RO-154513 (300 nM) prolonged decay of mIPSCs in TLE but not in controls. Postembedding EM for the [alpha]4 subunit revealed that is was located in the presynaptic regions of the symmetric synapses on the soma of DGCs of control rats, but in the center of symmetric synapses in rats with TLE., These studies demonstrate loss of tonic inhibition in DGCs in chronic temporal lobe epilepsy. Altered pharmacological sensitivity of synaptic receptors to a range for modulators suggested expression of [alpha]4 subunit in synapses. Furthermore, electron microscopic and immunohistochemical studies confirmed that [alpha]4 subunit was present at inhibitory synapses in DGCs of epileptic, but not in control rats., (Supported by Public Health Service Grants from NINDS (RO1 NS40337, and RO1NS044370).)