Membrane and circuit properties of lateral septum neurons: relationships with hippocampal rhythms

Abstract

The lateral septum receives the most important afferents from the hippocampus, has been proposed to contribute to theta ( θ) rhythm generation. Our aim was to study the membrane and circuital properties of lateral septum neurons and their relationship with hippocampal rhythms. Extra- and intracellular recordings ( n=81) were obtained in urethane-anesthetized rats. Two neuronal populations were found, one of them with hippocampal θ rhythm dependence ( θ-D, 68%), and the other group independent of hippocampal θ rhythm ( θ-I, 32%). Other differences were spontaneous firing rate ( θ-D=13.20±2.09, θ-I=6.99±1.18; p<0.005) with a bursting pattern in the θ-D group and single discharges in the θ-I group. Intracellular recordings showed higher synaptic activity in θ-D than in θ-I neurons. Both groups showed fast spikes while only θ-D neurons had high-threshold and low-threshold slow spikes. θ-D neurons had θ-oscillations in their membrane potential. Stimulation of the contralateral lateral septum resets the hippocampal θ rhythm and the θ rhythm recorded intracellularly in the lateral septum neurons. Some neurons (10.6%) showed rhythmic oscillations lasting a few seconds, at a higher frequency than those of the simultaneously recorded hippocampal EEG. This high frequency appeared spontaneously or could be evoked by stimulations of the fornix and reticularis pontis oralis nucleus (RPO). Homologous high frequency oscillations appeared in the simultaneously recorded hippocampal EEG, synchronized with the neuronal activity, during fornix stimulation. RPO stimulation evoked increments (57%) or decrements (43%) in the firing rate of lateral septum neurons. Thus, they could participate in different septal networks to modulate the θ rhythm. The marked functional relationship between lateral septum neurons and hippocampal θ rhythm supports the proposal that lateral septum represents a feedback system for the improvement of medial septum θ rhythm. The hypothalamic projections could be a way to introduce θ and higher rhythms into this structure that control many biological rhythms.