Convergence and segregation of septal and median raphe inputs onto different subsets of hippocampal inhibitory interneurons.

The convergence and segregation of medial septal and median raphe afferents in the innervation of different subpopulations of GABAergic interneurons was investigated in the rat hippocampal formation. Following local injections of 5,7-dihydroxytryptamine into the median raphe nucleus destroying all serotonergic neurons, iontophoretic injections of Phaseolus vulgaris leucoagglutinin (PHAL) into the medial septum resulted in anterograde labelling of axons in the hippocampus. The labelled varicose fibres made multiple contacts with calbindin D28K-, parvalbumin-, and cholecystokinin-immunoreactive interneurons. These results disproved the possibility that PHAL-labelled afferents innervating hippocampal interneurons following septal PHAL injections would have been raphe axons passing through the injection site. In the second set of experiments a double anterograde tracing technique (PHAL from the septum and biotinylated-PHAL from the median raphe) and a triple or double immunostaining procedure was used to determine the types of interneurons (calbindin D28K-, parvalbumin-, or cholecystokinin-immunoreactive) innervated by one, or the other, or both pathways. The results showed that parvalbumin-containing neurons were innervated by septal afferents but avoided by raphe axons, whereas calbindin D28K-containing cells, and to a smaller extent cholecystokinin-containing cells served as targets for both pathways. In some cases the same individual calbindin D28K- or cholecystokinin-containing neurons received multiple contacts from afferents of both septal and raphe origin. Thus, our results indicate that different subcortical nuclei modulate largely different inhibitory circuits in the hippocampal formation. However, considering the occasional convergence of the two subcortical nuclei not only onto the same type, but even onto the same individual calbindin D28K-containing interneurons, we propose that a particular inhibitory function, most probably feed-forward inhibition in the distal dendritic region, is under the control of both pathways.