What is Learned Determines How Pavlovian Conditioned Fear is Consolidated in the Brain.

Activity in the basolateral amygdala complex (BLA) is needed to encode fears acquired through contact with both innate sources of danger (i.e., things that are painful) and learned sources of danger (e.g., being threatened with a gun). However, within the BLA, the molecular processes required to consolidate the two types of fear are not the same: protein synthesis is needed to consolidate the first type of fear (so-called first-order fear) but not the latter (so-called second-order fear). The present study examined why first- and second-order fears differ in this respect. Specifically, it used a range of conditioning protocols in male and female rats, and assessed the effects of a BLA infusion of the protein synthesis inhibitor, cycloheximide, on first- and second-order conditioned fear. The results revealed that the differential protein synthesis requirements for consolidation of first- and second-order fears reflect differences in what is learned in each case. Protein synthesis in the BLA is needed to consolidate fears that result from encoding of relations between stimuli in the environment (stimulus-stimulus associations, typical for first-order fear) but is not needed to consolidate fears that form when environmental stimuli associate directly with fear responses emitted by the animal (stimulus-response associations, typical for second-order fear). Thus, the substrates of Pavlovian fear conditioning in the BLA depend on the way that the environment impinges upon the animal. This is discussed with respect to theories of amygdala function in Pavlovian fear conditioning, and ways in which stimulus-response associations might be consolidated in the brain.

The Opioid Receptor Antagonist Naloxone Enhances First-Order Fear Conditioning, Second-Order Fear Conditioning and Sensory Preconditioning in Rats.

The opioid receptor antagonist naloxone enhances Pavlovian fear conditioning when rats are exposed to pairings of an initially neutral stimulus, such as a tone, and a painful foot shock unconditioned stimulus (US; so-called first-order fear conditioning; Pavlov, 1927). The present series of experiments examined whether naloxone has the same effect when conditioning occurs in the absence of US exposure. In Experiments 1a and 1b, rats were exposed to tone-shock pairings in stage 1 (one trial per day for 4 days) and then to pairings of an initially neutral light with the already conditioned tone in stage 2 (one trial per day for 4 days). Experiment 1a confirmed that this training results in second-order fear of the light; and Experiment 1b showed that naloxone enhances this conditioning: rats injected with naloxone in stage 2 froze more than vehicle-injected controls when tested with the light alone (drug-free). In Experiments 2a and 2b, rats were exposed to light-tone pairings in stage 1 (one trial per day for 4 days) and then to tone-shock pairings in stage 2 (one trial per day for 2 days). Experiment 2a confirmed that this training results in sensory preconditioned fear of the light; and Experiment 2b showed that naloxone enhances sensory preconditioning when injected prior to each of the light-tone pairings: rats injected with naloxone in stage 1 froze more than vehicle-injected controls when tested with the light alone (drug-free). These results were taken to mean that naloxone enhances fear conditioning independently of its effect on US processing; and more generally, that opioids regulate the error-correction mechanisms that underlie associative formation.

Protein synthesis in the basolateral amygdala complex is required for consolidation of a first-order fear memory, but not for consolidation of a higher-order fear memory.

The present series of experiments pursued our recent findings that consolidation of a second-order fear memory requires neuronal activity, but not de novo protein synthesis, in the basolateral amygdala complex (BLA). It used a modified second-order conditioning protocol in which rats were exposed to S1-shock pairings in stage 1 and pairings of the serial S2-S1 compound and shock in stage 2. Experiment 1 showed that responding (freezing) to S2 in this protocol is conditional on its compounding with S1 in stage 2 (Experiment 1), and therefore, the result of associative formation. The remaining experiments then showed that the protein synthesis requirement for consolidation of new learning about S2 varied with the training afforded S1. When S1 was trained in stage 1 and present in stage 2, consolidation of the new S2 fear memory was unaffected by pre- or post-stage 2 infusions of the protein synthesis inhibitor, cycloheximide, into the BLA (Experiments 2 and 5). This result was observed independently of the number of S1-shock pairings in stage 1 (even a single pairing produced the result), and alongside demonstrations that cycloheximide infusions disrupt consolidation of a first-order fear memory (Experiments 2 and 5). However, when S1 was not conditioned in stage 1 (Experiment 3) or was omitted from conditioning in stage 2 (Experiment 4), consolidation of the new S2 fear memory was disrupted by post-stage 2 cycloheximide infusions into the BLA. These results were taken to imply that the consolidation of a higher-order fear memory exploits molecular events associated with consolidation of a reactivated first-order fear memory; hence it occurs independently of de novo protein synthesis in the BLA. Alternatively, the nature of the association formed in higher-order conditioning may be such as to not require de novo protein synthesis for its consolidation.