Neuronal circuitry in the basal septum and preoptic area of the rat.

Extracellular action potentials of spontaneously active units were studied in the basal septum and medial preoptic area (SPOA). Units responded in a similar way during stimulation of the amygdala, the reticular formation, the medial geniculate nucleus or the ventral hippocampus. The initial response was either short latency (6--35 msec) excitation or inhibition, the latter being predominant. A percentage of neurones (14.5%) did not respond. Only one response did not have an inhibitory component and responses have been further classified on the basis of the presence or absence of a postinhibitory excitation, and its rhythmicity. Single neurones tended to respond in a similar way to stimulation of different sites. Long inhibition (100--400 msec) is often preceded by excitation and may be a recurrent mechanism whilst short inhibition (18--55 msec) may be a 'direct' mechamism. Long inhibition may be mediated by GABA as it is weakened by intravenous bicuculline whilst short inhibition was not affected. A small amplitude barrage of action potentials at the onset of inhibition was suggested to be generated by inhibitory interneurones within the SPOA. Rhythmic postinhibitory excitatory responses suggest transmission of activity from stimulation sites to the SPOA via the thalamus. A hypothesis for local neuronal circuitry of spontaneously active neurones in the SPOA was proposed.