Differential effects of selective opioid peptide antagonists on the acquisition of pavlovian fear conditioning

DEIRA-FERNANDEZ. Differential effects of selective opioid peptide antagonists onthe acquisition of Pavlovian fear condition-ing. PEPTIDES 12(5) 1033-1037, 1991.--Pretreatment wi h opioid antagonists enhances acquisition of Pavlovian fear conditioning. The present experiments attempted to characterize the type of opioid receptor esponsible for this effect using a procedure that assessed the fear of rats to a chamber previously associated with electric shock (1 mA, 0.75 s). Freezing, a species-typical immo-bility, was employed as an index of fear. Two Ix opioid antagonists, CTOP (40 ng) and naloxonazine (10 p.g), enhanced condi-tioning. On the other hand, the K antagonist nor-binaltorphimine reduced conditioning. Two ~ antagonist treatments (16-methyl cyprenorphine and naltrindole) had no reliable effect on acquisition. Thus the enhancement of conditioning appears to be mediated by i ~ receptors. Previous research as shown that the conditional fear produced by these procedures caused an analgesia that is also mediated by I ~ receptors. It is argued that the enhancement effect occurs because of an antagonism of this analgesia nd that the analgesia normally acts to regulate the level of fear conditioning. Opioid antagonist Pavlovian conditioning Fear Freezing Mu opioid receptor Kappa opioid receptor Nor-binaltorphimine 16-Methyl cyprenorphine Naltrindole Stress-induced analgesia Cys2TyraOrnSPenT-amide Delta opioid receptor Naloxonazin