Induction by kainate of theta frequency rhythmic activity in the rat medial septum-diagonal band complex in vitro

The medial septum-diagonal band (MSDB) complex, via the septohippocampal pathway, is thought to be critical for the generation and/or maintenance of the hippocampal theta rhythm in vivo. The aim was to determine whether the MSDB is capable of generating and maintaining its own rhythmic firing activity, a mechanism by which it could impose a theta frequency oscillatory activity on the hippocampus. Bath application of 50-300 nm kainate to an in vitro preparation of 20- to 25-day-old rat MSDB elicited rhythmic extracellular field activity primarily within the theta frequency band (4-12 Hz). This activity was observed both at 33 degrees C and at 37 degrees C, and was localized to the midline part of the MSDB that is rich in parvalbumin-containing neurones. The application of neurotransmitter receptor antagonists and putative gap junction blockers showed that the oscillatory field activity was dependent upon the activation of GABA(A) receptors and possibly gap junctions, but not on the activation of NMDA, GABA(B), muscarinic or nicotinic receptors. The frequency of the oscillatory activity was reduced by the application of diazepam or low doses of baclofen. Intracellular recording showed that concomitant action potential firing activity in putative GABAergic and cholinergic neurone populations was of a single spiking rather than a bursting firing nature, and was coherent with extracellularly recorded oscillatory field activity. We conclude that kainate activation of neuronal circuitry in the MSDB is capable of synchronization of rhythmic activity in the MSDB, and that this may underlie the mechanism for phase-locking rhythmic burst activity in the MSDB in vivo.