Parvalbumin-immunoreactive, fast-spiking neurons in the medial septum diagonal band complex of the rat: Intracellular recordings in vitro

The medial septum/diagonal band complex is composed predominantly of cholin ergic and GABAergie neurons, and it projects to the hippocampal formation. A proportion of the GABAergic neurons contain parvalbumin, a calcium-bindin g protein that has previously been localized in fast-spiking, non-accommoda ting GABAergic neurons in the cerebral cortex and neostriatum. The aim of t he present study was to determine whether parvalbumin is localized preferen tially in a similar electrophysiological class of neuron in the medial sept um/diagonal band complex. The study was carried out using in vitro intracel lular recording, intracellular biocytin filling and parvalbumin immunocytoc hemistry. Three main classes of neurons were identified according to standa rd criteria: burst-firing, slow-firing and fast-firing neuronal populations . The fast-firing neurons were subdivided into two subpopulations based on whether or not they displayed accommodation. The fast-spiking, non-accommod ating cells were furthermore found to be spontaneously active at resting po tentials, and to possess action potentials of significantly (P < 0.05) shor ter duration (half width: 0.61 +/- 0.12 ms) than those of the regular-spiki ng. accommodating neurons (1.0 +/- 0.34 ms). Of the neurons that were succe ssfully filled with biocytin and processed for parvalbumin immunoreactivity , 82% of the fast-spiking, non-accommodating cells possessed parvalbumin im munoreactivity, while none of the regular-spiking, accommodating neurons we re found to be immunoreactive for parvalbumin. The slow-firing neurons, sho wn previously to be cholinergic, did not stain for parvalbumin immunoreacti vity, in agreement with studies showing parvalbumin to be localized solely in GABAergic neurons in the medial septum/diagonal band complex. In conclusion, these findings suggest the presence of a previously uncharac terized population of neurons in the medial septum/diagonal band complex th at generate high-frequency, non-adaptive discharge. This property correlate s with the localization of parvalbumin in these neurons, which suggests tha t parvalbumin fulfils the same role in the medial septum/diagonal band comp lex that it does in other parts of the brain. The fast-spiking neurons in t he medial septum/diagonal band complex may play an essential role in the GA BAergic influence of the septum on the hippocampal formation. (C) 1999 IBRO , Published by Elsevier Science Ltd.