Immediate-early gene expression in the amygdala following footshock stress and contextual fear conditioning.

This study investigated the increase in expression in the amygdala of 2 immediate-early genes, c-fos and NGFI-A, following contextual fear conditioning. The immediate-shock freezing deficit paradigm was used to compare rats that received footshock after exploring a context to rats that received footshock immediately after placement in the chamber. The former procedure produces contextual fear conditioning while the latter does not. Rats were either handled (handled group), placed in a test chamber without receiving footshock (context-no-footshock group), received footshock immediately upon being placed in the chamber (immediate-footshock group), or received footshock after a 1 min delay (delayed-footshock group). Only the delayed-footshock group displayed a fear response (freezing behavior). Rats were sacrificed either 15 min after the experience or after a retention test 24 h later. The c-fos mRNA was increased in the medial nucleus of the amygdala in all of the groups that were placed in the test chamber. However, rats that received footshock (immediate- and delayed-footshock groups) had greater levels of c-fos mRNA expression than rats of the context-no-footshock group. The c-fos mRNA expression in the immediate- and delayed-footshock groups did not differ. However, after the retention test, the expression of c-fos mRNA in the medial nucleus of the amygdala did not differ between groups. In contrast to c-fos, NGFI-A mRNA expression in the lateral nucleus of the amygdala was greater in the delayed-footshock group than the handled and context-no-footshock groups 15 min after the footshock. This elevation in NGFI-A mRNA was not seen in the immediate-footshock group. This suggests that NGFI-A mRNA in the lateral nucleus of the amygdala may play a role in contextual fear conditioning.

NMDA processes mediate anterograde amnesia of contextual fear conditioning induced by hippocampal damage: immunization against amnesia by context preexposure.

Hippocampal lesions in rats produce both a retrograde and an anterograde amnesia of contextual fear conditioning. The present experiments examined the anterograde deficit in context conditioning. The deficit produced by electrolytic hippocampal lesions was apparent when training occurred on 7, 14, or 28 days following surgery, confirming the durability of the amnesia. The role of the hippocampus in context conditioning may be related to an NMDA (N-methyl-D-aspartate) receptor-mediated process. Both NMDA hippocampal lesions and intrahippocampal administration of an NMDA antagonist produced anterograde amnesia. Animals preexposed to the conditioning context 28 days prior to hippocampal lesioning were protected from the deficit normally produced by the lesions. Thus, the hippocampus must form a contextual representation during preexposure that is subsequently stored elsewhere. Once formed this representation of the context can be associated with an unconditional stimulus.

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

Pretreatment with opioid antagonists enhances acquisition of Pavlovian fear conditioning. The present experiments attempted to characterize the type of opioid receptor responsible 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 immobility, was employed as an index of fear. Two mu opioid antagonists, CTOP (40 ng) and naloxonazine (10 micrograms), enhanced conditioning. On the other hand, the kappa antagonist nor-binaltorphimine reduced conditioning. Two delta antagonist treatments (16-methyl cyprenorphine and naltrindole) had no reliable effect on acquisition. Thus the enhancement of conditioning appears to be mediated by mu receptors. Previous research has shown that the conditional fear produced by these procedures caused an analgesia that is also mediated by mu receptors. It is argued that the enhancement effect occurs because of an antagonism of this analgesia and that the analgesia normally acts to regulate the level of fear conditioning.

Role of mu and kappa opioid receptors in conditional fear-induced analgesia: the antagonistic actions of nor-binaltorphimine and the cyclic somatostatin octapeptide, Cys2Tyr3Orn5Pen7-amide.

When rats are returned to an environment in which they previously received electric shock they show a reduction in sensitivity to painful stimuli. This conditional fear-induced analgesia was measured using the formalin test. The octapeptide Cys2Tyr3Orn5Pen7-amide (CTOP; 0, 10, 40 and 160 ng/rat) dose-dependently reversed conditional analgesia when administered i.c.v. The 40-ng dose partially attenuated fear-induced analgesia, whereas the 160-ng dose reversed it completely. Using similar procedures, CTOP was tested for its ability to reverse the analgesia produced by i.c.v. administered [D-Ala2,-NMPhe4, Glyol5]-enkephalin, [D-Pen2,D-Pen5]-enkephalin and U50488H, which are highly selective opioid agonists at mu, delta and kappa receptors, respectively. At 40 ng/rat, CTOP reversed the analgesia produced by the mu selective ligand but not that produced by the delta ligand or the kappa ligand. At 80 ng CTOP antagonized the analgesia produced both by both enkephalin analogs but not U50488H. Nor-binaltorphimine (0, 1, 3, 10 and 30 micrograms/rat) had no effect on conditional analgesia. However, the 10- and 30-micrograms doses could reverse completely the analgesia produced by U50488H. Therefore, it appears that mu and delta, but not kappa receptors, are involved in this opioid form of stress-induced analgesia.