Effects of three neurochemical stimuli on delayed feeding and energy metabolism

Infusions of norepinephrine (NE), the gamma-aminobutyric acid agonist, musc imol (MUS), or neuropeptide Y (NPY) into the paraventricular nucleus (PVN) of the hypothalamus all increase food intake. Such feeding may be due to di rect activation of behavioral processes driving ingestion and/or to alterat ions in nutrient metabolism that feeding serves to normalize. To examine th ese possibilities, male Sprague-Dawley rats received PVN infusions of vehic le, 20 nmol NE, 1 nmol MUS or 100 pmol NPY at dark onset, then food intake was measured under three feeding conditions: (1) I and 2 h immediately afte r injections, (2) I h after a I h delay between injections and access to fo od, and (3) 1 h after a 1 h feeding delay, bur with injections occurring ju st before presenting food. Measures of energy expenditure (EE) and respirat ory quotients (RQs) in the absence of food were made over 2 h in parallel e xperiments. Results confirmed that NE, MUS and NPY all increased dark-onset feeding, but only NPY increased intake above control levels after a 1 h fe eding delay. No neurochemically-induced changes in EE were observed, nor we re there changes in RQs after NE or MUS. However, NPY reliably enhanced RQs from 30 to 120 min of testing. Our findings imply that NE and MUS initiate relatively immediate, short-term feeding that is not associated with chang es in nutrient metabolism and does not summate with cues stimulated by dela yed access to food. NPY initiates more protracted feeding temporally linked to enhanced carbohydrate metabolism This may indicate that part of NPY's f eeding stimulatory effects are secondary to physiological processes driving ingestion. (C) 2000 Elsevier Science B.V. All rights reserved.