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Patterns of collateralization of the raphe nuclei and nucleus incertus to the septo-hippocampal system

Navarro Sánchez, M;Gil-Miravet, I;Montero Caballero, D;Chaima, L;Es-Safi, S;Olucha-Bordonau, F;

The relaxin 3 (rln3), rich nucleus incertus (ni), and the serotonin (5ht), rich raphe nuclei, are modular midline brainstem units that influence various brain functions, including context perception. This study aims to characterise the collateralisation pattern of the ni projection system on the raphe nuclei and the suprammamilar nucleus (sum) in its projections to the septohippocampal system. For this purpose, retrograde tracers fluorogold (fg) and cholera toxin subunit b (ctb) were injected into the medial septum (ms) and dentate gyrus (dg), combined with immunofluorescence of 5ht, rln3, fg and ctb. Our results show a high density of rln3 projection fibres in the medial and dorsal raphe regions. In this area, rln3 fibres were observed in close proximity to the 5ht neuronal soma, while others were observed in synaptic-like connections. Few 5ht neurons from the ms were revealed in the medial and dorsal raphe. Finally, a population of sum neurons sends collateral projections to the entorhinal/hippocampal system and to the ms, and this population is innervated by ni rln3 cells. These findings revealed the mapping of the modulatory circuitry between ni rln3 and raphe 5ht, and interactions with the septo-hippocampus, which receives projections from the sum. Based on this core evidence, it can be postulated that sum is a key component of projections from the ascending ni to the entorhinal-hippocampal circuitry that is involved in the generation, encoding and recall of contextual memories.