Endogenous opioids: Naloxone disrupts learned performance in rats

The effect of naloxone on learned performance reinforced by food was examined in 2 experiments. Male rats were trained to run down a short runway for 5 (45 mg) food pellets per trial and were then shifted either to 1 or 0 pellets. Following such an abrupt reinforcement shift, animals typically show an emotional disruption of performance (Crespi, 1942) referred to as “depression.” We examined the postshift depression-effect in groups treated either with saline (SAL) or naloxone (NAL). In experiment 1 NAL groups received a single 10 mg/kg (s.c.) injection prior to each postshift session. When compared with SAL controls, NAL animals showed an exaggerated postshift depression-effect. Furthermore, a single (0.3 mg/kg, s.c.) injection of the enkephalin analog FK 33-824 (D-Ala², MePhe⁴, Met-(0)⁵-o1) produced a dramatic recovery of performance. In the second experiment, these effects were replicated at a low NAL dose (1 mg/kg), which had no direct effect on motor performance. These findings suggest that opiate systems may modulate the incentive motivation that maintains learned performance.