Showing papers by "Tamás F. Freund published in 1990"

Serotonergic control of the hippocampus via local inhibitory interneurons.

Abstract: Information flow and processing in hippocampal neuronal networks is determined by a wide range of inhibitory mechanisms [e.g., feedforward or feedback, gamma-aminobutyrate (GABA) A or B receptor-mediated, perisomatic shunting, or distal dendritic inhibition], each subserving specialized functions. These forms of local inhibition are mediated by morphologically and neurochemically well-defined, mostly GABA-containing, interneurons, which control large populations of principal cells through their extensive axonal arborizations. These neurons can serve as ideal targets for subcortical pathways, such as those originating in the septum or raphe, which exercise a global control over hippocampal activity. This intriguing possibility prompted us to study whether the profound effect of the serotonergic raphe-hippocampal pathway is mediated by inhibitory interneurons or whether a direct diffuse action on the principal cells is dominant. We demonstrate that axons of this pathway form multiple synaptic contacts with hippocampal GABAergic interneurons. Interestingly, the serotonergic afferents selectively innervate the somata and dendritic trees of GABAergic neurons that contain the 28-kDa calcium-binding protein calbindin D28K, but never those that contain another calcium-binding protein, parvalbumin. These results show that the mechanism by which the serotonergic pathway may exert a powerful influence on hippocampal function involves the modulation of local inhibitory circuits. Furthermore, the selectivity in the choice of target GABAergic interneurons suggests a strong functional specialization among inhibitory circuits, as well as among the subcortical input pathways originating in the septum and raphe.

Distribution of septohippocampal neurons containing parvalbumin or choline acetyltransferase in the rat brain

Abstract: A combination of retrograde transport of horseradish peroxidase or wheat germ agglutinin-colloidal gold with either single or double-label immunohistochemistry is used to describe the comparative topographic distribution of parvalbumin- and choline acetyltransferase-immunoreactive septal neurons that project to the hippocampal formation of the rat. The morphometric parameters of the retrogradely labelled, parvalbumin-containing neurons were very similar, if not identical, to those neurons of the midline and medial part of the medial septum and the diagonal band regions that had previously been shown to be immunoreactive for gamma-aminobutyric acid or for glutamate decarboxylase following colchicine treatment. The total number of parvalbumin-immunoreactive and choline acetyltransferase-positive retrogradely labelled cells was counted at 9 representative levels through the rostrocaudal extension (from 2.4 mm anterior to the level of bregma) of the medial septal-diagonal band complex. In the whole medial septum-vertical limb of the diagonal band region, about 33% of the total retrogradely labelled neurons showed immunoreactivity to parvalbumin, whereas the parvalbumin-negative cells were mainly choline acetyltransferase-immunopositive. In comparison with the average figure, the proportion of the retrogradely labelled parvalbumin-containing neurons was higher in the middle part (around 1.5 mm anterior to the bregma) than in either the rostral or caudal ends. The reverse was true for the distribution of the cholinergic septohippocampal neurons. At the maximum levels the parvalbumin-immunoreactive neurons accounted for more than half of the total retrogradely labelled cells in 4 out of 6 rats. Moreover, within the complexity of the septal neurons, a marked regularity of topographic organisation was observed in the distribution of retrogradely labelled parvalbumin-containing GABAergic and choline acetyltransferase-positive cholinergic neurons as if they were subdivided cytoarchitectonically.