Is Experimental Evolution of an Increased Aerobic Exercise Performance in Bank Voles Mediated by Endocannabinoid Signaling Pathway?

Abstract

The level of physical activity achieved in a given situation depends on both physiological abilities and behavioral characteristics (motivation). We used a unique animal model to test a hypothesis that evolution of an increased aerobic exercise performance can be facilitated by evolution of motivation to undertake physical activity, mediated by brain endocannabinoid system. Bank voles (Myodes glareolus) from "aerobic" A lines selected for 22 generations for high swim-induced aerobic metabolism (VO₂swim) achieved 65% higher "voluntary maximum" VO₂swim than voles from unselected, "control" C lines. In C lines, VO₂swim was 24% lower than the maximum forced-running aerobic metabolism (VO₂run), while in A lines VO₂swim and VO₂run were practically the same. Thus, the selection changed both the aerobic capacity and motivation to exercise at the top performance level. We applied a pharmacological treatment manipulation to test a hypothesis that the endocannabinoid signaling pathway 1) affects the voles performance in the aerobic exercise trials, and 2) has been modified in the selection process. Administration of the CB1 receptor antagonist (Rimonabant) did not affect the level of metabolism, but administration of the endocannabinoid reuptake inhibitor (AM404) decreased VO₂swim both in A and C lines (4%, p = 0.03) and tended to decrease VO₂run (2%, p = 0.07). The significant effect of AM404 suggests the involvement of endocannabinoids in signaling pathways controlling the motivation to be active. However, the response to AM404 did not differ between A and C lines (interaction effect, p ≥ 0.29). Thus, the results did not provide a support to the hypothesis that modifications of endocannabinoid signaling have played a role in the evolution of increased aerobic exercise performance in our experimental evolution model system.

Summary statement: The results corroborated involvement of endocannabinoids in the regulation of physical activity, but did not support the hypothesis that modification of endocannabinoid signaling played a role in the evolution of increased aerobic exercise performance in our experimental evolution model.

Keywords: endocannabinoids; experimental evolution; motivation; physical activity; selective breeding; voluntary exercise.

FIGURE 1

The relationship between swim-induced (VO₂swim) or run-induced (VO₂run) 1-min maximum rate of oxygen consumption and body mass in bank voles, and the effects of selection, sex, and pharmacological manipulation. (A) VO₂swim in all individuals from generation 22 (C – Control lines, A – Aerobic lines) tested as a part of the selection experiment (selection-trial results). (B) The effect of Exercise Type (VO₂swim vs. VO₂run) in voles from C (N = 96) and A (N = 96) lines (voles used in AM404 and Rimonabant experiments combined). (C,D) The effect of AM404 [an endocannabinoid (eCB) reuptake inhibitor] on VO₂swim (C) and VO₂run (D): repeated trials in a subsample of voles from C (N = 48) and A (N = 48) lines. (E,F) The effect of Rimonabant (a cannabinoid receptor CB1 antagonist) on VO₂swim (E) and VO₂run (F): repeated trials in a subsample of voles from C (N = 48) and A (N = 48) lines. Dotted lines indicate the mean body mass for which the adjusted least-squares means were calculated (shown in Figure 2–4 and Supplementary Table S4).

FIGURE 2

The maximum rate of oxygen consumption achieved during selection trial and pharmacological trials: swim trials 1 and 2, and run trials 1 and 2 (only the trials after placebo-vehicle injections), in bank voles from selected A lines (n = 96) and unselected C lines (n = 96). The adjusted least squares means for the mean body mass (24 g) with 95% confidence limits are presented [LSM(95%CL) note: an overlap of the confidence limits does not indicate a lack of difference between repeated measurements at different conditions, because the confidence limits are based on among-individual variation at particular conditions, whereas the inferences concerning differences between conditions are based on within-individual comparisons. See Table 1 for results of the proper significance tests].

FIGURE 3

Summary of the main results from experiment with AM404 – an endocannabinoid reuptake inhibitor: repeated trials in a subsample of voles from selected A (n = 48) and unselected C lines (n = 48). (A) 1-min maximum swim-induced oxygen consumption (VO₂swim). (B) The time when voles achieved the VO₂swim. (C) Whole-trial mean VO₂swim (typically about 15 min). (D) 1-min maximum run-induced oxygen consumption (VO₂run). (E) The ratio of VO₂swim or VO₂run achieved after drug to the one after vehicle (proportional response). Bars represent adjusted least squares mean with 95% confidence limits (LSM[95%CL] Note: as explained in Figure 2 legend, an overlap of the confidence limits does not indicate that a difference is not significant. See Figure 5 and Tables 1, 2 for results of the proper significance tests).

FIGURE 4

Summary of the main results from experiment with Rimonabant – cannabinoid receptor CB1 antagonist: repeated trials in a subsample of voles from selected A (n = 48) and unselected C lines (n = 48). (A) 1-min maximum swim-induced oxygen consumption (VO₂swim). (B) The time when voles achieved the VO₂swim. (C) Whole-trial mean VO₂swim (typically about 15 min). (D) 1-min maximum run-induced oxygen consumption (VO₂run). (E) The ratio of VO₂swim or VO₂run achieved after drug to the one after vehicle (proportional response). Bars represent adjusted least squares mean with 95% confidence limits [LSM(95%CL) Note: as explained in Figure 2 legend, an overlap of the confidence limits does not indicate that a difference is not significant. See Figure 5 and Tables 1, 2 for results of the proper significance tests].

FIGURE 5

The effects of Rimonabant (cannabinoid receptor CB1 antagonist) and AM404 (endocannabinoid reuptake inhibitor on the measured traits): (A–D) Mean within-individual differences between the values obtained after drug treatment and vehicle injection [(A) 1-min maximum swim-induced oxygen consumption (VO₂swim); (B) the time when voles achieved the VO₂swim; (C) whole-trial mean VO₂swim; (D)1-min maximum run-induced oxygen consumption (VO₂run)]. Bars represent the least squares estimates of the mean differences with 95% confidence limits.