CONNECTIONS OF THE CAUDAL VENTROLATERAL MEDULLARY RETICULAR-FORMATIONIN THE CAT BRAIN-STEM

A region of the caudal ventrolateral medullary reticular formation (CV LM) participates in baroreceptor, vestibulosympathetic, and somatosymp athetic reflexes; the adjacent retroambigual area is involved in gener ating respiratory-related activity and is essential for control of the upper airway during vocalization. However, little is known about the connections of the CVLM in the cat. In order to determine the location s of terminations of CVLM neurons, the anterograde tracers Phaseolus v ulgaris leucoagglutinin and tetramethylrhodamine dextran amine were in jected into this region. These injections produced a dense concentrati on of labeled axons throughout the lateral medullary reticular formati on (lateral tegmental field), including the retrofacial nucleus and nu cleus ambiguus, regions of the rostral ventrolateral medulla, the late ral and ventrolateral aspects of the hypoglossal nucleus, nucleus inte rcalatus, and the facial nucleus. A smaller number of labeled axons we re located in the medial, lateral, and commissural subnuclei of nucleu s tractus solitarius, the A5 region of the pontine reticular formation , the ventral and medial portions of the spinal and motor trigeminal n uclei, locus coeruleus, and the parabrachial nucleus. We confirmed the projection from the CVLM to both the rostral ventrolateral medulla an d lateral tegmental field using retrograde tracing. Injections of biot inylated dextran amine or Fluorogold into these regions resulted in re trogradely labeled cell bodies in the CVLM. However, the neurons proje cting to the lateral tegmental field were located mainly dorsal to tho se projecting to the rostral ventrolateral medulla, suggesting that th ese neurons form two groups, possibly with different inputs. Injection s of retrograde tracers into the lateral tegmental field and rostral v entrolateral medulla also produced labeled cell bodies in other region s, including the medial and inferior vestibular nuclei and nucleus sol itarius. These data are consistent with the view that the CVLM of the cat is a multifunctional area that regulates blood pressure, produces vocalization, affects the shape of the oral cavity, and elicits contra ction of particular facial muscles.