Afferent projections to the nucleus cuneiformis in the rat.

Afferent projections to the nucleus cuneiformis (CNF) were examined in the rat with retrograde tracing techniques employing microinjection of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP). As the CNF is an extensive nucleus in its rostrocaudal extent, the rostral and caudal subdivisions of the CNF were studied separately. Projections to caudal CNF were observed from all subdivisions of the periaqueductal gray, the deep layers of the superior colliculus as well as the caudal levels of the CNF. In contrast, WGA-HRP microinjections into the rostral CNF retrogradely labeled more cells but they were located exclusively in the rostral CNF. The present data support the hypothesis that three adjacent brain stem nuclei, i.e. the caudal CNF, the periaqueductal gray and the deep layers of the superior colliculus, function in unison to control ventral medullary pain pathways. The relationship of these three structures to the rostral CNF is unclear and deserves further study.