The a9 Nicotinic Acetylcholine Receptor Shares Pharmacological Properties with Type A g-Aminobutyric Acid, Glycine, and Type 3 Serotonin Receptors

In the present study, we provide evidence that the a9 nicotinic acetylcholine receptor (nAChR) shares pharmacological properties with members of the Cys-loop family of receptors. Thus, the type A g-aminobutyric acid receptor antagonist bicuculline, the glycinergic antagonist strychnine, and the type 3 serotonin receptor antagonist ICS-205,930 block ACh-evoked currents in a9-injected Xenopus laevis oocytes with the following rank order of potency: strychnine . ICS-205,930 . bicuculline. Block by antagonists was reflected in an increase in the acetylcholine (ACh) EC50 value, with no changes in agonist maximal response or Hill coefficient, which suggests a competitive type of block. Moreover, whereas neither g-aminobutyric acid nor glycine modified ACh-evoked currents, serotonin blocked responses to ACh in a concentration-dependent manner. The present results suggest that the a9 nAChR must conserve in its primary structure some residues responsible for ligand binding common to other Cys-loop receptors. In addition, it adds further evidence that the a9 nAChR and the cholinergic receptor present at the base of cochlear outer hair cells have similar pharmacological properties. Nicotinic acetylcholine receptors (nAChRs) are complexes of protein subunits that coassemble to form an ion channel that is gated through the binding of the neurotransmitter acetylcholine (ACh) to its ligand-binding site (Changeux et al., 1987). A diversity of subunits have been cloned in recent years. The nAChR at the neuromuscular junction mediates fast synaptic transmission and is thought to have a (a1)2b1gd stoichiometry (Galzi et al., 1991). Ten genes that encode neuronal nAChR subunits have been identified in the vertebrate central or peripheral nervous system: a2 to a8, b2 to b4 (Sargent, 1993; McGehee and Role, 1995). In heterologous expression systems, the neuronal subunits a2, a3, a4, and a6 lead to the assembly of functional nAChR in combination with either b2 or b4. They preserve the structural motif of muscle nAChR, with a pentameric structure that includes two a and three b subunits (Anand et al., 1991; Cooper et al., 1991). The a7 and a8 subunits form part of a different group within the neuronal nAChR, because they can assemble into functional receptors in the absence of any other subunit and account for the a-bungarotoxin-binding sites in the central nervous system (Couturier et al., 1990; Gerzanich et al.,