OPIOID RECEPTORS REGULATE LEARNING ABOUT RECENTLY BUT NOT REMOTELY EXPERIENCED EVENTS: A STUDY OF FALSE FEAR CONDITIONING IN RATS

Opioid receptors regulate various forms of learning and memory. Here, we investigated whether they also regulate false context fear learning in rats. To do so, we used a three-stage protocol in which rats are pre-exposed to a context (A) in stage 1, shocked upon placement in a similar but different context (B) in stage 2, and tested in either the pre-exposed context (A) or shocked context (B) in stage 3. At test, rats display fear responses (freezing) in the pre-exposed context A but not in the shocked context B, suggesting that they form a false fear memory involving context A.
Across two experiments, we examined the effect of injecting naloxone (an opioid receptor antagonist) on formation of the false context A fear memory in stage 2. We found that the effect of naloxone depended on the recency of the context A pre-exposure in stage 1. When pre-exposure occurred just 5 min earlier, naloxone enhanced the false conditioning of A when rats were shocked in B: relative to vehicle-injected controls, naloxone-treated rats froze more in context A at test. By contrast, when pre-exposure occurred 24 hr earlier, naloxone had no effect on the false conditioning of A.
These results were taken to imply that opioid receptors are differentially involved in learning about recently and remotely experienced events. They are discussed with respect to the notions of self- and retrieval-generated priming, and theories that allow for conditioning to be supported by either of these processes.