Low frequency stimulation for seizure suppression: Identification of optimal targets in the entorhinal-hippocampal circuit

BACKGROUND

Mesial temporal lobe epilepsy (MTLE) with hippocampal sclerosis (HS) is a common form of drug-resistant focal epilepsy in adults. Treatment for pharmacoresistant patients remains a challenge, with deep brain stimulation (DBS) showing promise for alleviating intractable seizures. This study explores the efficacy of low frequency stimulation (LFS) on specific neuronal targets within the entorhinal-hippocampal circuit in a mouse model of MTLE.

OBJECTIVE

Our previous research demonstrated that LFS of the medial perforant path (MPP) fibers in the sclerotic hippocampus reduced seizures in epileptic mice. Here, we aimed to identify the critical neuronal population responsible for this antiepileptic effect by optogenetically stimulating presynaptic and postsynaptic compartments of the MPP-dentate granule cell (DGC) synapse at 1 Hz. We hypothesize that specific targets for LFS can differentially influence seizure activity depending on the cellular identity and location within or outside the seizure focus.

METHODS

We utilized the intrahippocampal kainate (ihKA) mouse model of MTLE and targeted specific neural populations using optogenetic stimulation. We recorded intracranial neuronal activity from freely moving chronically epileptic mice with and without optogenetic LFS up to 3 h.

RESULTS

We found that LFS of MPP fibers in the sclerotic hippocampus effectively suppressed epileptiform activity while stimulating principal cells in the MEC had no impact. Targeting DGCs in the sclerotic septal or non-sclerotic temporal hippocampus with LFS did not reduce seizure numbers but shortened the epileptiform bursts.

CONCLUSION

Presynaptic stimulation of the MPP-DGC synapse within the sclerotic hippocampus is critical for seizure suppression via LFS.

Rapid Aging in the Perforant Path Projections to the Rodent Dentate Gyrus

Why layers II/III of entorhinal cortex (EC) deteriorate in advance of other regions during the earliest stages of Alzheimer's disease is poorly understood. Failure of retrograde trophic support from synapses to cell bodies is a common cause of neuronal atrophy, and we accordingly tested for early-life deterioration in projections of rodent layer II EC neurons. Using electrophysiology and quantitative imaging, changes in EC terminals during young adulthood were evaluated in male rats and mice. Field excitatory postsynaptic potentials, input/output curves, and frequency following capacity by lateral perforant path (LPP) projections from lateral EC to dentate gyrus were unchanged from 3 to 8-10 months of age. In contrast, the unusual presynaptic form of long-term potentiation (LTP) expressed by the LPP was profoundly impaired by 8 months in rats and mice. This impairment was accompanied by a reduction in the spine to terminal endocannabinoid signaling needed for LPP-LTP induction and was offset by an agent that enhances signaling. There was a pronounced age-related increase in synaptophysin within LPP terminals, an effect suggestive of incipient pathology. Relatedly, presynaptic levels of TrkB-receptors mediating retrograde trophic signaling-were reduced in the LPP terminal field. LTP and TrkB content were also reduced in the medial perforant path of 8- to 10-month-old rats. As predicted, performance on an LPP-dependent episodic memory task declined by late adulthood. We propose that memory-related synaptic plasticity in EC projections is unusually sensitive to aging, which predisposes EC neurons to pathogenesis later in life.SIGNIFICANCE STATEMENT Neurons within human superficial entorhinal cortex are particularly vulnerable to effects of aging and Alzheimer's disease, although why this is the case is not understood. Here we report that perforant path projections from layer II entorhinal cortex to the dentate gyrus exhibit rapid aging in rodents, including reduced synaptic plasticity and abnormal protein content by 8-10 months of age. Moreover, there was a substantial decline in the performance of an episodic memory task that depends on entorhinal cortical projections at the same ages. Overall, the results suggest that the loss of plasticity and related trophic signaling predispose the entorhinal neurons to functional decline in relatively young adulthood.

Predictive role of subcomponents of the left arcuate fasciculus in prognosis of aphasia after stroke: A retrospective observational study

The relationship between the left arcuate fasciculus (AF) and stroke-related aphasia is unclear. In this retrospective study, we aimed to investigate the role of subcomponents of the left AF in predicting prognosis of aphasia after stroke. Twenty stroke patients with aphasia were recruited and received language assessment as well as diffusion tensor tractography scanning at admission. According to injury of the left AF, the participants were classified into four groups: group A (4 cases), the AF preserved intactly; group B (6 cases), the anterior segment injured; group C (4 cases), the posterior segment injured; and group D (6 cases), completely injured. After a consecutive speech therapy, language assessment was performed again. Changes of language functions among the groups were compared and the relation between these changes with segments injury of the AF was analyzed. After therapy, relatively high increase score percentage changes in terms of all the subcategories of language assessment were observed both in group A and C; by contrast, only naming in group B, and spontaneous speech in group D. Although no statistical difference was demonstrated among the four groups. In addition, there was no significant correlation between improvement of language function with segments injury of the AF. The predictive role of subcomponents of the left AF in prognosis of aphasia is obscure in our study. Nevertheless, it indicates the importance of integrity of the left AF for recovery of aphasia, namely that preservation of the left AF on diffusion tensor tractography could mean recovery potential of aphasia after stroke.

Removing entorhinal cortex input to the dentate gyrus does not impede low frequency oscillations, an EEG-biomarker of hippocampal epileptogenesis

Following prolonged perforant pathway stimulation (PPS) in rats, a seizure-free "latent period" is observed that lasts around 3 weeks. During this time, aberrant neuronal activity occurs, which has been hypothesized to contribute to the generation of an "epileptic" network. This study was designed to 1) examine the pathological network activity that occurs in the dentate gyrus during the latent period, and 2) determine whether suppressing this activity by removing the main input to the dentate gyrus could stop or prolong epileptogenesis. Immediately following PPS, continuous video-EEG monitoring was used to record spontaneous neuronal activity and detect seizures. During the latent period, low frequency oscillations (LFOs), occurring at a rate of approximately 1 Hz, were detected in the dentate gyrus of all rats that developed epilepsy. LFO incidence was apparently random, but often decreased in the hour preceding a spontaneous seizure. Bilateral transection of the perforant pathway did not impact the incidence of hippocampal LFOs, the latency to epilepsy, or hippocampal neuropathology. Our main findings are: 1) LFOs are a reliable biomarker of hippocampal epileptogenesis, and 2) removing entorhinal cortex input to the hippocampus neither reduces the occurrence of LFOs nor has a demonstrable antiepileptogenic effect.

[Coherence and phase analysis of theta-oscillations in the septohippocampal system during seizures]

Interrelations of the hippocampus and medial septal area (MSA) in the theta band (4-8 Hz) were studied during seizures produced by electrical kindling in waking guinea pigs. Field activity (EEG) was analyzed using the wavelet transform. A decrease in coherence of theta-oscillations in the hippocampus and MSA was observed during seizures. Phase analysis showed that in the beginning of kindling the MSA led in phase, but after formation of the pathological focus, MSA lagged the hippocampus. The data may contribute to understanding mechanisms of temporal lobe epilepsy.

A novel phospholipid-based drug formulation, VP025, modulates age- and LPS-induced microglial activity in the rat

BACKGROUND

A common change that occurs with age in the central nervous system is an increase in microglial-associated inflammation. This is usually coupled with an increase in the concentration of the inflammatory cytokine interleukin-1beta (IL-1beta) in the hippocampus and an inhibition in long-term potentiation.

OBJECTIVES

To assess the effects of a novel preparation of phospholipid nanoparticles incorporating phosphatidylglycerol, VP025, on inflammatory changes in hippocampus of aged and lipopolysaccharide (LPS)-treated rats.

METHODS/RESULTS

We report that a possible initial target cell of the putative anti-inflammatory actions of VP025 may be macrophages, as VP025 is engulfed by, and has the capacity to alter the activity of, these cells. VP025 reversed the increase in IFN-gamma concentration in supernatant taken from peritoneal macrophages harvested from LPS-treated rats. In addition, markers of microglial activity, major histocompatibility complex class II (MHC II) mRNA expression, CD40 expression and IL-1beta concentration were increased, and CD200 expression was reduced, in the hippocampus of these rats. VP025 reversed changes in CD40, IL-1beta and CD200 in aged rats, and also restored long-term potentiation in aged and LPS-treated rats.

CONCLUSIONS

We conclude that VP025 has the ability to modulate the activity of macrophage, microglia and neurons in response to stressors such as ageing and LPS treatment.

Disturbance of the correlation between hippocampal and septal EEGs during epileptogenesis

Field potentials of the hippocampus and the medial septal-diagonal band complex (MSDB) were recorded in the control and during the kindling stimulation of the perforant path in waking guinea pigs. Changes in the correlation of activities of these structures during stimulation-evoked seizures (model of acute epilepsy) and during epileptogenesis elicited by the kindling (model of chronic epilepsy) were analysed. In the control, a high correlation between the background activities of the hippocampus and MSDB was observed. In the first days of stimulation at the parameters that evoked seizure discharges in the hippocampus, the MSDB did not show the epileptiform activity; however, repeated daily stimulation gave rise to epileptiform discharges, which increased with time. As a result of kindling, the MSDB became capable of generating seizure activity irrespective of the hippocampus. The degree of correlation between the activities of the two structures sharply decreased during "acute" and "chronic" seizures. In the process of kindling, a progressive disintegration of activities of the hippocampus and MSDB was revealed, indicating the disturbance of the functioning of septohippocampal network during epileptogenesis. The data obtained add to the knowledge about the mechanisms of temporal lobe epilepsy and may help to develop new approaches to the therapy of this disease.

[Relationships between the hippocampus and medial septal region and their alterations during epileptogenesis]

EEGs of the hippocampus and the medial septal region (MSR) in the control conditions and during repeated stimulation of the perforant path were simultaneously recorded in awake guinea pigs. Changes in correlation of activity of these structures during seizures provoked by the stimulation (model of acute epilepsy) and in the process of epileptogenesis induced by the kindling (model of chronic epilepsy) were analyzed. A high correlation of the baseline activities of the hippocampus and MSR observed in the control sharply decreased during acute and chronic seizures. Kindling led to emergence of the MSR capability of hippocampus-independent generation of the field seizure discharges. In the process of kindling the progressive disintegration of activities of the hippocampus and the MSR was revealed being indicative of disorders in functioning of the septohippocampal network during epileptogenesis.

Effects of propofol and halothane on long-term potentiation in the rat hippocampus after transient cerebral ischaemia

BACKGROUND

Propofol is reported to have protective effects on cerebral ischaemia-induced neuronal death. The aim of this study was to explore whether propofol and halothane can protect hippocampal neuronal function from ischaemic injury during general anaesthesia in rats.

METHODS

Rats were divided into 2-vessel occlusion (incomplete cerebral ischaemia) and 4-vessel occlusion (complete cerebral ischaemia) groups consisting of three subgroups each (sham-operated, propofol and halothane groups). One hour after starting propofol 1 mg kg(-1) min(-1) with 30% O2 and N2 or halothane 0.8% in 30% O2 and N2 rats with or without bilateral vertebral artery occlusion had bilateral common carotid arteries occluded by vessel clips for 10 min. Anaesthesia was maintained for another 1 h. Seven days after ischaemia-reperfusion, hippocampal long-term potentiation in the perforant path-dentate gyrus synapse was determined as an index of cerebral outcome.

RESULTS

In the propofol groups, the formation of long-term potentiation was significantly impaired in the 2-vessel and 4-vessel occlusion groups compared to the respective sham-operated groups (P < 0.01 and P < 0.05, respectively). Impaired formation of long-term potentiation in propofol groups was comparable to that in halothane groups. The formation of long-team potentiation in the propofol and halothane 2-vessel group was not significantly different from that in the awake 2-vessel group.

CONCLUSIONS

Propofol and halothane administered during ischaemia do not possess protective effects against hippocampal neuronal dysfunction induced by cerebral ischaemia-reperfusion as evaluated by our transient ischaemic rat models.

Effect of different patterns of low-frequency stimulation on piriform cortex kindled seizures

Low-frequency stimulation (LFS) is an antiepileptic and antiepileptogenic electrical stimulation. In this study the effect of changes in some LFS (1Hz, monophasic square wave) parameters (intensity, pulse duration and train duration) on piriform cortex kindled seizures was investigated both in fully kindled rats and during kindling acquisition. In fully kindled animals, application of different patterns of LFS immediately before kindling stimulation had no significant effect on seizure parameters. However, daily (15 min) application of LFS (0.1 ms pulse duration at intensity equal to after-discharge threshold (ADT) and 1 ms pulse duration at intensity equal to 1/4 ADT) during inter-seizure interval of 7 days significantly reduced the stage 5 duration of the next kindled seizure. Application of the same two LFS protocols for 3 days and 2 weeks had no effect on seizure parameters. The effect of LFS was also tested using different paradigms during kindling acquisition. When LFS (0.1 and 1 ms pulse duration, intensity equal to ADT and 1/4 ADT) was delivered daily after each kindling stimulation, it could significantly decrease after-discharge duration in various days during kindling development. In this experiment, only LFS with 0.1 ms pulse duration and intensity equal to ADT significantly delayed the appearance of seizure stages 1 and 2. According to obtained results, it may be concluded that in fully kindled rats application of different patterns of LFS before kindling stimulation has no anticonvulsant effect, but it can exert an inhibitory effect when applied during an inter-seizure interval of 7 days. In addition, LFS has antiepileptogenic effect during kindling acquisition. These effects depend on the applied LFS parameters (e.g. intensity, pulse duration and train duration).

Neuronal structures involved in the induction and propagation of seizures caused by nerve agents: Implications for medical treatment

In epilepsy research, studies have been made to identify brain areas critical for triggering and/or controlling propagated seizure activity. The purpose of the present study was to focus on a similar approach in nerve agent research by reviewing relevant literature to map potential trigger sites and propagation pathways for seizures. The piriform cortex and medial septal area emerge as prime target areas for soman-induced seizures. The cholinergic hyperactivation in the latter structures seems to induce increased glutamatergic activity in the piriform, entorhinal, and perirhinal cortices along with the hippocampal region. For prophylactic or early treatment, mapping of muscarinic subreceptors in the piriform cortex and medial septum would be guiding for designing anticholinergic drugs with optimal properties. Sustained seizures governed by glutamatergic over-activity may primarily be terminated by drugs with optimal glutamatergic antagonism primarily in the piriform, entorhinal, and perirhinal cortices. Studies of radiolabeled ligands to map subreceptors may provide specification of wanted drug properties to guide the choice among existing agents or to synthesize novel ones.

Memory deficits in amyotrophic lateral sclerosis patients with dementia and degeneration of the perforant pathway A clinicopathological study

Degeneration of the perforant pathway, not extensively surveyed so far in amyotrophic lateral sclerosis (ALS) with dementia, was found in eight out of twelve autopsied patients with clinically detectable dementia. Because the severity of degeneration of the entorhinal cortex and that of spongiosis of the outer half of the molecular layer of the dentate gyrus were correlated in these eight patients, it is suspected that the degeneration of the perforant pathway may explain these concomitant lesions. This was further corroborated by occasional involvement of the parahippocampal white matter and subiculum, other components of this pathway. Moreover, six of them manifested clinically detectable memory deficits and three of them exhibited amnesia or episodic memory impairments similar to Alzheimer's disease (AD). Abnormal intensity restricted the dentate gyrus on brain magnetic resonance imaging in a severe case looks like the degeneration of the molecular layer. This involvement of the perforant pathway in ALS patients and its correlation to memory deficits should be taken in account for evaluation of dementia.

Dentate filter function is altered in a proepileptic fashion during aging

PURPOSE

The elderly have an increased incidence and prevalence for seizure disorders. Further, since up to 50% of these cases have no identifiable antecedent, it has been hypothesized that aging of the central nervous system itself may be epileptogenic. Aged rats, compared to adults, exhibit a greater susceptibility to and severity of seizures associated with hippocampal activation. Whether this aging-related change reflects proconvulsive changes in limbic circuitry is unknown and thus was the focus of this study.

METHODS

Hippocampal slices from adult and aged Fischer 344 rats were examined using electrophysiological techniques. The dentate gyrus was our model region since it is involved with both wet-dog shakes and limbic seizures, and it is affected preferentially with age.

RESULTS

No differences were noted between groups in field potential activity elicited with low frequency stimulation. In contrast, 5-Hz molecular layer stimulation could evoke multiple population spikes in approximately 40% of aged versus 0% of adult slices. Further, recording in CA3 revealed that this stimulation paradigm could elicit multiple spikes in aged, but not adult, slices that frequently evolved into spontaneous epileptiform bursts. This change in the capacity of the dentate to respond to and filter afferent input was associated with an aging-related decrease in the frequency of spontaneous IPSPs and an increased propensity for large amplitude prolonged EPSPs following disinhibition.

CONCLUSIONS

These epileptogenic changes in dentate function and circuitry could contribute to the exacerbated susceptibility for hippocampal seizures in aged rodents, as well as the aging-related decline in spatial learning and memory.

Modeling of context-dependent retrieval in hippocampal region CA1: implications for cognitive function in schizophrenia

The symptoms of schizophrenia may be associated with reductions in NMDA receptor (NMDAR) function. This is suggested by the psychotomimetic effects of NMDA antagonists, the ameliorative effects of NMDAR indirect agonists, elevated levels of the NMDA antagonist N-acetyl-aspartyl-glutamate (NAAG) in schizophrenic brain, and findings from recent genetic studies. However, the link between reduced NMDAR function and the behavioral features of schizophrenics has not been made explicit. Here we present a network simulation of hippocampal function, focused on retrieval of verbal stimuli in human memory tasks. Specifically, we trained a computational model of the hippocampal complex to perform a context-dependent paired associate task, a free recall task with category clustering, and the transitive inference (TI) task. In this network, direct perforant pathway input from entorhinal cortex to region CA1 provides the basis for semantic context cueing during initial encoding and retrieval, allowing selective retrieval on the basis of category cues. Alterations in the magnitude of this direct perforant pathway input to region CA1 causes impairments in use of organizational strategies for memory, accounting for specific features of memory dysfunction in schizophrenics and in normals treated with ketamine. This model provides a theoretical link between cellular physiological changes and specific cognitive symptoms. As such, it can shed light on the etiology of schizophrenia in a fundamental way, and also holds the promise of pointing the way to more effective treatments.

Tumor necrosis factor and its p55 and p75 receptors are not required for axonal lesion-induced microgliosis in mouse fascia dentata

Tumor necrosis factor (TNF) is a potent pro-inflammatory and neuromodulatory cytokine. In the CNS it is produced primarily by microglia and considered to regulate microglial activation. On the basis of previous observations of increased microglial TNF mRNA synthesis in areas of anterograde axonal and terminal degeneration in mice, we studied the effect of TNF and its p55 and p75 receptors on axonal lesion-induced microglial activation in fascia dentata following transection of the perforant path (PP) projection. Unexpectedly, cell counting showed that the axonal lesion-induced microglial response in TNF and TNF-p55p75 receptor knock out mice and C57BL/6 mice was similar 5 days after the lesion. In addition, the microglial expression of the lysosomal-associated antigen CD68, and the clearance of MBP(+) myelin debris appeared similar in TNF and TNF-p55p75 receptor knock out mice compared to C57BL/6 mice. Quantitative PCR and in situ hybridization showed the expression of TNF mRNA to be maximally upregulated 6 h after the lesion, and confirmed that TNF mRNA was still upregulated 5 days after lesion when microglial numbers, CD11b mRNA level, and cellular TNF-p55 and -p75 receptor mRNA level reached maximum. However, in spite of the induction of TNF mRNA, TNF protein level remained at base-line in fascia dentata using immunohistochemistry and ELISA. In conclusion, the results showed a lower than expected lesion-induced increase in TNF protein, and that neither TNF nor its receptors were required for the axonal lesion-induced microglial morphological transformation and proliferation or for the initial clearance of degenerated myelin in the PP-deafferented fascia dentata.

Decrease of hippocampal GABA B receptor-mediated inhibition after hyperthermia-induced seizures in immature rats

PURPOSE

Whether febrile seizures have detrimental consequences on the brain is still controversial. We hypothesized that neuronal inhibition in the hippocampus is altered after hyperthermia-induced seizures in immature rats.

METHODS

Rats were given a single seizure by a heat lamp on postnatal day (PND) 15, or repeated seizures by heated air on PND 13 to 15. Fourteen or 30 days after the seizure(s), laminar field potentials were recorded by 16-channel silicon probes in CA1 and the dentate gyrus (DG), in response to the paired-pulse stimulation of the CA3 and medial perforant path, and analyzed as current source density. Gamma-aminobutyric acid (GABA)(B)-receptor antagonist CGP35348 was injected intracerebroventricularly (icv).

RESULTS

At 14 but not at 30 days after a single or after repeated hyperthermia-induced seizures, paired-pulse facilitation (PPF) of the CA1 population spikes at 100 to 200 ms interpulse intervals (IPIs) was significantly increased in seizure as compared with control rats, irrespective of the types of induced seizures. CGP35348 icv also resulted in PPF at 100 to 200 ms IPIs in CA1 of control rats, but CGP35348 had no effect on PPF in seizure rats. At 30 days after repeated seizures, paired-pulse inhibition in the DG was significantly increased at 30-ms IPI, and PPF was increased at 200-ms IPI. CGP35348 increased paired-pulse inhibition in the DG in repeated-seizure rats but not in control rats.

CONCLUSIONS

We conclude that hyperthermia-induced seizures in immature rats induced a decrease of GABA(B) receptor-mediated inhibition in CA1 and DG that lasted > or =14 to 30 days after hyperthermic seizure(s).

Expression of a familial Alzheimer's disease-linked presenilin-1 variant enhances perforant pathway lesion-induced neuronal loss in the entorhinal cortex

Alzheimer's disease (AD) is characterized by neuronal loss in the hippocampus and entorhinal cortex that is manifested by progressive memory impairment and cognitive decline. Autosomal-dominant, familial forms of AD (FAD) are caused by mutations in genes encoding amyloid precursor protein, presenilin-1 (PS1), and presenilin 2. Although it is established that expression of mutant PS1 variants leads to increased production of highly fibrillogenic amyloidbeta42 (Abeta42) peptides that deposit in the brains of patients with AD, the mechanism(s) by which Abeta deposition and expression of mutant genes induce lamina- and region-specific vulnerability of neuronal populations is not known. We have examined the hypothesis that expression of transgene-encoded FAD-linked mutant PS1 variants in entorhinal cortex neurons exacerbates the vulnerability of these cells to lesion-induced neuronal loss. To test this notion, we transected the perforant pathway (PP) of transgenic mice harboring either wild-type human PS1 (PS1HWT) or the FAD-linked mutant PS1DeltaE9 variant and examined neuronal survival in layer II of the entorhinal cortex (ECL2). Remarkably, PP transections lead to marked reductions in the numbers of ECL2 neurons in the ECL2 of mice expressing mutant PS1, compared with ECL2 neurons in PP-lesioned PS1HWT mice. Finally, and in contrast to studies in nontransgenic mice and in mice expressing PS1HWT, ECL2 neurons that express mutant PS1 and the calcium binding protein calbindin-D28k in ECL2 are also susceptible to lesion-induced neuronal loss. We conclude that expression of FAD-linked mutant PS1 variants enhances the vulnerability of neurons in the entorhinal cortex to PP lesion-induced cytotoxicity.

Epileptogenesis Is Associated With Enhanced Glutamatergic Transmission in the Perforant Path

The perforant path provides the main excitatory input into the hippocampus and has been proposed to play a critical role in the generation of temporal lobe seizures. It has been hypothesized that changes in glutamatergic transmission in this pathway promote the epileptogenic process and seizure generation. We therefore asked whether epileptogenesis is associated with enhanced glutamatergic transmission from the perforant path to dentate granule cells. We used a rat model of temporal lobe epilepsy in which spontaneous seizures occur after an episode of pilocarpine-induced status epilepticus. Whole cell patch-clamp recordings were obtained from dentate granule cells in hippocampal slices from control and epileptic animals 3 wk after pilocarpine-induced status epilepticus. The paired pulse ratio of perforant path-evoked AMPA receptor-mediated excitatory postsynaptic currents (EPSCs) was reduced in tissue obtained from epileptic rats. This is consistent with an increase in release probability. N-methyl-d-aspartate (NMDA) receptor-mediated EPSCs were also prolonged. This prolongation could not be accounted for by decreased activity of glutamate transporters or by a change in NMDA receptor subunit composition in dentate granule cells, implying a change in NMDA receptor kinetics. This change in NMDA receptor kinetics was associated with the emergence of significant synaptic cross-talk, detected as a use-dependent block of receptors activated by medial perforant path synapses after lateral perforant path stimulation in MK-801. Enhanced glutamatergic transmission and the emergence of cross-talk among perforant path-dentate granule cell synapses may contribute to lowering seizure threshold.

Examining the gateway to the limbic system with diffusion tensor imaging: the perforant pathway in dementia

Current treatments for Alzheimer's disease (AD) are only able to slow the progression of mental deterioration, making early and reliable diagnosis an essential part of any promising therapeutic strategy. In the initial stages of AD, the first neuropathological alterations occur in the perforant pathway (PP), a large neuronal fiber tract located at the entrance to the limbic system. However, to date, there is no sensitive diagnostic tool for performing in vivo assessments of this structure. In the present bimodal magnetic resonance imaging (MRI) study, we examined 10 elderly controls, 10 subjects suffering from mild cognitive impairment (MCI), and 10 AD patients in order to evaluate the sensitivity of diffusion tensor imaging (DTI), a new MRI technique, for detecting changes in the PP. Furthermore, the diagnostic explanatory power of DTI data of the PP should be compared to high-resolution MRI volumetry and intervoxel coherences (COH) of the hippocampus and the entorhinal cortex, two limbic regions also involved in the pathophysiology of early AD. DTI revealed a marked decrease in COH values in the PP region of MCI (right side: 26%, left side: 29%, as compared to controls) and AD patients (right side: 37%, left side: 43%, as compared to controls). Reductions in COH values of the PP region were significantly correlated with cognitive impairment. DTI data of the PP zone were the only parameter differing significantly between control subjects and MCI patients, while the volumetric measures and the COH values of the hippocampus and the entorhinal cortex did not. DTI of medial temporal brain regions is a promising non-invasive tool for the in vivo diagnosis of the early/preclinical stages of AD.

Mouse models of impaired fear memory exhibit deficits in amygdalar LTP

Inbred mouse strains have different genetic backgrounds that can result in impairments of synaptic plasticity and memory. They are valuable models for probing the mechanisms of memory impairments. We examined fear memory in several inbred strains, along with synaptic plasticity that may underlie fear memory. Long-term potentiation (LTP) is a form of activity-dependent synaptic plasticity that is a candidate cellular mechanism for some forms of learning and memory. Strains with impaired contextual or cued fear memory may have selective LTP deficits in different hippocampal subregions, or in the amygdala. We measured fear memory and its extinction in five inbred strains: C57BL/6NCrlBR (B6), A/J, BALB/cByJ (BALB), C57BL/10J (B10), and SM/J (SM). We also measured LTP in the basolateral amygdala and in the hippocampal Schaeffer collateral-commissural (SC) and medial perforant pathways (MPP). All strains exhibited intact contextual fear memory 24 h post-training, but cued fear memory was impaired in strains A/J, BALB, and SM. At 1 h post-training, both contextual and cued fear memory deficits were more widespread: all strains except for B6 and B10 showed impairments of both types of memory. Contextual fear extinction was impaired in BALB and SM. We found that amygdalar LTP was reduced in strains A/J and BALB, but SC LTP was intact in all strains (except for a selective multi-train LTP impairment in BALB). MPPLTP was similar in all five strains. Thus, reduced amygdalar LTP is correlated with impaired cued fear memory in strains A/J and BALB. Also, hippocampal SC LTP is more strongly correlated with 24-h (long-term) than with 1-h (short-term) contextual fear memory. In this first conjoint study of amygdala-dependent memory and amygdalar LTP in inbred mice, we identified specific hippocampal and amygdalar LTP deficits that correlate with fear memory impairments. These deficits should be considered when selecting inbred strains for genetic modification.

Temporal effects of edaravone, a free radical scavenger, on transient ischemia-induced neuronal dysfunction in the rat hippocampus

We examined the effect of a free radical scavenger edaravone on ischemia/reperfusion-induced impairment of long-term potentiation in the perforant path-dentate gyrus synapses of the rat hippocampus, as a measure of functional outcome 4 days after transient global ischemia (2-vessel occlusion, 10 min). Edaravone (3 and 10 mg/kg, i.v.) immediately after reperfusion (Day 0) alleviated ischemia-induced impairment of long-term potentiation in a dose-related manner, whereas treatment on Day 1 or 4 after reperfusion failed to rescue the impaired long-term potentiation. Edaravone administration on Day 0 also prevented the post-ischemic increase in hydroxyl radical formation and the expression of vascular endothelial growth factor, basic fibroblast growth factor and neuronal and inducible nitric oxide synthases of the hippocampus. Thus, edaravone protected the rat hippocampus from ischemia-induced long-term potentiation impairment with a therapeutic time window, suggesting that free radical formation after ischemia/reperfusion is a pivotal trigger of neurofunctional complications after global ischemic stroke.

Patterns of dentate granule cell responses to perforant path stimulation in epileptic mice with granule cell dispersion

In adult mice, intrahippocampal administration of kainic acid induces a structural modification of the granule cell layer reminiscent of granule cell dispersion (GCD) seen in humans with temporal lobe epilepsy. We tested that GCD might be involved in the patterns of granule cell responses to perforant path stimulation by recording field potentials in vivo after kainic acid-induced status epilepticus until the phase of chronic seizure activity in presence of GCD or after its alteration by K252a co-treatment, an inhibitor of tyrosine kinase activities. Stimulation triggered bursts of multiple population spikes, the number of which progressively increased with time whereas their amplitude decreased in parallel with the progressive decrease in granule cell density. The population spike threshold was reached for a lower excitatory synaptic drive than in controls, as assessed by the initial slope of the field excitatory post-synaptic potential. This indicates that, for identical synaptic responses, granule cells were closer to the firing threshold. Fast inhibition, assessed by paired pulse stimulation, was compromised immediately after the initial status epilepticus, consistent with the rapid loss of most hilar cells. Neither the epileptic course nor the epileptiform responses of the granule cells were modified and manipulation by alteration following GCD manipulation while granule cell neuropeptide-Y immunostaining was substantially decreased. In this mouse model of TLE, granule cells display a progressive increase in epileptiform responses to afferent input until the occurrence of spontaneous seizures. The population spike amplitude decreases in parallel with GCD while the granule cell excitability is enhanced. Consequently, data from field potentials in epilepsy experiments should be interpreted with care, taking into account the possible variations in the neuronal density in the recorded area.

Differential actions of nerve growth factor receptors TrkA and p75NTR in a rat model of epileptogenesis

Kindling, an experimental model of epileptogenesis, and activation-induced synaptic reorganization are modulated by nerve growth factor (NGF), but whether NGF acts via its high-affinity receptor TrkA and/or the common neurotrophin receptor p75NTR is unknown. We previously demonstrated, and confirmed in this study, that inhibition of NGF binding to both TrkA and p75NTR inhibited kindling and decreased kindling-induced mossy fiber sprouting. We now report specific inhibition of TrkA.NGF binding, but not p75NTR.NGF binding, retarded perforant path kindling progression. However, mossy fiber sprouting was inhibited by either selective TrkA.NGF or p75NTR.NGF antagonists. Our results suggest that TrkA, but not p75NTR, plays a role in kindling, while both receptors modulate kindling-induced mossy fiber sprouting. This implicates different mechanisms of neurotrophin action on kindling (mediated by TrkA) and neuronal sprouting (mediated by both TrkA and p75NTR) and suggests that sprouting involves kindling-independent neurotrophin action via p75NTR.

Electrophysiological correlates of neural plasticity compensating for ischemia-induced damage in the hippocampus

Injury to the brain often results in loss of synapses or cell death in the damaged area. Subsequent to the injury, the areas that are not directly affected often exhibit enhanced neuronal plasticity. Although there are many reports of morphological changes resulting from such plasticity, their functional consequences are poorly understood. In this study we examined electrophysiological changes associated with ischemia-induced neurogenesis in the hippocampus, a brain region that is particularly vulnerable but also exceptionally plastic. Transient global ischemia was induced in Sprague-Dawley rats by occlusion of both carotid arteries and a reduction in blood pressure for 12 min. The procedure resulted in delayed cell death in the CA1 field of the hippocampus while the dentate gyrus (DG) was spared. To assess neurogenesis and synaptic changes in parallel we used both hemispheres from each animal. One side was used for immunohistochemistry and the other for in vitro electrophysiological experiments in brain slices. Synaptic field responses and synaptic plasticity (LTP) in perforant path within the DG were reduced by 50% at 10 days after the ischemic injury but recovered at 35 days. Synaptic responses in non-neurogenic CA1 were abolished in parallel with cell death and did not recover. Gamma irradiation applied focally to the head selectively prevented neurogenesis and the synaptic recovery in the DG. These experiments reveal electrophysiological changes associated with reactive neural plasticity in the hippocampus.

Limbic Self-sustaining Status Epilepticus in Rats Is Not Associated with Hyperthermia

PURPOSE

To evaluate the impact of limbic status epilepticus on temperature.

METHODS

The perforant path in freely moving rats was stimulated electrically for 120 min to induce self-sustaining status epilepticus (SSSE). For 150 min after the end of stimulation, epidural temperature and electrographic and clinical seizure activity were assessed in animals with limbic and motor SSSE, as well as in animals without development of SE.

RESULTS

Temperature in all animals with SSSE was elevated by 1.5+/-0.8 degrees C after the end of stimulation compared with baseline values (p<0.01). In animals with pure limbic SE, temperature decreased continuously to baseline values over the 150-min period of observation. In contrast, in animals with motor SSSE, temperature remained elevated during continuing epileptic activity and was still significantly higher 150 min after the end of stimulation compared with baseline (p<0.01). In animals that did not develop SSSE, temperature was not changed after the end of electrical stimulation and in the 150 min thereafter compared with baseline values.

CONCLUSIONS

The results indicate that hyperthermia as seen in SE is the consequence of motor convulsions and not of epileptic activity itself, as seen in limbic SSSE.

Rapid deletion of mossy cells does not result in a hyperexcitable dentate gyrus: implications for epileptogenesis

Loss of cells from the hilus of the dentate gyrus is a major histological hallmark of human temporal lobe epilepsy. Hilar mossy cells, in particular, are thought to show dramatic numerical reductions in pathological conditions, and one prominent theory of epileptogenesis is based on the assumption that mossy cell loss directly results in granule cell hyperexcitability. However, whether it is the disappearance of hilar mossy cells from the dentate gyrus circuitry after various insults or the subsequent synaptic-cellular alterations (e.g., reactive axonal sprouting) that lead to dentate hyperexcitability has not been rigorously tested, because of the lack of available techniques to rapidly remove specific classes of nonprincipal cells from neuronal networks. We developed a fast, cell-specific ablation technique that allowed the targeted lesioning of either mossy cells or GABAergic interneurons in horizontal as well as axial (longitudinal) slices of the hippocampus. The results demonstrate that mossy cell deletion consistently decreased the excitability of granule cells to perforant path stimulation both within and outside of the lamella where the mossy cell ablation took place. In contrast, ablation of interneurons caused the expected increase in excitability, and control aspirations of the hilar neuropil or of interneurons in the presence of GABA receptor blockers caused no alteration in granule cell excitability. These data do not support the hypothesis that loss of mossy cells from the dentate hilus after seizures or traumatic brain injury directly results in hyperexcitability.

Initiation of electrographic seizures by neuronal networks in entorhinal and perirhinal cortices in vitro

The hippocampus is often considered to play a major role in the pathophysiology of mesial temporal lobe epilepsy. However, emerging clinical and experimental evidence suggests that parahippocampal areas may contribute to a greater extent to limbic seizure initiation, and perhaps epileptogenesis. To date, little is known about the participation of entorhinal and perirhinal networks to epileptiform synchronization. Here, we addressed this issue by using simultaneous field potential recordings in horizontal rat brain slices containing interconnected limbic structures that included the hippocampus proper. Epileptiform discharges were disclosed by bath applying the convulsant drug 4-aminopyridine (50 microM) or by superfusing Mg(2+)-free medium. In the presence of 4-aminopyridine, slow interictal- (duration=2.34+/-0.29 s; interval of occurrence=25.75+/-2.11 s, n=16) and ictal-like (duration=31.25+/-3.34 s; interval of occurrence=196.96+/-21.56 s, n=17) discharges were recorded in entorhinal and perirhinal cortices after abating the propagation of CA3-driven interictal activity to these areas following extended hippocampal knife cuts. Simultaneous recordings obtained from the medial and lateral entorhinal cortex, and from the perirhinal cortex revealed that interictal and ictal discharges could initiate from any of these areas and propagate to the neighboring structure with delays of 8-66 ms. However, slow interictal- and ictal-like events more often originated in the medial entorhinal cortex and perirhinal cortex, respectively. Cutting the connections between entorhinal and perirhinal cortices (n=10), or functional inactivation of cortical areas by local application of a glutamatergic receptor antagonist (n=11) made independent epileptiform activity occur in all areas. These procedures also shortened ictal discharge duration in the entorhinal cortices, but not in the perirhinal area. Similar results could be obtained by applying Mg(2+)-free medium (n=7). These findings indicate that parahippocampal networks provide independent epileptiform synchronization sufficient to sustain limbic seizures as well as that the perirhinal cortex plays a preferential role in in vitro ictogenesis.

Translamellar disinhibition in the rat hippocampal dentate gyrus after seizure-induced degeneration of vulnerable hilar neurons

Longitudinally restricted axonal projections of hippocampal granule cells suggest that transverse segments of the granule cell layer may operate independently (the "lamellar" hypothesis). Longitudinal projections of excitatory hilar mossy cells could be viewed as antithetical to lamellar function, but only if longitudinal impulse flow effectively excites distant granule cells. We, therefore, determined the effect of focal granule cell discharges on granule cells located >2 mm along the longitudinal axis. During perforant pathway stimulation in urethane-anesthetized rats, passive diffusion of the GABA(A) receptor antagonist bicuculline methiodide from the tip of a glass recording electrode evoked granule cell discharges and c-Fos expression in granule cells, mossy cells, and inhibitory interneurons, within a approximately 400 microm radius. This focally evoked activity powerfully suppressed distant granule cell-evoked responses recorded simultaneously approximately 2.5-4.5 mm longitudinally. Three days after kainic acid-induced status epilepticus or prolonged perforant pathway stimulation, translamellar inhibition was intact in rats with <40% hilar neuron loss but was consistently abolished after extensive (>85%) hilar cell loss. Retrograde transport of Fluoro-Gold (FG) from the rostral dentate gyrus revealed that few inhibitory interneurons were among the many retrogradely labeled hilar neurons 2.5-4.5 mm longitudinally. Although many somatostatin-positive hilar interneurons effectively transported FG from the distant septum, few of these neurons transported detectable FG from much closer hippocampal injection sites. Inhibitory basket and chandelier cells also exhibited minimal longitudinal FG transport. These findings suggest that translamellar disinhibition may result from the loss of vulnerable, longitudinally projecting mossy cells and may represent a network-level mechanism underlying postinjury hippocampal dysfunction and epileptic network hyperexcitability.

The dual response of protein kinase Fyn to neural trauma: early induction in neurons and delayed induction in reactive astrocytes

In the developing central nervous system, a src-related protein-tyrosine kinase fyn participates in the myelination process, neuronal growth, and cytoskeletal organization. In adults, fyn has been implicated in learning and memory formation. To test if fyn expression is modulated by neuronal activity, we performed quantitative in situ hybridization (ISH) using brain sections of the adult rats that had undergone either kainic acid (KA)-induced seizures or neuronal deafferentation (entorhinal cortex lesion, ECL). In the KA model, a few hours after seizure activities, fyn mRNA was elevated in the dentate gyrus (DG) (+45%), cerebral cortex layer III (+35%), and piriform cortex (+25%). Conversely, fyn mRNA consistently decreased in the hippocampal neurons after transection of the major axonal inputs from the entorhinal cortex. Although fyn expression in the brain has been allegedly limited to neurons and oligodendrocytes, we provide in this study the first evidence that fyn mRNA is highly expressed in the astrocytes involved in reactive gliosis. In the KA model, the occurrence of fyn-overexpressing astrocytes increased with the progress of neuronal damage in the CA1 and CA3 regions of the hippocampus. In contrast, fyn-overexpressing astrocytes were not observed in the granular cell layer of dentate gyrus (DG), where neurons were not damaged. Likewise, in the ECL model, the most drastic change in fyn mRNA expression took place at the reactive astrocytes near the stab wound sites, where fyn mRNA levels were doubled 4-10 d after the lesion. Collectively, our data suggest that (i) an early induction of fyn mRNA in neurons is linked to neuronal activity, and (ii) the delayed induction of fyn mRNA in reactive astrocytes near the damaged cells may play novel signaling roles during glial response.

Alterations of heterogeneous nuclear RNP A2 and B1 in the hippocampus of the rat after perforant pathway lesion

We examined alterations in post-transcriptional regulation following deafferentation of the perforant pathway by focusing on heterogeneous nuclear ribonucleoprotein (hnRNP) A2 and B1 in rat hippocampi subjected to perforant pathway lesions. In control brains, immunoreactivity to both was observed in the nuclei of neurons throughout the hippocampus using immunohistochemical techniques. From 1 to 14 days post-lesion, a slight increase in A2 immunoreactivity was observed in neurons within the dentate granular layer as well in the pyramidal cells of the cornus Ammon fields ipsilateral to the lesion. In contrast, we observed a marked decrease in B1 immunoreactivity in the same regions at 1, 3 and 7 days post-lesion. All these alterations, however, were transient. A2 immunoreactivity returned to normal levels by 30 days post-lesion, and B1 immunoreactivity had completely recovered by 14 days post-lesion. The results of immunoblot analysis for A2 and B1 were wholly consistent with immunohistochemical observations. Our study suggests that post-transcriptional regulation in the hippocampal neurons changes after a perforant pathway lesion. Our study further suggests that the functions of hnRNPA2 and B1 are different, as each was differentially involved in the plastic response to deafferentation of the perforant pathway.

Dynamics of oligodendrocyte responses to anterograde axonal (Wallerian) and terminal degeneration in normal and TNF-transgenic mice

The inflammatory cytokine tumour necrosis factor (TNF) can both induce oligodendrocyte and myelin pathology and promote proliferation of oligodendrocyte progenitor cells and remyelination. We have compared the response of the oligodendrocyte lineage to anterograde axonal (Wallerian) and terminal degeneration and lesion-induced axonal sprouting in the hippocampal dentate gyrus in TNF-transgenic mice with the response in genetically normal mice. Transectioning of the entorhino-dentate perforant path axonal projection increased hippocampal TNF mRNA expression in both types of mice, but to significantly larger levels in the TNF-transgenics. At 5 days after axonal transection, numbers of oligodendrocytes and myelin basic protein (MBP) mRNA expression in the denervated dentate gyrus in TNF-transgenic mice had increased to the same extent as in nontransgenic littermates. At this time, transgenics showed a tendency towards a greater increase in the number of juxtaposed, potentially proliferating oligodendrocytes. Noteworthy, at day 5 we also observed upregulation of MBP mRNA expression in adjacent hippocampal subregions with lesion-induced axonal sprouting, which were devoid of axonal degeneration, raising the possibility that sprouting axons provide trophic stimuli to the oligodendrocyte lineage. Twenty-eight days after lesioning, oligodendrocyte numbers and MBP mRNA expression were reduced to near normal levels. However, oligodendrocyte densities in the TNF-transgenic mice were significantly lower than in nontransgenics. We conclude that the early response of the oligodendrocyte lineage to axonal lesioning and lesion-induced axonal sprouting appears unaffected by the supranormal TNF levels in the TNF-transgenic mice. TNF may, however, have long-term inhibitory effects on the oligodendrocyte response to axonal lesioning.

Increased dendritic excitability in hippocampal ca1 in vivo in the kainic acid model of temporal lobe epilepsy: a study using current source density analysis

We used kainic acid in rats as an animal model of temporal lobe epilepsy, and studied the synaptic transmission in hippocampal subfield CA1 of urethane-anesthetized rats in vivo. Dendritic currents were revealed by field potential mapping, using a single micropipette or a 16-channel silicon probe, followed by current source density analysis. We found that the population excitatory postsynaptic potentials in the basal dendrites and distal apical dendrites of CA1 were increased in kainate-treated as compared with control rats following paired-pulse, but not single-pulse, stimulation of CA3b or medial perforant path. In contrast, the trisynaptic midapical dendritic response in CA1 following medial perforant path stimulation was decreased in kainate-treated as compared with control rats. Increased coupling between excitatory postsynaptic potential and the population spike in CA1 was found after kainate seizures. Short-latency, presumably monosynaptic CA1 population spikes following medial perforant path stimulation was found in kainate-treated but not control rats. An enhancement of dendritic excitability was evidenced by population spikes that invaded into or originated from the distal apical dendrites of CA1 in kainate-treated but not control rats. Reverberation of hippocampo-entorhinal activity was evidenced by recurrent excitation of CA1 following CA3b stimulation in kainate-treated but not control rats. Blockade of inhibition by intraventricularly administered bicuculline induced excitatory potentials in CA1 that were stronger and more prolonged in kainate-treated than control rats. The bicuculline-induced excitation was mainly blocked by non-N-methyl-D-aspartate receptor antagonists. We conclude that kainate seizures induced disinhibition in CA1 that unveiled excitation at the basal and distal apical dendrites, resulting in enhancement of the direct entorhinal cortex to CA1 input and reverberations via the hippocampo-entorhinal loop. These changes in the output of the hippocampus from CA1 are likely detrimental to the behavioral functions of the hippocampus and they may contribute to increased seizure susceptibility after kainate seizures.

Chronic brain inflammation results in cell loss in the entorhinal cortex and impaired LTP in perforant path-granule cell synapses

Alzheimer's disease (AD) is characterized by chronic neuroinflammation, significant temporal lobe cell loss, and dementia. We investigated the influence of chronic neuroinflammation produced by chronic infusion of lipopolysaccharide (LPS) into the fourth ventricle for 4 weeks upon the induction and maintenance of long-term potentiation (LTP) in the dentate gyrus of the hippocampus, a well-characterized model of cellular synaptic plasticity. We also examined for pyramidal cell loss within the entorhinal cortex an area of the brain that contains the cell bodies of the perforant path. The results demonstrate that chronic neuroinflammation results in the loss of pyramidal cells within layers II and III of the entorhinal cortex and a significant attenuation of LTP within the dentate gyrus. Similar changes may underlie the temporal lobe pathology and dementia associated with AD.

Resistance of immature hippocampus to morphologic and physiologic alterations following status epilepticus or kindling

Seizures in adult rats result in long-term deficits in learning and memory, as well as an enhanced susceptibility to further seizures. In contrast, fewer lasting changes have been found following seizures in rats younger than 20 days old. This age-dependency could be due to differing amounts of hippocampal neuronal damage produced by seizures at different ages. To determine if there is an early developmental resistance to seizure-induced hippocampal damage, we compared the effects of kainic acid (KA)-induced status epilepticus and amygdala kindling on hippocampal dentate gyrus anatomy and electrophysiology, in immature (16 day old) and adult rats. In adult rats, KA status epilepticus resulted in numerous silver-stained degenerating dentate hilar neurons, pyramidal cells in fields CA1 and CA3, and marked numerical reductions in CA3c pyramidal neuron counts (-57%) in separate rats. Two weeks following the last kindled seizure, some, but significantly less, CA3c pyramidal cell loss was observed (-26%). Both KA status epilepticus and kindling in duced mossy-fiber sprouting, as evidenced by ectopic Timm staining in supragranular layers of the dentate gyrus. In hippocampal slices from adult rats, paired-pulse stimulation of perforant path axons revealed a persistent enhancement of dentate granule-cell inhibition following KA status epilepticus or kindling. While seizures induced by KA or kindling in 16-day-old rats were typically more severe than in adults, the immature hippocampus exhibited markedly less KA-induced cell loss (-22%), no kindling-induced loss, no detectable synaptic rearrangement, and no change in dentate inhibition. These results demonstrate that, in immature rats, neither severe KA-induced seizures nor repeated kindled seizures produce the kind of hippocampal damage and changes associated with even less severe seizures in adults. The lesser magnitude of seizure-induced hippocampal alterations in immature rats may explain their greater resistance to long-term effects of seizures on neuronal function, as well as future seizure susceptibility. Conversely, hippocampal neuron loss and altered synaptic physiology in adults may contribute to increased sensitivity to epileptogenic stimuli, spontaneous seizures, and behavioral deficits.

Donepezil reverses a mnemonic deficit produced by scopolamine but not by perforant path lesion or transient cerebral ischaemia

The purpose of these studies were threefold. Firstly, to further characterize the effect of perforant path transection on a test of short-term memory: delayed matching (or nonmatching)-to-position [D(N)MTP]. Secondly, to evaluate the effect of a transient cerebral ischaemia in the same task. Both surgical procedures were chosen as they produce a CNS lesion similar to that described in Alzheimer's Disease (AD). Thirdly, the effect of the acetylcholinesterase inhibitor, donepezil (Aricept(R), E2020), on the resulting cognitive impairment was studied. Perforant path transection produced a robust, delay-dependent impairment of choice accuracy in rats performing either a delayed matching- or nonmatching-to-position task. Sample latency was also reduced following lesion, yet the lesion-induced impairment was not affected by increasing the response requirement at the sample stage. An 11-min period of transient ischaemia (two-vessel occlusion model) resulted in almost complete loss of hippocampal CA1 pyramidal cells and a delay-dependent impairment in DMTP performance. However, unlike perforant path lesions, this deficit was unstable and declined in magnitude over the experimental period. Increasing the delay interval restored this deficit. Donepezil, at doses that robustly attenuated a scopolamine (0.06 mg/kg s.c.)-induced DMTP accuracy impairment in naïve, unoperated rats, had no effect against either lesion-induced impairment. The results are considered in terms of the effectiveness of acetylcholinesterase inhibitors in noncholinergic-based preclinical cognitive models.

Increased afterdischarge threshold during kindling in epileptic rats

The effects of daily electrical kindling stimulation of the perforant pathway were investigated in an excitotoxic rat model of epilepsy with chronic seizures in order to learn whether the preexisting epileptic condition would facilitate or retard kindling. Sprague-Dawley rats with recurrent spontaneous seizures 4-8 months after unilateral intrahippocampal kainic acid (KA) injection were implanted with recording electrodes in the hippocampus and stimulating electrodes in the perforant path. Daily stimulation for 10 s at 5 Hz was given for 15 days. The afterdischarge (AD) threshold and the AD duration of kindled KA rats were compared before and during kindling with those of a kindled control group. In the control group, as expected, mean AD thresholds decreased ( P<0.01), while AD duration progressively increased. Although AD threshold was the same in KA and control groups at the start of kindling, in the KA group a significant increase in threshold occurred from the beginning to the end of kindling ( P<0.01). Behaviorally, KA rats showed stage 4 or 5 seizures on the first stimulation, and stage 3-5 seizures during the remainder of kindling. Paired pulse testing showed facilitation of late components of the dentate gyrus field potential at the beginning of kindling, and suppression of late components at the end, in the KA rats. A significant decrease in the rate of spontaneous seizures in KA rats was noted during the period of kindling ( P=0.04). These results suggest that electrical stimulation of the perforant path may strengthen homeostatic seizure suppressing mechanisms, and may provide insights into novel approaches to the treatment of clinical seizures in temporal lobe epilepsy.

Perforant path lesioning induces sprouting of CA3-associated fibre systems in mouse hippocampal formation

In comparison to the rat, the anatomy of the mouse hippocampus, and in particular the response to entorhinal cortex lesioning, is less well characterised. Here we studied the axonal sprouting response after lesioning of the entorhinodentate perforant path projection in young adult SJL/J and C57BL/6 mice. We found that lesioning led to translaminar sprouting of Timm stained regio inferior hippocampus (CA3)-associated fibre systems into the denervated termination zones of the CA3 and dentate gyrus, from the adjacent non-denervated stratum radiatum of CA3. Differences were seen in the Timm staining pattern of the two strains of mice, while the response to lesioning appeared similar albeit less pronounced than that observed in the rat. We also observed an intensified acetylcholine esterase staining reflective of cholinergic sprouting in the denervated perforant path termination zones, which was particularly prominent in areas with sprouting of Timm stained CA3-associated fibres. Finally, we showed that some of the sprouting fibres within the CA3 were myelinated, due to an increased density of silver impregnated myelinated fibres in this region after lesioning. These results show that the basic characteristics of the response to perforant path lesioning in mice are similar to those in the rat, but suggest that the magnitude of the response in the two species is different.

Phosphorylation of P42/P44 MAP kinase and DNA fragmentation in the rat perforant pathway stimulation model of limbic epilepsy

The intracellular signaling pathways associated with neuronal injury after perforant pathway stimulation of the rodent hippocampus have not been examined. To determine whether activation of the p42/p44 (Erk1/2) MAP kinase (MAPK) phosphorylation cascade is linked to neuronal injury after perforant pathway stimulation (PPS), we stained for phosphorylated Erk1/2 (P-Erk1/2) and for DNA fragmentation, a marker of cell death after PPS. Eighteen Sprague-Dawley rats underwent PPS for 6 (n=6), 12 (n=6), or 24 (n=6) h and were sacrificed either immediately (n=9) or 48 h (n=9) after stimulation. Sham-operated non-stimulated control animals (n=2) and control animals receiving low frequency stimulation only (n=4) were also examined. Brain sections were stained for DNA fragmentation and P-Erk1/2. DNA fragmentation was evident only in granule cells and CA3 pyramidal cells of the stimulated side 48 h after 24 h of PPS. PPS resulted in robust phosphorylation of Erk1/2 that displayed a stereotyped timecourse, appearing first in hilar neurons on the ipsilateral side and later in hilar neurons, granule cells, hippocampal pyramidal and non-neuronal cell populations on both the stimulated and contralateral sides. Both Erk1/2 phosphorylation and DNA fragmentation show definite and reproducible staining patterns after PPS that vary based on duration of stimulation. Populations displaying Erk1/2 activation appeared to differ from those showing DNA fragmentation and neuronal injury.

Local generation of fast ripples in epileptic brain

Aperiodic high-frequency oscillations (>100 Hz) reflect a short-term synchronization of neuronal electrical activity. It has been shown in the epileptic brain that spontaneous oscillations in the frequency range of 250-600 Hz reflect action potential population bursts of synchronously discharging neuronal clusters. These oscillations occur in the early stages of epileptogenesis in areas adjacent to the brain lesion and may trigger the formation of seizure-generating neuronal networks. We studied the extent of the area generating oscillations in the frequency range of 250-600 Hz [fast ripples (FRs)] in intrahippocampal kainic acid-treated rats with spontaneous seizures, by analyzing voltage versus depth profiles of FRs in hippocampal and parahippocampal areas in freely moving animals and by spatial mapping in hippocampal slice preparations in vitro. The strength of inhibition was compared in areas with and without FRs using a paired-pulse paradigm. The extent of the areas generating FRs did not exceed 1 mm(3). The areas generating FRs became broader after the application of the GABA(A) receptor antagonist bicuculline. Paired-pulse fast inhibition at 15-30 msec intervals was similar in areas generating FRs and areas not generating FRs. Our data illustrate that hypothesized clusters of highly interconnected neurons are capable of overcoming interneuron feedback inhibition, resulting in generation of epileptiform bursts, eventually leading to seizure activity.

Limbic network interactions leading to hyperexcitability in a model of temporal lobe epilepsy

In mouse brain slices that contain reciprocally connected hippocampus and entorhinal cortex (EC) networks, CA3 outputs control the EC propensity to generate experimentally induced ictal-like discharges resembling electrographic seizures. Neuronal damage in limbic areas, such as CA3 and dentate hilus, occurs in patients with temporal lobe epilepsy and in animal models (e.g., pilocarpine- or kainate-treated rodents) mimicking this epileptic disorder. Hence, hippocampal damage in epileptic mice may lead to decreased CA3 output function that in turn would allow EC networks to generate ictal-like events. Here we tested this hypothesis and found that CA3-driven interictal discharges induced by 4-aminopyridine (4AP, 50 microM) in hippocampus-EC slices from mice injected with pilocarpine 13-22 days earlier have a lower frequency than in age-matched control slices. Moreover, EC-driven ictal-like discharges in pilocarpine-treated slices occur throughout the experiment (< or = 6 h) and spread to the CA1/subicular area via the temporoammonic path; in contrast, they disappear in control slices within 2 h of 4AP application and propagate via the trisynaptic hippocampal circuit. Thus, different network interactions within the hippocampus-EC loop characterize control and pilocarpine-treated slices maintained in vitro. We propose that these functional changes, which are presumably caused by seizure-induced cell damage, lead to seizures in vivo. This process is facilitated by a decreased control of EC excitability by hippocampal outputs and possibly sustained by the reverberant activity between EC and CA1/subiculum networks that are excited via the temporoammonic path.

Combination of hypothyroidism and stress abolishes early LTP in the CA1 but not dentate gyrus of hippocampus of adult rats

Clinical experience suggests that both hypothyroidism and stress interfere with mental concentration and memory. This electrophysiological study examined the effect of hypothyroidism and stress, separately or combined, on long-term potentiation (LTP), a widely accepted cellular model for learning and memory. Measurements of early LTP (E-LTP) were carried out in the hippocampus of urethane-anesthetized adult Wistar rats. Hypothyroidism was achieved by thyroidectomy, and the 'intruder' stress was used as a model of chronic psychosocial stress. Stimulating electrodes were placed in the left CA3 region and right angular bundle and a recording electrode was placed in the right CA1 or the dentate gyrus (DG). The results showed that in the CA1 region of the hippocampus, hypothyroid or stress partially blocked E-LTP. However, when hypothyroidism and stress were combined, they eliminated E-LTP. In contrast, no significant change in E-LTP was seen in the DG of the three groups of rats. These results suggest that impaired memory because of hypothyroidism or stress may be related to impairment of the E-LTP in the Schaffer collateral synapses but not of that of the perforant path synapses.

Intrinsic optical signals and electrographic seizures in the rat limbic system

We measured the intrinsic optical signals (IOSs) generated by rat hippocampus-entorhinal cortex (EC) slices in response to single shock electrical stimuli delivered in the EC deep layers during application of the convulsant drug 4-aminopyridine (50 microM). With field potential recordings the stimulus-induced responses had duration = 35 +/- 6.3 s mean +/- SEM, n = 7 slices) and characteristics resembling electrographic seizures. IOS changes reflecting an increase in light transmittance occurred in the EC and hippocampus following similar stimuli (n = 45). IOSs increased progressively to reach peak values 20-30 s after the stimulus and returned slowly to prestimulus values within 100 s, thus outlasting the field potential discharge. IOS changes initiated in the medial EC, near to the stimulation site, and spread to the lateral EC, the dentate, and the CA3/CA1 areas. IOS spread from EC to hippocampus was not seen after perforant path cut (n = 5). Moreover, field potential and IOS responses were markedly decreased by excitatory amino acid receptor antagonists (n = 12). The antiepileptic drugs topiramate (10-100 microM, n = 16) or lamotrigine (100-400 microM, n = 12) reduced the IOS changes in the EC and their spread to distant areas. These effects were reversible and dose-dependent (IC50 = 48 microM and 210 microM for topiramate and lamotrigine, respectively). Thus, in 4AP-treated hippocampus-EC slices, IOS changes accompany and outlast the field potential epileptiform responses, depend on glutamatergic transmission and are characterized by temporal and spatial distributions consistent with propagation through established anatomical pathways. We also propose that IOSs may represent a reliable tool for screening the effects of neuroactive compounds such as antiepileptic drugs.

Involvement of interleukin-1beta/nitric oxide pathway in the postischemic impairment of long-term potentiation of the rat hippocampus

To investigate whether postischemic cerebral dysfunction occurs via the interleukin-1 beta/nitric oxide (IL-1beta/NO) pathway, we examined the effects of an IL-1beta antagonist on long-term potentiation (LTP) impairment and excessive NO production in the rat hippocampus after 10-min global ischemia. Intracerebroventricilar administration of the IL-1beta antagonist attenuated NO production and rescued LTP impairment in the perforant path-dentate gyrus synapses, observed 1 day and 4 days after ischemic insult, respectively. There was an inverse relationship between LTP in the dentate gyrus synapses and hippocampal NO production. Centrally applied IL-1beta mimicked the consequences of transient ischemia in LTP formation and hippocampal NO production in non-ischemic rats. These findings indicate that the IL-1beta/NO pathway is involved in the hippocampal LTP impairment observed in the postischemic brain.

Disruption of inhibition in area CA1 of the hippocampus in a rat model of temporal lobe epilepsy

Previous studies have revealed a loss of neurons in layer III of the entorhinal cortex (EC) in patients with temporal lobe epilepsy. These neurons project to the hippocampus and may activate inhibitory interneurons, so that their loss could disrupt inhibitory function in the hippocampus. The present study evaluates this hypothesis in a rat model in which layer III neurons were selectively destroyed by focal injections of the indirect excitotoxin, aminooxyacetic acid (AOAA). Inhibitory function in the hippocampus was assessed by evaluating the discharge of CA1 neurons in response to stimulation of afferent pathways in vivo. In control animals, stimulation of the temporo-ammonic pathway leads to heterosynaptic inhibition of population spikes generated by subsequent stimulation of the commissural projection to CA1. This heterosynaptic inhibition was substantially reduced in animals that had received AOAA injections 1 mo previously. Stimulation of the commissural projection also elicited multiple population spikes in CA1 in AOAA-injected animals, and homosynaptic inhibition in response to paired-pulse stimulation of the commissural projection was dramatically diminished. These results suggest a disruption of inhibitory function in CA1 in AOAA-injected animals. To determine whether the disruption of inhibition occurred selectively in CA1, we assessed paired-pulse inhibition in the dentate gyrus. Both homosynaptic inhibition generated by paired-pulse stimulation of the perforant path, and heterosynaptic inhibition produced by activation of the commissural projection to the dentate gyrus appeared largely comparable in AOAA-injected and control animals; thus abnormalities in inhibitory function following AOAA injections occurred relatively selectively in CA1. Electrolytic lesions of the EC did not cause the same loss of inhibition as seen in animals with AOAA injections, indicating that the loss of inhibition in CA1 is not due to the loss of excitatory driving of inhibitory interneurons. Also, electrolytic lesions of the EC in animals that had been injected previously with AOAA had little effect on the abnormal physiological responses in CA1, suggesting that most alterations in inhibition in CA1 are not due to circuit abnormalities within the EC. Comparisons of control and AOAA-injected animals in a hippocampal kindling paradigm revealed that the duration of afterdischarges elicited by high-frequency stimulation of CA3, and the number of stimulations required to elicit kindled seizures were comparable. Taken together, our results reveal that the selective loss of layer III neurons induced by AOAA disrupts inhibitory function in CA1, but this does not create a circuit that is more prone to at least one form of kindling.

Physiological and structural evidence for hippocampal involvement in persistent seizure susceptibility after traumatic brain injury

Epilepsy is a common outcome of traumatic brain injury (TBI), but the mechanisms of posttraumatic epileptogenesis are poorly understood. One clue is the occurrence of selective hippocampal cell death after fluid-percussion TBI in rats, consistent with the reported reduction of hippocampal volume bilaterally in humans after TBI and resembling hippocampal sclerosis, a hallmark of temporal-lobe epilepsy. Other features of temporal-lobe epilepsy, such as long-term seizure susceptibility, persistent hyperexcitability in the dentate gyrus (DG), and mossy fiber synaptic reorganization, however, have not been examined after TBI. To determine whether TBI induces these changes, we used a well studied model of TBI by weight drop on somatosensory cortex in adult rats. First, we confirmed an early and selective cell loss in the hilus of the DG and area CA3 of hippocampus, ipsilateral to the impact. Second, we found persistently enhanced susceptibility to pentylenetetrazole-induced convulsions 15 weeks after TBI. Third, by applying GABA(A) antagonists during field-potential and optical recordings in hippocampal slices 3 and 15 weeks after TBI, we unmasked a persistent, abnormal APV-sensitive hyperexcitability that was bilateral and localized to the granule cell and molecular layers of the DG. Finally, using Timm histochemistry, we detected progressive sprouting of mossy fibers into the inner molecular layers of the DG bilaterally 2-27 weeks after TBI. These findings are consistent with the development of posttraumatic epilepsy in an animal model of impact head injury, showing a striking similarity to the enduring behavioral, functional, and structural alterations associated with temporal-lobe epilepsy.

A specific and sensitive method for visualization of tumor necrosis factor in the murine central nervous system

We present here sensitive, simple and robust methods for detection of tumor necrosis factor (TNF) mRNA and TNF in histological sections and homogenates of brain tissue from mice subjected to focal cerebral ischemia or hippocampal axonal lesioning. Both types of lesions are characterized by induction of TNF synthesis in resident microglial cells, which in the ischemic lesions are supplemented by TNF synthesizing, blood-borne macrophages. In situ hybridization for TNF mRNA is performed using alkaline phosphatase-labelled oligodeoxynucleotide probes. These probes show excellent rendition of individual cells, and can successfully be combined with immunohistochemical procedures. We also describe a sensitive immunohistochemical method for detection of TNF, which can be combined with visualization of an additional antigen. The specificity of the histological procedures are confirmed by RT-PCR and Western blot analysis on homogenates prepared from microdissected brain regions. Advantages and disadvantages of the methods are discussed with emphasis on the specificity and sensitivity of the histological procedures. Our strategy for detection of TNF mRNA and protein provides a solid basis for clarifying the cellular synthesis, regulation and function of TNF in the normal, injured or diseased CNS. Furthermore, the methodology can readily be applied in studies of other cytokines and growth factors in the CNS.

Identification of differentially expressed genes in the denervated rat hippocampus by cDNA arrays

To elucidate the molecular mechanism underlying the physiological responses to injury in the central nervous system, gene expression profiles in rodent hippocampus following perforant path transection were investigated using cDNA array hybridization. Of the 8000 arrayed clones, 47 exhibited differential expression by >3-fold difference in the denervated hippocampus from control, with 15 up-regulated and 22 down-regulated. They can be functionally assigned into several classes, among which the most prominent are those coding proteins involved in macromolecules synthesis and processing. Northern blot analysis verified the validation of the aforementioned array data. These results throw some new light on the physiological responses of the hippocampus to entorhinal deafferentation at molecular level.

Tau, beta-amyloid and beta-amyloid precursor protein distribution in the entorhinal-hippocampal alvear and perforant pathways in the Alzheimer's brain

It has been suggested that the pathological lesions of Alzheimer's disease (AD) spread along neuronal connections. This study was designed to examine this hypothesis in the alvear and perforant pathways, two well-defined neuroanatomical pathways that project from the entorhinal cortex to the hippocampus. Paraffin-sections of hippocampal-entorhinal cortex from 25 AD cases were immunolabelled for tau, beta-amyloid (Abeta) and beta-amyloid precursor protein (betaAPP). We used image-analysis to quantify immunolabelling at both ends of the alvear and perforant pathways. At the beginning and the end of the alvear pathway, area of immunolabelling in microm2 per area of field (72000 microm2) were as follows: tau 349 and 821 (P<0.01), Abeta 349 and 61 (P<0.05) and betaAPP 18 and 73 (P<0.01). Corresponding values for the perforant pathway were tau 421 and 387, Abeta 382 and 115 (P<0.05) and betaAPP 55 and 83. Tau was significantly greater at the end than at the beginning of the alvear pathway, but similar at both ends of the perforant pathway. There was significantly more Abeta at the beginning than at the end of the alvear and perforant pathway. These results at least in part reinforce previous work [19] that tau-rich areas may be neuronally connected to Abeta-rich areas.

Transforming growth factor-beta1 induces transforming growth factor-beta1 and transforming growth factor-beta receptor messenger RNAs and reduces complement C1qB messenger RNA in rat brain microglia

Transforming growth factor-beta1 is a multifunctional peptide with increased expression during Alzheimer's disease and other neurodegenerative conditions which involve inflammatory mechanisms. We examined the autoregulation of transforming growth factor-beta1 and transforming growth factor-beta receptors and the effects of transforming growth factor-beta1 on complement C1q in brains of adult Fischer 344 male rats and in primary glial cultures. Perforant path transection by entorhinal cortex lesioning was used as a model for the hippocampal deafferentation of Alzheimer's disease. In the hippocampus ipsilateral to the lesion, transforming growth factor-beta1 peptide was increased >100-fold; the messenger RNAs encoding transforming growth factor-beta1, transforming growth factor-beta type I and type II receptors were also increased, but to a smaller degree. In this acute lesion paradigm, microglia are the main cell type containing transforming growth factor-beta1, transforming growth factor-beta type I and II receptor messenger RNAs, shown by immunocytochemistry in combination with in situ hybridization. Autoregulation of the transforming growth factor-beta1 system was examined by intraventricular infusion of transforming growth factor-beta1 peptide, which increased hippocampal transforming growth factor-beta1 messenger RNA levels in a dose-dependent fashion. Similarly, transforming growth factor-beta1 increased levels of transforming growth factor-beta1 messenger RNA and transforming growth factor-beta type II receptor messenger RNA (IC(50), 5pM) and increased release of transforming growth factor-beta1 peptide from primary microglia cultures. Interactions of transforming growth factor-beta1 with complement system gene expression are also indicated, because transforming growth factor-beta1 decreased C1qB messenger RNA in the cortex and hippocampus, after intraventricular infusion, and in cultured glia. These indications of autocrine regulation of transforming growth factor-beta1 in the rodent brain support a major role of microglia in neural activities of transforming growth factor-beta1 and give a new link between transforming growth factor-beta1 and the complement system. The auto-induction of the transforming growth factor-beta1 system has implications for transgenic mice that overexpress transforming growth factor-beta1 in brain cells and for its potential role in amyloidogenesis.

Nitric oxide synthase immunoreactivity in the rat hippocampus after status epilepticus induced by perforant pathway stimulation

Nitric oxide has recently been implicated in mediation of neuronal excitotoxicity and damage. This study aimed at elucidating the changes in the expression of neuronal isoform of nitric oxide synthase (nNOS) in the hippocampus after status epilepticus induced by perforant pathway stimulation. nNOS-immunoreactivity (nNOS-ir) and neuronal damage, assessed by silver staining, were evaluated separately in different hippocampal subfields 2 weeks after induction of status epilepticus. Perforant pathway stimulation resulted in an increase in the number of nNOS-immunoreactive neurons in the stratum radiatum of the CA1 and CA3 subfields of the hippocampus proper, and the hilus of the dentate gyrus. The morphology and distribution of the nNOS-ir neurons resembled that of interneurons. No correlation of the number of nNOS-ir neurons to the neuronal damage score was observed. Our results suggest that status epilepticus provokes a de novo expression of nNOS protein, and the nNOS expressing neurons may be selectively resistant to epileptic brain injury.