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

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.