Response-dependent effects of morphine on reinforcing lateral hypothalamic self-stimulation.

The effects of morphine (10mg/kg) on intracranial self-stimulation were studied in three separate test situations, each requiring rats to perform different types of responses. Self-stimulation was depressed in a test of rate of bar-pressing, to a lesser extent in a test of rate of wall-pressing in which a wider range of movements were reinforced, but not in a shuttle-box, with brain stimulation continuously available on one side of box. This resonse dependency suggests that the depressive effect of morphine on bar-pressing for lateral hypothalamic stimulation reflects a performance deficit rather than an effect on the reinforcing value of the stimulation.