31. Effects of dextran on brain slice water, extracellular space and histology

Abstract

Conditioning electrical stimuli applied to the dorsal hippocampus of chloralose-anaesthetized, paralyzed cats, suppressed somatically evoked unit responses in the intralaminar thalamus. Mean onset latency for suppression was 29 msec (range: 10–45 msec), and mean duration of complete response suppression was 318 msec (range: 125–700 msec).Electrical stimulation at the level of the hippocampal pyramids or their proximal dendrites was required for suppression of intralaminar responses. Stimulation of the dentate fascia and area CA4 did not exert the same effect. The effect was not due to spread of current to supraventricular tissue, and was blocked by interruption of the fornix outflow pathway.Unit responses were suppressed by hippocampal stimulation more frequently in rostral centrolateral than in caudal centromedial and parafascicular intralaminar areas. A proportion of units in adjacent areas of laterodorsal, dorsomedial, ventrolateral, lateral posterior and limitants nuclei, and in the prerubral field, were also suppressed. No ventrobasal units, lemniscal or extralemniscal, were suppressed.Many of the same intralaminar units were suppressed by either hippocampal or caudate nucleus stimulation. Caudate stimulation was more likely than hippocampal to suppress units in caudal intralaminar nuclei.Suppression of intralaminar somatosensory responses is considered to provide a possible neural basis for reported similarities in the involvement of the hippocampus and caudate nucleus in behavioral plasticity.