Hypotension activates neuropeptide Y-containing neurons in the rat medullaoblongata

The present study was designed to determine whether neurons within cardiova scular control nuclei of the rat brainstem that become activated following a hypotensive insult also possess the capacity to utilize neuropeptide Y. A dult male Wistar-Kyoto rats were injected with glyceryl trinitrate (10 mg/k g, i.p.) or vehicle, and 4 h later anaesthetized (pentobarbitone, 60 mg/kg, i.p.) and transcardially perfused. The brains were removed and processed b y standard two-colour peroxidase immunohistochemistry. Activated cells were determined by incubation with a primary antibody to Fos protein, which was followed by a second incubation with a primary antibody to neuropeptide Y for double labelling of Fos-positive cells. Compared to vehicle, glyceryl t rinitrate-induced hypotension caused a marked induction of Fos protein in t he caudal one-third of the nucleus tractus solitarius (bregma -14 to -13.3 mm), which tailed off rapidly in more rostral sections. Following hypotensi on, significant populations of activated cells were also observed in the ro stral and caudal ventrolateral medulla. In the caudal nucleus tractus solit arius and the posterior part of the medial nucleus tractus solitarius, resp ectively, 15 of 104 and 40 of 120 Fos-positive cells exhibited cytoplasmic neuropeptide Y immunoreactivity following hypotension, compared to seven of 40 and 15 of 40 in vehicle-treated rats, indicating a significant (two- to three-fold) increase in double-labelled cells following systemic glyceryl trinitrate (P < 0.05, unpaired t-test). In contrast, in the anterior part o f the medial nucleus tractus solitarius, the number of double-labelled cell s did not change following hypotension. An increase in double-labelled cell s was also observed in the rostral ventrolateral medulla (2.5-fold increase compared to vehicle) and caudal ventrolateral medulla (5.8-fold increase c ompared to vehicle) following hypotension. These data indicate that, in the rat, neuropeptide Y-containing neurons are involved in the central response to a hypotensive challenge. The primary r egions where neuropeptide Y-containing neurons appear to be activated are t he caudal one-third of the nucleus tractus solitarius and the caudal ventro lateral medulla/rostral ventrolateral medulla, which are key nuclei associa ted with the integration of the baroreceptor heart rate reflex and sympathe tic vasomotor outflow. (C) 1999 IBRO. Published by Elsevier Science Ltd.