lucifer-yellow and Epilepsy

Hippocampal dentate granule cells in temporal lobe epilepsy (TLE) patients with mesial sclerosis (MTLE) are reported to be hyperexcitable compared to those in patients with a mass lesion outside the hippocampus (MaTLE) (Williamson, Clin Neurosci 1994;2: 47-52). To determine if such hyperexcitability is associated with an altered morphology of these neurons, Lucifer Yellow-filled granule cells from MTLE patients were compared with those from MaTLE. The morphology of granule cells in both subject groups resembles closely that of human granule cells described previously by Golgi studies. About 40% of human granule cells have basal dendrites. Additionally their apical dendrites are much more limited in their spread in the longitudinal axis of the hippocampus contributing perhaps to a much more narrow lamellar organization than in rats. Analysis of variance computed on 21 morphometric parameters reveals a significant increase in the length of the portion of the dendrite in the inner molecular layer (IML), and a decrease in length in the outer third of the molecular layer in MTLE, compared to MaTLE. Factor analysis performed on the morphometric features of each group of neurons reveals that in the MaTLE neurons the most distinctive feature is the total dendritic length and the overall distribution of spines on them, whereas in MTLE a lengthening and elaboration of the dendrites in the IML is most distinctive. Previous observations of increased synaptic terminals containing neuropeptides, and neurotransmitter receptors in the IML taken in conjunction with an elaboration of granule cell dendrites in this region, suggest considerable synaptic reorganization within the IML of the MTLE hippocampus which may contribute to its epileptogenicity.

Topics: Dendrites; Dentate Gyrus; Epilepsy; Epilepsy, Temporal Lobe; Fluorescent Dyes; Hippocampus; Humans; Isoquinolines; Neuronal Plasticity; Neurons; Sclerosis

To date, there is little experimental evidence supporting or refuting electrotonic interactions through gap junctions in the generation and/or spread of seizure activity in the mammalian brain. We have studied gap junctional mechanisms in the in vitro calcium-free induced model of epilepsy using electrophysiological and staining techniques in the CA1 area of the hippocampus. Lucifer yellow staining of CA1 pyramidal neurons revealed that dye coupling was increased 2.3 times in hippocampal slices made hyperexcitable by perfusion with calcium-free artificial cerebrospinal fluid (aCSF). Furthermore, multiple neuronal dye coupling (triplets, quintuplets) was observed in these conditions but never in control (standard aCSF). Under conditions that reduce gap junctional conductance (intracellular acidification, octanol, halothane), seizure-like activity was suppressed in the CA1 area in this epilepsy model, whereas increasing gap junctional conductance by intracellular alkalinization increased the frequency and duration of field burst events. Intracellular acidification also reduced dye coupling as well as the frequency of fast prepotentials (electrotonic potentials) without altering neuronal firing frequency. Simultaneous extracellular field and single whole-cell recordings revealed suppression of synchronization between neuronal firing and spontaneous field burst activity during acidification. These observations indicate an apparent increase in electrotonic coupling during calcium-free induced spontaneous rhythmic field burst activity in the CA1 area of the hippocampus and that electrotonic coupling may contribute substantially to the synchronization of neuronal firing underlying seizure-like events.

Topics: Acids; Animals; Calcium; Electrophysiology; Epilepsy; Fluorescent Dyes; Gap Junctions; Hippocampus; In Vitro Techniques; Isoquinolines; Neurons; Octanols; Rats; Rats, Wistar; Time Factors

Acute osmotic disturbances can lead to profound neurological problems, yet there has been little experimentation at a cellular level to assess if neurophysiological changes are induced by altered osmolality. Using extra- and intracellular recording in the rat neocortical slice preparation, we examined pyramidal neurons of layers II-III under changing osmotic conditions. Single cell properties, field potentials, synaptic transmission and epileptiform discharges were studied in control saline (295 mOsm) and compared with corresponding data collected during exposure to osmolalities between 245 and 375 mOsm. Single cell properties (resting membrane potential, cell input resistance, action potential threshold and duration) did not change significantly, but neuronal interactions were considerably influenced by osmotic change within minutes. Hyposmolality increased the amplitude of evoked field potentials and of excitatory postsynaptic potentials recorded intracellularly. Hyperosmolality, induced with mannitol, decreased these parameters. Electrotonic coupling, as gauged by the degree of dye coupling and by cell input resistance, was not influenced by shifts in osmolality. The clinical finding that overhydration promotes seizure onset was examined in slices made epileptogenic in Mg2(+)-free saline. Hyposmolality increased the frequency and decreased the duration of interictal bursts, whereas raising osmolality with mannitol had opposite effects. None of the aforementioned effects occurred when osmolality was increased with a freely permeable substance such as dimethylsulfoxide, nor could they be ascribed to changes in saline Na+ or Ca2+ concentrations. The results are consistent with hyposmotic solutions reducing extracellular space by causing cells to swell. Theoretically, during population discharge, this should both concentrate K+ released extracellularly and possibly increase field (ephaptic) interactions. How lowered osmolality strengthens spontaneous and evoked excitatory synaptic transmission in neocortex is not yet clear. However, it may be an important mechanism underlying the increased seizure susceptibility of patients and experimental animals with lowered plasma osmolality. Conversely, suppression of excitatory postsynaptic potentials by osmotically active substances may be involved in the lowered seizure susceptibility observed clinically.

Topics: Action Potentials; Animals; Cerebral Cortex; Epilepsy; Fluorescent Dyes; Isoquinolines; Male; Neurons; Osmolar Concentration; Rats; Rats, Inbred Strains; Synapses

Four general mechanisms can hypothetically contribute to or mediate localized synchronization of neuronal activity: (a) recurrent excitatory chemical synapses, (b) electrotonic coupling via gap junctions, (c) electrical field effects (ephaptic interactions), and (d) changes in the concentration of extracellular ions (e.g., K+). It has generally been believed that synchronization of epileptiform bursts derives primarily, if not exclusively, from recurrent excitatory chemical synapses. Dual intracellular recordings from the CA3 area of the hippocampus have been used to demonstrate the existence of recurrent synaptic excitation, and computer simulations have provided a theoretical framework for the idea that relatively sparse interactions through recurrent excitatory chemical synapses can generate synchronized bursting after inhibitory pathways are blocked with convulsant agents. Additional experimental studies have supported the hypothesis that a model for seizure discharge, the penicillin-induced paroxysmal depolarization shift (PDS), is associated with a large increase in excitatory synaptic conductance. However, recent studies have suggested that electrical interactions are also likely to play an important role in spike synchronization during epileptic discharges. Several research groups have used in vitro preparations to show that afterdischarges and spontaneous bursts of population spikes (which represent synchronized action potentials) can occur after chemical synaptic transmission has been blocked in solutions containing low [Ca2+]. Although this result was first observed in the CA1 area, it has recently been confirmed in other regions of the hippocampus. These experiments indicate that mechanisms other than chemical synaptic transmission are capable of synchronizing action potentials in the hippocampus. In this chapter, two forms of electrical interaction that could mediate synchronization will be considered: (a) electrotonic coupling through gap junctions and (b) electrical field effects through extracellular space. Changes in the concentration of extracellular ions are another mechanism not involving chemical synapses. However, it seems unlikely that ionic changes act on the rapid time scale of electrical interactions, and their contribution is discussed elsewhere in this volume. We review evidence for the existence of electrotonic coupling and electrical field effects in the hippocampus and neocortex, and discuss their possible involvement in the

Topics: Animals; Cerebral Cortex; Electrophysiology; Epilepsy; Extracellular Space; Hippocampus; Intercellular Junctions; Ions; Isoquinolines; Models, Neurological; Neural Inhibition; Neurology; Osmolar Concentration; Rats; Synapses; Time Factors