The hippocampus occupies a high level within the cortical hierarchy dedicated to the representation of spatial learning, episodic memory and associative learning. Its contributions vary along the septo-temporal axis. The subject of this thesis is the "ventral" hippocampus (at the temporal pole), that is involved in modulating stress responses and motivated or emotional behaviors, supported by widespread long-range projections to the related subcortical and cortical regions.
Previous tracing studies could not characterize projections of individual neurons together with their cellular identity and their activity patterns. This study focused on pyramidal neurons in the ventral CA1 hippocampus, which provides the main output of the hippocampus. I have identified three main long-range projection routes, enabling individual neurons to innervate the following targets: The first branch runs directly antero-dorsally through the fimbria/fornix system towards the septal complex, hypothalamus, ventral striatum and olfactory areas. Via the second (also direct) route, the axons leave the ventral hippocampus before travelling antero-ventrally to the amygdaloid complex, amygdalo-hippocampal transition areas and/or caudo-ventrally to the parahippocampal, temporal associational and olfactory-related caudal regions. The third route, which is the classical one, first branches in the str. oriens and innervates the local CA1 before running caudo-ventrally, giving rise to branches that innervate the ventral subiculum. After leaving this area it targets the caudal subicular complex and parahippocampal areas.
Calbindin positive neurons were shown to innervate the stratum moleculare of the ventral subiculum locally, the neurochemically distinct calbindin negative neurons avoided this subregion. Furthermore, I observed that calbindin negative cells are usually projecting more widespread and calbindin positive cells have a high local connectivity, but less extended extrahippocampal projections indicating a certain level of correlation between the neurochemical profile and the projection pattern of a neuron.
These results suggest that ventral hippocampal pyramidal neurons may form different subpopulations based on their local intrahippocampal and extrahippocampal projection fields and neurochemical profiles.