Ascending and descending projections from nucleus reticularis magnocellularis and nucleus reticularis gigantocellularis: an autoradiographic and horseradish peroxidase study in the rat.

The projections of the rostral medulla were studied using retrograde and orthograde transport techniques in the rat. The present horseradish peroxidase (HRP) studies indicate that the ventral portion of nucleus reticularis gigantocellularis (NGC) and nucleus reticularis magnocellularis (NMC) project to both rostral and caudal levels of the spinal cord, while dorsal NGC projects only to the rostral cord. A differential density distribution of labeled cells was observed, with the greatest density of NGC-spinal neurons located rostral to the level of the inferior olive; and the greatest density of NMC-spinal neurons located caudally. This differential density distribution, when coupled with microiontophoretic application of [3H]amino acids allowed relatively independent labeling of the adjacent NGC- and NMC-spinal systems. On the basis of the HRP and autoradiographic studies 3 separate regions were delineated: dorsal NGC, ventral NGC and NMC. Descending projections from NGC were observed to the lateral vestibular nucleus, facial nucleus, hypoglossal nucleus and nucleus cuneatus. At cervical levels NGC fibers projected through the ventral and ventrolateral columns. Terminal fields were observed in laminae VII, VIII and to a lesser extent in IX. Labeled NGC fibers became difficult to follow by thoracic levels, which is consistent with the present HRP results. A continuum of descending NGC projections was observed with dorsally located NGC neurons projecting bilaterally through the ventral columns, and ventrally located NGC cells projecting through the ipsilateral ventrolateral columns. Ascending projections from NGC to the motor nucleus of V, trochlear nucleus, oculomotor nucleus, Edinger-Westphal nucleus, the ventral aspect of the periaqueductal gray, the deep and intermediate layers of the superior colliculus, nucleus parafasicularis and centromedianus, the Fields of Forel and the dorsal and lateral hypothalamic nuclei were observed. Descending projections from NMC to the dorsal nucleus of the vagus, hypoglossal nucleus, nucleus commissuralis and intercalatus were observed. At cervical levels, fibers project through the ipsilateral lateral columns, particularly its dorsal aspect. Terminal fields are located ipsilaterally in laminae IV, V and VI, and bilaterally in VII, VIII and X. NMC projections continue through caudal levels of the spinal cord including a projection to the ipsilateral intermediolateral columns. Ascending NMC projections are limited to the ventral pontine reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)