Medullary catecholaminergic neurons projecting to lateral hypothalamic area and expressing Fos after chemical stimulation of the stomach in the rat.

Catecholaminergic neurons in the medulla projecting to lateral hypothalamic area (LH) and expressing Fos were investigated in the rat by a triple labelling method, in which horseradish peroxidase (HRP) was injected into the LH, and visceral noxious stimulation was induced by formalin injection into the stomach, sections of the medulla were stained histochemically for HRP and immunohistochemically for Fos and tyrosine hydroxylase (TH). Some neurons labeled with HRP and showing both Fos- and TH-like immunoreactivities were mainly found in the nucleus tractus solitarii (NTS) and ventrolateral medulla (VLM) at the middle and caudal levels of medulla, only a few of them were located in the reticular formation between NTS and VLM. These results indicated that some medullary catecholaminergic neurons projected to the LH and some of them might be involved in the transmissing processes of stress responses to the visceral nociceptive information produced by chemical stimulation.