title: Identification of a novel class of central reward sites showing a delayed and cumulative response to opiate blockade.
abstract: Heretofore it has been assumed that the brain's reward system shows a response to opiate receptor blockade that is (a) immediate in nature and (b) close to asymptote after the initial dose. On theoretical grounds a graded and gradual response to blockade might also be predicted. To test this, rats were implanted with subcortical electrodes for self-stimulation and trained to panel-press for contingent intracranial reinforcement. Chronic blockade of the opiate system was produced by repeated injections of naltrexone HCl. A small percentage of self-stimulation sites proved refractory to opiate blockade both acutely and chronically. Also, some sites showed an immediate decrease in response level, as previously reported. As was predicted, a number of sites originally showing no response alteration showed a graded response decrease over a 4-day test period. These findings indicate the existence of a novel class of opiate sensitive reward sites and suggest (a) some apparently insensitive sites may become sensitive to opiate blockade under appropriate testing circumstances and (b) opioid neuropeptides may control certain tonic as well as more immediate aspects of motivation.
mesh_terms: D000818:Animals; D004558:Electric Stimulation; D004567:Electrodes, Implanted; D008297:Male; D008474:Medial Forebrain Bundle / Q000502:physiology*; D009292:Narcotic Antagonists / Q000494:pharmacology*; D009434:Neural Pathways / Q000502:physiology*; D051381:Rats; D011919:Rats, Inbred Strains; D012201:Reward; D012653:Self Stimulation / Q000187:drug effects*
chemical_list: D009292:Narcotic Antagonists