Chronic morphine fails to enhance the reward value of prefrontal cortex self-stimulation

Dopamine (DA) plays an important role in the rewarding effects of drugs of abuse and intracranial self-stimulation (ICSS). We previously reported that ICSS derived from the prefrontal cortex appears insensitive to the reward-enhancing effects of amphetamine, a drug that increases DA release and reward at other ICSS sites. In the present study, rats with prefrontal electrodes were tested to see if morphine (7.5 or 10.0 mg/kg, IP) given once per day for 10 days enhanced prefrontal reward as assessed with the curve-shift method. Morphine initially produced sedation; however, after 3-4 days response rates increased sharply while frequency thresholds were unaffected. These results demonstrate that morphine does not enhance prefrontal ICSS reward and provide further evidence that prefrontal brain stimulation reward does not display the same characteristics as other ICSS sites.