Getting past the asterisk: the subunit composition of presynaptic nicotinic receptors that modulate striatal dopamine release.

Nicotinic acetylcholine receptors (nAChRs), despite wide distribution, do not play a major role in direct mediation of central synaptic transmission. Instead, these receptors are primarily involved in modulating the release of neurotransmitters, including glutamate, GABA, norepinephrine, dopamine, and acetylcholine itself (Role and Berg, 1996; Wonnacott, 1997). Modulation of dopamine release within the striatum is thought to underlie some of the reinforcing and rewarding aspects of nicotine (Dani et al., 2001). Neuronal nAChRs are localized somato-dendritically and on presynaptic terminals of dopaminergic neurons projecting from the substantia nigra (SN) and ventral tegmental area (VTA) to the striatum. Although there is much that we would like to know about these receptors, even the basic goal of understanding subunit composition turns out to be more challenging than it seems. In an article by Salminen et al. (2004) in this issue of Molecular Pharmacology, Grady and colleagues combine the use of knockout mice, a selective antagonist, and a dopamine release assay to present the culmination of this effort at the presynaptic terminal. We are now provided with what is likely to be the complete subunit composition of four distinct presynaptic neuronal nAChRs that modulate dopamine release in the striatum. The nAChRs form as pentameric arrangements of subunits around a central ion channel. Based on protein sequence and gene structure, these subunits are organized into several subfamilies (Corringer et al., 2000). The members of subfamily III ( 26, 24) and one member of subfamily II ( 7) will concern us here. Members of subfamilies I ( 910) and IV (muscle nAChR subunits), and one member of subfamily II ( 8), are not expressed in mammalian brain (Corringer et al., 2000; Elgoyhen et al., 2001). Subfamily III is further subdivided into tribes. An agonist-binding site is formed at the interface between a tribe 1 subunit ( 2, 3, 4, 6) and a tribe 2 subunit ( 2, 4). The receptor is formed from two of these subunit pairs and a fifth subunit from tribe 2 or tribe 3 ( 5, 3). Thus, subunit composition determines the pharmacological and functional properties of neuronal nAChRs. Many subunit combinations have been characterized using exogenous expression systems in the hope that pharmacological and functional properties could be used to identify the subunit composition of receptors in vivo (Role, 1992). Unfortunately, a major problem soon became apparent. For example, if a competitive antagonist is specific for a particular subunit combination in vitro ( 3 2, for example) and is found to antagonize a receptor in vivo, the most that can be said about the in vivo receptor is that it possesses one 3 2 subunit interface. The other three subunits cannot be definitively identified. This problem led to a provisional nomenclature in which subunits known to be present are stated and unidentified subunits represented by an asterisk (Lukas et al., 1999). The receptor in the example would be designated 3 2*, with the asterisk representing three of the five subunits. The goal is to eliminate the asterisk. Because of the limitations of pharmacological tools then available, presynaptic nAChRs modulating striatal dopamine release initially seemed to be homogeneous (Wonnacott, 1997). This picture began to change with the development of the -conotoxins ( -Ctx) as tools for investigating neuronal nAChRs (McIntosh et al., 1999). In particular, -CtxMII was found to be highly selective for the 3 2 subunit combination (Cartier et al., 1996) and became a useful tool for investigating receptor structure (Harvey et al., 1997). Use of -CtxMII allowed presynaptic nAChRs on striatal dopaminergic terminals to be divided into two classes: “ -CtxMII–sensitive” and “ -CtxMII–resistant” (Kulak et al., 1997; Kaiser et al., 1998). Because -CtxMII would be expected to block any nAChR containing the 3 2 subunit interface, the -CtxMII–sensitive receptors were identified as 3 2*, an exciting advance for the field. Thus it became confusing when deletion of the