Neuronal activity of the septum following various types of deafferentation.

Extracellular recording of neuronal activity of septal nuclei was performed in unanesthetized chronic rabbits in the following conditions: (1) intact septal nuclei; (2) complete interruption of connections with hippocampus; (3) complete basal undercutting of septum. Additional series of records were performed in transversal guinea pig septal slices incubated in vitro. Computer analysis of activity was used. Interruption of hippocampal connections increased the mean frequency of discharges in the lateral septal nucleus 2-fold. Theta-bursts were preserved and the proportion of theta-bursting neurons increased from 28 to 42%. The mean frequency of theta-bursts was slightly augmented (mean 5.4 cps versus 5.2 cps in intact septum). Twenty-six per cent of neurons were characterized by regular, pacemaker-like discharges, while in the normal septum such units were almost completely absent. After septal undercutting the mean frequency of spontaneous activity was increased 2-fold in the medial septal nucleus. Rhythmic bursts were again preserved, the proportion of bursting neurons was close to normal (25%), but the mean frequency of bursts was significantly lower (mean 3.3 cps). Rhythmic activity was extremely regular. Neurons (21%) with pacemaker-like activity were also present. In septal slices neurons possessed relatively high spontaneous activity (mean 10.2/sec). In the majority of units (64%) activity was highly regular. Twenty-two per cent of the units possessed rhythmic bursts in their activity with frequencies of 2.0-5.1 cps. The role of intrinsic septal mechanisms and afferent influences in generation and modulation of theta-bursts is discussed.