Pathophysiology of generalized photosensitive epilepsy in the cat

The role of dopamine in epilepsy

The clinical benefits of dopamine agonists in the management of epilepsy can be traced back over a century, whilst the introduction of neuroleptics into psychiatry practice 40 years ago witnessed the emergence of fits as a side effect of dopamine receptor blockade. Epidemiologists noticed a reciprocal relationship between the supposed dopaminergic overactivity syndrome of schizophrenia and epilepsy, which came to be regarded as a dopamine underactivity condition. Early pharmacological studies of epilepsy employed nonselective drugs, that often did not permit dopamine's antiepileptic action to be clearly dissociated from that of other monoamines. Likewise, the biochemical search for genetic abnormalities in brain dopamine function, as predeterminants of spontaneous epilepsy, proved largely inconclusive. The discovery of multiple dopamine receptor families (D1 and D2), mediating opposing influences on neuronal excitability, heralded a new era of dopamine-epilepsy research. The traditional anticonvulsant action of dopamine was attributed to D2 receptor stimulation in the forebrain, while the advent of selective D1 agonists with proconvulsant properties revealed for the first time that dopamine could also lower the seizure threshold from the midbrain. Whilst there is no immediate prospect of developing D2 agonists or D1 antagonists as clinically useful antiepileptics, there is a growing awareness that seizures might be precipitated as a consequence of treating other neurological disorders with D2 antagonists (schizophrenia) or D1 agonists (parkinsonism).

Effect of unilateral claustral lesion on intermittent light stimulation-induced convulsive response in D,L-allylglycine treated cats

The effect of unilateral lesions of the claustrum was examined in cats treated with D,L-allylglycine. Prior to the lesion, intermittent light stimulation (ILS) induced (a) myoclonic jerking associated with generalized spike, or polyspike and wave discharge, maximal in the subcortical structures monitored and the cortical visual area, and (b) bisymmetrical generalized-onset tonic-clonic convulsions associated with sustained spike discharge in the motor cortex bilaterally. Subsequent to a unilateral lesion of the claustrum, ILS-induced electro-clinical manifestations of myoclonic jerking remained unchanged. However, the bisymmetrical convulsive pattern transformed into a partial onset secondarily generalized convulsive pattern beginning in the intact hemisphere. It is concluded that the claustrum plays an important role for access of visual afferents to the motor mechanism responsible for ILS-induced convulsive seizure.

Pharmacological models of generalized absence seizures in rodents

A number of animal models of generalized absence seizures in rodents are described. These include absence seizures induced by gamma-hydroxybutyrate (GHB), low dose pentylenetetrazole, penicillin, THIP, and AY-9944. All of these models share behavioral and EEG similarity to human absence seizures and show pharmacologic specificity for antiabsence drugs such as ethosuximide and trimethadione. Moreover, the absence seizures induced by these agents are exacerbated by GABAergic agonists, a property unique to experimental absence seizures. These models are predictable, reproducible, and easy to standardize. They are useful both in studying mechanisms of pathogenesis of absence seizures as well as in screening for antiabsence activity of potential antiepileptic drugs.

Neuromagnetic topography of photoconvulsive response in man

The neuromagnetic method was applied to the study of photoconvulsive responses. The identification of specific magnetic field distributions over the scalp was achieved by; (a) a stimulation paradigm consisting of series of trains of flicker stimuli randomly presented to the epileptic patient, after eye closure, to get epileptic responses while avoiding seizures; (b) a novel procedure for data analysis, to select consistent responses. These patterns, when sufficiently stable in time and dipolar in shape, were used for source localization in the usual biomagnetic framework of the equivalent current dipole source representation. The results of this approach suggest that different specific cortical areas are repeatedly and randomly activated, involving mainly the frontal, occipital and temporal areas, often with a hemispheric prevalence.

gamma-Hydroxybutyrate model of generalized absence seizures: further characterization and comparison with other absence models

gamma-Hydroxybutyrate (GHB) produces a constellation of EEG and behavioral events that respond selectively to antiabsence antiepileptic drugs. The GHB-induced seizure was quantitated in the presence of three other absence seizure models: pentylenetetrazole, systemic penicillin, and the flash evoked afterdischarge (FEAD). Penicillin and pentylenetetrazole produced a significant prolongation of GHB-induced seizure in a dose-dependent fashion. This potentiation of GHB seizure was observed when these compounds were given either before administration of gamma-butyrolactone (GBL), the prodrug of GHB, or at the onset of GHB-induced seizure. Photic stimulation given in a manner to produce FEAD also resulted in a significant prolongation of GHB-induced seizure. All of these maneuvers lowered the threshold to GHB seizure, but none interfered with the brain kinetics of GHB in the animals treated with GBL. Ethosuximide pretreatment significantly shortened the GHB seizure and overcame the potentiating effect of penicillin and pentylenetetrazole in this model. These data confirm the GHB-treated animal as a model of generalized absence seizure. The GHB model meets appropriate criteria for an absence seizure model and compares favorably with other models of absence currently in use.

Role of the forebrain commissure and hemispheric independence in photosensitive response of epileptic baboon, Papio papio

The effect of monocular intermittent light stimulation (ILS) of either hemivisual field (HVF) of the full visual field (FVF) was examined in Papio papio with or without forebrain bisection. ILS of the HVF or the FVF in non-bisected baboons produced bisymmetrical and bisynchronous spike and wave which was followed by a self-sustained seizure without EEG evidence of hemispheric independence. ILS of the FVF in bisected baboons also produced bilateral spike and wave and self-sustained seizures of a similar nature. With ILS of the HVF in bisected baboons, EEG seizures lateralized largely to the contralateral hemisphere and when the ILS of the HVF was switched to the other eye similarly lateralized spike and wave and a self-sustained seizure were produced in the other hemisphere. These findings suggest that (a) the forebrain commissure, most probably the corpus callosum (and possibly the hippocampal commissure), plays a major but not unique role in the bisynchronization and generalization of the ILS-induced spike and wave and the self-sustained seizures, and (b) each hemisphere possesses independent cerebral excitability to the ILS.

Photic epilepsy problems raised in man and animals

The data gathered in 30 years' study in man and in several animal species, but especially in the Papio papio baboon, tend to show that the cortex plays a decisive part in the seizure and interval discharges induced by intermittent light stimulation in photic epilepsy. Two regions of the cortex predominate: the frontorolandic and occipital regions. The cortical cortex can, indeed, transmit or control the visual input to the frontorolandic region and can cause intermittent discharges in certain specific conditions in baboons and in certain human patients. The corticocortical pathway conveys the visual impulses to the frontorolandic cortex and is certainly modulated by deep structures like the reticular systems and the thalamus. At present no more can be said since further research is needed.