Abstract

Ghrelin is a newly discovered peptide that is released from the stomach and from neurons in the hypothalamic arcuate nucleus (ARC) and potently stimulates growth hormone release and food intake. Neuropeptide-Y (NPY) neurons in the ARC play an important role in the stimulation of food intake. The present study aimed to determine whether ghrelin directly activates NPY neurons and, if so, to explore its signaling mechanisms. Whether the neurons that respond to ghrelin could be regulated by orexin and leptin was also examined. We isolated single neurons from the ARC of rats and measured the cytosolic Ca²⁺ concentration ([Ca²⁺]_i) with fura-2 fluorescence imaging. Ghrelin (10⁻¹² to 10⁻⁸ mol/l) concentration-dependently increased [Ca²⁺]_i, which occurred in 35% of the ARC neurons. Approximately 80% of these ghrelin-responsive neurons were proved to be NPY-containing by immunocytochemical staining, and 58% of them were glucose-sensitive neurons as judged by their responses to lowering glucose concentrations. The [Ca²⁺]_i responses to ghrelin were markedly attenuated by inhibitors of protein kinase A (PKA) but not protein kinase C and by a blocker of N-type but not L-type Ca²⁺ channels. Orexin increased [Ca²⁺]_i and leptin attenuated ghrelin-induced [Ca²⁺]_i increases in the majority (80%) of ghrelin-responsive NPY neurons. These results demonstrate that ghrelin directly interacts with NPY neurons in the ARC to induce Ca²⁺ signaling via PKA and N-type Ca²⁺ channel-dependent mechanisms. The integration of stimulatory effects of ghrelin and orexin and inhibitory effect of leptin may play an important role in the regulation of the activity of NPY neurons and thereby feeding.