Glucocorticoid effects on memory consolidation depend on functional interactions between the medial prefrontal cortex and basolateral amygdala

Abstract

Considerable evidence indicates that the basolateral complex of the amygdala (BLA) interacts with efferent brain regions in mediating glucocorticoid effects on memory consolidation. Here, we investigated whether glucocorticoid influences on the consolidation of memory for emotionally arousing training depend on functional interactions between the BLA and the medial prefrontal cortex (mPFC), a brain region involved in higher-order cognitive and affective processing. The glucocorticoid receptor (GR) agonist 11beta,17beta-dihydroxy-6,21-dimethyl-17alpha-pregna-4,6-trien-20yn-3-one (RU 28362) administered unilaterally into the left mPFC of male Sprague Dawley rats immediately after inhibitory avoidance training enhanced 48 h retention performance. An ipsilateral, but not contralateral, lesion of the BLA blocked the memory enhancement. In a second experiment, RU 28362 infused into the mPFC after inhibitory avoidance training increased BLA levels of phosphorylated extracellular signal-regulated kinase 1/2 (pErk1/2). Blockade of this pErk1/2 activity in the BLA with the mitogen-activated protein kinase kinase inhibitor PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one] prevented the memory enhancement, suggesting that GR agonist administration into the mPFC enhances memory consolidation via modulation of BLA activity. Conversely, GR agonist infusions administered into the BLA posttraining increased pErk1/2 levels in the mPFC in regulating memory consolidation. Moreover, as assessed with a two-phase inhibitory avoidance procedure designed to separate modulatory influences on memory of context and footshock, posttraining GR agonist infusions into either the BLA or mPFC enhanced memory of the contextual as well as aversive information acquired during inhibitory avoidance training. These findings indicate that glucocorticoid effects on memory consolidation depend on bidirectional interactions between the BLA and mPFC.

Figure 1.

Ipsilateral, but not contralateral, lesions of the BLA block inhibitory avoidance retention enhancement induced by posttraining intra-mPFC infusions of a GR agonist. Step-through latencies (mean + SEM) in seconds on the 48 h inhibitory avoidance retention test of rats with unilateral sham- or NMDA-induced lesions of the BLA given the GR agonist RU 28362 (3 or 10 ng in 0.5 μl) into the left mPFC immediately after training. A, Ipsilateral BLA lesion. B, Contralateral BLA lesion. ^✷p < 0.05; ^✷✷p < 0.01 compared with the corresponding vehicle group; ^♦p < 0.05 compared with the corresponding sham lesion–RU 28362 group (n = 8–14 per group). C, Injection needle tips in the mPFC of 20 randomly selected sham-lesioned (black circles) and 20 BLA-lesioned rats (black diamonds) included in the experiment. D, Smallest (black area) and largest (gray area) ipsilateral and contralateral BLA lesions from rats used in the analysis. Adapted from Paxinos and Watson (2005).

Figure 2.

GR agonist RU 28362 (3 or 10 ng in 0.5 μl) infusions into the mPFC after inhibitory avoidance training increase pErk1/2 levels in the BLA. A, Mean ratio (+SEM) of pErk1/2 in the BLA ipsilateral to mPFC infusions versus pErk1/2 measured in the contralateral BLA is significantly greater in rats administered RU 28362 than the ratio of the comparison across two hemispheres of vehicle-treated rats. ^✷p < 0.05; ^✷✷p < 0.01 (n = 6–11 per group). No significant differences were observed across hemispheres in rats that were not trained on the inhibitory avoidance task (n = 5–6 per group). B, Density of pErk1/2 bands was normalized to density of total MAP kinase bands on the same blot. Left band is ipsilateral (i) to mPFC infusion, and right is contralateral (c). C, Immunohistochemical demonstration of location of pErk1/2 within the amygdala. Positive cells were present in the BLA as well as in the central part of the lateral division of the central nucleus (CEA).

Figure 3.

A MEK inhibitor infused into the BLA blocks inhibitory avoidance retention enhancement induced by posttraining GR agonist infusions into the mPFC. A, Step-through latencies (mean + SEM) in seconds on the 48 h inhibitory avoidance retention test of rats given immediate posttraining infusions of the GR agonist RU 28362 (3 or 10 ng in 0.5 μl) into the left mPFC either alone or together with the MEK inhibitor PD98059 (20 ng in 0.2 μl) into the ipsilateral BLA. ^✷✷p < 0.01 compared with the corresponding vehicle group; ^♦♦p < 0.01 compared with the corresponding saline–RU 28362 group (n = 10–13 per group). B, Injection needle tips in the left mPFC and BLA of 30 randomly selected rats included in the experiment.

Figure 4.

Infusions of the GR agonist RU 28362 (1 or 3 ng in 0.2 μl) into the BLA after inhibitory avoidance training increase pErk1/2 levels in the mPFC. A, Mean ratio (+SEM) of pErk1/2 in the mPFC ipsilateral to BLA infusions versus pErk1/2 measured in the contralateral mPFC is significantly greater in rats given RU 28362 than the ratio of the comparison across two hemispheres of vehicle-treated rats. ^✷✷p < 0.01 (n = 7–8 per group). No significant differences were observed across hemispheres in rats that were not trained on the inhibitory avoidance task (n = 4–6 per group). B, Density of pErk1/2 bands was normalized to density of total MAP kinase bands on the same blot. The left band is ipsilateral (i) to intra-BLA infusion, and right is contralateral (c). C, Immunohistochemical demonstration of location of pErk1/2 within the mPFC. Positive cells were present in neurons in both the prelimbic and infralimbic regions of the ipsilateral mPFC, particularly in pyramidal cells of layers 2 and 3. Note pErk1/2 immunoreactivity in apical dendrites in this area.

Figure 5.

A MEK inhibitor infused into the mPFC blocks inhibitory avoidance retention enhancement induced by posttraining GR agonist infusions into the BLA. Step-through latencies (mean + SEM) in seconds on the 48 h inhibitory avoidance retention test of rats given immediate posttraining infusions of the GR agonist RU 28362 (1 or 3 ng in 0.2 μl) into the left BLA either alone or together with the MEK inhibitor PD98059 (50 ng in 0.5 μl) into the ipsilateral mPFC. ^✷✷p < 0.01 compared with the corresponding vehicle group; ^♦♦p < 0.01 compared with the corresponding vehicle–RU 28362 group (n = 10–13 per group).

Figure 6.

GR agonist infusions into the BLA or mPFC enhance memory for similar components of inhibitory avoidance training. Step-through latencies (mean + SEM) in seconds on the 48 h retention test of rats given bilateral infusions of the GR agonist RU 28362 into the BLA or mPFC immediately after either context or shock training on the two-phase modified inhibitory avoidance task. A, Posttraining infusions of RU 28362 (1 or 3 ng in 0.2 μl) into the BLA enhanced inhibitory avoidance retention latencies when administered after either the context exposure or the shock experience. B, Posttraining infusions of the GR agonist RU 28362 (3 or 10 ng in 0.5 μl) into the mPFC enhanced inhibitory avoidance retention latencies when administered after either the context exposure or the shock experience. ^✷p < 0.05; ^✷✷p < 0.01 compared with the corresponding vehicle group (n = 10–18 per group).