Spinal projections of hypothalamic histidine decarboxylase-immunoreactive neurones.

The existence of a histidine decarboxylase (HDC)-immunoreactive diencephalo-spinal pathway in the rat was demonstrated using an antiserum raised against HDC from fetal rat liver. HDC-immunoreactive nerve cell bodies were numerous in the ventral and lateral caudal hypothalamus. More caudally, in the mesencephalon, no cell bodies were observed but fairly many, transversely cut nerve fibres were found in association with the fasiculus longitudinalis medialis bilaterally. At the most caudal medullary level these longitudinally passing fibres became displaced ventrally to a position just laterally to the pyramidal decussation. In the spinal cord the fibres were more dispersed and rather sparse in most areas. The existence of a diencephalo-spinal HDC-immunoreactive pathway was verified by analyzing material from rats which had received injections of the retrograde fluorescent tracer True Blue into the cervical spinal cord. True Blue fluorescence and HDC immunofluorescence were found to coexist in a subpopulation of the HDC-immunoreactive neurones in the hypothalamus.