title: Dual effect of morphine in long-term social memory in rat.
abstract: OBJECTIVE
  Bimodal dose-response relationships have been demonstrated in animals and humans following morphine administration. We examined if systemic administration of morphine, in extremely low (μg) and high (mg, analgesic) doses, changed the learning process.
  
  
  METHODS
  In the social learning test, an adult rat investigates a juvenile. The juvenile is submitted to a second encounter after a few days and investigation by the adult should be reduced. Morphine was administered before the first encounter between rats, and the critical test was performed 24, 72 or 168 h later, when animals were re-exposed to each other, in the absence of morphine.
  
  
  RESULTS
  Low doses of morphine, comparable with endogenous brain concentrations, enhanced long-term memory recognition; while high doses did the reverse, indicating the adult failed to recognize the juvenile. Recognition of a familiar rat appeared to be mediated within the brain accessory olfactory bulb (AOB) by an opioid system intrinsic to the olfactory system through μ-opioid receptors (MORs). At this supraspinal site, the PLC/PKC signalling pathway was activated by extremely low morphine doses.
  
  
  CONCLUSIONS
  Morphine treatment administration may either disrupt or facilitate social memory, depending on the dose, extending to memory formation the bimodal effects of morphine previously shown in pain. Social memory formation elicited by extremely low morphine doses, was mediated within the AOB by an opioid system, intrinsic to the olfactory system through MORs.
authors.affiliation: Department of Medical Surgical Sciences and Neuroscience, University of Siena, Siena, Italy. enrica.bianchi@unisi.it
authors.forename: Enrica
authors.initials: E
authors.lastname: Bianchi
authors.forename: Cristina
authors.initials: C
authors.lastname: Menicacci
authors.forename: Carla
authors.initials: C
authors.lastname: Ghelardini
mesh_terms: D000701:Analgesics, Opioid / Q000008:administration & dosage*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D057567:Memory, Long-Term / Q000187:drug effects*; D009020:Morphine / Q000008:administration & dosage*; D009830:Olfactory Bulb / Q000378:metabolism; D051381:Rats; D017450:Receptors, Opioid, mu / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects
chemical_list: D000701:Analgesics, Opioid; D017450:Receptors, Opioid, mu; D009020:Morphine