Regional properties of dorsal and ventral hippocampus in suppression of intralaminar thalamic unit responses.

In chloralose-anaesthetized, Flaxedil-paralysed cats, the suppression of extra-lemniscal thalamic units by dorsal and ventral hippocampus was investigated. Unitary responses to test somatic stimuli, recorded in centrolateral and neighbouring thalamic nuclei, were interacted with conditioning electric stimulation in different regions around the hippocampal arch, including parahippocampal gyrus (entorhinal and retrosplenial areas). Stimulation of dorsal (dhc) and ventral (VHC) hippocampus suppressed roughly equal proportions of responses. However, within each of DHC and VHC, effectiveness depended on the region stimulated. In DHC, fields CA1 and CA3, subiculum (SUB), and retrosplenial area, but not field CA4 or dentate gyrus, usually suppressed extralemniscal units at currents below 1.0 mA. In VHC, the most effective regions were entorhinal cortex, CA3, and CA4 with dentate gryus (FD), while stimulation of CA1 or subiculum was almost ineffective, at currents below 1.0 mA. In VHC, the regions were ranked for effectiveness: Entorhinal cortex=CA3 is greater than FD is greater than SUB is greater than CA1. No topographic relationship was found between hippocampal region and thalamic loci for unit suppression. Lemniscal-type unit responses in ventrobasal thalamus were unaffected by stimulation of the hippocampus or parahippocampal gryus. Interruption of the fornix-fimbria system prevented suppression elicited from CA1 of DHC, or from CA3 but not FD of VHC. It had no effect on suppression elicited from retrosplenial or entorhinal cortex. Hippocampal regional variation of effectiveness in suppressing extralemniscal pathways may contribute to the differential behavioural involvements reported for different hippocampal structures.