Deletion of the α7, β2, or β4 Nicotinic Receptor Subunit Genes Identifies Highly Expressed Subtypes with Relatively Low Affinity for [3H]Epibatidine

Expression pattern of nicotinic acetylcholine receptor subunit transcripts in neurons and astrocytes in the ventral tegmental area and locus coeruleus

Acetylcholine is the endogenous agonist for the neuronal nicotinic acetylcholine receptor (nAChR) system, which is involved in attention, memory, affective behaviours and substance use disorders. Brain nAChRs are highly diverse with 11 different subunits that can form multiple receptor subtypes, each with distinct receptor and pharmacological properties. Different neuronal cell types can also express different nAChR subtypes, resulting in highly complex cholinergic signalling. Identifying which nAChR subunit transcripts are expressed in cell types can provide an indication of which nAChR combinations are possible and which receptor subtypes may be most pharmacologically relevant to target. In addition to differences in expression across cell types, nAChRs also undergo changes in expression levels from adolescence to adulthood. In this study, we used fluorescent in situ hybridization to identify and quantify the expression of α4, α5, α6, β2 and β3 nAChR subunit transcripts in dopaminergic, GABAergic, glutamatergic and noradrenergic neurons and astrocytes in the ventral tegmental area (VTA) and locus coeruleus (LC) in adult and adolescent, male and female C57BL/6J mice. There were distinct differences in the pattern of nAChR subunit transcript expression between the two brain regions. LC noradrenergic neurons had high prevalence of α6, β2 and β3 expression, with very low expression of α4, suggesting the α6(non-α4)β2β3 receptor as a main subtype in these neurons. VTA astrocytes from adult mice showed greater prevalence of α5, α6, β2 and β3 transcript compared with adolescent mice. These data highlight the complex nAChR expression patterns across brain region and cell type.

Cholinergic Boutons are Distributed Along the Dendrites and Somata of VIP Neurons in the Inferior Colliculus

Cholinergic signaling shapes sound processing and plasticity in the inferior colliculus (IC), the midbrain hub of the central auditory system, but how cholinergic terminals contact and influence individual neuron types in the IC remains largely unknown. Using pharmacology and electrophysiology, we recently found that acetylcholine strongly excites VIP neurons, a class of glutamatergic principal neurons in the IC, by activating α3β4* nicotinic acetylcholine receptors (nAChRs). Here, we confirm and extend these results using tissue from mice of both sexes. First, we show that mRNA encoding α3 and β4 nAChR subunits is expressed in many neurons throughout the IC, including most VIP neurons, suggesting that these subunits, which are rare in the brain, are important mediators of cholinergic signaling in the IC. Next, by combining fluorescent labeling of VIP neurons and immunofluorescence against the vesicular acetylcholine transporter (VAChT), we show that individual VIP neurons in the central nucleus of the IC (ICc) are contacted by a large number of cholinergic boutons. Cholinergic boutons were distributed adjacent to the somata and along the full length of the dendritic arbors of VIP neurons, positioning cholinergic signaling to affect synaptic computations arising throughout the somatodendritic compartments of VIP neurons. In addition, cholinergic boutons were occasionally observed in close apposition to dendritic spines on VIP neurons, raising the possibility that cholinergic signaling also modulates presynaptic release onto VIP neurons. Together, these results strengthen the evidence that cholinergic signaling exerts widespread influence on auditory computations performed by VIP neurons and other neurons in the IC.

Effect of Intrahippocampal Administration of α7 Subtype Nicotinic Receptor Agonist PNU-282987 and Its Solvent Dimethyl Sulfoxide on the Efficiency of Hypoxic Preconditioning in Rats

We have previously suggested a key role of the hippocampus in the preconditioning action of moderate hypobaric hypoxia (HBH). The preconditioning efficiency of HBH is associated with acoustic startle prepulse inhibition (PPI). In rats with PPI > 40%, HBH activates the cholinergic projections of hippocampus, and PNU-282987, a selective agonist of α7 nicotinic receptors (α7nAChRs), reduces the HBH efficiency and potentiating effect on HBH of its solvent dimethyl sulfoxide (DMSO, anticholinesterase agent) when administered intraperitoneally. In order to validate the hippocampus as a key structure in the mechanism of hypoxic preconditioning and research a significance of α7nAChR activation in the hypoxic preconditioning, we performed an in vivo pharmacological study of intrahippocampal injections of PNU-282987 into the CA1 area on HBH efficiency in rats with PPI ≥ 40%. We found that PNU-282987 (30 μM) reduced HBH efficiency as with intraperitoneal administration, while DMSO (0.05%) still potentiated this effect. Thus, direct evidence of the key role of the hippocampus in the preconditioning effect of HBH and some details of this mechanism were obtained in rats with PPI ≥ 40%. The activation of α7nAChRs is not involved in the cholinergic signaling initiated by HBH or DMSO via any route of administration. Possible ways of the potentiating action of DMSO on HBH efficiency and its dependence on α7nAChRs are discussed.

α3β4 ∗ Nicotinic Acetylcholine Receptors Strongly Modulate the Excitability of VIP Neurons in the Mouse Inferior Colliculus

The inferior colliculus (IC), the midbrain hub of the central auditory system, receives extensive cholinergic input from the pontomesencephalic tegmentum. Activation of nicotinic acetylcholine receptors (nAChRs) in the IC can alter acoustic processing and enhance auditory task performance. However, how nAChRs affect the excitability of specific classes of IC neurons remains unknown. Recently, we identified vasoactive intestinal peptide (VIP) neurons as a distinct class of glutamatergic principal neurons in the IC. Here, in experiments using male and female mice, we show that cholinergic terminals are routinely located adjacent to the somas and dendrites of VIP neurons. Using whole-cell electrophysiology in brain slices, we found that acetylcholine drives surprisingly strong and long-lasting excitation and inward currents in VIP neurons. This excitation was unaffected by the muscarinic receptor antagonist atropine. Application of nAChR antagonists revealed that acetylcholine excites VIP neurons mainly via activation of α₃β₄^∗ nAChRs, a nAChR subtype that is rare in the brain. Furthermore, we show that acetylcholine excites VIP neurons directly and does not require intermediate activation of presynaptic inputs that might express nAChRs. Lastly, we found that low frequency trains of acetylcholine puffs elicited temporal summation in VIP neurons, suggesting that in vivo-like patterns of cholinergic input can reshape activity for prolonged periods. These results reveal the first cellular mechanisms of nAChR regulation in the IC, identify a functional role for α₃β₄^∗ nAChRs in the auditory system, and suggest that cholinergic input can potently influence auditory processing by increasing excitability in VIP neurons and their postsynaptic targets.

GABAB receptors blockage modulates somatic and aversive manifestations induced by nicotine withdrawal

There is substantial evidence that GABA_B agonist, baclofen, prevents somatic and motivational responses induced by nicotine withdrawal and may target drug cue vulnerabilities in humans. In this context, we explored different aspects associated with the possible mechanisms whereby the GABA_B receptors might influence nicotine withdrawal. Male mice received nicotine (2.5 mg/kg, s.c.) 4 times daily, for 7 consecutive days. Nicotine-treated mice received the nicotinic acetylcholine receptor antagonist, mecamylamine (MEC, 2 or 3.5 mg/kg, s.c.), to precipitate the withdrawal state. A second group of dependent mice received 2-hydroxysaclofen (GABA_B receptor antagonist, 1 mg/kg, s.c.) before MEC-precipitated abstinence. Somatic signs of nicotine withdrawal were measured for 30 min. Anxiogenic-like response associated to nicotine withdrawal was assessed by the elevated plus maze test. The dysphoric/aversive effect induced by nicotine withdrawal was evaluated using conditioned place aversion paradigm. Dopamine, serotonin and its metabolites concentrations were determined by HPLC in the striatum, cortex and hippocampus. Finally, α4β2 nicotinic acetylcholine receptor density was determined in several brain regions using autoradiography assays. The results showed that MEC-precipitated nicotine withdrawal induced somatic manifestations, anxiogenic-like response and dysphoric/aversive effect, and 2-hydroxysaclofen potentiated these behavioral responses. Additionally, 2-hydroxysaclofen was able to change striatal dopamine levels and α4β2 nicotinic acetylcholine receptor density, both altered by MEC-precipitated nicotine withdrawal. These findings provide important contributions to elucidate neurobiological mechanisms implicated in nicotine withdrawal. We suggest that GABA_B receptor activity is necessary to control alterations induced by nicotine withdrawal, which supports the idea of targeting GABA_B receptors to treat tobacco addiction in humans.

Differential effects of withdrawal from intermittent and continuous nicotine exposure on reward deficit and somatic aspects of nicotine withdrawal and expression of α4β2* nAChRs in Wistar male rats

BACKGROUND

Chronic nicotine exposure produces neuroadaptations in brain reward systems and α4β2 nicotinic acetylcholine receptors (nAChRs) in the corticolimbic brain areas. We previously demonstrated opposite effects of nicotine exposure delivered by self-administration or pumps on brain reward thresholds that can be attributed to the different temporal pattern and contingency of nicotine exposure. We investigated the effects of these two factors on reward thresholds and somatic signs during nicotine withdrawal, and on nAChRs binding in corticolimbic brain areas.

METHODS

The intracranial self-stimulation procedure was used to assess reward thresholds in rats prepared with pumps delivering various doses of nicotine continuously or intermittently. Separate group of rats were randomly exposed to nicotine via pumps (non-contingent) or nicotine self-administration (contingent) to determine [¹²⁵I]-epibatidine binding at α4β2* nAChRs.

RESULTS

Withdrawal from continuous non-contingent nicotine exposure led to significant elevations in thresholds and increases in somatic signs in rats, while there was no significant effect of withdrawal from intermittent non-contingent nicotine exposure at the same doses. nAChRs were upregulated during withdrawal from continuous non-contingent nicotine exposure. α4β2* nAChRs were upregulated in the ventral tegmental area and prelimbic cortex during withdrawal from non-contingent intermittent exposure and in the nucleus accumbens during withdrawal from contingent intermittent nicotine exposure to the same dose.

CONCLUSIONS

During non-contingent nicotine exposure, the temporal pattern of nicotine delivery differentially affected thresholds and somatic signs of withdrawal. Upregulation of α4β2* nAChRs was brain site-specific and depended on both temporal pattern and contingency of nicotine exposure.

TC299423, a Novel Agonist for Nicotinic Acetylcholine Receptors

(E)-5-(Pyrimidin-5-yl)-1,2,3,4,7,8-hexahydroazocine (TC299423) is a novel agonist for nicotinic acetylcholine receptors (nAChRs). We examined its efficacy, affinity, and potency for α6β2^∗ (α6β2-containing), α4β2^∗, and α3β4^∗ nAChRs, using [¹²⁵I]-epibatidine binding, whole-cell patch-clamp recordings, synaptosomal ⁸⁶Rb⁺ efflux, [³H]-dopamine release, and [³H]-acetylcholine release. TC299423 displayed an EC₅₀ of 30–60 nM for α6β2^∗ nAChRs in patch-clamp recordings and [³H]-dopamine release assays. Its potency for α6β2^∗ in these assays was 2.5-fold greater than that for α4β2^∗, and much greater than that for α3β4^∗-mediated [³H]-acetylcholine release. We observed no major off-target binding on 70 diverse molecular targets. TC299423 was bioavailable after intraperitoneal or oral administration. Locomotor assays, measured with gain-of-function, mutant α6 (α6L9′S) nAChR mice, show that TC299423 elicits α6β2^∗ nAChR-mediated responses at low doses. Conditioned place preference assays show that low-dose TC299423 also produces significant reward in α6L9′S mice, and modest reward in WT mice, through a mechanism that probably involves α6(non-α4)β2^∗ nAChRs. However, TC299423 did not suppress nicotine self-administration in rats, indicating that it did not block nicotine reinforcement in the dosage range that was tested. In a hot-plate test, TC299423 evoked antinociceptive responses in mice similar to those of nicotine. TC299423 and nicotine similarly inhibited mouse marble burying as a measure of anxiolytic effects. Taken together, our data suggest that TC299423 will be a useful small-molecule agonist for future in vitro and in vivo studies of nAChR function and physiology.

Interactions between nicotine and drugs of abuse: a review of preclinical findings

Polysubstance abuse is common among substance-use disorder patients, and nicotine is one of the most commonly co-used substances. Epidemiological and clinical laboratory studies suggest that nicotine, when combined with other drugs of abuse, increases intake of one or both substances. This review focuses on the preclinical literature regarding nicotine's interaction with alcohol, stimulants (i.e., cocaine, amphetamines), opioids (i.e., morphine, heroin), and Δ⁹-tetrahydrocannabinol (THC). The current understanding of how these various classes of abused drugs may interact with nicotine on behavioral, physiological, and pharmacological indices that may be important in maintaining co-use of one or both substances in human populations are highlighted. Suggestions as to future areas of research and gaps in knowledge are offered.

Cholinergic Regulation of Morphine Release from Human White Blood Cells: Evidence for a Novel Nicotinic Receptor via Pharmacological and Micro Array Analysis

Recent work from our laboratory has demonstrated that human white blood cells make morphine and that substances of abuse, i.e. nicotine, alcohol and cocaine have the ability to release this endogenous substance, suggesting a common mechanism of action. We now demonstrate that the nicotinic process is more complex than formerly envisioned. The incorporation rate of 125I-labeled morphine into PMN and MN are 7.85±0.36%, 1.42±0.19%, respectfully, suggesting in MN this process is of low activity. Separate incubations of PMN with varying concentrations of nicotine or the nicotine agonist epibatidine resulted in a statistically significant enhancement of 125I-trace labeled morphine released into the extracellular medium. In order to ascertain the specificity of the nicotine stimulated morphine release the following experiments were performed. Co-incubation of hexamethonium dichloride (5 μg/ml and at 10 μg/ml), which preferentially blocks nicotinic receptors at autonomic ganglia, with nicotine, exerted a very weak inhibitory effect. Co-incubation of α-BuTx or atropine or chlorisondamine diiodide or dihydro-β-erythroidine hydrobromide, an α4β2 receptor antagonist, did not block nicotine induced morphine release alone or in combination, suggesting either the response was not specific or it was mediated by a novel nicotinic receptor. Human leukocyte total RNA isolated from whole blood were analyzed, using the Human Genome Survey microarray (Applied Biosystems), for cholinergic receptor expression. PMN nicotinic receptor gene expression was present and contained numerous variants (eight). The number of variants suggests that indeed a novel nicotinic receptor may be mediating this effect, while simultaneously demonstrating the significance of the cholinergic receptor expression in these immune cells.

Engineered α4β2 nicotinic acetylcholine receptors as models for measuring agonist binding and effect at the orthosteric low-affinity α4-α4 interface

The nicotinic acetylcholine receptor α4β2 is important for normal mammalian brain function and is known to express in two different stoichiometries, (α4)2(β2)3 and (α4)3(β2)2. While these are similar in many aspects, the (α4)3(β2)2 stoichiometry differs by harboring a third orthosteric acetylcholine binding site located at the α4-α4 interface. Interestingly, the third binding site has, so far, only been documented using electrophysiological assays, actual binding affinities of nicotinic receptor ligands to this site are not known. The present study was therefore aimed at determining binding affinities of nicotinic ligands to the α4-α4 interface. Given that epibatidine shows large functional potency differences at α4-β2 vs. α4-α4 interfaces, biphasic binding properties would be expected at (α4)3(β2)2 receptors. However, standard saturation binding experiments with [(3)H]epibatidine did not reveal biphasic binding under the conditions utilized. Therefore, an engineered β2 construct (β2(HQT)), which converts the β(-) face to resemble that of an α4(-) face, was utilized to create (α4)3(β2(HQT))2 receptors harboring three α4-α4 interfaces. With this receptor, low affinity binding of epibatidine with a Kd of ∼5 nM was observed in sharp contrast to a Kd value of ∼10 pM observed for wild-type receptors. A strong correlation between binding affinities at the (α4)3(β2(HQT))2 receptor and functional potencies at the wild-type receptor of a range of nicotinic ligands highlighted the validity of using the mutational approach. Finally, large differences in activities at α4-β2 vs. α4-α4 interfaces were observed for structurally related agonists underscoring the need for establishing all binding parameters of compounds at α4β2 receptors.

Structure of neuronal nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are a critical component of the brain's cholinergic neurotransmission system that modulates important physiological processes and whose dysfunction has been observed in patients with neurodegenerative diseases and mental illness. nAChRs are a heterogeneous family of receptor subtypes consisting of pentameric combinations of α and β subunits, and are widely expressed throughout the central and peripheral nervous system. nAChR subtypesnAChR subtypes share a common basic structure but their biophysical and pharmacological properties depend on their subunit compositionSubunit composition , which is therefore central to understanding receptor function in the nervous system and discovering new subtype-selective drugs. We briefly review some recent findings concerning the structure and function of nAChRs, particularly the native subtypes.

Drug-repurposing identified the combination of Trolox C and Cytisine for the treatment of type 2 diabetes

Background

Drug-induced gene expression dataset (for example Connectivity Map, CMap) represent a valuable resource for drug-repurposing, a class of methods for identifying novel indications for approved drugs. Recently, CMap-based methods have successfully applied to identifying drugs for a number of diseases. However, currently few gene expression based methods are available for the repurposing of combined drugs. Increasing evidence has shown that the combination of drugs may valid for novel indications.

Method

Here, for this purpose, we presented a simple CMap-based scoring system to predict novel indications for the combination of two drugs. We then confirmed the effectiveness of the predicted drug combination in an animal model of type 2 diabetes.

Results

We applied the presented scoring system to type 2 diabetes and identified a candidate combination of two drugs, Trolox C and Cytisine. Finally, we confirmed that the predicted combined drugs are effective for the treatment of type 2 diabetes.

Conclusion

The presented scoring system represents one novel method for drug repurposing, which would provide helps for greatly extended the space of drugs.

Effectiveness of nicotinic agonists as desensitizers at presynaptic α4β2- and α4α5β2-nicotinic acetylcholine receptors

INTRODUCTION

Nicotine interacts with nicotinic acetylcholine receptors (nAChRs) and modifies neuronal functions. The net result of nicotine exposure is difficult to assess because multiple nAChR subtypes exist and are expressed on multiple classes of neurons. Nicotine, unlike the natural agonist acetylcholine, remains in tissues for hours, and during this extended exposure nAChRs desensitize. Therefore, agonists can block the natural functions of nAChRs. Higher nicotine concentrations are required to desensitize α4β2-nAChRs containing the α5 subunit. The aim of these experiments was to determine if this property holds true for compounds other than nicotine.

METHODS

[(3)H]-dopamine release from crude mouse striatal synaptosomal preparations was used to measure activation and desensitization of the [(α4β2)2β2] and [(α4β2)2α5] nAChR subtypes. Affinity was measured by competition with [(125)I]-epibatidine.

RESULTS

Nine compounds of varying affinity and efficacy were tested. All compounds partially desensitized both subtypes; concentration necessary for desensitization correlated with binding site affinity but not efficacy. All compounds showed a similar, significant shift in concentration necessary for a 50% effect when the α5 subunit was included (averaging 8-fold higher). The extent of desensitization produced by a 10-min exposure did not correlate with affinity or efficacy of compound.

CONCLUSION

Full or partial nicotinic agonists used as medications may effectively desensitize α4β2-nAChRs. However, significantly higher concentrations of all compounds tested were required to elicit desensitization of α4α5β2-nAChRs than α4β2-nAChRs. If desensitization is the important property for a smoking cessation drug, basic screening at both subtypes may provide a mechanistic foundation for effectiveness.

Mecamylamine-precipitated nicotine withdrawal syndrome and its prevention with baclofen: an autoradiographic study of α4β2 nicotinic acetylcholine receptors in mice

A previous study from our laboratory showed that baclofen (BAC, GABAB receptor agonist) was able to prevent the behavioral expression of nicotine (NIC) withdrawal syndrome. To further investigate the mechanisms underlying this effect, we conducted this study, with the aims of analyzing α4β2 nicotinic receptor density during NIC withdrawal and, in case we found any changes, of determining whether they could be prevented by pretreatment with BAC. Swiss Webster albino mice received NIC (2.5 mg/kg, s.c.) 4 times daily, for 7 days. On the 8th day, NIC-treated mice received the nicotinic antagonist mecamylamine (MEC; 2 mg/kg, i.p.) 1 h after the last dose of NIC. A second group of NIC-treated mice received BAC (2 mg/kg, i.p.) prior to MEC administration. Thirty minutes after MEC, mice were sacrificed and brain autoradiography with [(3)H]epibatidine was carried out at five different anatomical levels. Autoradiographic mapping showed a significant increase of α4β2 nicotinic receptor labeling during NIC withdrawal in the nucleus accumbens shell (AcbSh), medial habenular nucleus (HbM), thalamic nuclei, dorsal lateral geniculate (DLG) nucleus, fasciculus retroflexus (fr), ventral tegmental area, interpeduncular nucleus and superior colliculus. BAC pretreatment prevented the increased α4β2 nicotinic receptor binding sites in the AcbSh, MHb, thalamic nuclei, DLG nucleus and fr. The present results suggest a relationship between BAC's preventive effect of the expression of NIC withdrawal signs, and its ability to restore the changes in α4β2 nicotinic receptor labeling, evidenced in specific brain areas in NIC withdrawn animals.

Morphine dependence and withdrawal induced changes in cholinergic signaling

Cholinergic signaling is thought to be involved in morphine dependence and withdrawal, but the specific mechanisms involved remain unclear. The current study aimed to identify alterations in the cholinergic system that may contribute to the development of morphine dependence and withdrawal. Acetylcholinesterase (AChE) activity and [³H]-epibatidine binding were evaluated in order to determine if morphine dependence and withdrawal induces alterations in cholinergic signaling or expression of high affinity nicotinic acetylcholine receptors (nAChRs) in the midbrain (MB), medial habenula (MHb) and interpeduncular nucleus (IPN). The effect of cholinergic signaling through nAChRs on morphine-withdrawal induced jumping behavior was then determined. Lastly, the contribution of β4-containing nAChRs receptors in the MHb to morphine-withdrawal induced jumping behavior and neuronal activity as indicated by c-fos expression was assessed. Chronic morphine administration decreased AChE activity in MB and MHb, an effect that was no longer present following precipitated withdrawal. Morphine dependent mice showed increased nicotinic acetylcholine receptor (nAChR) levels in MB. Further, nicotine (0.4 mg/kg) and lobeline (3 mg/kg) decreased jumping behavior while mecamylamine (1 mg/kg) had no effect. Knock-down of β4 subunit-containing nAChRs in the MHb attenuated c-fos activation, but did not decrease morphine withdrawal-induced jumping. Thus, morphine withdrawal induces cholinergic signaling in the MHb, but this does not appear to be responsible for the effects of cholinergic drugs on somatic signs of opiate withdrawal, as measured by jumping behavior.

Adolescent nicotine exposure transiently increases high-affinity nicotinic receptors and modulates inhibitory synaptic transmission in rat medial prefrontal cortex

Adolescence is a critical developmental period during which most adult smokers initiate their habit. Adolescents are more vulnerable than adults to nicotine's long-term effects on addictive and cognitive behavior. We investigated whether adolescent nicotine exposure in rats modifies expression of nicotinic acetylcholine receptors (nAChRs) in medial prefrontal cortex (mPFC) in the short and/or long term, and whether this has functional consequences. Using receptor binding studies followed by immunoprecipitation of nAChR subunits, we showed that adolescent nicotine exposure, as compared with saline, caused an increase in mPFC nAChRs containing α4 or β2 subunits (24 and 18%, respectively) 24 h after the last injection. Nicotine exposure in adulthood had no such effect. This increase was transient and was not observed 5 wk following either adolescent or adult nicotine exposure. In line with increased nAChRs expression 1 d after adolescent nicotine exposure, we observed a 34% increase in amplitude of nicotine-induced spontaneous inhibitory postsynaptic currents in layer II/III mPFC pyramidal neurons. These effects were transient and specific, and observed only acutely after adolescent nicotine exposure, but not after 5 wk, and no changes were observed in adult-exposed animals. The acute nicotine-induced increase in α4β2-containing receptors in adolescents interferes with the normal developmental decrease (37%) of these receptors from early adolescence (postnatal day 34) to adulthood (postnatal day 104) in the mPFC. Together, this suggests that these receptors play a role in mediating the acute rewarding effects of nicotine and may underlie the increased sensitivity of adolescents to nicotine.

The duration of nicotine withdrawal-associated deficits in contextual fear conditioning parallels changes in hippocampal high affinity nicotinic acetylcholine receptor upregulation

A predominant symptom of nicotine withdrawal is cognitive deficits, yet understanding of the neural basis for these deficits is limited. Withdrawal from chronic nicotine disrupts contextual learning in mice and this deficit is mediated by direct effects of nicotine in the hippocampus. Chronic nicotine treatment upregulates nicotinic acetylcholine receptors (nAChR); however, it is unknown whether upregulation is related to the observed withdrawal-induced cognitive deficits. If a relationship between altered learning and nAChR levels exists, changes in nAChR levels after cessation of nicotine treatment should match the duration of learning deficits. To test this hypothesis, mice were chronically administered 6.3mg/kg/day (freebase) nicotine for 12 days and trained in contextual fear conditioning on day 11 or between 1 to 16 days after withdrawal of treatment. Changes in [(125)I]-epibatidine binding at cytisine-sensitive and cytisine-resistant nAChRs and chronic nicotine-related changes in α4, α7, and β2 nAChR subunit mRNA expression were assessed. Chronic nicotine had no behavioral effect but withdrawal produced deficits in contextual fear conditioning that lasted 4 days. Nicotine withdrawal did not disrupt cued fear conditioning. Chronic nicotine upregulated hippocampal cytisine-sensitive nAChR binding; upregulation continued after cessation of nicotine administration and the duration of upregulation during withdrawal paralleled the duration of behavioral changes. Changes in binding in cortex and cerebellum did not match behavioral changes. No changes in α4, α7, and β2 subunit mRNA expression were seen with chronic nicotine. Thus, nicotine withdrawal-related deficits in contextual learning are time-limited changes that are associated with temporal changes in upregulation of high-affinity nAChR binding.

Characterizing low affinity epibatidine binding to α4β2 nicotinic acetylcholine receptors with ligand depletion and nonspecific binding

Background

Along with high affinity binding of epibatidine (K_d1≈10 pM) to α4β2 nicotinic acetylcholine receptor (nAChR), low affinity binding of epibatidine (K_d2≈1-10 nM) to an independent binding site has been reported. Studying this low affinity binding is important because it might contribute understanding about the structure and synthesis of α4β2 nAChR. The binding behavior of epibatidine and α4β2 AChR raises a question about interpreting binding data from two independent sites with ligand depletion and nonspecific binding, both of which can affect equilibrium binding of [³H]epibatidine and α4β2 nAChR. If modeled incorrectly, ligand depletion and nonspecific binding lead to inaccurate estimates of binding constants. Fitting total equilibrium binding as a function of total ligand accurately characterizes a single site with ligand depletion and nonspecific binding. The goal of this study was to determine whether this approach is sufficient with two independent high and low affinity sites.

Results

Computer simulations of binding revealed complexities beyond fitting total binding for characterizing the second, low affinity site of α4β2 nAChR. First, distinguishing low-affinity specific binding from nonspecific binding was a potential problem with saturation data. Varying the maximum concentration of [³H]epibatidine, simultaneously fitting independently measured nonspecific binding, and varying α4β2 nAChR concentration were effective remedies. Second, ligand depletion helped identify the low affinity site when nonspecific binding was significant in saturation or competition data, contrary to a common belief that ligand depletion always is detrimental. Third, measuring nonspecific binding without α4β2 nAChR distinguished better between nonspecific binding and low-affinity specific binding under some circumstances of competitive binding than did presuming nonspecific binding to be residual [³H]epibatidine binding after adding a large concentration of cold competitor. Fourth, nonspecific binding of a heterologous competitor changed estimates of high and low inhibition constants but did not change the ratio of those estimates.

Conclusions

Investigating the low affinity site of α4β2 nAChR with equilibrium binding when ligand depletion and nonspecific binding are present likely needs special attention to experimental design and data interpretation beyond fitting total binding data. Manipulation of maximum ligand and receptor concentrations and intentionally increasing ligand depletion are potentially helpful approaches.

Decreased α4β2 nicotinic receptor number in the absence of mRNA changes suggests post-transcriptional regulation in the spontaneously hypertensive rat model of ADHD

The spontaneously hypertensive rat (SHR) is widely used as a model of attention-deficit/hyperactivity disorder (ADHD). Deficits in central nicotinic receptors (nAChRs) have been previously observed in SHRs, which is interesting since epidemiological studies have identified an association between smoking and ADHD symptoms in humans. Here, we examine whether nAChR deficits in SHRs compared with Wistar Kyoto rat (WKY) controls are nAChR subtype-specific and whether these deficits correlate with changes at the level of mRNA transcription in specific brain regions. Levels of binding sites (B(max) ) and dissociation constants (K(d)) for nAChRs were determined from saturation curves of high-affinity [³H]epibatidine- and [³H] Methyllycaconitine (MLA) binding to membranes from cortex, striatum, hippocampus and cerebellum. In additional brain regions, nAChRs were examined by autoradiography with [¹²⁵I]A-85380 and [¹²⁵I]α-bungarotoxin. Levels of mRNA encoding nAChR subunits were measured using quantitative real-time PCR (qPCR). We showed that the number of α4β2 nAChR binding sites is lower globally in the SHR brain compared with WKY in the absence of significant differences in mRNA levels, with the exception of lower α4 mRNA in cerebellum of SHR compared with WKY. Furthermore, nAChR deficits were subtype- specific because no strain difference was found in α7 nAChR binding or α7 mRNA levels. Our results suggest that the lower α4β2 nAChR number in SHR compared with WKY may be a consequence of dysfunctional post-transcriptional regulation of nAChRs.

An autoradiographic survey of mouse brain nicotinic acetylcholine receptors defined by null mutants

Nine nicotinic receptor subunits are expressed in the central nervous system indicating that a variety of nicotinic acetylcholine receptors (nAChR) may be assembled. A useful method with which to identify putative nAChR is radioligand binding. In the current study the binding of [(125)I]α-bungarotoxin, [(125)I]α-conotoxinMII, 5[(125)I]-3-((2S)-azetidinylmethoxy)pyridine (A-85380), and [(125)I]epibatidine has been measured autoradiographically to provide data on many nAChR binding sites. Each binding site was evaluated semi-quantitatively for samples prepared from wild-type and α2, α4, α6, α7, β2, β4, α5 and β3 null mutant mice. Deletion of the α7 subunit completely and selectively eliminated [(125)I]α-bungarotoxin binding. The binding of [(125)I]α-conotoxinMII was eliminated in most brain regions by deletion of either the α6 or β2 subunit and is reduced by deletion of either the α4 or β3 subunit. The binding of 5[(125)I]A-85380 was completely eliminated by deletion of the β2 subunit and significantly reduced by deletion of the α4 subunit. Most, but not all, α4-independent sites require expression of the α6 subunit. The effect of gene deletion on total [(125)I]epibatidine binding was very similar to that on [(125)I]A-85380 binding. [(125)I]Epibatidine also labels β4* nAChR, which was readily apparent for incubations conducted in the presence of 100nM cytisine. The effects of α3 gene deletion could not be evaluated, but persistence of residual sites implies the expression of α3* nAChR. Taken together these results confirm and extend previously published evaluations of the effect of nAChR gene deletion and help to define the nAChR subtypes measurable by ligand binding.

Smoking, nicotine and visual plasticity: does what you know, tell you what you can see?

Nicotine exposure alters activity-dependent synaptic plasticity processes. Effects on learning and memory outcomes, and the synaptic changes that underlie them, are well-documented. Parallels in hippocampal and visual system pharmacology suggest that nicotine has the potential to alter activity-dependent structural organization in visual areas. Such alterations may contribute to deficits in visual performance reported in smoking exposed individuals.

Differential region-specific regulation of α4β2* nAChRs by self-administered and non-contingent nicotine in C57BL/6J mice

Neuronal nAChR upregulation is the hallmark of chronic nicotine exposure. Neuroplasticity to abused drugs, however, depends on whether their administration is forced by the experimenter or is under the control of the experimental animal. Neuroadaptation to chronic nicotine self-administration was examined with a yoked-control paradigm, using nose-poking as the operating procedure. Freely moving C57BL/6J mice that responded for 0.03 mg/kg/infusion of intravenous nicotine under a continuous schedule of reinforcement (FR-1), had control over the rate and amount of drug intake that a yoked littermate passively received (n = 11). The impact of response dependency on neurobiological changes in nicotinic and dopaminergic systems was subsequently assessed using quantitative autoradiography. Cytisine-sensitive [(125)I]epibatidine binding, [³H]SCH23390, [³H]raclopride and [³H]mazindol were used to label nAChRs with α4β2* subtype properties, D1 and D2 dopaminergic receptors, and dopamine transporters, respectively. During a period of 12 days, self-administration was reliably initiated and maintained in animals receiving response-contingent nicotine. Region specific changes in the density of α4β2* nAChRs were found to be dependent on the contingency of nicotine treatment. Higher levels of α4β2* receptor binding were observed in the dorsal lateral geniculate nucleus and the ventral tegmental area of self-administering mice, compared to non-contingent animals. Moreover, response-independent increases in D2 binding were observed following chronic nicotine administration. No change in D1 and DAT binding was observed among groups. These findings indicate regional specific alterations in the regulation of the nicotinic cholinergic system following contingent and non-contingent nicotine exposure, and underline the importance of response dependency on the development of nicotine addiction.

Cholinergic modulation of locomotion and striatal dopamine release is mediated by alpha6alpha4* nicotinic acetylcholine receptors

Dopamine (DA) release in striatum is governed by firing rates of midbrain DA neurons, striatal cholinergic tone, and nicotinic ACh receptors (nAChRs) on DA presynaptic terminals. DA neurons selectively express alpha6* nAChRs, which show high ACh and nicotine sensitivity. To help identify nAChR subtypes that control DA transmission, we studied transgenic mice expressing hypersensitive alpha6(L9'S)* receptors. alpha6(L9'S) mice are hyperactive, travel greater distance, exhibit increased ambulatory behaviors such as walking, turning, and rearing, and show decreased pausing, hanging, drinking, and grooming. These effects were mediated by alpha6alpha4* pentamers, as alpha6(L9'S) mice lacking alpha4 subunits displayed essentially normal behavior. In alpha6(L9'S) mice, receptor numbers are normal, but loss of alpha4 subunits leads to fewer and less sensitive alpha6* receptors. Gain-of-function nicotine-stimulated DA release from striatal synaptosomes requires alpha4 subunits, implicating alpha6alpha4beta2* nAChRs in alpha6(L9'S) mouse behaviors. In brain slices, we applied electrochemical measurements to study control of DA release by alpha6(L9'S) nAChRs. Burst stimulation of DA fibers elicited increased DA release relative to single action potentials selectively in alpha6(L9'S), but not WT or alpha4KO/alpha6(L9'S), mice. Thus, increased nAChR activity, like decreased activity, leads to enhanced extracellular DA release during phasic firing. Bursts may directly enhance DA release from alpha6(L9'S) presynaptic terminals, as there was no difference in striatal DA receptor numbers or DA transporter levels or function in vitro. These results implicate alpha6alpha4beta2* nAChRs in cholinergic control of DA transmission, and strongly suggest that these receptors are candidate drug targets for disorders involving the DA system.

Structural differences determine the relative selectivity of nicotinic compounds for native alpha 4 beta 2*-, alpha 6 beta 2*-, alpha 3 beta 4*- and alpha 7-nicotine acetylcholine receptors

Mammalian brain expresses multiple nicotinic acetylcholine receptor (nAChR) subtypes that differ in subunit composition, sites of expression and pharmacological and functional properties. Among known subtypes of receptors, alpha 4 beta 2* and alpha 6 beta 2*-nAChR have the highest affinity for nicotine (where * indicates possibility of other subunits). The alpha 4 beta 2*-nAChRs are widely distributed, while alpha 6 beta 2*-nAChR are restricted to a few regions. Both subtypes modulate release of dopamine from the dopaminergic neurons of the mesoaccumbens pathway thought to be essential for reward and addiction. alpha 4 beta 2*-nAChR also modulate GABA release in these areas. Identification of selective compounds would facilitate study of nAChR subtypes. An improved understanding of the role of nAChR subtypes may help in developing more effective smoking cessation aids with fewer side effects than current therapeutics. We have screened a series of nicotinic compounds that vary in the distance between the pyridine and the cationic center, in steric bulk, and in flexibility of the molecule. These compounds were screened using membrane binding and synaptosomal function assays, or recordings from GH4C1 cells expressing h alpha 7, to determine affinity, potency and efficacy at four subtypes of nAChRs found in brain, alpha 4 beta 2*, alpha 6 beta 2*, alpha 7 and alpha 3 beta 4*. In addition, physiological assays in gain-of-function mutant mice were used to assess in vivo activity at alpha 4 beta 2* and alpha 6 beta 2*-nAChRs. This approach has identified several compounds with agonist or partial agonist activity that display improved selectivity for alpha 6 beta 2*-nAChR.

Nicotinic receptor-based therapeutics and candidates for smoking cessation

Tobacco dependence is the most preventable cause of death and is a chronic, relapsing disorder in which compulsive tobacco use persists despite known negative health consequences. All currently available cessation agents (nicotine, varenicline and bupropion) have limited efficacy and are associated with high relapse rates, revealing a need for more efficacious, alternative pharmacotherapies. The major alkaloid in tobacco, nicotine, activates nicotinic receptors (nAChRs) which increase brain extracellular dopamine producing nicotine reward leading to addiction. nAChRs are located primarily presynaptically and modulate synaptic activity by regulating neurotransmitter release. Subtype-selective nAChR antagonists that block reward-relevant mesocorticolimbic and nigrostriatal dopamine release induced by nicotine may offer advantages over current therapies. An innovative approach is to provide pharmacotherapies which are antagonists at nAChR subtypes mediating nicotine evoked dopamine release. In addition, providing multiple medications with a wider array of targets and mechanisms should provide more treatment options for individuals who are not responsive to the currently available pharmacotherapies. This review summarizes the currently available smoking cessation therapies and discusses emerging potential therapeutic approaches employing pharmacological agents which act as antagonists at nicotinic receptors.

John Daly's compound, epibatidine, facilitates identification of nicotinic receptor subtypes

The diversity of nicotinic acetylcholine receptor (nAChR) subtypes was explored by measuring the effects of gene deletion and pharmacological diversity of epibatidine binding sites in mouse brain. All epibatidine binding sites require expression of either the alpha7, beta2, or beta4 subunit. In agreement with general belief, the alpha4beta2*-nAChR and alpha7-nAChR subtypes are major components of the epibatidine binding sites. alpha4beta2*-nAChR sites account for approximately 70% of total high- and low-affinity epibatidine binding sites, while alpha7-nAChR accounts for 16% of the total sites all of which have lower affinity for epibatidine. The other subtypes are structurally diverse. Although these minor subtypes account for only 14% of total binding in whole brain, they are expressed at relatively high concentrations in specific brain areas indicating unique functional roles.

Selectivity of ABT-089 for alpha4beta2* and alpha6beta2* nicotinic acetylcholine receptors in brain

Numerous pharmaceutical efforts have targeted neuronal nicotinic receptors (nAChRs) for amelioration of cognitive deficits. While alpha4beta2 and alpha7 are the more prominent nAChR in brain, other heteromeric nAChR can have important impact on agonist pharmacology. ABT-089 is a pioneer nAChR agonist found to enhance cognitive function with an exceptionally low incidence of adverse effects. To further investigate the mechanism of action of ABT-089, we evaluated its function in mouse brain preparations in which we have characterized the subunit composition of native nAChR. Among alpha4beta2*-nAChR, ABT-089 had partial agonist activity (7-23% of nicotine) and high selectivity for alpha4alpha5beta2 nAChR as evidenced by loss of activity in thalamus of alpha5(-/-) mice. ABT-089 stimulated [(3)H]-dopamine release (57%) exceeded the activity at alpha4beta2* nAChR, that could be explained by the activity at alpha6beta2* nAChR. The concentration-response relationship for ABT-089 stimulation of alpha6beta2* nAChR was biphasic. EC(50) and efficacy values for ABT-089, respectively, were 28 microM and 98% at the less sensitive alpha6beta2* nAChR and 0.11 microM and 36% at the more sensitive subtype (the most sensitive target for ABT-089 identified to date). ABT-089 had essentially no agonist or antagonist activity at concentrations <or=300 microM at alpha3beta4-nAChR measured by [(3)H]-acetylcholine release from interpeduncular nucleus. Thus, ABT-089 is a beta2* nAChR ligand with demonstrable agonist activity at alpha4beta2* and alpha6beta2* receptors. As one form of alpha6beta2* nAChR is sensitive to sub-muM concentrations, we propose that this receptor in particular may contribute to the enhanced cognitive performance following low doses of ABT-089.

Structural and functional diversity of native brain neuronal nicotinic receptors

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of ligand-gated ion channels present in the central and peripheral nervous systems, that are permeable to mono- and divalent cations. They share a common basic structure but their pharmacological and functional properties arise from the wide range of different subunit combinations making up distinctive subtypes. nAChRs are involved in many physiological functions in the central and peripheral nervous systems, and are the targets of the widely used drug of abuse nicotine. In addition to tobacco dependence, changes in their number and/or function are associated with neuropsychiatric disorders, ranging from epilepsy to dementia. Although some of the neural circuits involved in the acute and chronic effects of nicotine have been identified, much less is known about which native nAChR subtypes are involved in specific physiological functions and pathophysiological conditions. We briefly review some recent findings concerning the structure and function of native nAChRs, focusing on the subtypes identified in the mesostriatal and habenulo-interpeduncular pathways, two systems involved in nicotine reinforcement and withdrawal. We also discuss recent findings concerning the effect of chronic nicotine on the expression of native subtypes.

The road to discovery of neuronal nicotinic cholinergic receptor subtypes

The discovery that mammalian brain expresses the mRNAs for nine different nicotinic cholinergic receptor subunits (alpha2-alpha7, beta2-beta4) that form functional receptors when expressed in Xenopus laevis oocytes suggests that many different types of nicotinic cholinergic receptors (nAChRs) might be expressed in the mammalian brain., Using an historical approach, this chapter reviews some of the progress made in identifying the nAChR subtypes that seem to play a vital role in modulating dopaminergic function. nAChR subtypes that are expressed in dopamine neurons, as well as neurons that interact with dopamine neurons (glutamatergic, GABAergic), serve as the focus of this review. Subjects that are highlighted include the discovery of a low affinity alpha4beta2* nAChR, the identity of recently characterized alpha6* nAChRs, and the finding that these alpha6* receptors have the highest affinity for receptor activation of any of the native receptors that have been characterized to date. Topics that have been ignored in other recent reviews of this area, such as the discovery and potential importance of alternative transcripts, are presented along with a discussion of their potential importance.

Radioligand binding characterization of neuronal nicotinic acetylcholine receptors

Described in this unit are radioligand binding protocols for three neuronal nicotinic acetylcholine receptor (nAChR) subtypes. Detailed descriptions of binding protocols are presented for the two predominant CNS subtypes of nAChRs, α4β2 and α7, as well as the ganglionic α3β4 nAChR. [(3)H]Cytisine is utilized for α4β2 nAChRs, while [(3)H]methyllycaconitine is utilized for α7 nAChRs, both in rat brain. α3β4 nAChRs in IMR-32 cells are labeled with [(3)H]epibatidine. Reference data are presented, as well as commentary on the current state of neuronal nicotinic receptor research.

The alpha 3 subunit gene of the nicotinic acetylcholine receptor is a candidate gene for ethanol stimulation

Alcohol and nicotine are coabused, and preclinical and clinical data suggest that common genes may influence responses to both drugs. A gene in a region of mouse chromosome 9 that includes a cluster of three nicotinic acetylcholine receptor (nAChR) subunit genes influences the locomotor stimulant response to ethanol. The current studies first used congenic mice to confirm the influential gene on chromosome 9. Congenic F(2) mice were then used to more finely map the location. Gene expression of the three subunit genes was quantified in strains of mice that differ in response to ethanol. Finally, the locomotor response to ethanol was examined in mice heterozygous for a null mutation of the alpha 3 nAChR subunit gene (Chrna3). Congenic data indicate that a gene on chromosome 9, within a 46 cM region that contains the cluster of nAChR subunit genes, accounts for 41% of the genetic variation in the stimulant response to ethanol. Greater expression of Chrna3 was found in whole brain and dissected brain regions relevant to locomotor behavior in mice that were less sensitive to ethanol-induced stimulation compared to mice that were robustly stimulated; the other two nAChR subunit genes in the gene cluster (alpha 5 and beta 4) were not differentially expressed. Locomotor stimulation was not expressed on the genetic background of Chrna3 heterozygous (+/-) and wild-type (+/+) mice; +/- mice were more sensitive than +/+ mice to the locomotor depressant effects of ethanol. Chrna3 is a candidate gene for the acute locomotor stimulant response to ethanol that deserves further examination.

Alpha-conotoxin Arenatus IB[V11L,V16D] [corrected] is a potent and selective antagonist at rat and human native alpha7 nicotinic acetylcholine receptors

A recently developed alpha-conotoxin, alpha-conotoxin Arenatus IB-[V11L,V16D] (alpha-CtxArIB[V11L,V16D]) [corrected], is a potent and selective competitive antagonist at rat recombinant alpha7 nicotinic acetylcholine receptors (nAChRs), making it an attractive probe for this receptor subtype. alpha7 nAChRs are potential therapeutic targets that are widely expressed in both neuronal and non-neuronal tissues, where they are implicated in a variety of functions. In this study, we evaluate this toxin at rat and human native nAChRs. Functional alpha7 nAChR responses were evoked by choline plus the allosteric potentiator PNU-120596 [1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxazol-3-yl)-urea] in rat PC12 cells and human SH-SY5Y cells loaded with calcium indicators. alpha-CtxArIB[V11L,V16D] specifically inhibited alpha7 nAChR-mediated increases in Ca2+ in PC12 cells. Responses to other stimuli, 5-I-A-85380 [5-iodo-3-(2(S)-azetidinylmethoxy)pyridine dihydrochloride], nicotine, or KCl, that did not activate alpha7 nAChRs were unaffected. Human alpha7 nAChRs were also sensitive to alpha-CtxArIB[V11L, V16D]; acetylcholine-evoked currents in Xenopus laevis oocytes expressing human alpha7 nAChRs were inhibited by alpha-CtxArIB[V11L,V16D] (IC(50), 3.4 nM) in a slowly reversible manner, with full recovery taking 15 min. This is consistent with the time course of recovery from blockade of rat alpha7 nAChRs in PC12 cells. alpha-CtxArIB[V11L,V16D] inhibited human native alpha7 nAChRs in SHSY5Y cells, activated by either choline or AR-R17779 [(2)-spiro[1-azabicyclo[2.2.2]octane-3,59-oxazolidin]-29-one] plus PNU-120596. Rat brain alpha7 nAChRs contribute to dopamine release from striatal minces; alpha-CtxArIB[V11L,V16D] (300 nM) selectively inhibited choline-evoked dopamine release without affecting responses evoked by nicotine that activates heteromeric nAChRs. This study establishes that alpha-CtxArIB[V11L,V16D] selectively inhibits human and rat native alpha7 nAChRs with comparable potency, making this a potentially useful antagonist for investigating alpha7 nAChR functions.

Varenicline ameliorates ethanol-induced deficits in learning in C57BL/6 mice

Ethanol is a frequently abused drug that impairs cognitive processes such as learning. Varenicline, an alpha4beta2 nicotinic receptor partial agonist and alpha7 nicotinic receptor full agonist prescribed for smoking cessation, has been shown to decrease ethanol consumption. The current study investigated whether varenicline could ameliorate ethanol-induced deficits in learning and whether varenicline alters blood alcohol concentration in C57BL/6 mice. Conditioning consisted of two auditory conditioned stimulus (CS; 30s, 85dB white noise)-foot shock unconditioned stimulus (US; 2s, 0.57mA) pairings. For all studies, saline or ethanol (1.0, 1.5, 2.0g/kg i.p.) was administered 15min before training, and saline or varenicline (0.05, 0.1, 0.2mg/kg i.p.) was administered 60min before either training or testing. For blood alcohol analysis, saline or varenicline (0.1mg/kg) was administered 60min before collection, and saline or ethanol (1.0, 1.5, 2.0g/kg) was administered 15min before collection. Varenicline dose-dependently ameliorated ethanol-induced conditioning deficits for all three doses of ethanol when administered before training but not when administered 24h later, before testing. In addition, varenicline did not alter blood alcohol concentration. The smoking cessation aid varenicline may have therapeutic uses for treating ethanol-associated disruptions in cognitive processes.

High affinity binding of epibatidine to serotonin type 3 receptors

Epibatidine and mecamylamine are ligands used widely in the study of nicotinic acetylcholine receptors (nAChRs) in the central and peripheral nervous systems. In the present study, we find that nicotine blocks only 75% of (125)I-epibatidine binding to rat brain membranes, whereas ligands specific for serotonin type 3 receptors (5-HT(3)Rs) block the remaining 25%. (125)I-Epibatidine binds with a high affinity to native 5-HT(3)Rs of N1E-115 cells and to receptors composed of only 5-HT(3A) subunits expressed in HEK cells. In these cells, serotonin, the 5-HT(3)R-specific antagonist MDL72222, and the 5-HT(3)R agonist chlorophenylbiguanide readily competed with (125)I-epibatidine binding to 5-HT(3)Rs. Nicotine was a poor competitor for (125)I-epibatidine binding to 5-HT(3)Rs. However, the noncompetitive nAChR antagonist mecamylamine acted as a potent competitive inhibitor of (125)I-epibatidine binding to 5-HT(3)Rs. Epibatidine inhibited serotonin-induced currents mediated by endogenous 5-HT(3)Rs in neuroblastoma cell lines and 5-HT(3A)Rs expressed in HEK cells in a competitive manner. Our results demonstrate that 5-HT(3)Rs are previously uncharacterized high affinity epibatidine binding sites in the brain and indicate that epibatidine and mecamylamine act as 5-HT(3)R antagonists. Previous studies that depended on epibatidine and mecamylamine as nAChR-specific ligands, in particular studies of analgesic properties of epibatidine, may need to be reinterpreted with respect to the potential role of 5-HT(3)Rs.

Cytisine for smoking cessation: a research agenda

Cytisine has a molecular structure somewhat similar to that of nicotine and varenicline. The concept for the new smoking cessation drug varenicline was based partly on cytisine. Like varenicline, cytisine is a partial agonist of nicotinic acetylcholine receptors, with high affinity for alpha4beta2 receptors. Cytisine has been used since the 1960s as a smoking cessation drug in Eastern and Central Europe, but has remained largely unnoticed elsewhere. Three placebo-controlled trials, conducted in East and West Germany in the 1960s and 1970s, suggest that cytisine, even with minimal behavioural support, may be effective in aiding smoking cessation. Cytisine tablets are very inexpensive to produce and could be a more affordable treatment than nicotine replacement, bupropion and varenicline. There is however a dearth of scientific research on the properties of cytisine, including safety, abuse liability and efficacy. This paper seeks to identify research priorities for molecular, animal and clinical studies. In particular, new studies are necessary to define the nicotinic receptor interaction profile of cytisine, to establish its pharmacokinetics and pharmacodynamics in humans, to determine whether animals self-administer cytisine, and to ascertain whether cytisine is safe and effective as a smoking cessation drug. Potentially, this research effort, contributing to wider use of an inexpensive drug, could save many lives.

Nicotine improves cognitive deficits of dopamine transporter knockout mice without long-term tolerance

Various studies suggest a dysfunction of nicotinic neurotransmission in schizophrenia and establish that patients suffering from schizophrenia and attention deficit hyperactivity disorder (ADHD) have a high tobacco consumption, potentially for the purpose of self-medication. Owing to its neuroprotective and procognitive effects, transdermal nicotine was proposed to be an effective treatment of some neurodegenerative and psychiatric diseases. Mice deficient in the dopamine transporter (DAT KO) exhibit a phenotype reminiscent of schizophrenia and ADHD, including hyperdopaminergia, hyperactivity, paradoxical calming by methylphenidate and cognitive deficits, some of which being improved by antipsychotic agents. We recently demonstrated that nicotinic receptor content and function were profoundly modified in DAT KO mice. In this study, we assessed the effects of a chronic nicotine treatment in the drinking water on the nicotine-induced locomotion, anxiety status and learning performance. Chronically nicotine-treated DAT KO mice were always hypersensitive to the hypolocomotor effect of nicotine without tolerance and did not exhibit the anxiogenic effect of nicotine treatment observed in WT mice. Very interestingly, both acute and chronic nicotine treatments greatly improved their deficits in the cued and spatial learning, without eliciting tolerance. We speculate that the procognitive effects of nicotine in DAT KO mice are related to the upregulation of alpha7 nicotinic receptors in the hippocampus, amygdala, and prelimbic cortex, all areas involved in cognition. Data from our studies on DAT KO mice shed light on the nicotine self-medication in psychiatric patients and suggest that nicotinic agonists could favorably lead to additional therapy of psychiatric diseases.

Nicotine-induced dystonic arousal complex in a mouse line harboring a human autosomal-dominant nocturnal frontal lobe epilepsy mutation

We generated a mouse line harboring an autosomal-dominant nocturnal frontal lobe epilepsy (ADNFLE) mutation: the alpha4 nicotinic receptor S248F knock-in strain. In this mouse, modest nicotine doses (1-2 mg/kg) elicit a novel behavior termed the dystonic arousal complex (DAC). The DAC includes stereotypical head movements, body jerking, and forelimb dystonia; these behaviors resemble some core features of ADNFLE. A marked Straub tail is an additional component of the DAC. Similar to attacks in ADNFLE, the DAC can be partially suppressed by the sodium channel blocker carbamazepine or by pre-exposure to a very low dose of nicotine (0.1 mg/kg). The DAC is centrally mediated, genetically highly penetrant, and, surprisingly, not associated with overt ictal electrical activity as assessed by (1) epidural or frontal lobe depth-electrode electroencephalography or (2) hippocampal c-fos-regulated gene expression. Heterozygous knock-in mice are partially protected from nicotine-induced seizures. The noncompetitive antagonist mecamylamine does not suppress the DAC, although it suppresses high-dose nicotine-induced wild-type-like seizures. Experiments on agonist-induced 86Rb+ and neurotransmitter efflux from synaptosomes and on alpha4S248Fbeta2 receptors expressed in oocytes confirm that the S248F mutation confers resistance to mecamylamine blockade. Genetic background, gender, and mutant gene expression levels modulate expression of the DAC phenotype in mice. The S248F mouse thus appears to provide a model for the paroxysmal dystonic element of ADNFLE semiology. Our model complements what is seen in other ADNFLE animal models. Together, these mice cover the spectrum of behavioral and electrographic events seen in the human condition.

Multiple brain pathways and receptors underlying tobacco addiction

Over the last 20 years much progress has been made in understanding the pharmacologic basis of tobacco addiction. In particular, the role of nicotine in reinforcing smoking behavior has been studied from a variety of perspectives. This article discusses two important aspects of this topic: (1) brain pathways underlying tobacco addiction; and (2) the actions of nicotine at nicotinic cholinergic receptors. Recent evidence will be reviewed indicating that nicotine reinforces smoking behavior by acting on more than one subtype of nicotinic receptor. Similarly, the role of several brain pathways in tobacco addiction will be considered. Tobacco addiction may thus be seen as a complex neuropsychopharmacological disorder; further progress in smoking cessation treatment may require that we address the multiple molecular and brain components of this addiction.

The subtypes of nicotinic acetylcholine receptors on dopaminergic terminals of mouse striatum

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]-alpha-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 alpha-conotoxin MII (alpha-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 alpha6 subunit (alpha4alpha6beta2beta3, alpha6beta2beta3, alpha6beta2) and bind alpha-CtxMII with high affinity. One of these three subtypes (alpha4alpha6beta2beta3) 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 alpha-CtxMII (alpha4beta2, alpha4alpha5beta2) are somewhat more numerous than the alpha6* 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.

Cell autonomy, receptor autonomy, and thermodynamics in nicotine receptor up-regulation

Chronic nicotine exposure, in smokers or in experimental rodents administered nicotine, produces elevated levels of nicotinic acetylcholine receptors in several brain regions. However, there are few data on up-regulation of receptors in specific neuronal subtypes. We tested whether functional up-regulation of nicotinic responses occurs in cultured GABAergic neurons of the ventral midbrain. Fura-2 measurements of nicotinic responses were made on ventral midbrain neurons from knock-in mice heterozygous for the alpha4-M2 domain Leu9'Ala mutation, which confers nicotine hypersensitivity. Chronic nicotine exposure at a concentration (10 nM for 3 days) that activates only the hypersensitive alpha4* (Leu9'Ala) receptors, but not wild-type receptors, resulted in significant potentiation of ACh (100 microM)-elicited responses. Experiments were also performed on midbrain neuronal cultures heterozygous for the alpha4* (Leu9'Ala) mutation as well as for a GFP protein fused to a GABA transporter that reliably reveals GABAergic neurons. In cultures chronically treated with 10nM nicotine, there was significantly increased alpha4* nicotinic-induced Ca(2+) influx elicited by low concentration of ACh (3 microM). Furthermore, chronic exposure to the competitive antagonist dihydro-beta-erythroidine, but not to the noncompetitive antagonist mecamylamine, induced up-regulation of ACh elicited nicotinic responses. These results suggest that occupation of alpha4* nicotinic receptor binding site(s), at the interface between two subunits, is sufficient to promote assembly and/or up-regulation of functional receptors in GABAergic neurons. Up-regulation in neurons is both "cell-autonomous", occurring at the cell itself, and "receptor autonomous", occurring at the receptor itself, and may be a thermodynamic necessity of ligand-protein interactions.

Gene targeting demonstrates that alpha4 nicotinic acetylcholine receptor subunits contribute to expression of diverse [3H]epibatidine binding sites and components of biphasic 86Rb+ efflux with high and low sensitivity to stimulation by acetylcholine

[3H]Epibatidine binds to nAChR subtypes in mouse brain with higher (KD approximately 0.02 nM) and lower affinity (KD approximately 7 nM), which can be further subdivided through inhibition by selected agonists and antagonists. These subsets are differentially affected by targeted deletion of alpha7, beta2 or beta4 subunits. Most, but not all, higher and lower affinity binding sites require beta2 (Marks, M.J., Whiteaker, P., Collins, A.C., 2006. Deletion of the alpha7, beta2 or beta4 nicotinic receptor subunit genes identifies highly expressed subtypes with relatively low affinity for [3H]epibatidine. Mol. Pharmacol. 70, 947-959). Effects of functional alpha4 gene deletion are reported here. Deletion of alpha4 virtually eliminated cytisine-sensitive, higher-affinity [3H]epibatidine binding as did beta2 deletion, confirming that these sites are alpha4beta2*-nAChR. Cytisine-resistant, higher-affinity [3H]epibatidine binding sites are diverse and some of these sites require alpha4 expression. Lower affinity [3H]epibatidine binding sites are also heterogeneous and can be subdivided into alpha-bungarotoxin-sensitive and -resistant components. Deleting alpha4 did not affect the alpha-bungarotoxin-sensitive component, but markedly reduced the alpha-bungarotoxin-resistant component. This effect was similar, but not quite identical, to the effect of beta2 deletion. This provides the first evidence that lower-affinity epibatidine binding sites in the brain require expression of alpha4 subunits. The effects of alpha4 gene targeting on receptor function were measured using a 86Rb+ efflux assay. Concentration-effect curves for ACh-stimulated 86Rb+ efflux are biphasic (EC50 values=3.3 microM and 300 microM). Targeting alpha4 produced substantial gene-dose dependent reductions in both phases in whole brain and in most of the 14 brain regions assayed. These effects are very similar to those following deletion of beta2. Thus, alpha4beta2*-nAChRs mediate a significant fraction of both phases of ACh stimulated 86Rb+ efflux.

Sustained increase of alpha7 nicotinic receptors and choline-induced improvement of learning deficit in STOP knock-out mice

Mice deficient in the microtubule stabilizing protein STOP (stable tubule only polypeptide) show synaptic plasticity anomalies in hippocampus, dopamine hyper-reactivity in the limbic system and severe behavioral deficits. Some of these disturbances are alleviated by long-term antipsychotic treatment. Therefore, this mouse line represents a pertinent model for some aspects of schizophrenia symptomatology. Numerous data support dysfunction of nicotinic neurotransmission in schizophrenia and epidemiological studies show increased tobacco use in schizophrenic patients, in whom nicotine has been reported to improve cognitive deficits and impairment in sensory gating. In this study, we examined potential alterations in cholinergic (ACh) and nicotinic components and functions in STOP mutant mice. STOP KO mice displayed no variation of the density of ACh esterase and beta2* nicotinic receptors (nAChRs), large reductions in the density of vesicular ACh transporter and alpha6* nAChRs and marked increases in the density of alpha7 nAChRs, in some brain areas. STOP KO mice were hypersensitive to the stimulating locomotor effect of nicotine and, interestingly, their impaired performance in learning the cued version of the water maze were improved by administration of the preferential alpha7 nAChR agonist choline. Altogether, our data show that the deletion of the ubiquitous STOP protein elicited restricted alterations in ACh components. They also suggest that nicotinic neurotransmission can be deficient in STOP KO mice and that mutant mice can represent a meaningful model to study some nicotinic dysfunctions and therapeutic treatments.

Genetic Approaches Identify Differential Roles for α4β2*Nicotinic Receptors in Acute Models of Antinociception in Mice

The effects of nicotine on the tail-flick and hot-plate tests were determined to identify nicotinic receptor subtypes responsible for spinally and supraspinally mediated nicotine analgesia in knockin mice expressing hypersensitive alpha(4) nicotinic receptors (L9'S), in seven inbred mouse strains (C57BL/6, DBA/2, A/2, CBA/2, BALB/cByJ, C3H/HeJ, and 129/SvEv), and in two F1 hybrids (B6CBAF1 and B6D2F1). L9'S heterozygotes were approximately 6-fold more sensitive to the antinociceptive effects of nicotine than the wild-type controls in the hot-plate test but not in the tail-flick assay. Large differences in the effects of nicotine were also observed with both tests for the seven mouse strains. A/J and 129 mice were 6- to 8-fold more sensitive than CBA and BALB mice. In addition, B6CBAF1 hybrid mice were even less sensitive than CBA mice. Nicotinic binding sites were measured in three spinal cord regions and the hindbrain of the inbred strains. Significant differences in cytisine-sensitive, high affinity [(125)I]epibatidine binding site levels (alpha(4)beta(2)(*) subtypes), but not in (125)I-alpha-bungarotoxin binding (alpha(7)(*) subtypes), were observed. Significant negative correlations between cytisine-sensitive [(125)I]epibatidine binding and nicotine ED(50) for both tests were noted. Our results indicate that alpha(4)beta(2)(*) acetylcholine nicotinic receptors (nAChR) are important in mediating nicotine analgesia in supraspinal responses, while also showing that alpha(4)beta(2)(*)-nAChR and at least one other nAChR subtype appear to modulate spinal actions.

Functional alterations of nicotinic neurotransmission in dopamine transporter knock-out mice

Mice lacking the dopamine (DA) transporter (DAT) gene exhibit a phenotype reminiscent of schizophrenia and attention deficit hyperactivity disorder (ADHD), including hyperDAergia, hyperactivity and deficits in cognitive performance, which are alleviated by antipsychotic agents. Numerous studies suggest a dysfunction of nicotinic neurotransmission in schizophrenia and show increased tobacco intake in schizophrenic and ADHD patients, possibly as a self-medication. Thus, we examined the potential alteration of nicotinic neurotransmission in DAT knock-out (KO) mice. We showed that constitutively hyperDAergic DAT KO mice exhibited modifications in nicotinic receptor density in an area- and subtype-dependent manner. In some DAergic areas, the small decrease in the beta2* nicotinic subunit (nAChR) density contrasted with the higher decrease and increase in the alpha6* and alpha7 nAChR densities, respectively. Mutant mice were hypersensitive to the stimulant locomotor effects of nicotine at low doses, probably due to enhanced nicotine-induced extracellular DA level. They also showed hypersensitivity to the hypolocomotion induced by nicotine. In contrast, no hypersensitivity was observed for other nicotine-induced behavioral effects, such as anxiety or motor activity in the elevated plus maze. Co-administration of nicotinic agonists at sub-active doses elicited opposite locomotor effects in wild-type and DAT KO mice, as reported previously for methylphenidate. Interestingly, such a co-administration of nicotinic agonists induced synergistic hypolocomotion in DAT KO mice. These findings show that a targeted increase of DA tone can be responsible for significant adaptations of the cholinergic/nicotinic neurotransmission. This study may provide potential leads for the use of nicotine or combined nicotinic agonists for the therapy of psychiatric disorders.