Chan, Dennis; (1992) A topographical analysis of hippocampal field connectivity. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The intrinsic connectivity of the hippocampus in the rat was studied using the retrograde tracers Fast Blue and rhodamine microspheres. In an initial double-labelling study the bilateral labelling within the CA3 field was observed following injection of the two dyes into homologous regions in the two CA1 fields. The labelling in the contralateral CA3 field was found to be identical both in distribution and in density to that observed in ipsilateral CAS, suggesting that the CAS-CA1 projection is bilaterally symmetrical. The observation that nearly 100% of backfilled cells were double-labelled indicates that homologous regions in the two CA1 fields receive information from the same cells in CAS, which indicates that the two CA1 fields act as one functional unit. Evidence for the existence of a sparse association/commissural system within CA1 was provided by the presence of a small number of labelled cells in the contralateral CA1 field. The topographic organization of the CAS-CA1 projection was studied following a series of injections of the two dyes into different locations across the extent of the CA1 field. In this experiment a new technique for the injections of tracers into the hippocampus was introduced, in which a microelectrode positioned adjacent to the tip of the injection pipette was used to record neuronal activity during the operation. This allowed the complex spike firing of hippocampal cells to be used as a marker for the dorsoventral location of the injection, thus greatly improving the accuracy of the injections. An analysis of the labelling in the extended hippocampus reveals that projections to CA1 arise from bands of cells organized diagonally across the CAS field. The CAS-CA1 projection was topographically organized in the septotemporal plane, with progressively more temporal parts of CA1 receiving inputs from bands located more temporally in CA3, Further analysis showed that cells positioned along a diagonal axis in CA1 were found to receive fibres from the same band of cells in CA3, and that this "CA1 axis" is similar to the axis of the diagonal bands in CAB. It therefore appears that the CA3-CA1 projection is organized in a diagonal fashion across the septotemporal length of the hippocampus both in terms of projecting and recipient cells. A number of injections was also placed into the CA3 field in order to study the CA3 association projection. In these cases the pattern of labelling in the CA3 field in the extended hippocampus was organized parallel to the septotemporal axis, and was topographically organized such that any given region in CA3 preferentially received fibres from cells located along the Scime transverse segment of CA3. These findings support the notion that the CA3 association pathway interconnects cells spread over a large fraction of the septotemporal extent of CAS. Preliminary data concerning the organization of the projection from the dentate gyrus to CA3 were collected following an injection of biocytin into the dentate gyrus. Additional data was taken from 3 animals in which rhodamine microspheres were found to be transported anterogradely along the mossy fibres in a previously unreported fashion. In all cases the projection was organized orthogonal to the septotemporal axis for most of the its length, with a shift in trajectory in proximal CA3 in the temporal direction. These results show that these three intrahippocampal pathways are angled with respect to each other, thus forming a lattice framework of connections similar to the "crossing fibre arrays" described by Tamamaki and Nojyo (1991a). Each pathway displays a specific directionality which, in contrast to the original lamellar hypothesis, allows the spread of information across all parts of the hippocampus. (Abstract shortened by ProQuest.)

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