DISINHIBITION OF RAT HIPPOCAMPAL PYRAMIDAL CELLS BY GABAERGIC AFFERENTS FROM THE SEPTUM

1. Slices were prepared from rat forebrain to include both the septum and the hippocampus in order to examine the effects of septal stimulat ion on hippocampal inhibitory circuits. 2. Repetitive stimulation of s epto-hippocampal fibres caused a maintained decrease in the frequency of spontaneous IPSPs recorded from CA3 pyramidal cells in the presence of antagonists of excitatory amino acid receptors and of muscarine re ceptors. 3. In records made from pyramidal cells with CsCl-filled elec trodes, IPSPs were examined at potentials both more positive and more negative than their reversal potential. Single septal stimuli hyperpol arized pyramidal cells when IPSPs were depolarizing events and depolar ized them wiles IPSPs were hyperpolarizing. The GABA(A) receptor antag onist picrotoxin abolished the effects of septal stimulation. 4. Activ ation of septal afferents initiated an IPSP in hippocampal inhibitory cells but not in pyramidal cells. Septal IPSPs had similar kinetics to those initiated by local hippocampal stimulation and could suppress i nhibitory cell discharge. 5. In pyramidal cells recorded with potassiu m acetate-filled electrodes, septal stimuli initiated a depolarization that increased with the driving force for Cl- and that could cause fi ring. 6. Rhythmic stimulation of septo-hippocampal fibres at 5 Hz init iated, in the hippocampus, a maintained out-of-phase oscillation of py ramidal cell discharge and inhibitory cell firing, as detected by the occurrence of spontaneous IPSPs. 7. These results suggest that GABAerg ic septo-hippocampal afferents selectively inhibit hippocampal inhibit ory cells and so disinhibit pyramidal cells. This disinhibition could contribute to the transmission of the theta rhythm from the septum to the hippocampus.