Memory-enhancing corticosterone treatment increases amygdala norepinephrine and Arc protein expression in hippocampal synaptic fractions

Abstract

Considerable evidence indicates that glucocorticoid hormones enhance the consolidation of memory for emotionally arousing events through interactions with the noradrenergic system of the basolateral complex of the amygdala (BLA). We previously reported that intra-BLA administration of a beta-adrenoceptor agonist immediately after inhibitory avoidance training enhanced memory consolidation and increased hippocampal expression of the protein product of the immediate early gene activity-regulated cytoskeletal-associated protein (Arc). In the present experiments corticosterone (3 mg/kg, i.p.) was administered to male Sprague-Dawley rats immediately after inhibitory avoidance training to examine effects on long-term memory, amygdala norepinephrine levels, and hippocampal Arc expression. Corticosterone increased amygdala norepinephrine levels 15 min after inhibitory avoidance training, as assessed by in vivo microdialysis, and enhanced memory tested at 48 h. Corticosterone treatment also increased expression of Arc protein in hippocampal synaptic tissue. The elevation in BLA norepinephrine appears to participate in corticosterone-influenced modulation of hippocampal Arc expression as intra-BLA blockade of beta-adrenoceptors with propranolol (0.5 microg/0.2 microL) attenuated the corticosterone-induced synaptic Arc expression in the hippocampus. These findings indicate that noradrenergic activity at BLA beta-adrenoceptors is involved in corticosterone-induced enhancement of memory consolidation and expression of the synaptic-plasticity-related protein Arc in the hippocampus.

Figure 1

Effect of immediate posttraining corticosterone (3 mg/kg, i.p.) treatment on 48-h inhibitory avoidance memory retention latencies. Corticosterone-treated rats (n=10) had significantly longer retention latencies than did the vehicle treated rats (n=7;**p<.01). Results represent mean + SEM for latency in seconds.

Figure 2

Effect of immediate post-training corticosterone treatment on norepinephrine (NE) levels in the BLA. Microdialysis samples were collected every 15 minutes. Norepinephrine levels (mean ± SEM) are expressed as a percentage change from average baseline levels. (A) Corticosterone treatment (n=6; 3 mg/kg, i.p.) significantly increased norepinephrine release in the amygdala of animals trained on an inhibitory avoidance task compared with vehicle-injected animals (n=5; *p<.05). (B) No difference was observed in corticosterone- or vehicle-injected animals (n=4; n=4 respectively) that were not trained on the task

Figure 3

Western blot analysis at different time points following training for the effect of immediate post-training systemic corticosterone on Arc protein expression in synaptoneurosome fractions of the dorsal hippocampus. Memory-enhancing corticosterone (3 mg/kg) does not appear to increase Arc protein expression at 15 min or 30 min after training and injection. There is a significant increase with corticosterone injections in Arc protein expression at 45 min, and a slightly larger increase at 1 h after training and injection (*p<.05). Densitometry values for Arc are normalized to actin and to cage control and then presented as a percentage of the control vehicle group. Data are presented as mean + SEM.

Figure 4

A. Immunohistochemical analysis of the effect of systemic corticosterone on Arc protein in the hippocampus of animals trained on an inhibitory avoidance task. Memory-enhancing injections of corticosterone (3 mg/kg, i.p.) appear to increase Arc immunoreactivity in the dorsal hippocampus, particularly in the dendrites of the CA1 region and dentate gyrus. B. Western blot quantification of corticosterone effect on Arc protein expression in synaptoneurosome fractions of the hippocampus. In animals trained on an inhibitory avoidance task, memory-enhancing injections of corticosterone (3 mg/kg, i.p.) significantly increase Arc expression in synaptoneurosome fractions (corticosterone, n=8; vehicle, n=7; *p<.05) C. Corticosterone treatment (3 mg/kg) did not significantly affect Arc protein expression in synaptoneurosome fractions (corticosterone, n=8; vehicle, n=7) taken from animals that were not trained. D. Western blot from synaptoneurosome fractions of rats receiving either vehicle (V) or corticosterone (C) injections. E. Western blot for acetyl H3. Acetyl H3 is present in the whole hippocampal homogenate (WHH) but not in synaptoneurosome fractions (SNS). F. Western blot for PSD-95. There is a 5-fold enrichment of PSD-95 in synaptoneurosome (SNS) fractions compared to whole hippocampal homogenate (WHH). Densitometry values for Arc are normalized to actin and to cage control and then presented as a percentage of the control vehicle group. Data are presented as mean + SEM.

Figure 5

Intra-basolateral amygdala infusions of propranolol or vehicle. A. Rats were infused with vehicle into the BLA of one hemisphere and propranolol into the BLA of the other hemisphere immediately after inhibitory avoidance training. The side of the vehicle/drug infusion was counter-balanced. B. Representative location of cannulae and needle tips within the BLA C. Injection needle tips in the BLA of all rats included in the experiment. Adapted from Paxinos & Watson (2005).

Figure 6

Effect of post-training intra-BLA infusions of propranolol (0.5 μg/ 0.2 μL) on corticosterone-induced Arc expression in hippocampal synaptoneurosome fractions. A. Western blot densitometry values for Arc were normalized to actin, expressed as a ratio of the drug-infused hemisphere to the vehicle-infused hemisphere (or ratio of left-right hemisphere for bilateral, vehicle-infused rats), and are finally expressed as a percentage of the bilateral control group. For propranolol-treated rats (n=10), the percentage of Arc expression was significantly lower in hippocampal synaptoneurosome fractions than in synaptoneurosome fractions taken from bilateral vehicle-treated rats (n=7; *p<.05). B. In the top panel, immunohistochemistry illustrates a moderate reduction in Arc staining in the hippocampus ipsilateral to the intra-BLA propranolol infusion. Here, the reduction appears to be greater in the dentate gyrus and CA3 regions, however this pattern was not reliably observed. C. 20× magnification of the CA1 region of the dorsal hippocampus reveals a reduction in Arc staining in the dendritic regions of the hippocampus ipsilateral to the intra-BLA propranolol infusion. Data are presented as mean + SEM.