The identity of nicotinic receptor subtypes sufficient to elicit both the acute and chronic effects of nicotine dependence is unknown. We engineered mutant mice with a4 nicotinic subunits containing a single point mutation, Leu9' --> Ala9' in the pore-forming M2 domain, rendering a4* receptors hypersensitive to nicotine. Selective activation of a4* nicotinic acetylcholine receptors with low doses of agonist recapitulates nicotine effects thought to be important in dependence, including reinforcement in response to acute nicotine administration, as well as tolerance and sensitization elicited by chronic nicotine administration. These data indicate that activation of a4* receptors is sufficient for nicotine-induced reward, tolerance, and sensitization.

Summary α6-containing (α6*) nicotinic ACh receptors (nAChRs) are selectively expressed in dopamine (DA) neurons and participate in cholinergic transmission. We generated and studied mice with gain-of-function α6* nAChRs, which isolate and amplify cholinergic control of DA transmission. In contrast to gene knockouts or pharmacological blockers, which show necessity, we show that activating α6* nAChRs and DA neurons is sufficient to cause locomotor hyperactivity. α6L9’S mice are hyperactive in their home cage and fail to habituate to a novel environment. Selective activation of α6* nAChRs with low doses of nicotine, by stimulating DA but not GABA neurons, exaggerates these phenotypes and produces a hyperdopaminergic state in vivo. Experiments with additional nicotinic drugs show that altering agonist efficacy at α6* provides fine-tuning of DA release and locomotor responses. α6*-specific agonists or antagonists may, by targeting endogenous cholinergic mechanisms, provide a new method for manipulating DA transmission in Parkinson’s disease, nicotine dependence, or attention deficit hyperactivity disorder.

Understanding effects of chronic nicotine requires identifying the neurons and synapses whose responses to nicotine itself, and to endogenous acetylcholine, are altered by continued exposure to the drug. To address this problem, we developed mice whose ␣4 nicotinic receptor subunits are replaced by normally functioning fluorescently tagged subunits, providing quantitative studies of receptor regulation at micrometer resolution. Chronic nicotine increased ␣4 fluorescence in several regions; among these, midbrain and hippocampus were assessed functionally. Although the midbrain dopaminergic system dominates reward pathways, chronic nicotine does not change ␣4* receptor levels in dopaminergic neurons of ventral tegmental area (VTA) or substantia nigra pars compacta. Instead, upregulated, functional ␣4* receptors localize to the GABAergic neurons of the VTA and substantia nigra pars reticulata. In consequence, GABAergic neurons from chronically nicotine-treated mice have a higher basal firing rate and respond more strongly to nicotine; because of the resulting increased inhibition, dopaminergic neurons have lower basal firing and decreased response to nicotine. In hippocampus, chronic exposure to nicotine also increases ␣4* fluorescence on glutamatergic axons of the medial perforant path. In hippocampal slices from chronically treated animals, acute exposure to nicotine during tetanic stimuli enhances induction of long-term potentiation in the medial perforant path, showing that the upregulated ␣4* receptors in this pathway are also functional. The pattern of cell-specific upregulation of functional ␣4* receptors therefore provides a possible explanation for two effects of chronic nicotine: sensitization of synaptic transmission in forebrain and tolerance of dopaminergic neuron firing in midbrain.

This review summarizes studies that attempted to determine the subtypes of nicotinic acetylcholine receptors (nAChR) expressed in the dopaminergic nerve terminals in the mouse. A variety of experimental approaches has been necessary to reach current knowledge of these subtypes, including in situ hybridization, agonist and antagonist binding, function measured by neurotransmitter release from synaptosomal preparations, and immunoprecipitation by selective antibodies. Early developments that facilitated this effort include the radioactive labeling of selective binding agents, such as [ 125 I]-α-bungarotoxin and [ 3 H]-nicotine, advances in cloning the subunits, and expression and evaluation of function of combinations of subunits in Xenopus oocytes. The discovery of epibatidine and α-conotoxin MII (α-CtxMII), and the development of nAChR subunit null mutant mice have been invaluable in determining which nAChR subunits are important for expression and function in mice, as well as allowing validation of the specificity of subunit specific antibodies. These approaches have identified five nAChR subtypes of nAChR that are expressed on dopaminergic nerve terminals. Three of these contain the α6 subunit (α4α6β2β3, α6β2β3, α6β2) and bind α-CtxMII with high affinity. One of these three subtypes (α4α6β2β3) also has the highest sensitivity to nicotine of any native nAChR that has been studied, to date. The two subtypes that do not have high affinity for α-CtxMII (α4β2, α4α5β2) are somewhat more numerous than the α6* subtypes, but do bind nicotine with high affinity. Given that our first studies detected readily measured differences in sensitivity to agonists and antagonists among these five nAChR subtypes, it seems likely that subtype selective compounds could be developed that would allow therapeutic manipulation of diverse nAChRs that have been implicated in a number of human conditions. Alterations in nicotinic cholinergic receptor (nAChRs) number or function have been implicated in psychopathologies such as anxiety, attention deficit hyperactivity disorder, depression, schizophrenia (reviewed in [1,2,3]), at least one form of familial epilepsy [4], and Parkinson's and Alzheimer's diseases [5]. It is not particularly surprising that nAChRs might play important roles in modulating several human diseases given that binding sites for nicotinic ligands such as [ 125 I]-α-bungarotoxin [6,7,8], [ 3 H]-nicotine [7,8] Corresponding author: Sharon R Grady e-mail: sharon.grady@colorado.edu phone: 303-492-9677 fax: 303-492-8063 mailing address: Institute for Behavioral Genetics University of Colorado 447UCB Boulder, CO 80309. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered w...

Neuronal nicotinic acetylcholine receptors (nAChRs) both mediate direct cholinergic synaptic transmission and modulate synaptic transmission by other neurotransmitters. Novel ligands are needed as probes to discriminate among structurally related nAChR subtypes. ␣-Conotoxin MII, a selective ligand that discriminates among a variety of nAChR subtypes, fails to discriminate well between some subtypes containing the closely related ␣3 and ␣6 subunits. Structure-function analysis of ␣-conotoxin MII was performed in an attempt to generate analogs with preference for ␣6-containing [␣6* (asterisks indicate the possible presence of additional subunits)] nAChRs. Alanine substitution resulted in several analogs with decreased activity at ␣3* versus ␣6* nAChRs heterologously expressed in Xenopus laevis oocytes. From the initial analogs, a series of mutations with two alanine substitutions was synthesized. 125 I]␣-conotoxin MII binding to putative ␣6␤2* nAChRs in mouse brain homogenates (K i ϭ 3.3 nM). Thus, structure-function analysis of ␣-conotoxin MII enabled the creation of novel selective antagonists for discriminating among nAChRs containing ␣3 and ␣6 subunits.nAChRs activated by the endogenous neurotransmitter acetylcholine belong to the superfamily of ligand-gated ion channels that also includes GABA A , 5-hydroxytryptamine-3, and glycine receptors (Changeux, 1993). These different ligand-gated ion channels show considerable sequence and structural homology. Each of the subunits has a relatively hydrophilic amino terminal half (ϳ200 amino acids) that constitutes an extracellular domain. This is followed by three hydrophobic transmembrane domains, a large intracellular loop, and then a fourth hydrophobic transmembrane span.A large number of genes have been cloned that encode subunits of nAChRs. It has been proposed that these subunits may be divided into subfamilies on the basis of both gene structure and mature protein sequence. The subunits ␣2, ␣3, ␣4, and ␣6 belong to subfamily III, tribe 1; ␤2 and ␤4 belong to tribe III-2; and the putative structural subunits ␣5 and ␤3 belong to tribe III-3 (Corringer et al., 2000). Within tribe III-1, subunits ␣3 and ␣6 show considerable sequence identity (ϳ80% in the ligand-binding extracellular domain). Thus, designing ligands to distinguish between ␣3* 1 and ␣6* is particularly challenging.␣-Conotoxin MII is a 16 amino acid peptide originally isolated from the venom of the marine snail Conus magus. This peptide potently targets neuronal in preference to the muscle subtype of nicotinic receptor with high affinity for both ␣3␤2 and ␣6* nAChRs. Unfortunately, ␣-conotoxin MII may not distinguish well between ␣3* and ␣6* nAChRs (Kuryatov et al., 2000). In an effort to remedy this situation and produce a selective ligand for ␣6* nAChRs, we have generated a series of ␣-conotoxin MII analogs.The ␣6 subunit is expressed in catecholaminergic neurons and in retina (Le Novère et al., 1996Vailati et al., 1999). In striatum, ␣6* nAChRs seem to play a central role in the modulation ...

Nigrostriatal dopaminergic neurons express many nicotinic acetylcholine receptor (nAChR) subunits capable of forming multiple nAChR subtypes. These subtypes are expressed differentially along the neuron and presumably mediate diverse responses. beta3 subunit mRNA has restricted expression but is abundant in the substantia nigra and ventral tegmental areas. To investigate the potential role(s) of nicotinic receptors containing the beta3 subunit in dopaminergic tracts, we generated mice with a null mutation in the beta3 gene. We were thereby able to identify a population of beta3-dependent alpha-conotoxin MII-binding nAChRs that modulate striatal dopamine release. Changes were also observed in locomotor activity and prepulse inhibition of acoustic startle, behaviors that are controlled, in part, by nigrostriatal and mesolimbic dopaminergic activity, respectively, suggesting that beta3-containing nAChRs modulate these behaviors.

[125I]‐Epibatidine binds to multiple nicotinic acetylcholine receptor (nAChR) subtypes with high affinity. In this study, [125I]‐epibatidine was used to label and characterize a novel nAChR subtype found in mouse brain inferior colliculus, interpeduncular nucleus, and olfactory bulb homogenates. Binding of [125I]‐epibatidine was saturable and apparently monophasic in each brain region (KD=71±12 pM mean±s.e.mean across regions) but inhibition of [125I]‐epibatidine binding (200 pM) by A85380, cytisine and (−)‐nicotine was biphasic, indicating the presence of multiple binding sites. The sites with lower agonist affinity comprised 30.0±2.2, 58.6±0.1 and 48.7±3.3% of specific [125I]‐epibatidine (200 pM) binding in inferior colliculus, interpeduncular nucleus, and olfactory bulb homogenates, respectively. The affinity difference between A85380‐sensitive and ‐resistant binding sites was particularly marked (approximately 1000 fold). Thus A85380 was used to differentiate agonist‐sensitive and ‐resistant sites. The pharmacological profiles of the A85380‐resistant sites in each region were assessed with inhibition binding experiments, using 14 agonists and five antagonists. The profiles were indistinguishable across regions, implying that A85380‐resistant [125I]‐epibatidine binding sites in inferior colliculus, interpeduncular nucleus, and olfactory bulb represent a single nAChR subtype. The pharmacological profile of the A85380‐resistant sites is very different from that previously reported for high affinity (−)‐[3H]‐nicotine‐, [125I]‐α‐bungarotoxin‐, or [125I]‐α‐conotoxin MII‐binding sites, suggesting that they represent a novel nAChR population in mouse brain. British Journal of Pharmacology (2000) 131, 729–739; doi:

The alpha3 subunit gene was one of the first neuronal nicotinic acetylcholine receptor (nAChR) subunits to be cloned (Boulter et al., 1986), but direct evidence of alpha3 subunit contributions to mammalian central nAChR populations has not been presented. The studies reported here used mice engineered to contain a null mutation in the alpha3 nAChR subunit gene (Xu et al., 1999) to examine the involvement of the alpha3 subunit in central nAChR populations. Heterologously expressed alpha3beta2 and alpha3beta4 nAChRs are pharmacologically similar to native [125I]alpha-conotoxin MII (alpha-CtxMII)-binding and 3-(2(S)-azetidinylmethoxy)pyridine dihydrochloride (A85380)-resistant [125I]epibatidine-binding nAChR subtypes, respectively. The hypothesis that both native sites are alpha3-subtype nAChRs was tested using quantitative autoradiography in alpha3-null mutant mice. Somewhat surprisingly, deletion of the alpha3 nAChR subunit gene did not affect expression of the great majority of [125I]alpha-CtxMII-binding sites, indicating that they do not correspond to heterologously expressed alpha3beta2 nAChRs. The only exception to this was observed in the habenulointerpeduncular tract, where alpha3-dependent [125I]alpha-CtxMII binding was observed. This finding may suggest the presence of an additional, minor nicotinic population in this pathway. In contrast, most -resistant [125I]epibatidine-binding nAChRs were dependent on alpha3 gene expression, suggesting that they do indeed correspond to an alpha3 nAChR subtype. However, widespread but lower levels of alpha3-independent -resistant [125I]epibatidine binding were also seen. Again, this may indicate the existence of an additional, minor population of non-alpha3 -resistant sites.