scholarly journals Septal Modulation of Excitatory Transmission in Hippocampus

2003 ◽  
Vol 90 (4) ◽  
pp. 2358-2366 ◽  
Author(s):  
Laura Lee Colgin ◽  
Enikö A. Kramár ◽  
Christine M. Gall ◽  
Gary Lynch
Keyword(s):  
Granule Cells ◽  
Cannabinoid Receptor ◽  
Mossy Fiber ◽  
Acetylcholinesterase Inhibitor ◽  
Molecular Layer ◽  
Hippocampal Slices ◽  
Muscarinic Acetylcholine Receptor ◽  
Single Pulse ◽  
Perforant Path ◽  
Synaptic Responses

Application of the acetylcholinesterase inhibitor physostigmine to conventional hippocampal slices caused a significant reduction of field excitatory postsynaptic potentials (EPSPs) elicited by single pulse stimulation to the medial perforant path. Similar but smaller effects were obtained in the lateral perforant path and other excitatory pathways within hippocampus. The reductions were blocked by atropine, were not accompanied by evident changes in the EPSP waveform, and were eliminated by lesions to the cholinergic septo-hippocampal projections. Antidromic responses to mossy fiber stimulation, recorded in stratum granulosum, were not affected by the drug. However, paired-pulse facilitation was reliably increased, indicating that the depressed synaptic responses were secondary to reductions in transmitter release. The absence of cholinergic axo-axonic connections in the molecular layer suggests that physostigmine reduces presynaptic release by increasing retrograde signaling from the granule cells. In accord with this, an antagonist of the CB1 cannabinoid receptor eliminated the effects of physostigmine on synaptic responses, while an antagonist of the presynaptically located m2 muscarinic acetylcholine receptor did not. This is in contrast to previously reported effects involving application of cholinergic agonists, in which presynaptic inhibition likely results from direct activation of presynaptically located muscarinic receptors. In summary, it is proposed that the cholinergic inputs from the septum to the middle molecular layer modulate, via endocannabinoid release, the potency of the primary excitatory afferent of hippocampus.

Effect of Chronic Stress on Synaptic Currents in Rat Hippocampal Dentate Gyrus Neurons

2003 ◽  
Vol 89 (1) ◽  
pp. 625-633 ◽  
Author(s):  
Henk Karst ◽  
Marian Joëls
Keyword(s):  
Dentate Gyrus ◽  
Chronic Stress ◽  
Granule Cells ◽  
Hippocampal Slices ◽  
Perforant Path ◽  
Maximal Response ◽  
Stress Exposure ◽  
Dentate Granule Cells ◽  
Prior Stress ◽  
Synaptic Responses

We investigated the effect of chronic stress on synaptic responses of rat dentate granule cells to perforant path stimulation. Rats were subjected for 3 wk to unpredictable stressors twice daily or to control handling. One day after the last stressor, hippocampal slices were prepared and synaptic responses were determined with whole-cell recording. At that time, adrenal weight was found to be increased and thymus weight as well as gain in body weight were decreased in the stressed versus control animals, indicative of corticosterone hypersecretion during the stress period. In slices from rats with basal corticosteroid levels (at the circadian trough, under rest), no effect of prior stress exposure was observed on synaptic responses. However, synaptic responses of dentate granule cells from chronically stressed and control rats were differently affected by in vitro activation of glucocorticoid receptors, i.e., 1–4 h after administration of 100 nM corticosterone for 20 min. Thus the maximal response to synaptic activation of dentate cells at holding potential of −70 mV [when N-methyl-d-aspartate (NMDA) receptors are blocked by magnesium] was significantly enhanced after corticosterone administration in chronically stressed but not in control animals. In accordance, the amplitude of α-amino-3-hydroxy-5-methylisolazole-4-propionic acid (AMPA) but not of NMDA receptor-mediated currents was increased by corticosterone in stressed rats, over the entire voltage range. Corticosterone treatment also decreased the time to peak of AMPA currents, but this effect did not depend on prior stress exposure. The data indicate that following chronic stress exposure synaptic excitation of dentate granule cells may be enhanced when corticosterone levels rise. This enhanced synaptic flow could contribute to enhanced excitation of projection areas of the dentate gyrus, most notably the CA3 hippocampal region.

Recurrent Excitatory Connectivity in the Dentate Gyrus of Kindled and Kainic Acid–Treated Rats

2000 ◽  
Vol 83 (2) ◽  
pp. 693-704 ◽  
Author(s):  
Michael Lynch ◽  
Thomas Sutula
Keyword(s):  
Dentate Gyrus ◽  
Kainic Acid ◽  
Granule Cell ◽  
Time Course ◽  
Granule Cells ◽  
Mossy Fiber ◽  
Hippocampal Slices ◽  
Granule Cell Layer ◽  
Perforant Path ◽  
Inhibitory Circuits

Repeated seizures induce mossy fiber axon sprouting, which reorganizes synaptic connectivity in the dentate gyrus. To examine the possibility that sprouted mossy fiber axons may form recurrent excitatory circuits, connectivity between granule cells in the dentate gyrus was examined in transverse hippocampal slices from normal rats and epileptic rats that experienced seizures induced by kindling and kainic acid. The experiments were designed to functionally assess seizure-induced development of recurrent circuitry by exploiting information available about the time course of seizure-induced synaptic reorganization in the kindling model and detailed anatomic characterization of sprouted fibers in the kainic acid model. When recurrent inhibitory circuits were blocked by the GABAAreceptor antagonist bicuculline, focal application of glutamate microdrops at locations in the granule cell layer remote from the recorded granule cell evoked trains of excitatory postsynaptic potentials (EPSPs) and population burst discharges in epileptic rats, which were never observed in slices from normal rats. The EPSPs and burst discharges were blocked by bath application of 1 μM tetrodotoxin and were therefore dependent on network-driven synaptic events. Excitatory connections were detected between blades of the dentate gyrus in hippocampal slices from rats that experienced kainic acid–induced status epilepticus. Trains of EPSPs and burst discharges were also evoked in granule cells from kindled rats obtained after ≥1 wk of kindled seizures, but were not evoked in slices examined 24 h after a single afterdischarge, before the development of sprouting. Excitatory connectivity between blades of the dentate gyrus was also assessed in slices deafferented by transection of the perforant path, and bathed in artificial cerebrospinal fluid (ACSF) containing bicuculline to block GABAA receptor–dependent recurrent inhibitory circuits and 10 mM [Ca2+]o to suppress polysynaptic activity. Low-intensity electrical stimulation of the infrapyramidal blade under these conditions failed to evoke a response in suprapyramidal granule cells from normal rats ( n = 15), but in slices from epileptic rats evoked an EPSP at a short latency (2.59 ± 0.36 ms) in 5 of 18 suprapyramidal granule cells. The results are consistent with formation of monosynaptic excitatory connections between blades of the dentate gyrus. Recurrent excitatory circuits developed in the dentate gyrus of epileptic rats in a time course that corresponded to the development of mossy fiber sprouting and demonstrated patterns of functional connectivity corresponding to anatomic features of the sprouted mossy fiber pathway.

NPY Inhibits Glutamatergic Excitation in the Epileptic Human Dentate Gyrus

1999 ◽  
Vol 82 (1) ◽  
pp. 478-483 ◽  
Author(s):  
Peter R. Patrylo ◽  
Anthony N. van den Pol ◽  
Dennis D. Spencer ◽  
Anne Williamson
Keyword(s):  
Metabotropic Glutamate Receptor ◽  
Granule Cells ◽  
Molecular Layer ◽  
Hippocampal Slices ◽  
Perforant Path ◽  
Cell Excitability ◽  
Metabotropic Glutamate ◽  
Normal Rat ◽  
Group 3 ◽  
Excitatory Responses

Neuropeptide Y (NPY) has been shown to depress hyperexcitable activity that has been acutely induced in the normal rat brain. To test the hypothesis that NPY can also reduce excitability in the chronically epileptic human brain, we recorded intracellularly from dentate granule cells in hippocampal slices from patients with hippocampal seizure onset. NPY had a potent and long-lasting inhibitory action on perforant path-evoked excitatory responses. In comparison, the group 3 metabotropic glutamate receptor agonistl-2-amino-4-phosphonobutyric acid (l-AP4) evoked a mild and transient decrease. NPY-containing axons were found throughout the hippocampus, and in many epileptic patients were reorganized, particularly in the dentate molecular layer. NPY may therefore play a beneficial role in reducing granule cell excitability in chronically epileptic human tissue, and subsequently limit seizure severity.

scholarly journals Somatostatin Depresses Long-Term Potentiation and Ca2+ Signaling in Mouse Dentate Gyrus

2002 ◽  
Vol 88 (6) ◽  
pp. 3078-3086 ◽  
Author(s):  
Michael V. Baratta ◽  
Tyra Lamp ◽  
Melanie K. Tallent
Keyword(s):  
Dentate Gyrus ◽  
High Frequency ◽  
Granule Cells ◽  
Molecular Layer ◽  
Long Term Potentiation ◽  
Perforant Path ◽  
Functional Consequence ◽  
Term Potentiation ◽  
Synaptic Responses

The selective loss of somatostatin (SST)-containing interneurons from the hilus of the dentate gyrus is a hallmark of epileptic hippocampus. The functional consequence of this loss, including its contribution to postseizure hyperexcitability, remains unclear. We address this issue by characterizing the actions of SST in mouse dentate gyrus using electrophysiological techniques. Although the majority of dentate SST receptors are located in the outer molecular layer adjacent to lateral perforant path (LPP) synapses, we found no consistent action of SST on standard synaptic responses generated at these synapses. However, when SST was present during application of high-frequency trains that normally generate long-term potentiation (LTP), the induction of LTP was impaired. SST did not alter the maintenance of LTP when applied after its induction. To examine the mechanism by which SST inhibits LTP, we recorded from dentate granule cells and examined the actions of this neuropeptide on synaptic transmission and postsynaptic currents. Unlike findings in the CA1 hippocampus, we observed no postsynaptic actions on K+ currents. Instead, SST inhibited Ca2+/Ba2+ spikes evoked by depolarization. This inhibition was dependent on N-type Ca2+currents. Blocking these currents also blocked LTP, suggesting a mechanism through which SST may inhibit LTP. Our results indicate that SST reduction of dendritic Ca2+ through N-type Ca2+ channels may contribute to modulation of synaptic plasticity at LPP synapses. Therefore the loss of SST function postseizure could result in abnormal synaptic potentiation that contributes to epileptogenesis.

Effects of aging on axon terminals in the dentate molecular layer

1987 ◽  
Vol 45 ◽  
pp. 928-929
Author(s):  
K. Cullen-Dockstader ◽  
E. Fifkova
Keyword(s):  
Granule Cells ◽  
Molecular Layer ◽  
Age Groups ◽  
Long Term Potentiation ◽  
Male Rats ◽  
Perforant Path ◽  
Axon Terminals ◽  
Fischer 344 ◽  
Spatial Memory Deficit ◽  
Effects Of Aging

Normal aging results in a pronounced spatial memory deficit associated with a rapid decay of long-term potentiation at the synapses between the perforant path and spines in the medial and distal thirds of the dentate molecular layer (DML), suggesting the alteration of synaptic transmission in the dentate fascia. While the number of dentate granule cells remains unchanged, and there are no obvious pathological changes in these cells associated with increasing age, the density of their axospinous contacts has been shown to decrease. There are indications that the presynaptic element is affected by senescence before the postsynaptic element, yet little attention has been given to the fine structure of the remaining axon terminals. Therefore, we studied the axon terminals of the perforant path in the DML across three age groups.5 Male rats (Fischer 344) of each age group (3, 24 and 30 months), were perfused through the aorta.

Optical Recording Study of Granule Cell Activities in the Hippocampal Dentate Gyrus of Kainate-Treated Rats

2000 ◽  
Vol 83 (4) ◽  
pp. 2421-2430 ◽  
Author(s):  
Yo Otsu ◽  
Eiichi Maru ◽  
Hisayuki Ohata ◽  
Ichiro Takashima ◽  
Riichi Kajiwara ◽  
...  
Keyword(s):  
Dentate Gyrus ◽  
Granule Cell ◽  
Granule Cells ◽  
Mossy Fiber ◽  
Molecular Layer ◽  
Mossy Fibers ◽  
Optical Recording ◽  
Granule Cell Layer ◽  
Gabaergic Inhibition ◽  
Mossy Fiber Sprouting

In the epileptic hippocampus, newly sprouted mossy fibers are considered to form recurrent excitatory connections to granule cells in the dentate gyrus and thereby increase seizure susceptibility. To study the effects of mossy fiber sprouting on neural activity in individual lamellae of the dentate gyrus, we used high-speed optical recording to record signals from voltage-sensitive dye in hippocampal slices prepared from kainate-treated epileptic rats (KA rats). In 14 of 24 slices from KA rats, hilar stimulation evoked a large depolarization in almost the entire molecular layer in which granule cell apical dendrites are located. The signals were identified as postsynaptic responses because of their dependence on extracellular Ca2+. The depolarization amplitude was largest in the inner molecular layer (the target area of sprouted mossy fibers) and declined with increasing distance from the granule cell layer. In the inner molecular layer, a good correlation was obtained between depolarization size and the density of mossy fiber terminals detected by Timm staining methods. Blockade of GABAergic inhibition by bicuculline enlarged the depolarization in granule cell dendrites. Our data indicate that mossy fiber sprouting results in a large and prolonged synaptic depolarization in an extensive dendritic area and that the enhanced GABAergic inhibition partly masks the synaptic depolarization. However, despite the large dendritic excitation induced by the sprouted mossy fibers, seizurelike activity of granule cells was never observed, even when GABAergic inhibition was blocked. Therefore, mossy fiber sprouting may not play a critical role in epileptogenesis.

scholarly journals Dendritic spikes in hippocampal granule cells are necessary for long-term potentiation at the perforant path synapse

2018 ◽  
Author(s):  
Sooyun Kim ◽  
Yoonsub Kim ◽  
Suk-Ho Lee ◽  
Won-Kyung Ho
Keyword(s):  
Granule Cells ◽  
Memory Function ◽  
Brain Slices ◽  
Long Term Potentiation ◽  
Perforant Path ◽  
Term Potentiation ◽  
Dendritic Spikes ◽  
Voltage Attenuation ◽  
Synaptic Responses

AbstractLong-term potentiation (LTP) of synaptic responses is essential for hippocampal memory function. Perforant-path (PP) synapses on hippocampal granule cells (GCs) contribute to the formation of associative memories, which are considered the cellular correlates of memory engrams. However, the mechanisms of LTP at these synapses are not well understood. Due to sparse firing activity and the voltage attenuation in their dendrites, it remains unclear how associative LTP at distal synapses occurs. Here we show that NMDA receptor-dependent LTP can be induced at PP-GC synapses without backpropagating action potentials (bAPs) in acute rat brain slices. Dendritic recordings reveal substantial attenuation of bAPs as well as local dendritic Na + ‐spike generation during PP-GC input. Inhibition of Na+ ‐spikes impairs LTP suggesting that LTP at PP-GC synapse requires local Na + ‐spikes. Thus, dendritic spikes are essential for LTP induction at PP-GC synapse and may constitute a key cellular mechanism for memory formation in the dentate gyrus.

Recurrent Mossy Fiber Pathway in Rat Dentate Gyrus: Synaptic Currents Evoked in Presence and Absence of Seizure-Induced Growth

1999 ◽  
Vol 81 (4) ◽  
pp. 1645-1660 ◽  
Author(s):  
Maxine M. Okazaki ◽  
Péter Molnár ◽  
J. Victor Nadler
Keyword(s):  
Status Epilepticus ◽  
Dentate Gyrus ◽  
Granule Cells ◽  
Mossy Fiber ◽  
Molecular Layer ◽  
Mossy Fibers ◽  
Fiber Growth ◽  
Antidromic Stimulation ◽  
Mossy Fiber Pathway ◽  
Stimulation Of

Recurrent mossy fiber pathway in rat dentate gyrus: synaptic currents evoked in presence and absence of seizure-induced growth. 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 ∼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.

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