Anne Williamson

Delayed K+ clearance associated with aquaporin-4 mislocalization: Phenotypic defects in brains of α-syntrophin-null mice

Recovery from neuronal activation requires rapid clearance of potassium ions (K+) and restoration of osmotic equilibrium. The predominant water channel protein in brain, aquaporin-4 (AQP4), is concentrated in the astrocyte end-feet membranes adjacent to blood vessels in neocortex and cerebellum by association with α-syntrophin protein. Although AQP4 has been implicated in the pathogenesis of brain edema, its functions in normal brain physiology are uncertain. In this study, we used immunogold electron microscopy to compare hippocampus of WT and α-syntrophin-null mice (α-Syn-/-). We found that <10% of AQP4 immunogold labeling is retained in the perivascular astrocyte end-feet membranes of the α-Syn-/- mice, whereas labeling of the inwardly rectifying K+ channel, Kir4.1, is largely unchanged. Activity-dependent changes in K+ clearance were studied in hippocampal slices to test whether AQP4 and K+ channels work in concert to achieve isosmotic clearance of K+ after neuronal activation. Microelectrode recordings of extracellular K+ ([K+]o) from the target zones of Schaffer collaterals and perforant path were obtained after 5-, 10-, and 20-Hz orthodromic stimulations. K+ clearance was prolonged up to 2-fold in α-Syn-/- mice compared with WT mice. Furthermore, the intensity of hyperthermia-induced epileptic seizures was increased in approximately half of the α-Syn-/-mice. These studies lead us to propose that water flux through perivascular AQP4 is needed to sustain efficient removal of K+ after neuronal activation.

A Retrospective Analysis of Hippocampal Pathology in Human Temporal Lobe Epilepsy: Evidence for Distinctive Patient Subcategories

Summary:  Purpose: This study is a retrospective analysis of the pathology of the hippocampus from patients with medically intractable temporal lobe epilepsy. We attempted to relate neuronal density, immunohistochemistry, electrophysiologic data, and surgical outcome.

Glutamate and astrocytes—Key players in human mesial temporal lobe epilepsy?

Approximately one‐third of all patients with epilepsy continue to suffer from seizures even after appropriate treatment with antiepileptic drugs. Medically refractory epilepsies are associated with considerable morbidity and mortality, and more efficacious therapies against these disorders are clearly needed. However, the discovery of better therapies has been lagging due to an incomplete understanding of the mechanisms underlying the development of epilepsy (epileptogensis) and seizures (ictogenesis) in humans. An increasing number of studies have suggested that an abnormal amplification of glutamatergic activity—often referred to as the “glutamate hypothesis”—is involved in the pathophysiology of seizures and certain types of medically refractory epilepsies. For example, elevated levels of extracellular glutamate in hyperexcitable areas of the brain, up‐regulation of glutamate receptors, and loss of the glutamate‐metabolizing enzyme, glutamine synthetase (GS), have all been reported in patients with mesial temporal lobe epilepsy (MTLE). Moreover, it appears that glial cells, particularly the astrocyte, may play a key role in the glutamate overflow in MTLE. Proliferation of astrocytes is a hallmark of the epileptogenic focus in MTLE, and the proliferated cells are characterized by several unique features that are permissive for the excessive accumulation and release of astrocytic glutamate. Here, we assess recent data regarding the glutamate excess in epilepsy, review the role of glutamine synthetase, and discuss the implications of astrocytes in the pathophysiology of MTLE.

Neurometabolism in human epilepsy

Purpose: Because of the large and continuous energetic requirements of brain function, neurometabolic dysfunction is a key pathophysiologic aspect of the epileptic brain. Additionally, neurometabolic dysfunction has many self‐propagating features that are typical of epileptogenic processes, that is, where each occurrence makes the likelihood of further mitochondrial and energetic injury more probable. Thus abnormal neurometabolism may be not only a chronic accompaniment of the epileptic brain, but also a direct contributor to epileptogenesis.

Doublecortin expression in adult cat and primate cerebral cortex relates to immature neurons that develop into GABAergic subgroups

DCX-immunoreactive (DCX+) cells occur in the piriform cortex in adult mice and rats, but also in the neocortex in adult guinea pigs and rabbits. Here we describe these cells in adult domestic cats and primates. In cats and rhesus monkeys, DCX+ cells existed across the allo- and neocortex, with an overall ventrodorsal high to low gradient at a given frontal plane. Labeled cells formed a cellular band in layers II and upper III, exhibiting dramatic differences in somal size (5-20 microm), shape (unipolar, bipolar, multipolar and irregular), neuritic complexity and labeling intensity. Cell clusters were also seen in this band, and those in the entorhinal cortex extended into deeper layers as chain-like structures. Densitometry revealed a parallel decline of the cells across regions with age in cats. Besides the cellular band, medium-sized cells with weak DCX reactivity resided sparsely in other layers. Throughout the cortex, virtually all DCX+ cells co-expressed polysialylated neural cell adhesion molecule. Medium to large mature-looking DCX+ cells frequently colocalized with neuron-specific nuclear protein and gamma-aminobutyric acid (GABA), and those with a reduced DCX expression also partially co-labeled for glutamic acid decarboxylase, parvalbumin, calbindin, beta-nicotinamide adenine dinucleotide phosphate diaphorase and neuronal nitric oxide synthase. Similar to cats and monkeys, small and larger DCX+ cells were detected in surgically removed human frontal and temporal cortices. These data suggest that immature neurons persist into adulthood in many cortical areas in cats and primates, and that these cells appear to undergo development and differentiation to become functional subgroups of GABAergic interneurons.

Decrease in inhibition in dentate granule cells from patients with medial temporal lobe epilepsy

Alterations in synaptic inhibition are associated with epileptiform activity in several acute animal models; however, it is not clear if there are changes in inhibition in chronically epileptic tissue. We have used intracellular recordings from granule cells of patients with temporal lobe epilepsy to determine whether synaptic inhibition is compromised. Two groups of patients with medial temporal lobe epilepsy were used, those with medial temporal lobe sclerosis (MTLE), and those with extrahippocampal masses (MaTLE) where the cell loss and synaptic reorganization that characterize MTLE are not seen. Although the level of tonic inhibition at the somata was not significantly different in the two patient groups, there was a reduction in the conductance of polysynaptic perforant path–evoked fast and slow inhibitory postsynaptic potentials (IPSPs) (53% and 66%, respectively). We found that there was a comparable decrease in the monosynaptic IPSP conductances examined in the presence of glutamatergic antagonists as that seen for the polysynaptically evoked IPSPs. These data suggest that the decrease in inhibition seen in normal artificial cerebrospinal fluid in MTLE granule cells cannot be solely explained by a decrease in excitatory input onto inhibitory interneurons and may reflect changes at the interneuron–granule cells synapse or in the number of specific inhibitory interneurons. Ann Neurol 1999;45:92–99

Gabapentin and vigabatrin increase GABA in the human neocortical slice

The effects of antiepileptic drugs, gabapentin and vigabatrin, on gamma-aminobutyric acid (GABA) concentrations were studied in human (n=14) and rat (n=6) neocortical slice preparations. In this study, neocortical slices were incubated with gabapentin, vigabatrin or no drugs for 3 h in an oxygenated environment. Proton magnetic resonance spectroscopy (MRS) of perchloric acid (PCA) extracts was used to measure GABA concentrations. Vigabatrin increased cellular GABA concentrations in both human and rat neocortical slices by 62% (P<0.001) and 88% (P<0.03), respectively. Gabapentin significantly increased GABA concentrations by 13% (P<0.02) in human neocortical slices made from tissue resected during epilepsy surgery. However, in the rat neocortical slice exposed to the same conditions as the human tissue, gabapentin did not increase GABA significantly. These results confirm our MRS studies in vivo that gabapentin increases GABA levels in epileptic patients, but has minimal or no effect in a healthy rodent model. Caution must be used in extrapolating negative results obtained in rodent models to the human condition.

Physiology of Human Cortical Neurons Adjacent to Cavernous Malformations and Tumors

Summary:  Purpose: Focal neocortical seizures can be associated with a number of specific pathologies including supratentorial tumors and cavernous malformations (CMs), both of which are highly epileptogenic.

Comparison between the membrane and synaptic properties of human and rodent dentate granule cells

We have compared the cellular and synaptic properties of rodent dentate granule cells with those of humans. The human tissue was obtained from neurosurgical procedures which necessitated removal of the hippocampus for treatment of extra-hippocampal tumors which presented clinically with seizures. The hippocampi studied here were neuroanatomically similar to autopsy controls. The present studies have demonstrated that there are few differences between rodent and human granule cells as regards either their membrane properties or their synaptic physiology and pharmacology. The differences we noted were (1) less spike frequency adaptation in the human relative to rodent cells; and (2) perforant path stimulation reliably elicited both feedforward and feedback inhibition in the rodent cells, while in the human tissue feedback inhibition appeared to predominate. It is unclear if these changes are due to the seizure experience or if they represent true species differences.

Zinc reduces dentate granule cell hyperexcitability in epileptic humans

THE hippocampi of epileptic patients with medial temporal lobe epilepsy exhibit a characteristic pattern of anatomical changes including cell loss and sprouting of the granule cell axons, the mossy fibers, into the inner molecular layer of the dentate. In addition to glutamate, mossy fibers release Zn2+. In the present study we investigated the action of Zn2+ on excitatory synaptic potentials in the dentate granule cells of patients wkh medial temporal lobe epilepsy. We show here that Zn2+ limits the duration of excitatory responses in these cells, probably by blocking the N-methyl-D-aspartate (NMDA) receptors. Zinc may, therefore, play an important role in limiting epileptiform activity in this tissue.