In Rat Medial Septal Nuclear Injection Of Amyloid ¦Â Protein In The Hippocampal Theta Rhythm And Long-term Potentiation In Vivo Electrical Physiology Study

Alzheimer's disease (AD) is the most common form of dementia in the elderly, and characterized clinically by a progressive decline in cognitive function. The major pathological hallmarks of AD are extracellular accumulation of senile plaques, the intracellular appearance of neurofibrillary tangles and synaptic and neuronal loss. In AD, high density of senile plaques was found mainly in specific neocortical and hoppocampal regions, and composed of amyloidβ-protein (Aβ), which is derived form a larger amyloid precursor protein (APP). Naturally secreted Aβcontains 40 or 42 amino acids and has a considerable neurotoxicity. The synthetic Aβ25-35 fragments of Aβ, and even Aβ31-35, also have neurotoxicity. The production and deposition of Aβis widely believed to the major pathogenesis of AD. It was found that amyloid deposits were prevalent in not only the hippocampus and cerebral cortex but also the septum by quantitative analysis of a transgenic mouse model. And in the AD brain, a profound loss of cholinergic neurons in basal forebrain including septum was also found. Basal forebrain cholinergic dysfunction has been suggested to be one of important causes for the cognitive deficits in AD patients according to the cholinergic hypothesis of AD.The theta rhythm of the hippocampus, a component of hippocampal local field potential (LFP), is a nearly sinusoidal oscillation of 3 to 12 Hz. The theta rhythm has attracted significant attention based on its reported involvement in memory processing functions of the hippocampus. Medial septum (MS), a part of basal forebrain cholinergic system, projects the cholinergic, GABAergic and glutamatergic fibers into the hippocampus through the fornix and acts as a pacemaker of the hippocampal theta rhythm which plays a role in the maintenance of spatial working memory. As reported, the impairment of medial septal neurons impaired the hipocampal theta rhythm. In behavioral experiments, selective lesions of medial septal cholinergic or GABAergic neurons, in some experiments, impaired the spatial working memory; but in other studies, which failed to cause any impairment in spatial working memory. Importantly, the relationship between hippocampal theta rhythm and AD is not clear at present. In addition, hippocampal long-term potentiation (LTP) has been recognized as a cellular mechanism underlying hippocampus-dependent long-term memory. Although it has been reported that theta rhythm may involve in the formation of hippocampal LTP, whether or not the deposition of Aβinto medial septum affect the induction or maintenance of hippocampal LTP has not been reported.Therefore, by using the in vivo electrophysiological methods, we investigated the effects of MS injection of Aβ25-35 or KA on the hippocampal theta rhythm and LTP to implore the relationship between the amyloid hypothesis and the cholinergic hypothesis of AD and to try to explain the electrophysiological mechanism of the impairment of medial septum in the pathogenesis of AD. Saline, Aβ25-35 or Kainic acid (KA) were administered into the MS of adult male SD rats. Two-week later, the theta rhythm induced by tail pinch (TP) and LTP of field excitatory postsynaptic potential (fEPSP) induced by high frequency stimulation (HFS) in the hippocampal CA1 region of urethane anesthetized rats were recorded separately.Rats were anesthetized with chloral hydrate and fixed in the stereotaxic instrument. Then Aβ25-35, KA or saline was injected into MS according to the stereotaxic atlas. After two-week recovery, rats were anesthetized with urethane and fixed in the stereotaxic instrument again. A tungsten microelectrode was inserted through the dura into hippocampus to record the LFP of area CA1. The LFP signals were sampled at 500 Hz with an bandpass of 0.1 to 250 Hz. TP was used to induce theta rhythm in area CA1 of anesthetized rat, and then the theta power before and after TP were analyzed using the Chronux toolbox under Matlab.We found that: (1) there was a apparent peak power value (P_peak ) of post-TP LFP in the 3-4 Hz band in control group, and the P_peak of post-TP LFP was significant greater than the power of pre-TP LFP (p<0.05). (2) the averaged peak power frequency (F_peak) of TP-induced theta in control (n=19), Aβ(n=15) and KA (n=17) groups were 3.56±0.21 Hz, 3.56±0.26 Hz and 3.62±0.19 Hz, separately, without significant differences among them (P>0.05). (3) in Aβgroup, the P_peak of post-TP LFP was also significant greater than the power of pre-TP LFP (p<0.05), but the averaged P_peak(22.7±1.84 dB) was significantly lower than that of control group (30.6±1.91 dB, p<0.01). (4) in KA group, no significant difference between the Pp eak of post-TP LFP and the power of pre-TP LFP was found. However, the averaged P_peak (22.6±2.1 dB) was significantly lower than that of control group (p<0.01).The results indicate that TP is an effective method to induce hippocampal theta rhythm in anesthetized rats. MS injection of Aβ25-35 or KA can suppress the theta rhythm in area CA1 while not affecting the TP-induced theta frequency. It also suggests that both Aβ25-35 and KA can impair the MS neurons, but the neural impairment may be not the same.After recording LFP of one side of the hippocampus, we used another electrophysiological equipment to record the fEPSPs in another side of the hippocampus. Test stimulation or HFS was applied to the Schaffer collateral to record the baseline fEPSP and HFS-induced LTP for studying the effect of MS injection of Aβ25-35, KA or saline on basic fEPSP and LTP. The baseline fEPSPs induced by test stimulation were recorded for 30 minutes. Then HFS was given to induce LTP. After HFS, the fEPSPs were recorded again with test stimulation for 1 hour. The fEPSP amplitudes between three groups at 1 hour post-HFS were compared.The results showed: (1) the averaged fEPSP amplitude at 1 hour post-HFS was 146.7±3.8% of basine fEPSP in control group (n=6), indicating that hippocampal LTP could be successfully induced by HFS. (2) the baseline fEPSP in Aβgroup (n=7) did not differ from that of control group (p>0.05), but the LTP was significantly lower than that of control group (p<0.05). (3) there was no significant difference in baseline fEPSP amplitude and LTP between KA group (n=6) and control group (p>0.05).So, the results indicate that the MS injections of Aβ25-35 but not KA can depress the LTP in area CA1, suggesting that the depression of LTP may be related to the Aβ25-35-induced lesions of MS neurons.In short, MS injection of Aβ25-35 or KA can impair the MS neurons and the theta rhythm in hippocampal CA1 area. And, the LTP in area CA1 can also be depressed by the injection of Aβ25-35 into MS. Our results obviously contribute to explain the role of MS in the formation of hippocampal theta rhythm, the mechanism of impairment of hippocampal LTP by the deposition of Aβin MS and the cognitive deficits observed in AD patients. The results also suggest that the deposits of Aβin MS, no less than in cortex or hippocampus, may also play a key role in AD.