Contextual Fear Extinction and Re-Extinction in Carioca High- and Low-Conditioned Freezing Rats

We recently reported two novel breeding lines of rats known as Carioca Highand Low-conditioned Freezing (CHF and CLF), based on defensive freezing responses to contextual cues previously associated with electric footshock. The present study used animals of the 8th generation of our selective breeding program to investigate both contextual fear extinction and re-extinction. The results consistently showed that CHF animals froze more than CLF animals. Long extinction training was able to extinguish phenotypic differences between lines, but the divergence was restored after just one fear reacquisition training session. These differences disappeared again during re-extinction training. The possible neural mechanisms involved in these two types of learning are discussed.