Inhibition Of Dentate Granule Cells Research Articles

Decrease in inhibition in dentate granule cells from patients with medial temporal lobe epilepsy.

Alterations in synaptic inhibition are associated with epileptiform activity in several acute animal models; however, it is not clear if there are changes in inhibition in chronically epileptic tissue. We have used intracellular recordings from granule cells of patients with temporal lobe epilepsy to determine whether synaptic inhibition is compromised. Two groups of patients with medial temporal lobe epilepsy were used, those with medial temporal lobe sclerosis (MTLE), and those with extrahippocampal masses (MaTLE) where the cell loss and synaptic reorganization that characterize MTLE are not seen. Although the level of tonic inhibition at the somata was not significantly different in the two patient groups, there was a reduction in the conductance of polysynaptic perforant path-evoked fast and slow inhibitory postsynaptic potentials (IPSPs) (53% and 66%, respectively). We found that there was a comparable decrease in the monosynaptic IPSP conductances examined in the presence of glutamatergic antagonists as that seen for the polysynaptically evoked IPSPs. These data suggest that the decrease in inhibition seen in normal artificial cerebrospinal fluid in MTLE granule cells cannot be solely explained by a decrease in excitatory input onto inhibitory interneurons and may reflect changes at the interneuron-granule cells synapse or in the number of specific inhibitory interneurons.

Strong paired pulse depression of dentate granule cells in slices from patients with temporal lobe epilepsy.

Recurrent, feedback excitation by sprouted mossy fibers may contribute to the hyperexcitability observed in human temporal lobe epilepsy. Observations in rodent models of epilepsy mimic the findings in human tissue and reveal that dentate granule cells sprout axons which innervate fibers in their own dendritic layer. However, recent evidence in rodents suggest that these sprouted fibers may form connections which cause inhibition of dentate granule cells, not excitation. Thus, the net effect of sprouting in human epileptic tissue may not only be recurrent excitation. We analyzed paired pulse depression in dentate slices from 9 patients with temporal lobe epilepsy and found evidence for strong feedback inhibition. We also noted failure of high frequency stimulation induced inhibition in our human specimens. These data challenge the concept that human epileptic dentate granule cells are excited by recurrent mossy fiber sprouting.

Decrement of GABAA receptor-mediated inhibitory postsynaptic currents in dentate granule cells in epileptic hippocampus.

1. Inhibitory postsynaptic currents (IPSCs) were studied in hippocampal dentate granule cells (DGCs) in the pilocarpine model and human temporal lobe epilepsy, with the use of the whole cell patch-clamp recording technique in slice preparations. 2. In the pilocarpine model, hippocampal slices were prepared from rats that were allowed to experience spontaneous seizures for 2 mo. Human hippocampal specimens were obtained from epileptic patients who underwent surgical treatment for medically intractable seizures. 3. IPSCs were generated by single perforant path stimulation and recorded at a membrane potential (Vm) of 0 mV near the reversal potential of glutamate excitatory postsynaptic currents in the voltage-clamp recording. IPSCs were pharmacologically identified as gamma-aminobutyric acid-A (GABAA) IPSCs by 10 microM bicuculline methiodide. 4. During low-frequency stimulation, IPSCs were not different in amplitude among non-seizure-experienced rat hippocampi, human nonsclerotic hippocampi, seizure-experienced rat hippocampi, and human sclerotic hippocampi. In the last two groups of DGCs, current-clamp recordings indicated the presence of prolonged excitatory postsynaptic potentials (EPSPs) mediated by the N-methyl-D-aspartate (NMDA) receptor. 5. High-frequency stimulation, administered at Vm = -30 mV to activate NMDA currents, reduced GABAA IPSC amplitude specifically in seizure-experienced rat hippocampi (t = 2.5, P < 0.03) and human sclerotic hippocampi (t = 7.7, P < 0.01). This reduction was blocked by an NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (APV) (50 microM). The time for GABAA IPSCs to recover to their original amplitude was also shortened by the application of APV. 6. I conclude that, when intensively activated, NMDA receptor-mediated excitatory transmission may interact with GABAergic synaptic inhibition in DGCs in seizure-experienced hippocampus to transiently reduce GABA(A) receptor-channel function. Such interactions may contribute to give rise to epileptic excitation in chronically seizure-prone hippocampus.

Altered inhibition of dentate granule cells during spatial learning in an exploration task

To investigate the extent to which inhibitory interneurons control impulse flow through the dentate gyrus during spatial learning in an exploration task, dentate field potentials were recorded in response to paired stimulation of the perforant path while rats rested or explored. Recurrent inhibition of the granule cells was measured as the reduction of the second waveform when a population spike was present in the first. Both the population spike and the field EPSP (fEPSP) were suppressed at interstimulus intervals shorter than approximately 40 msec. Consistent differences were observed between potentials recorded at equivalent brain temperature in the exploration and resting (reference) conditions. During exploration, the fEPSP of the second (test) waveform was reduced further compared with reference potentials with a similar response to the first (conditioning) stimulus. This reduction was observed only when the first pulse elicited a population spike. The population spike of the second waveform was facilitated compared with reference potentials with similar fEPSP slopes. These observations suggest that exploration is coupled to increased inhibition on the perforant-path terminals or the dendrites of the granule cells, whereas the inhibition on the somata is decreased. The two phenomena were not correlated and followed different time courses. The suppression of the fEPSP decayed gradually, although it was still present at 15 min, whereas the facilitation of the population spike was stable. Together, these changes, which likely involve different populations of interneurons, may focus and amplify incoming signals from the entorhinal cortex.

Selective protection of neuropeptide containing dentate hilar interneurons by non-NMDA receptor blockade in an animal model of status epilepticus

We used a 24 h perforant path stimulation model of status epilepticus to study the role of non-NMDA receptors in the loss of hilar interneurons and paired pulse inhibition associated with the model. In one experiment, NBQX administered i.v. at 1.0 mg/kg/h significantly reduced the loss of hematoxylin and eosin-stained hilar neurons from 360.2 to 125.3 but failed to protect against the loss of paired pulse inhibition. In a second experiment, i.v. NBQX at 1.5 mg/kg/h significantly protected against loss of SS- and NPY-positive hilar interneurons but also failed to protect against loss of paired pulse nhibition. These results demonstrate that the neuronal loss associated with sustained stimulation of this excitatory pathway is mediated in part through non-NMDA receptors. The lack of protection against loss of paired pulse inhibition suggests that SS- and NPY-immunoreactive interneurons may not be responsible for frequency-dependent paired-pulse inhibition of dentate granule cells.

Effects of LTP-inducing tetanic stimulations of the perforant path on the commissural inhibition and facilitation of dentate granule cell discharge

Effects of diazepam (1 mg/kg, i.p.) on the commissural influences on granule cells were first examined to further assess its GABAergic inhibitory mechanism. Whereas the commissural inhibition at an interval of 5–8 ms of the contralateral dentate-perforant path (CP) combined stimulus was not enhanced by diazepam, the commissural facilitation at a CP interval of 11–20 ms was reduced, suggesting that the GABAergic inhibition is involved in rather a part of the commissural facilitation at a CP interval of 11–20 msec than a part of the ‘commissural’ inhibition at a shorter CP interval. Based on the results of diazepam, effects of high-frequency stimulations of the perforant path on the commissural inhibition of dentate granule cells were then examined, in relation to their effects on the dentate paired-pulse depression. These stimulations produced the long-term potentiation of the perforant path-dentate excitatory synapse and significant reduction of the paired-pulse depression. The commissural inhibition at a CP interval of 5–8 ms remained unchanged following tetanization. The commissural facilitation at a CP interval of 11–20 ms was, however, slightly enhanced by tetanic stimulations and a statistical significance was obtained at a CP interval of 19 ms. These results imply that tetanic stimulations of the perforant path selectively reduce the GABAergic component of the commissural inhibition, as well as that of the paired-pulse depression.

Gamma-Aminobutyric acid and benzodiazepine receptors in the kindling model of epilepsy: a quantitative radiohistochemical study

Quantitative radiohistochemistry was utilized to study alterations of gamma-aminobutyric acid (GABA) and benzodiazepine receptors in the kindling model of epilepsy. The radioligands used for GABA and benzodiazepine receptors were [3H] muscimol and [3H]flunitrazepam, respectively. GABA receptor binding was increased by 22% in fascia dentata of the hippocampal formation but not in neocortex or substantia nigra of kindled rats. Within fascia dentata, GABA receptor binding was increased to an equivalent extent in stratum granulosum and throughout stratum moleculare; no increase was found in dentate hilus or stratum lacunosummoleculare or stratum radiatum of CA1. The increased binding was present at 24 hr but not at 28 days after the last kindled seizure. The direction, anatomic distribution, and time course of the increased GABA receptor binding were paralleled by increased benzodiazepine receptor binding. Unexpectedly, GABA receptor-mediated enhancement of benzodiazepine receptor binding was slightly attenuated in fascia dentata of kindled compared to control rats. The anatomic distribution of the increased GABA receptor binding is consistent with a localization to somata and dendritic trees of dentate granule cells. We suggest that increased GABA and benzodiazepine receptor binding may contribute to enhanced inhibition of dentate granule cells demonstrated electrophysiologically in kindled animals.