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Widespread cortical projections of the hippocampal formation in the cat
Insights
The subicular cortex extensively projects to medial cortical areas, including the medial prefrontal cortex. This pathway may link neocortical association areas with memory-related limbic system regions.
Area of Science:
- Neuroscience
- Neuroanatomy
Background:
- The hippocampal formation plays a crucial role in memory and cognition.
- Understanding its cortical projections is vital for deciphering brain circuitry.
Purpose of the Study:
- To map the efferent projections from the hippocampal formation to the cat's cortex.
- To identify specific cortical targets of hippocampal subregions.
Main Methods:
- Retrograde horseradish peroxidase (HRP) tracing technique.
- Stereotactic injections of HRP into various cortical areas of 31 cats.
- Screening of hippocampal subregions for labeled neurons.
Main Results:
- Subicular areas of the hippocampal formation contain labeled neurons projecting to the cortex, except for entorhinal injections.
- Dense projections from the subicular cortex target medial cortical areas: cingulate, retrosplenial, and medial prefrontal cortex.
- The entorhinal cortex receives projections from the subicular area.
- Sparse projections also reach dorsal prefrontal, temporal, parietal, and prepiriform cortex.
Conclusions:
- The subicular cortex is a major source of hippocampal efferents to the neocortex.
- The projection to the medial prefrontal cortex is significant, potentially linking neocortical association areas with memory-related limbic structures.
- This pathway may be crucial for integrating cognitive and memory functions.
Abstract:
Efferent projections from the hippocampal formation to the cat's cortex were traced with the retrograde horseradish peroxidase technique. Different areas of the cortex of 31 cats were injected with small amounts of horseradish peroxidase. All subregions of the hippocampal formation were screened for labeled cells. It was found that, with the exception of the entorhinal injections, only subicular areas of the hippocampal formation contain labeled neurons. When HRP was injected into the entorhinal cortex, labeled cells are also found in the hippocampus proper. The most dense projection from the subicular cortex is directed to the medial part of the cortical hemisphere. Here, cingulate, retrosplenial and medial prefrontal fields receive a substantial number of subicular efferents. Furthermore, the entorhinal cortex is reached by a number of axons originating in the subicular area. Scarce projections from the subicular cortex terminate in the dorsal prefrontal, temporal, parietal and prepiriform cortex. It is suggested that the projection from the subicular cortex to the neocortical areas of the frontal pole (medial prefrontal cortex) is of special importance as it may constitute a link between the association areas of the neocortex and those regions of the limbic system thought to play a role in memory (subicular cortex, mamillary bodies, anterior thalamus, cingulate gyrus).