strychnine and Epilepsies--Partial

In the anaesthetized rat, regions of the somatosensory cortex have been subpially isolated, leaving intact the cortical blood supply and the connectivity via the white matter. Application of penicillin or strychnine into layer IV of intact cortex resulted in enhancement of amplitude and prolongation of evoked potentials together with the appearance of spontaneous epileptiform discharges. Within a partially isolated region of cortex, spontaneous and evoked potentials occurred as in normal cortex, but application of convulsant drug resulted in no changes in evoked potentials and in no spontaneous spiking. With incisions for which the surface profile measured 0.9 x 0.9 mm, full-depth isolation resulted in interruption of the propensity for epilepsy, whereas half-depth incisions left epileptic manifestations unimpaired. With the surface profile measuring 0.5 x 0.5 mm, half-depth isolation was sufficient to prevent epileptic activity. Results from isolated regions of various geometries and sizes indicated that the ability of cortical neurones to generate epileptic activity depends on the amount of connectivity with surrounding cortex. The propensity of cortex to become epileptic is thus a mass action effect and the 'epileptic neuronal aggregate' is operationally different from anatomically based modular organizations such as thalamo-cortical or cortico-cortical columns. In the small barrel field of the somatosensory cortex, partial isolations that prevented the appearance of spontaneous epileptiform spiking contained many barrels, indicating that a single thalamo-cortical module contains insufficient inherent lateral connectivity to support epileptiform activity. Theoretical considerations indicated that the excitability of a neurone depends both on its monosynaptic connections with other neurones and on the connectivity of these latter with neurones further afield. The interruption of epileptiform activity by partial isolation could be mimicked by a computer model in which connectivity was mediated via short synaptic paths. The model exhibited self-sustaining synchronized neural activity that could be prevented by interruption solely of polysynaptic paths.

Topics: Animals; Computer Simulation; Disease Models, Animal; Electroencephalography; Epilepsies, Partial; Evoked Potentials, Somatosensory; Male; Microelectrodes; Neural Pathways; Penicillins; Rats; Rats, Sprague-Dawley; Somatosensory Cortex; Strychnine; Thalamus

Acute experiments in rats showed that SC electrical stimulation (ES) (100 Hz, 5 V, 0.25 ms) resulted in suppression of the epileptic activity in contralateral strychnine-induced (0.1%; 1.0%) focus and shortening of the focus existence time. After electrocoagulation of SC ES-zone the EpA in the focus was restored. ES of SC ipsilateral to strychnine-induced (0.1%) focus caused an increase in the spike frequencies but significant part of the antiepileptic system.

Topics: Action Potentials; Animals; Cerebral Cortex; Electric Stimulation; Electroencephalography; Epilepsies, Partial; Male; Rats; Rats, Wistar; Strychnine; Superior Colliculi; Time Factors

Foci of increased excitability were created in acute experiments on rats by means of weak strychnine solutions working at independent regimens. The hyperactive excitability focus induced by means of concentrated strychnine solutions played the role of a determinant structure. The importance of the latter is in the fact that it determines the activity character of other epileptogenic foci, enhances their convulsive activity, unites them into a single functional complex and determines the behaviour of the complex as a whole. This complex can be destroyed by depression of the determinant focus activity. Switching off any dependent foci included into this complex fails to destroy that latter. Results of the investigations confirmed on the new model the general concept of the role played by the determinant structures in the brain activity.

Topics: Acute Disease; Animals; Cerebral Cortex; Disease Models, Animal; Electroencephalography; Epilepsies, Partial; Evoked Potentials; Rats; Seizures; Strychnine

In experiments on cats foci of enhanced excitation working in independent regimes were created in the neuronal-isolated cortex using weak strychnine solutions. Creation of a hyperactive focus by means of application of strychnine concentrated solutions or crystals leads to an increase in the amplitude and discharge frequency in other foci, synchronism in discharges of these foci and their unification into a single functional complex, working in the hyperactive focus regime. The latter, in such a way, presented the determinant structure. Under the influence of the determinaant focus there was also a marked generalization of the convulsive activity manifested by an embrace with epileptical discharges of the cortex area intact from strychnine. The results obtained indicate that the established relations between the foci of hyperactivity in the cortx as well as the effects of the determinant structure may be realized by the cortex mechanisms proper.

Topics: Action Potentials; Animals; Cats; Cerebral Cortex; Decerebrate State; Epilepsies, Partial; Strychnine

It was shown in experiments on cats that a lesion of the medial forebrain bundle (MFB) and partly of the preoptic region (RPO) at the side of local strychnine application of the cortex (g. suprasylvius medius) resulted in depression of the epileptiform activity in the strychnine-induced focus, as well as in the second "mirror" focus appearing in the symmetrical cortical area of the other hemisphere. This effect could also be obtained under conditions of injury of the MFB alone. The lesion of the MFB and partly of the RPO at the side of the "Mirror" focus leads to depression of the spike potentials in this focus only and does not influence the activity in the primary epileptiform focus. The described effects are considered from the aspect of conceptions of the role played by the determinant dispatch station (DDS) in the central nervous system activity: the primary epileptiform focus plays the role of the hyperactive DDS which induces the development of the secondary foci and maintaines and determines the character of their activity. The results of these studies suggest the participation of the MFB in the modulation of the epileptiform activity in the cortex.

Topics: Action Potentials; Animals; Brain Injuries; Cats; Cerebral Cortex; Epilepsies, Partial; Hypothalamus; Medial Forebrain Bundle; Neural Pathways; Preoptic Area; Strychnine