Parallel memory processing by the CA1 region of the dorsal hippocampus and the basolateral amygdala

Abstract

There is abundant literature on the role of the basolateral amygdala (BLA) and the CA1 region of the hippocampus in memory formation of inhibitory avoidance (IA) and other behaviorally arousing tasks. Here, we investigate molecular correlates of IA consolidation in the two structures and their relation to NMDA receptors (NMDArs) and beta-adrenergic receptors (beta-ADrs). The separate posttraining administration of antagonists of NMDAr and beta-ADr to BLA and CA1 is amnesic. IA training is followed by an increase of the phosphorylation of calcium and calmodulin-dependent protein kinase II (CaMKII) and ERK2 in CA1 but only an increase of the phosphorylation of ERK2 in BLA. The changes are blocked by NMDAr antagonists but not beta-ADr antagonists in CA1, and they are blocked by beta-ADr but not NMDAr antagonists in BLA. In addition, the changes are accompanied by increased phosphorylation of tyrosine hydroxylase in BLA but not in CA1, suggesting that beta-AD modulation results from local catecholamine synthesis in the former but not in the latter structure. NMDAr blockers in CA1 do not alter the learning-induced neurochemical changes in BLA, and beta-ADr blockade in BLA does not hinder those in CA1. When put together with other data from the literature, the present findings suggest that CA1 and BLA play a role in consolidation, but they operate to an extent in parallel, suggesting that each is probably involved with different aspects of the task studied.

Fig. 1.

Consolidation of IA memory requires functional NMDAr and β-ADr in BLA and the CA1 region of the dorsal hippocampus. Effect of the bilateral intra-BLA (A) or intra-CA1 (B) infusion of vehicle (VEH), AP5 (5 μg per side), MK-801 (MK; 1 μg per side), TIM (1 μg per side), or PRO (1 μg per side) on IA memory retention as evaluated 24 h (test 1) or 72 h (test 2) after training. Data are expressed as median ± interquartile range. ***, P < 0.001 and **, P < 0.01 vs. VEH in the respective session in Dunn's multiple comparison analysis after Kruskal-Wallis test; n = 10 per group.

Fig. 2.

IA training induces the activation of ERK2 and CaMKII in the CA1 region of the dorsal hippocampus and the activation of ERK2 in the BLA. Representative immunoblots and densitometric quantification show the time course of the IA training-induced increase in αCaMKII (pCaMKII), GluR1 (pGluR1), ERK2 (pERK2), and TH (pTH) phosphorylation levels in total homogenates prepared from the BLA or the CA1 region of the dorsal hippocampus (CA1). IA-trained rats were killed immediately (0), 30 or 180 min after training. n = Naïve animals, i.e., rats that were killed without submitting them to any specific behavioral protocol. Data are expressed as mean ± SEM. ***, P < 0.001, **, P < 0.01, and *, P < 0.05 vs. naïve in Dunnett's test after ANOVA; n = 5 per group.

Fig. 3.

Inhibition of TH in the BLA but not in the CA1 region of the hippocampus blocks consolidation of IA memory. Effect of the bilateral intra-BLA (Left) or intra-CA1 (Right) infusion of vehicle (VEH) or AMPT on IA memory retention as evaluated 24 h posttraining. Data are expressed as median ± interquartile range. ***, P < 0.001 vs. VEH in Dunn's multiple comparison analysis after Kruskal-Wallis test; n = 10 per group.

Fig. 4.

Activation of the ERK signaling pathway induced by IA training requires functional β-ADr in BLA and NMDAr in the CA1 region of the dorsal hippocampus. (A) Representative immunoblots show the effect of the intra-BLA infusion of TIM, U0126 (U0), and AP5 on the increase in ERK2 (pERK2) and TH (pTH) phosphorylation induced in that region by IA training. Total homogenates prepared from the BLA of IA-trained rats that received TIM (1 μg per side) or U0126 (1 μg per side) immediately after training and were killed 30 min thereafter, exhibited significantly lower levels of pERK2 (t = 5.51, P < 0.05 for TIM; t = 9.76, P < 0.001 for U0; n = 4 per group) and pTH (t = 3.25, P < 0.05 for TIM; t = 7.23, P < 0.01 for U0; n = 4 per group) than those obtained from trained animals that received intra-BLA vehicle (T groups). The intra-BLA infusion of AP5 (5 μg per side) immediately posttraining had no effect on the training-induced increase in pERK2 (t = 0.89, P > 0.1; n = 4) and pTH levels (t = 1.49, P > 0.1; n = 4) in that region. N = Naïve animals, i.e., rats that were killed without submitting them to any specific behavioral protocol. (B) Representative immunoblots show the effect of the intra-CA1 infusion of TIM, U0126, and AP5 on the increase in pERK2 induced in that region by IA training. Total homogenates prepared from the dorsal CA1 region of trained rats that received U0126 (1 μg per side) or AP5 (5 μg per side) immediately after training and were killed 30 min thereafter exhibited significantly lower levels of pERK2 (t = 10.47, P < 0.001 for U0; t = 3.58, P < 0.05 for AP5; n = 4 per group) than those obtained from trained animals that received intra-CA1 vehicle (T groups). The intra-CA1 infusion of TIM (1 μg per side) immediately posttraining had no effect on the training-induced increase in pERK2 in that region (t = 0.71, P > 0.1; n = 4). (C and D) Densitometric quantification (C) and representative immunoblots (D) show the time course of the increase in pERK2 and pTH levels induced in total homogenates prepared from BLA by a 0.5-mA, 2-s electric foot shock. Shocked animals were killed immediately (0), 30 or 180 min after the electric footshock. Data are expressed as mean ± SEM. ***, P < 0.001 vs. Naïve in Dunnett's test after ANOVA; n = 5 per group. (E) Representative immunoblots show the effect of the intra-BLA administration of TIM on the increase in pERK2 and pTH induced in that region by a 0.5-mA, 2-s electric foot shock. Total homogenates prepared from the BLA of shocked rats that received TIM (1 μg per side) immediately after the shock and were killed 30 min thereafter exhibited significantly lower levels of pERK2 (t = 4.2, P < 0.05; n = 4 per group) and pTH (t = 3.45, P < 0.05; n = 4 per group) than those obtained from shocked animals that received intra-BLA vehicle (S groups).

Fig. 5.

Blockade of β-ADr and NMDAr in dorsal CA1 does not affect the biochemical changes induced by IA training in BLA, and blockade of β-ADr and NMDAr in BLA does not affect the biochemical changes induced by IA training in dorsal CA1. (A) Representative immunoblots show the lack of effect of the intra-CA1 infusion of TIM and AP5 on the increase in ERK2 (pERK2; t = 0.95, P > 0.1 for TIM; t = 0.27, P > 0.1 for AP5; n = 4 per group) and TH (pTH; t = 0.16, P > 0.1 for TIM; t = 1.19, P > 0.1 for AP5; n = 4 per group) phosphorylation induced in BLA by IA training. (B) Representative immunoblots show the effect of the intra-CA1 administration of TIM and AP5 on the increase in CaMKII (pCaMKII; t = 0.24, P > 0.1 for TIM; t = 5.68, P < 0.01 for AP5; n = 4 per group) and GluR1 (pGluR1; t = 0.12, P > 0.1 for TIM; t = 3.54, P < 0.05 for AP5; n = 4 per group) phosphorylation induced in that region by IA training. (C) Representative immunoblots show the lack of effect of the intra-BLA infusion of TIM and AP5 on the increase in pCaMKII (t = 0.84, P > 0.1 for TIM; t = 0.40, P > 0.1 for AP5; n = 4 per group), pGluR1 (t = 0.03, P > 0.1 for TIM; t = 0.34, P > 0.1 for AP5; n = 4 per group), and pERK2 (t = 0.32, P > 0.1 for TIM; t = 0.66, P > 0.1 for AP5; n = 4 per group) phosphorylation induced in the dorsal CA1 region by IA training.