Critical Role of Presynaptic Nicotinic ACh Receptor in the Formation of Long‐Term Potentiation: Implications for Development of Anti‐Dementia Drugs

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Abstract
Background: Since neuronal nicotinic ACh receptors are involved in the cognitive function, they have been studied as a target of anti‐dementia drugs. The present study was designed to understand the role of nicotinic ACh receptors in the expression of long‐term potentiation (LTP), a cellular model of learning and memory.Methods: The ultrastructural localization of neuronal nicotinic ACh receptors in the rat hippocampus was examined electron‐immunohistochemically using an antibody against the α7 subunit, forming a brain‐type nicotinic ACh receptor. Miniature excitatory postsynaptic currents (mEPSCs) were monitored in cultured rat hippocampal neurons. Schaffer collateral‐CA1 LTP and perforant path LTP were analyzed by recording field excitatory postsynaptic potentials (fEPSPs) and population spikes (PSs) in the CA1 region and the dentate gyrus of rat hippocampal slices or in the intact mouse hippocampus.Results: α7 receptors are preferentially localized on presynaptic terminals, where the receptors are employed in the release of the excitatory neurotransmitter, glutamate. The probability of LTP development was markedly reduced in the presence of the neuronal nicotinic ACh receptor antagonists, α‐bungarotoxin and mecamylamine, in both the CA1 region and the dentate gyrus of rat hippocampal slices. Perforant path LTP was never induced in slices with selective cholinergic denervation using 192 IgG‐saporin, while it was not affected by atropine, a selective muscarinic ACh receptor antagonist, in normal slices. Nicotine facilitated hippocampal neurotransmission with the saturation occluding the potentiation induced by tetanic stimulation, and vice versa. A similar occlusion was also obtained with an intact mouse hippocampus. These types of LTP, which are dependent uponN‐methyl‐D‐aspartate (NMDA) receptors, were still induced by treatment with nicotine in the presence of D‐2‐amino‐5‐phosphonovaleric acid (APV), a selective NMDA receptor antagonist.Conclusion: The results of the present study suggest that presynaptic nicotinic ACh receptors play a critical role as a target of retrograde messengers in the formation of NMDA receptor‐dependent LTP. This may account for the involvement of nicotinic ACh receptors in cognitive function. Drugs enhancing the activity of neuronal nicotinic ACh receptors, therefore, are capable of expressing LTP, conversely, ameliorating dementia.