Effects of morphine on spontaneous multiple-unit activity: Possible relation to mechanisms of analgesia and reward

The effects of morphine (15 mg/kg) on multiple-unit activity in the awake rat were investigated at brain sites previously characterized by their ability to support stimulation-produced analgesia (SPA) and intracranial self-stimulation (ICSS). Of the SPA and SPA + ICSS sites, most of which were located in the periaqueductal gray matter, 91% showed increased multiple-unit activity after morphine administration (median increase = 80%). In contrast, only 50% of the ICSS-only sites, most of which were located in the lateral hypothalamus, and only 29% of sites supporting neither behavior showed this effect. All increases in multiple-unit activity were at least partly reversed by naloxone (1 mg/kg). Latencies to their onset and to analgesia measured by the tail-flick method were significantly correlated. A significant negative correlation was found between ICSS current thresholds and increases in multiple-unit activity after morphine at ICSS-only sites. These data lend further support to the suggestion that morphine exerts its analgesic action by activating an endogenous analgesic system and that the periaqueductal gray constitutes an important part of such a system. Furthermore, it is suggested that morphine's excitatory effect at self-stimulation loci may reflect its rewarding properties.