Potassium 2-(1-hydroxypentyl)-benzoate promotes long-term potentiation in Aβ1-42-injected rats and APP/PS1 transgenic mice

Abstract

Aim: Potassium 2-(1-hydroxypentyl)-benzoate (dl-PHPB) is a new drug candidate for ischemic stroke. The aim of this study was to investigate the effects of dl-PHPB on memory deficits and long-term potentiation (LTP) impairment in animal models of Alzheimer's disease.

Methods: The expression of NMDA receptor subunits GluN1 and GluN2B in the hippocampus and cortex of APP/PS1 transgenic mice were detected using Western blot analysis. Memory deficits of the mice were evaluated with the passive avoidance test. LTP impairment was studied in the dentate region of Aβ1-42-injected rats and APP/PS1 transgenic mice.

Results: APP/PS1 transgenic mice showed significantly lower levels of GluN1 and p-GluN2B in hippocampus, and chronic administration of dl-PHPB (100 mg · kg(-1) · d(-1), po) reversed the downregulation of p-GluN2B, but did not change GluN1 level in the hippocampus. Furthermore, chronic administration of dl-PHPB reversed the memory deficits in APP/PS1 transgenic mice. In the dentate region of normal rats, injection of dl-PHPB (100 μmol/L, icv) did not change the basal synaptic transmission, but significantly enhanced the high-frequency stimulation (HFS)-induced LTP, which was completely prevented by pre-injection of APV (150 μmol/L, icv). Chronic administration of dl-PHPB (100 mg · kg(-1) · d(-1), po) reversed LTP impairment in Aβ1-42-injected normal rats and APP/PS1 transgenic mice.

Conclusion: Chronic administration of dl-PHPB improves learning and memory and promotes LTP in the animal models of Alzheimer's disease, possibly via increasing p-GluN2B expression in the hippocampus.

Figure 1

Long-term oral treatment with dl-PHPB ameliorated memory deficits in APP/PS1 Tg mice. Escape latency and error frequency were evaluated via step-down passive avoidance test. (A) Escape latency was recorded for the retention test 24 h after training, representing the time spent by animals before stepping down from the platform onto the grid floor. Animals in the Tg control group spent significantly less time on the platform compared with WT control group (^bP<0.05 vs WT control group, n=13–15). dl-PHPB treatment (100 mg·kg⁻¹·d⁻¹ for 1 month, po) ameliorated this effect and significantly prolonged escape latency (^fP<0.01 vs Tg control group, n=13–15). (B) Compared with the WT control group, animals in the Tg control group showed higher error frequencies (^bP<0.05 vs WT control group, n=13–15), and treatment with dl-PHPB reduced the error frequency (^fP<0.01 vs Tg control group, n=13–15). All data are presented as means±SEM.

Figure 2

Effects of dl-PHPB on basal synaptic transmission and LTP in the DG of normal rats. dl-PHPB was applied intracerebroventricularly. (A) dl-PHPB had no effect on basal synaptic transmission. After a baseline recording of 15 min, rats were icv injected (white arrow) with dl-PHPB (10 μmol/L) or vehicle. The time course of changes in the PS amplitude is expressed as the percentage of the mean baseline value. Statistical significance was evaluated at 30 min after injection of dl-PHPB. There was no significant change after drug application (n=7). (B) Typical PS waveforms recorded in different treatments. (C) Rats were icv injected with dl-PHPB (10 μmol/L) or vehicle followed by a series of HFS (black arrow) 30 min later. dl-PHPB (10 μmol/L) can significantly increase LTP induced by HFS compared with the control group. Example data traces in the upper panel are responses recorded at the times prior to HFS and 60 min after HFS. Statistical significance was evaluated at 60 min after induction of LTP. All data were presented as means±SEM. ^hP<0.05 vs control, n=8–9.

Figure 3

Protective effects of oral treatment with dl-PHPB on Aβ-mediated inhibition of LTP in vivo. dl-PHPB was applied orally. (A) Typical PS waveforms recorded in different treatments. (B) In the control group, HFS induced robust LTP, whereas injection of soluble Aβ_1–42 (1 nmol, icv) 60 min prior to HFS strongly inhibited the induction of LTP. In animals orally pretreated for 1 week with 100 mg/kg dl-PHPB, the injection of Aβ_1–42 (1 nmol, icv) did not impair the induction of LTP compared with the Aβ_1–42 injected group. Statistical significance was evaluated at 60 min after induction of LTP. All data are presented as means±SEM. ⁱP<0.01 vs control group, n=5. ^kP<0.05 vs Aβ_1–42 group, n=5.

Figure 4

dl-PHPB reversed the LTP impairment in APP/PS1 Tg mice in vivo. (A) Typical PS waveforms recorded from different treatments. (B) LTP was decreased in APP/PS1 mice compared with non-Tg mice, but this reduction was ameliorated by dl-PHPB treatment (100 mg·kg⁻¹·d⁻¹ for 1 month, po). Data are expressed as the mean percentage change in PS. Analysis of the mean percentage changes in PS at 60 min after HFS. This result revealed a significant decrease in the LTP level in the APP/PS1 control group compared with non-Tg mice (^cP<0.01 vs WT control group, n=5); dl-PHBP treatment significantly attenuated the LTP reduction. ^eP<0.05 vs Tg control group, n=5.

Figure 5

The effects of dl-PHPB on LTP were related to NMDA receptors. (A) Example data traces recorded at the times prior to HFS and 60 min after HFS. Injection of D-APV (icv, white arrow) 40 min prior to HFS significantly inhibited LTP induction. Application of 10 μmol/L dl-PHPB (white arrow) 10 min after D-APV injection could not reverse the inhibition of LTP levels even though 10 μmol/L dl-PHPB could facilitate LTP induction when pretreated alone. (B) Changes of PS amplitude following HFS in the presence and absence of 10 μmol/L dl-PHPB and DNQX. Icv injection of DNQX 40 min prior to HFS significantly inhibited LTP induction. Application of 10 μmol/L dl-PHPB 10 min after DNQX injection slightly reversed the inhibition of LTP levels. Statistical significance was evaluated at 60 min after induction of LTP. All data are presented as means±SEM. ^hP<0.05, ⁱP<0.01 vs control group, ^oP<0.01 vs dl-PHPB, n=5–8.

Figure 6

dl-PHPB restored the phosphorylation level of NMDARs in the hippocampus of APP/PS1 Tg mice. (A–B) Quantitative analysis of GluN1 in cortex (A) and hippocampus (B) of WT or APP/PS1 mice treated with vehicle or dl-PHPB. Representative Western blots for GluN1 are shown in the upper panel. Quantified results were normalized to β-actin expression. (C–D) Quantitative analysis of p-GluN2B in the cortex (C) and hippocampus (D) of WT or APP/PS1 mice treated with vehicle or dl-PHPB. Representative Western blots of p-GluN2B are shown in the upper panel. Quantified results were normalized to GluN2B expression. All values are expressed as percentages compared to vehicle-treated WT control mice and represented as the group mean±SEM (n=4). ^bP<0.05 vs WT control group. ^eP<0.05 vs Tg control group.