Paradoxical [correction of 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), IBAT 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.