Effects of morphine and naloxone on play-rewarded spatial discrimination in juvenile rats.

Juvenile rats were trained on a spatial discrimination task (T-maze) rewarded by the opportunity to play with a conspecific. Neither morphine (MS; 1 mg/kg) nor naloxone (NX; 1 mg/kg) administration affected choice or running time during the acquisition of the task, even though in the goal box, MS-treated animals played more than and NX-treated animals less than vehicle-treated controls. Thus, brain opioid systems seem to influence the expression of play without affecting the apparent appetitive strength of play motivation. When play reward was no longer available in the goal box, animals that continued to be treated with MS were more resistant to extinction than either vehicle- or NX-treated animals. They continued to complete the task more often and in less time than the controls. NX-treated rats, on the other hand, extinguished faster than controls. The evidence supports the conclusion that opioid systems are important in the maintenance of social habits.