Effects of Selective Breeding, Exercise, and Sex on Endocannabinoid Levels in the Mouse Small Intestine

Abstract

Only half of adults in the United States get the physical activity they need to help prevent certain chronic diseases. Such diseases as cardiovascular disease and diabetes are metabolism‐related and closely tied to insufficient physical activity. The endocannabinoid (EC) system serves many physiological roles, including in the regulation of voluntary locomotion, energy balance, and food reward. Signaling at the cannabinoid type 1 receptor (CB1) has been specifically implicated in acute and long‐term rodent voluntary exercise on wheels. Plasma levels of endocannabinoids in mice are known to change in association with wheel running, but EC concentrations in the gut have not been studied in the context of exercise. We studied four replicate lines of high runner (HR) mice that had been selectively bred for 74 generations based on the average number of wheel revolutions on days 5 and 6 of a 6‐day period of wheel access. Four additional replicate lines have been bred without regard to wheel running and serve as controls (C). On average, mice from HR lines voluntarily run on wheels ~3 times more than C mice on a daily basis. HR mice have higher home‐cage activity when housed without wheels, reduced body mass and fat, eat more, and their wheel running responds differently to a CB1 agonist & antagonist compared to C mice. A recent study showed that circulating levels (during peak running) of primary endocannabinoids 2‐arachidonoylglycerol (2‐AG) and anandamide (AEA) are altered by wheel running, differ between HR and C mice, and have sex‐specific interactions (Thompson et al. 2017. Physiology & Behavior 170:141–150). The EC system in the gut is of particular interest because consuming dietary fats leads to increased EC content in rodent small‐intestinal epithelium, which in turn, drives overeating. A focus on HR gut endocannabinoids may present novel insights concerning exercise metabolism of fats and gut‐brain reward signaling. We hypothesized that small intestine [EC] would differ between HR and C mice, between the sexes, between mice housed with vs. without wheels for 6 days, and would covary with acute running amounts (during the previous 30 minutes). We used 196 total mice, half males and half females, half HR and half C, and half either given or not given access to wheels for 6 days. Jejunum gut mucosa was collected for lipid extractions and subsequent EC analysis via ultra‐performance liquid chromatography coupled to tandem mass spectrometry (UPLC/MS/MS). 2‐AG, AEA, as well as their analogs docosahexaenoylglycerol (DHG), oleoylethanolamide (OEA) and docosahexaenoylethanolamide (DHEA), were measured. DHG showed a significant 3‐way interaction of linetype, wheel access, and sex, with no consistent effects of any of these factors. A trend also showed HR mice having lower 2‐AG, which suggests that HR mice may have lower rates of production of intestinal ECs and/or faster rates of degradation by monoacylglycerol lipase (MGL). Functionally, lower 2‐AG in HR jejunum might be related to differences from C mice with respect to locomotor activity, body composition, and/or food consumption.