Immunohistochemical evidence for the adrenergic medullary longitudinal bundle as a major ascending pathway to the hypothalamus.

Three weeks after unilateral electrolytic lesion of the longitudinal axon bundle in the medulla oblongata of the rat, we observed a decrease in the number of phenylethanolamine-N-methyltransferase (PNMT)-immunoreactive (IR) nerve fibers in virtually all the regions of the diencephalon ipsilaterally to the lesion, especially in the dorsomedial nucleus and the paraventricular nucleus of the hypothalamus. These results indicate that the hypothalamic PNMT-IR terminal-like fibers originate in the ipsilateral medulla oblongata presumptive adrenaline-containing (Ad) neurons especially through ascending projections provided in majority by the longitudinal axon bundle. Further, no PNMT-IR cell bodies were detected in the hypothalamus even after colchicine treatment.