cgp-55845a and Epilepsy--Temporal-Lobe

Effects of pre- and postsynaptic γ-aminobutyric acid B (GABA(B)) receptor activation were characterized in human tissue from epilepsy surgery.. Slices of human cortical tissue were investigated in a submerged-type chamber with intracellular recordings in layers II/III. Parallel experiments were performed in rat neocortical slices with identical methods. Synaptic responses were elicited with single or paired stimulations of incrementing intervals.. Neurons in human epileptogenic tissue exhibited usually small inhibitory postsynaptic potentials (IPSP) mediated by GABA(B) receptor, verified by the sensitivity to the selective antagonist CGP 55845A. The IPSP(B) conductance averaged 5.8 nS in neurons from epileptogenic tissues and 15.9 nS in neurons from nonepileptogenic tissues (p < 0.0001). Application of baclofen caused small conductance increases in human neurons, which were linearly related to IPSP(B) conductances. Paired-pulse stimulation revealed constant synaptic responses in human temporal lobe epilepsy (TLE) slices at all interstimulus intervals (ISIs). Pharmacologically isolated IPSP(A) in the human tissue exhibited a small paired-pulse depression (average 10% at 500 ms ISI). Bicuculline-induced paroxysmal depolarization shifts (PDSs) were transiently depressed by 24% in human TLE tissue; and by 74% in rat neocortical slices (200 ms ISI; p = 0.015). The depressions of bicuculline-induced PDSs were antagonized by CGP 55845A in both species. Staining for GABA(B) receptors revealed significantly smaller numbers of immunopositive dots in human epileptogenic neurons versus human control neurons.. The small IPSP(B), baclofen-conductances, and paired-pulse depression of PDSs and IPSPs in human TLE tissue indicate a reduced density of post- and presynaptic GABA(B) receptors. The reduced efficacy of presynaptic GABA(B) receptors facilitates the occurrence of repetitive synaptic activity.

Topics: Animals; Anticonvulsants; Baclofen; Bicuculline; Cerebral Cortex; Drug Resistance; Epilepsy; Epilepsy, Temporal Lobe; Excitatory Postsynaptic Potentials; GABA-B Receptor Antagonists; gamma-Aminobutyric Acid; Humans; Immunohistochemistry; Mice; Neurons; Phosphinic Acids; Propanolamines; Rats; Rats, Wistar; Receptors, GABA-B; Synaptic Transmission

Temporal lobe epilepsy is common and difficult to treat. Reduced inhibition of dentate granule cells may contribute. Basket cells are important inhibitors of granule cells. Excitatory synaptic input to basket cells and unitary IPSCs (uIPSCs) from basket cells to granule cells were evaluated in hippocampal slices from a rat model of temporal lobe epilepsy. Basket cells were identified by electrophysiological and morphological criteria. Excitatory synaptic drive to basket cells, measured by mean charge transfer and frequency of miniature EPSCs, was significantly reduced after pilocarpine-induced status epilepticus and remained low in epileptic rats, despite mossy fiber sprouting. Paired recordings revealed higher failure rates and a trend toward lower amplitude uIPSCs at basket cell-to-granule cell synapses in epileptic rats. Higher failure rates were not attributable to excessive presynaptic inhibition of GABA release by activation of muscarinic acetylcholine or GABA(B) receptors. High-frequency trains of action potentials in basket cells generated uIPSCs in granule cells to evaluate readily releasable pool (RRP) size and resupply rate of recycling vesicles. Recycling rate was similar in control and epileptic rats. However, quantal size at basket cell-to-granule cell synapses was larger and RRP size smaller in epileptic rats. Therefore, in epileptic animals, basket cells receive less excitatory synaptic drive, their pools of readily releasable vesicles are smaller, and transmission failure at basket cell-to-granule cell synapses is increased. These findings suggest dysfunction of the dentate basket cell circuit could contribute to hyperexcitability and seizures.

Topics: Analysis of Variance; Animals; Atropine Derivatives; Dentate Gyrus; Disease Models, Animal; Electric Stimulation; Epilepsy, Temporal Lobe; GABA Antagonists; In Vitro Techniques; Lysine; Male; Muscarinic Agonists; Muscarinic Antagonists; Nerve Net; Neurons; Patch-Clamp Techniques; Phosphinic Acids; Pilocarpine; Propanolamines; Rats; Rats, Sprague-Dawley; Synaptic Potentials; Synaptophysin; Time Factors

Intracellular recording techniques were used to examine GABA(A) receptor-mediated synaptic inhibition in pyramidal cells of the CA1 region of the rat hippocampus in the post-self sustaining limbic status epilepticus model of temporal lobe epilepsy. Orthodromically evoked, monosynaptic inhibitory postsynaptic potentials were recorded in vitro following pharmacological blockade of ionotropic glutamate and GABA(B) receptors. Inhibitory postsynaptic potentials from epileptic tissue were kinetically altered relative to controls; both the 10-90% rise-time and width (measured at half-maximum amplitude) were reduced by approximately 50% resulting in significant shortening of duration. The degree of pyramidal cell hyperexcitability, assessed before pharmacological treatment as the number of action potentials evoked by maximum intensity afferent stimulation, correlated significantly with the magnitude of synaptic potential duration reduction determined following blockade of glutamatergic neurotransmission. Bath application of the benzodiazepine type 1 receptor agonist zolpidem reduced post-self sustaining limbic status epilepticus CA1 pyramidal cell hyperexcitability substantially (but not completely) via a marked increase in inhibitory postsynaptic potential area. Post-self-sustaining limbic status epilepticus inhibitory postsynaptic potentials which exhibited the most pronounced shortening were augmented by zolpidem to a greater degree than longer duration synaptic potentials. In contrast, zolpidem-induced augmentation of control inhibitor, postsynaptic potential area was much less robust. It is suggested that a deficiency in post-self-sustaining limbic status epilepticus GABA(A) receptor-mediated synaptic inhibition contributes to a state of partial disinhibition which is a major factor in enhanced CA1 excitability in chronic limbic epilepsy. Possible mechanisms underlying post-self-sustaining limbic status epilepticus kinetic abnormalities are discussed.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Animals; Disease Models, Animal; Electric Stimulation; Epilepsy, Temporal Lobe; Evoked Potentials; GABA Antagonists; GABA-A Receptor Antagonists; GABA-B Receptor Antagonists; Hippocampus; Hypnotics and Sedatives; Male; Phosphinic Acids; Propanolamines; Pyramidal Cells; Pyridines; Rats; Rats, Sprague-Dawley; Receptors, GABA-A; Receptors, GABA-B; Regression Analysis; Synapses; Zolpidem