The Role of the Septal Nuclei in Controlling the Activity of Vagosensitive Neurons in the Solitary Tract Nucleus in Cats

Studies on cats anesthetized with a mixture of Nembutal and chloralose were performed to study the descending influences of single, paired, and frequent stimulation of the lateral septal nucleus (LSN) and bed nucleus of the stria terminalis (BNST) on the activity of viscerosensory neurons in the solitary tract nucleus, identified by stimulation of the cervical part of the vagus nerve. Of 70 units recorded in the solitary tract nucleus, 50 were identified as primary and secondary input vagus neurons. Single stimuli applied to the septal nuclei induced initial excitation in 30% (15 units) of vagus neurons. The latent period of these responses was 5-25 msec. Presentation of paired stimuli showed that loss of the ability to respond to the second stimulus occurred at interstimulus intervals of 10-200 and sometimes 300 msec. A total of 34% (17 units) of solitary tract nucleus neurons showed tonic changes in spontaneous activity in response to rhythmic stimulation. Increasing the stimulation frequency to 10-20 Hz led to very different changes in the spontaneous rhythm, i.e., wave-like changes (decreases and increases in frequency, secondary suppression) or complete inhibition, sometimes lasting up to 10 sec after stimulation ended. A small number of units (five) showed a blocking effect of septal discharges on the visceral afferent input in conditions of paired stimulation. These results lead to the conclusion that the LSN and BNST are involved in modulating the activity of bulbar viscerosensory neurons, though their influences are mediated mainly via oligo- and polysynaptic pathways via other limbic structures (hypothalamus, amygdala).