Bulbospinal neuropeptide Y-immunoreactive neurons in the rat: comparison with adrenaline-synthesising neurons

Immunohistochemistry and retrograde tracing using cholera toxin B subunit colloidal gold (CTB-gold) has been used to identify neurons in the medulla that contain neuropeptide Y and project to the area of the intermediolateral cell column in either the upper (T2–T4) or the lower (T8–T9) thoracic spinal cord. The rostrocaudal distributions of neuropeptide Y neurons and neuropeptide Y/CTB-gold neurons have been compared with the distributions of adrenaline-synthesising, phenylethanolamine N-methyltransferase-containing neurons and phenylethanolamine N-methyltransferase/CTB-gold neurons visualised in adjacent sections. In particular areas of the rostral medulla similarities in the numbers and distributions of neuropeptide Y neurons and phenylethanolamine N-methyltransferase neurons suggested a coexistence of the peptide within the catecholamine neurons. However, at the most rostral levels of the rostral ventral medulla, the large numbers of phenylethanolamine N-methyltransferase neurons were not matched by similar numbers of neuropeptide Y neurons, so that the phenylethanolamine N-methyltransferase neurons in this area could not all contain neuropeptide Y. In the rostral ventral medulla fewer neuropeptide Y/CTB-gold neurons than phenylethanolamine N-methyltransferase/CTB-gold neurons were observed, so that these bulbospinal peptide neurons might define a subset of the phenylethanolamine N-methyltransferase/CTB-gold neurons, accounting for 25% of the total phenylethanolamine N-methyltransferase bulbospinal projection from the rostral ventral medulla. Other neuropeptide Y/CTB-gold neurons in the dorsal medulla are also likely to contain phenylethanolamine N-methyltransferase. Finally, a population of neuropeptide Y/CTB-gold neurons was identified in the caudal ventral medulla, these neurons appear not to contain catecholamine synthesising enzymes.