| Alzheimer's disease (AD), with about 35.6 million patients worldwide, is a progressive, neurodegenerative disorder manifesting as a severe impairment of learning and memory. One of prominent pathological hallmarks of AD is presence of high density of senile plaques in the brain of patients with AD, which is composed of amyloidβprotein (Aβ). The neurotoxicity of natural Aβ(Aβ1-40 and Aβ1-42) has been widely reported, but the mechanism by which Aβimpairs cognitive function and the active center of Aβis still not well clarified now. Although the sequence 25-35 in Aβhas been recognized to be the active center of the whole molecule of Aβfor its similarity in neurotoxicity to natural Aβmolecule, our recent studies in electrophysiology, membrane patch and cultured neuron show that Aβ31-35, similar to Aβ25-35 or Aβ1-40, also impairs the function of neurons, suggesting that the sequence 31-35 may be a shorter active center than 25-35 in Aβmolecules. However, it is still an open question up to now whether Aβ31-35 can impair the cognitive function of rats in learning and memory.Another prominent pathological hallmark of AD is the loss of cholinergic neurons in the AD brain. It is reported that the nicotinic acetylcholine receptors (nAChRs) were decreased at cortex and hippocampus which are strong related to learning and memory. As we know, nAChRs in the brain are related to cognitive functions, including learning and memory. The most abundant forms of nAChRs in the mammalian brain areα4β2-nAChRs andα7-nAChRs. Interestingly, it has been demonstrated that Aβcould induce cholinergic dysfunction and cognitive deficits. In our previous study, we also found thatα4β2-nAChRs were required in Aβ-induced depression of hippocampal LTP. Recently, some researches indicate thatα7 nAChRs play more important roles in the development, differentiation, and pathophysiology of the nervous system. Above all, it has been reported that Aβ1-42 binds toα7 nAChRs with a high affinity, suggesting thatα7 nAChRs may be closely involved in the neurotoxicity of Aβ. However, different results about the function ofα7 nAChRs have been reported. For example, some investigators found thatα7 nAChRs subunit protein and mRNA are up-regulated in human brain samples from Alzheimer patients and AD animal models; nicotine, a non-selective nAChR agonist, was found to be able to enhance the depressive actions of Aβ1-40 on LTP in the rat hippocampal CA1 region in vivo. In contrast, it is also reported that loss ofα7 nAChRs enhanced beta-amyloid oligomer accumulation and exacerbated early-stage cognitive decline in a mouse model of AD. These results indicate that the exact roles ofα7 nAChRs in the AD pathogenesis are still controversial, and whether/howα7 nAChRs are involved in Aβ-induced impairment of hippocampal LTP and cognitive function is still to be further clarified.Therefore, the present study, by using electrophysiological and behavioral methods, investigated the effects of intracerebroventricular (i.c.v.) injection of Aβ25-35 and Aβ31-35 on the hippocampal LTP and spatial learning and memory of rats. At the same time, by application ofα7 nAChRs agonist choline and antagonist, MLA, we investigated the possibleα7 nAChR mechanisms underlying Aβneurotoxicity.Objective: To examine the effects of different Aβpeptide on in vivo LTP in rat hippocampal CA1 region and its probableα7 nAChRs mechanism by acute i.c.v. injection of Aβ25-35,Aβ31-35 and the specificα7 nAChRs agonist and antagonist and recording hippocampal field excitatory postsynaptic potentials (fEPSPs).Method: Adult-male SD rats were placed in a stereotaxic device after anesthetized. A stainless steel guide cannula was implanted into the right lateral cerebral ventricle stereotaxically and a bound stimulating/recording electrode was inserted into the hippocampal Schaffer-collateral/CA1 region. The baseline fEPSPs, HFS-induced LTP and paired pulses-induced PPF were recorded in hippocampal CA1 region by delivering test stimuli, high-frequency stimulus and paired test stimuli to Schaffer-collateral/commissural pathway. Different drugs such as Aβ25-35, Aβ31-35, choline and MLA were slowly delivered through the implanted cannula into right lateral cerebral ventricle.Results: (1) Aβ25-35 and Aβ31-35 did not affect baseline fEPSPs but significantly suppressed HFS-induced LTP. Injection of 25 nmol Aβ25-35 or 25 nmol Aβ31-35 produced a significant and similar suppression on LTP following HFS application. For example, at 15 min, 30 min and 60 min after HFS, the averaged amplitude of fEPSPs decreased to 131.5±3.0%, 129.6±3.7% and 123.7±2.8% in Aβ25-35 group from 161.0±4.1%, 154.2±4.0% and 145.3±2.6% in control group (P<0.01), respectively. Similarly, the amplitude of fEPSPs in Aβ31-35 group with the same molar concentration (25 nmol) decreased to 138.7±5.6%, 137.9±3.7% and 129.0±3.8%, respectively, significantly lower than that in control group (P<0.01). (2) Choline, a selectiveα7 nAChRs agonist, showed no effect on the induction of LTP, but enhanced the suppression of LTP induced by Aβ31-35. The averaged amplitude of fEPSPs in co-injection of choline and Aβ31-35 decreased to 123.3±4.0%, 117.4±4.9% and 112.8±3.7% compared with Aβ31-35 only group (P<0.05). (3) Methyllycaconitine (MLA), a specificα7 nAChRs antagonist, slightly suppressed hippocamal LTP but significantly reversed the Aβ31-35-induced depression of LTP. The average fEPSPs amplitudes in MLA group were 139.8±4.2%, 134.5±2.8% and 125.8±1.9% at 15 min, 30 min and 60 min after HFS, significantly lower than that in control group (P<0.01). But the average fEPSPs amplitudes increased to 154.2±7.6%, 152.1±5.8% and 143.8±2.7% in the co-application group (P<0.05 or P< 0.01). (4) All drugs used in the study did not affect PPF (P>0.05).Conclusion: i.c.v. injection of Aβ25-35 or Aβ31-35 significantly suppressed HFS-induced LTP, while choline enhanced the Aβ31-35 induced LTP suppression and MLA partly protect against the LTP impairment induced by Aβ31-35. The results suggest thatα7 nAChRs are required for Aβ-induced depression of hippocampal LTP in CA1 region.Objective: To investigate whether Aβ31-35 could affect spatial learning and memory of rats, and clarify the probable cholinergic mechanism by which Aβimpairs cognitive function, the present study observed the effects of Aβ25-35, Aβ31-35, choline, MLA, and co-application of choline or MLA and Aβ31-35 on the spatial learning and memory by using a Morris water maze test.Method: SD rats (230-250 g) were divided randomly into 7 groups: control, Aβ25-35, Aβ31-35, choline, MLA and co-application of choline or MLA and Aβ31-35 groups. All drugs were injected by i.c.v. path using a Hamilton micro-syringe. Morris water maze tests (Hidden platform and visible platform tests) were performed 2 weeks after drugs injection to obtain the ability of spatial learning and memory.Results: (1) In control group, the average escape latencies and distances of rats for searching for the underwater platform were 113.7±3.6 s, 44.6±3.7 s, 29.2±2.3 s, 23.1±1.3 s and 18.2±0.7 s as well as 2461.4±130.1 cm, 738.1±59.4 cm, 475.9±50.6 cm, 362.9±22.5 cm and 270.2±14.0 cm on training days 1-5, respectively. The total time elapsed and distance swum in target quadrant were 49.6±1.9 s and 1090.5±83.4 cm, respectively. (2) Aβ31-35, similar to Aβ25-35, impaired spatial learning and memory of rats, with longer latencies and distances for searching for the platform under water on training days 2-5 (P<0.01 or P<0.05). In the probe trails, the total time elapsed and distance swum in target quadrant were 35.9±1.2 s and 654.2±32.8 cm for Aβ25-35 group, and 37.9±1.8 s and 655.9±25.5 cm for Aβ31-35 group, significantly less than the values of control group (P<0.01). (3) Administration of 100μg choline alone did not affect the behavior of learning and memory, but co-application of choline and Aβ31-35 enhanced the impairment of spatial learning and memory induced by Aβ31-35. The average escape latencies and distances in choline plus Aβ31-35 group were significantly larger than the corresponding values in Aβ31-35 alone group on training days 3-5 (P<0.01). In the probe trials, the total time elapsed and distance swum in the target quadrant were 33.2±1.3 s and 588.3±26.9 cm, respectively, significantly less than Aβ31-35 only group (P<0.05). (4) ICV injection of 10 ug MLA impaired spatial learning and memory of rats, with longer latencies and distances for searching for the platform under water (P<0.01 or P<0.05). However, co-application MLA and Aβ31-35 partly reversed the impairment of cognitive function induced by Aβ31-35. The average escape latencies and distances were significantly decreased on training days 2-5 (P<0.01 or P<0.05). In probe trials, the total time and distance spend in the target quadrant in the co-application group were 52.8±1.6 s and 926.4±42.8 cm, respectively, significantly large than the values of Aβ31-35 alone group (P<0.01). (5) The visible platform tests showed that all drugs did not affect the vision and the swimming speeds of rats (P>0.05).Conclusion: Aβ31-35, similar to Aβ25-35, impaired spatial learning and memory of rats, providing a strong behavioral evidence that 31-35 sequence of Aβmay be a shorter active center in whole molecule of Aβ. Most importantly, our results indicate that choline can enhance the impairment of spatial learning and memory induced by Aβ31-35 and MLA can partly reverse the Aβ31-35-induced cognitive decline, suggesting a close involvement ofα7 nAChRs in Aβinduced deficit in cognitive function and a beneficial effect ofα7 nAChRs up-regulation in AD treatment.
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