Ghrelin counteracts insulin-induced activation of vagal afferent neurons via growth hormone secretagogue receptor

Highlights

• Ghrelin inhibits insulin-induced [Ca^2 +]_i increase in nodose ganglion (NG) neurons.

• Ghrelin interacts with GHSR to counteract insulin in NG neurons.

• Ghrelin counteracts insulin but not CCK in increasing [Ca^2 +]_i in NG neurons.

• This interaction may inform brain of ghrelin- vs. insulin-dominant metabolic states.

Abstract

Vagal afferent nerves sense meal-related gastrointestinal and pancreatic hormones and convey their information to the brain, thereby regulating brain functions including feeding. We have recently demonstrated that postprandial insulin directly acts on the vagal afferent neurons. Plasma concentrations of orexigenic ghrelin and anorexigenic insulin show reciprocal dynamics before and after meals. The present study examined interactive effects of ghrelin and insulin on vagal afferent nerves. Cytosolic Ca^2 + concentration ([Ca^2 +]_i) in isolated nodose ganglion (NG) neurons was measured to monitor their activity. Insulin at 10^− 7 M increased [Ca^2 +]_i in NG neurons, and the insulin-induced [Ca^2 +]_i increase was inhibited by treatment with ghrelin at 10^− 8 M. This inhibitory effect of ghrelin was attenuated by [D-Lys³]-GHRP-6, an antagonist of growth hormone-secretagogue receptor (GHSR). Des-acyl ghrelin had little effect on insulin-induced [Ca^2 +]_i increases in NG neurons. Ghrelin did not affect [Ca^2 +]_i increases in response to cholecystokinin (CCK), a hormone that inhibits feeding via vagal afferent neurons, indicating that ghrelin selectively counteracts the insulin action. These results demonstrate that ghrelin via GHSR suppresses insulin-induced activation of NG neurons. The action of ghrelin to counteract insulin effects on NG might serve to efficiently inform the brain of the systemic change between fasting-associated ghrelin-dominant and fed-associated insulin-dominant states for the homeostatic central regulation of feeding and metabolism.

Introduction

Insulin, released from the pancreas, regulates peripheral glucose and lipid metabolism via acting on the peripheral organs such as the liver, skeletal muscle and adipose tissue. Insulin is also known to influence the brain, thereby regulating glucose and lipid metabolisms (Obici et al., 2002, Scherer et al., 2011) and inhibiting feeding (Filippi et al., 2013, Niswender et al., 2004, Woods et al., 1979). Ghrelin, released primarily from the stomach (Date et al., 2000, Kojima et al., 1999) and to a lesser extent from the pancreas (Dezaki et al., 2004, Dezaki et al., 2006), stimulates feeding (Nakazato et al., 2001), promotes growth hormone release (Kojima et al., 1999), inhibits insulin release (Dezaki et al., 2004), and regulates glucose and lipid metabolism (Dezaki et al., 2006, Gahete et al., 2014). Plasma concentrations of orexigenic ghrelin and anorexigenic insulin change in a reciprocal manner both before and after meals (Cummings et al., 2001), which is thought to contribute to dynamic regulation of feeding, glucose and energy metabolism. Regarding possible mechanisms for the diurnal reciprocal changes of these hormones, it has been demonstrated that ghrelin interacts with the pancreatic β-cells to suppress insulin release (Dezaki et al., 2004), and that insulin interacts with gastric X/A-like cells to suppress ghrelin release (Sakata et al., 2012). Prior to meal intake, plasma ghrelin level is high and plasma insulin level is low, and this ghrelin-dominant state stimulates appetite and increases food intake. Following food intake, plasma ghrelin and insulin levels change in a reciprocal manner (Cummings et al., 2001), and the resultant insulin-dominant state could induce satiety and decrease food intake.

The mechanisms by which the peripheral insulin and ghrelin inform the brain are still obscure. It has been reported that both insulin and ghrelin can cross the blood–brain barrier (BBB) and enter the brain (Banks and Kastin, 1998, Banks et al., 2002), and their direct actions on the brain have been suggested. Furthermore, these hormones are thought to regulate feeding via acting on the first order neurons in the hypothalamic arcuate nucleus (ARC) that senses systemic metabolic signals (Niswender et al., 2004, Schwartz et al., 2000). In the ARC, the neurons co-expressing orexigenic neuropeptide Y (NPY) and agouti-related peptide (AgRP) play a pivotal role in regulation of feeding. Ghrelin directly activates ARC NPY neurons by increasing cytosolic Ca^2 + concentration ([Ca^2 +]_i) (Kohno et al., 2003), and the ghrelin-induced [Ca^2 +]_i increase is suppressed by insulin (Iwasaki et al., 2013, Maejima et al., 2011).

Insulin and ghrelin enter the brain across BBB in a limited manner (Banks and Kastin, 1998, Banks et al., 2002). The vagal afferent nerves, one of the primary visceral sensory nerves, sense peripheral factors including gastrointestinal and pancreatic hormones and transmit their signals to the brain, thereby regulating food intake (Iwasaki and Yada, 2012). Peripheral administration of ghrelin reportedly increases food intake via the vagal afferents (Date et al., 2002) that express GHRS (Burdyga et al., 2006, Date et al., 2002, Sakata et al., 2003). Moreover, peripheral administration of ghrelin increases dopamine β hydroxylase mRNA expression in the nucleus tractus solitaries (NTS) to which vagal afferents project, elevates noradrenaline release in the ARC where the NTS noradrenergic neurons innervate, and activates the ARC NPY neurons through adrenergic receptors, thereby increasing food intake (Date et al., 2006). We have recently demonstrated that insulin directly activates the vagal afferents including those innervating the pancreas, via signaling cascade of insulin receptor (IR) – insulin receptor substrate 2 (IRS2) – phosphatidylinositol 3 kinase (PI3 kinase) (Iwasaki et al., 2013). In IRS2 knockout mice that exhibit hyperphagic obesity, insulin action in the vagal afferent neurons was impaired while it was intact in the ARC NPY neurons, suggesting that the impaired insulin sensing by the vagal afferents is linked to hyperphagic obesity in IRS2 knockout mice (Iwasaki et al., 2013).

Inverse relationship between ghrelin and insulin has been demonstrated; ghrelin and insulin are released in a reverse diurnal pattern, ghrelin suppresses insulin secretion from pancreatic β-cells, and ghrelin and insulin reciprocally regulate NPY neurons in the hypothalamus. Based on these findings, we propose the insulin-counteracting nature of ghrelin (Yada et al., 2008, Yada et al., 2014). To further substantiate it, it is of particular importance to verify whether ghrelin counteracts the insulin action on the vagal afferents, the essential pathway for information flow from the periphery to the brain. In this study, we investigated the effect of ghrelin on insulin-induced [Ca^2 +]_i increases in the vagal afferents neurons isolated from the nodose ganglion (NG) in mice. We also investigated the involvement of GHSR in the effects of ghrelin.