Brain stem melanocortinergic modulation of meal size and identification of hypothalamic POMC projections.

Metabolic, cognitive, and environmental factors processed in the forebrain modulate food intake by changing the potency of direct controls of meal ingestion in the brain stem. Here, we behaviorally and anatomically test the role of the hypothalamic proopiomelanocortin (POMC) system in mediating some of these descending, indirect controls. Melanotan II (MTII), a stable melanocortin 4 receptor (MC4R) and melanocortin 3 receptor (MC3R) agonist injected into the fourth ventricle near the dorsal vagal complex, potently inhibited 14-h food intake by decreasing meal size but not meal frequency; SHU9119, an antagonist, increased food intake by selectively increasing meal size. Furthermore, MTII injected into the fourth ventricle increased and SHU9119 tended to decrease heart rate and body temperature measured telemetrically in freely moving rats. Numerous alpha-melanocyte-stimulating hormone-immunoreactive axons were in close anatomical apposition to nucleus tractus solitarius neurons showing c-Fos in response to gastric distension, expressing neurochemical phenotypes implicated in ingestive control, and projecting to brown adipose tissue. In retrograde tracing experiments, a small percentage of arcuate nucleus POMC neurons was found to project to the dorsal vagal complex. Thus melanocortin signaling in the brain stem is sufficient to alter food intake via changing the potency of satiety signals and to alter sympathetic outflow. Although the anatomical findings support the involvement of hypothalamomedullary POMC projections in mediating part of the descending, indirect signal, they do not rule out involvement of POMC neurons in the nucleus tractus solitarius in mediating part of the direct signal.