Pinniped electroencephalography: Methodology and findings in California sea lions (Zalophus californianus)

This study was designed to identify abnormalities in the electroencephalograms (EEGs) recorded from stranded California sea lions (Zalophus californianus) with suspected domoic acid (DA) toxicosis. Recordings from animals presenting for non-neurological issues were also obtained to better understand the normal EEG (background activity and transient events) in this species, as, to date, studies have focused on examining natural sleep in pinnipeds. Most animals were sedated for electrode placement and EEG acquisition with some receiving antiepileptic medications or isoflurane during the procedure. A total of 103 recordings were read and scored from 0 (normal) to 3 (severely abnormal). Epileptiform discharges, consisting of spikes, sharp waves, slow waves, and/or spike waves, were present in all EEGs with scores of 1, 2, or 3. The distribution of these events over the scalp varied. While often generalized, others were lateralized over one hemisphere, bifrontal, bioccipital, and/or bitemporal, while some discharges were multifocal. Findings were different between sea lions and occasionally changed within the EEG on a given sea lion. No clinical seizures were observed during the recording but a few sea lions had findings consistent with electroencephalographic seizures. When available, supporting diagnostic results obtained from magnetic resonance imaging (MRI) and/or necropsy/histopathology were described, as well as the status of those sea lions that recovered and were released with satellite tags.

Ventral Hippocampal Formation Is the Primary Epileptogenic Zone in a Rat Model of Temporal Lobe Epilepsy

Temporal lobe epilepsy is common, but mechanisms of seizure initiation are unclear. We evaluated seizure initiation in female and male rats that had been systemically treated with pilocarpine, a widely used model of temporal lobe epilepsy. Local field potential (LFP) recordings from many brain regions revealed variable sites of earliest recorded seizure activity, but mostly the ventral hippocampal formation. To test whether inactivation of the ventral hippocampal formation would reduce seizures, mini-osmotic pumps were used to continually and focally deliver TTX. High doses of TTX infused unilaterally into the ventral hippocampal formation blocked seizures reversibly but also reduced LFP amplitudes in remote brain regions, indicating distant effects. A lower dose did not reduce LFP amplitudes in remote brain regions but did not reduce seizures when infused unilaterally. Instead, seizures tended to initiate in the contralateral ventral hippocampal formation. Bilateral infusion of the lower dose into the ventral hippocampal formation reduced seizure frequency 85%. Similar bilateral treatment in the amygdala was not effective. Bilateral infusion of the dorsal hippocampus reduced seizure frequency, but only 17%. Together, these findings reveal that the ventral hippocampal formation is a primary bilaterally independent epileptogenic zone, and the dorsal hippocampus is a secondary epileptogenic zone in pilocarpine-treated rats. This is consistent with many human patients, and the results further validate the LFP method for identifying seizure onset zones. Finally, the findings are more consistent with a focal mechanism of ictogenesis rather than one involving a network of interdependent nodes.SIGNIFICANCE STATEMENT To better understand how seizures start, investigators need to know where seizures start in the animal models they study. In the widely used pilocarpine-treated rat model of temporal lobe epilepsy, earliest seizure activity was most frequently recorded in the ventral hippocampal formation. Confirming the primary role of the ventral hippocampal formation, seizure frequency was reduced most effectively when it was inactivated focally, bilaterally, and continually with infused TTX. These findings suggest that the ventral hippocampal formation is the primary site of seizure initiation in this animal model of temporal lobe epilepsy, consistent with findings in many human patients.

Non-invasive, neurotoxic surgery reduces seizures in a rat model of temporal lobe epilepsy

Surgery can be highly effective for treating certain cases of drug resistant epilepsy. The current study tested a novel, non-invasive, surgical strategy for treating seizures in a rat model of temporal lobe epilepsy. The surgical approach uses magnetic resonance-guided, low-intensity focused ultrasound (MRgFUS) in combination with intravenous microbubbles to open the blood-brain barrier (BBB) in a transient and focal manner. During the period of BBB opening, a systemically administered neurotoxin (Quinolinic Acid: QA) that is normally impermeable to the BBB gains access to a targeted area in the brain, destroying neurons where the BBB has been opened. This strategy is termed Precise Intracerebral Non-invasive Guided Surgery (PING). Spontaneous recurrent seizures induced by pilocarpine were monitored behaviorally prior to and after PING or under control conditions. Seizure frequency in untreated animals or animals treated with MRgFUS without QA exhibited expected seizure rate fluctuations frequencies between the monitoring periods. In contrast, animals treated with PING targeting the intermediate-temporal aspect of the hippocampus exhibited substantial reductions in seizure frequency, with convulsive seizures being eliminated entirely in two animals. These findings suggest that PING could provide a useful alternative to invasive surgical interventions for treating drug resistant epilepsy, and perhaps for treating other neurological disorders in which aberrant neural circuitries play a role.

Ictal onset sites and γ-aminobutyric acidergic neuron loss in epileptic pilocarpine-treated rats

OBJECTIVE

The present study tested whether ictal onset sites are regions of more severe interneuron loss in epileptic pilocarpine-treated rats, a model of human temporal lobe epilepsy.

METHODS

Local field potential recordings were evaluated to identify ictal onset sites. Electrode sites were visualized in Nissl-stained sections. Adjacent sections were processed with proximity ligation in situ hybridization for glutamic acid decarboxylase 2 (Gad2). Gad2 neuron profile numbers at ictal onset sites were compared to contralateral regions. Other sections were processed with immunocytochemistry for reelin or nitric oxide synthase (NOS), which labeled major subtypes of granule cell layer-associated interneurons. Stereology was used to estimate numbers of reelin and NOS granule cell layer-associated interneurons per hippocampus.

RESULTS

Ictal onset sites varied between and within rats but were mostly in the ventral hippocampus and were frequently bilateral. There was no conclusive evidence of more severe Gad2 neuron profile loss at sites of earliest seizure activity compared to contralateral regions. Numbers of granule cell layer-associated NOS neurons were reduced in the ventral hippocampus.

SIGNIFICANCE

In epileptic pilocarpine-treated rats, ictal onset sites were mostly in the ventral hippocampus, where there was loss of granule cell layer-associated NOS interneurons. These findings suggest the hypothesis that loss of granule cell layer-associated NOS interneurons in the ventral hippocampus is a mechanism of temporal lobe epilepsy.

Proportional loss of parvalbumin-immunoreactive synaptic boutons and granule cells from the hippocampus of sea lions with temporal lobe epilepsy

One in 26 people develop epilepsy and in these temporal lobe epilepsy (TLE) is common. Many patients display a pattern of neuron loss called hippocampal sclerosis. Seizures usually start in the hippocampus but underlying mechanisms remain unclear. One possibility is insufficient inhibition of dentate granule cells. Normally parvalbumin-immunoreactive (PV) interneurons strongly inhibit granule cells. Humans with TLE display loss of PV interneurons in the dentate gyrus but questions persist. To address this, we evaluated PV interneuron and bouton numbers in California sea lions (Zalophus californianus) that naturally develop TLE after exposure to domoic acid, a neurotoxin that enters the marine food chain during harmful algal blooms. Sclerotic hippocampi were identified by the loss of Nissl-stained hilar neurons. Stereological methods were used to estimate the number of granule cells and PV interneurons per dentate gyrus. Sclerotic hippocampi contained fewer granule cells, fewer PV interneurons, and fewer PV synaptic boutons, and the ratio of granule cells to PV interneurons was higher than in controls. To test whether fewer boutons was attributable to loss versus reduced immunoreactivity, expression of synaptotagmin-2 (syt2) was evaluated. Syt2 is also expressed in boutons of PV interneurons. Sclerotic hippocampi displayed proportional losses of syt2-immunoreactive boutons, PV boutons, and granule cells. There was no significant difference in the average numbers of PV- or syt2-positive boutons per granule cell between control and sclerotic hippocampi. These findings do not address functionality of surviving synapses but suggest reduced granule cell inhibition in TLE is not attributable to anatomical loss of PV boutons.

Testing Different Combinations of Acoustic Pressure and Doses of Quinolinic Acid for Induction of Focal Neuron Loss in Mice Using Transcranial Low-Intensity Focused Ultrasound

The goal of this study was to test different combinations of acoustic pressure and doses of quinolinic acid (QA) for producing a focal neuronal lesion in the murine hippocampus without causing unwanted damage to adjacent brain structures. Sixty male CD-1 mice were divided into 12 groups that underwent magnetic resonance-guided focused ultrasound at high (0.67 MPa), medium (0.5 MPa) and low (0.33 MPa) acoustic peak negative pressures and received QA at high (0.012 mmol), medium (0.006 mmol) and low (0.003 mmol) dosages. Neuronal loss occurred only when magnetic resonance-guided focused ultrasound with adequate acoustic power (0.67 or 0.5 MPa) was combined with QA. The animals subjected to the highest acoustic power had larger lesions than those treated with medium acoustic power, but two mice had evidence of bleeding. When the intermediate acoustic power was used, medium and high dosages of QA produced lesions larger than those produced by the low dosage.

A single subconvulsant dose of domoic acid at mid-gestation does not cause temporal lobe epilepsy in mice

Harmful blooms of domoic acid (DA)-producing algae are a problem in oceans worldwide. DA is a potent glutamate receptor agonist that can cause status epilepticus and in survivors, temporal lobe epilepsy. In mice, one-time low-dose in utero exposure to DA was reported to cause hippocampal damage and epileptiform activity, leading to the hypothesis that unrecognized exposure to DA from contaminated seafood in pregnant women can damage the fetal hippocampus and initiate temporal lobe epileptogenesis. However, development of epilepsy (i.e., spontaneous recurrent seizures) has not been tested. In the present study, long-term seizure monitoring and histology was used to test for temporal lobe epilepsy following prenatal exposure to DA. In Experiment One, the previous study's in utero DA treatment protocol was replicated, including use of the CD-1 mouse strain. Afterward, mice were video-monitored for convulsive seizures from 2 to 6 months old. None of the CD-1 mice treated in utero with vehicle or DA was observed to experience spontaneous convulsive seizures. After seizure monitoring, mice were evaluated for pathological evidence of temporal lobe epilepsy. None of the mice treated in utero with DA displayed the hilar neuron loss that occurs in patients with temporal lobe epilepsy and in the mouse pilocarpine model of temporal lobe epilepsy. In Experiment Two, a higher dose of DA was administered to pregnant FVB mice. FVB mice were tested as a potentially more sensitive strain, because they have a lower seizure threshold, and some females spontaneously develop epilepsy. Female offspring were monitored with continuous video and telemetric bilateral hippocampal local field potential recording at 1-11 months old. A similar proportion of vehicle- and DA-treated female FVB mice spontaneously developed epilepsy, beginning in the fourth month of life. Average seizure frequency and duration were similar in both groups. Seizure frequency was lower than that of positive-control pilocarpine-treated mice, but seizure duration was similar. None of the mice treated in utero with vehicle or DA displayed hilar neuron loss or intense mossy fiber sprouting, a form of aberrant synaptic reorganization that develops in patients with temporal lobe epilepsy and in pilocarpine-treated mice. FVB mice that developed epilepsy (vehicle- and DA-treated) displayed mild mossy fiber sprouting. Results of this study suggest that a single subconvulsive dose of DA at mid-gestation does not cause temporal lobe epilepsy in mice.

Seizure frequency correlates with loss of dentate gyrus GABAergic neurons in a mouse model of temporal lobe epilepsy

Epilepsy occurs in one of 26 people. Temporal lobe epilepsy is common and can be difficult to treat effectively. It can develop after brain injuries that damage the hippocampus. Multiple pathophysiological mechanisms involving the hippocampal dentate gyrus have been proposed. This study evaluated a mouse model of temporal lobe epilepsy to test which pathological changes in the dentate gyrus correlate with seizure frequency and help prioritize potential mechanisms for further study. FVB mice (n = 127) that had experienced status epilepticus after systemic treatment with pilocarpine 31-61 days earlier were video-monitored for spontaneous, convulsive seizures 9 hr/day every day for 24-36 days. Over 4,060 seizures were observed. Seizure frequency ranged from an average of one every 3.6 days to one every 2.1 hr. Hippocampal sections were processed for Nissl stain, Prox1-immunocytochemistry, GluR2-immunocytochemistry, Timm stain, glial fibrillary acidic protein-immunocytochemistry, glutamic acid decarboxylase in situ hybridization, and parvalbumin-immunocytochemistry. Stereological methods were used to measure hilar ectopic granule cells, mossy cells, mossy fiber sprouting, astrogliosis, and GABAergic interneurons. Seizure frequency was not significantly correlated with the generation of hilar ectopic granule cells, the number of mossy cells, the extent of mossy fiber sprouting, the extent of astrogliosis, or the number of GABAergic interneurons in the molecular layer or hilus. Seizure frequency significantly correlated with the loss of GABAergic interneurons in or adjacent to the granule cell layer, but not with the loss of parvalbumin-positive interneurons. These findings prioritize the loss of granule cell layer interneurons for further testing as a potential cause of temporal lobe epilepsy.

Hilar somatostatin interneuron loss reduces dentate gyrus inhibition in a mouse model of temporal lobe epilepsy

OBJECTIVE

In patients with temporal lobe epilepsy, seizures usually start in the hippocampus, and dentate granule cells are hyperexcitable. Somatostatin interneurons are a major subpopulation of inhibitory neurons in the dentate gyrus, and many are lost in patients and animal models. However, surviving somatostatin interneurons sprout axon collaterals and form new synapses, so the net effect on granule cell inhibition remains unclear.

METHODS

The present study uses optogenetics to activate hilar somatostatin interneurons and measure the inhibitory effect on dentate gyrus perforant path-evoked local field potential responses in a mouse model of temporal lobe epilepsy.

RESULTS

In controls, light activation of hilar somatostatin interneurons inhibited evoked responses up to 40%. Epileptic pilocarpine-treated mice exhibited loss of hilar somatostatin interneurons and less light-induced inhibition of evoked responses.

SIGNIFICANCE

These findings suggest that severe epilepsy-related loss of hilar somatostatin interneurons can overwhelm the surviving interneurons' capacity to compensate by sprouting axon collaterals.

More Docked Vesicles and Larger Active Zones at Basket Cell-to-Granule Cell Synapses in a Rat Model of Temporal Lobe Epilepsy

Temporal lobe epilepsy is a common and challenging clinical problem, and its pathophysiological mechanisms remain unclear. One possibility is insufficient inhibition in the hippocampal formation where seizures tend to initiate. Normally, hippocampal basket cells provide strong and reliable synaptic inhibition at principal cell somata. In a rat model of temporal lobe epilepsy, basket cell-to-granule cell (BC→GC) synaptic transmission is more likely to fail, but the underlying cause is unknown. At some synapses, probability of release correlates with bouton size, active zone area, and number of docked vesicles. The present study tested the hypothesis that impaired GABAergic transmission at BC→GC synapses is attributable to ultrastructural changes. Boutons making axosomatic symmetric synapses in the granule cell layer were reconstructed from serial electron micrographs. BC→GC boutons were predicted to be smaller in volume, have fewer and smaller active zones, and contain fewer vesicles, including fewer docked vesicles. Results revealed the opposite. Compared with controls, epileptic pilocarpine-treated rats displayed boutons with over twice the average volume, active zone area, total vesicles, and docked vesicles and with more vesicles closer to active zones. Larger active zones in epileptic rats are consistent with previous reports of larger amplitude miniature IPSCs and larger BC→GC quantal size. Results of this study indicate that transmission failures at BC→GC synapses in epileptic pilocarpine-treated rats are not attributable to smaller boutons or fewer docked vesicles. Instead, processes following vesicle docking, including priming, Ca(2+) entry, or Ca(2+) coupling with exocytosis, might be responsible.

SIGNIFICANCE STATEMENT

One in 26 people develops epilepsy, and temporal lobe epilepsy is a common form. Up to one-third of patients are resistant to currently available treatments. This study tested a potential underlying mechanism for previously reported impaired inhibition in epileptic animals at basket cell-to-granule cell (BC→GC) synapses, which normally are reliable and strong. Electron microscopy was used to evaluate 3D ultrastructure of BC→GC synapses in a rat model of temporal lobe epilepsy. The hypothesis was that impaired synaptic transmission is attributable to smaller boutons, smaller synapses, and abnormally low numbers of synaptic vesicles. Results revealed the opposite. These findings suggest that impaired transmission at BC→GC synapses in epileptic rats is attributable to later steps in exocytosis following vesicle docking.

Surviving mossy cells enlarge and receive more excitatory synaptic input in a mouse model of temporal lobe epilepsy

Numerous hypotheses of temporal lobe epileptogenesis have been proposed, and several involve hippocampal mossy cells. Building on previous hypotheses we sought to test the possibility that after epileptogenic injuries surviving mossy cells develop into super-connected seizure-generating hub cells. If so, they might require more cellular machinery and consequently have larger somata, elongate their dendrites to receive more synaptic input, and display higher frequencies of miniature excitatory synaptic currents (mEPSCs). To test these possibilities pilocarpine-treated mice were evaluated using GluR2-immunocytochemistry, whole-cell recording, and biocytin-labeling. Epileptic pilocarpine-treated mice displayed substantial loss of GluR2-positive hilar neurons. Somata of surviving neurons were 1.4-times larger than in controls. Biocytin-labeled mossy cells also were larger in epileptic mice, but dendritic length per cell was not significantly different. The average frequency of mEPSCs of mossy cells recorded in the presence of tetrodotoxin and bicuculline was 3.2-times higher in epileptic pilocarpine-treated mice as compared to controls. Other parameters of mEPSCs were similar in both groups. Average input resistance of mossy cells in epileptic mice was reduced to 63% of controls, which is consistent with larger somata and would tend to make surviving mossy cells less excitable. Other intrinsic physiological characteristics examined were similar in both groups. Increased excitatory synaptic input is consistent with the hypothesis that surviving mossy cells develop into aberrantly super-connected seizure-generating hub cells, and soma hypertrophy is indirectly consistent with the possibility of axon sprouting. However, no obvious evidence of hyperexcitable intrinsic physiology was found. Furthermore, similar hypertrophy and hyper-connectivity has been reported for other neuron types in the dentate gyrus, suggesting mossy cells are not unique in this regard. Thus, findings of the present study reveal epilepsy-related changes in mossy cell anatomy and synaptic input but do not strongly support the hypothesis that mossy cells develop into seizure-generating hub cells.

Hippocampal neuropathology of domoic acid-induced epilepsy in California sea lions (Zalophus californianus)

California sea lions (Zalophus californianus) are abundant human-sized carnivores with large gyrencephalic brains. They develop epilepsy after experiencing status epilepticus when naturally exposed to domoic acid. We tested whether sea lions previously exposed to DA (chronic DA sea lions) display hippocampal neuropathology similar to that of human patients with temporal lobe epilepsy. Hippocampi were obtained from control and chronic DA sea lions. Stereology was used to estimate numbers of Nissl-stained neurons per hippocampus in the granule cell layer, hilus, and pyramidal cell layer of CA3, CA2, and CA1 subfields. Adjacent sections were processed for somatostatin immunoreactivity or Timm-stained, and the extent of mossy fiber sprouting was measured stereologically. Chronic DA sea lions displayed hippocampal neuron loss in patterns and extents similar but not identical to those reported previously for human patients with temporal lobe epilepsy. Similar to human patients, hippocampal sclerosis in sea lions was unilateral in 79% of cases, mossy fiber sprouting was a common neuropathological abnormality, and somatostatin-immunoreactive axons were exuberant in the dentate gyrus despite loss of immunopositive hilar neurons. Thus, hippocampal neuropathology of chronic DA sea lions is similar to that of human patients with temporal lobe epilepsy.

Blockade of excitatory synaptogenesis with proximal dendrites of dentate granule cells following rapamycin treatment in a mouse model of temporal lobe epilepsy

Inhibiting the mammalian target of rapamycin (mTOR) signaling pathway with rapamycin blocks granule cell axon (mossy fiber) sprouting after epileptogenic injuries, including pilocarpine-induced status epilepticus. However, it remains unclear whether axons from other types of neurons sprout into the inner molecular layer and synapse with granule cell dendrites despite rapamycin treatment. If so, other aberrant positive-feedback networks might develop. To test this possibility stereological electron microscopy was used to estimate the numbers of excitatory synapses in the inner molecular layer per hippocampus in pilocarpine-treated control mice, in mice 5 days after pilocarpine-induced status epilepticus, and after status epilepticus and daily treatment beginning 24 hours later with rapamycin or vehicle for 2 months. The optical fractionator method was used to estimate numbers of granule cells in Nissl-stained sections so that numbers of excitatory synapses in the inner molecular layer per granule cell could be calculated. Control mice had an average of 2,280 asymmetric synapses in the inner molecular layer per granule cell, which was reduced to 63% of controls 5 days after status epilepticus, recovered to 93% of controls in vehicle-treated mice 2 months after status epilepticus, but remained at only 63% of controls in rapamycin-treated mice. These findings reveal that rapamycin prevented excitatory axons from synapsing with proximal dendrites of granule cells and raise questions about the recurrent excitation hypothesis of temporal lobe epilepsy.

Does mossy fiber sprouting give rise to the epileptic state?

Many patients with temporal lobe epilepsy display structural changes in the seizure initiating zone, which includes the hippocampus. Structural changes in the hippocampus include granule cell axon (mossy fiber) sprouting. The role of mossy fiber sprouting in epileptogenesis is controversial. A popular view of temporal lobe epileptogenesis contends that precipitating brain insults trigger transient cascades of molecular and cellular events that permanently enhance excitability of neuronal networks through mechanisms including mossy fiber sprouting. However, recent evidence suggests there is no critical period for mossy fiber sprouting after an epileptogenic brain injury. Instead, findings from stereological electron microscopy and rapamycin-delayed mossy fiber sprouting in rodent models of temporal lobe epilepsy suggest a persistent, homeostatic mechanism exists to maintain a set level of excitatory synaptic input to granule cells. If so, a target level of mossy fiber sprouting might be determined shortly after a brain injury and then remain constant. Despite the static appearance of synaptic reorganization after its development, work by other investigators suggests there might be continual turnover of sprouted mossy fibers in epileptic patients and animal models. If so, there may be opportunities to reverse established mossy fiber sprouting. However, reversal of mossy fiber sprouting is unlikely to be antiepileptogenic, because blocking its development does not reduce seizure frequency in pilocarpine-treated mice. The challenge remains to identify which, if any, of the many other structural changes in the hippocampus are epileptogenic.

High-dose rapamycin blocks mossy fiber sprouting but not seizures in a mouse model of temporal lobe epilepsy

PURPOSE

The role of granule cell axon (mossy fiber) sprouting in temporal lobe epileptogenesis is unclear and controversial. Rapamycin suppresses mossy fiber sprouting, but its reported effects on seizure frequency are mixed. The present study used high-dose rapamycin to more completely block mossy fiber sprouting and to measure the effect on seizure frequency.

METHODS

Mice were treated with pilocarpine to induce status epilepticus. Beginning 24 h later and continuing for 2 months, vehicle or rapamycin (10 mg/kg/day) was administered. Starting 1 month after status epilepticus, mice were monitored by video 9 h per day, every day, for 1 month to measure the frequency of spontaneous motor seizures. At the end of seizure monitoring, a subset of mice was prepared for anatomic analysis. Mossy fiber sprouting was measured as the proportion of the granule cell layer and molecular layer that displayed black labeling in Timm-stained sections.

KEY FINDINGS

Extensive mossy fiber sprouting developed in mice that experienced status epilepticus and were treated with vehicle. In rapamycin-treated mice, mossy fiber sprouting was blocked almost to the level of naive controls. Seizure frequency was similar in vehicle-treated and rapamycin-treated mice.

SIGNIFICANCE

These findings suggest that mossy fiber sprouting is not necessary for epileptogenesis in the mouse pilocarpine model. They also reveal that rapamycin does not have antiseizure or antiepileptogenic effects in this model.

Mossy cell dendritic structure quantified and compared with other hippocampal neurons labeled in rats in vivo

Mossy cells are likely to contribute to normal hippocampal function and to the pathogenesis of neurologic disorders that involve the hippocampus, including epilepsy. Mossy cells are the least well-characterized excitatory neurons in the hippocampus. Their somatic and dendritic morphology has been described qualitatively but not quantitatively. In the present study rat mossy cells were labeled intracellularly with biocytin in vivo. Somatic and dendritic structure was reconstructed three-dimensionally. For comparison, granule cells, CA3 pyramidal cells, and CA1 pyramidal cells were labeled and analyzed using the same approach. Among the four types of hippocampal neurons, granule cells had the smallest somata, fewest primary dendrites and dendritic branches, and shortest total dendritic length. CA1 pyramidal cells had the most dendritic branches and longest total dendritic length. Mossy cells and CA3 pyramidal cells both had large somata and similar total dendritic lengths. However, mossy cell dendrites branched less than CA3 pyramidal cells, especially close to the soma. These findings suggest that mossy cells have dendritic features that are not identical to any other type of hippocampal neuron. Therefore, electrotonic properties that depend on soma-dendritic structure are likely to be distinct in mossy cells compared to other neurons.

Identification of new epilepsy treatments: issues in preclinical methodology

Preclinical research has facilitated the discovery of valuable drugs for the symptomatic treatment of epilepsy. Yet, despite these therapies, seizures are not adequately controlled in a third of all affected individuals, and comorbidities still impose a major burden on quality of life. The introduction of multiple new therapies into clinical use over the past two decades has done little to change this. There is an urgent demand to address the unmet clinical needs for: (1) new symptomatic antiseizure treatments for drug-resistant seizures with improved efficacy/tolerability profiles, (2) disease-modifying treatments that prevent or ameliorate the process of epileptogenesis, and (3) treatments for the common comorbidities that contribute to disability in people with epilepsy. New therapies also need to address the special needs of certain subpopulations, that is, age- or gender-specific treatments. Preclinical development in these treatment areas is complex due to heterogeneity in presentation and etiology, and may need to be formulated with a specific seizure, epilepsy syndrome, or comorbidity in mind. The aim of this report is to provide a framework that will help define future guidelines that improve and standardize the design, reporting, and validation of data across preclinical antiepilepsy therapy development studies targeting drug-resistant seizures, epileptogenesis, and comorbidities.

Is there a critical period for mossy fiber sprouting in a mouse model of temporal lobe epilepsy?

PURPOSE

Dentate granule cell axon (mossy fiber) sprouting creates an aberrant positive-feedback circuit that might be epileptogenic. Presumably, mossy fiber sprouting is initiated by molecular signals, but it is unclear whether they are expressed transiently or persistently. If transient, there might be a critical period when short preventative treatments could permanently block mossy fiber sprouting. Alternatively, if signals persist, continuous treatment would be necessary. The present study tested whether temporary treatment with rapamycin has long-term effects on mossy fiber sprouting.

METHODS

Mice were treated daily with 1.5 mg/kg rapamycin or vehicle (i.p.) beginning 24 h after pilocarpine-induced status epilepticus. Mice were perfused for anatomic evaluation immediately after 2 months of treatment ("0 delay") or after an additional 6 months without treatment ("6-month delay"). One series of sections was Timm-stained, and an adjacent series was Nissl-stained. Stereologic methods were used to measure the volume of the granule cell layer plus molecular layer and the Timm-positive fraction. Numbers of Nissl-stained hilar neurons were estimated using the optical fractionator method.

KEY FINDINGS

At 0 delay, rapamycin-treated mice had significantly less black Timm staining in the granule cell layer plus molecular layer than vehicle-treated animals. However, by 6-month delay, Timm staining had increased significantly in mice that had been treated with rapamycin. Percentages of the granule cell layer plus molecular layer that were Timm-positive were high and similar in 0 delay vehicle-treated, 6-month delay vehicle-treated, and 6-month delay rapamycin-treated mice. Extent of hilar neuron loss was similar among all groups that experienced status epilepticus and, therefore, was not a confounding factor. Compared to naive controls, average volume of the granule cell layer plus molecular layer was larger in 0 delay vehicle-treated mice. The hypertrophy was partially suppressed in 0 delay rapamycin-treated mice. However, 6-month delay vehicle- and 6-month delay rapamycin-treated animals had similar average volumes of the granule cell layer plus molecular layer that were significantly larger than those of all other groups.

SIGNIFICANCE

Status epilepticus-induced mossy fiber sprouting and dentate gyrus hypertrophy were suppressed by systemic treatment with rapamycin but resumed after treatment ceased. These findings suggest that molecular signals that drive mossy fiber sprouting and dentate gyrus hypertrophy might persist for >2 months after status epilepticus in mice. Therefore, prolonged or continuous treatment might be required to permanently suppress mossy fiber sprouting.

Rapamycin suppresses axon sprouting by somatostatin interneurons in a mouse model of temporal lobe epilepsy

PURPOSE

In temporal lobe epilepsy many somatostatin interneurons in the dentate gyrus die. However, some survive and sprout axon collaterals that form new synapses with granule cells. The functional consequences of γ-aminobutyric acid (GABA)ergic synaptic reorganization are unclear. Development of new methods to suppress epilepsy-related interneuron axon sprouting might be useful experimentally.

METHODS

Status epilepticus was induced by systemic pilocarpine treatment in green fluorescent protein (GFP)-expressing inhibitory nerurons (GIN) mice in which a subset of somatostatin interneurons expresses GFP. Beginning 24 h later, mice were treated with vehicle or rapamycin (3 mg/kg intraperitoneally) every day for 2 months. Stereologic methods were then used to estimate numbers of GFP-positive hilar neurons per dentate gyrus and total length of GFP-positive axon in the molecular layer plus granule cell layer. GFP-positive axon density was calculated. The number of GFP-positive axon crossings of the granule cell layer was measured. Regression analyses were performed to test for correlations between GFP-positive axon length versus number of granule cells and dentate gyrus volume.

KEY FINDINGS

After pilocarpine-induced status epilepticus, rapamycin- and vehicle-treated mice had approximately half as many GFP-positive hilar neurons as did control animals. Despite neuron loss, vehicle-treated mice had over twice the GFP-positive axon length per dentate gyrus as controls, consistent with GABAergic axon sprouting. In contrast, total GFP-positive axon length was similar in rapamycin-treated mice and controls. GFP-positive axon length correlated most closely with dentate gyrus volume.

SIGNIFICANCE

These findings suggest that rapamycin suppressed axon sprouting by surviving somatostatin/GFP-positive interneurons after pilocarpine-induced status epilepticus in GIN mice. It is unclear whether the effect of rapamycin on axon length was on interneurons directly or secondary, for example, by suppressing growth of granule cell dendrites, which are synaptic targets of interneuron axons. The mammalian target of rapamycin (mTOR) signaling pathway might be a useful drug target for influencing GABAergic synaptic reorganization after epileptogenic treatments, but additional side effects of rapamycin treatment must be considered carefully.

Excitatory input onto hilar somatostatin interneurons is increased in a chronic model of epilepsy

The density of somatostatin (SOM)-containing GABAergic interneurons in the hilus of the dentate gyrus is significantly decreased in both human and experimental temporal lobe epilepsy. We used the pilocarpine model of status epilepticus and temporal lobe epilepsy in mice to study anatomical and electrophysiological properties of surviving somatostatin interneurons and determine whether compensatory functional changes occur that might offset loss of other inhibitory neurons. Using standard patch-clamp techniques and pipettes containing biocytin, whole cell recordings were obtained in hippocampal slices maintained in vitro. Hilar SOM cells containing enhanced green fluorescent protein (EGFP) were identified with fluorescent and infrared differential interference contrast video microscopy in epileptic and control GIN (EGFP-expressing Inhibitory Neurons) mice. Results showed that SOM cells from epileptic mice had 1) significant increases in somatic area and dendritic length; 2) changes in membrane properties, including a small but significant decrease in resting membrane potential, and increases in time constant and whole cell capacitance; 3) increased frequency of slowly rising spontaneous excitatory postsynaptic currents (sEPSCs) due primarily to increased mEPSC frequency, without changes in the probability of release; 4) increased evoked EPSC amplitude; and 5) increased spontaneous action potential generation in cell-attached recordings. Results suggest an increase in excitatory innervation, perhaps on distal dendrites, considering the slower rising EPSCs and increased output of hilar SOM cells in this model of epilepsy. In sum, these changes would be expected to increase the inhibitory output of surviving SOM interneurons and in part compensate for interneuronal loss in the epileptogenic hippocampus.

Initial loss but later excess of GABAergic synapses with dentate granule cells in a rat model of temporal lobe epilepsy

Many patients with temporal lobe epilepsy display neuron loss in the dentate gyrus. One potential epileptogenic mechanism is loss of GABAergic interneurons and inhibitory synapses with granule cells. Stereological techniques were used to estimate numbers of gephyrin-positive punctae in the dentate gyrus, which were reduced short-term (5 days after pilocarpine-induced status epilepticus) but later rebounded beyond controls in epileptic rats. Stereological techniques were used to estimate numbers of synapses in electron micrographs of serial sections processed for postembedding GABA-immunoreactivity. Adjacent sections were used to estimate numbers of granule cells and glutamic acid decarboxylase-positive neurons per dentate gyrus. GABAergic neurons were reduced to 70% of control levels short-term, where they remained in epileptic rats. Integrating synapse and cell counts yielded average numbers of GABAergic synapses per granule cell, which decreased short-term and rebounded in epileptic animals beyond control levels. Axo-shaft and axo-spinous GABAergic synapse numbers in the outer molecular layer changed most. These findings suggest interneuron loss initially reduces numbers of GABAergic synapses with granule cells, but later, synaptogenesis by surviving interneurons overshoots control levels. In contrast, the average number of excitatory synapses per granule cell decreased short-term but recovered only toward control levels, although in epileptic rats excitatory synapses in the inner molecular layer were larger than in controls. These findings reveal a relative excess of GABAergic synapses and suggest that reports of reduced functional inhibitory synaptic input to granule cells in epilepsy might be attributable not to fewer but instead to abundant but dysfunctional GABAergic synapses.

Synaptic input to dentate granule cell basal dendrites in a rat model of temporal lobe epilepsy

In patients with temporal lobe epilepsy some dentate granule cells develop basal dendrites. The extent of excitatory synaptic input to basal dendrites is unclear, nor is it known whether basal dendrites receive inhibitory synapses. We used biocytin to intracellularly label individual granule cells with basal dendrites in epileptic pilocarpine-treated rats. An average basal dendrite had 3.9 branches, was 612 microm long, and accounted for 16% of a cell's total dendritic length. In vivo intracellular labeling and postembedding GABA-immunocytochemistry were used to evaluate synapses with basal dendrites reconstructed from serial electron micrographs. An average of 7% of 1,802 putative synapses were formed by GABA-positive axon terminals, indicating synaptogenesis by interneurons. Ninety-three percent of the identified synapses were GABA-negative. Most GABA-negative synapses were with spines, but at least 10% were with dendritic shafts. Multiplying basal dendrite length/cell and synapse density yielded an estimate of 180 inhibitory and 2,140 excitatory synapses per granule cell basal dendrite. Based on previous estimates of synaptic input to granule cells in control rats, these findings suggest an average basal dendrite receives approximately 14% of the total inhibitory and 19% of excitatory synapses of a cell. These findings reveal that basal dendrites are a novel source of inhibitory input, but they primarily receive excitatory synapses.

Prolonged infusion of inhibitors of calcineurin or L-type calcium channels does not block mossy fiber sprouting in a model of temporal lobe epilepsy

PURPOSE

It would be useful to selectively block granule cell axon (mossy fiber) sprouting to test its functional role in temporal lobe epileptogenesis. Targeting axonal growth cones may be an effective strategy to block mossy fiber sprouting. L-type calcium channels and calcineurin, a calcium-activated phosphatase, are critical for normal growth cone function. Previous studies have provided encouraging evidence that blocking L-type calcium channels or inhibiting calcineurin during epileptogenic treatments suppresses mossy fiber sprouting.

METHODS

Rats were treated systemically with pilocarpine to induce status epilepticus, which lasted at least 2 h. Then, osmotic pumps and cannulae were implanted to infuse calcineurin inhibitors (FK506 or cyclosporin A) or an L-type calcium channel blocker (nicardipine) into the dorsal dentate gyrus. After 28 days of continuous infusion, extent of mossy fiber sprouting was evaluated with Timm staining and stereological methods.

RESULTS

Percentages of volumes of the granule cell layer plus molecular layer that were Timm-positive were similar in infused and noninfused hippocampi.

CONCLUSIONS

These findings suggest inhibiting calcineurin or L-type calcium channels does not block mossy fiber sprouting in the pilocarpine-treated rat model of temporal lobe epilepsy.

Changes in Granule Cell Firing Rates Precede Locally Recorded Spontaneous Seizures by Minutes in an Animal Model of Temporal Lobe Epilepsy

Although much is known about persistent molecular, cellular, and circuit changes associated with temporal lobe epilepsy, mechanisms of seizure onset remain unclear. The dentate gyrus displays many persistent epilepsy-related abnormalities and is in the mesial temporal lobe where seizures initiate in patients. However, little is known about seizure-related activity of individual neurons in the dentate gyrus. We used tetrodes to record action potentials of multiple, single granule cells before and during spontaneous seizures in epileptic pilocarpine-treated rats. Subsets of granule cells displayed four distinct activity patterns: increased firing before seizure onset, decreased firing before seizure onset, increased firing only after seizure onset, and unchanged firing rates despite electrographic seizure activity in the immediate vicinity. No cells decreased firing rate immediately after seizure onset. During baseline periods between seizures, action potential waveforms and firing rates were similar among the four subsets of granule cells in epileptic rats and in granule cells of control rats. The mean normalized firing rate of granule cells whose firing rates increased before seizure onset deviated from baseline earliest, beginning 4 min before dentate gyrus electrographic seizure onset, and increased progressively, more than doubling by seizure onset. It is generally assumed that neuronal firing rates increase abruptly and synchronously only when electrographic seizures begin. However, these findings show heterogeneous and gradually building changes in activity of individual granule cells minutes before spontaneous seizures.

Recurrent circuits in layer II of medial entorhinal cortex in a model of temporal lobe epilepsy

Patients and laboratory animal models of temporal lobe epilepsy display loss of layer III pyramidal neurons in medial entorhinal cortex and hyperexcitability and hypersynchrony of less vulnerable layer II stellate cells. We sought to test the hypothesis that loss of layer III pyramidal neurons triggers synaptic reorganization and formation of recurrent, excitatory synapses among layer II stellate cells in epileptic pilocarpine-treated rats. Laser-scanning photo-uncaging of glutamate focally activated neurons in layer II while excitatory synaptic responses were recorded in stellate cells. Photostimulation revealed previously unidentified, functional, recurrent, excitatory synapses between layer II stellate cells in control animals. Contrary to the hypothesis, however, control and epileptic rats displayed similar levels of recurrent excitation. Recently, hyperexcitability of layer II stellate cells has been attributed, at least in part, to loss of GABAergic interneurons and inhibitory synaptic input. To evaluate recurrent inhibitory circuits in layer II, we focally photostimulated interneurons while recording inhibitory synaptic responses in stellate cells. IPSCs were evoked more than five times more frequently in slices from control versus epileptic animals. These findings suggest that in this model of temporal lobe epilepsy, reduced recurrent inhibition contributes to layer II stellate cell hyperexcitability and hypersynchrony, but increased recurrent excitation does not.

Neuron-specific nuclear antigen NeuN is not detectable in gerbil subtantia nigra pars reticulata

NeuN (Neuronal Nuclei), the neuron-specific marker of nuclear protein is used extensively in histological procedures to identify major cell-types in adult vertebrate nervous systems of a variety of species including rodents and humans. Some notable exceptions (i.e., NeuN-negative neurons) include Purkinje cells in cerebellum, mitral cells in olfactory bulb, and photoreceptors in retina. Here we report that neurons in gerbil (Meriones unguiculatus) substantia nigra pars reticulata (SNr), whose "neuronal" phenotype was confirmed via electrophysiology, biocytin-labeling, histology, and in situ hybridization, are also devoid of NeuN-immunoreactivity as assayed with the widely used monoclonal antibody A60. Immunohistochemistry of rat SNr using the same antibody yielded robust staining. These data suggest lack of NeuN-immonoreactivity observed in certain cell-types and brain regions can be species-specific.

Hyperexcitability, interneurons, and loss of GABAergic synapses in entorhinal cortex in a model of temporal lobe epilepsy

Temporal lobe epilepsy is the most common type of epilepsy in adults, and its pathophysiology remains unclear. Layer II stellate cells of the entorhinal cortex, which are hyperexcitable in animal models of temporal lobe epilepsy, provide the predominant synaptic input to the hippocampal dentate gyrus. Previous studies have ascribed the hyperexcitability of layer II stellate cells to GABAergic interneurons becoming "dormant" after disconnection from their excitatory synaptic inputs, which has been reported to occur during preferential loss of layer III pyramidal cells. We used whole-cell recording from slices of entorhinal cortex in pilocarpine-treated epileptic rats to test the dormant interneuron hypothesis. Hyperexcitability appeared as multiple action potentials and prolonged depolarizations evoked in layer II stellate cells of epileptic rats but not controls. However, blockade of glutamatergic synaptic transmission caused similar percentage reductions in the frequency of spontaneous IPSCs in layer II stellate cells of control and epileptic rats, suggesting similar levels of excitatory synaptic input to GABAergic interneurons. Direct recordings and biocytin labeling revealed two major types of interneurons in layer III whose excitatory synaptic drive in epileptic animals was undiminished. Interneurons in layer III did not appear to be dormant; therefore, we tested whether loss of GABAergic synapses might underlie hyperexcitability of layer II stellate cells. Stereological evidence of fewer GABAergic interneurons, fewer gephyrin-immunoreactive punctae, and reduced frequency of spontaneous IPSCs and miniature IPSCs (recorded in tetrodotoxin) confirmed that layer II stellate cell hyperexcitability is attributable, at least in part, to reduced inhibitory synaptic input.

GABAA receptor-mediated IPSCs and alpha1 subunit expression are not reduced in the substantia nigra pars reticulata of gerbils with inherited epilepsy

Domestic Mongolian gerbils, a model of inherited epilepsy, begin having spontaneous seizures at approximately 1.5 mo of age, making it possible to evaluate them during epileptic and pre-epileptic stages. Previous studies have shown that GABA binding is reduced in the substantia nigra pars reticulata (SNr) of both epileptic and pre-epileptic gerbils compared with controls, suggesting that reduced expression of GABAA receptors in SNr might be epileptogenic in this model. To test this hypothesis, we measured the expression of the GABAA receptor alpha1 subunit, the dominant alpha subunit expressed in the SNr, and evaluated GABAA receptor-mediated postsynaptic currents in SNr neurons. GABA(A) alpha1 subunit mRNA levels in substantia nigra-rich tissue from pre-epileptic animals were similar to controls, and immunocytochemistry for the alpha1 subunit showed similar strong expression in the SNr in both groups. Western analysis confirmed that expression of the alpha1 subunit protein was similar in substantia nigra-rich tissue from pre-epileptic and control gerbils. The frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) and the frequency of miniature (m)IPSCs in SNr neurons of pre-epileptic gerbil were similar to those of controls. The amplitude of mIPSCs in the pre-epileptics was significantly larger than controls. Zolpidem, an alpha1 subunit-specific modulator of the GABAA receptor, was equally efficacious in prolonging the decay time of mIPSCs in both groups. Hence, contrary to the predictions of the hypothesis, mRNA and protein expression levels of the major GABAA receptor alpha subunit were normal, and neurons of the SNr in pre-epileptic gerbils displayed normal or enhanced IPSC frequencies and amplitudes. Therefore reduced expression of GABAA receptors in SNr is not likely to be an epileptogenic mechanism in this model.

Stereological analysis of forebrain regions in kainate-treated epileptic rats

Patients and models of temporal lobe epilepsy display neuron loss in the hippocampal formation, but neuropathological changes also occur in other forebrain regions. We sought to evaluate the specificity and extent of volume loss of the major forebrain regions in epileptic rats months after kainate-induced status epilepticus. In systematic series of Nissl-stained sections, the areas of major forebrain regions were measured, and volumes were estimated using the Cavalieri principle. In some regions, the optical fractionator method was used to estimate neuron numbers. Most kainate-treated rats showed significant volume loss in the amygdala, olfactory cortex, and septal region, but others displayed different patterns, with significant loss only in the hippocampus or thalamus, for example. Average volume loss was most severe in the amygdala and olfactory cortex (82-83% of controls), especially the caudal parts of both regions. In the piriform cortex (including the endopiriform nucleus) of epileptic rats, an average of approximately one-third of Nissl-stained neurons and one-third of the GABAergic interneurons labeled by in situ hybridization for GAD67 mRNA were lost, and the extent of neuron loss was correlated with the extent of volume loss. Volumetric analysis of major forebrain regions was insensitive to specific neuron loss in subregions such as layer III of the entorhinal cortex and the hilus of the dentate gyrus. These findings provide quantitative evidence that kainate-treated rats tend to display extensive neuron and volume loss in the amygdala and olfactory cortex, although the patterns and extent of loss in forebrain regions vary considerably among individuals. In this status epilepticus-based model, extrahippocampal damage appears to be more extensive and hippocampal damage appears to be less extensive than that reported for patients with temporal lobe epilepsy.

Prolonged infusion of cycloheximide does not block mossy fiber sprouting in a model of temporal lobe epilepsy

PURPOSE

The role of protein synthesis in mossy fiber sprouting is unclear. Conflicting reports exist on whether a single dose of the protein synthesis-blocker cycloheximide administered around the time of an epileptogenic injury can block the eventual development of mossy fiber sprouting.

METHODS

In rats, osmotic minipumps and cannulae were implanted to deliver 8 mg/ml cycloheximide to one dentate gyrus and vehicle to the other. This method has been used to block protein synthesis in the infused region for up to 5 days with minimal neurotoxic effects (Taha and Stryker, Neuron 2002;34:425-36). After 2 days of infusion, rats were treated with pilocarpine to induce status epilepticus. Pumps were removed 3 days later. Thirty days after pilocarpine treatment, rats were perfused, and hippocampal sections were processed for Timm staining.

RESULTS

Timm staining revealed aberrant mossy fiber sprouting in the inner molecular layer regardless of whether hippocampi were treated with cycloheximide or vehicle. Cycloheximide-treated hippocampi displayed more aberrant Timm staining and more tissue damage around the infusion site than did vehicle-treated hippocampi.

CONCLUSIONS

Prolonged infusion of cycloheximide, spanning the period of pilocarpine treatment, did not block mossy fiber sprouting. This finding suggests that protein-dependent mechanisms around the time of an epileptogenic injury are not necessary for the eventual development of synaptic reorganization.

Does a unique type of CA3 pyramidal cell in primates bypass the dentate gate?

The predominant excitatory synaptic input to the hippocampus arises from entorhinal cortical axons that synapse with dentate granule cells, which in turn synapse with CA3 pyramidal cells. Thus two highly excitable brain areas--the entorhinal cortex and the CA3 field--are separated by dentate granule cells, which have been proposed to function as a gate or filter. However, unlike rats, primates have "dentate" CA3 pyramidal cells with an apical dendrite that extends into the molecular layer of the dentate gyrus, where they could receive strong, monosynaptic, excitatory synaptic input from the entorhinal cortex. To test this possibility, the dentate gyrus molecular layer was stimulated while intracellular recordings were obtained from CA3 pyramidal cells in hippocampal slices from neurologically normal macaque monkeys. Stimulus intensity of the outer molecular layer of the dentate gyrus was standardized by the threshold intensity for evoking a dentate gyrus field potential population spike. Recorded proximal CA3 pyramidal cells were labeled with biocytin, processed with diaminobenzidine for visualization, and classified according to their dendritic morphology. In response to stimulation of the dentate gyrus molecular layer, action potential thresholds were similar in proximal CA3 pyramidal cells with different dendritic morphologies. These findings do not support the hypothesis that dentate CA3 pyramidal cells receive stronger synaptic input from the entorhinal cortex than do other proximal CA3 pyramidal cells.

Laboratory animal models of temporal lobe epilepsy

Temporal lobe epilepsy is a common human disease that is difficult to treat. The pathogenesis of temporal lobe epilepsy, which holds many unresolved questions, and opportunities for creating more effective treatments and preventative strategies are reviewed herein. Laboratory animal models are essential to meet these challenges. How models are created, how they compare with each other and with the disease in human patients, and how they advance our understanding of temporal lobe epilepsy are described.

Prolonged infusion of tetrodotoxin does not block mossy fiber sprouting in pilocarpine-treated rats

PURPOSE

Mossy fiber sprouting is a common abnormality found in patients and models of temporal lobe epilepsy. The role of mossy fiber sprouting in epileptogenesis is unclear, and its blockade would be useful experimentally and perhaps therapeutically. Results from previous attempts to block mossy fiber sprouting have been disappointing or controversial. In some brain regions, prolonged application of the sodium channel blocker tetrodotoxin prevents axon sprouting and posttrauma epileptogenesis. The present study tested the hypothesis that prolonged, focal infusion of tetrodotoxin would block mossy fiber sprouting after an epileptogenic treatment.

METHODS

Adult rats were treated with pilocarpine to induce status epilepticus. Several hours to 3 days after pilocarpine treatment, a pump with a cannula directed toward the dentate gyrus was implanted to deliver 10 microM tetrodotoxin or vehicle alone at 0.25 microl/h. This method blocks local EEG activity in the hippocampus (Galvan et al. J Neurosci 2000; 20:2904-16). After 28 days of continuous infusion, rats were perfused with fixative, and their hippocampi analyzed anatomically with stereologic techniques.

RESULTS

Tetrodotoxin infusion was verified immunocytochemically in tetrodotoxin-treated but not vehicle-treated hippocampi. Tetrodotoxin-infused and vehicle-infused hippocampi displayed similar levels of hilar neuron loss. The Timm stain revealed mossy fiber sprouting regardless of whether hippocampi were treated with tetrodotoxin infusion, vehicle infusion, or neither.

CONCLUSIONS

Prolonged infusion of tetrodotoxin did not block mossy fiber sprouting. This finding suggests that sodium channel-mediated neuronal activity is not necessary for mossy fiber sprouting after an epileptogenic treatment.

Dendritic morphology, local circuitry, and intrinsic electrophysiology of principal neurons in the entorhinal cortex of macaque monkeys

Little is known about the neuroanatomical or electrophysiological properties of individual neurons in the primate entorhinal cortex. We have used intracellular recording and biocytin-labeling techniques in the entorhinal slice preparation from macaque monkeys to investigate the morphology and intrinsic electrophysiology of principal neurons. These neurons have previously been studied most extensively in rats. In monkeys, layer II neurons are usually stellate cells, as in rats, but they occasionally have a pyramidal shape. They tend to discharge trains, not bursts, of action potentials, and some display subthreshold membrane potential oscillations. Layer III neurons are pyramidal, and they do not appear to display membrane potential oscillations. The distribution of dendrites and of axon collaterals suggests that neurons in layers II and III are interconnected by a network of associational fibers. Layer V and VI neurons are pyramidal and tend to discharge trains of action potentials. The distribution of dendrites and axon collaterals suggests that there is an associative network of principal neurons in layers V and VI, and they also project axon collaterals toward superficial layers. Importantly, entorhinal cortical neurons in monkeys appear to exhibit significant differences from those in rats. Morphologically, neurons in monkey entorhinal layers II and III have more primary dendrites, more dendritic branches, and greater total dendritic length than in rats. Electrophysiologically, layer II neurons in monkeys exhibit less sag, and subthreshold oscillations are less robust and slower. Some monkey layer III neurons discharge bursts of action potentials that are not found in rats. The interspecies differences revealed by this study may influence information processing and pathophysiological processes in the primate entorhinal cortex. J. Comp. Neurol. 470:317-329, 2004.

Recurrent excitation of granule cells with basal dendrites and low interneuron density and inhibitory postsynaptic current frequency in the dentate gyrus of macaque monkeys

Temporal lobe epilepsy is often associated with pathological changes in the dentate gyrus, and such changes may be more common in humans than in some nonprimate species. To examine species-specific characteristics that might predispose the dentate gyrus to epileptogenic damage, we evaluated recurrent excitation of granule cells with and without basal dendrites in macaque monkeys, measured miniature inhibitory postsynaptic currents (mIPSCs) of granule cells in macaque monkeys and compared them to rats, and estimated the granule cell-to-interneuron ratio in macaque monkeys and rats. In hippocampal slices from monkeys, whole-cell patch recording revealed antidromically evoked excitatory PSCs that were four times larger and inhibitory PSCs that were over two times larger in granule cells with basal dendrites than without. These findings suggest that granule cells with basal dendrites receive more recurrent excitation and, to a lesser degree, more recurrent inhibition. Miniature IPSC amplitude was slightly larger in monkey granule cells with basal dendrites than in those without, but mIPSC frequency was similar and only 26% of that reported for rats. In situ hybridization for glutamic acid decarboxylase and immunocytochemistry for somatostatin, parvalbumin, and neuronal nuclei revealed interneuron proportions and distributions in monkeys that were similar to those reported for rats. However, the interneuron-to-granule cell ratio was lower in monkeys (1:28) than in rats (1:11). These findings suggest that in the primate dentate gyrus, recurrent excitation is enhanced and inhibition is reduced compared with rodents. These primate characteristics may contribute to the susceptibility of the human dentate gyrus to epileptogenic injuries.

Reduced inhibition and increased output of layer II neurons in the medial entorhinal cortex in a model of temporal lobe epilepsy

Temporal lobe epilepsy is the most common type of epilepsy in adults, and its underlying mechanisms are unclear. To investigate how the medial entorhinal cortex might contribute to temporal lobe epilepsy, we evaluated the histology and electrophysiology of slices from rats 3-7 d after an epileptogenic injury (pilocarpine-induced status epilepticus). Nissl staining, NeuN immunocytochemistry, and in situ hybridization for GAD65 mRNA were used to verify the preferential loss of glutamatergic neurons and the relative sparing of GABAergic interneurons in layer III. From slices adjacent to those that were used for anatomy, we obtained whole-cell patch recordings from layer II medial entorhinal cortical neurons. Recordings under current-clamp conditions revealed similar intrinsic electrophysiological properties (resting membrane potential, input resistance, single spike, and repetitive firing properties) to those of controls. Spontaneous IPSCs were less frequent (68% of controls), smaller in amplitude (57%), and transferred less charge (51%) than in controls. However, the frequency, amplitude, and rise time of miniature IPSCs were normal. These findings suggest that after epileptogenic injuries the layer II entorhinal cortical neurons receive less GABA(A) receptor-mediated synaptic input because presynaptic inhibitory interneurons become less active. To investigate the possible consequences of reduced spontaneous inhibitory input to layer II neurons, we recorded field potentials in the dentate gyrus, their major synaptic target. At 5 d after pilocarpine-induced status epilepticus the spontaneous field potentials recorded in vivo were over three times more frequent than in controls. These findings suggest that an epileptogenic injury reduces inhibition of layer II neurons and results in excessive synaptic input to the dentate gyrus.

Reduced inhibition of dentate granule cells in a model of temporal lobe epilepsy

Patients and models of temporal lobe epilepsy have fewer inhibitory interneurons in the dentate gyrus than controls, but it is unclear whether granule cell inhibition is reduced. We report the loss of GABAergic inhibition of granule cells in the temporal dentate gyrus of pilocarpine-induced epileptic rats. In situ hybridization for GAD65 mRNA and immunocytochemistry for parvalbumin and somatostatin confirmed the loss of inhibitory interneurons. In epileptic rats, granule cells had prolonged EPSPs, and they discharged more action potentials than controls. Although the conductances of evoked IPSPs recorded in normal ACSF were not significantly reduced and paired-pulse responses showed enhanced inhibition of granule cells from epileptic rats, more direct measures of granule cell inhibition revealed significant deficiencies. In granule cells from epileptic rats, evoked monosynaptic IPSP conductances were <40% of controls, and the frequency of GABA(A) receptor-mediated spontaneous and miniature IPSCs (mIPSCs) was <50% of controls. Within 3-7 d after pilocarpine-induced status epilepticus, miniature IPSC frequency had decreased, and it remained low, without functional evidence of compensatory synaptogenesis by GABAergic axons in chronically epileptic rats. Both parvalbumin- and somatostatin-immunoreactive interneuron numbers and the frequency of both fast- and slow-rising GABA(A) receptor-mediated mIPSCs were reduced, suggesting that loss of inhibitory synaptic input to granule cells occurred at both proximal/somatic and distal/dendritic sites. Reduced granule cell inhibition in the temporal dentate gyrus preceded the onset of spontaneous recurrent seizures by days to weeks, so it may contribute, but is insufficient, to cause epilepsy.

BDNF overexpression increases dendrite complexity in hippocampal dentate gyrus

There is increasing evidence that brain-derived neurotrophic factor (BDNF) modulates synaptic and morphological plasticity in the developing and mature nervous system. Plasticity may be modulated partially by BDNF's effects on dendritic structure. Utilizing transgenic mice where BDNF overexpression was controlled by the beta-actin promoter, we evaluated the effects of long-term overexpression of BDNF on the dendritic structure of granule cells in the hippocampal dentate gyrus. BDNF transgenic mice provided the opportunity to investigate the effects of modestly increased BDNF levels on dendrite structure in the complex in vivo environment. While the elevated BDNF levels were insufficient to change levels of TrkB receptor isoforms or downstream TrkB signaling, they did increase dendrite complexity of dentate granule cells. These cells showed an increased number of first order dendrites, of total dendritic length and of total number of branch points. These results suggest that dendrite structure of granule cells is tightly regulated and is sensitive to modest increases in levels of BDNF. This is the first study to evaluate the effects of BDNF overexpression on dendrite morphology in the intact hippocampus and extends previous in vitro observations that BDNF influences synaptic plasticity by increasing complexity of dendritic arbors.

Evoked responses of the dentate gyrus during seizures in developing gerbils with inherited epilepsy

When they are 1-2 mo old, domesticated Mongolian gerbils begin having initially mild seizures which become more severe with age. To evaluate the development of this increasing seizure severity, we obtained field potential responses of the dentate gyrus to paired-pulse stimulation of the perforant path during seizures. In 18 gerbils that were 1.5-8.0 mo old, 73 seizures were analyzed. We measured population spike amplitude, the slope of the field excitatory postsynaptic potential (fEPSP), and the population spike amplitude ratio (2nd/1st) to evaluate excitatory and inhibitory synaptic processes. In gerbils <2 mo old, exposure to a novel environment was followed by an increase in population spike amplitude and then by seizure onset, but population spike amplitude ratio and fEPSP slope remained at baseline levels, and multiple population spikes were never evoked. As previously reported for chronically epileptic gerbils, these findings provide little evidence of a disinhibitory seizure-initiating mechanism in the dentate gyrus when young gerbils begin having seizures. In young gerbils evoked responses changed little during the behaviorally mild seizures. In contrast, most seizures in older gerbils included generalized convulsions, postictal depression, and evoked responses that changed dramatically. In older gerbils, shortly after seizure onset the dentate gyrus became hyperexcitable. Population spike amplitude and fEPSP slope peaked, and multiple population spikes were evoked, suggesting that mechanisms for seizure amplification and spread are more developed in older gerbils. Next, dentate gyrus excitability decreased precipitously, and population spike amplitude and fEPSP slope diminished. This period of hypoexcitability began before the end of the seizure, suggesting it may contribute to seizure termination. After the convulsive phase of the seizure, older gerbils remained motionless during a period of postictal depression, and population spike amplitude remained suppressed until the abrupt switch to normal exploratory activity. These findings suggest that the mechanisms of postictal depression may suppress granule cell excitability. The population spike amplitude ratio peaked after the convulsive phase and then gradually returned to the baseline level an average of 12 min after seizure onset, suggesting that granule cell inhibition recovers within minutes after a spontaneous seizure. Although it is unclear whether the seizure-related changes in evoked responses are a cause or an effect of increased seizure severity in older gerbils, their analysis provides clues about developmental changes in the mechanisms of seizure spread and termination.

Axon arbors and synaptic connections of a vulnerable population of interneurons in the dentate gyrus in vivo

The predominant gamma-aminobutyric acid (GABA)ergic neuron class in the hilus of the dentate gyrus consists of spiny somatostatinergic interneurons. We examined the axon projections and synaptic connections made by spiny hilar interneurons labeled with biocytin in gerbils in vivo. Axon length was 152-497 mm/neuron. Sixty to 85% of the axon concentrated in the outer two thirds of the molecular layer of the dentate gyrus. The septotemporal span of the axon arbor extended over 48-82% of the total hippocampal length, which far exceeds the septotemporal span of axons of granule cells whose complete axon arbors extended over 15-29%. A three-dimensionally reconstructed 216-microm-long spiny hilar interneuron axon segment in the outer third of the molecular layer formed an average of 1 synapse every 5.1 microm. Of the 42 symmetric (inhibitory) synapses formed by the reconstructed segment, 88% were with spiny dendrites of presumed granule cells, and 67% were with dendritic spines that also receive an asymmetric (excitatory) contact from an unlabeled axon terminal. Postembedding GABA-immunocytochemistry revealed that 55% of the GABAergic synapses in the outer third of the molecular layer were with spines. Therefore, in the outer molecular layer, spiny hilar interneurons form synaptic contacts that appear to be positioned to exert inhibitory control near sites of excitatory synaptic input from the entorhinal cortex to granule cell dendritic spines. These findings demonstrate far-reaching, yet highly specific, connectivity of individual interneurons and suggest that the loss of spiny hilar interneurons, as occurs in temporal lobe epilepsy, may contribute to hyperexcitability in the hippocampus.

Somatostatin-immunoreactive interneurons contribute to lateral inhibitory circuits in the dentate gyrus of control and epileptic rats

Lateral inhibition, a feature of neuronal circuitry that enhances signaling specificity, has been demonstrated in the rat dentate gyrus. However, neither the underlying neuronal circuits, nor the ways in which these circuits are altered in temporal lobe epilepsy, are completely understood. This study examines the potential contribution of one class of inhibitory interneurons to lateral inhibitory circuits in the dentate gyrus of both control and epileptic rats. The retrograde tracer wheat germ ag-glutinin-apo-horse radish peroxidase-gold (WGA-apo-HRP-gold) was injected into the septal dentate gyrus. Neurons double-labeled for glutamic acid decarboxylase (GAD) and the retrograde tracer are concentrated in the hilus and may contribute to lateral inhibition. Neurons double-labeled for somatostatin and the retrograde tracer account for at least 28% of GAD-positive neurons with axon projections appropriate for generating lateral inhibition in control rats. Despite an overall loss of somatostatin-expressing cells in epileptic animals, the number of somatostatin-positive interneurons with axon projections appropriate for generating lateral inhibition is similar to that seen in controls. These findings suggest that somatostatinergic interneurons participate in lateral inhibitory circuits in the dentate gyrus of both control and epileptic rats, and that surviving somatostatinergic interneurons might sprout new axon collaterals in epileptic animals.

Heightened seizure severity in somatostatin knockout mice

Patients and experimental models of temporal lobe epilepsy display loss of somatostatinergic neurons in the dentate gyrus. To determine if loss of the peptide somatostatin contributes to epileptic seizures we examined kainate-evoked seizures and kindling in somatostatin knockout mice. Somatostatin knockout mice were not observed to experience spontaneous seizures. Timm staining, acetylcholinesterase histochemistry, and immunocytochemistry for NPY, calbindin, calretinin, and parvalbumin revealed no compensatory changes or developmental abnormalities in the dentate gyrus of somatostatin knockout mice. Optical fractionator counting of Nissl-stained hilar neurons showed similar numbers of neurons in wild type and somatostatin knockout mice. Mice were treated systemically with kainic acid to evoke limbic seizures. Somatostatin knockout mice tended to have a shorter average latency to stage 5 seizures, their average maximal behavioral seizure score was higher, and they tended to be more likely to die than controls. In response to kindling by daily electrical stimulation of the perforant path, to more specifically challenge the dentate gyrus, mean afterdischarge duration in somatostatin knockout mice was slightly longer, but the number of treatments to five stage 4-5 seizures was similar to controls. Although we cannot exclude the possibility of undetected compensatory mechanisms in somatostatin knockout mice, these findings suggest that somatostatin may be mildly anticonvulsant, but its loss alone is unlikely to account for seizures in temporal lobe epilepsy.

Absence of temporal lobe epilepsy pathology in dogs with medically intractable epilepsy

Epilepsy is a common neurological problem in dogs. In some dogs, seizures cannot be controlled adequately with anticonvulsant medication. Temporal lobe epilepsy is the most common type of epilepsy in adult humans, it is frequently resistant to anticonvulsant therapy, and it is commonly associated with characteristic neuropathological abnormalities in the hippocampal dentate gyrus. We sought to test the hypothesis that dogs with medically intractable epilepsy have temporal lobe epilepsy. The hippocampi of 6 dogs that were euthanized because of chronic, recurrent seizures were compared with those of 8 nonepileptic controls. In control and epileptic dogs, stereological cell counting showed similar numbers of neurons in the hilus of the dentate gyrus, somatostatin immunoreactivity identified numerous immunopositive neurons in the hilus, and Timm staining showed the normal pattern of granule cell axon projections. These findings demonstrate a lack of hilar neuron loss and granule cell axon reorganization, suggesting that temporal lobe epilepsy is not a common cause of medically intractable epilepsy in dogs.

Intracellular recording and labeling of mossy cells and proximal CA3 pyramidal cells in macaque monkeys

Little is known about the morphological characteristics and intracellular electrophysiological properties of neurons in the primate hippocampus and dentate gyrus. We have therefore begun a program of studies using intracellular recording and biocytin labeling in hippocampal slices from macaque monkeys. In the current study, we investigated mossy cells and proximal CA3 pyramidal cells. As in rats, macaque mossy cells display fundamentally different traits than proximal CA3 pyramidal cells. Interestingly, macaque mossy cells and CA3 pyramidal neurons display some morphological differences from those in rats. Macaque monkey mossy cells extend more dendrites into the molecular layer of the dentate gyrus, have more elaborate thorny excrescences on their proximal dendrites, and project more axon collaterals into the CA3 region. In macaques, three types of proximal CA3 pyramidal cells are found: classical pyramidal cells, neurons with their dendrites confined to the CA3 pyramidal cell layer, and a previously undescribed cell type, the "dentate" CA3 pyramidal cell, whose apical dendrites extend into and ramify within the hilus, granule cell layer, and molecular layer of the dentate gyrus. The basic electrophysiological properties of mossy cells and proximal CA3 cells are similar to those reported for the rodent. Mossy cells have a higher frequency of large amplitude spontaneous depolarizing postsynaptic potentials, and proximal CA3 pyramidal cells are more likely to discharge bursts of action potentials. Although mossy cells and CA3 pyramidal cells in macaque monkeys display many morphological and electrophysiological features described in rodents, these findings highlight significant species differences, with more heterogeneity and the potential for richer interconnections in the primate hippocampus.