All three structures play a major role in extinction learning with differential involvement over time and across contexts. Inhibitory circuits comprised of intercalated neurons in the amygdala, relay inhibitory outputs to the central nucleus in the amygdala preventing neuronal excitation to the same brain regions that control fear. Additionally, “infra-limbic” cortex, which lies ventral to the prelimbic cortex in the rodent, appears to attenuate the expression of fear responses through connections with these so-called intercalated inhibitory cells within the amygdala (Quirk & Mueller, 2008). Lastly, findings also suggest that the hippocampus plays a role in mediating context-specific learning and recall of fear extinction .
The amygdala plays a role in fear extinction processes across development. In adult rats undergoing extinction, the amygdala supports forming of the initial CS-no-UCS association. However, once this association is formed, the amygdala is no longer needed for subsequent extinction processes (Laurent, Marchand, & Westbrook, 2008). Unlike at older ages (24-day-old rats), these re-extinction processes continue to be dependent on the amygdala in younger animals (i.e., 17-day-old rats) (Kim & Richardson, 2008). Thus, development results in a shift from amygdala-dependent to amygdala-independent extinction.
Likewise, developmental findings emerge for the involvement of vmPFC in extinction recall. In adult rats, vmPFC damage does not affect within-session extinction, but impacts extinction recall 24 h after extinction . These results further emphasize the difference between the process of extinction learning and extinction recall in both the neural and the behavioral levels. Similarly, no developmental differences emerge in extinction learning between 23-day-old (preadolescent), 35-day-old (adolescent) and 70-day-old (adult) rats. However, adolescent rats elicited greater freezing behavior compared to their younger and older counterparts when tested 24 h following extinction, indicating that adolescents failed to retrieve the extinction memory of the CS . These findings may reflect a non-linear developmental trajectory of the PFC function during extinction recall. Some controversy exists concerning the presence of such non-linearity, which may also manifest in changes in PFC volume during adolescence, in both rats and humans ). Other work more consistently finds linear changes in brain volume and behavior during adolescence, without clear evidence of non-linear discontinuities . Regardless, development influences extinction recall within the infralimic cortex.
Finally, developmental differences also emerge for the hippocampus, a region involved in context learning and context modulation of extinction (Corcoran, Desmond, Frey, & Maren, 2005). During fear conditioning and extinction, spatial aspects of the surroundings are also integrated in the learning processes to form a long-term contextual memory (Maren, 2011). Although evidence of long-term contextual memories emerges in preadolescent rats between 18 and 23 days after birth, young rats are impaired in forming these long term contextual memories (Rudy & Morledge, 1994). Extinction learning in the younger rats might be context-independent (Gogolla et al., 2009, Kim and Richardson, 2007a, Kim and Richardson, 2007b and Storsve and Richardson, 2009). For example, 24-day-old rats show renewal and reinstatement effects following extinction; whereas, 17-day-old rats do not. If extinction learning in young rats is indeed context-independent, they may fail to express successful extinction recall even if within-session extinction was observed because retrieving context information may be necessary for extinction recall (Delamater, 2004). Alternatively, the hippocampus could mediate within-session extinction even before it reaches full maturation, but is not involved in retrieving extinction memory at later assessments (Corcoran et al., 2005, Delamater, 2004 and Kim and Richardson, 2010).
Taken together, the available developmental data from research in animal models suggest an essential difference in the neural architecture underlying fear extinction across development. More specifically, fear extinction during early development may depend primarily on the amygdala, whereas joint roles for the amygdala, vmPFC and the hippocampus may occur at later ages. These findings may reflect neural processes that are undergoing maturation (amygdala and hippocampus) as well as structural changes (PFC) across the developing rodent.
Developmental differences in rat models have also emerged from pharmacological studies. Studies using adult rats examining the formation of long-term extinction memory have implicated N-methyl-d-asparate (NMDA) involvement in fear conditioning and extinction (Lattal et al., 2006 and Miserendino et al., 1990). Interestingly, a strong NMDA antagonist (MK-801) impairs long-term extinction in pre-adolescent (23-day-old), as indicated by higher freezing behavior (CR) during extinction retention, but not in younger (16-day-old) rats (Langton, Kim, Nicholas, & Richardson, 2007). Moreover, differential alterations of inhibitory neurotransmission mechanisms involved in fear extinction have been associated with increased γ-aminobutyric acid (GABA) binding in the amygdala (Chhatwal, Myers, Ressler, & Davis, 2005). Similar to NMDA results, GABA antagonist (FG7142) has been shown to attenuate extinction in adult rats (Harris & Westbrook, 1998), and in pre-adolescent rats (23-day-old) as indicated by higher levels of freezing when exposed to the extinguished CS in the same context, but not in younger rats (16-day-old) (Kim & Richardson, 2007b). These pharmacological findings further support the differences in extinction learning processes and the underpinning mechanisms across development.