Hippocampus has been regarded as a brain region crucial in the consolidation of memory. The synapses within the region exhibit a characteristic nature, the long term potentiation (LTP) of synaptic transmission efficiency, which has been studied as a model system for analysis of the basic processes underlying the synaptic plasticity. The increase in the intracellular concentration of Ca2+ ([Ca2+]i) during the LTP-inducible stimulation was demonstrated, and this increase triggers the intracellular enzymatic cascade leading to the modulation of transmission efficiency. In this context, a subtype of glutamate receptor activatable by N-methyl-D-aspartic acid (NMDA) has attracted attention. But beside glutamatergic neurotransmission within the hippocampus, the region receives an intense cholinergic input from the septum. It has been reported in various cells and tissues that the activation of muscarinic acetylcholine receptors (mAChRs) produces the increase in [Ca2+]i, through an activation of voltage-sensitive Ca2+ channels or an activation of G-protein-coupled phosphatidylinositol turnover system. Thus, it is important to know whether mAChR coupled with Ca2+ mobilization machinery are present in the hippocampus and to reveal their causal relationship to the plastic nature of hippocampal synapses. This chapter describes a heterogeneous expression of the mAChRs in the hippocampal neurons, variation in the pattern of [Ca2+]i change during the stimulation of these receptors and topographical patterns of their distribution in this brain area.
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