Wheel access duration in rats: II. Day-night and within-session changes.

Time of day and duration of wheel access were manipulated to see how running changed over days. Young male rats were given 2 hr of wheel access, during either the light or dark phase. Over 24 days, running increased 4-fold in the dark group but remained low and stable in the light group. With rats given either 1 or 4 hr of dark phase wheel access, running increased much more in the 4-hr group (even in Hour 1) than in the 1-hr group. The within-session running pattern for high runners changed from a habituation to a sensitization profile. Running and cocaine self-administration (S. H. Ahmed & G. F. Koob, 1998, 1999) are affected similarly by session length, suggesting running as an interesting addiction model.