Efferent projections of the A1 catecholamine cell group in the rat: an autoradiographic study

Efferent connections of the region of the A1 catecholamine cell group were investigated by the anterograde autoradiographic method in rats, some of which had been pretreated with intraventricular injections of 6-hydroxydopamine (6-OHDA). Spinal projections were further studied by combining histofluorescence and horseradish peroxidase staining in the same sections. Projections from the A1 region ascend through the lateral hypothalamus to the bed nucleus of the stria terminalis, the medial preoptic area, and several hypothalamic nuclei: the dorsomedial nucleus, the dorsal hypothalamic area, the paraventricular and supraoptic nuclei, and the median eminence. These projections are predominantly ipsilateral. Since they are sensitive to 6-OHDA, they presumably arise from catecholamine cells of the A1 group. Other pathways are not noticeably affected by 6-OHDA. These include projections through the reticular formation to the contralateral nucleus ambiguus, the ipsilateral facial nucleus, and the Kölliker-Fuse nuclei, the parabrachial nuclei and the periaqueductal gray on both sides, as well as the intralaminar nuclei of the thalamus. These pathways probably do not arise from the A1 group, although a minor noradrenergic component cannot be ruled out. Spinal projections extend to the intermediolateral cell column and the ventral horn, and especially to the phrenic motor nucleus. However, these projections arise almost exclusively from non-catecholamine cells. Only a small minority of the fluorescent A1 cells are retrogradely labeled after injections of HRP into the upper thoracic spinal cord. They lie at the level of the pyramidal decussation. Likewise only a few fluorescent cells of the A2 group are labeled. Although the A1 and A2 groups were long thought to be principal sources of spinal norepinephrine, a review of the literature shows that this belief was based on equivocal evidence.