Electrophysiological characteristics of non-bursting, glutamate decarboxylase messenger RNA-positive neurons of the medial septum/diagonal band nuclei of guinea-pig and rat

Nuclei of the medial septum/diagonal band region of the mammalian forebrain contain neurons that give rise to the septohippocampal pathway, which has separate cholinergic and GABAergic components. This pathway is known to inf luence hippocampal-dependent memory and learning processes, but the precise role of each component is unclear. In this study, we tested the hypothesis that fast-firing, non-bursting medial septum/diagonal band neurons are GAB Aergic. We used brain slice preparations from young adult guinea-pigs and r ats, or from weanling rats, to perform current-clamp recordings from medial septum/diagonal band neurons. Recorded neurons were injected with biocytin for subsequent visualization with fluorescent avidin, and then hybridized with a S-35-labeled riboprobe for glutamate decarboxylase-67 messenger RNA. As a positive control, guinea-pig cerebellar Purkinje cells were labeled a nd hybridized with the riboprobe. As expected, labeled Purkinje cells were glutamate decarboxylase-67 messenger RNA positive. Slow-firing, cholinergic (choline acetyltransferase-positive) guinea-pig medial septum/diagonal ban d neurons were glutamate decarboxylase-67 messenger RNA negative. Contrary to our hypothesis, of the guinea-pig neurons, only three of 11 fast-firing neurons were glutamate decarboxylase-67 positive. Of the rat medial septum/ diagonal band neurons, three of four were positive for glutamate decarboxyl ase-67 messenger RNA. These data suggest that fast-firing, non-bursting neurons of the medial sep tum/diagonal band, as sampled by sharp-electrode intracellular recordings i n brain slices, may be a heterogeneous group of neurons, some of which are GABAergic. Together with recent data demonstrating the presence of another GABAergic marker, parvalbumin, in fast-firing septal neurons, we conclude t hat GABAergic septohippocampal neurons include a population of fast-firing, non-bursting neurons. The influence of these neurons on the hippocampus is likely to occur on a shorter time-scale and over a wider range of firing f requencies as compared to slowly firing cholinergic septohippocampal neuron s. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.