Search

Selective breeding of house mice has been used to study the evolution of locomotor behavior. Our model consists of 4 replicate lines selectively bred for high voluntary wheel running (High-Runner) and 4 bred randomly (Control). The major changes in High-Runner lines appear to have taken place in the brain rather than in capacities for exercise. Their neurobiological profile resembles features of human Attention Deficit Hyperactivity Disorder (ADHD) and is also consistent with high motivation for exercise as a natural reward. Both ADHD and motivation for natural rewards (such as food and sex), as well as drugs of abuse, have been associated with alterations in function of the neuromodulator dopamine, and High-Runner mice respond differently to dopamine drugs. In particular, drugs that block the dopamine transporter protein (such as Ritalin and cocaine) reduce the high-intensity running of High-Runner mice but have little effect on Control mice. In preliminary studies of mice exercised on a treadmill, brain dopamine concentrations did not differ, suggesting that changes in the dopamine system may have occurred downstream of dopamine production (e.g., receptor expression or transduction). Brain imaging by immunohistochemical detection of c-Fos identified several key regions (prefrontal cortex, nucleus accumbens, caudateputamen, lateral hypothalamus) that appear to play a role in the differential response to Ritalin and in the increased motivation for running in High-Runner mice. The activation of other brain regions, such as the hippocampus, was closely associated with wheel running itself. Chronic wheel running (several weeks) also increased the production of new neurons to apparently maximal levels in the hippocampus, but impaired learning in High-Runner mice. We discuss the biomedical implications of these findings.

We used a novel mouse model to study the effects of selective breeding for high locomotor activity (14 generations) on relative organ sizes, hematocrit (Hct), and blood hemoglobin (Hb) concentration. We also examined effects of exercise training and genotype-by-environment interactions by housing animals for 8 weeks with wheels that were either free to rotate or locked. Mice from the four replicate High-Runner (HR) lines were smaller in total body mass but had larger body mass-adjusted kidneys relative to the four Control lines (P < 0.05). Control and HR lines did not differ significantly for mass-adjusted tail length or masses of the 'triceps surae' hindlimb muscle group, heart (ventricle), spleen, liver, adrenal glands or gonads. Wheel access caused a reduction in body mass and an increase in relative heart mass. In females only, wheel access caused a reduction in relative spleen mass. Wheel access did not affect relative tail length or relative mass of the triceps surae, liver, adrenal gland or gonads. Significant interactions between selection history and wheel access were observed in females for spleen, liver, and gonad mass as well as Hct and Hb. Wheel access caused increases in both Hct and Hb, mainly in the HR lines. The mini-muscle phenotype, caused by a Mendelian recessive allele that halves hindlimb muscle mass, was significantly associated with several other body composition traits, including reduced body mass, increased tail length, increased heart mass, increased liver mass (females only), increased mean adrenal gland mass (females only), increased mean kidney mass (males only), and reduced Hct (wheel-access females only). Results are discussed in context of the beneficial acclimation hypothesis, genotype-by-environment interactions, and the potential for 'nurture' to be self-reinforcing of 'nature' in some complex behavioral-physiological phenotypes.

Research has been conducted on the locomotor behavior evolution. The genetic basis and discrete polymorphism in hindlimb muscle mass have been investigated and the results indicate that muscle mass variations affect locomotor activities.