Parodoxical effect of orexin A: hypophagia induced by hyperthermia

This experiment tested the effect of the sympathetic and thermogenic activation induced by orexin A on eating behavior. The food intake, firing rate (FR) of the sympathetic nerves to interscapular brown adipose tissue (IBAT), MAT and abdominal temperatures (T-IBAT and T-ab), and heart rate (HR) were monitored in 24 h-fasting male Sprague-Dawley rats for 15 h after food presentation. Orexin A (1.5 nmol) was injected into the lateral cerebral ventricle 6 h before food presentation while FR, T-IBAT, and T-ab, and HR were also monitored. The same variables were controlled in rats receiving orexin A contemporaneously to food presentation. Two other groups of control animals were tested with the same procedure, however orexin A was substituted by saline. The results showed that food intake was significantly lower in the group receiving orexin A 6 h before food presentation in comparison to all the other groups. FR, T-IBAT and T-ab, and HR were significantly higher in the rats receiving orexin A with respect to rats receiving saline. These findings demonstrate that orexin A, so-called for its orexigen action, can also induce hypophagia. On the other hand, orexin A always induces an activation of the thermogenesis. These results suggest a revision of the role played by orexin A in the control of food intake, assigning to this peptide a primary role in the thermoregulation. The possibility that orexin A can induce hypophagia is well demonstrated by this experiment, so that the scientific community should use a different name for this peptide. An appropriate name could be 'hyperthermine' A. (C) 2002 Elsevier Science B.V. All rights reserved.