Diurnal-stimulated and stress-induced ACTH release in rats is mediated by ventral noradrenergic bundle

Female rats were bilaterally injected with 3 micrograms of 6-hydroxydopamine (6-OHDA) dissolved in 0.2 microliter saline, via a glass micropipet stereotaxically implanted into the ventral noradrenergic-ascending bundle (VNAB). This bundle conveys most of the catecholaminergic innervation to the paraventricular nuclei and originates from the locus coeruleus and from two medullary groups of neurons (A1 and A2). Two weeks after injection, and 1 wk after the subsequent implantation of an arterial cannula, serial blood samples were taken from each rat over a 36-h period for assay of basal secretion patterns of ACTH and corticosterone (C) by radioimmunoassay and radiocompetition, respectively. Other blood samples were collected at short intervals over a 2-h period to explore the stress-ether responses of both hormones. Effects of 6-OHDA injections on catecholaminergic innervation were attested by the striking decrease in the histofluorescence of hypothalamic catecholamines and by the 86% drop in the hypothalamic noradrenaline concentrations measured by high-performance liquid chromatography at constant dopamine titers. Compared with control, sham-lesioned rats, pharmacological destruction of the VNAB by 6-OHDA led to 1) obliteration of the circadian patterns for ACTH and C and the emergence in their place of ultradian fluctuations of reduced amplitude above base-line levels and 2) 80% inhibition of the ACTH stress response which correlated with a short-lived, depressed C response. These results are discussed within the framework of the controversial literature on the mechanisms by which catecholamines may control corticotropic function.