RHYTHMIC FIRING OF MEDIAL SEPTUM NONCHOLINERGIC NEURONS

The presence of theta rhythm(3) (5-10 Hz) in the hippocampus has been shown to enable long-term potentiation,(12,18) a synaptic mechanism wh ich has been proposed to underlie learning and memory. Medial septum c holinergic and GABAergic neurons that project to the hippocampus(7,11, 17,24,26,34) have been hypothesized to play conjointly a major role in the genesis of this rhythm.(3,19,31) Building upon previous studies t hat have established the electrophysiological criteria for distinguish ing cholinergic and non-cholinergic neurons in this area,(1,2,8,9,14,2 3) it is demonstrated here that medial septum non-cholinergic neurons, putatively GABAergic, have the ability to discharge in rhythmic clust ers of action potentials occurring at frequencies ranging from 1 to 8 Hz. Within the clusters, the firing frequency of action potentials var ied between 13 and 57 Hz in a voltage-dependent manner. In addition, s mall voltage-dependent subthreshold membrane potential oscillations (1 6-54 Hz) were observed between clusters. Both subthreshold oscillation s and clusters mere eliminated by tetrodotoxin at 1 mu M. These result s indicate that non-cholinergic medial septum neurons could convey to the hippocampus not only theta but also higher frequency rhythmicity i n the beta-gamma range (20-60 Hz). Copyright (C) 1996 IBRO.