Delayed backward conditioning of place preference induced by wheel running in rats

Backward conditioning of place preference has been obtained in hungry rats when wheel running (the rewarding US) is followed immediately by exposure to a distinctive chamber (the CS). We tested whether such backward conditioning would occur with a 10-min delay. In each of four paired exposures, a period of wheel running (2 or 22 h, in separate experiments) was followed by 10 min in the home cage and then 30 min in the CS chamber. Control rats were put in the CS chamber without wheel running. In other similar experiments, a 0-, 10-, or 30-min delay separated wheel running from exposure to the CS chamber. Reliable conditioned place preference (CPP) occurred when the delay was 0 or 10 but not 30 min. The strength of CPP decreased as the delay increased. These findings imply that the reward process initiated by wheel running remains active for some time after running stops. They support the view that the suppression of feeding produced by wheel running in hungry rats (activity anorexia) is mediated by this reward process.