The normal morphology and distribution of parvalbumin-containing neurons (shown in a previous study to be GABAergic nerve cells) of the medial septal-diagonal band region of the adult rat brain have been studied, and the findings compared with observations on choline acetyltransferase-immunoreactive neurons. The two antigens were visualized either in the same sections using a double-label immunohistochemical procedure for the simultaneous localization of parvalbumin and choline acetyl-transferase, or in immediately adjacent sections. In double-stained sections of the whole medial septal-diagonal band complex, about 34% of the total neurons showed immunoreactivity to parvalbumin; the proportion of parvalbumin-labelled neurons was slightly higher in the medial septal-vertical limb of the diagonal band region, and much lower in the horizontal limb of the diagonal band region. The distribution of parvalbumin- and choline acetyltransferase-containing neurons also varied markedly between different mediolateral subdivisions of the medial septum: about 30, 65 and 2% of the parvalbumin-immunoreactive neurons were present in the midline, medial and lateral part of the medial septum, respectively. At different rostrocaudal levels, the proportion of parvalbumin- and choline acetyltransferase-positive neurons varied in a consistent manner, and the largest number of parvalbumin-containing neurons was found at the level 1.9 mm anterior to the bregma. In the absence of reliable immunocytochemical methods for the localization of glutamate decarboxylase and GABA, parvalbumin may serve as a good marker for studying the distribution of GABAergic neurons in the medial septum-diagonal band region. Moreover, the precise maps reported in the present study of the topographic localization of parvalbumin-containing GABAergic and choline acetyltransferase-immunoreactive cholinergic nerve cells in the medial septal-diagonal band complex will serve as a useful guide in future morphological and electrophysiological studies on the septum and its efferents.
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