The interactive and individual effects of orexin A and neurotensin in the brain on spontaneous physical activity.

Abstract

Within the brain, there are key feeding and locomotion circuits that feature the signaling of numerous peptides. Orexin A has been shown to have effects on feeding and physical activity behaviors when administered centrally or in several brain regions. This effect is site-specific and requires the presence of orexin receptors. Our lab has studied the behavioral effects of injecting orexin A into the dorsal rostral portion of the lateral hypothalamus. We have previously observed an acute increase in spontaneous physical activity as well as food intake following injection of orexin A into this area. The acute effect is also observed with bicuculline-mediated stimulation of neurons in the rostrolateral hypothalamus. It is likely that there are other peptide signaling systems which are downstream of orexin receptor activation in the rostrolateral hypothalamus that are responsible for the observed increase in spontaneous physical activity. In chapter one, we chose to study neurotensin signaling in the ventral tegmental area as a possible downstream event of orexin receptor activation in the rostrolateral hypothalamus. Firstly we performed immunohistochemical studies which confirmed the colocalization of orexin-receptor and neurotensin-expressing neurons in the rostrolateral hypothalamus. Secondly, we coinjected orexin A in the rostrolateral hypothalamus and a neurotensin receptor antagonist in the ventral tegmental area. In this case we observed an attenuation of orexin A responsivity in the time spent ambulating in the first hour post-injection in the presence of the antagonist. Thirdly we administered a dose range of both neurotensin and the neurotensin antagonist in the ventral tegmental area. High doses of the antagonist and an intermediate dose of neurotensin both increased spontaneous physical activity within two hours post-injection. The equivalent directionality of the effects of neurotensin and the antagonist are hypothesized to occur through several potential mechanisms including partial agonist-like qualities of the antagonist and a differential activation of dopamine receptors in the nucleus accumbens. Finally after coinjecting orexin A and neurotensin in the rostrolateral hypothalamus, we found that neurotensin antagonizes the effect of orexin A on spontaneous physical activity in the rostrolateral hypothalamus. We confirm here an involvement of neurotensin signaling in the orexin A-mediated response. Further studies are needed to elucidate the dynamics of the pathways within the rostrolateral hypothalamus and the ventral tegmental area as well as to determine if other peptide signaling systems and brain sites are involved in orexin signaling. In chapter two we sought to determine which other brain sites may respond to orexin A. We chose two sites, the ventral tegmental area and the paraventricular nucleus of the thalamus, which contain orexin receptors. To determine if an increase in spontaneous physical activity was observed with orexin A injection into these sites, we performed a dose response experiment in both sites. We found that orexin A increased the time spent moving when injected into the ventral tegmental area at a high dose, whereas orexin A had iii no effect on spontaneous physical activity at any dose when injected into the paraventricular nucleus of the thalamus. We conclude that the presence of orexin receptors in a brain site does not ascertain physical activity responsiveness to orexin A, but that the effect is site-specific as we had hypothesized.