New findings on extinction of conditioned fear early in development: theoretical and clinical implications.

Research with adult animals suggests that extinction depends, at least partly, on new inhibitory learning that is specific to the context in which it is learned. However, several recent studies show that extinction processes are dissociated across development. The present article reviews research on the behavioral and neurobiological mechanisms underlying extinction in developing rats. To summarize, postweanling aged rats (i.e., 24-day-olds) display adult-like extinction in that they show renewal, reinstatement, spontaneous recovery, and compound summation of extinguished stimuli. However, preweanling aged rats (i.e., 17-day-olds) do not show any of those behavioral phenomena. Pharmacological studies also show that reducing N-methyl-D-aspartate, gamma-aminobutryic acid, and opioid neurotransmission impairs extinction in 24-day-old rats, but extinction in P17 rats is only affected by the blocking of opioid neurotransmission. Lastly, extinction in 24-day-old rats involves the amygdala and the ventromedial prefrontal cortex (vmPFC), which are critical brain areas in the neural circuitry of extinction in adult rats. Interestingly, extinction in 17-day-old rats involves the amygdala but not the vmPFC. The existing models of extinction cannot account for these developmental differences. These findings showing that distinct processes mediate extinction at different stages of development may have significant clinical implications, which are discussed in this review. 2010 Society of Biological Psychiatry. Published by Elsevier Inc. All rights reserved.