Date of Award

9-1996

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Psychology

Supervisor

Ronald J. Racine

Abstract

Human epilepsies are a family of disorders of the nervous system, characterized by transient, recurring episodes of neuronal hypersynchronous seizure activity. These often begin locally, butthey may generalize to produce convulsions. The basic mechanisms responsible for the study of the neuronal abnormalities associated with these events are unknown, but several animal models of epilepsy are available for the study of neuronal abnormalities associated with these events. One of these is the kindling model in which repeated application of electrical stimulation to certain brain regions results in the progressive development of electroencephalographic and behavioural seizures.

In this thesis, we compared two excitatory monosynaptic systems for rates of transfer kindling effects, levels and alterations in inhibition, kindling-induced cell loss and mossy fiber sprouting. Animals were kindled for 30 or more stage 5 seizures. The two monosynaptic systems studied were: 1) entorhinal cortex-perforant path-dentate gyrus system and 2) olfactory bulb-lateral olfactory tract-piriform cortex.

Our major findings may be summarized as follows: a) there was immediate transfer in the olfactory bulb-lateral olfactory tract-piriform cortex system but no in the entorhinal cortex-perforant path-dentate gyrus system. We hypothesized that the difference in tranfer rates was due to differences in the spatio-temporal patterns of discharge at the network level. b) In both systems, the levels of inhibition were increased following kindling, remained increased throughout administration of kindling stimulations and then returned to baseline levels after discontinuation of the kindling stimulations. This suggests that a disturbance in the inhibitory system of the dentate gyrus or the piriform cortex can not explain the epileptogenesis. c) We found a decrease in the number of hilar cells of the denate gyrus following kindling in the entorhinal cortex-perforant path-dentate gyrus system but not following kindling of the olfactory bulb-lateral olfactory tract-pirifrom cortex system. d) Sprouting of mossy fibers was found in the IML of the dentate gyrus following kindling of the lateral olfactory tract but only after at least 14 stage 5 seizures had been elicited. There was no evidence of sprouting these fibers into the stratum oriens of area CA3.

These findings suggest that kindling results in a remarkable specificity in the seizure of circuitry dependent on the site of stimulation that does not require cell loss in the hilus or mossy fiber sprouting into either the CA3 or the IML for it to occur. Since we found enhanced inhibition in both systems with kindling, a failure of inhibition, at least in these systems, can not explain epileptogenesis.

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