Endocannabinoid regulation of gastric vagal afferent signalling

Abstract

Background: Gastric vagal afferents (GVAs) play a role in peripheral appetite control. Tension sensitive GVAs respond to stretch or distension of the stomach sending signals to the hindbrain to produce feelings of fullness and satiety. The sensitivity of tension sensitive GVAs is reduced in diet-induced obesity which may contribute to increased meal size. Further, transient receptor potential vanilloid 1 (TRPV1) knockout mice also show decreased GVA tension receptor sensitivity. Endocannabinoids (ECs) regulate appetite via cannabinoid 1 (CB1) receptors and are also endogenous ligands for TRPV1. The CB1 receptor and TRPV1 are expressed on GVAs and the CB1 receptor is also expressed on gastric ghrelin cells. Further, the endocannabinoid anandamide (AEA; ligand for CB1) is expressed in the stomach. However, it is not known if ECs, ghrelin, and TRPV1 interact to regulate GVA sensitivity. Aims: To determine in lean and diet induced obese mice: 1. The co-expression of CB1, TRPV1, and ghrelin receptor (growth hormone secretagogue receptor; GHSR) in individual GVA cell bodies. 2. The effect of AEA on GVA sensitivity and secondary messenger pathways involved. 3. The effect of AEA and ghrelin on the expression of orexigenic receptors in nodose ganglia. Methods: Retrograde tracing was used to identify single GVA cell bodies in the nodose ganglia which was then combined with single cell QRT-PCR. An in vitro electrophysiology preparation was used to determine the effects of methanandamide (mAEA; stable analogue of AEA) on the sensitivity of GVAs in C57BL/6 mice fed ad libitum with either a standard laboratory diet (SLD) or high fat diet (HFD). This was performed with mAEA alone or in combination with antagonists to determine possible secondary messenger pathways. Nodose ganglia were cultured for 14 hours in a medium containing mAEA or ghrelin and then analysed via QRT-PCR for changes in receptor or ion channel expression. Results: CB1, TRPV1, and GHSR were expressed and co-expressed in individual tension sensitive GVA neurons in a diet-dependent manner. In SLD-mice mAEA exhibited concentration-dependent dual inhibitory and excitatory effects on the mechanosensitivity of tension sensitive GVAs. This was abolished to a single inhibitory effect regardless of concentration in HFD-mice. In cultured vagal afferent cell bodies, exposure to mAEA and ghrelin altered the expression of CB1, TRPV1, and GHSR mRNA in a diet dependent manner. Conclusions: ECs, acting through CB1 and TRPV1, have a pivotal role in modulating GVA satiety signals depending on the second messenger pathway utilised. In HFD-mice only an inhibitory effect is observed. These changes may contribute to the development and/or maintenance of obesity.