Ghrelin transport across the blood-brain barrier can occur independently of the growth hormone secretagogue receptor

Abstract

Objective: The blood-brain barrier (BBB) regulates the entry of substrates and peptides into the brain. Ghrelin is mainly produced in the stomach but exerts its actions in the central nervous system (CNS) by crossing the BBB. Once present in the CNS, ghrelin can act in the hypothalamus to regulate food intake, in the hippocampus to regulate neurogenesis, and in the olfactory bulb to regulate food-seeking behavior. The goal of this study was to determine whether the primary signaling receptor for ghrelin, the growth hormone secretagogue receptor (GHSR), mediates the transport of ghrelin from blood to brain.

Methods: We utilized the sensitive and quantitative multiple-time regression analysis technique to determine the transport rate of mouse and human acyl ghrelin (AG) and desacyl ghrelin (DAG) in wildtype and Ghsr null mice. We also measured the regional distribution of these ghrelin peptides throughout the brain. Lastly, we characterized the transport characteristics of human DAG by measuring the stability in serum and brain, saturability of transport, and the complete transfer across the brain endothelial cell.

Results: We found the transport rate across the BBB of both forms of ghrelin, AG, and DAG, were not affected by the loss of GHSR. We did find differences in the transport rate between the two isoforms, with DAG being faster than AG; this was dependent on the species of ghrelin, human being faster than mouse. Lastly, based on the ubiquitous properties of ghrelin throughout the CNS, we looked at regional distribution of ghrelin uptake and found the highest levels of uptake in the olfactory bulb.

Conclusions: The data presented here suggest that ghrelin transport can occur independently of the GHSR, and ghrelin uptake varies regionally throughout the brain. These findings better our understanding of the gut-brain communication and may lead to new understandings of ghrelin physiology.

Keywords: Blood–brain barrier; Ghrelin; Growth hormone secretagogue receptor; Pharmacokinetics.

Figure 1

Blood-to-brain influx of ¹²⁵I-ghrelin peptides in WT and Ghsr null mice. No significant difference in A) mAG, B) hAG, C) mDAG, or D) hDAG influx was observed between WT (closed circles, n = 10–11) and Ghsr null mice (open circles, n = 6–9).

Figure 2

Characterization of ¹²⁵I-hDAG. A) Stability in serum and whole brain (Whole Brain *p < 0.05 vs 2 and 6 min). B) Complete transfer across the brain endothelial cell (n = 3). C) Lack of saturable transport (One-Way ANOVA: p = 0.89, n = 9).

Figure 3

Regional distribution of ghrelin peptides. Levels of ¹²⁵I-ghrelin peptides were measured in each brain region after 10 min circulation. There were significant differences in the regional distribution of A) mAG, B) hAG, C) mDAG, and D) hDAG levels. There was also a difference due to genotype with B) hAG. (n = 5–7 per region per genotype).

Figure 4

Pictorial representation of ¹²⁵I-ghrelins brain distribution. A heat map was generated based on the data in Table 2. A–C) Data are collapsed across genotype. D–E) Data are separated for ¹²⁵I-hAG due to statistically significant differences between genotypes.