The influence of intra-hippocampal gamma-aminobutyric acid on the orienting reflex

Abstract

The spontaneous electrical background and evoked orienting EEG activities were tested before and after intra-hippocampal administration of gamma-aminobutyric acid (GABA) to eleven adult cats with electrodes chronically implanted in the ectosylvian and suprasylvian gyri, mesencephalic reticular formation (MRF), posterior hypothalamic area and dorsal hippocampus. Concomitantly the spontaneous and auditorily induced behaviour were observed. The results were as follows: 1. 1. The effects of intra-hippocampal GABA appeared 2 min following uni- or bilateral administration, were, mostly evident between 2 and 7 min and disappeated after 35–40 min. The rapid effect of the drug was due to its intra-hippocampal administration, which permitted avoidance of the blood-brain barrier and thus immediately induced a high concentration of GABA. 2. 2. No changes in the animals behaviour and in the electrical activity of the neocortex were observed (the neocortical arousal reaction and the potentials evoked by repetitive auditory stimulation were not altered). Thus intra-hippocampal GABA exerts its influence exclusively upon the hippocampus. 3. 3. Intra-hippocampal GABA induced changes in hippocampal activity (slight increase of spontaneous electrical activity, faster extinction of the hippocampal evoked response to repetitive auditory stimulation and depression, or even suppression, of the spontaneous or evoked arousal reaction), delayed extinction of orienting behaviour and convulsive activity in MRF. The EEG and behavioural changes were simultaneous. 4. 4. The authors assume that the alterations are due to a depressing action of the drug on hippocampal neurones. The local depressing effect is responsible for suppression of the modulating effect of the hippocampus on the MRF and therefore upon the orienting reflex.