Peripheral injection of ghrelin induces Fos expression in the dorsomedial hypothalamic nucleus in rats

Peripheral ghrelin has been shown to act as a gut–brain peptide exerting a potent orexigenic effect on food intake. The dorsomedial nucleus of the hypothalamus (DMH) is innervated by projections from other brain areas being part of the network of nuclei controlling energy homeostasis, among others NPY/AgRP-positive fibers arising from the arcuate nucleus (ARC). The aim of the study was to determine if peripherally administered ghrelin affects neuronal activity in the DMH, as assessed by Fos expression. The number of Fos positive neurons was determined in the DMH, paraventricular nucleus of the hypothalamus (PVN), ARC, ventromedial hypothalamic nucleus (VMH), nucleus of the solitary tract (NTS) and in the area postrema (AP) in non-fasted Sprague–Dawley rats in response to intraperitoneally (ip) injected ghrelin (3 nmol/rat) or vehicle (0.15 M NaCl). Peripheral ghrelin induced a significant increase in the number of Fos-ir positive neurons/section compared with vehicle in the ARC (mean ± SEM: 49 ± 2 vs. 23 ± 2 neurons/section, p = 0.001), PVN (69 ± 5 vs. 34 ± 3, p = 0.001), and DMH (142 ± 5 vs. 83 ± 5, p < 0.001). Fos-ir positive neurons were mainly localized within the ventral part of the DMH. No change in Fos expression was observed in the VMH (53 ± 8 vs. 48 ± 6, p = 0.581), NTS (42 ± 2 vs. 40 ± 3, p = 0.603), and in the AP (7 ± 1 vs. 5 ± 1, p = 0.096). Additional double-labelling with anti-Fos and anti-AgRP revealed that Fos positive neurons in the DMH were encircled by a network of AgRP-ir positive fibers. These data indicate that peripheral ghrelin activates DMH neurons and that NPY-/AgRP-positive fibers may be involved in the response.