Pairings of a conditioned (CS) and unconditioned (US) stimulus can result in approach to either the CS (sign tracking) or the US (goal tracking). The distance between the conditioned stimulus and the site of US delivery affects the sign / goal tracking response and impairs the associative learning; the addictive drugs can enhance the incentive effects of a reward and thus increase individual reward seeking behavior. There are no reports on the relationship between impairments of associative learning by the distance and the incentive amplifying effects of addictive drugs. Therefore, this experiment attempted to investigate the effects of administration of morphine on sign-tracking and goal-tracking when the spatial separation between the CS and a US food was 8、30 and 60 cm.

Fifty-six male Sprague-Dawley rats participated in the approach conditioning experiment of different distances consisted of habituation, food receptacle training, acquisition, administration of morphine which was given daily injection of morphine (5mg/kg) or saline for 7 days, extinction phases and reversal learning. Two different material objects served as CS+ and CS−, and a 10-sec food as US while approach duration of the CS (sign-tracking) and US (goal-tracking) was measured. Each CS+ was always followed by food delivery any of the experiment phases except the extinction phases, whereas the CS− was not.

The results were as following: (1) Sign tracking of rats developed when the CS was 60 cm from the US. Increasing the spatial separation between the CS and US resulted in a decline in sign tracking and had no effect on goal tracking under non-drug conditions. (2) Acute administration of morphine (5.0 mg/kg) decreased measures of sign-tracking, from 8 to 60cm, while simultaneously increasing measures of goal-tracking when the CS was 8 and 60 cm from the source of the US. Repeated administration of morphine decreased measures of sign-tracking while simultaneously increasing measures of goal-tracking when the CS was 8 and 30 cm from the source of the US. In the extinction test, prior morphine exposure decreased sign-tracking when the CS was 8 and 60cm from the source of the US, and increased goal-tracking when the spatial separation between the CS and US was 60 cm. (3) In the discrimination reversal learning, rats pre-exposed to morphine showed less contact to new CS+ compared to saline controls from 8 to 60cm, showed worse discrimination and biased to old CS+ when the spatial separation was 30 and 60 cm.

The results of this study suggested that distance had little effect on suppression of sign-tracking but had enhancing effects on goal-tracking by morphine. Morphine pre-exposure impairing discrimination reversal learning was facilitated by distance. Research shows the distance is an important factor, which facilitates impairments of associative learning and has no effect on the incentive amplifying effects of following administration of addictive drug.