Perineuronal nets ensheath fast spiking, parvalbumin-immunoreactive neurons in the medial septum/diagonal band complex

Perineuronal nets, composed of extracellular matrix material, have previous ly been associated with parvalbumin-immunoreactive neurons in the medial se ptum/diagonal band (MS/DB) complex of the rat. The aim of this study was to correlate the presence of perineuronal nets with electrophysiological prop erties and parvalbumin immunoreactivity in MS/DB neurons. Intracellular rec ordings were made from cells in a brain slice preparation maintained in vit ro, and neurons were characterized into four populations: (i) slow-firing n eurons, (ii) burst-firing neurons, (iii) fast spiking neurons with narrow a ction potentials and a small degree of spike frequency adaptation, and (iv) regular spiking neurons with broader action potentials and a high degree o f spike frequency adaptation. Following electrophysiological characterizati on, neurons were filled with biocytin, processed for parvalbumin immunoreac tivity and stained for perineuronal nets using Wisteria floribunda lectin. The three substances were viewed with triple fluorescence. Fast spiking, no nadapting neurons, shown previously to contain parvalbumin immunoreactivity , were nearly all ensheathed by perineuronal nets. There was a population o f small parvalbumin-immunoreactive neurons which did not possess perineuron al nets, and which were not encountered with the intracellular electrodes. The other three neuron types in the MS/DB did not contain parvalbumin immun oreactivity or perineuronal nets. In keeping with this neurochemical profil e for electrophysiologically identified neurons, burst-firing neurons had a ction potential parameters more similar to those of regular spiking than of fast spiking neurons. We conclude that fast spiking neurons, presumed to b e GABAergic septohippocampal projection neurons, are surrounded by supporti ve structures to enable the high level of neuronal discharge required for p roducing disinhibition of hippocampal pyramidal neurons.