Effects of naloxone on rewarding and aversive brain sites

The effects of naloxone on the rewarding and aversive properties of brain stimulation derived from the ventral tegmental area and the nucleus reticularis gigantocellularis, respectively, were assessed in rats, based on the following measures-the current threshold for latency to escape aversive nucleus reticularis gigantocellularis stimulation, the frequency threshold for rewarding ventral tegmental area stimulation, and the frequency threshold for self-stimulation obtained from delivery of concurrent ventral tegmental area and nucleus reticularis gigantocellularis stimulation, before and after three systemic doses of naloxone (0, 10, and 20mg/kg); in the latter case, the stimulation trains were interdigitated with an interpulse interval of 2 ms. Initially, thresholds for concurrent stimulation were elevated relative to the values obtained for ventral tegmental area stimulation alone, returning to baseline values only when the nucleus reticularis gigantocellularis stimulation no longer induced escape. After each pairing of the two sites, the current threshold for escape gradually increased until the maximum value administered, 700 microA, at which point aversive responses were no longer observed. This required very few pairings, between one and five trials across animals. Drug tests were then begun and produced a significant dose-response threshold increase across animals, without reinstating the latency to escape nucleus reticularis gigantocellularis stimulation. These findings are discussed in terms of a dissociation between the analgesic and rewarding properties of ventral tegmental area stimulation.