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Summary Blood-borne factors regulate adult hippocampal neurogenesis and cognition in mammals. We report that elevating circulating unacylated-ghrelin (UAG), using both pharmacological and genetic methods, reduced hippocampal neurogenesis and plasticity in mice. Spatial memory impairments observed in ghrelin-O-acyl transferase-null (GOAT −/− ) mice that lack acyl-ghrelin (AG) but have high levels of UAG were rescued by acyl-ghrelin. Acyl-ghrelin-mediated neurogenesis in vitro was dependent on non-cell-autonomous BDNF signaling that was inhibited by UAG. These findings suggest that post-translational acylation of ghrelin is important to neurogenesis and memory in mice. To determine relevance in humans, we analyzed circulating AG:UAG in Parkinson disease (PD) patients diagnosed with dementia (PDD), cognitively intact PD patients, and controls. Notably, plasma AG:UAG was only reduced in PDD. Hippocampal ghrelin-receptor expression remained unchanged; however, GOAT + cell number was reduced in PDD. We identify UAG as a regulator of hippocampal-dependent plasticity and spatial memory and AG:UAG as a putative circulating diagnostic biomarker of dementia.

Acyl-ghrelin (AG): A Form of ghrelin that has been acylated by GOAT, which enables it to bind to GHS-R1a. Acyl-protein thioesterase 1 (APT1): An endogenous enzyme known to de-acylate acylghrelin Allosteric mechanism: Regulation of a protein or receptor independent of its active site. Brain derived neurotrophic factor (BDNF): A growth factor in the brain, involved in cell proliferation, survival, learning and memory. Bromodeoxyuridine (BrdU): A synthetic thymidine analogue, incorporated into DNA during replication. It can be used as a marker of proliferation during a specific timeframe. Butyrylcholinesterase: An enzyme that hydrolyses many different choline-based esters, but can also de-acylate acyl-ghrelin, (AG) to form unacylated ghrelin (UAG). Endogenous esterase enzyme present in the plasma known to de-acylate acyl-ghrelin. Caloric restriction (CR): A reduction of food intake (typically 30% less) in the absence of malnutrition. Calorie Restriction mimetics: endogenous or exogenous factors that mimic the effects of CR cAMP response element binding protein (CREB): A transcription factor regulating BDNF expression and important in neuronal plasticity and memory. Dentate gyrus (DG): A sub-region of the hippocampus where neurogenesis occurs. Ghrelin-O-Acyl transferase (GOAT): a member of the membrane bound O-Acyl transferase (MBOAT) family, the only enzyme known to acylate ghrelin. GluA1-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPAR) receptors: Glutamate receptor and ion channels responsible for fast excitatory synaptic transmission. Important for synaptic plasticity and long-term potentiation (LTP). Growth hormone secretagogue receptor (GHSR): A G-protein-coupled receptor (GPCR) also known as the ghrelin receptor. It is the only known receptor for acyl-ghrelin. GHSR-eGFP mice: genetically mutated mice that co-express green fluorescent protein (GFP) with the Growth Hormone Secretagogue Receptor (GHS-R) gene Heterodimers: A protein complex, consisting of two different proteins. Hormesis: A bi-phasic dose-response to a drug or intervention, in which a low dose produces a beneficial effect and a high dose produces a harmful effect. Long-term potentiation (LTP): An important process in memory formation, resulting in a long-term increase in the strength of electrical activity or transmission between two neurones. Neural stem progenitor cells (NSPCs): Multipotent stem cells that reside in the brain with the capacity to differentiate into any neural cell type. Novel object recognition (NOR): Behaviour test of cognitive function that requires recognising novel from familiar objects. This is a hippocampal-dependent task. Passive avoidance learning (PAL): Behaviour test that evaluate memory and learning in rodents with neurological disorders. The rodents learn to avoid an environment in which an aversive stimulus was previously delivered. Pattern separation: The ability to distinguish similar inputs into separate discrete outputs. SAMP8 mice: a naturally occurring mouse model of accelerated ageing wi...

Growing evidence highlights a complex interaction between olfaction and metabolism with impaired olfactory function observed in obesity and increased olfactory sensitivity during hunger. The mechanisms linking metabolic state and olfaction remain unknown, but increased accessibility of hormones, such as ghrelin, and the diverse expression of hormone receptors such as those for ghrelin (GHSRs) in the olfactory system suggests an underappreciated neuroendocrine role. Here, we examined the hypothesis that GHSRs in the olfactory bulb (OB) link hunger with olfactory sensitivity to influence foraging behaviours and metabolism. Selective deletion of OBGHSRs in adult male mice was achieved with adeno-associated viral expression of cre-recombinase in the OB of floxed-Ghsr mice. OBGHSR deletion significantly affected olfactory discrimination and habituation to both food and pheromone odours, with greatest effect under fasted conditions. Anxiety-like and depression-like behaviour was significantly greater after OBGHSR deletion using 3 independent anxiety behavioural tasks and testing for anhedonia, whereas exploratory behaviour was reduced. No effect on spatial navigation and memory was observed. Although OBGHSR deletion did not affect cumulative food intake, it significantly impacted feeding behaviour as evidenced by altered bout number and duration. Moreover, food-finding after fasting or ip ghrelin was attenuated. Intriguingly, OBGHSR deletion caused an increase in body weight and fat mass, spared fat utilisation on a chow diet and impaired glucose metabolism indicating metabolic dysfunction. We conclude that OBGHSRs maintain olfactory sensitivity, particularly during hunger, and facilitate behavioural adaptations that optimise food-seeking in anxiogenic environments, priming metabolic pathways in preparation for food consumption.

New neurones are generated throughout life in the mammalian brain in a process known as adult hippocampal neurogenesis (AHN). Since this phenomenon grants a high degree of neuroplasticity influencing learning and memory and mood related behaviour, identifying factors that regulate AHN may be important for ameliorating age-related cognitive decline and neurodegeneration. Calorie restriction (CR), in the absence of malnutrition, has been shown to enhance AHN and improve hippocampal-dependent memory, mediated by the stomach hormone, ghrelin. Intermittent fasting (IF), a dietary strategy offering more flexibility than conventional CR, also promotes aspects of AHN. The 5:2 diet is a popular form of IF linked to a range of health benefits, however its effects on AHN and spatial memory are not well characterised. We hypothesised that the 5:2 diet would enhance AHN in a ghrelin-dependent manner. To assess this, we used immunohistochemistry to quantify new adult-born neurones and new neural stem cells (NSCs) in the hippocampal DG of adolescent and adult wild-type and mice lacking the ghrelin receptor following six weeks on a 5:2 diet. We report an age-related decline in neurogenic processes and identify a novel role for ghrelin-receptor in regulating the formation of new adult born NSCs in an age-dependent manner. However, the 5:2 diet did not affect new neurone or NSC formation in the DG. Consistent with this finding the 5:2 diet did not alter performance on a spatial learning and memory task. These data suggest that the 5:2 diet used in this study does not increase AHN or improve associated spatial memory function.