NMDA-dependent currents in granule cells of the dentate gyrus contribute to induction but not permanence of kindling.
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Single-electrode voltage-clamp techniques and bath application of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV) were used to study the time course of seizure-induced alterations in NMDA-dependent synaptic currents in granule cells of the dentate gyrus in hippocampal slices from kindled and normal rats. In agreement with previous studies, granule cells from kindled rats examined within 1 wk after the last of 3 or 30-35 generalized tonic-clonic (class V) seizures demonstrated an increase in the NMDA receptor-dependent component of the perforant path-evoked synaptic current. Within 1 wk of the last kindled seizure, NMDA-dependent charge transfer underlying the perforant path-evoked current was increased by 63-111% at a holding potential of -30 mV. In contrast, the NMDA-dependent component of the perforant-evoked current in granule cells examined at 2.5-3 mo after the last of 3 or 90-120 class V seizures did not differ from age-matched controls. Because the seizure-induced increases in NMDA-dependent synaptic currents declined toward control values during a time course of 2.5-3 mo, increases in NMDA-dependent synaptic transmission cannot account for the permanent susceptibility to evoked and spontaneous seizures induced by kindling. The increase in NMDA receptor-dependent transmission was associated with the induction of kindling but was not responsible for the maintenance of the kindled state. The time course of alterations in NMDA-dependent synaptic current and the dependence of the progression of kindling and kindling-induced mossy fiber sprouting on repeated NMDA receptor activation are consistent with the possibility that the NMDA receptor is part of a transmembrane signaling pathway that induces long-term cellular alterations and circuit remodeling in response to repeated seizures, but is not required for permanent seizure susceptibility in circuitry altered by kindling. (+info)
In vivo intracellular analysis of granule cell axon reorganization in epileptic rats.
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In vivo intracellular recording and labeling in kainate-induced epileptic rats was used to address questions about granule cell axon reorganization in temporal lobe epilepsy. Individually labeled granule cells were reconstructed three dimensionally and in their entirety. Compared with controls, granule cells in epileptic rats had longer average axon length per cell; the difference was significant in all strata of the dentate gyrus including the hilus. In epileptic rats, at least one-third of the granule cells extended an aberrant axon collateral into the molecular layer. Axon projections into the molecular layer had an average summed length of 1 mm per cell and spanned 600 microm of the septotemporal axis of the hippocampus-a distance within the normal span of granule cell axon collaterals. These findings in vivo confirm results from previous in vitro studies. Surprisingly, 12% of the granule cells in epileptic rats, and none in controls, extended a basal dendrite into the hilus, providing another route for recurrent excitation. Consistent with recurrent excitation, many granule cells (56%) in epileptic rats displayed a long-latency depolarization superimposed on a normal inhibitory postsynaptic potential. These findings demonstrate changes, occurring at the single-cell level after an epileptogenic hippocampal injury, that could result in novel, local, recurrent circuits. (+info)
Postnatal development of hippocampal dentate granule cell gamma-aminobutyric acidA receptor pharmacological properties.
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Postnatal development of hippocampal dentate granule cell gamma-aminobutyric acidA (GABAA) receptor pharmacological properties was studied. Granule cells were acutely isolated from hippocampi of 7- to 14- and 45- to 52-day-old rats, and whole cell patch-clamp recordings were obtained. The sensitivity of GABAA receptors to GABA and modulation of GABAA receptor currents by benzodiazepines (BZ), zinc, furosemide, and loreclezole was studied. Multiple changes in the pharmacological properties of dentate granule-cell GABAA receptors occurred during the first 52 days of postnatal development: GABA-evoked maximal current increased with postnatal age; GABAA receptors changed from BZ type 3 in young rats to BZ type 1 in adult rats; furosemide and zinc inhibited GABAA receptor currents in young rats but not in adult rats; the fraction of cells that expressed loreclezole-sensitive GABAA receptors increased with postnatal age. These findings suggest that dentate granule cells in young and adult animals express pharmacologically distinct GABAA receptors and that the postnatal development of these receptors is prolonged, lasting at least 45 days. Comparison with the previously reported pharmacological properties of GABAA receptors on dentate granule cells acutely isolated from hippocampi of 28- to 35-day-old rats suggests that receptors expressed at that age have properties intermediate between young and adult rats. (+info)
Overexpression of a Shaker-type potassium channel in mammalian central nervous system dysregulates native potassium channel gene expression.
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The nervous system maintains a delicate balance between excitation and inhibition, partly through the complex interplay between voltage-gated sodium and potassium ion channels. Because K+ channel blockade or gene deletion causes hyperexcitability, it is generally assumed that increases in K+ channel gene expression should reduce neuronal network excitability. We have tested this hypothesis by creating a transgenic mouse that expresses a Shaker-type K+ channel gene. Paradoxically, we find that addition of the extra K+ channel gene results in a hyperexcitable rather than a hypoexcitable phenotype. The presence of the transgene leads to a complex deregulation of endogenous Shaker genes in the adult central nervous system as well as an increase in network excitability that includes spontaneous cortical spike and wave discharges and a lower threshold for epileptiform bursting in isolated hippocampal slices. These data suggest that an increase in K+ channel gene dosage leads to dysregulation of normal K+ channel gene expression, and it may underlie a mechanism contributing to the pathogenesis of human aneuploidies such as Down syndrome. (+info)
Modification of postsynaptic densities after transient cerebral ischemia: a quantitative and three-dimensional ultrastructural study.
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Abnormal synaptic transmission has been hypothesized to be a cause of neuronal death resulting from transient ischemia, although the mechanisms are not fully understood. Here, we present evidence that synapses are markedly modified in the hippocampus after transient cerebral ischemia. Using both conventional and high-voltage electron microscopy, we performed two- and three-dimensional analyses of synapses selectively stained with ethanolic phosphotungstic acid in the hippocampus of rats subjected to 15 min of ischemia followed by various periods of reperfusion. Postsynaptic densities (PSDs) from both area CA1 and the dentate gyrus were thicker and fluffier in postischemic hippocampus than in controls. Three-dimensional reconstructions of selectively stained PSDs created using electron tomography indicated that postsynaptic densities became more irregular and loosely configured in postischemic brains compared with those in controls. A quantitative study based on thin sections of the time course of PSD modification indicated that the increase in thickness was both greater and more long-lived in area CA1 than in dentate gyrus. Whereas the magnitude of morphological change in dentate gyrus peaked at 4 hr of reperfusion (140% of control values) and declined thereafter, changes in area CA1 persisted and increased at 24 hr of reperfusion (191% of control values). We hypothesize that the degenerative ultrastructural alteration of PSDs may produce a toxic signal such as a greater calcium influx, which is integrated from the thousands of excitatory synapses onto dendrites, and is propagated to the neuronal somata where it causes or contributes to neuronal damage during the postischemic phase. (+info)
On the mechanism of histaminergic inhibition of glutamate release in the rat dentate gyrus.
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1. Histaminergic depression of excitatory synaptic transmission in the rat dentate gyrus was investigated using extracellular and whole-cell patch-clamp recording techniques in vitro. 2. Application of histamine (10 microM, 5 min) depressed synaptic transmission in the dentate gyrus for 1 h. This depression was blocked by the selective antagonist of histamine H3 receptors, thioperamide (10 microM). 3. The magnitude of the depression caused by histamine was inversely related to the extracellular Ca2+ concentration. Application of the N-type calcium channel blocker omega-conotoxin (0. 5 or 1 microM) or the P/Q-type calcium channel blocker omega-agatoxin (800 nM) did not prevent depression of synaptic transmission by histamine. 4. The potassium channel blocker 4-aminopyridine (4-AP, 100 microM) enhanced synaptic transmission and reduced the depressant effect of histamine (10 microM). 4-AP reduced the effect of histamine more in 2 mM extracellular calcium than in 4 mM extracellular calcium. 5. Histamine (10 microM) did not affect the amplitude of miniature excitatory postsynaptic currents (mEPSCs) and had only a small effect on their frequency. 6. Histaminergic depression was not blocked by an inhibitor of serine/threonine protein kinases, H7 (100 microM), or by an inhibitor of tyrosine kinases, Lavendustin A (10 microM). 7. Application of adenosine (20 microM) or the adenosine A1 agonist N6-cyclopentyladenosine (CPA, 0.3 microM) completely occluded the effect of histamine (10 microM). 8. We conclude that histamine, acting on histamine H3 receptors, inhibits glutamate release by inhibiting presynaptic calcium entry, via a direct G-protein-mediated inhibition of multiple calcium channels. Histamine H3 receptors and adenosine A1 receptors act upon a common final effector to cause presynaptic inhibition. (+info)
Quantal amplitude and quantal variance of strontium-induced asynchronous EPSCs in rat dentate granule neurons.
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1. Excitatory postsynaptic currents (EPSCs) were recorded from granule cells of the dentate gyrus in acute slices of 17- to 21-day-old rats (22-25 C) using tissue cuts and minimal extracellular stimulation to selectively activate a small number of synaptic contacts. 2. Adding millimolar Sr2+ to the external solution produced asynchronous EPSCs (aEPSCs) lasting for several hundred milliseconds after the stimulus. Minimally stimulated aEPSCs resembled miniature EPSCs (mEPSCs) recorded in the same cell but differed from them in ways expected from the greater range of dendritic filtering experienced by mEPSCs. aEPSCs had the same stimulus threshold as the synchronous EPSCs (sEPSCs) that followed the stimulus with a brief latency. aEPSCs following stimulation of distal inputs had a slower mean rise time than those following stimulation of proximal inputs. These results suggest that aEPSCs arose from the same synapses that generated sEPSCs. 3. Proximally elicited aEPSCs had a mean amplitude of 6.7 +/- 2.2 pA (+/- s.d., n = 23 cells) at -70 mV and an amplitude coefficient of variation of 0. 46 +/- 0.08. 4. The amplitude distributions of sEPSCs never exhibited distinct peaks. 5. Monte Carlo modelling of the shapes of aEPSC amplitude distributions indicated that our data were best explained by an intrasite model of quantal variance. 6. It is concluded that Sr2+-evoked aEPSCs are uniquantal events arising at synaptic terminals that were recently invaded by an action potential, and so provide direct information about the quantal amplitude and quantal variance at those terminals. The large quantal variance obscures quantization of the amplitudes of evoked sEPSCs at this class of excitatory synapse. (+info)
Recurrent mossy fiber pathway in rat dentate gyrus: synaptic currents evoked in presence and absence of seizure-induced growth.
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A common feature of temporal lobe epilepsy and of animal models of epilepsy is the growth of hippocampal mossy fibers into the dentate molecular layer, where at least some of them innervate granule cells. Because the mossy fibers are axons of granule cells, the recurrent mossy fiber pathway provides monosynaptic excitatory feedback to these neurons that could facilitate seizure discharge. We used the pilocarpine model of temporal lobe epilepsy to study the synaptic responses evoked by activating this pathway. Whole cell patch-clamp recording demonstrated that antidromic stimulation of the mossy fibers evoked an excitatory postsynaptic current (EPSC) in approximately 74% of granule cells from rats that had survived >10 wk after pilocarpine-induced status epilepticus. Recurrent mossy fiber growth was demonstrated with the Timm stain in all instances. In contrast, antidromic stimulation of the mossy fibers evoked an EPSC in only 5% of granule cells studied 4-6 days after status epilepticus, before recurrent mossy fiber growth became detectable. Notably, antidromic mossy fiber stimulation also evoked an EPSC in many granule cells from control rats. Clusters of mossy fiber-like Timm staining normally were present in the inner third of the dentate molecular layer at the level of the hippocampal formation from which slices were prepared, and several considerations suggested that the recorded EPSCs depended mainly on activation of recurrent mossy fibers rather than associational fibers. In both status epilepticus and control groups, the antidromically evoked EPSC was glutamatergic and involved the activation of both AMPA/kainate and N-methyl-D-aspartate (NMDA) receptors. EPSCs recorded in granule cells from rats with recurrent mossy fiber growth differed in three respects from those recorded in control granule cells: they were much more frequently evoked, a number of them were unusually large, and the NMDA component of the response was generally much more prominent. In contrast to the antidromically evoked EPSC, the EPSC evoked by stimulation of the perforant path appeared to be unaffected by a prior episode of status epilepticus. These results support the hypothesis that recurrent mossy fiber growth and synapse formation increases the excitatory drive to dentate granule cells and thus facilitates repetitive synchronous discharge. Activation of NMDA receptors in the recurrent pathway may contribute to seizure propagation under depolarizing conditions. Mossy fiber-granule cell synapses also are present in normal rats, where they may contribute to repetitive granule cell discharge in regions of the dentate gyrus where their numbers are significant. (+info)