Differential Age-dependent Mechanisms of High-frequency Stimulation-induced Potentiation in the Prefrontal Cortex-Basolateral Amygdala Pathway Following Fear Extinction.

Post-weaning is a critical period for brain maturation in the rat and is comparable to childhood and adolescences in humans. The basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC) are two brain regions that continue to mature during post-weaning and establish a critical circuit regulating the acquisition and extinction of conditioned fear. We previously demonstrated that exposure to stress leads to significant differences between adults and PWs in the kinetics of extinction behavior as well as differential effects on long-term potentiation. In the current experiments, we aimed to investigate whether prior fear or extinction learning would elicit differences in the ability to induce electrical LTP in the mPFC-BLA pathway in the adult and PW animals. To that end, we subjected adult and PW rats to auditory fear conditioning and extinction, followed by high-frequency stimulation (HFS) to induce LTP. The results indicate that when the conditioning protocol is adjusted to produce comparable extinction kinetics in both age groups, no LTP can be induced after fear conditioning in the mPFC-BLA pathway. Importantly, after extinction, LTP was successfully induced, and a significant difference was observed in the levels of potentiation between adults and PW rats. Further, freezing levels during extinction positively correlated with the magnitude of LTP only in adult animals. These results suggest that the changes occurring at the synaptic level following fear extinction are dissimilar in adult and PW animals. Our results further strengthen the assertion that PW and adult fear extinction learning may rely on different mechanisms.

Perturbation of GABAergic Synapses at the Axon Initial Segment of Basolateral Amygdala Induces Trans-regional Metaplasticity at the Medial Prefrontal Cortex.

GABAergic synapses in the basolateral amygdala (BLA) play an important role in fear memory generation. We have previously reported that reduction in GABAergic synapses innervating specifically at the axon initial segment (AIS) of principal neurons of BLA, by neurofascin (NF) knockdown, impairs fear extinction. BLA is bidirectionally connected with the medial prefrontal cortex (mPFC), which is a key region involved in extinction of acquired fear memory. Here, we showed that reducing AIS GABAergic synapses within the BLA leads to impairment of synaptic plasticity in the BLA-mPFC pathway, as well as in the ventral subiculum (vSub)-mPFC pathway, which is independent of BLA involvement. The results suggest that the alteration within the BLA subsequently resulted in a form of trans-regional metaplasticity in the mPFC. In support of that notion, we observed that NF knockdown induced a severe deficit in behavioral flexibility as measured by reversal learning. Interestingly, reversal learning similar to extinction learning is an mPFC-dependent behavior. In agreement with that, measurement of the immediate-early gene, c-Fos immunoreactivity after reversal learning was reduced in the mPFC and BLA, supporting further the notion that the BLA GABAergic manipulation resulted in trans-regional metaplastic alterations within the mPFC.

Dissociation in the effects of stress and D1 receptors activation on basolateral amygdalar LTP in juvenile and adult animals.

Post-weaning or juvenility is a critical developmental stage during which neural structures as the medial prefrontal cortex (mPFC) and amygdala and the neurotransmitter systems undergo major reorganization and thus are susceptible to the effects of environmental factors. Interactions between the mPFC and the amygdala are critical for fear regulation. Changes in plasticity in the projections from the mPFC to the basolateral amygdala (BLA) were previously shown following stress and fear. In the present study, we examined possible differences between adult and PW animals in the (1) effects of exposure to stress on BLA- LTP and (2) the possible dependency of mPFC-BLA LTP on N-methyl-d-aspartate (NMDA) and Dopamine (D) receptors activation. The results show that similarly to the adults, the mPFC- BLA pathway in the PW animals is not amenable to the induction of LTP. However, in contrast to adult animals, exposure to stress in PW animals did not result in enhanced BLA-LTP. Interestingly, only under activation of the D1 receptors, PW animals expressed LTP in the BLA. In both groups of age, the NMDA partial agonist d-cycloserine (DCS) did not result in any change in the levels of potentiation. Our results confirm differences between the PW and the adult animal and show that stress and the activation of Dopamine are associated with different effects; which may have potential implications for the treatment of anxiety and stress disorders across development.

Differences in stress-induced changes in extinction and prefrontal plasticity in postweanling and adult animals.

BACKGROUND: Postweaning is a critical developmental stage during which the medial prefrontal cortex (mPFC) undergoes major changes and the brain is vulnerable to the effects of stress. Surprisingly, the engagement of the mPFC in extinction of fear was reported to be identical in postweanling (PW) and adult animals. Here, we examined whether the effect of stress on extinction and mPFC plasticity would be similar in PW and adult animals. METHODS: PW and adult animals were fear conditioned and exposed to the elevated platform stress paradigm, and extinction and long-term potentiation were examined. The dependency of stress-induced modulation of extinction and plasticity on N-methyl-D-aspartate receptors was examined as well. RESULTS: We show that exposure to stress is associated with reduction of fear and enhanced induction of long-term potentiation (LTP) in PW pups, in contrast to its effects in adult animals. Furthermore, we report opposite effects in the occlusion of LTP following the enhanced or impaired extinction in the two age groups and that the reversal of the effects of stress is independent of N-methyl-D-aspartate receptor activation in PW animals. CONCLUSIONS: Our results show that qualitatively different mechanisms control the modulatory effects of stress on extinction and plasticity in postweanling pups compared with adult rats. Our results point to significant differences between young and adult brains, which may have potential implications for the treatment of anxiety and stress disorders across development.