Rats were trained to respond according to a modification of the discrete-trial current-threshold BSR procedure of Kornetsky and Esposito5,6 in an operant box equipped with a wheel manipulandum and ICSS stimulator (Med Associates). Briefly, a trial was initiated by the delivery of a non-contingent electrical stimulus. This electrical reinforcer had a duration of 500 ms and consist of 0.1 ms rectangular cathodal pulses that delivered at a frequency of 50–100 Hz. The frequency of the stimulation was selected for individual rats so that threshold elevation and lowering may be detected, and this frequency was held constant throughout the experiment. A one-quarter turn of the wheel manipulandum within 7.5 sec of the delivery of the non-contingent stimulation resulted in the delivery of an electrical stimulus identical in all parameters to the non-contingent stimulus that initiated the trial. After a variable inter-trial interval (7.5–12.5 sec, mean of 10 sec), another trial was initiated with the delivery of a non-contingent electrical stimulus. Failure to respond to the non-contingent stimulus within 7.5 sec resulted in the onset of the inter-trial interval. Responding during the
