Corticosterone levels during extinction of runway response in rats

Abstract This investigation examined whether the hypothalamic-pituitary-adrenocortical axis would activate during extinction of a straight runway response. Four groups of rats were trained to run in a straight runway for 3 (for 3S and 3L groups) or 10 (for 10S and 10L groups) food pellets as a reward for either 15 (for 3S and 10S groups) or 25 (for 3L and 10L groups) acquisition days followed by 3 extinction days. Blood samples for determination of serum corticosterone concentration were taken from all animals under 5 different situations: 10 min after entering the experimental room, after the 24th acquisition day, after the first and third extinction days and under the deprivation control. Running speed of 10S and 10L groups in extinction days were reduced more quickly than that of 3S and 3L groups. After the third extinction day, serum corticosterone concentrations were significantly elevated in 10S and 10L, but not in 3S and 3L groups. These results indicate that the amount of reward in the acquisition phase influences not only the running speed, but also the corticosterone concentration in the extinction phase.