Projections to the cardioinhibitory region of the nucleus ambiguus of rat

The nucleus ambiguus is a brainstem structure which sends projections through the vagus nerve to the viscera, primarily heart, lung, and gut. The anatomical relationship between the nucleus ambiguus and other brain structures has not been elucidated nor has the cardiac region been identified physiologically in rats. We have attempted to clarify which areas of the nucleus ambiguus are cardioinhibitory and to determine other regions of the brain which send direct projections to this physiologically identified cardiac region. Stimulating electrodes were positioned stereotaxically in the medulla of anesthetized rats. Small currents were passed through the electrodes to locate regions in the ventrolateral medulla which slowed heart rate. In each rat, the area found was small (less than 200 micron in diameter), very specific, and located in the rostral portion of the nucleus ambiguus. Micro-quantities of horseradish peroxidase were then iontophoretically ejected into this brainstem area; 24-72 hours following the HRP injection, the rats were processed for HRP reaction product using the tetramethybenzidine method. The major brain area which sent projections to the rostral nucleus ambiguus was the ipsilateral medial subnucleus of the solitary tract. A few labeled cells were found in the ipsilateral ventrolateral subnucleus of the solitary tract, parabrachial complex, the paraventricular nucleus of the hypothalamus, and the contralateral nucleus ambiguus. Control injections in reticular areas surrounding the rostral nucleus ambiguus showed no label in the medial solitary nucleus.