JoAnn E. Franck

Physiologic and Morphologic Characteristics of Granule Cell Circuitry in Human Epileptic Hippocampus

Summary: Morphological and electrophysiological techiques were used to examine granule cells and their mossy fiber axons in nine surgically resected hippocampal specimens from temporal lobe epilepsy (TLE) patients. Timm histochemistry showed mossy fiber sprouting into the inner molecular layer (IML) of the dentate in a subset of tissue samples. In slices from five tissue samples, stimulus‐induced bursting activity could be induced with a low concentration (2.5 μM) of bicuculline; bursts were sensitive to the N‐methyl‐d‐aspartate (NMDA) blocker, APV. There was a general correlation between such sprouting and experimentally induced yperexcit ability. Fourteen granule cells from five tissue samples were intracellularly stained [with lucifer yellow (LY) or neurobiotin]. Axons from a subset of these neurons showed axon collaterals reaching into the IML, but this axon projection pattern for single cells was not directly correlated with degree of mossy fiber sprouting shown grossly by Timm staining. Electron microscopic examination of intracellularly stained elements showed mossy fiber axon terminals making asymmetric synaptic contacts (including autapses on the granule cell dendrite) with dendritic shafts and spines in both apical and basal domains. These data are consistent with the hypothesis that mossy fiber sprouting provides a structural basis for recurrent excitation of granule cells, but does not provide direct support of the hypothesis that mossy fiber sprouting causes hyperexcitability. The data suggest that granule cell bursting activity is at least in part a function of compromised synaptic inhibition, since levels of γ‐aminobutyric acid (GABA) blockade that are generally subthreshold for burst induction were epileptogenic in some tissue samples from human epileptic hippocampus.

Do kainate-lesioned hippocampi become epileptogenic?

Kainic acid lesions of hippocampal subfields CA3-CA4 produced dramatic synchronous afterdischarge activity in subfield CA1 when studied 2-4 weeks post-lesion in the in vitro slice preparation. This epileptiform discharge was correlated with a loss of intrinsic firing-induced afterhyperpolarizations and synaptic IPSPs. Two to 4 months post-lesion, intrinsic afterpotentials and synaptic inhibition appeared normal in most cells studied.

Glutamate-mediated selective vulnerability to ischemia is present in organotypic cultures of hippocampus.

Ischemic damage to the brain, whether induced experimentally or observed clinically, often produces a pattern of delayed selective cell death in subfield CA1 of hippocampus which has been associated with significant neurologic deficits. The present study demonstrates that this selective vulnerability of CA1 neurons to ischemia, with relative preservation of their neighbors, is expressed in organotypic tissue culture and is prevented by the N-methyl-D-aspartate (NMDA) receptor blocker, MK-801. These data provide conclusive evidence that this selective cell death does not have a vascular etiology but is mediated by factors intrinsic to the hippocampal neurons and/or local circuitry. This model system provides an opportunity both to examine mechanisms of ischemic cell death in an avascular environment and to study methods of prevention in the absence of systemic variables.

Na,K-ATPase is decreased in hippocampus of kainate-lesioned rats

The effects of intraventricular injection of kainic acid on the Na,K-ATPase (Na,K pump) were examined in discrete pyramidal cell regions of rat hippocampus. [3H]Ouabain binding was used to quantitate Na,K-ATPase catalytic subunits and in situ hybridization was used to determine Na,K-ATPase mRNA levels. Large decreases were found in both [3H]ouabain binding and alpha 3 isoform mRNA in hippocampus areas, especially in the CA3 pyramidal cell layer, which sustains heavy cell losses as a result of bilateral, intraventricular injection of kainic acid. Substantial decreases in the high affinity component of ouabain binding and in the alpha 3 isoform mRNA (but not isoforms for other Na,K-ATPase subunits) were also observed in the CA1 region of hippocampus, an area preserved in this model. High affinity [3H]ouabain binding was decreased 25-33% in the stratum pyramidale and stratum radiatum after treatment with kainic acid, and alpha 3 mRNA was decreased by 26-50%. To further characterize the decrease in alpha 3 mRNA, animals were killed at 1, 2, and 3 weeks after injection of kainate and results show a large decrease in alpha 3 mRNA only at 2 weeks recovery time. While the pathology underlying temporal lobe epilepsy is unclear, changes in the Na,K-ATPase may be involved in abnormal firing characteristics of cells in epileptic tissue.

Combined kainate and ischemia produces ‘mesial temporal sclerosis’

In the hippocampi of individuals with temporal lobe epilepsy, cells in CA1, CA3, and the dentate hilus are often gone, but dentate granule cells and CA2 are relatively spared. Several animal models have been developed which mimic portions of this damage. In the present work, combined intraventricular kainate and forebrain ischemia produced a lesion most like that observed clinically; the dentate granule cells and a resistant cluster of pyramidal cells, which received mossy fiber input, were the only principle neurons remaining. This preparation may be valuable both in determining the nature of cells that survive and in understanding the consequences of such damage.

Developmental Hyperthermic Seizures Alter Adult Hippocampal Benzodiazepine Binding and Morphology

Summary: Experimental hyperthermic seizures in 15‐day‐old rats resulted in both an alteration in the benzodiazepine (BDZ) binding site in the hippocampal region and morphologic changes in hippocampal neurons. These alterations were not apparent until adulthood. Experimental adult animals also showed a reduced anticonvulsant response to a selective BDZ ligand when challenged with pentylenetetrazol. Developmental hyperthermic seizures may initiate dynamic changes in the hippocampus that contribute to adult functional impairment.

Benzodiazepine binding increases in the superficial laminae of the trigeminal subnucleus caudalis following central rhizotomy

Benzodiazepine (BZ) binding is being studied in the spinal trigeminal nucleus of the cat 3 and 11 days following unilateral retrogasserian rhizotomy using in vitro autoradiography and computer-assisted densitometry. At 3 days following rhizotomy there is an increase in number and decrease in affinity of flunitrazepam binding sites in the superficial laminae of subnucleus caudalis of the spinal trigeminal complex. By 11 days, affinity remains below control values and binding site number shows an insignificant but detectable increase. There is no change in binding site characteristics in deeper laminae at either survival time. The results are discussed in relation to the physiological hyperactivity and synaptic changes which occur following such lesions and to other conditions of deafferentation.