Circulating levels of endocannabinoids respond acutely to voluntary exercise, are altered in mice selectively bred for high voluntary wheel running, and differ between the sexes

Abstract

The endocannabinoid system serves many physiological roles, including in the regulation of energy balance, food reward, and voluntary locomotion. Signaling at the cannabinoid type 1 receptor has been specifically implicated in motivation for rodent voluntary exercise on wheels. We studied four replicate lines of high runner (HR) mice that have been selectively bred for 81 generations based on average number of wheel revolutions on days five and six of a six-day period of wheel access. Four additional replicate lines are bred without regard to wheel running, and serve as controls (C) for random genetic effects that may cause divergence among lines. On average, mice from HR lines voluntarily run on wheels three times more than C mice on a daily basis. We tested the general hypothesis that circulating levels of endocannabinoids (i.e., 2-arachidonoylglycerol [2-AG] and anandamide [AEA]) differ between HR and C mice in a sex-specific manner. Fifty male and 50 female mice were allowed access to wheels for six days, while another 50 males and 50 females were kept without access to wheels (half HR, half C for all groups). Blood was collected by cardiac puncture during the time of peak running on the sixth night of wheel access or no wheel access, and later analyzed for 2-AG and AEA content by ultra-performance liquid chromatography coupled to tandem mass spectrometry. We observed a significant three-way interaction among sex, linetype, and wheel access for 2-AG concentrations, with females generally having lower levels than males and wheel access lowering 2-AG levels in some but not all subgroups. The number of wheel revolutions in the minutes or hours immediately prior to sampling did not quantitatively predict plasma 2-AG levels within groups. We also observed a trend for a linetype-by-wheel access interaction for AEA levels, with wheel access lowering plasma concentrations of AEA in HR mice, while raising them in C mice. In addition, females tended to have higher AEA concentrations than males. For mice housed with wheels, the amount of running during the 30 minutes before sampling was a significant positive predictor of plasma AEA within groups, and HR mice had significantly lower levels of AEA than C mice. Our results suggest that voluntary exercise alters circulating levels of endocannabinoids, and further demonstrate that selective breeding for voluntary exercise is associated with evolutionary changes in the endocannabinoid system.

Maternal exposure to Western diet affects adult body composition and voluntary wheel running in a genotype-specific manner in mice

“…Interestingly, we found that individuals with the mini-muscle phenotype (found only in two of the HR lines in the present sample of mice) had statistically higher VO 2 max. Previous studies have generally not found this to be the case [40,50], although mini-muscle individuals did have higher VO 2 max when tested in hypoxia [34] and in a comparison of only one control and two HR lines [51]. …”

“…Tiredness was graded by how many seconds (<1, 1-5, or >5s) elapsed after the trial before the mouse was able to walk, groom, or engage in exploratory activities. The wheel apparatus for measuring VO 2 max was chosen over the more traditional treadmill-based test because they obtain equivalent values and the wheel apparatus closely mimics the behavior for which HR mice have been bred [40]. …”

Reduced non-bicarbonate skeletal muscle buffering capacity in mice with the mini-muscle phenotype

“…All procedures in this study were approved by and are in accordance with guidelines set forth by the Institutional Animal Care and Use Committee at the University of California, Riverside. Mice used in the present study are the same as in Thompson et al (2017), which came from generation 74 of an ongoing artificial selection experiment in which mice are bred for high voluntary wheelrunning behavior (Careau et al, 2013;Swallow et al, 1998). The founding population was 224 outbred Hsd:ICR mice (Mus domesticus).…”

“…A period of 6 days was chosen in order to gain insight regarding their running abilities in a way that matched the criterion used for selective breeding. As in Thompson et al (2017), experimental animals were on a reverse photoperiod, with lights on from 19:00 h to 07:00 h. Mini-muscle status was determined by dissection of the triceps surae muscle complex (Garland et al, 2002). Because the mini-muscle phenotype includes altered fiber type of the hindlimb muscles (Bilodeau et al, 2009;Guderley et al, 2006Guderley et al, , 2008Talmadge et al, 2014), it was used as an additional main effect in all statistical analyses (see below).…”

“…Home-cage activity (HCA, an indicator of spontaneous physical activity; Garland et al, 2011) was measured using passive infrared motion sensors placed inside each cage. As previously described (Acosta et al, 2017;Copes et al, 2015;Thompson et al, 2017), HCA sensors were interfaced with a computer and recorded activity 3 times s −1 as a binary output (0=no movement, 1=movement) and then readings were averaged across every 1 min interval. As for wheel running, HCA was measured for 23 h day −1 .…”