In vitro and in vivo neuronal nicotinic receptor properties of (+)- and (-)-pyrido[3,4]homotropane [(+)- and (-)-PHT]: (+)-PHT is a potent and selective full agonist at α6β2 containing neuronal nicotinic acetylcholine receptors

Pyrido[3,4]homotropane (PHT) is a conformationally rigid, high affinity analogue of nicotine. (+)-PHT was previously shown to be 266 times more potent than (-)-PHT for inhibition of [(3)H]epibatidine binding to nAChRs but had no antinociceptive activity in mouse tail-flick or hot-plate tests and was not a nicotinic antagonist even when administered intrathecally. While (-)-PHT had no agonist activity, it was a potent, nicotinic antagonist in the test. Here, electrophysiological studies with rat nAChRs show (+)-PHT to be a low efficacy partial agonist selective for α4β2-nAChRs, relative to α3β4-nAChRs (15-fold) and α7-nAChRs (45-fold). (-)-PHT was an antagonist with selectivity for α3β4, relative to α4β2- (3-fold) and α7- (11-fold) nAChRs. In [(3)H]DA release studies in mice, (+)-PHT was 10-fold more potent than (-)-PHT at α4β2*-nAChRs and 30-fold more potent at α6β2*-nAChRs. Studies using α5KO mice suggested that much of the activity at α4β2*-nAChRs is mediated by the α4β2α5-nAChR subtype. In conditioned place preference studies, (-)-PHT was more potent than (+)-PHT in blocking nicotine reward. Off-target screens showed (+)- and (-)-PHT to be highly selective for nAChRs. The high potency, full agonism of (+)- and (-)-PHT at α6*-nAChR contrasts with the partial agonism observed for α4*-nAChR, making these ligands intriguing probes for learning more about the pharmacophores for various nAChRs.

Effects of orally-bioavailable short-acting kappa opioid receptor-selective antagonist LY2456302 on nicotine withdrawal in mice

Kappa opioid receptor (KOR) signaling has been implicated in mediating behavioral and biochemical effects associated with drug dependence. The most commonly used KOR antagonists, norbinaltorphimine (norBNI) and (3R)-7-Hydroxy-N{(1S)-1-{[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-2-methylpropyl}-1,2,3,4-tetrahydro-3-isoquinoline-carboxamide (JDTic), have provided a wealth of information in this area; however, the delayed onset and long-lasting effects of these antagonists complicate experimental design and interpretation of results, and make them less than ideal for clinical studies. Initial studies with the recently developed KOR antagonist, LY2456302, show that the compound is a short acting, high-affinity, selective KOR antagonist with therapeutic potential for mood disorders and ethanol use in animal models, and is well tolerated in humans. The goal of the current study was to evaluate the effectiveness of LY2456302 in alleviating the nicotine withdrawal syndrome in mice. Mice were chronically treated with nicotine for 14 days and physical and affective nicotine withdrawal signs were measured using a spontaneous nicotine withdrawal model and conditioned place aversion (CPA) following pre-treatment with LY2456302, administered orally. Vehicle treated nicotine withdrawn mice displayed significant anxiety-related behavior, somatic signs, hyperalgesia, and CPA. Similar to previous studies with norBNI and JDTic, LY2456302 alleviated the nicotine withdrawal syndrome, as evidenced by decreased expression of nicotine withdrawal induced anxiety-related behavior, somatic signs, and CPA, and increased hotplate latency in nicotine withdrawn mice following pre-treatment. Given the current results, and with its favorable pharmacokinetic and pharmacodynamic profile, LY2456302 may be a useful therapeutic agent for treatment of multiple aspects of the nicotine withdrawal syndrome.

The nicotinic α6 subunit gene determines variability in chronic pain sensitivity via cross-inhibition of P2X2/3 receptors

Chronic pain is a highly prevalent and poorly managed human health problem. We used microarray-based expression genomics in 25 inbred mouse strains to identify dorsal root ganglion (DRG)-expressed genetic contributors to mechanical allodynia, a prominent symptom of chronic pain. We identified expression levels of Chrna6, which encodes the α6 subunit of the nicotinic acetylcholine receptor (nAChR), as highly associated with allodynia. We confirmed the importance of α6* (α6-containing) nAChRs by analyzing both gain- and loss-of-function mutants. We find that mechanical allodynia associated with neuropathic and inflammatory injuries is significantly altered in α6* mutants, and that α6* but not α4* nicotinic receptors are absolutely required for peripheral and/or spinal nicotine analgesia. Furthermore, we show that Chrna6's role in analgesia is at least partially due to direct interaction and cross-inhibition of α6* nAChRs with P2X2/3 receptors in DRG nociceptors. Finally, we establish the relevance of our results to humans by the observation of genetic association in patients suffering from chronic postsurgical and temporomandibular pain.

The role of alpha5 nicotinic acetylcholine receptors in mouse models of chronic inflammatory and neuropathic pain

The aim of the present study was to determine the impact of α5 nicotinic acetylcholine receptor (nAChR) subunit deletion in the mouse on the development and intensity of nociceptive behavior in various chronic pain models. The role of α5-containing nAChRs was explored in mouse models of chronic pain, including peripheral neuropathy (chronic constriction nerve injury, CCI), tonic inflammatory pain (the formalin test) and short and long-term inflammatory pain (complete Freund's adjuvant, CFA and carrageenan tests) in α5 knock-out (KO) and wild-type (WT) mice. The results showed that paw-licking time was decreased in the formalin test, and the hyperalgesic and allodynic responses to carrageenan and CFA injections were also reduced. In addition, paw edema in formalin-, carrageenan- or CFA-treated mice were attenuated in α5-KO mice significantly. Furthermore, tumor necrosis factor-alpha (TNF-α) levels of carrageenan-treated paws were lower in α5-KO mice. The antinociceptive effects of nicotine and sazetidine-A but not varenicline were α5-dependent in the formalin test. Both hyperalgesia and allodynia observed in the CCI test were reduced in α5-KO mice. Nicotine reversal of mechanical allodynia in the CCI test was mediated through α5-nAChRs at spinal and peripheral sites. In summary, our results highlight the involvement of the α5 nAChR subunit in the development of hyperalgesia, allodynia and inflammation associated with chronic neuropathic and inflammatory pain models. They also suggest the importance of α5-nAChRs as a target for the treatment of chronic pain.

The α3β4* nicotinic ACh receptor subtype mediates physical dependence to morphine: mouse and human studies

BACKGROUND AND PURPOSE

Recent data have indicated that α3β4* neuronal nicotinic (n) ACh receptors may play a role in morphine dependence. Here we investigated if nACh receptors modulate morphine physical withdrawal.

EXPERIMENTAL APPROACHES

To assess the role of α3β4* nACh receptors in morphine withdrawal, we used a genetic correlation approach using publically available datasets within the GeneNetwork web resource, genetic knockout and pharmacological tools. Male and female European-American (n = 2772) and African-American (n = 1309) subjects from the Study of Addiction: Genetics and Environment dataset were assessed for possible associations of polymorphisms in the 15q25 gene cluster and opioid dependence.

KEY RESULTS

BXD recombinant mouse lines demonstrated an increased expression of α3, β4 and α5 nACh receptor mRNA in the forebrain and midbrain, which significantly correlated with increased defecation in mice undergoing morphine withdrawal. Mice overexpressing the gene cluster CHRNA5/A3/B4 exhibited increased somatic signs of withdrawal. Furthermore, α5 and β4 nACh receptor knockout mice expressed decreased somatic withdrawal signs compared with their wild-type counterparts. Moreover, selective α3β4* nACh receptor antagonists, α-conotoxin AuIB and AT-1001, attenuated somatic signs of morphine withdrawal in a dose-related manner. In addition, two human datasets revealed a protective role for variants in the CHRNA3 gene, which codes for the α3 nACh receptor subunit, in opioid dependence and withdrawal. In contrast, we found that the α4β2* nACh receptor subtype is not involved in morphine somatic withdrawal signs.

CONCLUSION AND IMPLICATIONS

Overall, our findings suggest an important role for the α3β4* nACh receptor subtype in morphine physical dependence.

Differential roles of α6β2* and α4β2* neuronal nicotinic receptors in nicotine- and cocaine-conditioned reward in mice

Mesolimbic α6* nicotinic acetylcholine receptors (nAChRs) are thought to have an important role in nicotine behavioral effects. However, little is known about the role of the various α6*-nAChRs subtypes in the rewarding effects of nicotine. In this report, we investigated and compared the role of α6*-nAChRs subtypes and their neuro-anatomical locus in nicotine and cocaine reward-like effects in the conditioned place preference (CPP) paradigm, using pharmacological antagonism of α6β2* nAChRs and genetic deletion of the α6 or α4 subunits in mice. We found that α6 KO mice exhibited a rightward shift in the nicotine dose-response curve compared with WT littermates but that α4 KO failed to show nicotine preference, suggesting that α6α4β2*-nAChRs are involved. Furthermore, α6β2* nAChRs in nucleus accumbens were found to have an important role in nicotine-conditioned reward as the intra-accumbal injection of the selective α6β2* α-conotoxin MII [H9A; L15A], blocked nicotine CPP. In contrast to nicotine, α6 KO failed to condition to cocaine, but cocaine CPP in the α4 KO was preserved. Intriguingly, α-conotoxin MII [H9A; L15A], blocked cocaine conditioning in α4 KO mice, implicating α6β2* nAChRs in cocaine reward. Importantly, these effects did not generalize as α6 KO showed both a conditioned place aversion to lithium chloride as well as CPP to palatable food. Finally, dopamine uptake was not different between the α6 KO or WT mice. These data illustrate that the subjective rewarding effects of both nicotine and cocaine may be mediated by mesolimbic α6β2* nAChRs and that antagonists of these receptor subtypes may exhibit therapeutic potential.

Differential effects of endocannabinoid catabolic inhibitors on morphine withdrawal in mice

BACKGROUND

Inhibition of endocannabinoid catabolic enzymes fatty acid amide hydrolase (FAAH) and/or monoacylglycerol lipase (MAGL) reduces somatic morphine withdrawal signs, but its effects on aversive aspects of withdrawal are unknown. The present study investigated whether Δ(9)-tetrahydrocannabinol (THC), the MAGL inhibitor JZL184, the FAAH inhibitor PF-3845, or the dual FAAH/MAGL inhibitor SA-57 would reduce acquisition of morphine withdrawal-induced conditioned place avoidance (CPA) and jumping.

METHODS

Mice were implanted with placebo or 75 mg morphine pellets, 48 h later injected with naloxone or saline and placed in the conditioning apparatus, and assessed for CPA at 72 h. Subjects were also observed for jumping behavior following naloxone challenge.

RESULTS

Naloxone (0.056 mg/kg) produced robust CPA in morphine-pelleted, but not placebo-pelleted, mice. Morphine pretreatment prevented the occurrence of withdrawal CPA and withdrawal jumping, while clonidine (an α2 adrenergic receptor agonist) only blocked withdrawal CPA. THC, JZL184, and SA-57 significantly reduced the percentage of mice that jumped during the conditioning session, but did not affect acquisition of withdrawal CPA. PF-3845 did not reduce morphine withdrawal CPA or jumping. Finally, neither THC nor the endocannabinoid catabolic enzyme inhibitors in non-dependent mice elicited a conditioned place preference or aversion.

CONCLUSIONS

These findings suggest that inhibiting endocannabinoid catabolic enzymes reduces somatic morphine withdrawal signs, but not aversive aspects as inferred in the CPA paradigm. The observation that non-dependent mice administered inhibitors of endocannabinoid degradation did not display place preferences is consistent with the idea that that endocannabinoid catabolic enzymes might be targeted therapeutically, with reduced risk of abuse.

The analgesic-like properties of the alpha7 nAChR silent agonist NS6740 is associated with non-conducting conformations of the receptor

The α7 nicotinic acetylcholine receptor (nAChR) is a promising drug target for a number of neurological disorders including chronic pain and inflammatory diseases. Since α7 can function as a ligand-gated ion channel, drug development initially focused on ligands that were selective activators of the α7 ion channel. However, the best α7 drugs for chronic pain and inflammation indications may not be ion channel activators but rather "silent agonists", which bind to the receptor but preferentially induce non-conducting states that modulate signal transduction in non-neuronal cells. One such compound is NS6740. We show that NS6740 selectively induces prolonged desensitization of α7 nAChRs. There are two forms of α7 desensitization that can be distinguished by their sensitivity to the positive allosteric modulators (PAMs). At high concentrations, NS6740 preferentially induces PAM-insensitive desensitization, which over the course of several minutes reverts to the sensitive form. NS6740 was tested in several pain models after in vivo administration in the mouse. Although it had no effects in acute thermal pain, NS6740 induced significant dose- and time-dependent antinociceptive activity in formalin- and acetic acid-induced nociceptive behaviors as well as in the chronic constrictive nerve injury (CCI) model for neuropathic pain. The antinociceptive activity of NS6740 in these models was α7-dependent. In addition, NS6740 administration reversed pain-induced aversion, an important affective component of pain. The time and concentration dependence of the effects were consistent with NS6740 induction of PAM-insensitive non-conducting states, suggesting that signal transduction required for analgesia is accomplished by α7 receptors in that conformation.

New mechanisms and perspectives in nicotine withdrawal

Diseases associated with tobacco use constitute a major health problem worldwide. Upon cessation of tobacco use, an unpleasant withdrawal syndrome occurs in dependent individuals. Avoidance of the negative state produced by nicotine withdrawal represents a motivational component that promotes continued tobacco use and relapse after smoking cessation. With the modest success rate of currently available smoking cessation therapies, understanding mechanisms involved in the nicotine withdrawal syndrome are crucial for developing successful treatments. Animal models provide a useful tool for examining neuroadaptative mechanisms and factors influencing nicotine withdrawal, including sex, age, and genetic factors. Such research has also identified an important role for nicotinic receptor subtypes in different aspects of the nicotine withdrawal syndrome (e.g., physical vs. affective signs). In addition to nicotinic receptors, the opioid and endocannabinoid systems, various signal transduction pathways, neurotransmitters, and neuropeptides have been implicated in the nicotine withdrawal syndrome. Animal studies have informed human studies of genetic variants and potential targets for smoking cessation therapies. Overall, the available literature indicates that the nicotine withdrawal syndrome is complex, and involves a range of neurobiological mechanisms. As research in nicotine withdrawal progresses, new pharmacological options for smokers attempting to quit can be identified, and treatments with fewer side effects that are better tailored to the unique characteristics of patients may become available. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Effects of methoxsalen, a CYP2A5/6 inhibitor, on nicotine dependence behaviors in mice

Metabolism of nicotine to inactive cotinine by hepatic enzyme CYP2A6 is the principal pathway by which active nicotine is removed from circulation. We therefore hypothesized that inhibition of mouse CYP2A5, the ortolog of human CYP2A6, by methoxsalen (8-methoxypsoralen) alter dependence-related behaviors of nicotine in the mouse. Conditioned place preference (CPP) test was used to assess the appetitive reward-like properties and precipitated nicotine withdrawal to assess physical (somatic and hyperalgesia) and affective (anxiety-related behaviors) measures. The nicotine plasma levels were also measured with or without methoxsalen pretreatment. Methoxsalen (15 and 30 mg/kg, intraperitoneally) pretreatment enhanced nicotine-induced preference in mice (p<0.05). However, there was a lack of enhancement of nicotine in the CPP test after the highest dose of the CYP-2A5 inhibitor. Similarly to the CPP results, repeated administration of methoxsalen increased the intensity of mecamylamine-precipitated withdrawal signs. The potentiation of nicotine preference and withdrawal intensity by methoxsalen was accompanied by significant increase in nicotine plasma levels in mice (p<0.05). Finally, methoxsalen enhanced the ability of a very low dose of nicotine (0.05 mg/kg) to reverse withdrawal signs in mice undergoing spontaneous withdrawal after chronic nicotine infusion (p<0.05). In conclusion, inhibition of nicotine metabolism by methoxsalen alters the behavioral effects of nicotine in the mouse. Combining CYP2A6 inhibitors with low dose nicotine replacement therapies may have a beneficial role in smoking cessation because it will decrease the drug elimination rate and maintain plasma and brain nicotine levels.

Synthesis, nicotinic acetylcholine receptor binding, and antinociceptive properties of 2'-fluoro-3'-(substituted pyridinyl)-7-deschloroepibatidine analogues

2′-Fluoro-3-(substituted pyridine)epibatidine analogues 7a–e and 8a–e were synthesized, and their in vitro and in vivo nAChR properties were determined. 2′-Fluoro-3′-(4″-pyridinyl)deschloroepibatidine (7a) and 2′-fluoro-3′-(3″-pyridinyl)deschloroepibatidine (8a) were synthesized as bioisosteres of the 4′-nitrophenyl lead compounds 5a and 5g. Comparison of the in vitro nAChR properties of 7a and 8a to those of 5a and 5g showed that 7a and 8a had in vitro nAChR properties similar to those of 5a and 5g but both were more selective for the α4β2-nAChR relative to the α3β4- and α7-nAChRs than 5a and 5g. The in vivo nAChR properties in mice of 7a were similar to those of 5a. In contrast, 8a was an agonist in all four mouse acute tests, whereas 5g was active only in a spontaneous activity test. In addition, 5g was a nicotine antagonist in both the tail-flick and hot-plate tests, whereas 8a was an antagonist only in the tail-flick test.

Genetic variation within the Chrna7 gene modulates nicotine reward-like phenotypes in mice

Mortality from tobacco smoking remains the leading cause of preventable death in the world, yet current cessation therapies are only modestly successful, suggesting new molecular targets are needed. Genetic analysis of gene expression and behavior identified Chrna7 as potentially modulating nicotine place conditioning in the BXD panel of inbred mice. We used gene targeting and pharmacological tools to confirm the role of Chrna7 in nicotine conditioned place preference (CPP). To identify molecular events downstream of Chrna7 that may modulate nicotine preference, we performed microarray analysis of α7 knock-out (KO) and wild-type (WT) nucleus accumbens (NAc) tissue, followed by confirmation with quantitative polymerase chain reaction (PCR) and immunoblotting. In the BXD panel, we found a putative cis expression quantitative trait loci (eQTL) for Chrna7 in NAc that correlated inversely to nicotine CPP. We observed that gain-of-function α7 mice did not display nicotine preference at any dose tested, whereas conversely, α7 KO mice demonstrated nicotine place preference at a dose below that routinely required to produce preference. In B6 mice, the α7 nicotinic acetylcholine receptor (nAChR)-selective agonist, PHA-543613, dose-dependently blocked nicotine CPP, which was restored using the α7 nAChR-selective antagonist, methyllycaconitine citrate (MLA). Our genomic studies implicated a messenger RNA (mRNA) co-expression network regulated by Chrna7 in NAc. Mice lacking Chrna7 demonstrate increased insulin signaling in the NAc, which may modulate nicotine place preference. Our studies provide novel targets for future work on development of more effective therapeutic approaches to counteract the rewarding properties of nicotine for smoking cessation.

Similar activity of mecamylamine stereoisomers in vitro and in vivo

A previous characterization of mecamylamine stereoisomers using nicotinic acetylcholine receptors expressed in Xenopus oocytes revealed only small differences between the activity of the R and S forms of mecamylamine. However, that work was limited in the breadth of receptor subtypes tested, especially in regard to the discrimination of high and low sensitivity receptors, which differ in the ratios of alpha and beta subunits. We report new data using subunit concatamers, which produce uniform populations of high-sensitivity or low-sensitivity receptors, as well as alpha2, alpha5, and alpha6-containing receptors, which were not studied previously. Consistent with previous studies, we found that beta4-containing receptors were most sensitive to mecamylamine and that the IC50 values for the inhibition of net charge were lower than for inhibition of peak currents. No large differences were seen between the activities of the mecamylamine isomers. Additionally, a previously reported potentiation of high-sensitivity α4β2 receptors by S-mecamylamine could not be reproduced in the oocyte system, even with mutants that had greatly reduced sensitivity to mecamylamine inhibition or when the selective agonist TC-2559 was used. In vivo studies suggested that the R-isomer might be somewhat more potent than the S isomer at blocking CNS effects of nicotine. Although the potency difference was no more than a factor of two, it is consistent with lower LD50 estimates previously reported for the R isomer. Our results significantly extend knowledge of the nicotinic acetylcholine receptor activity profile of mecamylamine and support the hypothesis that these effects are not strongly stereoisomer selective.

The α4 nicotinic receptor promotes CD4+ T-cell proliferation and a helper T-cell immune response

Smoking is a common addiction and a leading cause of disease. Chronic nicotine exposure is known to activate nicotinic acetylcholine receptors (nAChRs) in immune cells. We demonstrate a novel role for α4 nAChRs in the effect of nicotine on T-cell proliferation and immunity. Using cell-based sorting and proteomic analysis we define an α4 nAChR expressing helper T-cell population (α4(+)CD3(+)CD4(+)) and show that this group of cells is responsive to sustained nicotine exposure. In the circulation, spleen, bone marrow, and thymus, we find that nicotine promotes an increase in CD3(+)CD4(+) cells via its activation of the α4 nAChR and regulation of G protein subunit o, G protein regulated-inducer of neurite outgrowth, and CDC42 signaling within T cells. In particular, nicotine is found to promote a helper T cell 2 adaptive immunologic response within T cells that is absent in α4(-/-) mice. We thus present a new mechanism of α4 nAChR signaling and immune regulation in T cells, possibly accounting for the effect of smoking on the immune system.

The cannabinoid CB2 receptor is necessary for nicotine-conditioned place preference, but not other behavioral effects of nicotine in mice

RATIONALE

Whereas cannabinoid CB1 receptors have long been known to contribute to the rewarding effects and dependence liability of many drugs of abuse, recent studies have implicated the involvement of cannabinoid CB2 receptors.

OBJECTIVE

Here, we evaluated the role of CB2 receptors in the rewarding properties of nicotine, as assessed in the conditioned place preference (CPP) paradigm and mecamylamine-precipitated withdrawal in nicotine dependent mice.

METHODS

Using complementary pharmacological and genetic approaches, we investigated the involvement of CB2 receptors in nicotine- and cocaine-induced CPP in mice and mecamylamine-precipitated withdrawal in nicotine-dependent mice. We also determined whether deletion of CB2 receptors affects nicotine-induced hypothermia and hypoalgesia.

RESULTS

Nicotine-induced (0.5 mg/kg) CPP was completely blocked by selective CB2 antagonist, SR144528 (3 mg/kg) in wild-type mice, and was absent in CB2 (-/-) mice. Conversely, the CB2 receptor agonist, O-1966 (1, 3, 5, 10, 20 mg/kg) given in combination with a subthreshold dose of nicotine (0.1 mg/kg) elicited a place preference. In contrast, O-1966 (20 mg/kg) blocked cocaine (10 mg/kg)-induced CPP in wild type mice, while CB2 (-/-) mice showed unaltered cocaine CPP. CB2 (+/+) and (-/-) nicotine-dependent mice showed almost identical precipitated withdrawal responses and deletion of CB2 receptor did not alter acute somatic effects of nicotine.

CONCLUSIONS

Collectively, these results indicate that CB2 receptors are required for nicotine-induced CPP in the mouse, while it is not involved in nicotine withdrawal or acute effects of nicotine. Moreover, these results suggest that CB2 receptors play opposing roles in nicotine- and cocaine-induced CPP.

Functional role of alpha7 nicotinic receptor in chronic neuropathic and inflammatory pain: studies in transgenic mice

A growing body of evidence indicates that α7 nicotinic receptor subtypes play an important role in chronic inflammatory and neuropathic pain signaling. In the present study, we investigated the role of the endogenous α7 nicotinic receptors (nAChRs) signaling in pain and inflammation using transgenic mice. For that we evaluated pain-related behaviors in the α7 mutant mice (KO) and its complementary α7 hypersensitive mice (KI) expressing the L250T α7 nAChRs and their respective WT mice in acute, chronic inflammatory and neuropathic mouse models. α7 KO and KI mice showed no significant changes in pain responses evoked by acute noxious thermal and mechanical stimuli as compared with WT littermates. While α7 KO mice showed no alterations in thermal and mechanical allodynia compared to WT mice after chronic nerve injury in the CCI test, α7 KI mice showed a significant reduction in these pain-related responses. However, marked increase in edema, hyperalgesia, and allodynia associated with intraplantar CFA injection was observed in the α7 KO mice compared with the WT littermates. In contrast, α7 KI mice displayed lesser degree of hyperalgesia and allodynia after CFA injection. Finally, the ability of systemic nicotine to reverse already-developed mechanical allodynia produced by intraplantar CFA seen in WT mice was lost in the α7 KO animals. Overall, our results demonstrate that endogenous α7 nAChRs mechanisms play an important role in chronic inflammatory and neuropathic pain models. This provides an additional rationale for the utility of α7 nAChR agonists in the treatment of inflammatory and chronic pain.

In vivo pharmacological interactions between a type II positive allosteric modulator of α7 nicotinic ACh receptors and nicotinic agonists in a murine tonic pain model

BACKGROUND AND PURPOSE

The α7 nicotinic ACh receptor subtype is abundantly expressed in the CNS and in the periphery. Recent evidence suggests that α7 nicotinic ACh receptor (nAChR) subtypes, which can be activated by an endogenous cholinergic tone comprising ACh and the α7 agonist choline, play an important role in chronic pain and inflammation. In this study, we evaluated whether type II α7 positive allosteric modulator PNU-120596 induces antinociception on its own and in combination with choline in the formalin pain model.

EXPERIMENTAL APPROACH

We assessed the effects of PNU-120596 and choline and the nature of their interactions in the formalin test using an isobolographic analysis. In addition, we evaluated the interaction of PNU-120596 with PHA-54613, an exogenous selective α7 nAChR agonist, in the formalin test. Finally, we assessed the interaction between PNU-120596 and nicotine using acute thermal pain, locomotor activity, body temperature and convulsing activity tests in mice.

KEY RESULTS

We found that PNU-120596 dose-dependently attenuated nociceptive behaviour in the formalin test after systemic administration in mice. In addition, mixtures of PNU-120596 and choline synergistically reduced formalin-induced pain. PNU-120596 enhanced the effects of nicotine and α7 agonist PHA-543613 in the same test. In contrast, PNU-120596 failed to enhance nicotine-induced convulsions, hypomotility and antinociception in acute pain models. Surprisingly, it enhanced nicotine-induced hypothermia via activation of α7 nAChRs.

CONCLUSIONS AND IMPLICATIONS

Our results demonstrate that type II α7 positive allosteric modulators produce antinociceptive effects in the formalin test through a synergistic interaction with the endogenous α7 agonist choline.

Early smoking onset and risk for subsequent nicotine dependence: a monozygotic co-twin control study

OBJECTIVE

Early onset of regular smoking is associated with an elevated risk for later nicotine dependence. Whether or not this association is causal is unknown and has substantial public policy implications.

METHOD

The authors used a monozygotic co-twin control study design. Pairs were selected from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders for discordance in age at onset of regular smoking. Nicotine dependence was measured by the Fagerström test for nicotine dependence and level of craving.

RESULTS

The authors identified 175 male-male and 69 female-female monozygotic twin pairs who differed by at least 2 years in age at onset of regular smoking. During their period of heaviest smoking, the twin who began smoking earlier had significantly higher Fagerström test scores in both the male-male (Cohen's d=0.20) and female-female twin pairs (d=0.26). Craving for cigarettes when unable to smoke was also higher in the early-onset member in both groups (male pairs, d=0.38; female pairs, d=0.25). The early-onset smoking twin did not differ from the later-onset twin in symptoms of alcohol or cannabis abuse or dependence, current alcohol use, or maximal level of cannabis, sedative, stimulant, or cocaine use.

CONCLUSIONS

Controlling for genetic and familial-environmental effects, age at onset of regular smoking predicted level of nicotine dependence. Consistent with the animal literature, these findings suggest that in humans, early nicotine exposure directly increases level of later nicotine dependence. These results should be interpreted in the context of the methodological strengths and limitations of the monozygotic co-twin design.

The role of fatty acid amide hydrolase inhibition in nicotine reward and dependence

The endogenous cannabinoid anandamide (AEA) exerts the majority of its effects at CB1 and CB2 receptors and is degraded by fatty acid amide hydrolase (FAAH). FAAH KO mice and animals treated with FAAH inhibitors are impaired in their ability to hydrolyze AEA and other non-cannabinoid lipid signaling molecules, such as oleoylethanolamide (OEA) and palmitoylethanolamide (PEA). AEA and these other substrates activate non-cannabinoid receptor systems, including TRPV1 and PPAR-α receptors. In this mini review, we describe the functional consequences of FAAH inhibition on nicotine reward and dependence as well as the underlying endocannabinoid and non-cannabinoid receptor systems mediating these effects. Interestingly, FAAH inhibition seems to mediate nicotine dependence differently in mice and rats. Indeed, pharmacological and genetic FAAH disruption in mice enhances nicotine reward and withdrawal. However, in rats, pharmacological blockade of FAAH significantly inhibits nicotine reward and has no effect in nicotine withdrawal. Studies suggest that non-cannabinoid mechanisms may play a role in these species differences.

The β2 nicotinic acetylcholine receptor subunit differentially influences ethanol behavioral effects in the mouse

The high co-morbidity between alcohol (ethanol) and nicotine abuse suggests that nicotinic acetylcholine receptors (nAChRs), thought to underlie nicotine dependence, may also be involved in alcohol dependence. The β2* nAChR subtype serves as a potential interface for these interactions since they are the principle mediators of nicotine dependence and have recently been shown to modulate some acute responses to ethanol. Therefore, the aim of this study was to more fully characterize the role of β2* nAChRs in ethanol-responsive behaviors in mice after acute exposure to the drug. We conducted a battery of tests in mice lacking the β2* coding gene (Chrnb2) or pretreated with a selective β2* nAChR antagonist for a range of ethanol-induced behaviors including locomotor depression, hypothermia, hypnosis, and anxiolysis. We also tested the effect of deletion on voluntary escalated ethanol consumption in an intermittent access two-bottle choice paradigm to determine the extent of these effects on drinking behavior. Our results showed that antagonism of β2* nAChRs modulated some acute behaviors, namely by reducing recovery time from hypnosis and enhancing the anxiolytic-like response produced by acute ethanol in mice. Chrnb2 deletion had no effect on ethanol drinking behavior, however. We provide further evidence that β2* nAChRs have a measurable role in mediating specific behavioral effects induced by acute ethanol exposure without affecting drinking behavior directly. We conclude that these receptors, along with being key components in nicotine dependence, may also present viable candidates in the discovery of the molecular underpinnings of alcohol dependence.

The α3β4* nicotinic acetylcholine receptor subtype mediates nicotine reward and physical nicotine withdrawal signs independently of the α5 subunit in the mouse

The 15q25 gene cluster contains genes that code for the α5, α3, and β4 nicotinic acetylcholine receptor (nAChRs) subunits, and in human genetic studies, has shown the most robust association with smoking behavior and nicotine dependence to date. The limited available animal studies implicate a role for the α5 and β4 nAChR subunits in nicotine dependence and withdrawal; however studies focusing on the behavioral role of the α3β4* nAChR receptor subtype in nicotine dependence are lacking. Because of the apparent role of the α3β4* nAChR subtype in nicotine dependence, the goal of the current study was to better evaluate the involvement of this subtype in nicotine mediated behavioral responses. Using the selective α3β4* nAChR antagonist, α-conotoxin AuIB, we assessed the role of α3β4* nAChRs in acute nicotine, nicotine reward, and physical and affective nicotine withdrawal. Because α5 has also been implicated in nicotine dependence behaviors in mice and can form functional receptors with α3β4*, we also evaluated the role of the α3β4α5* nAChR subtype in nicotine reward and somatic nicotine withdrawal signs by blocking the α3β4* nAChR subtype in α5 nAChR knockout mice with AuIB. AuIB had no significant effect on acute nicotine behaviors, but dose-dependently attenuated nicotine reward and physical withdrawal signs, with no significant effect in affective withdrawal measures. Interestingly, AuIB also attenuated nicotine reward and somatic signs in α5 nAChR knockout mice. This study shows that α3β4* nAChRs mediate nicotine reward and physical nicotine withdrawal, but not acute nicotine behaviors or affective nicotine withdrawal signs in mice. The α5 subunit is not required in the receptor assembly to mediate these effects. Our findings suggest an important role for the α3β4* nAChR subtype in nicotine reward and physical aspects of the nicotine withdrawal syndrome.

Nicotine reward and affective nicotine withdrawal signs are attenuated in calcium/calmodulin-dependent protein kinase IV knockout mice

The influx of Ca(2+) through calcium-permeable nicotinic acetylcholine receptors (nAChRs) leads to activation of various downstream processes that may be relevant to nicotine-mediated behaviors. The calcium activated protein, calcium/calmodulin-dependent protein kinase IV (CaMKIV) phosphorylates the downstream transcription factor cyclic AMP response element binding protein (CREB), which mediates nicotine responses; however the role of CaMKIV in nicotine dependence is unknown. Given the proposed role of CaMKIV in CREB activation, we hypothesized that CaMKIV might be a crucial molecular component in the development of nicotine dependence. Using male CaMKIV genetically modified mice, we found that nicotine reward is attenuated in CaMKIV knockout (-/-) mice, but cocaine reward is enhanced in these mice. CaMKIV protein levels were also increased in the nucleus accumbens of C57Bl/6 mice after nicotine reward. In a nicotine withdrawal assessment, anxiety-related behavior, but not somatic signs or the hyperalgesia response are attenuated in CaMKIV -/- mice. To complement our animal studies, we also conducted a human genetic association analysis and found that variants in the CaMKIV gene are associated with a protective effect against nicotine dependence. Taken together, our results support an important role for CaMKIV in nicotine reward, and suggest that CaMKIV has opposing roles in nicotine and cocaine reward. Further, CaMKIV mediates affective, but not physical nicotine withdrawal signs, and has a protective effect against nicotine dependence in human genetic association studies. These findings further indicate the importance of calcium-dependent mechanisms in mediating behaviors associated with drugs of abuse.

The antinociceptive effects of nicotinic receptors α7-positive allosteric modulators in murine acute and tonic pain models

The α7 nicotinic acetylcholine receptor (nAChR) subtype is abundantly expressed in the central nervous system and in the periphery. Recent evidence suggests that α7 nAChR subtypes, which can be activated by an endogenous cholinergic tone, comprising acetylcholine and the α7 nAChR agonist choline, play an important role in subchronic pain and inflammation. This study's objective was to test whether α7 nAChR positive allosteric modulators (PAMs) produce antinociception in in vivo mouse models of acute and persistent pain. Testing type I [N-(5-chloro-2-hydroxyphenyl)-N'-[2-chloro-5-(trifluoromethyl)phenyl] (NS1738)] and type II [1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxazol-3-yl) (PNU-120596)] α7 nAChR PAMs in acute and persistent pain, we found that, although neither reduced acute thermal pain, only PNU-120596 dose-dependently attenuated paw-licking behavior in the formalin test. The long-acting effect of PNU-120596 in this test was in discordance with its pharmacokinetic profile in mice, which suggests the involvement of postreceptor signaling mechanisms. Our results with selective mitogen-activated protein kinase kinase inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene monoethanolate (U0126) argues for an important role of extracellular signal-regulated kinase-1/2 pathways activation in PNU-120596's antinociceptive effects. The α7 antagonist MLA, administered intrathecally, reversed PNU-120596's effects, confirming PNU-120596's action, in part, through central α7 nAChRs. Importantly, tolerance to PNU-120596 was not developed after subchronic treatment of the drug. Surprisingly, PNU-120596's antinociceptive effects were blocked by NS1738. Our results indicate that type II α7 nAChR PAM PNU-120596, but not type I α7 nAChR PAM NS1738, shows significant antinociception effects in persistent pain models in mice.

Novel insights on the effect of nicotine in a murine colitis model

Studies showed that nicotine has a positive influence on symptoms of ulcerative colitis. In the present study, we explored the effect of nicotine treatment using different routes of administration in the dextran sodium sulfate (DSS) colitis mouse model. We also investigated the effects of cotinine, a major metabolite of nicotine, in the model. C57BL6 adult male mice were given DSS solution freely in the drinking water for seven consecutive days, and tap water was given thereafter. Disease severity, length of the colon, colon tissue histology, and inflammatory markers, including colonic myeloperoxidase activity and colonic tumor necrosis factor-α levels, were evaluated. The effect of nicotine and cotinine treatments via various different routes of administration were examined the DSS model. In addition, we measured the plasma levels of nicotine and cotinine in our treatment protocols. Administration of low, but not high, doses of oral nicotine in DSS-treated mice resulted in a significant decrease in disease severity, histologic damage scores, as well as colonic level of tumor necrosis factor-α. However, the anti-inflammatory effect of nicotine was not seen after chronic s.c. or minipump infusion of the drug. Differences in plasma levels of nicotine and cotinine do not seem to account for this lack of effect. Finally, oral cotinine alone failed to show a significant effect in the DSS model of colitis. These results highlight that dose and route of administration play a critical role in the protective effect of nicotine in the DSS mouse colitis model.

Effects of α7 positive allosteric modulators in murine inflammatory and chronic neuropathic pain models

Agonists and positive allosteric modulators (PAMs) of α7 nicotinic acetylcholine receptors (nAChRs) are currently being considered as novel therapeutic approaches for managing cognitive deficits in schizophrenia and Alzheimer's disease. Though α7 agonists were recently found to possess antinociceptive and anti-inflammatory properties in rodent models of chronic neuropathic pain and inflammation, the effects of α7 nAChRs PAMs on chronic pain and inflammation remain largely unknown. The present study investigated whether PAMs, by increasing endogenous cholinergic tone, potentiate α7 nAChRs function to attenuate inflammatory and chronic neuropathic pain in mice. We tested two types of PAMS, type I (NS1738) and type II (PNU-120596) in carrageenan-induced inflammatory pain and chronic constriction injury (CCI) neuropathic pain models. We found that both NS1738 and PNU-120596 significantly reduced thermal hyperalgesia, while only PNU-120596 significantly reduced edema caused by a hind paw infusion of carrageenan. Importantly, PNU-120596 reversed established thermal hyperalgesia and edema induced by carrageenan. In the CCI model, PNU-120596 had long-lasting (up to 6 h), dose-dependent anti-hyperalgesic and anti-allodynic effects after a single injection, while NS1738 was inactive. Systemic administration of the α7 nAChR antagonist MLA reversed PNU-120596's effects, suggesting the involvement of central and peripheral α7 nAChRs. Furthermore, PNU-120596 enhanced an ineffective dose of selective agonist PHA-543613 to produce anti-allodynic effects in the CCI model. Our results indicate that the type II α7 nAChRs PAM PNU-120596, but not the type I α7 nAChRs PAM NS1738, shows significant anti-edematous and anti-allodynic effects in inflammatory and CCI pain models in mice.

Acute behavioral effects of nicotine in male and female HINT1 knockout mice

Human genetic association and brain expression studies, and mouse behavioral and molecular studies implicate a role for the histidine triad nucleotide-binding protein 1 (HINT1) in schizophrenia, bipolar disorder, depression and anxiety. The high comorbidity between smoking and psychiatric disorders, schizophrenia in particular, is well established. Associations with schizophrenia and HINT1 are also sex specific, with effects more predominant in males; however, it is unknown if sex differences associated with the gene extend to other phenotypes. Thus, in this study, using a battery of behavioral tests, we elucidated the role of HINT1 in acute nicotine-mediated behaviors using male and female HINT1 wild-type (+/+) and knockout (-/-) mice. The results show that male HINT1 -/- mice were less sensitive to acute nicotine-induced antinociception in the tail-flick, but not hot-plate test. At low nicotine doses, male and female HINT1 -/- mice were less sensitive to nicotine-induced hypomotility, although the effect was more pronounced in females. Baseline differences in locomotor activity observed in male HINT1 +/+ and -/- mice were absent in females. Nicotine did not produce an anxiolytic effect in male HINT1 -/- mice, but rather an anxiogenic response. Diazepam also failed to induce an anxiolytic response in these mice, suggesting a general anxiety phenotype not specific to nicotine. Differences in anxiety-like behavior were not observed in female mice. These results further support a role for HINT1 in nicotine-mediated behaviors and suggest that alterations in the gene may have differential effects on phenotype in males and females.

Function of human α3β4α5 nicotinic acetylcholine receptors is reduced by the α5(D398N) variant

Genome-wide studies have strongly associated a non-synonymous polymorphism (rs16969968) that changes the 398th amino acid in the nAChR α5 subunit from aspartic acid to asparagine (D398N), with greater risk for increased nicotine consumption. We have used a pentameric concatemer approach to express defined and consistent populations of α3β4α5 nAChR in Xenopus oocytes. α5(Asn-398; risk) variant incorporation reduces ACh-evoked function compared with inclusion of the common α5(Asp-398) variant without altering agonist or antagonist potencies. Unlinked α3, β4, and α5 subunits assemble to form a uniform nAChR population with pharmacological properties matching those of concatemeric α3β4* nAChRs. α5 subunit incorporation reduces α3β4* nAChR function after coinjection with unlinked α3 and β4 subunits but increases that of α3β4α5 versus α3β4-only concatemers. α5 subunit incorporation into α3β4* nAChR also alters the relative efficacies of competitive agonists and changes the potency of the non-competitive antagonist mecamylamine. Additional observations indicated that in the absence of α5 subunits, free α3 and β4 subunits form at least two further subtypes. The pharmacological profiles of these free subunit α3β4-only subtypes are dissimilar both to each other and to those of α3β4α5 nAChR. The α5 variant-induced change in α3β4α5 nAChR function may underlie some of the phenotypic changes associated with this polymorphism.

Synthesis and nicotinic acetylcholine receptor in vitro and in vivo pharmacological properties of 2'-fluoro-3'-(substituted phenyl)deschloroepibatidine analogues of 2'-fluoro-3'-(4-nitrophenyl)deschloroepibatidine

Herein, we report the synthesis and nicotinic acetylcholine receptor (nAChR) in vitro and in vivo pharmacological properties of 2'-fluoro-3'-(substituted phenyl)deschloroepibatidines 5b-g, analogues of 3'-(4-nitrophenyl) compound 5a. All compounds had high affinity for α4β2-nAChR and low affinity for α7-nAChR. Initial electrophysiological studies showed that all analogues were antagonists at α4β2-, α3β4-, and α7-nAChRs. The 4-carbamoylphenyl analogue 5g was highly selective for α4β2-nAChR over α3β4- and α7-nAChRs. All the analogues were antagonists of nicotine-induced antinociception in the tail-flick test. Molecular modeling docking studies using the agonist-bound form of the X-ray crystal structure of the acetylcholine binding protein suggested several different binding modes for epibatidine, varenicline, and 5a-g. In particular, a unique binding mode for 5g was suggested by these docking simulations. The high binding affinity, in vitro efficacy, and selectivity of 5g for α4β2-nAChR combined with its nAChR functional antagonist properties suggest that 5g will be a valuable pharmacological tool for studying the nAChR and may have potential as a pharmacotherapy for addiction and other central nervous system disorders.

The antinociceptive effects of nicotinic partial agonists varenicline and sazetidine-A in murine acute and tonic pain models

Nicotinic agonists display a wide-range profile of antinociceptive activity in acute, tonic, and chronic pain models. However, their effectiveness is limited by their unacceptable side effects. We investigated the antinociceptive effects of two new α4β2* nicotinic partial agonists, varenicline and sazetidine-A, in acute thermal and tonic pain mouse models. Both drugs failed to induce significant effects in the tail-flick and hot-plate tests after subcutaneous administration. However, they blocked nicotine's effects in these tests at very low doses. In contrast to acute pain tests, varenicline and sazetidine-A dose-dependently induced an analgesic effect in the mouse formalin test after systemic administration. Their antinociceptive effects were mediated, however, by different nicotinic acetylcholine receptor (nAChR) subtypes. Sazetidine-A effects were mediated by β2* nAChR subtypes, whereas varenicline actions were attributed to α3β4 nAChRs. Moreover, low inactive doses of varenicline blocked nicotine's actions in phase II of the formalin test. Overall, our results suggest that the antagonistic actions of varenicline at low doses are mediated by β2*-nAChRs and at higher doses as an agonist by α3β4*-nAChRs. In contrast, both actions of sazetidine-A are mediated by β2*-nAChR subtypes. These results suggest that nicotinic partial agonists possess analgesic effects in a rodent tonic pain model and may provide a potential treatment for the treatment of chronic pain disorders.

Nicotine suppresses hyperexcitability of colonic sensory neurons and visceral hypersensivity in mouse model of colonic inflammation

Recently, we reported that nicotine in vitro at a low 1-μM concentration suppresses hyperexcitability of colonic dorsal root ganglia (DRG; L(1)-L(2)) neurons in the dextran sodium sulfate (DSS)-induced mouse model of acute colonic inflammation (1). Here we show that multiple action potential firing in colonic DRG neurons persisted at least for 3 wk post-DSS administration while the inflammatory signs were diminished. Similar to that in DSS-induced acute colitis, bath-applied nicotine (1 μM) gradually reduced regenerative multiple-spike action potentials in colonic DRG neurons to a single action potential in 3 wk post-DSS neurons. Nicotine (1 μM) shifted the activation curve for tetrodotoxin (TTX)-resistant sodium currents in inflamed colonic DRG neurons (voltage of half-activation changed from -37 to -32 mV) but did not affect TTX-sensitive currents in control colonic DRG neurons. Further, subcutaneous nicotine administration (2 mg/kg b.i.d.) in DSS-treated C57Bl/J6 male mice resulted in suppression of hyperexcitability of colonic DRG (L(1)-L(2)) neurons and the number of abdominal constrictions in response to intraperitoneal injection of 0.6% acetic acid. Collectively, the data suggest that neuronal nicotinic acetylcholine receptor-mediated suppression of hyperexcitability of colonic DRG neurons attenuates reduction of visceral hypersensitivity in DSS mouse model of colonic inflammation.

The neuropeptide galanin and variants in the GalR1 gene are associated with nicotine dependence

The neuropeptide galanin and its receptors are expressed in brain regions implicated in drug dependence. Indeed, several lines of evidence support a role for galanin in modulating the effects of drugs of abuse, including morphine, cocaine, amphetamine, and alcohol. Despite these findings, the role of galanin and its receptors in the effects of nicotine is largely underexplored. Here, using mouse models of nicotine reward and withdrawal, we show that there is a significant correlation between mecamylamine-precipitated nicotine withdrawal somatic signs and basal galanin or galanin receptor 1 (GALR1) expression in mesolimbocortical dopamine regions across the BXD battery of recombinant inbred mouse lines. The non-peptide galanin receptor agonist, galnon, also blocks nicotine rewarding effects and reverses mecamylamine-precipitated nicotine withdrawal signs in ICR mice. Additionally, we conducted a meta-analysis using smoking information from six European-American and African-American data sets. In support of our animal data, results from the association study show that variants in the GALR1 gene are associated with a protective effect in nicotine dependence (ND). Taken together, our data suggest that galanin has a protective role against progression to ND, and these effects may be mediated through GALR1.

Exploring behavioral and molecular mechanisms of nicotine reward in adolescent mice

Tobacco smoking during adolescence has become a prominent preventable health problem faced in the United States. Addictive properties of smoking are thought to have a pronounced effect at a young age, thereby increasing vulnerability to a life-long addiction and decreasing the likelihood of smoking cessation during adulthood. Learning and memory involvement in nicotine reward was assessed in early adolescent (PND 28-34) and adult (PND 70+) male ICR mice by conducting conditioning sessions of nicotine (0.5mg/kg) acquisition at varying time-spans, and evaluating extinction and reinstatement of nicotine preference using Conditioned Place Preference. Acquisition studies resulted in a significant preference for nicotine after 3 days of conditioning for both age groups, but not after only 1 or 2 conditioning days. In the extinction study, adolescent mice exhibited preference for nicotine 72 h after the last conditioning session, whereas preference for nicotine was extinct in adult mice by 72 h. Reinstatement studies showed adolescent mice, but not adult mice, recovering nicotine preference after a priming injection of 0.1mg/kg nicotine on day 9 after the mice underwent extinction. No significant differences were found when nAChRs were quantified in both early adolescent and adult mice using binding techniques including cytisine sensitive, α-conotoxin-MII sensitive, and α-bungarotoxin sensitive nAChRs. Levels of striatal dopamine release were measured in both age groups using a dopamine release assay over a range of nicotine doses, which also resulted in no significant differences. More sensitive assays may facilitate in understanding the mechanisms of nicotine reward in adolescent mice.

Synthesis of 2-(substituted phenyl)-3,5,5-trimethylmorpholine analogues and their effects on monoamine uptake, nicotinic acetylcholine receptor function, and behavioral effects of nicotine

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.

Varenicline blocks nicotine intake in rats with extended access to nicotine self-administration

RATIONALE

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine. Varenicline, a partial α4β2 nicotinic acetylcholine receptor (nAChR) agonist, is effective in reducing nicotine craving and relapse in smokers, suggesting that α4β2 nAChRs may play a key role in nicotine dependence. In rats, the effect of varenicline on nicotine intake has only been studied with limited access to the drug, a model of the positive reinforcing effect of nicotine. Varenicline has not been tested on the increase in motivation to take nicotine in nicotine-dependent rats.

OBJECTIVES

The present study evaluated the effects of varenicline on nicotine intake in rats with extended access to nicotine self-administration (23 h/day), a condition leading to the development of nicotine dependence. We hypothesized that varenicline's effects on nicotine self-administration would be greater in rats with extended than limited access to the drug and after forced abstinence rather than during baseline self-administration.

RESULTS

Varenicline dose-dependently decreased nicotine self-administration in rats with limited (1 h/day) and extended (23 h/day) access. Despite an increased sensitivity to the motivational effects of abstinence on nicotine intake compared with limited-access rats, varenicline was equally effective in decreasing nicotine intake in dependent rats with extended access to nicotine.

CONCLUSION

These results suggest that α4β2 nAChRs are critical in mediating the positive reinforcing effects of nicotine but may not be a key element underlying the negative reinforcement process responsible for the increased nicotine intake after abstinence in dependent subjects.

Nicotinic acetylcholine receptor efficacy and pharmacological properties of 3-(substituted phenyl)-2β-substituted tropanes

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.

Synthesis and biological evaluation of bivalent ligands for the cannabinoid 1 receptor

Dimerization or oligomerization of many G-protein-coupled receptors (GPCRs), including the cannabinoid 1 (CB1) receptor, is now widely accepted and may have significant implications for medications development targeting these receptor complexes. A library of bivalent ligands composed of two identical CB1 antagonist pharmacophores derived from SR141716 linked by spacers of various lengths were developed. The affinities of these bivalent ligands at CB1 and CB2 receptors were determined using radiolabeled binding assays. Their functional activities were measured using GTP-γ-S accumulation and intracellular calcium mobilization assays. The results suggest that the nature of the linker and its length are crucial factors for optimum interactions of these ligands at CB1 receptor binding sites. Finally, selected bivalent ligands (5d and 7b) were able to attenuate the antinociceptive effects of the cannabinoid agonist CP55,940 (21) in a rodent tail-flick assay. These novel compounds may serve as probes that will enable further characterization of CB1 receptor dimerization and oligomerization and its functional significance and may prove useful in the development of new therapeutic approaches to G-protein-coupled receptor mediated disorders.

The CB2 cannabinoid receptor-selective agonist O-3223 reduces pain and inflammation without apparent cannabinoid behavioral effects

Although Δ(9)-tetrahydrocannabinol (THC) and other mixed CB(1)/CB(2) receptor agonists are well established to elicit antinociceptive effects, their psychomimetic actions and potential for abuse have dampened enthusiasm for their therapeutic development. Conversely, CB(2) receptor-selective agonists have been shown to reduce pain and inflammation, without eliciting apparent cannabinoid behavioral effects. In the present study, we developed a novel ethyl sulfonamide THC analog, O-3223, and compared its pharmacological effects to those of the potent, mixed CB(1)/CB(2) receptor agonist, CP55,940, in a battery of preclinical pain models. Competitive cannabinoid receptor binding experiments revealed that O-3223 was approximately 80-fold more selective for CB(2) than CB(1) receptors. Additionally, O-3223 behaved as a full CB(2) receptor agonist in [(35)S]GTPγS binding. O-3223 reduced nociceptive behavior in both phases of the formalin test, reduced thermal hyperalgesia in the chronic constriction injury of the sciatic nerve (CCI) model, and reduced edema and thermal hyperalgesia elicited by intraplantar injection of LPS. These effects were blocked by pretreatment with the CB(2) receptor-selective antagonist SR144528, but not by the CB(1) receptor antagonist, rimonabant. Unlike CP55,940, O-3223 did not elicit acute antinociceptive effects in the hot-plate test, hypothermia, or motor disturbances, as assessed in the rotarod test. These data indicate that the CB(2) receptor-selective agonist, O-3223, reduces inflammatory and neuropathic nociception, without affecting basal nociception or eliciting overt behavioral effects. Moreover, this compound can serve as a template to develop new CB(2) receptor agonists with increased receptor selectivity and increased potency in treating inflammatory and neuropathic pain.

{alpha}7-nAChR-mediated suppression of hyperexcitability of colonic dorsal root ganglia neurons in experimental colitis

Controlled clinical trials of nicotine transdermal patch for treatment of ulcerative colitis have been shown to improve histological and global clinical scores of colitis. Here we report that nicotine (1 microM) suppresses in vitro hyperexcitability of colonic dorsal root ganglia (DRG) (L(1)-L(2)) neurons in the dextran sodium sulfate (DSS)-induced mouse model of acute colonic inflammation. Nicotine gradually reduced regenerative multiple-spike action potentials in colitis mice to a single action potential. Nicotine's effect on hyperexcitability of inflamed neurons was blocked in the presence of an alpha(7)-nicotinic acetylcholine receptor (nAChR) antagonist, methyllicaconitine, while choline, the alpha(7)-nAChR agonist, induced a similar effect to that of nicotine. Consistent with these findings, nicotine failed to suppress hyperexcitability in colonic DRG neurons from DSS-treated alpha(7) knockout mice. Furthermore, colonic DRG neurons from DSS-treated alpha(7) knockout mice were characterized by lower rheobase (10 +/- 5 vs. 77 +/- 13 pA, respectively) and current threshold (28 +/- 4 vs. 103 +/- 8 pA, respectively) levels than DSS-treated C57BL/J6 mice. An interesting observation of this study is that 8 of 12 colonic DRG (L(1)-L(2)) neurons from control alpha(7) knockout mice exhibited multiple-spike action potential firing while no wild-type neurons did. Overall, our findings suggest that nicotine at low 1 microM concentration suppresses in vitro hyperexcitability of inflamed colonic DRG neurons in a mouse model of acute colonic inflammation via activation of alpha(7)-nAChRs.

In vitro and in vivo characterization of a novel negative allosteric modulator of neuronal nAChRs

In this study, we compared the in vitro and in vivo neuronal nicotinic acetylcholine receptor (nAChR) properties of 1,2,3,3a,4,8b-hexahydro-2-benzyl-6-N,N-dimethylamino-1-methylindeno[1,2,-b]pyrrole (HDMP, 4) to that of negative allosteric modulator (NAM), PCP. Patch-clamp experiments showed that HDMP exhibited an inhibitory functional activity at α7 nAChRs with an IC(50) of 0.07 μM, and was 357- and 414-fold less potent at α4β2 and α3β4 nAChRs, with IC(50)s of 25.1 and 29.0 μM, respectively. Control patch-clamp experiments showed that PCP inhibited α7, α4β2 and α3β4 nAChRs with IC(50)s of to 1.3, 29.0 and 6.4 μM, respectively. Further, HDMP did not exhibit any appreciable binding affinity to either α7 or α4β2 nAChRs, suggesting its action via a non-competitive mechanism at these neuronal nAChR subtypes. The in vivo study showed that HDMP was a potent antagonist of nicotine-induced analgesia in the tail-flick (AD(50)=0.008 mg/kg), but not in the hot-plate test. All together, our in vitro and in vivo data suggest that HDMP is a novel NAM of neuronal nAChRs with potent inhibitory activity at α7 nAChR subtype at concentrations ≤ 1μM that are not effective for α4β2 and α3β4 nAChRs.

Synthesis and characterization of in vitro and in vivo profiles of hydroxybupropion analogues: aids to smoking cessation

To create potentially superior aids to smoking cessation and/or antidepressants and to elucidate bupropion's possible mechanisms of action(s), 23 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 alpha4beta2-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 alpha4beta2-nAChR can potentially be new pharmacotherapeutics for treatment of nicotine dependence.

Effects of hydroxymetabolites of bupropion on nicotine dependence behavior in mice

Bupropion is an atypical antidepressant that also has utility as a smoking cessation aid. Hydroxybupropions are major metabolites of bupropion and are believed to contribute to antidepressant and perhaps smoking cessation activities. Because bupropion metabolism is more similar in humans and mice than in humans and rats, the present study investigated effects of hydroxybupropion enantiomers in mouse behavioral models measuring various aspects of nicotine dependence. Bupropion and (2S,3S)-hydroxybupropion, but not (2R,3R)-hydroxybupropion, significantly decreased the development of nicotine reward as measured in the conditioned place preference and withdrawal paradigm in mice. Bupropion and both of its metabolites reversed affective and somatic withdrawal signs in nicotine-dependent mice, but the (2S,3S)-hydroxymetabolite had higher potency. Bupropion and (2S,3S)-, but not (2R,3R)-hydroxybupropion, produced partial substitution for nicotine in drug discrimination tests. Our findings support the hypothesis that the effects of bupropion on measures of nicotine dependence reflect actions of bupropion itself, its hydroxymetabolites, or a combination and suggest that the (2S,3S)-hydroxy isomer is the most active principle, making it a potentially better drug candidate for smoking cessation than bupropion.

Role of alpha5 nicotinic acetylcholine receptors in pharmacological and behavioral effects of nicotine in mice

Incorporation of the alpha5 nicotinic acetylcholine receptor (nAChR) subunit can greatly influence nAChR function without altering receptor number. Although few animal studies have assessed the role of the alpha5 nAChR in nicotine-mediated behaviors, recent evidence suggests an association between polymorphisms in the alpha5 nAChR gene and nicotine dependence phenotypes in humans. Thus, additional studies are imperative to elucidate the role and function of the alpha5 nAChR subunit in nicotine dependence. Using alpha5(-/-) mice, the current study aimed to examine the role of alpha5 nAChRs in the initial pharmacological effects of nicotine, nicotine reward using the conditioned place preference model, and the discriminative effects of nicotine using a two-lever drug discrimination model. (86)Rb(+) efflux and (125)I-epibatidine binding assays were conducted to examine the effect of alpha5 nAChR subunit deletion on expression and activity of functional nAChRs. Results show that alpha5(-/-) mice are less sensitive to the initial effects of nicotine in antinociception, locomotor activity, and hypothermia measures and that the alpha5 nAChR is involved in nicotine reward. Alternatively, alpha5(-/-) mice did not differ from wild-type littermates in sensitivity to the discriminative stimulus effects of nicotine. Furthermore, deletion of the alpha5 nAChR subunit resulted in a statistically significant decrease in function in the thalamus and hindbrain, but the decreases noted in spinal cord were not statistically significant. Receptor number was unaltered in all areas tested. Taken together, results of the study suggest that alpha5 nAChRs are involved in nicotine-mediated behaviors relevant to development of nicotine dependence.

Synthesis and biological evaluation of bupropion analogues as potential pharmacotherapies for smoking cessation

Bupropion (2a) analogues were synthesized and tested for their ability to inhibit monoamine uptake and to antagonize the effects of human alpha3beta4*, alpha4beta2, alpha4beta4, and alpha1* 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 alpha3beta4*-nAChR. Nine analogues have higher affinity at alpha3beta4*-nAChRs than 2a. Four analogues also had higher affinity for alpha4beta2 nAChR. Analogues 2r, 2m, and 2n with AD(50) 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 IC(50) values of 31 and 180 nM for DA and NE, respectively, and with IC(50) of 0.62 and 9.8 microm for antagonism of alpha3beta4 and alpha4beta2 nAChRs had the best overall in vitro profile relative to 2a.

Synthesis, nicotinic acetylcholine receptor binding, and antinociceptive properties of 3'-(substituted phenyl)epibatidine analogues. Nicotinic partial agonists

In 1992, John Daly et al. reported the isolation and structure determination of epibatidine. Epibatidine's unique structure and its potent nicotinic agonist activity have had a tremendous impact on nicotine receptor research. This research has led to a better understanding of the nicotinic acetylcholine receptor (nAChR) pharmacophore and to epibatidine analogues with potential as pharmacotherapies for treating various CNS disorders. In this study, we report the synthesis, receptor binding ([(3)H]epibatidine and [(125)I]iodoMLA), and in vivo pharmacological properties (mouse tail flick, hot plate, hypothermia, and spontaneous activity) of a series of 3'-(substituted phenyl)epibatidine analogues (5a-m). Results from these studies have added to the understanding of the nAChR pharmacophore and led to nicotinic partial agonists that may have potential for smoking cessation. All the analogues had affinities for the alpha4beta2 nAChR similar to epibatidine (1). 3'-(3-Dimethylaminophenyl)epibatidine (5m) has a nicotinic partial agonist pharmacological profile similar to the smoking cessation drug varenicline. Other analogues are partial agonists with varying degrees of nicotinic functional agonist and antagonist activity. 3'-(3-Aminophenyl)epibatidine (5j) is a more potent functional agonist and antagonist in all tests than varenicline. 3'-(3-Fluorophenyl)epibatidine and 3'-(3-chlorophenyl)epibatidine (5c and 5e) are more potent than varenicline when tested as agonists in four pharmacological tests and antagonists when evaluated against nicotine in the analgesia hot-plate test.

The role of alpha6-containing nicotinic acetylcholine receptors in nicotine reward and withdrawal

The alpha6 nicotinic acetylcholine receptor (nAChR) subunit is involved in nicotine-stimulated dopamine release in the striatum. It is expressed in brain regions and coexpressed with nAChR subtypes implicated in nicotine dependence behaviors; hence, this subunit may play a role in nicotine dependence. Using the alpha6-selective antagonist alpha-conotoxin H9A;L15A (MII[H9A;L15A]), we determined the role of alpha6* nAChRs in the pharmacological and behavioral effects of nicotine. We measured effects of pretreatment with MII[H9A;L15A] on analgesia, locomotion, and body temperature after a single injection of nicotine. Effects of MII[H9A;L15A] on nicotine reward were measured using the conditioned place preference (CPP) paradigm. We further measured physical (somatic signs and hyperalgesia) and affective [anxiety-related behavior and conditioned place aversion (CPA)] nicotine withdrawal behaviors after extended nicotine exposure. Results showed that MII[H9A;L15A] did not block acute nicotine effects on the behaviors measured. Conversely, MII[H9A:l15A] blocked the expression of nicotine CPP, as well as withdrawal-associated CPA and anxiety-related behavior in the elevated plus maze, but not withdrawal-induced somatic signs or hyperalgesia. These results suggest a role for the alpha6 nAChR subunit in nicotine reward and affective nicotine withdrawal but not acute nicotine-induced or physical withdrawal behaviors.

Characterization of pharmacological and behavioral differences to nicotine in C57Bl/6 and DBA/2 mice

Approximately 50-70% of the risk for developing nicotine dependence is attributed to genetics; therefore, it is of great significance to characterize the genetic mechanisms involved in nicotine reinforcement and dependence in hopes of generating better smoking cessation therapies. The overall goal of these studies was to characterize behavioral and pharmacological responses to nicotine in C57Bl/6 (B6) and DBA/2 (D2) mice, two inbred strains commonly used for genetic studies on behavioral traits. B6 and D2 mice where subjected to a battery of behavioral tests to measure nicotine's acute effects, calcium-mediated antinociceptive responses, tolerance to chronic treatment with osmotic mini pumps, and following three days of nicotine withdrawal. In general, D2 mice were less sensitive than B6 mice to the acute effects of nicotine, but were more sensitive to blockade of nicotine-induced antinociceptive responses by a calcium/calmodulin-dependent protein kinase II (CaMKII) inhibitor. B6, but not D2 mice, developed tolerance to nicotine and nicotine conditioned place preference (CPP). While B6 and D2 mice both expressed some physical withdrawal signs, affective withdrawal signs were not evident in D2 mice. These results provide a thorough, simultaneous evaluation of the pharmacological and behavioral differences to experimenter-administered nicotine as measured in several behavioral tests of aspects that contribute to smoking behavior. The B6 and D2 strains show wide phenotypic differences in their responses to acute or chronic nicotine. These results suggest that these strains may be useful progenitors for future genetic studies on nicotine behaviors across batteries of mouse lines such as the BXD recombinant inbred panel.

Beta 2 subunit-containing nicotinic receptors mediate acute nicotine-induced activation of calcium/calmodulin-dependent protein kinase II-dependent pathways in vivo

Nicotine is the addictive component of tobacco, and successful smoking cessation therapies must address the various processes that contribute to nicotine addiction. Thus, understanding the nicotinic acetylcholine receptor (nAChR) subtypes and subsequent molecular cascades activated after nicotine exposure is of the utmost importance in understanding the progression of nicotine dependence. One possible candidate is the calcium/calmodulin-dependent protein kinase II (CaMKII) pathway. Substrates of this kinase include the vesicle-associated protein synapsin I and the transcription factor cAMP response element-binding protein (CREB). The goal of these studies was to examine these postreceptor mechanisms after acute nicotine treatment in vivo. We first show that administration of nicotine increases CaMKII activity in the ventral tegmental area (VTA), nucleus accumbens (NAc), and amygdala. In beta2 nAChR knockout (KO) mice, nicotine does not induce an increase in kinase activity, phosphorylated (p)Synapsin I, or pCREB. In contrast, alpha7 nAChR KO mice show nicotine-induced increases in CaMKII activity and pCREB, similar to their wild-type littermates. Moreover, we show that when animals are pretreated with the CaMKII inhibitors 4-[(2S)-2-[(5-isoquinolinylsulfonyl) methylamino]-3-oxo-3-(4-phenyl-1-piperazinyl)propyl]phenyl isoquinolinesulfonic acid ester (KN-62) and N-[2-[[[3-(4-chlorophenyl)-2 propenyl]methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulphonamide (KN-93), nicotine-induced increase in the kinase activity and pCREB was attenuated in the VTA and NAc, whereas pretreatment with (2-[N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine, phosphate) (KN-92), the inactive analog, did not alter the nicotine-induced increase in pCREB. Taken together, these data suggest that the nicotine-induced increase in CaMKII activity may correlate with the nicotine-induced increase in pSynapsin I and pCREB in the VTA and NAc via beta2 subunit-containing nAChRs.

L-type calcium channels and calcium/calmodulin-dependent kinase II differentially mediate behaviors associated with nicotine withdrawal in mice

Smoking is a widespread health problem. Because the nicotine withdrawal syndrome is a major contributor to continued smoking and relapse, it is important to understand the molecular and behavioral mechanisms of nicotine withdrawal to generate more effective smoking cessation therapies. Studies suggest a role for calcium-dependent mechanisms, such as L-type calcium channels and calcium/calmodulin-dependent protein kinase II (CaMKII), in the effects of nicotine dependence; however, the role of these mechanisms in nicotine-mediated behaviors is unclear. Thus, the goal of this study was to elucidate the role of L-type calcium channels and CaMKII in nicotine withdrawal behaviors. Using both pharmacological and genetic methods, our results show that L-type calcium channels are involved in physical, but not affective, nicotine withdrawal behaviors. Although our data do provide evidence of a role for CaMKII in nicotine withdrawal behaviors, our pharmacological and genetic assessments yielded different results concerning the specific role of the kinase. Pharmacological data suggest that CaMKII is involved in somatic signs and affective nicotine withdrawal, and activity level is decreased after nicotine withdrawal, whereas the genetic assessments yielded results suggesting that CaMKII is involved only in the anxiety-related response, yet the kinase activity may be increased after nicotine withdrawal; thus, future studies are necessary to clarify the precise behavioral specifics of the relevance of CaMKII in nicotine withdrawal behaviors. Overall, our data show that L-type calcium channels and CaMKII are relevant in nicotine withdrawal and differentially mediate nicotine withdrawal behaviors.