The integrity of the ventral noradrenergic bundle (VNAB) is not necessary for a normal neuroendocrine stress response

Abstract

The paraventricular nucleus of the hypothalamus (PVN) receives a dense noradrenergic innervation originating in the caudal brainstem and conveyed by the ventral noradrenergic bundle (VNAB). To evaluate the importance of this pathway, rats were bilaterally injected with 6-hydroxydopamine (6-OHDA) into the VNAB, posterior to the locus coeruleus to avoid the lesion of the dorsal noradrenergic system. These lesions reduced noradrenaline (NA) levels in the PVN by 60% without any significant change of NA levels in the cortex or of dopamine or serotonin in any part of the brain, indicating the specificity of the lesion. After one or three weeks, the neuroendocrine responses to stress were monitored. The secretion of adrenocorticotropic hormone (ACTH), corticosterone and prolactin were studied under basal conditions and after exposure to a novel environment. The activity of the sympathetic nervous system (SNS) was studied in catheterized rats. Plasma catecholamines were measured in basal conditions, and in response to gentle handling or exposure to footshocks. Apart from a transient increase of the adrenocortical axis activity which disappeared 3 weeks after surgery, the lesion did not change either basal levels of the hormones measured or their response to stress, indicating that the noradrenergic input to the PVN conveyed by the VNAB is not necessary for a normal neuroendocrine stress response to occur.