The role of ghrelin-responsive mediobasal hypothalamic neurons in mediating feeding responses to fasting

Abstract

ObjectiveGhrelin is a stomach-derived hormone that affects food intake and regulates blood glucose. The best-characterized actions of ghrelin are mediated by its binding to and activation of the growth hormone secretagogue receptor (GHSR; ghrelin receptor). Adequate examination of the identity, function, and relevance of specific subsets of GHSR-expressing neurons has been hampered by the absence of a suitable Cre recombinase (Cre)-expressing mouse line with which to manipulate gene expression in a targeted fashion within GHSR-expressing neurons. The present study aims to characterize the functional significance and neurocircuitry of GHSR-expressing neurons in the mediobasal hypothalamus (MBH), as they relate to ghrelin-induced food intake and fasting-associated rebound hyperphagia, using a novel mouse line in which Cre expression is controlled by the Ghsr promoter.MethodsA Ghsr-IRES-Cre mouse line that expresses Cre directed by the Ghsr promoter was generated. The line was validated by comparing Cre activity in reporter mice to the known brain distribution pattern of GHSR. Next, the requirement of MBH GHSR-expressing neuronal activity in mediating food intake in response to administered ghrelin and in response to fasting was assessed after stereotaxic delivery of inhibitory designer receptor exclusively activated by designer drugs (DREADD) virus to the MBH. In a separate cohort of Ghsr-IRES-Cre mice, stereotaxic delivery of stimulatory DREADD virus to the MBH was performed to assess the sufficiency of MBH GHSR-expressing neuronal activity on food intake. Finally, the distribution of MBH GHSR-expressing neuronal axonal projections was assessed in the DREADD virus-injected animals.ResultsThe pattern of Cre activity in the Ghsr-IRES-Cre mouse line mostly faithfully reproduced the known GHSR expression pattern. DREADD-assisted inhibition of MBH GHSR neuronal activity robustly suppressed the normal orexigenic response to ghrelin and fasting-associated rebound food intake. DREADD-assisted stimulation of MBH GHSR neuronal activity was sufficient to induce food intake. Axonal projections of GHSR-expressing MBH neurons were observed in a subset of hypothalamic and extra-hypothalamic regions.ConclusionsThese results suggest that 1) activation of GHSR-expressing neurons in the MBH is required for the normal feeding responses following both peripheral administration of ghrelin and fasting, 2) activation of MBH GHSR-expressing neurons is sufficient to induce feeding, and 3) axonal projections to a subset of hypothalamic and/or extra-hypothalamic regions likely mediate these responses. The Ghsr-IRES-Cre line should serve as a valuable tool to further our understanding of the functional significance of ghrelin-responsive/GHSR-expressing neurons and the neuronal circuitry within which they act.

Ghrelin as a Survival Hormone

“…Within the brain, GHSRs are strongly expressed in several sites involved in mediating homeostatic feeding, hedonic feeding and other reward behaviors, energy homeostasis, and blood glucose [18, 20]. Not only are GHSRs activated upon binding by acyl-ghrelin, but also they possess other fascinating biology the characterization of which is still in its early stages.…”

“…True to the hallmark of enteroendocrine cells, ghrelin cells respond directly to nutrients, with glucose, amino acids and fatty acids negatively influencing its secretion, and the deficiency of glucose stimulating ghrelin secretion [20, 34, 37–39]. Also of note, the increase in plasma ghrelin associated with both short-term and more chronic caloric restriction requires sympathetic activation of the highly-expressed β 1 -adrenergic receptors on ghrelin cells [32].…”

“…Indeed, while food intake, body weight and/or body composition in ad lib chow-fed g hrl −/− , ghsr −/− and g oat −/− mice were comparable to their respective wild-type littermates [33, 47, 49, 50, 52–55], a double g hrl −/− /ghsr −/− exhibited lower body weight when fed standard chow diet, even though there was no difference in food intake [55]. While fasting-induced rebound food intake was unaltered in g hrl −/− or the ghsr −/− mice [47, 49, 56, 57], pharmacological antagonism of GHSR blunted fasting-induced rebound food intake [56] as did chemogenetic inhibition of mediobasal hypothalamic GHSR-expressing neurons [20]. Although g hrl −/− mice became obese when they were switched to high fat diet (HFD) during adulthood [48, 49], g hrl −/− and ghsr −/− mice resisted the full development of diet induced obesity (DIO), accumulating significantly less fat mass and demonstrating increased energy expenditure, when exposed to HFD immediately after weaning [53, 58].…”

Growth hormone secretagogue receptor constitutive activity impairs voltage‐gated calcium channel‐dependent inhibitory neurotransmission in hippocampal neurons

“…; Mani et al . , ), where exogenous ghrelin application modulates behavioural responses such as the reward value of food, response to stress, memory retention, spatial memory and learning behaviour (Diano et al . ; Atcha et al .…”

Estudio de los efectos de la ghrelina sobre los circuitos neuronales del tallo encefálico

“…análisis de las imágenes de ISHH del Allen Mouse Brain Atlas confirmaron que las neuronas del AVT de ratón expresan el GHSR, mientras que el NAcb no presenta neuronas que expresen el GHSR. Estos resultados son congruentes con lo que ya se ha publicado en estudios previos que emplean ISHH, PCR tiempo real, ensayos de pegado de ghrelina o modelos de ratones modificados genéticamente en los que las neuronas que expresan GHSR son marcadas fluorescentemente(Cabral et al, 2013;Chuang et al, 2011;Mani et al, 2014Mani et al, , 2017Perello M et al, 2012;Zigman et al, 2006). Así, el blanco de la acción de la ghrelina en el circuito mesolímbico serían las neuronas del AVT.…”