Presynaptic excitability changes induced in the solitary tract endings of laryngeal primary afferents by stimulation of nucleus raphe magnus and locus coeruleus.

To determine if the nucleus raphe magnus (NRM) and locus coeruleus (LC) might exert presynaptic modulatory influences on the endings in the medullary solitary tract nucleus of laryngeal afferents, multi-unit and single unit laryngeal afferent activity was recorded in the nodose ganglion in chloralose-anaesthetized cats. The afferents could be orthodromically activated by superior laryngeal nerve stimulation and antidromically activated from the solitary tract nucleus. An increase in antidromic excitability of laryngeal afferents, which was considered a reflection of primary afferent depolarization, could be induced by either NRM or LC conditioning stimuli and lasted for 300-400 msec. The findings suggest that presynaptic mechanisms may contribute to the suppression of solitary tract neuron or reflex activities induced by NRM or LC stimulation.