Endogenous catecholamines modulate growth hormone release in the conscious rat during hypoglycaemia but not in the basal state

Hindbrain noradrenergic A2 neurons: diverse roles in autonomic, endocrine, cognitive, and behavioral functions

Central noradrenergic (NA) signaling is broadly implicated in behavioral and physiological processes related to attention, arousal, motivation, learning and memory, and homeostasis. This review focuses on the A2 cell group of NA neurons, located within the hindbrain dorsal vagal complex (DVC). The intra-DVC location of A2 neurons supports their role in vagal sensory-motor reflex arcs and visceral motor outflow. A2 neurons also are reciprocally connected with multiple brain stem, hypothalamic, and limbic forebrain regions. The extra-DVC connections of A2 neurons provide a route through which emotional and cognitive events can modulate visceral motor outflow and also a route through which interoceptive feedback from the body can impact hypothalamic functions as well as emotional and cognitive processing. This review considers some of the hallmark anatomical and chemical features of A2 neurons, followed by presentation of evidence supporting a role for A2 neurons in modulating food intake, affective behavior, behavioral and physiological stress responses, emotional learning, and drug dependence. Increased knowledge about the organization and function of the A2 cell group and the neural circuits in which A2 neurons participate should contribute to a better understanding of how the brain orchestrates adaptive responses to the various threats and opportunities of life and should further reveal the central underpinnings of stress-related physiological and emotional dysregulation.

Hindbrain catecholamine neurons modulate the growth hormone but not the feeding response to ghrelin

The gastrointestinal peptide, ghrelin, elicits feeding and secretion when administered systemically or centrally. Previous studies have suggested that hypothalamic projections of hindbrain catecholamine neurons are involved in both of these actions of ghrelin. The purpose of this study was to further assess the role of hindbrain catecholamine neurons in ghrelin-induced feeding and GH secretion and to determine the anatomical distribution of the catecholamine neurons involved. We lesioned noradrenergic and adrenergic neurons that innervate the medial hypothalamus by microinjecting the retrogradely transported immunotoxin, saporin (SAP) conjugated to antidopamine-beta-hydroxylase (DSAP) into the paraventricular nucleus of the hypothalamus. Controls were injected with unconjugated SAP. We found that the DSAP lesion did not impair the feeding response to central or peripheral ghrelin administration, indicating that these neurons are not required for ghrelin's orexigenic effect. However, the GH response to ghrelin was prolonged significantly in DSAP-lesioned rats. We also found that expression of Fos, an indicator of neuronal activation, was significantly enhanced over baseline levels in A1, A1/C1, C1, and A5 cell groups after ghrelin treatment and in A1, A1/C1, and A5 cell groups after GH treatment. The similar pattern of Fos expression in catecholamine cell groups after GH and ghrelin and the prolonged GH secretion in response to ghrelin in DSAP rats together suggest that activation of hindbrain catecholamine neurons by ghrelin or GH could be a component of a negative feedback response controlling GH levels.