Identification of ultrasonic vocalization substrates determined by electrical stimulation applied to the medulla oblongata in the rat.

The aim of the present investigation was to identify the neural structures, within the rat medulla, that are responsible for rodent ultrasound production. Sound producing substrates were found to be located in the reticular formation and some cranial nerve nuclei as well as several other nuclei situated in the lateral and dorsomedial portions of the medulla. To estimate the degree of involvement in the generation of ultrasound signals, the sound response latencies were measured for each structure. The lateral reticular nucleus and the facial nucleus showed latencies that were significantly shorter than those for other nuclei, and they were assumed to have a primary part in rodent ultrasound production. Audible sounds of considerably longer latencies were produced exclusively by stimulation of the trigeminal spinal tract nucleus. No ultrasounds could be obtained in this region. These results were discussed in terms of innervations of the facial and laryngeal musculature by the specific neural structures. Present results were also discussed with reference to the roles of the bulbar monoaminergic neurons projecting to the spinal cord and the role of ascending nociceptive pathways.