Increased seizure susceptibility of the hippocampus compared with the neocortex of the immature mouse brain in vitro

Increased risk of epilepsy after transient global amnesia: A population-based study in South Korea

PURPOSE

This study aimed to investigate the risk of epilepsy after transient global amnesia (TGA).

METHODS

Study population was recruited using the International Classification of Diseases codes from the Korean National Health Insurance Service database between 2002 and 2020. The incidence of epilepsy was compared between the TGA (n=12,390) and non-TGA (n=33,868) groups, determined using 1:3 propensity score matching. Using Cox proportional hazard regression model, we obtained adjusted hazard ratios (aHRs) and 95% confidence intervals (CIs) for incident epilepsy in the TGA compared with non-TGA group. Logistic regression was performed to examine the independent variables determining incident epilepsy in the TGA group, and adjusted odds ratios (aORs) and 95% CIs were calculated.

RESULTS

The TGA group had a significantly higher cumulative incidence of epilepsy than controls (p <0.001, log-rank test). TGA was significantly associated with incident epilepsy in the Cox model (adjusted HR 1.46, 95% CI 1.36-1.56). The adjusted logistic regression showed that age (per 1 year, aOR 1.02, 95% CI 1.01-1.02), female sex (aOR 0.68, 95% CI 0.60-0.77), hypertension (aOR 1.14, 95% CI 1.00-1.30), diabetes (aOR 1.26, 95% CI 1.10-1.44), stroke (aOR 1.22, 95% CI 1.06-1.40), depression (aOR 1.44, 95% CI 1.22-1.69), anxiety (aOR 1.31, 95% CI 1.14-1.51), alcohol-related disease (aOR 1.96, 95% CI 1.38-2.78), low income (aOR 1.18, 95% CI 1.02-1.36) and rural residence (aOR 1.20, 95% CI 1.02-1.42) were associated with incident epilepsy.

CONCLUSIONS

Our results suggest a longitudinal association of TGA with incident epilepsy.

Axonal connections between S1 barrel, M1, and S2 cortex in the newborn mouse

The development of functionally interconnected networks between primary (S1), secondary somatosensory (S2), and motor (M1) cortical areas requires coherent neuronal activity via corticocortical projections. However, the anatomical substrate of functional connections between S1 and M1 or S2 during early development remains elusive. In the present study, we used ex vivo carbocyanine dye (DiI) tracing in paraformaldehyde-fixed newborn mouse brain to investigate axonal projections of neurons in different layers of S1 barrel field (S1Bf), M1, and S2 toward the subplate (SP), a hub layer for sensory information transfer in the immature cortex. In addition, we performed extracellular recordings in neocortical slices to unravel the functional connectivity between these areas. Our experiments demonstrate that already at P0 neurons from the cortical plate (CP), layer 5/6 (L5/6), and the SP of both M1 and S2 send projections through the SP of S1Bf. Reciprocally, neurons from CP to SP of S1Bf send projections through the SP of M1 and S2. Electrophysiological recordings with multi-electrode arrays in cortical slices revealed weak, but functional synaptic connections between SP and L5/6 within and between S1 and M1. An even lower functional connectivity was observed between S1 and S2. In summary, our findings demonstrate that functional connections between SP and upper cortical layers are not confined to the same cortical area, but corticocortical connection between adjacent cortical areas exist already at the day of birth. Hereby, SP can integrate early cortical activity of M1, S1, and S2 and shape the development of sensorimotor integration at an early stage.

Hypoglycaemic events resembling focal seizures -A case report and literature review

PURPOSE

To review the literature, for cases of hypoglycaemia misdiagnosed as epilepsy, including our interesting case of a patient with Type 1 Diabetes Mellitus, diagnosed with focal epilepsy.

METHODS

A literature search was completed. 20 of 473 studies, with a total of 22 cases found using specified search terms were relevant to this review. The papers identified and reviewed were those that dealt with hypoglycaemia misdiagnosed as epilepsy. The majority are isolated case reports given the rarity of this entity.

RESULTS

An underlying insulinoma is the most common cause for hypoglycaemic episodes to be misdiagnosed as epilepsy. Early morning seizures were prominent in 9 of the 22 cases.

CONCLUSION

Although rare, hypoglycaemia is an important differential diagnosis for drug-resistant epilepsy and early morning events may be an indication. We report the first case of recurrent hypoglycaemia from exogenous insulin, misdiagnosed as focal epilepsy with an available video EEG. The unusual presentation appeared clinically indistinct from recurrent focal seizures.

Potentiating Hemorrhage in a Periadolescent Rat Model of Closed-Head Traumatic Brain Injury Worsens Hyperexcitability but Not Behavioral Deficits

Post-traumatic epilepsy (PTE) and neurocognitive deficits are devastating sequelae of head injuries that are common in adolescents. Investigating desperately needed treatments is hindered by the difficulties in inducing PTE in rodents and the lack of established immature rat models of pediatric PTE. Hemorrhage is a significant risk factor for PTE, but compared to humans, rats are less prone to bleeding because of their rapid blood coagulation system. In this study, we promoted bleeding in the controlled cortical impact (CCI) closed-head injury model with a 20 min pre-impact 600 IU/kg intraperitoneal heparin injection in postnatal day 35 (P35) periadolescent rats, given the preponderance of such injuries in this age group. Temporo-parietal CCI was performed post-heparin (HTBI group) or post-saline (TBI group). Controls were subjected to sham procedures following heparin or saline administration. Continuous long-term EEG monitoring was performed for 3 months post-CCI. Sensorimotor testing, the Morris water maze, and a modified active avoidance test were conducted between P80 and P100. Glial fibrillary acidic protein (GFAP) levels and neuronal damage were also assessed. Compared to TBI rats, HTBI rats had persistently higher EEG spiking and increased hippocampal GFAP levels (p < 0.05). No sensorimotor deficits were detected in any group. Compared to controls, both HTBI and TBI groups had a long-term hippocampal neuronal loss (p < 0.05), as well as contextual and visuospatial learning deficits (p < 0.05). The hippocampal astrogliosis and EEG spiking detected in all rats subjected to our hemorrhage-promoting procedure suggest the emergence of hyperexcitable networks and pave the way to a periadolescent PTE rat model.

Transitions between neocortical seizure and non-seizure-like states and their association with presynaptic glutamate release

The transition between seizure and non-seizure states in neocortical epileptic networks is governed by distinct underlying dynamical processes. Based on the gamma distribution of seizure and inter-seizure durations, over time, seizures are highly likely to self-terminate; whereas, inter-seizure durations have a low chance of transitioning back into a seizure state. Yet, the chance of a state transition could be formed by multiple overlapping, unknown synaptic mechanisms. To identify the relationship between the underlying synaptic mechanisms and the chance of seizure-state transitions, we analyzed the skewed histograms of seizure durations in human intracranial EEG and seizure-like events (SLEs) in local field potential activity from mouse neocortical slices, using an objective method for seizure state classification. While seizures and SLE durations were demonstrated to have a unimodal distribution (gamma distribution shape parameter >1), suggesting a high likelihood of terminating, inter-SLE intervals were shown to have an asymptotic exponential distribution (gamma distribution shape parameter <1), suggesting lower probability of cessation. Then, to test cellular mechanisms for these distributions, we studied the modulation of synaptic neurotransmission during, and between, the in vitro SLEs. Using simultaneous local field potential and whole-cell voltage clamp recordings, we found a suppression of presynaptic glutamate release at SLE termination, as demonstrated by electrically- and optogenetically-evoked excitatory postsynaptic currents (EPSCs), and focal hypertonic sucrose application. Adenosine A1 receptor blockade interfered with the suppression of this release, changing the inter-SLE shape parameter from asymptotic exponential to unimodal, altering the chance of state transition occurrence with time. These findings reveal a critical role for presynaptic glutamate release in determining the chance of neocortical seizure state transitions.

A two-site, open-label, non-randomized trial comparing Focal Electrically-Administered Seizure Therapy (FEAST) and right unilateral ultrabrief pulse electroconvulsive therapy (RUL-UBP ECT)

BACKGROUND

Focal Electrically-Administered Seizure Therapy (FEAST) is a form of electroconvulsive therapy (ECT) that spatially focuses the electrical stimulus to initiate seizure activity in right prefrontal cortex. Two open-label non-comparative studies suggested that FEAST has reduced cognitive side effects when compared to historical data from other forms of ECT. In two different ECT clinics, we compared the efficacy and cognitive side effects of FEAST and Right Unilateral Ultrabrief Pulse (RUL-UBP) ECT.

METHODS

Using a non-randomized, open-label design, 39 depressed adults were recruited after referral for ECT. Twenty patients received FEAST (14 women; age 45.2 ± 12.7), and 19 received RUL-UBP ECT (16 women; age 43.2 ± 16.4). Key cognitive outcome measures were the postictal time to reorientation and the Columbia University Autobiographical Memory Interview: Short-Form (CUAMI-SF). Antidepressant effects were assessed using the Hamilton Rating Scale for Depression (HRSD24).

RESULTS

In the Intent-to-treat sample, a repeated measures mixed model suggested no between group difference in HRSD24 score over time (F1,35 = 0.82, p = 0.37), while the response rate favored FEAST (FEAST: 65%; RUL-UBP ECT: 57.9%), and the remission rate favored RUL-UBP ECT (FEAST: 35%; RUL-UBP ECT: 47.4%). The FEAST group had numeric superiority in average time to reorientation (FEAST: 6.6 ± 5.0 min; RUL-UBP ECT: 8.8 ± 5.8 min; Cohens d = 0.41), and CUAMI-SF consistency score (FEAST: 69.2 ± 14.2%; RUL-UBP ECT: 63.9 ± 9.9%; Cohens d = 0.43); findings that failed to meet statistical significance.

CONCLUSIONS

FEAST exerts similar efficacy relative to an optimal form of conventional ECT and may have milder cognitive side effects. A blinded, randomized, non-inferiority trial is needed.

Allopregnanolone augments epileptiform activity of an in-vitro mouse hippocampal preparation in the first postnatal week

In the immature brain the neurotransmitter γ-amino butyric acid (GABA) mediates a membrane depolarization and can contribute to both, inhibition and excitation. Therefore the consequences of a positive modulation of GABA(A) receptors by neurosteroids on epileptiform activity are hard to predict. In order to analyze whether neurosteroids attenuate or exaggerate epileptiform activity in the immature brain, we investigated the effect of the neurosteroid allopregnanolone on epileptiform activity in an in-toto hippocampus preparation of early postnatal mice (postnatal days 4-7) using field potential recordings. These in-vitro experiments revealed that 0.5 μmol/L allopregnanolone had no effect on ictal-like epileptiform activity, but increased the occurrence of interictal epileptiform events. The allopregnanolone-induced enhancement of interictal epileptiform activity could be blocked by a selective inhibition of synaptic GABA_A receptors. In contrast, allopregnanolone had no effect on interictal epileptiform activity upon enhanced extrasynaptic GABAergic activity. Patch-clamp experiments demonstrated that allopregnanolone prolonged the decay of GABAergic postsynaptic currents, but had no effect on tonic GABAergic currents. We conclude from these results that allopregnanolone can enhance excitability in the immature hippocampus viaprolonged synaptic GABAergic currents. This potential effect of neurosteroids on brain excitability should be considered if they are applied as anticonvulsants to premature or early postnatal babies.

Adenosine A1 Receptor Agonist 2-chloro-N6-cyclopentyladenosine and Hippocampal Excitability During Brain Development in Rats

Objective: The adenosinergic system may influence excitability in the brain. Endogenous and exogenous adenosine has anticonvulsant activity presumably by activating A1 receptors. Adenosine A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA) may thus bolster anticonvulsant effects, but its action and the number of A1 receptors at different developmental stages are not known.

Methods: Hippocampal epileptic afterdischarges (ADs) were elicited in 12-, 15-, 18-, 25-, 45-, and 60-day-old rats. Stimulation and recording electrodes were implanted into the dorsal hippocampus. The A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA, 0.5 or 1 mg/kg) was administered intraperitoneally 10 min before the first stimulation. Control animals were injected with saline. All rats were stimulated with a 2-s series of 1-ms biphasic pulses delivered at 60 Hz with increasing stepwise intensity (0.05–0.6 mA). Each age and dose group contained 9–14 animals. The AD thresholds and durations were evaluated, and the A1 receptors were detected in the hippocampus in 7-, 10-, 12-, 15-, 18-, 21-, 25-, 32-, and 52-day-old rats.

Results: Both CCPA doses significantly increased hippocampal AD thresholds in 12-, 15-, 18-, and 60-day-old rats compared to controls. In contrast, the higher dose significantly decreased AD threshold in the 25-day-old rats. The AD durations were significantly shortened in all age groups except for 25-day-old rats where they were significantly prolonged. A1 receptor expression in the hippocampus was highest in 10-day-old rats and subsequently decreased.

Significance: The adenosine A1 receptor agonist CCPA exhibited anticonvulsant activity at all developmental stages studied here except for 25-day-old rats. Age-related differences might be due to the development of presynaptic A1 receptors in the hippocampus.

Dietary Omega-3 Polyunsaturated Fatty Acid Deprivation Does Not Alter Seizure Thresholds but May Prevent the Anti-seizure Effects of Injected Docosahexaenoic Acid in Rats

Background: Brain concentrations of omega-3 docosahexaenoic acid (DHA, 22:6n-3) have been reported to positively correlate with seizure thresholds in rodent seizure models. It is not known whether brain DHA depletion, achieved by chronic dietary omega-3 polyunsaturated fatty acid (PUFA) deficiency, lowers seizure thresholds in rats.

Objective: The present study tested the hypothesis that lowering brain DHA concentration with chronic dietary n-3 PUFA deprivation in rats will reduce seizure thresholds, and that compared to injected oleic acid (OA), injected DHA will raise seizure thresholds in rats maintained on n-3 PUFA adequate and deficient diets.

Methods: Rats (60 days old) were surgically implanted with electrodes in the amygdala, and subsequently randomized to the AIN-93G diet containing adequate levels of n-3 PUFA derived from soybean oil or an n-3 PUFA-deficient diet derived from coconut and safflower oil. The rats were maintained on the diets for 37 weeks. Afterdischarge seizure thresholds (ADTs) were measured every 4–6 weeks by electrically stimulating the amygdala. Between weeks 35 and 37, ADTs were assessed within 1 h of subcutaneous OA or DHA injection (600 mg/kg). Seizure thresholds were also measured in a parallel group of non-implanted rats subjected to the maximal pentylenetetrazol (PTZ, 110 mg/kg) seizure test. PUFA composition was measured in the pyriform-amygdala complex of another group of non-implanted rats sacrificed at 16 and 32 weeks.

Results: Dietary n-3 PUFA deprivation did not significantly alter amygdaloid seizure thresholds or latency to PTZ-induced seizures. Acute injection of OA did not alter amygdaloid ADTs of rats on the n-3 PUFA adequate or deficient diets, whereas acute injection of DHA significantly increased amygdaloid ADTs in rats on the n-3 PUFA adequate control diet as compared to rats on the n-3 PUFA deficient diet (P < 0.05). Pyriform-amygdala DHA percent composition did not significantly differ between the groups, while n-6 docosapentaenoic acid, a marker of n-3 PUFA deficiency, was significantly increased by 2.9-fold at 32 weeks.

Conclusion: Chronic dietary n-3 PUFA deficiency does not alter seizure thresholds in rats, but may prevent the anti-seizure effects of DHA.

Early seizures and temporal lobe trauma predict post-traumatic epilepsy: A longitudinal study

OBJECTIVE

Injury severity after traumatic brain injury (TBI) is a well-established risk factor for the development of post-traumatic epilepsy (PTE). However, whether lesion location influences the susceptibility of seizures and development of PTE longitudinally has yet to be defined. We hypothesized that lesion location, specifically in the temporal lobe, would be associated with an increased incidence of both early seizures and PTE. As secondary analysis measures, we assessed the degree of brain atrophy and functional recovery, and performed a between-group analysis, comparing patients who developed PTE with those who did not develop PTE.

METHODS

We assessed early seizure incidence (n = 90) and longitudinal development of PTE (n = 46) in a prospective convenience sample of patients with moderate-severe TBI. Acutely, patients were monitored with prospective cEEG and a high-resolution Magnetic Resonance Imaging (MRI) scan for lesion location classification. Chronically, patients underwent a high-resolution MRI, clinical assessment, and were longitudinally monitored for development of epilepsy for a minimum of 2 years post-injury.

RESULTS

Early seizures, occurring within the first week post-injury, occurred in 26.7% of the patients (n = 90). Within the cohort of subjects who had evidence of early seizures (n = 24), 75% had a hemorrhagic temporal lobe injury on admission. For longitudinal analyses (n = 46), 45.7% of patients developed PTE within a minimum of 2 years post-injury. Within the cohort of subjects who developed PTE (n = 21), 85.7% had a hemorrhagic temporal lobe injury on admission and 38.1% had early (convulsive or non-convulsive) seizures on cEEG monitoring during their acute ICU stay. In a between-group analysis, patients with PTE (n = 21) were more likely than patients who did not develop PTE (n = 25) to have a hemorrhagic temporal lobe injury (p < 0.001), worse functional recovery (p = 0.003), and greater temporal lobe atrophy (p = 0.029).

CONCLUSION

Our results indicate that in a cohort of patients with a moderate-severe TBI, 1) lesion location specificity (e.g. the temporal lobe) is related to both a high incidence of early seizures and longitudinal development of PTE, 2) early seizures, whether convulsive or non-convulsive in nature, are associated with an increased risk for PTE development, and 3) patients who develop PTE have greater chronic temporal lobe atrophy and worse functional outcomes, compared to those who do not develop PTE, despite matched injury severity characteristics. This study provides the foundation for a future prospective study focused on elucidating the mechanisms and risk factors for epileptogenesis.

APP Causes Hyperexcitability in Fragile X Mice

Amyloid-beta protein precursor (APP) and metabolite levels are altered in fragile X syndrome (FXS) patients and in the mouse model of the disorder, Fmr1KO mice. Normalization of APP levels in Fmr1KO mice (Fmr1KO /APPHET mice) rescues many disease phenotypes. Thus, APP is a potential biomarker as well as therapeutic target for FXS. Hyperexcitability is a key phenotype of FXS. Herein, we determine the effects of APP levels on hyperexcitability in Fmr1KO brain slices. Fmr1KO /APPHET slices exhibit complete rescue of UP states in a neocortical hyperexcitability model and reduced duration of ictal discharges in a CA3 hippocampal model. These data demonstrate that APP plays a pivotal role in maintaining an appropriate balance of excitation and inhibition (E/I) in neural circuits. A model is proposed whereby APP acts as a rheostat in a molecular circuit that modulates hyperexcitability through mGluR5 and FMRP. Both over- and under-expression of APP in the context of the Fmr1KO increases seizure propensity suggesting that an APP rheostat maintains appropriate E/I levels but is overloaded by mGluR5-mediated excitation in the absence of FMRP. These findings are discussed in relation to novel treatment approaches to restore APP homeostasis in FXS.

Extracellular Potassium and Seizures: Excitation, Inhibition and the Role of Ih

Seizure activity leads to increases in extracellular potassium concentration ([K[Formula: see text]]o), which can result in changes in neuronal passive and active membrane properties as well as in population activities. In this study, we examined how extracellular potassium modulates seizure activities using an acute 4-AP induced seizure model in the neocortex, both in vivo and in vitro. Moderately elevated [K[Formula: see text]]o up to 9[Formula: see text]mM prolonged seizure durations and shortened interictal intervals as well as depolarized the neuronal resting membrane potential (RMP). However, when [K[Formula: see text]]o reached higher than 9[Formula: see text]mM, seizure like events (SLEs) were blocked and neurons went into a depolarization-blocked state. Spreading depression was never observed as the blockade of ictal events could be reversed within 1-2[Formula: see text]min after the raised [K[Formula: see text]]o was changed back to control levels. This concentration-dependent dual effect of [K[Formula: see text]]o was observed using in vivo and in vitro mouse brain preparations as well as in human neocortical tissue resected during epilepsy surgery. Blocking the Ih current, mediated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, modulated the elevated [K[Formula: see text]]o influence on SLEs by promoting the high [K[Formula: see text]]o inhibitory actions. These results demonstrate biphasic actions of raised [K[Formula: see text]]o on neuronal excitability and seizure activity.

Spontaneous epileptiform activity in rat neocortex after controlled cortical impact injury

A hallmark of severe traumatic brain injury (TBI) is the development of post-traumatic epilepsy (PTE). However, the mechanisms underlying PTE remain poorly understood. In this study, we used a controlled cortical impact (CCI) model in rats to examine post-traumatic changes in neocortical excitability. Neocortical slices were prepared from rats at 7-9 days (week 1) and 14-16 days (week 2) after CCI injury. By week 2, we observed a substantial gray matter lesion with a cavity that extended to the hippocampal structure. Fluoro-Jade B staining of slices revealed active neuronal degeneration during weeks 1 and 2. Intracellular and extracellular recordings obtained from layer V revealed evoked and spontaneous epileptiform discharges in neocortices of CCI-injured rats. At week 1, intracellular recordings from pyramidal cells revealed evoked epileptiform firing that was synchronized with population events recorded extracellularly, suggestive of increased excitability. This activity was characterized by bursts of action potentials that were followed by recurrent, repetitive after-discharges. At week 2, both spontaneous and evoked epileptiform firing were recorded in slices from injured rats. The evoked discharges resembled those observed at week 1, but with longer burst durations. Spontaneous activity included prolonged, ictal-like discharges lasting up to 8-10 sec, and briefer interictal-like burst events (<1 sec). These results indicate that during the first 2 weeks following severe CCI injury, there is a progressive development of neocortical hyperexcitability that ultimately leads to spontaneous epileptiform firing, suggesting a rapid epileptogenic process.

Recurrent neonatal seizures result in long-term increases in neuronal network excitability in the rat neocortex

Neonatal seizures are associated with a high likelihood of adverse neurological outcomes, including mental retardation, behavioral disorders, and epilepsy. Early seizures typically involve the neocortex, and post-neonatal epilepsy is often of neocortical origin. However, our understanding of the consequences of neonatal seizures for neocortical function is limited. In the present study, we show that neonatal seizures induced by flurothyl result in markedly enhanced susceptibility of the neocortex to seizure-like activity. This change occurs in young rats studied weeks after the last induced seizure and in adult rats studied months after the initial seizures. Neonatal seizures resulted in reductions in the amplitude of spontaneous inhibitory postsynaptic currents and the frequency of miniature inhibitory postsynaptic currents, and significant increases in the amplitude and frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and in the frequency of miniature excitatory postsynaptic currents (mEPSCs) in pyramidal cells of layer 2/3 of the somatosensory cortex. The selective N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovalerate eliminated the differences in amplitude and frequency of sEPSCs and mEPSCs in the control and flurothyl groups, suggesting that NMDA receptors contribute significantly to the enhanced excitability seen in slices from rats that experienced recurrent neonatal seizures. Taken together, our results suggest that recurrent seizures in infancy result in a persistent enhancement of neocortical excitability.

Acute administration of docosahexaenoic acid increases resistance to pentylenetetrazol-induced seizures in rats

OBJECTIVE

Docosahexaenoic acid (DHA), an omega-3 fatty acid, has been reported to raise seizure thresholds. The purpose of the present study was to test the acute anticonvulsant effects of unesterified DHA in rats, using the maximal pentylenetetrazol (PTZ) seizure model, and also to examine DHA incorporation and distribution into blood serum total lipids and brain phospholipids and unesterified fatty acids. Sedation was measured to monitor for the potential toxicity of DHA.

METHODS

Male Wistar rats received subcutaneous injections of saline, oleic acid (OA), or DHA. An initial pilot study (Experiment 1) established 400mg/kg as an effective dose of DHA in the maximal PTZ seizure test. A subsequent time-response study, using 400mg/kg (Experiment 2), established 1 hour as an effective postinjection interval for administering DHA subcutaneously. A final study (Experiment 3) comprised two different groups. The first group ("seizure-tested rats") received saline, OA, or DHA (400mg/kg) subcutaneously, and were seizure tested in the maximal PTZ test 1 hour later to confirm the seizure latency measurements at that time. The second group ("assay rats") received identical subcutaneous injections of saline, OA, or DHA (400mg/kg). One hour postinjection, however, they were sacrificed for assay rather than being seizure tested. Assays involved the analysis of serum and brain DHA. Sedation was measured in both Experiment 3 groups during the 1-hour period prior to seizure testing or sacrifice.

RESULTS

As noted above, 400mg/kg proved to be an effective subcutaneous dose of DHA (Experiment 1), and 1 hour proved to be the most effective injection-test interval (Experiment 2). In Experiment 3, in the seizure-tested animals, subcutaneous administration of 400mg/kg of DHA significantly increased latency to PTZ seizure onset 1 hour postinjection relative to the saline- and OA-injected controls, which did not differ significantly from each other (P>0.05). In the assay animals, no significant effects of treatment on blood serum total lipids or on brain phospholipid or unesterified fatty acid profiles (P>0.05) were observed. There were also no differences in sedation among the three groups (P>0.05).

CONCLUSION

DHA increases resistance to PTZ-induced seizures without altering measures of sedation and, apparently, without changing DHA concentrations in serum or brain.

Whole isolated neocortical and hippocampal preparations and their use in imaging studies

This study shows that two whole isolated preparations from the young mouse, the neocortical 'slab' and the hippocampal formation, are useful for imaging studies requiring both global monitoring using light transmittance (LT) imaging and high resolution cellular monitoring using 2-photon laser scanning microscopy (2PLSM). These preparations share advantages with brain slices such as maintaining intrinsic neuronal properties and avoiding cardiac or respiratory movement. Important additional advantages include the maintenance of all local input and output pathways, the absence of surfaces injured by slicing and the preservation of three-dimensional tissue structure. Using evoked extracellular field recording, we demonstrate long-term (hours) viability of both whole preparations. We then show that propagating cortical events such as anoxic depolarization (AD) and spreading depression (SD) can be imaged in both preparations, yielding results comparable to those in brain slices but retaining the tissue's three-dimensional structure. Using transgenic mice expressing green fluorescent protein (GFP) in pyramidal and granule cell neurons, 2PLSM confirms that these preparations are free of the surface damage observed in sliced brain tissue. Moreover the neurons undergo swelling with accompanying dendritic beading following AD induced by simulated ischemia, similar to cortical damage described in vivo.

Factors which abolish hypoglycemic seizures do not increase cerebral glycogen content in vitro

The brain is heavily dependant on glucose for its function and survival. Hypoglycemia can have severe, irreversible consequences, including seizures, coma and death. However, the in vivo content of brain glycogen, the storage form of glucose, is meager and is a function of both neuronal activity and glucose concentration. In the intact in vitro hippocampus isolated from mice aged postnatal days 8-13, we have recently characterized a novel model of hypoglycemic seizures, wherein seizures were abolished by various neuroprotective strategies. We had hypothesized that these strategies might act, in part, by increasing cerebral glycogen content. In the present experiments, it was found that neither decreasing temperature nor increasing glucose concentrations (above 2 mM) significantly increased hippocampal glycogen content. Preparations of isolated frontal neocortex in vitro do not produce hypoglycemic seizures yet it was found they contained significantly lower glycogen content as compared to the isolated intact hippocampus. Further, the application of either TTX, or a cocktail containing APV, CNQX and gabazine, to block synaptic activity, did not increase, but paradoxically decreased, hippocampal glycogen content in the isolated intact hippocampus. Significant decreases in glycogen were noted when neuronal activity was increased via incubation with l-aspartate (500 muM) or low Mg(2+). Lastly, we examined the incidence of hypoglycemic seizures in hippocampi isolated from mice aged 15-19 and 22-24 days, and compared it to the incidence of hypoglycemic seizures of hippocampi isolated from mice aged 8-13 days described previously (Abdelmalik et al., 2007 Neurobiol Dis 26(3):646-660). It was noted that hypoglycemic seizures were generated less frequently, and had less impact on synaptic transmission in hippocmpi from PD 22-24 as compared to hippocampi from mice PD 15-19 or PD 8-13. However, hippocampi from 8- to 13-day-old mice had significantly more glycogen than the other two age groups. The present data suggest that none of the interventions which abolish hypoglycemic seizures increases glycogen content, and that low glycogen content, per se, may not predispose to the generation of hypoglycemic seizures.

Hypoglycemic seizures during transient hypoglycemia exacerbate hippocampal dysfunction

Severe hypoglycemia constitutes a medical emergency, involving seizures, coma and death. We hypothesized that seizures, during limited substrate availability, aggravate hypoglycemia-induced brain damage. Using immature isolated, intact hippocampi and frontal neocortical blocks subjected to low glucose perfusion, we characterized hypoglycemic (neuroglycopenic) seizures in vitro during transient hypoglycemia and their effects on synaptic transmission and glycogen content. Hippocampal hypoglycemic seizures were always followed by an irreversible reduction (>60% loss) in synaptic transmission and were occasionally accompanied by spreading depression-like events. Hypoglycemic seizures occurred more frequently with decreasing "hypoglycemic" extracellular glucose concentrations. In contrast, no hypoglycemic seizures were generated in the neocortex during transient hypoglycemia, and the reduction of synaptic transmission was reversible (<60% loss). Hypoglycemic seizures in the hippocampus were abolished by NMDA and non-NMDA antagonists. The anticonvulsant, midazolam, but neither phenytoin nor valproate, also abolished hypoglycemic seizures. Non-glycolytic, oxidative substrates attenuated, but did not abolish, hypoglycemic seizure activity and were unable to support synaptic transmission, even in the presence of the adenosine (A1) antagonist, DPCPX. Complete prevention of hypoglycemic seizures always led to the maintenance of synaptic transmission. A quantitative glycogen assay demonstrated that hypoglycemic seizures, in vitro, during hypoglycemia deplete hippocampal glycogen. These data suggest that suppressing seizures during hypoglycemia may decrease subsequent neuronal damage and dysfunction.

Shift of adenosine kinase expression from neurons to astrocytes during postnatal development suggests dual functionality of the enzyme

Adenosine is a potent modulator of excitatory neurotransmission, especially in seizure-prone regions such as the hippocampal formation. In adult brain ambient levels of adenosine are controlled by adenosine kinase (ADK), the major adenosine-metabolizing enzyme, expressed most strongly in astrocytes. Since ontogeny of the adenosine system is largely unknown, we investigated ADK expression and cellular localization during postnatal development of the mouse brain, using immunofluorescence staining with cell-type specific markers. At early postnatal stages ADK immunoreactivity was prominent in neurons, notably in cerebral cortex and hippocampus. Thereafter, as seen best in hippocampus, ADK gradually disappeared from neurons and appeared in newly developed nestin- and glial fibrillary acidic protein (GFAP)-positive astrocytes. Furthermore, the region-specific downregulation of neuronal ADK coincided with the onset of myelination, as visualized by myelin basic protein staining. After postnatal day 14 (P14), the transition from neuronal to astrocytic ADK expression was complete, except in a subset of neurons that retained ADK until adulthood in specific regions, such as striatum. Moreover, neuronal progenitors in the adult dentate gyrus lacked ADK. Finally, recordings of excitatory field potentials in acute slice preparations revealed a reduced adenosinergic inhibition in P14 hippocampus compared with adult. These findings suggest distinct roles for adenosine in the developing and adult brain. First, ADK expression in young neurons may provide a salvage pathway to utilize adenosine in nucleic acid synthesis, thus supporting differentiation and plasticity and influencing myelination; and second, adult ADK expression in astrocytes may offer a mechanism to regulate adenosine levels as a function of metabolic needs and synaptic activity, thus contributing to the differential resistance of young and adult animals to seizures.