Ghrelin, an endogenous ligand for the growth hormone secretagogue (GHS) receptor, stimulates feeding and increases body weight. The primary action site of ghrelin has been reported to be the neuropeptide Y (NPY)/agouti-related peptide (AgRP) neurons in the hypothalamic arcuate nucleus (ARC). In addition to the hypothalamus, the caudal brainstem also appears to be an important mediator for the orexigenic activity of ghrelin. However, it is not clear whether ghrelin applied directly to the caudal brainstem activates forebrain structures. The aim of this study was to determine whether recruitment of forebrain structures was required for hyperphagic responses stimulated by ghrelin delivery within the caudal brainstem. In our experiment, all rats were surgically implanted with indwelling cannulas in the dorsal vagal complex (DVC), and ghrelin (20 pmol in 0.5 μL) was delivered to the DVC. After the injection, the orexigenic response to ghrelin was recorded by Feeding and Activity Analyser, and NPY/AgRP mRNA expressions in rat hypothalamus were detected by real-time PCR. In addition, the NPY immunoreactive neurons in the ARC were assayed by immunohistochemistry. The results showed that ghrelin significantly increased cumulative food intake at 1, 2 and 3 h after ghrelin injection, maximal response occurring at 2 h after injection. NPY/AgRP mRNA levels in ARC treated with ghrelin increased significantly compared with those in control group (injected with saline). The highest levels of NPY and AgRP mRNA were detected at 2 h after injection. The total number and mean optical density of NPY-positive neurons increased in ghrelin treated rats compared with those in control group. Consistently, ghrelin's effect was most pronounced at 2 h after injection. Taken together, we conclude that the activation of NPY/AgRP neurons in the ARC is involved in the mediation of the hyperphagic response to brainstem ghrelin administration in neurologically intact rats.
