Abstract
The Hippocampal Formation (HF) is the brain system that supports declarative memory in human and nonhuman primates. The HF is made up of dentate gyrus (DG), Cornu Ammonis fields CA3, CA2 and CA1, subiculum (S), presubiculum (PrS), parasubiculum (PaS) and entorhinal cortex (EC). The unidirectional circuit that links all these structures through several synaptic steps ultimately, end up in stable memories, very likely, in the cerebral cortex. This consolidation process takes place once the brainstem activity is cancelled (Logothetis et al., 2012). The HF receives cortical input from polymodal association areas, as well as subcortical brain centers, which are monoaminergic brainstem nuclei. The main centers with direct access to the HF, are dopamine containing cell groups such as the ventral tegmental area (VTA) and the substantia nigra (SN) and adjacent mesencephalic reticular formation, serotonergic (centralis superior and dorsal raphe nuclei), and noradrenergic (Locus coeruleus) as retrograde tracing studies demonstrated. The paths and termination of those brainstem projections to the different HF fields are unknown. In the course of an ongoing study aiming at the study of brainstem projections to the HF, we describe the resulting labeling after deposits into the SN and the in different parts of the VTA. 1) Direct nigro-hipocampal fibers are scarce, but present in all components of the HF in particular rostrally, while VTA are somewhat denser. 2) Both VTA and SN fibers were observed in non-cellular strata of the HF. 3) Both SN and VTA present fibers that change course and adopt a direction in a transversal plane to the main axis of the hippocampus, in stratum radiatum and lacunosum-moleculare, usually orthogonal to the direction of the dendrites. 4) The SN, was observed giving off fibers to the polymorphic cell layer of the DG, which crossed the granule cell layer, into innermost portion of the molecular layer of the DG. Our results are in agreement with retrograde studies in which scarce retrograde labeled neurons were found, and suggest that direct and presumably through modulation of the excitability of the dendritic field of neurons in the HF, they possibly produce an effect on the HF function in memory, as it has been shown already in the monkey (Logothetis et al., 2012).
