INVESTIGATION OF THE BEHAVIORAL PROCESSES AND NEUROBIOLOGICAL SUBSTRATES INVOLVED IN THE MOTIVATION FOR VOLUNTARY WHEEL RUNNING IN THE RAT

Abstract

Voluntary wheel running in rodents has been used extensively to induce both overt changes in behavior and covert changes in the body and brain, but few studies have examined in a systematic and quantitatively comparable manner how variables such as gender, hormonal state, wheel availability, and wheel apparatus affect wheel running behavior. We studied wheel running in male and female Sprague Dawley rats (each group, n=4-20) by determining the distance run, time spent running, rate of running, and bouting patterns from first exposure to the wheel to acquisition of stabilized or habitual running. Gender differences emerged during the first 24 hours of wheel interaction and remained after running patterns stabilized (2-3 weeks). Females acquired the behavior more quickly and traveled significantly farther daily distances by running longer and at faster rates. The stage of the ovarian cycle markedly influenced running, with proestrus females running farther, longer, and faster in more frequent bouts than metestrus females or males. Gonadectomy significantly decreased all aspects of running; however, gender differences not attributable to circulating hormones remained: female running still surpassed that in males. In experienced runners, after weeks of deprivation of wheel availability, they quickly regained maximal running patterns, whereas months of deprivation returned them to a wheel-naive state. Limited daily wheel availability (30 minutes or 2 hours) led to running a greater percentage of wheel access time, even eliminating bout running, but the rate of running was always slower than in the ad libitum runner. Subjects with alternateday wheel access ran the same distance as ad libitum runners on wheel access days but adopted a different pattern of rate of running during wheel availability. Overall, rats with limited wheel access showed remarkable alterations in time and rate of running, which appeared to be an anticipatory response to removal of the wheel. Wheel running also was impacted by the type of apparatus utilized, which must be taken into consideration when comparing data across studies. Together, this information provides new evidence that voluntary wheel running habits are flexible and can be shaped by specific protocol choices. These data will help to inform future investigations of the CNS mechanisms underlying the motivation to engage in voluntary wheel running.