The presence of theta rhythm (5-10 Hz) in the hippocampus has been shown to enable long-term potentiation, a synaptic mechanism which has been proposed to underlie learning and memory. Medial septum cholinergic and GABAergic neurons that project to the hippocampus have been hypothesized to play conjointly a major role in the genesis of this rhythm. Building upon previous studies that have established the electrophysiological criteria for distinguishing cholinergic and non-cholinergic neurons in this area, it is demonstrated here that medial septum non-cholinergic neurons, putatively GABAergic, have the ability to discharge in rhythmic clusters of action potentials occurring at frequencies ranging from 1 to 8 Hz. Within the clusters, the firing frequency of action potentials varied between 13 and 57 Hz in a voltage-dependent manner. In addition, small voltage-dependent subthreshold membrane potential oscillations (16-54 Hz) were observed between clusters. Both subthreshold oscillations and clusters were eliminated by tetrodotoxin at 1 μM. These results indicate that non-cholinergic medial septum neurons could convey to the hippocampus not only theta but also higher frequency rhythmicity in the beta-gamma range (20-60 Hz).
Bibliographical noteFunding Information:
Acknowledgements-We thank Mrs Daniele Machard for her excellent technical assistance. This work was supported by grants from the Swiss Fonds National, the Sandoz and Roche Foundations (to MM), as well as by a postdoctoral fellowship from the Medical Research Council of Canada (to SW) and grants from the Fondation Fyssen and La Region Rhone-Alpes (to PF).
- 40 Hz oscillations
- basal forebrain
- theta rhythm
ASJC Scopus subject areas
- Neuroscience (all)