Nicotinic acetylcholine receptors
(nAChRs) have been investigated
for developing drugs that can potentially treat various central nervous
system disorders. Considerable evidence supports the hypothesis that
modulation of the cholinergic system through activation and/or desensitization/inactivation
of nAChR holds promise for the development of new antidepressants.
The introductory portion of this Miniperspective discusses the basic
pharmacology that underpins the involvement of α4β2-nAChRs
in depression, along with the structural features that are essential
to ligand recognition by the α4β2-nAChRs. The remainder
of this Miniperspective analyzes reported nicotinic ligands in terms
of drug design considerations and their potency and selectivity, with
a particular focus on compounds exhibiting antidepressant-like effects
in preclinical or clinical studies. This Miniperspective aims to provide
an in-depth analysis of the potential for using nicotinic ligands
in the treatment of depression, which may hold some promise in addressing
an unmet clinical need by providing relief from depressive symptoms
in refractory patients.
The nicotinic acetylcholine receptor (nAChR) β3 subunit is thought to serve an accessory role in nAChR subtypes expressed in dopaminergic regions implicated in drug dependence and reward. When β3 subunits are expressed in excess, they have a dominant-negative effect on function of selected nAChR subtypes. In this study, we show, in Xenopus oocytes expressing α2, α3 or α4 plus either β2 or β4 subunits, that in the presumed presence of similar amounts of each nAChR subunit, co-expression with wild-type β3 subunits generally (except for α3*-nAChR) lowers amplitudes of agonist-evoked, inward peak currents by 20–50% without having dramatic effects (≤ 2-fold) on agonist potencies. By contrast, co-expression with mutant β3V9'S subunits generally (except for α4β2*-nAChR) increases agonist potencies, consistent with an expected gain-of-function effect. This most dramatically demonstrates formation of complexes containing three kinds of subunit. Moreover, for oocytes expressing nAChR containing any α subunit plus β4 and β3V9'S subunits, there is spontaneous channel opening sensitive to blockade by the open channel blocker, atropine. Collectively, the results indicate that β3 subunits integrate into all of the studied receptor assemblies and suggest that natural co-expression with β3 subunits can influence levels of expression and agonist sensitivities of several nAChR subtypes.
ligand-gated ion channel; nicotinic acetylcholine receptor(s); receptor structure-function
A cytosine to thymidine (C→T) missense mutation in the signal peptide (SP) sequence (rs2472553) of the nicotinic acetylcholine receptor (nAChR) α2 subunit produces a threonine-to-isoleucine substitution (T22I) often associated with nicotine dependence (ND). We assessed effects on function of α2*-nAChR (‘*’indicates presence of additional subunits) of this mutation, which could alter SP cleavage, RNA/protein secondary structure, and/or efficiency of transcription, translation, subunit assembly, receptor trafficking or cell surface expression. Two-electrode voltage clamp analyses indicate peak current responses to ACh or nicotine are decreased 2.8–5.8-fold for putative low sensitivity (LS; 10:1 ratio of α:β subunit cRNAs injected) α2β2- or α2β4- nAChR and increased for putative high sensitivity (HS; 1:10 α:β subunit ratio) α2β2- (5.7–15-fold) or α2β4- (1.9–2.2-fold) nAChR as a result of the mutation. Agonist potencies are decreased 1.6–4-fold for putative LS or HS α2(T22I)β2-nAChR or for either α2*-nAChR subtype formed in the presence of equal amounts of subunit cRNA, slightly decreased for LS α2(T22I)β4-nAChR, but increased 1.4–2.4-fold for HS α2(T22I)β4-nAChR relative to receptors containing wild-type α2 subunits. These effects suggest that the α2 subunit SP mutation generally favors formation of LS receptor isoforms. We hypothesize that lower sensitivity of human α2*-nAChR to nicotine could contribute to increased susceptibility to ND. To our knowledge this is the first report of a SP mutation having a functional effect in a member of cys-loop family of ligand-gated ion channels.
Nicotinic acetylcholine receptor; signal peptide; single nucleotide polymorphism; missense mutation; receptor structure-function; electrophysiology
A considerable number of in vivo studies have demonstrated that the cholinergic system can dampen the peripheral immune response, and it is thought that the α7-nicotinic acetylcholine receptor (nAChR) subtype is a key mediator of this process. The goal of the present study was to determine if nicotine modulates immunological mechanisms known to be involved in the development of experimental autoimmune encephalomyelitis (EAE), a mouse model for CNS autoimmune disease, via α7-nAChRs. Here we show that nicotine exposure attenuates EAE severity and that this effect is largely abolished in nAChR α7 subunit knock-out mice. However, nicotine exposure partially retains the ability to reduce lymphocyte infiltration into the CNS, inhibit auto-reactive T cell proliferation and helper T cell cytokine production, down-regulate co-stimulatory protein expression on myeloid cells, and increase the differentiation and recruitment of regulatory T cells, even in the absence of α7-nAChRs. Diverse effects of nicotine on effector and regulatory T cells, as well as antigen presenting cells, may be linked to differential expression patterns of nAChR subunits across these cell types. Taken together, our data show that although α7-nAChRs indeed seem to play an important role in nicotine-conferred reduction of the CNS inflammatory response and protection against EAE, other nAChR subtypes also are involved in the anti-inflammatory properties of the cholinergic system.
Preclinical and clinical studies demonstrated that the inhibition of cholinergic supersensitivity through nicotinic antagonists and partial agonists can be used successfully to treat depressed patients, especially those who are poor responders to selective serotonin reuptake inhibitors (SSRIs). In our effort to develop novel antidepressant drugs, LF-3-88 was identified as a potent nicotinic acetylcholine receptor (nAChR) partial agonist with subnanomolar to nanomolar affinities for β2-containing nAChRs (α2β2, α3β2, α4β2, and α4β2*) and superior selectivity away from α3β4 − (Ki > 104 nmol/L) and α7-nAChRs (Ki > 104 nmol/L) as well as 51 other central nervous system (CNS)-related neurotransmitter receptors and transporters. Functional activities at different nAChR subtypes were characterized utilizing 86Rb+ ion efflux assays, two-electrode voltage-clamp (TEVC) recording in oocytes, and whole-cell current recording measurements. In mouse models, administration of LF-3-88 resulted in antidepressive-like behavioral signatures 15 min post injection in the SmartCube® test (5 and 10 mg/kg, i.p.; about 45-min session), decreased immobility in the forced swim test (1–3 mg/kg, i.p.; 1–10 mg/kg, p.o.; 30 min pretreatment, 6-min trial), and decreased latency to approach food in the novelty-suppressed feeding test after 29 days chronic administration once daily (5 mg/kg but not 10 mg/kg, p.o.; 15-min trial). In addition, LF-3-88 exhibited a favorable profile in pharmacokinetic/ADME-Tox (absorption, distribution, metabolism, excretion, and toxicity) assays. This compound was also shown to cause no mortality in wild-type Balb/CJ mice when tested at 300 mg/kg. These results further support the potential of potent and selective nicotinic partial agonists for use in the treatment of depression.
Antidepressive-like behavior; nicotinic acetylcholine receptor; partial agonist; selectivity
A 3-pyridyl ether scaffold bearing a cyclopropane-containing side chain was recently identified in our efforts to create novel antidepressants that act as partial agonists at α4β2-nicotinic acetylcholine receptors. In this study, a systematic structure-activity relationship investigation was carried out on both the azetidine moiety present in compound 3 and its right-hand side chain, thereby discovering a variety of novel nicotinic ligands that retain bioactivity and feature improved chemical stability. The most promising compounds 24, 26, and 30 demonstrated comparable or enhanced pharmacological profiles compared to the parent compound 4, and the N-methylpyrrolidine analogue 26 also exhibited robust antidepressant-like efficacy in the mouse forced swim test. The favorable ADMET profile and chemical stability of 26 further indicate this compound to be a promising lead as a drug candidate warranting further advancement down the drug discovery pipeline.
In our continued efforts to develop α4β2-nicotinic acetylcholine receptor (nAChR) partial agonists as novel antidepressants having a unique mechanism of action, structure activity relationship (SAR) exploration of certain isoxazolylpyridine ethers is presented. In particular, modifications to both the azetidine ring present in the starting structure 4 and its metabolically liable hydroxyl side chain substituent have been explored to improve compound druggability. The pharmacological characterization of all new compounds has been carried out using [3H]epibatidine binding studies together with functional assays based on 86Rb+ ion flux measurements. We found that the deletion of the metabolically liable hydroxyl group or its replacement by a fluoromethyl group not only maintained potency and selectivity, but also resulted in compounds showing antidepressant-like properties in the mouse forced swim test. These isoxazolylpyridine ethers appear to represent promising lead candidates in the design of innovative chemical tools containing reporter groups for imaging purposes and of possible therapeutics.
There is a significantly elevated incidence of epilepsy in Alzheimer's disease (AD). Moreover, there is neural hyperexcitation/synchronization in transgenic mice expressing abnormal levels or forms of amyloid precursor protein and its presumed, etiopathogenic product, amyloid-β1–42 (Aβ). However, the underlying mechanisms of how Aβ causes neuronal hyperexcitation remain unclear. Here, we report that exposure to pathologically relevant levels of Aβ induces Aβ form-dependent, concentration-dependent, and time-dependent neuronal hyperexcitation in primary cultures of mouse hippocampal neurons. Similarly, Aβ exposure increases levels of nicotinic acetylcholine receptor (nAChR) α7 subunit protein on the cell surface and α7-nAChR function, but not α7 subunit mRNA, suggesting post-translational upregulation of functional α7-nAChRs. These effects are prevented upon coexposure to brefeldin A, an inhibitor of endoplasmic reticulum-to-Golgi protein transport, consistent with an effect on trafficking of α7 subunits and assembled α7-nAChRs to the cell surface. Aβ exposure-induced α7-nAChR functional upregulation occurs before there is expression of neuronal hyperexcitation. Pharmacological inhibition using an α7-nAChR antagonist or genetic deletion of nAChR α7 subunits prevents induction and expression of neuronal hyperexcitation. Collectively, these results, confirmed in studies using slice cultures, indicate that functional activity and perhaps functional upregulation of α7-nAChRs are necessary for production of Aβ-induced neuronal hyperexcitation and possibly AD pathogenesis. This novel mechanism involving α7-nAChRs in mediation of Aβ effects provides potentially new therapeutic targets for treatment of AD.
Structure-based drug design can potentially accelerate the development of new therapeutics. In this study, a co-crystal structure of the acetylcholine binding protein (AChBP) from Capitella teleta (Ct) in complex with a cyclopropane-containing, selective α4β2-nicotinic acetylcholine receptor (nAChR) partial agonist (compound 5) was acquired. The structural determinants required for ligand binding obtained from this AChBP X-ray structure were used to refine our previous model of the human α4β2-nAChR, thus possibly providing a better understanding of the structure of the human receptor. In order to validate the potential application of the structure of the Ct-AChBP in the engineering of new α4β2-nAChR ligands, homology modeling methods, combined with in silico ADME calculations, were used to design analogs of compound 5. The most promising compound 12, exhibited an improved metabolic stability in comparison to the parent compound 5 while retaining favorable pharmacological parameters together with appropriate behavioral endpoints in the rodent studies.
Nicotine is a potent inhibitor of the immune response and is protective against experimental autoimmune encephalomyelitis (EAE). Initial studies suggested that the cholinergic system modulates inflammation via the α7-nicotinic acetylcholine receptor (nAChR) subtype. We recently have shown that effector T cells and myeloid cells constitutively express mRNAs encoding nAChR α9 and β2 subunits and found evidence for immune system roles for non-α7-nAChRs. In the present study, we assessed the effects of nAChR α9 or β2 subunit gene deletion on EAE onset and severity, with or without nicotine treatment. We report again that disease onset is delayed and severity is attenuated in nicotine-treated, wild-type mice, an effect that also is observed in α9 subunit knock-out (KO) mice irrespective of nicotine treatment. On the other hand, β2 KO mice fail to recover from peak measures of disease severity regardless of nicotine treatment, despite retaining sensitivity to nicotine’s attenuation of disease severity. Prior to disease onset, we found significantly less reactive oxygen species production in the CNS of β2 KO mice, elevated proportions of CNS myeloid cells but decreased ratios of CNS macrophages/microglia in α9 or β2 KO mice, and some changes in iNOS, TNF-α and IL-1β mRNA levels in α9 KO and/or β2 KO mice. Our data thus suggest that β2*- and α9*-nAChRs, in addition to α7-nAChRs, play different roles in endogenous and nicotine-dependent modulation of immune functions and could be exploited as therapeutic targets to modulate inflammation and autoimmunity.
auto-immunity; cholinergic anti-inflammatory pathway; experimental autoimmune encephalomyelitis; inflammation; multiple sclerosis; nicotinic acetylcholine receptors
Systemic administration of nicotine increases dopaminergic (DA) neuron firing in the ventral tegmental area (VTA), which is thought to underlie nicotine reward. Here, we report that the medial prefrontal cortex (mPFC) plays a critical role in nicotine-induced excitation of VTA DA neurons. In chloral hydrate-anesthetized rats, extracellular single-unit recordings showed that VTA DA neurons exhibited two types of firing responses to systemic nicotine. After nicotine injection, the neurons with type-I response showed a biphasic early inhibition and later excitation, whereas the neurons with type-II response showed a monophasic excitation. The neurons with type-I, but not type-II, response exhibited pronounced slow oscillations (SO) in firing. Pharmacological or structural mPFC inactivation abolished SO and prevented systemic nicotine-induced excitation in the neurons with type-I, but not type-II, response, suggesting that these VTA DA neurons are functionally coupled to the mPFC and nicotine increases firing rate in these neurons in part through the mPFC. Systemic nicotine also increased the firing rate and SO in mPFC pyramidal neurons. mPFC infusion of a non-α7 nAChR antagonist mecamylamine blocked the excitatory effect of systemic nicotine on the VTA DA neurons with type-I response, but mPFC infusion of nicotine failed to excite these neurons. These results suggest that nAChR activation in the mPFC is necessary, but not sufficient, for systemic nicotine-induced excitation of VTA neurons. Finally, systemic injection of bicuculline prevented nicotine-induced firing alterations in the neurons with type-I response. We propose that the mPFC plays a critical role in systemic nicotine-induced excitation of VTA DA neurons.
nicotine; prefrontal cortex; ventral tegmental area; dopamine neuron; in vivo recording; slow oscillation
There is considerable evidence to support the hypothesis that the blockade of nAChR is responsible for the antidepressant action of nicotinic ligands. The nicotinic acetylcholine receptor (nAChR) antagonist, mecamylamine, has been shown to be an effective add-on in patients that do not respond to selective serotonin reuptake inhibitors. This suggests that nAChR ligands may address an unmet clinical need by providing relief from depressive symptoms in refractory patients. In this study, a new series of nAChR ligands based on an isoxazole-ether scaffold have been designed and synthesized for binding and functional assays. Preliminary structure-activity relationship (SAR) efforts identified a lead compound 43, which possesses potent antidepressant-like activity (1 mg/kg, IP; 5 mg/kg, PO) in the classical mouse forced swim test. Early stage absorption, distribution, metabolism, excretion, and toxicity (ADME-Tox) studies also suggested favorable drug-like properties, and broad screening towards other common neurotransmitter receptors indicated that compound 43 is highly selective for nAChRs over the other 45 neurotransmitter receptors and transporters tested.
Despite their discovery in the early 20th century and intensive study over the last twenty years, nicotinic acetylcholine receptors (nAChRs) are still far from being well understood. Only a few chemical entities targeting nAChRs are currently undergoing clinical trials, and even fewer have reached the marketplace. In our efforts to discover novel and truly selective nAChR ligands, we designed and synthesized a series of chiral cyclopropane-containing α4β2-specific ligands that display low nanomolar binding affinities and excellent subtype selectivity, while acting as partial agonists at α4β2-nAChRs. Their favorable antidepressant-like properties were demonstrated in the classical mouse forced swim test. Preliminary ADMET studies and broad screening towards other common neurotransmitter receptors were also carried out to further evaluate their safety profile and eliminate their potential off-target activity. These highly potent cyclopropane ligands possess superior subtype selectivity compared to other α4β2-nAChR agonists reported to date, including the marketed drug varenicline, and therefore may fully satisfy the crucial prerequisite for avoiding adverse side effects. These novel chemical entities could potentially be advanced to the clinic as new drug candidates for treating depression.
Depression, a common neurological condition, is one of the leading causes of disability and suicide worldwide. Standard treatment targeting monoamine transporters selective for the neurotransmitters serotonin and noradrenalin are not able to help many patients that are poor responders. This study advances the development of sazetidine-A analogs that interact with α4β2-nAChR as partial agonists and that possess favorable antidepressant profiles. The resulting compounds that are highly selective for the α4β2 subtype of nAChR over α3β4-nAChRs are partial agonists at the α4β2 subtype and have excellent antidepressant behavioral profiles as measured by the mouse forced swim test. Preliminary ADMET studies for one promising ligand revealed an excellent plasma protein binding (PPB) profile, low CYP450 related metabolism, and low cardiovascular toxicity, suggesting it is a promising lead as well as a drug candidate to be advanced through the drug discovery pipeline.
Nicotinic acetylcholine receptors (nAChRs) warrant attention, as they play many critical roles in brain and body function and have been implicated in a number of neurological and psychiatric disorders, including nicotine dependence. nAChRs are composed as diverse subtypes containing specific combinations of genetically-distinct subunits and that have different functional properties, distributions, and pharmacological profiles. There had been confidence that the rules that define ranges of assembly partners for specific subunits were well-established, especially for the more prominent nAChR subtypes. However, we review here some newer findings indicating that nAChRs having largely the same, major subunits exist as isoforms with unexpectedly different properties. Moreover, we also summarize our own studies indicating that novel nAChR subtypes exist and/or have distributions not heretofore described. Importantly, the nAChRs that exist as new isoforms or subtypes or have interesting distributions require alteration in thinking about their roles in health and disease.
nicotine; nicotinic receptor; acetylcholine; Alzheimer’s disease; drug dependence
In women, medroxyprogesterone acetate (MPA) is the most commonly used progestin component of HT. In vitro, MPA negatively impacts markers of neuronal health and exacerbates experimentally-induced neurotoxicity. There is in vitro evidence that these factors are driven by GABAergic and neurotrophic systems. Whether these effects translate to a negative impact on brain function has not been tested in vivo, clinically or preclinically. Here we evaluate the mnemonic and neurobiological effects of MPA in the surgically menopausal rat. Aged ovariectomized (OVX) rats were given subcutaneous vehicle, natural progesterone, low-dose MPA or high-dose MPA. Multiple cognitive domains were analyzed via the water radial-arm maze (WRAM), and Morris maze (MM). Cognitive brain regions were assayed for changes in the GABAergic system by evaluating GAD protein, the synthesizing enzyme for GABA, and neurotrophins. On the WRAM, both progestin types impaired learning. Further, high-dose MPA impaired delayed memory retention on the WRAM, and exacerbated overnight forgetting on the MM. While neurotrophins were not affected by progesterone or MPA treatment, both progestin types altered GAD levels. MPA significantly and progesterone marginally decreased GAD levels in the hippocampus, and both MPA and progesterone significantly increased GAD levels in the entorhinal cortex. These findings suggest that MPA, the most commonly used progestin in HT, is detrimental to learning and two types of memory, and modulates the GABAergic system in cognitive brain regions, in aged menopausal rats. These findings, combined with in vitro evidence that MPA is detrimental to neuronal health, indicates that MPA has negative effects for brain health and function.
Hormone Therapy; Cognition; Progestins; Menopause; Aging; Learning and Memory
There is a need for different and better aids to tobacco product use cessation. Useful smoking cessation aids, bupropion (2) and varenicline (3), share some chemical features with 3-phenyltropanes (4), which have promise in cocaine dependence therapy. Here we report studies to generate and characterize pharmacodynamic features of 3-phenyltropane analogues. These studies extend our work on the multiple molecular target model for aids to smoking cessation. We identified several new 3-phenyltropane analogues that are superior to 2 in inhibition of dopamine, norepinephrine, and sometimes serotonin reuptake. All of these ligands also act as inhibitors of nicotinic acetylcholine receptor (nAChR) function with a selectivity profile that favors, like 2, inhibition of α3β4*-nAChR. Many of these ligands also block acute effects of nicotine-induced antinociception, locomotor activity, and hypothermia. Importantly, all except one of the analogues tested have better potencies in inhibition of nicotine conditioned place preference than 2. We have identified new compounds that have utility as research tools and possible promise for treatment of nicotine dependence.
Nicotine; 3-phenyltropanes; structure activity relationship; dopamine uptake norepinephrine uptake; nAChR antagonism; antinociception; locomotor activity; hypothermia multiple target; conditioned place preference
Toward development of smoking cessation aids superior to bupropion (2), we describe synthesis of 2-(substituted phenyl)-3,5,5-trimethylmorpholine analogues 5a–5h and their effects on inhibition of dopamine, norepinephrine, and serotonin uptake, nicotinic acetylcholine receptor (nAChR) function, acute actions of nicotine, and nicotine-conditioned place preference (CPP). Several analogues encompassing aryl substitutions, N-alkylation, and alkyl extensions of the morpholine ring 3-methyl group provided analogues more potent in vitro than (S,S)-hydroxybupropion (4a) as inhibitors of dopamine or norepinephrine uptake and antagonists of nAChR function. All of the new (S,S)-5 analogues had better potency than (S,S)-4a as blockers of acute nicotine analgesia in the tail-flick test. Two analogues with highest potency at α3β4*-nAChR and among the most potent transporter inhibitors have better potency than (S,S)-4a in blocking nicotine-CPP. Collectively, these findings illuminate mechanisms of action of 2 analogues and identify deshydroxybupropion analogues 5a–5h as possibly superior candidates as aids to smoking cessation.
Nicotine; bupropion; hydroxybupropion; structure activity relationship; dopamine uptake; norepinephrine uptake; nAChR antagonism; antinociception; locomotor activity; hypothermia
In order to advance therapeutic applications of nicotinic ligands, continuing research efforts are being directed toward the identification and characterization of novel nicotinic acetylcholine receptor (nAChR) ligands that are both potent and subtype selective. Herein we report the synthesis and pharmacological evaluation of members of a new series of 3-alkoxy-5-aminopyridine derivatives that display good selectivity for the α4β2-nAChR subtype based on ligand binding and functional evaluations. The most potent ligand in this series, compound 64, showed high radioligand binding affinity and selectivity for rat α4β2-nAChR with a Ki value of 1.2 nM and 4700-fold selectivity for α4β2-over α3β4-nAChR, and ~100-fold selectivity for functional, high-sensitivity, human α4β2-nAChR over α3β4*-nAChR. In the mouse forced swim test, compound 64 exhibited antidepressant-like effects. Structure-activity relationship (SAR) analyses suggest that the introduction of additional substituents to the amino group present on the pyridine ring of the N-demethylated analogue of compound 17 can provide potent α4β2-nAChR-selective ligands for possible use in treatment of neurological and psychiatric disorders including depression.
Diverse nicotinic acetylcholine receptor (nAChR) subtypes containing different subunit combinations can be placed on nerve terminals or soma/dendrites in the ventral tegmental area (VTA). nAChR α6 subunit message is abundant in the VTA, but α6*-nAChR cellular localization, function, pharmacology, and roles in cholinergic modulation of dopaminergic (DA) neurons within the VTA are not well-understood. Here, we report evidence for α6β2*-nAChR expression on GABA neuronal boutons terminating on VTA DA neurons. α-Conotoxin (α-Ctx) MII labeling coupled with immunocytochemical staining localizes putative α6*-nAChRs to presynaptic GABAergic boutons on acutely dissociated, rat VTA DA neurons. Functionally, acetylcholine (ACh) induces increases in the frequency of bicuculline-, picrotoxin-, and 4-aminopyridine-sensitive miniature inhibitory postsynaptic currents (mIPSCs) mediated by GABAA receptors. These increases are abolished by α6*-nAChR-selective α-Ctx MII or α-Ctx PIA (1 nM), but not by α7-(10 nM methyllycaconitine) or α4*-(1 μM dihydro-β-erythroidine) nAChR-selective antagonists. ACh also fails to increase mIPSC frequency in VTA DA neurons prepared from nAChR β2 knockout mice. Moreover, ACh induces an α-Ctx PIA-sensitive elevation in intraterminal Ca2+ in synaptosomes prepared from the rat VTA. Subchronic exposure to 500 nM nicotine reduces ACh-induced GABA release onto the VTA DA neurons, as does 10 days of systemic nicotine exposure. Collectively, these results indicate that α6β2*-nAChRs are located on presynaptic GABAergic boutons within the VTA and modulate GABA release onto DA neurons. These presynaptic α6β2*-nAChRs likely play important roles in nicotinic modulation of DA neuronal activity.
α6-nicotinic acetylcholine receptor; miniature inhibitory postsynaptic current; ventral tegmental area; dopamine neuron; α-conotoxin; patch clamp
To create potentially superior aids to smoking cessation and/or antidepressants and to elucidate bupropion’s possible mechanisms of action(s), several analogues based on its active hydroxymetabolite (2S,3S)-4a were synthesized and tested for their abilities to inhibit monoamine uptake and nAChR subtype activities in vitro and acute effects of nicotine in vivo. The 3′,4′-dichlorophenyl [(±)-4n], naphthyl (4r), and 3-chlorophenyl or 3-propyl analogues 4s and 4t, respectively, had higher inhibitory potency and/or absolute selectivity than (2S,3S)-4a for inhibition of DA, NE, or 5HT uptake. The 3′-fluorophenyl, 3′-bromophenyl, and 4-biphenyl analogues 4c, 4d, and 4l, respectively, had higher potency for antagonism of α4β2-nAChR than (2S,3S)-4a. Several analogues also had higher potency than (2S,3S)-4a as antagonists of nicotine-mediated antinociception in the tail-flick assay. The results suggest that compounds acting via some combination of DA, NE, or 5HT inhibition and/or antagonism of α4β2-nAChR can potentially be new pharmacotherapeutics for treatment of nicotine dependence.
Nicotine; bupropion; hydroxybupropion; structure activity relationship; dopamine uptake; norepinephrine uptake; nAChR antagonism; antinociception; locomotor activity; hypothermia
Systemic exposure to nicotine induces glutamatergic synaptic plasticity on dopamine (DA) neurons in the ventral tegmental area (VTA), but mechanisms are largely unknown. Here, we report that single, systemic exposure in rats to nicotine (0.17 mg/kg free base) increases the ratio of DA neuronal currents mediated by AMPA relative to NMDA receptors (AMPA/NMDA ratio) assessed 24 hr later, based on slice patch recording. The AMPA/NMDA ratio increase is evident within 1 hr and lasts for at least 72 hr after nicotine exposure (and up to 8 days after repeated nicotine administration). This effect cannot be prevented by systemic injection of either α7-nAChR-selective (methyllycaconitine, MLA) or β2*-nAChR-selective (mecamylamine, MEC) antagonists but is prevented by co-injection of MLA and MEC. In either nAChR α7 or β2 subunit knock-out mice, systemic exposure to nicotine still increases the AMPA/NMDA ratio. Pre-injection in rats of a NMDA receptor antagonist (MK801), but neither DA receptor antagonists (SCH23390 plus haloperidol) nor a calcineurin inhibitor (cyclosporine), prevents the nicotine-induced increase in AMPA/NMDA ratio. After systemic exposure to nicotine, glutamatergic (but not GABAergic) transmission onto rat VTA DA neuronal inputs is enhanced. Correspondingly, DA neuronal firing measured 24 hr after nicotine exposure using extracellular single unit recording in vivo is significantly faster, and there is conversion of silent to active DA neurons. Collectively, these findings demonstrate that systemic nicotine acting via either α7- or β2*-nAChRs increases pre- and post-synaptic glutamatergic function, and consequently initiates glutamatergic synaptic plasticity, which may be an important, early neuronal adaptation in nicotine reward and reinforcement.
nicotine; nicotinic acetylcholine receptor; AMPA receptor; synaptic plasticity; dopamine neuron; patch-clamp
Bupropion (2a) analogues were synthesized and tested for their ability to inhibit monoamine uptake and to antagonize the effects of human α3β4*, α4β2, α4β4, and α1* nAChRs. The analogues were evaluated for their ability to block nicotine-induced effects in four tests in mice. Nine analogues showed increased monoamine uptake inhibition. Similar to 2a all but one analogue show inhibition of nAChR function selective for human α3β4*-nAChR. Nine analogues have higher affinity at α3β4*-nAChRs than 2a. Four analogues also had higher affinity for α4β2 nAChR. Analogues 2r, 2m, and 2n with AD50 values of 0.014, 0.015, and 0.028 mg/kg were 87, 81, and 43 times more potent than 2a in blocking nicotine-induced antinociception in the tail-flick test. Analogue 2x with IC50 values of 31 and 180 nM for DA and NE, respectively, and IC50 = 0.62 and 9.8 μm for antagonism of α3β4 and α4β2 nAChRs had the best overall in vitro profile relative to 2a.
Nicotine; bupropion; structure activity relationship; dopamine uptake; norepinephrine uptake; nAChR antagonism; antinociception; locomotor activity; hypothermia; multiple target
AMOP-H-OH (6-[5-(azetidin-2-ylmethoxy)pyridin-3-yl]hex-5-yn-1-ol) and some of its sulfur-bearing analogs were tested for their actions in vitro at human α4β2-, α4β4-, α3β4*- and α1*-nicotinic acetylcholine receptors (nAChRs). AMOP-H-OH also was assessed in a model of antidepressant efficacy. AMOP-H-OH and some of its analogs have high potency and selectivity for α4β2-nAChRs over other nAChR subtypes. Effects are manifest as partial agonism, perhaps reflecting selectivity for high sensitivity (α4)3(β2)2-nAChRs. More prolonged exposure to AMOP-H-OH and its analogs produces inhibition of subsequent responses to acute challenges with nicotinic full agonists, again selectively for α4β2-nAChRs over other nAChR subtypes. The inhibition is mediated either via antagonism or desensitization of nAChR function, but the degree of inhibition of α4β2-nAChRs is limited by the drugs’ activities as partial agonists. Certain aspects of the in vitro pharmacology suggest that AMOP-H-OH and some of its analogs have a set of binding sites on α4β2-nAChRs that are distinct from those for full agonists. The in vitro pharmacological profile suggests that peripheral side effects of AMOP-H-OH or its analogs would be minimal and that their behavioral effects would be dominated by central nAChR actions. AMOP-H-OH also has profound and high potency antidepressant-like effects in the forced swim test. The net action of prolonged exposure to AMOP-H-OH or its analogs, as for nicotine, seems to be a selective decrease in α4β2-nAChR function. Inactivation of nAChRs may be a common neurochemical endpoint for nicotine dependence, its treatment, and some of its manifestations, including relief from depression.
nicotine; nAChRs; depression; α4β2; Sazetidine-A; AMOP-H-OH
Nicotinic acetylcholine receptors (nAChRs) containing α7 subunits are thought to assemble as homomers. α7-nAChR function has been implicated in learning and memory, and alterations of α7-nAChR have been found in patients with Alzheimer’s disease (AD). Here we report findings consistent with a novel, naturally-occurring nAChR subtype in rodent, basal forebrain cholinergic neurons. In these cells, α7 subunits are co-expressed, co-localize and co-assemble with β2 subunit(s). Compared to homomeric α7-nAChRs from ventral tegmental area neurons, functional, presumably heteromeric α7β2-nAChRs on cholinergic neurons freshly dissociated from medial septum/diagonal band (MS/DB) exhibit relatively slow kinetics of whole-cell current responses to nicotinic agonists and are more sensitive to the β2 subunit-containing nAChR-selective antagonist, dihydro-β-erythroidine (DHβE). Interestingly, presumed, heteromeric α7β2-nAChRs are highly sensitive to functional inhibition by pathologically-relevant concentrations of oligomeric, but not monomeric or fibrillar forms of amyloid β1-42 (Aβ1-42). Slow whole-cell current kinetics, sensitivity to DHβE, and specific antagonism by oligomeric Aβ1-42 also are characteristic of heteromeric α7β2-nAChRs, but not of homomeric α7-nAChRs, heterologously expressed in Xenopus oocytes. Moreover, choline-induced currents have faster kinetics and less sensitivity to Aβ when elicited from MS/DB neurons derived from nAChR β2 subunit knockout mice rather than from wild-type mice. The presence of novel, functional, heteromeric α7β2-nAChRs on basal forebrain cholinergic neurons and their high sensitivity to blockade by low concentrations of oligomeric Aβ1-42 suggests possible mechanisms for deficits in cholinergic signaling that could occur early in the etiopathogenesis of AD and might be targeted by disease therapies.
nicotinic receptor; basal forebrain; cholinergic neurons; patch clamp; amyloid beta; Alzheimer’s disease