BULBOSPINAL NEUROPEPTIDE Y-IMMUNOREACTIVE NEURONS IN THE RAT - COMPARISON WITH ADRENALINE-SYNTHESIZING NEURONS

Immunohistochemistry and retrograde tracing using cholera toxin B subu nit colloidal gold (CIB-gold) has been used to identify neurons in the medulla that contain neuropeptide Y and project to the area of the in termediolateral cell column in either the upper (T2-T4) or the lower ( T8-T9) thoracic spinal cord. The rostrocaudal distributions of neurope ptide 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-methy ltransferase/CTB-gold neurons visualised in adjacent sections. In part icular areas of the rostral medulla similarities in the numbers and di stributions of neuropeptide Y neurons and phenylethanolamine N-methylt ransferase neurons suggested a coexistence of the peptide within the c atecholamine neurons. However, at the most rostral levels of the rostr al ventral medulla, the large numbers of phenylethanolamine N-methyltr ansferase 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 vent ral medulla fewer neuropeptide Y/CTB-gold neurons than phenylethanolam ine N-methyltransferase/ CTB-gold neurons were observed, so that these bulbospinal peptide neurons might define a subset of the phenylethano lamine N-methyltransferase/CTB-gold neurons, accounting for 25% of the total phenylethanolamine N-methyltransferase bulbospinal projection f rom 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 n eurons was identified in the caudal ventral medulla, these neurons app ear not to contain catecholamine synthesising enzymes.