Overview of nicotinic receptors and their roles in the central nervous system.

Alzheimer's disease is a complex disorder affecting multiple neurotransmitters. In particular, the degenerative progression is associated with loss within the cholinergic systems. It should be anticipated that both muscarinic and nicotinic mechanisms are affected as cholinergic neurons are lost. This review focuses on the basic roles of neuronal nicotinic receptors, some subtypes of which decrease during Alzheimer's disease. Nicotinic acetylcholine receptors belong to a superfamily of ligand-gated ion channels that play key roles in synaptic transmission throughout the central nervous system. Neuronal nicotinic receptors, however, are not a single entity, but rather there are many different subtypes constructed from a variety of nicotinic subunit combinations. This structural diversity and the presynaptic, axonal, and postsynaptic locations of nicotinic receptors contribute to the varied roles these receptors play in the central nervous system. Presynaptic and preterminal nicotinic receptors enhance neurotransmitter release, and postsynaptic nicotinic receptors mediate a small minority of fast excitatory transmission. In addition, some nicotinic receptor subtypes have roles in synaptic plasticity and development. Nicotinic receptors are distributed to influence many neurotransmitter systems at more than one location, and the broad, but sparse, cholinergic innervation throughout the brain ensures that nicotinic acetylcholine receptors are important modulators of neuronal excitability.