Interestingly, microinjection of anisomycin at the time of later IS did not reduce the immunizing effects of earlier ES, even though muscimol does so (see above). These data support the selleck inhibitor idea that the original experience of control induces plastic changes in mPFC neurons that then respond to even uncontrollable stressors and inhibit
the DRN. In further support, Christianson et al. (2014) found that ES, but not IS increases phosphorylated ERK in the PL, and that the immunizing effects of ES are prevented by PL microinjection of AP5 or the MEK inhibitor U0126. It might be noted that the role of the DMS in control-induced plasticity is still under investigation. The PL and the PL-DMS act/outcome system are engaged under numerous MAPK Inhibitor Library conditions, and instrumental learning occurs frequently during development. Clearly, these experiences do not produce immunization against the impact of severe stressors. Thus, it must be the engagement of this system during an aversive experience that is critical. It is often stated that “neurons that fire together wire together”. This all suggests a
scheme as depicted in Fig. 6. Imagine a set of neurons that are activated by intense stressors and PL neurons that are activated by control or contingency. Only when both occur is the plasticity/connection process initiated, so that later, stressors themselves will activate the PL and its projecting neurons. If this model is correct, then simply activating PL projection neurons during exposure to even IS, should lead to immunization. Thus, intra-PL picrotoxin or vehicle was administered during
ES, yoked IS or control treatment. IS in a different environment Calpain occurred 7 days later. The critical finding (Amat et al., 2008) was that even IS blocked the later DRN activating and behavioral effects of subsequent IS if the PL was activated during the experience. Consistent with the model, intra-PL picrotoxin was without effect if it was given in the absence of a stressor. That is, PL activation plus uncontrollable stressor was immunizing, whereas neither were by themselves. The mPFC projects to many structures other than the DRN, and the glutamatergic pyramidal projections often synapse on GABAergic interneurons that inhibit the principal cells in the region. For example, pyramidal neurons from the infralimbic cortex (IL) region of the vmPFC project to an intercalated cell cluster (ITC) in the amygdala (Vertes, 2006). The ITC consists of GABAergic cells that inhibit output from the central nucleus (Berretta et al., 2005). Thus, stimulation of ITC cells inhibits conditioned fear responses. Although we have conducted far less work here, stressor control also appears to activate this mPFC-to-amygdala pathway.