An autoradiographic study of the organization of intrahippocampal association pathways in the rat

Abstract

The longer associational connections of the hippocampal formation have been studied autoradiographically in a series of adult rats after small injections of 3H-amino acids into each of its various cytoarchitectonic fields. The major findings can be summarized as follows. The dentate gyrus projects in a topographically ordered manner upon the pyramidal cells of the regio inferior by way of the supra-and infrapyramidal bundles of mossy fibers. Certain cells in the hilar region of the dentate gyrus (which operationally may be defined as constituting field CA, of Ammon's horn) give rise to a hippocampodentate projection to the inner one-quarter of the molecular layer of the dentate gyrus. Either the same or closely related cells give rise to fibers which join the Schaffer collateral system from field CA3 to the stratum radiatum and stratum oriens of the regio superior. The regio inferior is also characterized by a longitudinally directed associational bundle which runs throughout the septotemporal extent of the hippocampus and is centered in the region of subfield C h a . The regio superior has no reciprocal projection to the regio inferior but sends a substantial projection back to the subiculum and to the entorhinal area. There is also a projection to the subiculum from the regio inferior, and the subiculum itself probably contributes significantly to the projection to the entorhinal and perirhinal cortices.There is a striking parallelism between certain of these associational connections and the commissural projections to the hippocampus and dentate gyrus. Each cytoarchitectonic field that contributes a commissural projection also gives rise to an ipsilateral associational pathway which in its intrahippocampal course and its mode of termination exactly matches that of the commissural projection, although in general, the associational connections are more extensive in their distribution along the septo-temporal extent of the hippocampus than the corresponding commissural connections. The reverse is not true; there are a number of associational projections which are not paralleled by a commissural projection. All of the associational projections are topographically arranged, but those which extend across the transverse axis of the hippocampus usually show considerable divergence so that afferents from different levels overlap fairly considerably within their respective projection fields.Of the various classes of hippocampal connections, the intrahippocampal association pathways are by far the least well understood. Indeed, the most complete account of these connections is still that of Lorente de NO ('34), whose careful analysis of Golgi impregnated material of t h e rodent hippocampus has served as the starting point for most later studies. However, a t best the Golgi technique is useful only for the study of relatively short axonal trajectories (usually within a single hippocampal field) and our knowledge of the longer associational systems linking the vari- (Schaffer, 1892; Cajal, '11; Lorente de NO, '3...

Temporal sequence of ictal discharges propagation in the corticolimbic basal ganglia system during amygdala kindled seizures in freely moving rats

“…The delayed activation of hippocampal neurons does not necessarily indicate that cortical or SNr activity drives hippocampal synchronized firing. The delay may be a result of the trisynaptic circuit and dentate filtering within the hippocampal complex (Coulter, 2000), whereby the signals traveling from the amygdala and entorhinal cortex to the dentate gyrus, and from there to the CA3 and CA1 regions, experience delays at each step (Anderson et al, 1971;Swanson et al, 1978). On the other hand, tightly coupled synchronization between both sides of the hippocampus was found in the present study.…”

The Promnesic Effect of G-protein-Coupled 5-HT4 Receptors Activation Is Mediated by a Potentiation of Learning-Induced Spine Growth in the Mouse Hippocampus

“…Apical and oblique dendrites of CA1 neurons receive afferents from the Schaffer's collaterals laying in the same hemisphere and therefore connect CA3 to CA1 regions within the left or the right hippocampus. Basal dendrites also ensure CA3 to CA1 intrahemispheric connections but receive a stronger contralateral input making them extensively involved in intrahemispheric neurotransmission (Swanson et al, 1978). Another characteristic of basal dendrites is their major propensity to undergo plastic changes (Kaibara and Leung, 1993).…”

Novel two-dimensional morphometric maps and quantitative analysis reveal marked growth and structural recovery of the rat hippocampal regions from early hypothyroid retardation

“…Although CA2 and CA4 regions are small compared to CA1 and CA3, their total surface area (15%) warrant a closer attention to the contribution of these regions to HC function. Swanson et al (1978) and Gaarskjaer (1978) developed the first unfolded and extended maps of the rat HC (see also Swanson, 1998). Swanson et al (1978) constructed a 3D, large-scale model and realigned tracings of the cell layer in frontal sections in such a way as to provide a representation of the HC as longitudinal cortical fields in a septo-temporal direction.…”

“…Swanson et al (1978) and Gaarskjaer (1978) developed the first unfolded and extended maps of the rat HC (see also Swanson, 1998). Swanson et al (1978) constructed a 3D, large-scale model and realigned tracings of the cell layer in frontal sections in such a way as to provide a representation of the HC as longitudinal cortical fields in a septo-temporal direction. Our 2D maps attempt to unfold, flatten and extend the HC using the midline of the brain as a reference point.…”