Methamphetamine Inhibits Long-Term Memory Acquisition and Synaptic Plasticity by Evoking Endoplasmic Reticulum Stress

Abstract

Methamphetamine (MA), an illicit drug abused worldwide, leads to cognitive impairment and memory loss. However, the detailed mechanisms of MA-induced neurologic impairment are still unclear. The present study aimed to investigate the mechanisms of MA-induced inhibition of memory acquisition from the perspective of endoplasmic reticulum (ER) stress. ER stress, caused by the accumulation of wrongly folded proteins in the ER, is important for new protein synthesis, which further influence the formation of long-term memory. A subacute MA poisoning model of mice was established and several behavioral experiments were performed, including elevated plus maze, Morris water maze, electro-stimulus Y-maze, and novel object recognition tasks. The present results suggested that 4 days exposure to MA induced significant memory loss. Whereas, this damage to memory formation could be protected when mice were pre-treated with ER stress inhibitor, tauroursodeoxycholic acid (TUDCA). The results of Western blotting showed that subacute exposure to MA increased the expression levels of ER stress marker proteins, such as binding immunoglobulin protein, phosphorylated eukaryotic translation initiation factor 2α, cyclic AMP-dependent transcription factor (ATF)-4, ATF-6, and CCAAT-enhancer binding protein homologous protein. Meanwhile, the enhanced expression levels of these proteins were reversed by TUDCA, indicating that MA administration induced memory loss by evoking ER stress in the hippocampus. We also found that MA inhibited the induction of long-term potentiation (LTP) in the hippocampus. Nevertheless, LTP could be induced when mice were pre-treated with TUDCA. In conclusion, MA inhibited long-term memory acquisition and synaptic plasticity via ER stress.

Keywords: endoplasmic reticulum stress; memory; methamphetamine; neurotoxicity; tauroursodeoxycholic acid.

FIGURE 1

MA administration has no effect on the anxiety-like behavior of mice. (A) The experimental flow diagram of elevated plus maze task. (B) There was no significant difference in the percent time spent in the open arms between the four groups. Data was presented as the mean ± SEM, n = 8 per group, ns indicated p > 0.05.

FIGURE 2

MA-induced deficits of spatial memory are protected by TUDCA. (A) The experimental flow diagram of Morris water maze task. (B) MA-treated mice exhibited increased escaped latencies on days 1, 2, and 4 during the training session, in comparison with that of saline-treated mice. The escaped latency of TUDCA + MA-treated mice was less than that of MA-treated mice on day 4. (C) There was no difference in swimming speed between the four groups in the probe test. (D) MA-treated mice exhibited increased crossing latency, compared with that of saline-treated mice in the probe test, whereas mice of the TUDCA + MA group exhibited decreased latency, compared with that of MA-treated mice. (E) The platform site crossings of MA group were less than the Saline group, while mice of the TUDCA + MA group exhibited more platform position crossings than the MA group. *p < 0.05, **p < 0.01, ***p < 0.001 vs. the Saline group. ^#p < 0.05 vs. the MA group, ns indicated p > 0.05. Data was presented as the mean ± SEM, n = 10 per group.

FIGURE 3

MA-induced impairment of recognition memory is protected by TUDCA in electro-stimulus Y-maze. (A) The experimental flow diagram of electro-stimulus Y-maze task. (B) The number of mice exhibiting learnt and unlearnt in different groups. The present results indicated that a lower percent of MA-treated mice exhibited learnt in comparison with the saline group, whereas the percentage of mice exhibiting learnt in the TUDCA + MA group was higher than that of the MA group. (C) The percentage of correct trials in MA group was lower than that of the Saline group, while mice of the TUDCA + MA group represented a higher percent of correct trials in comparison with the MA group. *p < 0.05, **p < 0.01 vs. the Saline group. ^#p < 0.05, ^##p < 0.01 vs. the MA group. Data was presented as the mean ± SEM, n = 8 per group.

FIGURE 4

MA-induced long-term recognition memory loss in novel object recognition task is avoided by TUDCA. (A) The experimental flow diagram of NOR task. (B) In Test 1, there was no difference in the novel object index (NOI) between the four groups. (C) In Test 2, MA-treated mice exhibited a lower NOI, compared with that of the Saline group, TUDCA + MA-treated mice exhibited a higher NOI in comparison with that of the MA group. ***p < 0.001 vs. the Saline group. ^##p < 0.01 vs. the MA group, ns indicated p > 0.05. Data was expressed as the mean ± SEM, n = 8 per group.

FIGURE 5

MA-induced ER stress is reversed by TUDCA pre-treatment. (A,B) Four-day injections of MA increased the expression levels of Bip, ATF-4, ATF-6, p-eIF2α, and Chop, all of which were reversed by TUDCA pre-treatment. *p < 0.05, **p < 0.01 vs. the Saline group. ^#p < 0.05, ^##p < 0.01 vs. the MA group. Data was presented as the mean ± SEM, n = 3 per group.

FIGURE 6

MA-induced inhibition of LTP in vivo is reversed by TUDCA pre-treatment. (A) The position of stimulating electrodes and recording electrodes in the electrophysiological experiments. (B) A typical form of the population spike (PS) was consisted of a descending branch (AB) and an ascending branch (BC). The amplitude of PS was calculated by the average of the potential difference of AB and BC. (C) A typical enhancement of PS amplitude post-HFS, compared with the baseline. (D,E) The HFS induced a significant increase of the PS amplitude in the Saline group. (F,G) When mice were treated with MA, there was no difference in the PS amplitude post-HFS in comparison with the baseline. (H,I) When mice were pretreated with TUDCA, the HFS induced a prominent enhancement of the PS amplitude. **p < 0.01, ***p < 0.001 vs. the baseline, ns indicated p > 0.05. Data was presented as the mean ± SEM, n = 5 per group.