Hippocampo-prefrontal cortex pathway in the rat: multi-approaches data for research perspectives

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The hippocampus and the prefrontal cortex are similarly critical in various aspects of cognition. Therefore, a lot of studies focused on the communication between these two neural substrates. The hippocampo-prefrontal connexion of the rat is now well documented through anatomical,electrophysiological and behavioural approaches. These studies all reveal the complexity of this pathway and therefore underlie the complexity of the involvement of the hippocampus and the prefrontal cortex in cognition through many aspects, such as the regional (subregional) distribution, the cognitive demand of the task (nature and degree), the temporal aspect (from the different times in a given cognitivetask, to long-term mechanisms such as the consolidation), and pharmacological influences (dopamine concentrations). We will review the different approaches to recall the latest knowledge about the hippocampo-prefrontal pathway, and aim at guiding new research perspectives towards the functional regionalisation of the medial prefrontal cortex and the involvement of the dopamine in its cognitivefunction in rodents.

1) Anatomical and electrophysiological characteristics of the hippocampo-prefrontal pathway:

Due to the critical implication of both of the hippocampus and the prefrontal cortex in various cognitive processes, the detailed anatomical description of the hippocampo-prefrontal pathway appears to be fundamental to lead further any complementary electrophysiological,pharmacological and behavioural studies, with the aim of determining the nature of their functional implication in cognition.

The different subregions involved in the hippocampo-prefrontal cortex pathway:
The anatomical relationships between the different substrates have been investigated a lot using anterograde and retrograde tracers. The latters allowed to provide the evidence of the complexanatomical connexions, mostly reciprocal, that the prefrontal cortex combines with the thalamic mediodorsal nucleus, and with distinct cortical and subcortical structures in the rat (Groenewegen, 1988 ; Sesack et al., 1989 ; Condé et al., 1995).
In 1981, autoradiographic and fluorescent retrograde tracers allowed Swanson to give the first evidence of a direct projection from the Ammon’s horn of thehippocampus to cortical regions outside the hippocampal formation. He described a direct ipsilateral pathway from the pyramidal cells of CA1 that project to the infralimbic area of the prefrontal cortex (Swanson et al., 1981). The evidence of another direct projection from the hippocampus to the prefrontal cortex was then given by Ferino in 1987. Not only confirming Swanson’s discovery, he reportedlabelled in ipsilateral CA1, and in the adjacent ventral and dorsal subiculum after injections into different sites of the medial prefrontal cortex that did not necessary include the infralimbic area, i.e., the prelimbic area. Mostly found in temporal parts of the hippocampus, the labelled cells were distributed along a caudo-rostral axis, leaving the most septal part of CA1 and of the subiculum,CA2-3, and the dentate gyrus devoid of labelled cells (Ferino at al., 1987). A few years later, combining retrograde and anterograde tracers, Jay and Witter demonstrated that the hippocampus projects from restricted portions of CA1 (mostly ventral part) and the subiculum (adjacent part to CA1) to distinct areas of the prefrontal cortex: the prelimbic and medial-orbital areas Jay et al., 1989; Jay andWitter, 1991). They also detailed the density and the distribution of the connexions through the different regions involved. Indeed, the intermediate portion of the subiculum projects more densely and more diffusely than its most dorsal and ventral portions. Similarly, different innervations patterns exist in the prelimbic area of the prefrontal cortex. Its ventral portion is innervated by the...
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