The Ameliorative Effect Of Potassium2-(1-hydroxypentyl)-benzoate On Cognitive Functions And Long-term Potentiation

Alzheimer's disease (AD) is the most common form of dementia, affecting5%of the population above65years old in China. The clinical manifestations of AD include progressive memory loss and impairment in behavior, speech and visual perception. After cardiovascular disease, cancer and stroke, AD has become the fourth disease that severely affects the longevity and life quality of aged people. The neuropathological characteristics of AD include neuronal loss, extracellular senile plaques and intracellular neurofibrillary tangles. Although some drugs can alleviate the symptoms of AD, there are no effective ones that can cure or stop the progression of the disease.Potassium2-(1-hydroxypentyl)-benzoate (dl-PHPB) is a novel compound synthesized by Institute of Materia Medica, CAMS. Previous studies showed that dl-PHPB has protective effects on ischemia stroke through reducing infarct volume and improving regional cerebral blood in a rat model of transient focal cerebral ischemic. Recently, we found that dl-PHPB could significantly improve the learning and memory deficits in cerebral hypoperfused rats, Aβ intracerebroventricularly infused rats and the senescence accelerated mice (SAM). In order to further identify the ameliorative effects of dl-PHPB on memory capability and to investigate the possible mechanisms, we observed the effects of dl-PHPB on long-term potentiation (LTP) in hippocampus in vivo under physiological and chemical factors induced pathological conditions. In addition, we investigate the effects of dl-PHPB on cognitive functions and LTP in APP/PS1transgenic mice. It is well known that one of the most important properties of the mammalian brain is its plasticity which is specifically referred to the activity-dependent modification of the structure of synapses and strength or efficacy of synaptic transmission. LTP is considered to be a form of synaptic plasticity that has been widely used as a cellular model of learning and memory mechanisms. There are several physiological and pathological factors associated with AD that can influence LTP induction and maintenance. Therefore, it is of great significance for explaining the influence of drugs on synaptic plasticity through investigating their effects on LTP. In the present study, we observed the effect of dl-PHPB on synaptic plasticity under physiological and chemical factors induced pathological conditions including muscarinic inhibition induced memory impairment, oligomeric Aβ1-42injected neurotoxicity and LPS induced neuronal inflammation.Firstly, we investigated the effects of dl-PHPB on LTP in PP-DG pathway of hippocampus in rats. Our current study showed that10μM dl-PHPB could facilitate HFS-induced LTP in the hippocampus DG region in vivo without any influence on basal synaptic transmission. The facilitation could persist for at least60min. In order to further investigate the mechanism of dl-PHPB induced LTP facilitation, we applied NMDA antagonist D-APV. The results showed that D-APV(150μM) strongly inhibited HFS-induced LTP and pre-injection of D-APV (150μM)10min before dl-PHPB (10μM) application could completely prevent dl-PHPB induced LTP improvement. However, non-NMDA antagonist DNQX could not block dl-PHPB induced LTP facilitation. As indicated above, dl-PHPB might display its effect of memory improvement through the NMDA receptor pathway in hippocampus in vivo.Then we observed the effects of dl-PHPB treatment on different chemical factor induced pathological animal models.It is known that non-selective muscarinic acetylcholine antagonists such as scopolamine could impair the encoding of new memories in animal models and produce pronounced cognitive deficits in most learning and memory tasks. Therefore, the scopolamine induced animal model of memory impairment has been widely used for drug evaluation. In the Morris water maze task, comparing with control group, scopolamine-injected rats spent more cumulative distances to find the hidden platform in the training phase and less distances in the platform-quadrant in the probe trial, indicating their spatial learning abilities and memory abilities were significantly impaired. Oral treatment of dl-PHPB100mg/kg and Galantamine2.5mg/kg showed a significant effect in shortening the cumulative distances and increasing distances in the platform-quadrant compared with the control group, indicating that dl-PHPB could improve scopolamine induced learning and memory behavioral deficits. In the LTP experiments, hippocampal LTP was dramatically reduced by scopolamine injection following high frequency stimulation. Importantly, we found that oral treatment of dl-PHPB could recess the LTP inhibition induced by scopolamine, suggesting a possible memory facilitating mechanism mediated by muscarinic ACh receptor.Alzheimer's disease is a progressive neurological disorder that is characterized by accumulation of the β-amyloid proteins in its pathogenesis. Aβ is derived by the proteolytic processing of amyloid precursor protein (APP), resulting in a peptide predominantly40or42amino acids in length. There is substantial evidence indicating that the oligomeric Aβ peptides display neuronal toxicity through selective actions on synaptic plasticity such as LTP. Taken together, it seems important to identify the effects of drugs on Aβ oligomers induced LTP impairment. In the current study, we observed that acute injection of oligomeric Aβ1-4260min prior to HFS could strongly suppress LTP induction. Oral treatment of dl-PHPB could profoundly rescue the Aβ1-42caused LTP inhibition.It is well known that inflammation is a major causative factor for a progressive decline in motor and memory functions during the progress of AD. Lipopolysaccharide (LPS), a component of the outer membrane of most Gram-negative bacteria, could stimulate inflammation in brains. There are results showing that LPS induced inflammation could affect neurons and glial cells in brains, and LPS injection could inhibit LTP in perforant path granule cell synapses. Therefore, we established a model of LPS-induccd LTP deficits, and observed the effect of dl-PHVB on it. Our results showed that there was a tendency of attenuation in hippocampal LTP1day after LPS injection, and LTP was inhibited strongly3days and30days after LPS injection, indicating that LPS could significantly impair the efficacy of synaptic transmission. Oral treatment of dl-PHPB could not ameliorate this impairment.In sum, in this part of our study, we found that dl-PHPB could facilitate hippocampal LTP under physiological condition through the NMDA pathway possibly. In addition, oral treatment of dl-PHPB could recess the LTP deficits induced by muscarinic inhibition or oligomeric Aβ1-42injection. Genetic aberrancies are thought to be one of the main mechanisms of AD. There are several genes that are found either cause or increase the risk of AD, such as amyloid precursor protein(APP), presenilin-1(PS1), presenilin-2(PS2), apolipoprotein E(apo E) and so on. With the rapid development of transgenic technology, more and more AD transgenic animal models are utilized in pathogenesis research and therapeutic drug evaluation. In the current study, we applied the APP/PS1transgenic mice overexpressing mutant human amyloid precursor protein (APP) and presenilin-1(PS1). The transgenic mice exhibit age-dependent increases in Aβ peptides levels in brains and eventually develop robust amyloid plaque pathology which could well mimics the pathogenesis of AD. We investigated the effect of oral dl-PUPB treatment on behavioral performance, level of HFS induced LTP and related protein expression in APP/PS1transgenic mice.Increasingly cognitive decline in AD transgenic mice was also observed with age compared with wildtype littermates. And it is well known that passive avoidance learning is based on hippocampus-related contextual memory. In the present study, we conducted passive avoidance test on APP/PS1mice. Notably, transgenic mice aged14months spent significantly less time on the platform and had more error times compared with wildtype littermates, showing profound memory impairment. Chronic oral treatment of dl-PHPB could not only increase the escape latency time but also decrease error times happened in transgenic mice.In the electrophysiological experiment, we found that LTP level was attenuated in APP/PS1transgenic mice aged14months compared with wildtype littermates. Treatment of dl-PHPB orally for one month showed an increment of LTP in transgenic mice, indicating a therapeutic role in ameliorating the synaptic dysfunction in aged AD transgenic mice. Treatment of dl-PHPB also showed a trend in promoting LTP induction in aged wild type mice.Biochemistry study showed that the expression of NMDAR1and p-NMDAR2B(Tyr1472) were significantly decreased in hippocampus of APP/PS1 transgenic mice comparing with wild type littermates. The changes of protein expression could be related to the deficits of cognitive function and LTP. Oral treatment with dl-PHPB significantly increased the protein expression of p-NMDAR2B(Tyr1472).There were also changes of potassium channel expression level in brains of APP/PS1transgenic mice. Compared with wild type littermates, the protein expression of Kv3.4was much higher in the hippocampus of APP/PS1transgenic mice. There was a similar tendency of Kv3.4expression in cortex of APP/PS1transgenic mice without statistical significance. Oral treatment with dl-PHPB could significantly decrease the protein expression of Kv3.4in hippocampus.Protein expression of Kir6.1was strongly augmented in hippocampus of APP/PS1transgenic mice comparing with wild type littermates. In addition, oral treatment with dl-PHPB could significantly decrease the protein expression of Kir6.1in hippocampus. There was a similar tendency of the protein expression of Kir6.1in cortex without statistical significance.There were no significant differences between APP/PS1transgenic mice and wildtype littermates in the protein expression of Kv2.1and Kv3.1.In summary, our current study demonstrated that dl-PHPB could reverse the memory impairment performed in passive avoidance task and rescue LTP deficits in APP/PS1transgenic mice, indicating a therapeutic potential of dl-PHPB in the treatment of Alzheimer's diseases and explained a synaptic basis for its effect on cognitive amelioration. The expression of different subtypes of NMDA receptor and potassium channels indicate distinct pathophysiological roles in aged APP/PS1transgenic mice, which further substantiates the possible mechanism associated with progress of AD.