In vivo pharmacological effects of dihydro-beta-erythroidine, a nicotinic antagonist, in mice

Insights Into the Differential Desensitization of α4β2 Nicotinic Acetylcholine Receptor Isoforms Obtained With Positive Allosteric Modulation of Mutant Receptors

The development of highly efficacious positive allosteric modulators (PAMs) of α7 nicotinic acetylcholine receptors (nAChR) has proven useful in defining the ligand dependence of the conformational dynamics of α7 receptors. No such effective modulators are known to exist for the α4β2 nAChR of the brain, limiting our ability to understand the importance of desensitization for the activity profile of specific ligands. In this study, we used mutant β2 subunits that allowed the use of the α7 PAM 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS) to probe the desensitizing effects of nicotinic ligands on the two forms of α4β2 receptors; high sensitivity (HS) (two α4 and three β2 subunits) and low sensitivity (LS) (three α4 and two β2 subunits). A total of 28 different ligands of 8 different categories, based on activity and selectivity, were tested for their ability to induce TQS-sensitive desensitization of HS and LS α4β2 receptors. Results confirm that HS α4β2 receptor responses are strongly limited by desensitization, by at least an order of magnitude more so than the responses of LS receptors. The activation of α4β2 receptors by the smoking-cessation drugs cytisine and varenicline is strongly limited by desensitization, as is the activation of LS receptors by the HS-selective agonists 6-[5-[(2S)-2-Azetidinylmethoxy]-3-pyridinyl]-5-hexyn-1-ol dihydrochloride and 4-(5-ethoxy-3-pyridinyl)-N-methyl-(3E)-3-buten-1-amine difumarate. The evaluation of drugs previously identified as α7-selective agonists revealed varying patterns of α4β2 cross-desensitization that were predictive of the effects of these drugs on the activation of wild-type α4β2 receptors by acetylcholine, supporting the utility of TQS-sensitive receptors for the development of focused therapeutics. SIGNIFICANCE STATEMENT: To varying degrees, ligands regulate the balance of active and desensitized states of the two forms of the primary nAChR subtypes in brain. Using mutant beta subunits, an allosteric modulator can reverse ligand-induced desensitization, revealing the differential desensitization of the receptors by specific ligands. This study shows that drugs believed to be selective for therapeutic targets may cross-desensitize other targets and that, within a class of drugs, improved specificity can be achieved by using agents that reduce such cross-desensitization.

Further pharmacological characterization of a preclinical model of the early development of nicotine withdrawal

BACKGROUND

Establishing preclinical models of the development of nicotine withdrawal following acute nicotine exposure could inform tobacco addiction-related research, treatment, and policy. To this end, this lab has previously reported that rats exhibit withdrawal-like elevations in intracranial self-stimulation (ICSS) thresholds (anhedonia-like behavior) following acute nicotine exposure. The goal of this study was to provide further pharmacological characterization of ICSS as a measure of spontaneous and antagonist-precipitated withdrawal from acute nicotine.

METHODS AND RESULTS

Rats exhibited a small increase in ICSS thresholds over time following a single nicotine injection (1.0 mg/kg, s.c.), suggesting a modest spontaneous withdrawal effect (Experiment 1). In Experiment 2, the antidepressant bupropion (5.0 mg/kg, i.p.), which is used to treat tobacco addiction and attenuates nicotine withdrawal in both humans and rodents, blocked elevations in ICSS thresholds induced by a single injection of nicotine (0.5 mg/kg, s.c.) followed ≈ 2 h later by the non-selective, non-competitive nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (3.0 mg/kg, s.c.). In Experiment 3a, s.c. administration of the competitive, relatively selective α4ß2 nAChR antagonist dihydro-beta-erythroidine (DHßE) (5.6 mg/kg, but not 3.0 mg/kg) following each of 5 daily injections of nicotine (0.5 mg/kg, s.c.) elevated ICSS thresholds. Mecamylamine (3.0 mg/kg, s.c.) also elevated ICSS thresholds when administered following all 5 daily nicotine injections (0.5 mg/kg, s.c., Experiment 3b).

CONCLUSIONS

These findings provide further characterization of elevations in ICSS thresholds as a measure of withdrawal from acute nicotine exposure. Further use of these models may be useful for understanding the early development of nicotine withdrawal.

The Cholinergic System Contributes to the Immunopathological Progression of Experimental Pulmonary Tuberculosis

The cholinergic system is present in both bacteria and mammals and regulates inflammation during bacterial respiratory infections through neuronal and non-neuronal production of acetylcholine (ACh) and its receptors. However, the presence of this system during the immunopathogenesis of pulmonary tuberculosis (TB) in vivo and in its causative agent Mycobacterium tuberculosis (Mtb) has not been studied. Therefore, we used an experimental model of progressive pulmonary TB in BALB/c mice to quantify pulmonary ACh using high-performance liquid chromatography during the course of the disease. In addition, we performed immunohistochemistry in lung tissue to determine the cellular expression of cholinergic system components, and then administered nicotinic receptor (nAChR) antagonists to validate their effect on lung bacterial burden, inflammation, and pro-inflammatory cytokines. Finally, we subjected Mtb cultures to colorimetric analysis to reveal the production of ACh and the effect of ACh and nAChR antagonists on Mtb growth. Our results show high concentrations of ACh and expression of its synthesizing enzyme choline acetyltransferase (ChAT) during early infection in lung epithelial cells and macrophages. During late progressive TB, lung ACh upregulation was even higher and coincided with ChAT and α7 nAChR subunit expression in immune cells. Moreover, the administration of nAChR antagonists increased pro-inflammatory cytokines, reduced bacillary loads and synergized with antibiotic therapy in multidrug resistant TB. Finally, in vitro studies revealed that the bacteria is capable of producing nanomolar concentrations of ACh in liquid culture. In addition, the administration of ACh and nicotinic antagonists to Mtb cultures induced or inhibited bacterial proliferation, respectively. These results suggest that Mtb possesses a cholinergic system and upregulates the lung non-neuronal cholinergic system, particularly during late progressive TB. The upregulation of the cholinergic system during infection could aid both bacterial growth and immunomodulation within the lung to favor disease progression. Furthermore, the therapeutic efficacy of modulating this system suggests that it could be a target for treating the disease.

Functional α6β4 acetylcholine receptor expression enables pharmacological testing of nicotinic agonists with analgesic properties

The α6β4 nicotinic acetylcholine receptor (nAChR) is enriched in dorsal root ganglia neurons and is an attractive non-opioid therapeutic target for pain. However, difficulty expressing human α6β4 receptors in recombinant systems has precluded drug discovery. Here, genome-wide screening identified accessory proteins that enable reconstitution of human α6β4 nAChRs. BARP, an auxiliary subunit of voltage-dependent calcium channels, promoted α6β4 surface expression while IRE1α, an unfolded protein response sensor, enhanced α6β4 receptor assembly. Effects on α6β4 involve BARP's N-terminal region and IRE1α's splicing of XBP1 mRNA. Furthermore, clinical efficacy of nicotinic agents in relieving neuropathic pain best correlated with their activity on α6β4. Finally, BARP-knockout, but not NACHO-knockout mice lacked nicotine-induced antiallodynia, highlighting the functional importance of α6β4 in pain. These results identify roles for IRE1α and BARP in neurotransmitter receptor assembly and unlock drug discovery for the previously elusive α6β4 receptor.

Activating α4β2 Nicotinic Acetylcholine Receptors Alleviates Fentanyl-induced Respiratory Depression in Rats

BACKGROUND

Opioid analgesics are widely used for treatment of acute, postoperative, and chronic pain. However, activation of opioid receptors can result in severe respiratory depression. There is an unmet clinical need to develop a pharmacologic therapy to counter opioid-induced respiratory depression without interfering with analgesia. Further, additional advances to confront accidental lethal overdose with the use of fentanyl and other opioids are needed. Here, the authors test the hypothesis that activation of nicotinic receptors expressed within respiratory rhythm-generating networks would counter opioid-induced respiratory depression without compromising analgesia.

METHODS

Respiratory neural discharge was measured using in vitro brainstem-spinal cord and medullary slice rat preparations. In vivo, plethysmographic recording, nociception testing, and righting reflexes were used to examine respiratory ventilation, analgesia, and sedation, respectively.

RESULTS

The administration of nicotine, selective α4β2 nicotinic receptor agonist A85380, but not α7 nicotinic receptor agonist PNU282987, reversed opioid-induced respiratory depression in neonatal pups in vitro and in vivo. In adult rats in vivo, administration of A85380 (0.03 mg/kg), but not PNU282987, provides a rapid and robust reversal of fentanyl-induced decrease in respiratory rate (93.4 ± 33.7% of control 3 min after A85380 vs. 31 ± 20.5% of control after vehicle, n = 8 each, P < 0.001), without marked side effects. The coadministration of A85380 (0.06 mg/kg) with fentanyl or remifentanil markedly reduced respiratory depression and apneas, and enhanced the fentanyl-induced analgesia, as evidenced by increased paw withdrawal latency in Hargreaves plantar test (14.4 ± 2.8 s vs. vehicle: 11.3 ± 2.4 s, n = 8 each, P = 0.013) and decreased formalin-induced nocifensive duration (2.5 ± 2.4 min vs. vehicle: 5.4 ± 2.7 min, n = 8 each, P = 0.029).

CONCLUSIONS

The novel strategy of targeting α4β2 nicotinic acetylcholine receptors has the potential for advancing pain control and reducing opioid-induced respiratory depression and overdose.

All-Optical Electrophysiology Reveals the Role of Lateral Inhibition in Sensory Processing in Cortical Layer 1

Cortical layer 1 (L1) interneurons have been proposed as a hub for attentional modulation of underlying cortex, but the transformations that this circuit implements are not known. We combined genetically targeted voltage imaging with optogenetic activation and silencing to study the mechanisms underlying sensory processing in mouse barrel cortex L1. Whisker stimuli evoked precisely timed single spikes in L1 interneurons, followed by strong lateral inhibition. A mild aversive stimulus activated cholinergic inputs and evoked a bimodal distribution of spiking responses in L1. A simple conductance-based model that only contained lateral inhibition within L1 recapitulated the sensory responses and the winner-takes-all cholinergic responses, and the model correctly predicted that the network would function as a spatial and temporal high-pass filter for excitatory inputs. Our results demonstrate that all-optical electrophysiology can reveal basic principles of neural circuit function in vivo and suggest an intuitive picture for how L1 transforms sensory and modulatory inputs. VIDEO ABSTRACT.

Blockade of nicotinic acetylcholine receptor enhances the responsiveness to bupropion in the mouse forced swim test

The objective of the present study is to investigate the role of α4, α5, α6 or β2 nAChR subunits in the antidepressant-like effect of bupropion. Adult male mice were treated with subcutaneous acute doses of bupropion (3 and 10 mg/kg) 30 min before the forced swim test (FST) in α4, α5, α6, or β2 nAChR subunit knockout (KO) and wild-type (WT) mice. In addition, the effects of β2* antagonist dihydro-β-erythroidine (DHβE, 3 mg/kg) on antidepressant-like effects of bupropion in C57BL/6 J mice were assessed. Our results showed that baseline immobility and climbing time did not differ between KO and corresponding WT mice except for β2 KO. Bupropion significantly decreased immobility time and increased climbing time in the α4, α6 and β2 nAChR KO mice in comparison to WT littermates, indicating that lack of these nAChR subunits enhanced antidepressant effects of bupropion. On the contrary, the α5 nAChR subunit deletion did not alter the FST behavior in the bupropion-treated mice. Not only in the transgenic mice, bupropion also showed antidepressant-like effects in the WT mice. In addition, DHβE pretreatment before bupropion administration resulted in decreased immobility time and increased climbing time. Taken together, the present study provides evidence on the involvement of α4*, α6*, and β2* (* indicates possible presence of other subunits) nAChRs in the antidepressant-like effects of bupropion in the FST.

Augmenting the antinociceptive effects of nicotinic acetylcholine receptor activity through lynx1 modulation

Neuronal nicotinic acetylcholine receptors (nAChRs) of the cholinergic system have been linked to antinociception, and therefore could be an alternative target for pain alleviation. nAChR activity has been shown to be regulated by the nicotinic modulator, lynx1, which forms stable complexes with nAChRs and has a negative allosteric action on their function. The objective in this study was to investigate the contribution of lynx1 to nicotine-mediated antinociception. Lynx1 contribution was investigated by mRNA expression analysis and electrophysiological responses to nicotine in the dorsal raphe nucleus (DRN), a part of the pain signaling pathway. In vivo antinociception was investigated in a test of nociception, the hot-plate analgesia assay with behavioral pharmacology. Lynx1/α4β2 nAChR interactions were investigated using molecular dynamics computational modeling. Nicotine evoked responses in serotonergic and GABAergic neurons in the DRN are augmented in slices lacking lynx1 (lynx1KO). The antinociceptive effect of nicotine and epibatidine is enhanced in lynx1KO mice and blocked by mecamylamine and DHβE. Computer simulations predict preferential binding affinity of lynx1 to the α:α interface that exists in the stoichiometry of the low sensitivity (α4)₃(β2)₂ nAChRs. Taken together, these data point to a role of lynx1 in mediating pain signaling in the DRN through preferential affinity to the low sensitivity α4β2 nAChRs. This study suggests that lynx1 is a possible alternative avenue for nociceptive modulation outside of opioid-based strategies.

The nicotinic receptor drug sazetidine-A reduces alcohol consumption in mice without affecting concurrent nicotine consumption

Alcohol and nicotine addiction are frequently co-morbid. The nicotinic acetylcholine receptors (nAChRs) are critical for both alcohol and nicotine addiction mechanisms, since nAChR drugs that reduce nicotine consumption have been shown to also reduce alcohol consumption. Sazetidine-A, a pre-clinical nAChR drug with agonist and desensitizing effects at α4β2 and α7 nAChRs, has been reported to reduce alcohol consumption and nicotine self-administration in rats when administered at high doses. However, this effect has not been replicated in mice. In this study, we examined the effect of sazetidine-A on alcohol and nicotine consumption in male and female mice utilizing voluntary oral consumption procedures previously developed in our lab. We found that sazetidine-A (1 mg/kg, i.p) reduced overnight alcohol consumption, but did not affect nicotine consumption when presented either alone or concurrently with alcohol. Sazetidine-A did not reduce water or saccharin consumption at any dose tested. In a chronic co-consumption experiment in which either alcohol or nicotine was re-introduced after one week of forced abstinence, sazetidine-A attenuated post-abstinence consumption of alcohol but not nicotine. Sazetidine-A also significantly reduced alcohol consumption in an acute, binge drinking-in-the-dark procedure. Finally, we tested the effect of sazetidine-A on alcohol withdrawal, and found that sazetidine-A significantly reduced handling-induced convulsions during alcohol withdrawal. Collectively, these data suggest a novel role for the nAChR targets of sazetidine-A in specifically mediating alcohol consumption, separate from the involvement of nAChRs in mediating nicotine consumption. Delineation of this pathway may provide insight into novel therapies for the treatment of alcohol use disorders.

An Update of Erythrinan Alkaloids and Their Pharmacological Activities

The period of the past 5 years has witnessed a remarkable increase in all of the number, structural variety, and complexity of erythrinan alkaloids reported. This structural diversity seems to be most pronounced in the alkaloids reported from the two species Erythrina arborescens and Erythrina variegata. Between them, work-up of these taxa yielded new polymeric (dimeric and trimeric) erythrinan alkaloids, a first example in one case where a normal 6,5,6,6-membered indoloisoquinoline spirocylic core has rearranged to a spiro-fused 6,5,7,6-skeleton. Furthermore, erythrinan alkaloids with a fifth ring containing a 2H-imidazole functionality were also reported for the first time, together with some new structures having an unusual substitution and with functionalities at positions C-3 and C-7 of the erythrinan core. This contribution has included 40 more erythrinan alkaloids that are either new or were omitted in the most recent major reviews on the same topic, leading to a total of 154 known erythrinan alkaloids to date. There are a few cases where the structures of the new alkaloids are contestable due to insufficient data having been obtained on isolation. To facilitate easier reference and identification, all structures having a common core have been placed in the same table or figure in this chapter.The reported pharmacological activities of the new and known erythrinan alkaloids documented have shown a considerable bias towards central nervous system and related activities. Other prominent activities that have been reported are antifeedant, insecticidal, cytotoxic, antiprotozoal, anti-inflammatory, antioxidant, antifungal, and antiviral effects. Erythrinan alkaloids generally seem to lack antibacterial activity. Several new polymeric alkaloids were found to lack cytotoxicity against a number of human cancer cell lines, although two of them showed moderate aphicidal activity and one exhibited weak to moderate acetylcholinesterase inhibition. The biological activity of erythrinan alkaloids seems to be influenced by basic substructural requirements, such as a conjugated diene (Δ^1,2, Δ^6,7) system and is modulated by the presence (or absence) of other groups in rings A, B, C, and D of the erythrinan core. The erythrinan core may provide potential leads to structures that eventually may be useful therapeutically.In recent years, a number of stereoselective chemical synthesis methods have been applied towards the erythinan alkaloids, and these are described in this contribution.

Synthesis, Nicotinic Acetylcholine Receptor Binding, and in Vitro and in Vivo Pharmacological Properties of 2'-Fluoro-(substituted thiophenyl)deschloroepibatidine Analogues

The synthesis, nAChR in vitro and in vivo pharmacological properties of 2'-fluoro-3'-(substituted thiophenyl)deschloroepibatidine analogues (5a-f, 6a-d, and 7a-c) are presented herein. All had subnanomolar affinity at α4β2*-nAChRs. Contrary to lead structure epibatidine, a potent nAChR agonist, all were potent α4β2- and α3β4-AChR antagonists in an in vitro functional assay. In vivo, the compounds were also nAChR antagonists with various degrees of agonist activity. Compounds 5e, 5f, 6a, 6c, 6d, and 7c had no agonist effects in the tail-flick, hot-plate, hypothermia, or spontaneous activity tests, whereas 5a-d, 7a and 7b did not have agonist activity in the tail-flick and hot-plate tests but, like varenicline, were agonists in the hypothermia and spontaneous activity tests. Compound 6b had agonist activity in all four in vivo tests. All the compounds were antagonists of nicotine-induced antinociception in the tail-flick test, and all except 5c, 5d, 5f, and 6b were antagonists of nicotine-induced antinociception in the hot-plate test. Compound 7c, which had a K_i = 0.86 nM in the binding assay similar potency at α4β2/α3β4 with selectivity relative to α7 nAChRs, had an AD₅₀ value of 0.001 μg/kg in the tail-flick test with no agonist activity in the in vitro or in vivo test had one of the more interesting profiles.

Synthesis, Nicotinic Acetylcholine Binding, and in Vitro and in Vivo Pharmacological Properties of 2'-Fluoro-(carbamoylpyridinyl)deschloroepibatidine Analogues

In this study, we report the synthesis, nAChR in vitro and in vivo pharmacological properties of 2'-fluoro-(carbamoylpyridinyl)deschloroepibatidine analogues (5, 6a,b, and 7a,b), which are analogues of our lead structure epibatidine. All of the analogues had subnanomolar binding affinity for α4β2*-nAChRs, and all were potent antagonists of α4β2-nAChRs in an in vitro functional assay. Analogues 6a,b were also highly selective for α4β2- relative to α3β4- and α7-nAChRs. Surprisingly, all of the analogues were exceptionally potent antagonists of nicotine-induced antinociception in the mouse tail-flick test, relative to standard nAChR antagonists such as DHβE. 2'-Fluoro-(4-carbamoyl-3-pyridinyl)deschloroepitabidine (6a) displayed an attractive combination of properties, including subnanomolar binding affinity (Ki = 0.07 nM), submicromolar inhibition of α4β2-nAChRs in the functional assay (IC50 = 0.46 μM) with a high degree of selectivity for α4β2- relative to the α3β4/α7-nAChRs (54-/348-fold, respectively), potent inhibition of [(3)H]dopamine release mediated by α4β2*- and α6β2*-nAChRs in a synaptosomal preparation (IC50 = 21 and 32 nM, respectively), and an AD50 of 0.007 μg/kg as an antagonist of nicotine induced antinociception in the mouse tail-flick test which is 64 250 times more potent than DHβE. These data suggest that compound 6a will be highly useful as a pharmacological tool for studying nAChRs and merits further development.

Effects of nicotinic acetylcholine receptor agonists in assays of acute pain-stimulated and pain-depressed behaviors in rats

Agonists at nicotinic acetylcholine receptors (nAChRs) constitute one drug class being evaluated as candidate analgesics. Previous preclinical studies have implicated α4β2 and α7 nAChRs as potential mediators of the antinociceptive effects of (–)-nicotine hydrogen tartrate (nicotine) and other nAChR agonists; however, these studies have relied exclusively on measures of pain-stimulated behavior, which can be defined as behaviors that increase in frequency, rate, or intensity after presentation of a noxious stimulus. Pain is also associated with depression of many behaviors, and drug effects can differ in assays of pain-stimulated versus pain-depressed behavior. Accordingly, this study compared the effects of nicotine, the selective α4/6β2 agonist 5-(123I)iodo-3-[2(S)-2-azetidinylmethoxy]pyridine (5-I-A-85380), and the selective α7 agonist N-(3R)-1-azabicyclo(2.2.2)oct-3-yl-4-chlorobenzamide in assays of pain-stimulated and pain-depressed behavior in male Sprague-Dawley rats. Intraperitoneal injection of dilute lactic acid served as an acute noxious stimulus to either stimulate a stretching response or depress the operant responding, which is maintained by electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. Nicotine produced a dose-dependent, time-dependent, and mecamylamine-reversible blockade of both acid-stimulated stretching and acid-induced depression of ICSS. 5-I-A-85380 also blocked both acid-stimulated stretching and acid-induced depression of ICSS, whereas N-(3R)-1-azabicyclo(2.2.2)oct-3-yl-4-chlorobenzamide produced no effect in either procedure. Both nicotine and 5-I-A-85380 were ≥10-fold more potent in blocking the acid-induced depression of ICSS than in blocking the acid-induced stimulation of stretching. These results suggest that stimulation of α4β2 and/or α6β2 nAChRs may be especially effective to alleviate the signs of pain-related behavioral depression in rats; however, nonselective behavioral effects may contribute to apparent antinociception.

Modulation of aggressive behavior in mice by nicotinic receptor subtypes

Aggression is frequently comorbid with neuropsychiatric conditions and is a predictor of worse outcomes, yet current pharmacotherapies are insufficient and have debilitating side effects, precluding broad use. Multiple models of aggression across species suggest that the nicotinic acetylcholine receptor (nAChR) agonist nicotine has anti-aggressive (serenic) properties. Here we demonstrate dose-dependent serenic effects of acute nicotine administration in three distinct mouse strains: C57BL/6, BALB/c, and CD1. While acute nicotine administration (0.25mg/kg) modestly reduced solitary homecage locomotion, this could not account for nicotine's serenic effects since social encounters eliminated the hypolocomotor effect, and nicotine did not alter social interaction times. Pretreatment with the homomeric (α7 subunit) nAChR antagonist methyllycaconitine (5mg/kg), but not the heteromeric (β2 or β4 subunit-containing) nAChR antagonist dihydro-β-erythroidine (DHβE, 3mg/kg), blocked the serenic effects of nicotine. By contrast, pretreatment with DHβE blocked the effect of acute nicotine administration on locomotion, uncoupling nicotine's serenic and hypolocomotor effects. Finally, the α7 nAChR partial agonist GTS-21 reduced aggression in C57BL/6 mice. These results support the idea that acute nicotine administration has serenic effects and provide evidence for specificity of this effect distinct from effects on locomotion. Furthermore, pharmacological studies suggest that activation of α7 nAChRs underlies the serenic effects of nicotine. Further studies of nAChRs could enhance understanding of the neurobiology of aggression and may lead to the development of novel, more specific treatments for pathological aggression.

New Insights into the Mechanisms of Action of Cotinine and its Distinctive Effects from Nicotine

Tobacco consumption is far higher among a number of psychiatric and neurological diseases, supporting the notion that some component(s) of tobacco may underlie the oft-reported reduction in associated symptoms during tobacco use. Popular dogma holds that this component is nicotine. However, increasing evidence support theories that cotinine, the main metabolite of nicotine, may underlie at least some of nicotine's actions in the nervous system, apart from its adverse cardiovascular and habit forming effects. Though similarities exist, disparate and even antagonizing actions between cotinine and nicotine have been described both in terms of behavior and physiology, underscoring the need to further characterize this potentially therapeutic compound. Cotinine has been shown to be psychoactive in humans and animals, facilitating memory, cognition, executive function, and emotional responding. Furthermore, recent research shows that cotinine acts as an antidepressant and reduces cognitive-impairment associated with disease and stress-induced dysfunction. Despite these promising findings, continued focus on this potentially safe alternative to tobacco and nicotine use is lacking. Here, we review the effects of cotinine, including comparisons with nicotine, and discuss potential mechanisms of cotinine-specific actions in the central nervous system which are, to date, still being elucidated.

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.

Neuroactive effects of cotinine on the hippocampus: behavioral and biochemical parameters

The present work evaluated the effects of nicotine (NIC), cotinine (COT), mecamylamine (MEC), methyllycaconitine (MLA) and dihydro-beta-eritroidine (DHβE) on memory extinction and the following biochemical parameters of the hippocampus: lipid peroxidation (LPO), antioxidant capacity (AC) and the phosphorylation of Extracellular-Signal-Regulated Kinase (ERK 1/2). Young male rats that were implanted bilaterally with cannulae were submitted to memory extinction tests sessions, and their hippocampi were dissected for biochemical assays. The extinction of fear memory was significantly improved by both nicotine and its metabolite. Cotinine significantly increased LPO, while nicotine significantly decreased it. Antioxidant capacity was increased by all treatments. Our results showed that cotinine, unlike nicotine, may increase oxidative stress in the hippocampus, but this increase depends upon the dose used and happens without causing corresponding impairments in cognitive function. Cotinine also increased the phosphorylation of ERK 1/2 in a similar fashion as nicotine. Considering these results, it is plausible to wonder to what extent nicotine-attributed effects are really due to the actions of this alkaloid and whether they could be due instead to cotinine or to cotinine-nicotine interactions within the brain.

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 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.

The role of nicotinic acetylcholine receptor (nAChR) α7 subtype in the functional interaction between nicotine and ethanol in mouse cerebellum

BACKGROUND

Many epidemiological studies report that alcoholics overwhelmingly smoke tobacco and vice versa, which suggests a possible functional interaction between ethanol and nicotine. Although nicotine-ethanol interaction is well documented within the central nervous system, the mechanism is not well understood. Therefore, it is important from a public health standpoint to understand the mechanisms involved in nicotine and ethanol functional interaction. The intracerebellar (ICB) administration of nicotine significantly attenuates ethanol ataxia through nicotinic acetylcholine receptor (nAChR) α(4)β(2) subtype. This study, an extension of earlier work, was intended to investigate the possible role of nAChR subtype α(7) in mitigating ethanol ataxia.

METHODS

The effect of ICB injection of PNU-282987 (α(7) agonist; 25 ng to 2.5 μg) and the antagonist methyllycaconitine was evaluated on ethanol (2 g/kg; i.p.)-induced ataxia with a Rotorod. Cerebellar nitric oxide was determined fluorometrically in the presence of ethanol and/or PNU-282987.

RESULTS

Attenuation of ethanol-induced ataxia following PNU-282987 microinfusion was dose-dependent suggesting the participation of α(7) subtype in nicotine and ethanol interaction. Intracerebellar pretreatment with methyllycaconitine (α(7) -selective antagonist; 6 ng) virtually abolished the attenuating effect of PNU-282987 as well as the effect of nicotine, but not of RJR-2403 (α(4)β(2) -selective agonist; 125 ng) on ethanol-induced ataxia. Finally, ethanol administration significantly decreased cerebellar NO(x), whereas ICB PNU-282987 significantly increased and/or opposed ethanol-induced decrease in NO(x). These results were functionally in agreement with our Rotorod data.

CONCLUSIONS

These observations confirmed the following: (i) α(7) participation in nicotine-ethanol interaction and (ii) α(7) selectivity of methyllycaconitine. Overall, the results demonstrate the role of cerebellar nAChR α(7) subtype in nicotine-induced attenuation of ethanol-induced ataxia in cerebellar NO(x)-sensitive manner.

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.

Measurement of affective state during chronic nicotine treatment and withdrawal by affective taste reactivity in mice: the role of endocannabinoids

Despite tobacco being highly addictive, it is unclear if nicotine has significant affective properties. To address this, we studied taste reactions to gustatory stimuli, palatable sucrose and unpalatable quinine, which are believed to reflect ongoing affective state. Taste reactivity was assessed during chronic nicotine administration and spontaneous withdrawal and the role of the endogenous cannabinoids was also investigated. C57BL6J mice were implanted with intraoral fistula to allow passive administration of solutions. In the first study, taste reactivity was tracked throughout chronic vehicle or nicotine (12 mg/kg/day) infusion via osmotic minipumps and spontaneous withdrawal following removal of minipumps. In the second study, the endocannabinoid CB1-receptor antagonist AM251 (1, 3 and 10mg/kg, intraperitoneal) or vehicle was acutely administered before taste reactivity measurement during chronic nicotine administration. Chronic nicotine treatment and spontaneous withdrawal did not influence taste reactions to sucrose or quinine. AM251 decreased positive reactions to sucrose and increased negative reactions to quinine. The effects of AM251 were respectively attenuated and enhanced in nicotine infused mice. These results suggest chronic nicotine exposure and withdrawal has no apparent affective sequelae, as probed by taste reactivity, and thus may not explain the difficulty tobacco-users have in achieving abstinence. In contrast, endocannabinoids elevate affective state in drug-naïve animals and changes in endogenous endocannabinoid tone may underlie compensations in affective state during chronic nicotine exposure.

Galantamine improves apomorphine-induced deficits in prepulse inhibition via muscarinic ACh receptors in mice

BACKGROUND AND PURPOSE

Galantamine, a weak acetylcholine esterase (AChE) inhibitor and allosteric potentiator of nicotinic ACh receptors (nAChRs), improves apomorphine-induced deficits in prepulse inhibition (PPI), sensory information-processing deficits, via a nAChR-independent mechanism. The present study examined the role of muscarinic ACh receptors (mAChRs) in the effect of galantamine, and studied the mechanism of galantamine-induced increases in prefrontal ACh levels in mice.

EXPERIMENTAL APPROACH

Apomorphine (1 mg kg(-1)) was administered to male ddY mice (9-10 weeks old) to create a PPI deficit model. Extracellular ACh concentrations in the prefrontal cortex were measured by in vivo microdialysis.

KEY RESULTS

Galantamine- and donepezil-mediated improvements in apomorphine-induced PPI deficits were blocked by the preferential M(1) mAChR antagonist telenzepine. The mAChR agonist oxotremorine also improved apomorphine-induced PPI deficits. Galantamine, like donepezil, increased extracellular ACh concentrations in the prefrontal cortex. Galantamine-induced increases in prefrontal ACh levels were partially blocked by the dopamine D(1) receptor antagonist SCH23390, but not by antagonists of mAChRs (telenzepine) and nAChRs (mecamylamine). Galantamine increased dopamine, but not 5-HT, release in the prefrontal cortex.

CONCLUSIONS AND IMPLICATIONS

Galantamine improves apomorphine-induced PPI deficits by stimulating mAChRs through increasing brain ACh levels via a dopamine D(1) receptor-dependent mechanism and AChE inhibition.

Effects of subunit selective nACh receptors on operant ethanol self-administration and relapse-like ethanol-drinking behavior

RATIONALE AND OBJECTIVES

The sensitivity to ethanol central effects is partially determined by the subunit composition of brain nicotinic acetylcholine receptors (nAChRs). Thus, the effects of intraventral tegmental area (VTA) administration of the nicotinic subunit-specific antagonist, alpha-conotoxin MII (alphaCtxMII, alpha(3)beta(2)*, beta(3)*, alpha(6)*), were compared to those of systemic mecamylamine (MEC, an allosteric negative modulator of the nAChR), dihydro-beta-erythroidine (DHbetaE, alpha(4)beta(2)*), and methyllycaconitine (MLA, alpha(7)*) to elucidate involvement of different subunits of nAChRs in operant ethanol self-administration and relapse-like activation of ethanol consumption after ethanol deprivation in rats.

METHODS

The effects of drugs were studied in rats trained for operant oral self-administration of ethanol (FR = 1). For ethanol deprivation, trained animals were subjected to a period of alcohol deprivation for 10 days. alphaCtxMII was given directly into the VTA through implanted permanent intracranial cannulae, whereas MEC, DHbetaE, and MLA were administered systemically.

RESULTS

alphaCtxMII reduced operant ethanol self-administration and blocked the deprivation-induced relapse-like ethanol consumption. MEC reduced operant ethanol self-administration and inhibited the deprivation-induced increase in alcohol consumption. DHbetaE did not alter ethanol self-administration in the lower-dose range but inhibited ethanol intake at a higher dose (4 mg/kg), although this effect might have been nonspecific. MLA failed to block self-administration of ethanol and relapse-like drinking after deprivation.

CONCLUSIONS

Our results indicate that nAChRs are involved in the modulation of operant alcohol self-administration and relapse-like alcohol drinking behavior in rats. Our observations support the working hypothesis that systemically active selective ligands for nAChR alpha(3)beta(2)*, beta(3), and/or alpha(6)* receptor subunits might be of therapeutic value for the treatment of alcoholism.

Attenuation of ethanol-induced ataxia by alpha(4)beta(2) nicotinic acetylcholine receptor subtype in mouse cerebellum: a functional interaction

Many epidemiological studies support the notion that people who drink alcohol also smoke cigarettes and vice versa thereby suggesting a possible functional interaction between these two most widely used psychoactive substances. We have earlier demonstrated that direct intracerebellar (ICB) microinfusion of nicotine dose-dependently antagonizes ethanol-induced ataxia and further that this antagonism occurs in a glutamate-nitric oxide-cyclic guanylyl monophosphate (cGMP) sensitive manner. The present study was designed to determine the possible involvement of specific nicotinic acetylcholine receptor (nAChR) subtype alpha(4)beta(2) in nicotine-induced attenuation of ethanol ataxia. Using the Rotorod test and direct ICB microinfusion technique in stereotaxically cannulated CD-1 male mice, we performed the Rotorod test following ICB administration of the alpha(4)beta(2)-selective agonist, (E)-N-methyl-4-(3-pyridinyl)-3-buten-1-amine (RJR-2403; 31.25, 62.5, 125 ng) on ethanol (2 g/kg; i.p.) ataxia at 15, 30, 45, 60 min post-ethanol injection. RJR-2403 dose-dependently attenuated ethanol ataxia suggesting a role of alpha(4)beta(2) subtype in ameliorating ethanol-induced ataxia. Pretreatment with ICB dihydro-beta-erythroidine (DHbetaE: 125, 250, 500, 750 ng), a potent alpha(4)beta(2)-selective antagonist, significantly reduced RJR-2403's effect further supporting the alpha(4)beta(2) involvement. DHbetaE (ICB) also antagonized ICB nicotine-induced attenuation of ethanol ataxia again reinforcing the role of alpha(4)beta(2) subtype. Additional evidence for the role of alpha(4)beta(2) subtype was provided when ICB alpha(4)beta(2) antisense oligodeoxynucleotide treatment markedly antagonized RJR 2403-induced attenuation of ethanol ataxia compared with missense-treated animals. This was confirmed with an associated decrease in the expression of alpha(4)beta(2) subtypes indicated by immunoblot experiments. In conclusion, the results of the present investigation support an important role of alpha(4)beta(2) nAChR subtype in the expression of nicotine-induced attenuation of ethanol ataxia.

Nicotinic acetylcholine receptors in the ventral tegmental area mediate the dopamine activating and reinforcing properties of ethanol cues

RATIONALE

Cues associated with alcohol can elicit craving, support drug-seeking and precipitate relapse.

OBJECTIVES

We investigated the possible involvement of nicotinic acetylcholine receptors (nAChRs) in the ventral tegmental area (VTA) in the conditioned reinforcing properties of ethanol-associated stimuli in the rat.

MATERIALS AND METHODS

First, using in vivo microdialysis, we analyzed the effect of VTA perfusion of the nonselective nAChR antagonist mecamylamine (MEC) or the selective alpha4beta2* nAChR antagonist dihydro-beta-erythroidine (DHbetaE) on the nucleus accumbens (nAc) dopaminergic response to the presentation of an ethanol-associated conditioned stimulus (CS). Second, rats were trained to associate a tone+light CS with the presentation of 10% ethanol and were subsequently tested on the acquisition of a new instrumental response with conditioned reinforcement (CR) after local VTA infusion of MEC, DHbetaE, or alpha-Conotoxin MII (alpha-CtxMII, a selective alpha3beta2* and alpha6* nAChR antagonist).

RESULTS

The ethanol-associated CS elevated nAc dopamine, an effect that was blocked by VTA perfusion of MEC but not DHbetaE. Systemic administration of MEC or local VTA infusion of MEC or alpha-CtxMII selectively blocked ethanol-associated CR, whereas systemic DHbetaE had no effect.

CONCLUSIONS

We hypothesize a novel mechanism by which alcohol-associated cues promote drug-seeking behavior via activation of dopamine-stimulating alpha-CtxMII-sensitive nAChRs in the VTA. Pharmacological manipulations of selective nAChRs may thus be possible treatment strategies to prevent cue-induced relapse.

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

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

Nicotinic regulation of calcium/calmodulin-dependent protein kinase II activation in the spinal cord

Recent studies have implicated the involvement of Ca2+-dependent mechanisms, in particular, calcium/calmodulin-protein kinase II in nicotine-induced antinociception using the tail-flick test. The spinal cord was suggested as a possible site of this involvement. The present study was undertaken to investigate the hypothesis that the beta2 nicotinic receptor subunit plays a central role in nicotine-induced spinal antinociception via calcium/calmodulin-dependent calmodulin protein kinase II activation. The antinociceptive effects of i.t. nicotine in the tail-flick test did not significantly differ in wild-type and alpha7 knockout (KO) animals but were lost in beta2 knockout mice. When calcium/calmodulin-dependent calmodulin protein kinase II activity in the lumbar spinal cord after acute i.t. administration of nicotine was investigated in wild-type and beta2 and alpha7 knockout mice, the increase in calcium/calmodulin-dependent calmodulin protein kinase II activity was not significant reduced in alpha7 KO mice but was eliminated in the beta2 KO mice. In addition, L-type calcium channel blockers nimodipine and verapamil but not the N-methyl-D-aspartate antagonist MK-801 (dizocilpine maleate) blocked the increase in the kinase activity induced by nicotine. Taken together, these results are consistent with the hypothesis that increases in intracellular calcium result in activation of calcium-mediated second messengers in the spinal cord that play an important role in nicotine-induced antinociception as measured in the tail-flick test. Furthermore, our findings indicate that nicotinic stimulation of beta2-containing acetylcholine nicotinic receptors in the spinal cord can activate calcium/calmodulin-dependent calmodulin protein kinase II and produce nicotinic analgesia, which may require L-type calcium voltage and gated channels but not the intervention of glutamatergic transmission.

Nicotine self-administration acutely activates brain reward systems and induces a long-lasting increase in reward sensitivity

Nicotine is a major component of tobacco smoke contributing to the initiation and persistence of the harmful tobacco habit in human smokers. The reinforcing effects of nicotine likely arise through its ability to stimulate brain circuitry mediating the detection and experiencing of natural rewards. Nevertheless, remarkably little is known concerning the acute or long-lasting actions of nicotine on brain reward systems in vivo. Here, we investigated the effects of intravenously self-administered nicotine (0.03 mg/kg/infusion, free base) on the sensitivity of brain reward systems, reflected in alterations of intracranial self-stimulation (ICSS) thresholds in rats. Rats self-administered nicotine during 1 or 12 h daily sessions, with reward thresholds assessed 1 h before and 15 min after each self-administration session. Control rats remained nicotine naïve throughout. Nicotine self-administration increased the sensitivity of brain reward systems, detected by post-nicotine lowering of reward thresholds in 1 and 12 h rats. This nicotine-enhanced sensitivity of reward systems was reversed by the high-affinity nicotinic receptor antagonist dihydro-beta-erythroidine (DHbetaE; 3 mg/kg). Surprisingly, nicotine-induced excitation of reward systems persisted for at least 36 days after nicotine self-administration had ceased. Overall, these data demonstrate that rats can voluntarily consume quantities of nicotine sufficient to increase the sensitivity of brain reward systems, an action likely crucial in establishing and maintaining the nicotine habit. Moreover, self-administered nicotine resets the sensitivity of reward systems to a new increased level, thereby imprinting an indelible 'memory' of its effects in reward systems, an action that so far appears unique to nicotine among drugs of abuse.

Nicotine place preference in the mouse: influences of prior handling, dose and strain and attenuation by nicotinic receptor antagonists

RATIONALE

Although conditioned place preferences (CPPs) are seen with most abused drugs, nicotine does not always produce a preference in this design.

OBJECTIVES

The goals of the present experiment were to (1) examine various factors that could contribute to these inconsistent results and (2) begin to evaluate the specific nicotinic receptors involved in the nicotine CPP.

METHODS

The influences of prior handling, environmental habituation, and injection habituation on a nicotine CPP were first evaluated in ICR mice. Subsequently, various nicotine doses were assessed for their abilities to produce a CPP, and the effectiveness of nicotinic receptor antagonists in attenuating this preference was examined. Finally, nicotine CPPs were assessed in C57BL/6J and DBA/2J mice to examine the influence of strain in this design.

RESULTS

Nicotine CPPs were seen in handled/environmentally habituated, but not in unhandled, ICR mice. Habituation to the injection techniques failed to strengthen the preference. In ICR mice, a CPP was seen with one intermediate dose of nicotine. This CPP was attenuated by mecamylamine and dihydro-beta-erythroidine (DHbetaE). A nicotine CPP was also seen in C57BL/6J, but not in DBA/2J, mice.

CONCLUSION

Earlier handling experience and strain are important factors when evaluating a nicotine CPP in the mouse. In addition, certain nicotinic receptors underlie the nicotine CPP, indicating that this model can elucidate underlying mediators of nicotine reward.

The beta2 but not alpha7 subunit of the nicotinic acetylcholine receptor is required for nicotine-conditioned place preference in mice

RATIONALE

Tobacco use is implicated in approximately 440,000 deaths per year, making it the leading cause of preventable death in the United States. Although it is generally recognized that tobacco use is correlated with a variety of health-related complications, many smokers are unsuccessful in their efforts to stop smoking using current cessation therapies.

OBJECTIVES

Given that nicotine is the addictive component of tobacco, successful smoking cessation therapies must address the various processes, including reward, which contribute to nicotine addiction. As such, determining the nicotinic receptor subtypes involved in nicotine reward is of utmost importance to understanding how nicotine addiction progresses.

METHODS

Conditioned place preference (CPP) in three-chamber conditioning boxes was performed. For antagonist studies, drug was given on all conditioning sessions 10 min before nicotine or saline injection and placement in the boxes.

RESULTS

We have demonstrated that a pretreatment with the alpha4beta2 subunit of the nicotinic acetylcholine receptor (nAChR) antagonist dihydro-beta-erythroidine (2.0 mg/kg, s.c.) blocked nicotine (0.5 mg/kg, s.c.) CPP in wild-type mice (C57BL/6 mice). In contrast, pretreatment with an antagonist of the alpha7 subunit of the nAChR, methyllycaconitine (MLA, 5.0 or 10.0 mg/kg, s.c.), had no effect on this behavior. Finally, we showed that mice lacking the beta2 subunit of the nAChR did not exhibit nicotine CPP while alpha7 knock-out mice did.

CONCLUSION

Taken together, these data suggest that the beta2 subunit of the nAChR is critically involved in nicotine reward as measured by CPP.

2-Fluoro-3-(4-nitro-phenyl)deschloroepibatidine is a novel potent competitive antagonist of human neuronal alpha4beta2 nAChRs

A patch-clamp technique in a whole-cell configuration was used to examine the functional activity of recently developed 2-fluoro-3-(substituted phenyl)deschloroepibatidine analogs on two major subtypes of neuronal nicotinic acetylcholine receptors (nAChRs), alpha4beta2 and alpha3beta4, that predominate in the central and peripheral nervous systems, respectively. These epibatidine analogs have been shown previously to possess high binding affinity to alpha4beta2 but not to alpha7 nAChRs and to inhibit nicotine-induced analgesia in behavioral pain tests. The 2-fluoro-3-(4-nitro-phenyl)deschloroepibatidine (4-nitro-PFEB) exhibited the most pronounced antagonist activity among these analogs when tested electrophysiologically on alpha4beta2 nAChRs. It inhibited acetylcholine (ACh)-induced currents in a concentration-dependent manner with an IC(50) value of 0.1 microM and produced complete inhibition at approximately 1 microM concentration. 4-Nitro-PFEB at 0.1 microM concentration produced a 4-fold rightward shift in the ACh concentration-response curve without altering maximum ACh-induced response. This inhibitory effect of 4-nitro-PFEB was voltage- and use-independent and was partially reversible at its 1 microM concentration. The rise and decay kinetics of ACh-induced currents was not altered in the presence of 4-nitro-PFEB. In contrast to alpha4beta2 nAChRs, this compound did not affect alpha3beta4 nAChR-mediated currents at < or =1 microM (IC(50) approximately 63.9 microM). Overall, these functional data agree with previous binding and behavioral findings and suggest collectively that 4-nitro-PFEB is the most effective and selective antagonist of alpha4beta2 versus alpha3beta4 and alpha7 nAChRs among the tested analogs, acting on alpha4beta2 nAChR through a competitive mechanism with a potency 17-fold higher than that of dihydro-beta-erythroidine.

In vivo characterization of a novel inhibitor of CNS nicotinic receptors

There are multiple types of nicotine acetylcholine receptors (nAChR) in the brain associated with synaptic function, signal processing, or cell survival. The therapeutic targeting of nicotinic receptors in the brain will benefit from the identification of drugs, which may be selective for their ability to activate or inhibit a limited range of these receptor subtypes. We previously identified a family of bis-tetramethylpiperidine compounds as selective inhibitors of neuronal-type nicotinic receptors. In the present study we describe the in vivo effects and properties of 2,2,6,6-tetramethylpiperidin-4-yl heptanoate (TMPH), a novel inhibitor of neuronal nicotinic receptors. Delivered systemically, this drug can block central nervous system effects of nicotine, indicating that this drug is able to cross the blood-brain barrier and access sites in the brain. Unlike the prototype CNS-active nicotinic inhibitor, mecamylamine, TMPH blocked some but not all of the CNS effects of nicotine, indicating that it has a unique selectivity for specific receptor subtypes in the brain. The nAChR subtypes that mediate the locomotor effects and hypothermic effects of nicotine appear to be less sensitive to TMPH than those which mediate analgetic effects and discriminative stimuli. These results indicate that TMPH may possess unique selectivity for specific nicotinic receptor subtypes.

In vitro and in vivo effects of an alpha3 neuronal nicotinic acetylcholine receptor antisense oligonucleotide

In the mammalian central nervous system (CNS), a family of alpha and beta subunits (alpha2-7, beta2-4) assemble to form both hetero- and homopentameric neuronal nicotinic acetylcholine receptors (nAChRs). In contrast to alpha4beta2 and alpha7, the predominant brain subtypes, far less is known regarding the functional expression and significance of alpha3-containing nAChRs in the CNS. In trying to better understand the role alpha3 in the CNS, an antisense knockdown strategy was utilized in the present studies. Specifically, Isis 106567 was identified out of 80 antisense oligonucleotides (aONs) designed and screened for their ability to reduce alpha3 mRNA expression in PC-12 cells. In addition to reducing alpha3 mRNA by greater than 75%, Isis 106567 attenuated nicotine-induced calcium influx in alpha3-expressing F11 cells. In vivo studies revealed significant reduction of alpha3 mRNA levels in both thalamus and medial habenula, regions known to express alpha3, following continuous (7 days) intracerebroventricular (i.c.v.) infusion of Isis 106567 in rats. Consistent with functional alpha3 knockdown, epibatidine-induced c-Fos expression in the medial habenula was attenuated in aON-treated rats. Known physiological responses elicited by epibatidine, such as hypothermia and micturition, were not affected by alpha3 aON treatment. However, the incidence of epibatidine-induced seizures was reduced in alpha3-antisense aON-treated rats, suggesting that alpha3 may be involved in mediating seizures produced by the nAChR agonist. Results of our studies suggest that Isis 106567 may be a useful in vivo tool for characterizing the functional significance of alpha3 expression in the CNS.

Nicotine physical dependence and tolerance in the mouse following chronic oral administration

RATIONALE

Although nicotine dependence and tolerance develop in rats, few studies have examined these processes in the mouse. Establishing such mouse models would eventually allow for an examination of the role of specific nicotinic receptor subtypes in mediating these processes (i.e. through the use of receptor knockouts).

OBJECTIVES

The goals of the present study were to establish mouse models of nicotine dependence and tolerance.

METHODS

Mice were chronically exposed to nicotine (0-200 mug/ml) in their drinking solution and assayed for plasma nicotine and cotinine levels, withdrawal signs following nicotine cessation (spontaneous withdrawal) or nicotinic antagonist administration (precipitated withdrawal), or nicotine tolerance. Dependence assays included somatic sign observations (paw tremors, backing and head shakes), tail-flick, plantar stimulation, elevated plus-maze and spontaneous activity. Tolerance was assayed using tail-flick, hot-plate and body temperature tests.

RESULTS

Plasma nicotine and cotinine levels were elevated during oral nicotine exposure (15.85 ng/ml and 538.00 ng/ml, respectively) and quickly declined following nicotine cessation (<1 ng/ml and <2 ng/ml, respectively), providing evidence that the oral route was pharmacologically relevant. Nicotine withdrawal increased numbers of somatic signs (spontaneous and mecamylamine-precipitated withdrawal) and/or hyperalgesia (spontaneous withdrawal only). Chronic nicotine exposure also produced tolerance, as indicated by reduced responsivity to acute nicotine in assays of analgesia and hypothermia.

CONCLUSIONS

These results indicate that chronic oral nicotine produces dependence and tolerance in the mouse. Further, nicotine dependence may be mediated by multiple nicotinic receptor subtypes, since specific nicotinic receptor antagonists failed to precipitate withdrawal.

Synthesis, Nicotinic Acetylcholine Receptor Binding, and Antinociceptive Properties of 2‘-Fluoro-3‘-(substituted phenyl)deschloroepibatidine Analogues. Novel Nicotinic Antagonist

A series of 2'-fluoro-3'-(substituted phenyl)deschloroepibatidine analogues (5a-k) showed high affinity for alpha4beta2 binding with no affinity at alpha7 nAChRs. The most potent compound was 2'-fluoro-3'-(4-nitrophenyl)deschloroepibatidine (5g) which possessed a Ki value of 0.009 nM. Surprisingly, none of the compounds showed agonist effects in pain tests and body temperature changes in mice even when tested at 10-15 mg/kg with the exception of 5b, which showed only very weak agonist effects. In contrast, all the compounds were potent functional antagonists of nicotine-induced antinociception. Interestingly, the 3'-substituted phenyl analogues 5b-k were 10-870-fold more effective as antagonists in the tail-flick test versus the hot-plate procedure. They failed to antagonize nicotine-induced hypothermia. The 4-chlorophenyl analogue (5e) (AD50 = 0.0003 in the tail-flick test) was the most potent and selective analogue. These results suggest that these compounds will be highly useful for identifying which specific receptor subtypes are involved in each of nicotine's pharmacological effects. These compounds also deserve consideration as potential pharmacotherapies for treatment of smoking cessation.

The role of nicotinic receptor α7 subunits in nicotine discrimination

The subtypes of nicotinic receptors at which the behavioural effects of nicotine originate are not fully understood. The experiments described here use mice lacking the alpha7 subunit of nicotinic receptors to investigate the role of alpha7-containing receptors in nicotine discrimination. Wild-type and alpha7-knockout mice were trained in a two-lever nicotine discrimination procedure using a tandem schedule of food reinforcement. Mutant mice exhibited baseline rates of lever-pressing as low as 52.2% of rates in wild-type controls (n=21-24). Mutant and wild-type mice acquired discrimination of nicotine (0.4 or 0.8 mg/kg) at a similar rate (n=10-12) and reached similar final levels of accuracy (71.9 +/- 4.4% and 90.8 +/- 3.1% after 60 training sessions for 0.4 and 0.8 mg/kg training doses, respectively, in mutant mice, as compared with 75.0 +/- 6.5% and 87.6 +/- 4.8% for wild types). The genotypes exhibited similar steep dose-response curves for nicotine discrimination. In both genotypes, dose-response curves for mice trained with 0.8 mg/kg of nicotine were displaced three- to four-fold to the right as compared with those for the mice trained with the smaller dose. The predominant effect of nicotine on the overall rate of responding was a reduction at the largest doses tested and there was no difference between the genotypes. The results suggest that nicotinic receptors containing the alpha7 subunit do not contribute to the discriminative stimulus or response-rate-depressant effects of nicotine, although they may regulate baseline rates of operant responding.

Participation of the cholinergic system in the excessive grooming behavior induced by neuropeptide (N) glutamic acid (E) isoleucine (I) amide (NEI)

The neuropeptide (N) glutamic acid (E) isoleucine (I) amide (NEI) injected into the ventral tegmental area (VTA) or intraventricularly (icv) induces excessive grooming behavior (EGB) and motor activity (MA). Here, we studied whether the cholinergic system is involved in the NEI-induced behavior. The present results demonstrate that atropine, a general muscarinic antagonist, injected icv previous to NEI, suppresses the behavior provoked by icv injections of the peptide, whereas the prior icv injection of dyhidro-beta-erythroidine, a general nicotinic antagonist, did not affect the EGB and MA induced by the peptide. From the experimental evidence, it is suggested that NEI may act specifically on a cholinergic afferent to dopaminergic cells. Also, the results appear to indicate that a neural target, different from the dopamine system, may be activated by the peptide to elicit behavioral changes, such as EGB.

Role of different nicotinic acetylcholine receptors in mediating behavioral and neurochemical effects of ethanol in mice

Ethanol and nicotine are the most abused drugs, and it is well known that co-abuse of ethanol and nicotine is frequent in human beings. We have previously obtained results indicating that the ethanol-induced stimulation of both the mesolimbic dopamine system and locomotor activity may involve activation of central nicotinic acetylcholine receptors (nAChRs), especially those located in the ventral tegmental area. Different subpopulations of nAChRs have been identified, and, in the present series of experiments, we have studied the effects of various nAChR antagonists on the stimulation of dopamine overflow in the nucleus accumbens and on locomotor activity induced by ethanol in male mice. Ethanol (2.0 g/kg, i.p.) enhanced dopamine overflow in the nucleus accumbens by approximately 40%, measured by means of in vivo microdialysis in awake, freely moving mice. Mecamylamine (negative allosteric modulator of nAChR; 2.0 mg/kg, i.p.) blocked the ethanol-induced stimulation of both locomotor activity and accumbal dopamine overflow. Methyllycaconitine citrate (alpha(7) antagonist; 2.0 mg/kg, i.p.) and dihydro-beta-erythroidine (competitive and selective alpha(4)beta(2) antagonist; 0.5 mg/kg, s.c.), in doses that had no marked effects per se, did not significantly reduce the behavioral and neurochemical stimulation caused by ethanol. The present results support the suggestion that the stimulatory effects of ethanol on locomotor activity and dopamine release do not involve the alpha(4)beta(2) or alpha(7) subunit compositions of the nAChR and that the effects of mecamylamine are mediated through a site not directly associated with the alpha(4)beta(2) or alpha(7) nAChR subunits.

Synthesis, nicotinic acetylcholine receptor binding, and antinociceptive properties of 2-exo-2-(2',3'-disubstituted 5'-pyridinyl)-7-azabicyclo[2.2.1]heptanes: epibatidine analogues

A number of 2',3'-disubstituted epibatidine analogues were synthesized and evaluated in vitro for potency at nicotinic acetylcholine receptors (nAChRs) and in vivo for antinociception activity in the tail-flick and hot-plate models of acute pain and for their ability to affect core body temperature. Compounds that possessed electron-withdrawing groups (F, Cl, Br, and I) in both the 2'- and the 3'-positions showed affinities at the nAChR similar to epibatidine. However, in vivo efficacy did not correlate with affinity. 2-exo-(3'-Amino-2'-chloro-5'-pyridinyl)-7-azabicyclo[2.2.1]heptane (2i), an epibatidine analogue possessing an electron-releasing amino group in the 3'-position, produced the highest affinity. Compound 2i was also the most selective epibatidine analogue with a K(i) of 0.001 nM at alphabeta nAChRs, which is 26 times greater than that of epibatidine, and a alphabeta/alpha(7) K(i) ratio of 14,000, twice that of epibatidine. In vivo testing revealed that this compound potently inhibited nicotine-induced antinociception with AD(50) values below 1 microg/kg. Surprisingly, this same compound was also an agonist at higher doses (ED(50) approximately 20 microg/kg). Thus, the addition of the 3'-amino group to epibatidine confers potent antagonist activity to the compound with little effect on agonist activity. 2,3-Disubstituted epibatidine analogues possessing a 2'-amino group combined with a 3'-bromo or 3'-iodo group showed in vitro and in vivo nAChR properties similar to nicotine.

Contributory role for nornicotine in nicotine neuropharmacology: nornicotine-evoked [3H]dopamine overflow from rat nucleus accumbens slices

Nornicotine is a tobacco alkaloid and an active nicotine metabolite, which accumulates in brain to pharmacologically relevant concentrations following repeated nicotine administration to rats. Furthermore, nornicotine is self-administered by rats, indicating that it has reinforcing efficacy and may contribute to nicotine dependence. Since drugs of abuse activate the mesolimbic dopamine (DA) system to produce rewarding effects, the present study tested the hypothesis that nornicotine evokes DA release from nucleus accumbens in a nicotinic receptor-mediated manner. Rat nucleus accumbens slices were preloaded with [3H]DA and superfused for 60 min in the absence and presence of a range of alkaloid concentrations. Superfusate samples were collected and alkaloid-evoked [3H]overflow was determined. S(-)-Nornicotine (EC(50) value = 3.0 microM), R(+)-nornicotine (EC(50) value = 0.48 microM), and S(-)-nicotine (EC(50) value = 70 nM) evoked [3H]overflow in a concentration-dependent manner. For each nornicotine enantiomer, 0.3 microM was the lowest concentration to evoke significant [3H]overflow. Dihydro-beta-erythroidine (DHbetaE, 10 microM), a classical nicotinic receptor antagonist, inhibited the S(-)-nornicotine-evoked [3H]overflow, indicating the involvement of nicotinic receptors. Furthermore, the effect of S(-)-nornicotine was calcium-dependent, consistent with a nicotinic receptor-mediated mechanism. Whereas S(-)-nornicotine was found previously to be more potent in the striatum, R(+)-nornicotine was more potent than its enantiomer in nucleus accumbens, suggesting the involvement of different nicotinic receptor subtypes in these brain regions. Thus, the results of the current study indicate that nornicotine stimulated DA release from nucleus accumbens in a nicotinic receptor-mediated manner, further supporting the hypothesis that nornicotine contributes to tobacco dependence.

Neurobiology of the nicotine withdrawal syndrome

The aversive aspects of withdrawal from chronic nicotine exposure are thought to be an important motivational factor contributing to the maintenance of the tobacco habit in human smokers. Much emphasis has been placed on delineating the underlying neurobiological mechanisms mediating different components of the nicotine withdrawal syndrome. Recent studies have shown that both central and peripheral populations of nicotinic acetylcholine receptors (nAChRs) are involved in mediating somatic signs of nicotine withdrawal as measured by the rodent nicotine abstinence scale. However, only central populations of nAChRs are involved in mediating affective aspects of nicotine withdrawal, as measured by elevations in brain-stimulation reward thresholds and conditioned place aversion. Nicotine interacts with several neurotransmitter systems, including acetylcholine, dopamine, opioid peptides, serotonin, and glutamate systems. Evidence so far suggests that these neurotransmitters play a role in nicotine dependence and withdrawal processes. The available evidence also suggests that different underlying neurochemical deficits mediate somatic and affective components of nicotine withdrawal. The aim of the present review is to discuss preclinical findings concerning the neuroanatomical and neurochemical substrates involved in these different aspects of nicotine withdrawal.

Synthesis, nicotinic acetylcholine receptor binding, and antinociceptive properties of 2-exo-2-(2'-substituted-3'-phenyl-5'-pyridinyl)-7-azabicyclo[2.2.1]heptanes. Novel nicotinic antagonist

A series of 2'-substituted-3'-phenyl epibatidine analogues were synthesized and evaluated for inhibition of binding at nicotine acetylcholine receptors and for antinociceptive properties in mice. The introduction of a bulky phenyl group at the 3'-position exerted a profound influence on both receptor binding and antinociceptive effects. Substitution of different groups at the 2'-position distinguished between agonist and antagonist properties. These results demonstrate that structural requirements for receptor activities and recognition are distinctively different.

In vitro effects of nicotine on mitochondrial respiration and superoxide anion generation

In this study, we investigated the effects of nicotine on rat brain mitochondria. The polarographic studies determined the effects on the respiratory chain, whereas enzymatic assays and [3H]-nicotine binding allowed us to precisely identify its target and site of action. The measurements of oxygen consumption showed a significantly concentration-dependent inhibition by nicotine (EC50 was 4.95x10(-11) M), and a maximal decrease of 23.90% at 10(-7) M. Nicotine bound to complex I of the respiratory chain and inhibited the NADH-Ubiquinone reductase activity. We also showed that nicotine and NADH were competitive on complex I. Effects of cotinine, the main nicotine metabolite, and nornicotine, were also investigated: nornicotine inhibited the mitochondrial respiration whereas cotinine did not. Because the complex I generates superoxide anion, we investigated the effects of nicotine, following NBT oxidation, and showed that nicotine was able to inhibit this reactive oxygen species (ROS) generation by 15.74% with an EC50 of 2.02x10(-11) M. In conclusion, the present study shows that nicotine interacts with the complex I of brain mitochondrial respiratory chain and decreases ROS generation. This may explain a part of the beneficial and protective effects of nicotine in few neurodegenerative diseases, as suggested by many epidemiological studies.

Antagonism of the discriminative and aversive stimulus properties of nicotine in C57BL/6J mice

Mice of the C56BL/6J strain were trained to discriminate between nicotine (1.2 mg/kg) and saline in a two-lever drug discrimination procedure under a tandem variable-interval 60 s fixed-ratio 10 schedule of food reinforcement. Mice of the same strain were trained in conditioned taste aversion (CTA) experiments where drinking a saccharin or saline solution was paired with injection of nicotine or vehicle. During testing with both flavours presented simultaneously, a reduction in the intake of the nicotine-paired solution indicated CTA. The nicotine discrimination was acquired successfully and nicotine yielded a steep dose-response curve. The competitive nicotinic antagonist dihydro-beta-erythroidine (DHbetaE, 0.6-3.0 mg/kg) shifted the dose-response for the discriminative stimulus effect of nicotine to the right; the alpha7 nicotinic receptor antagonist methyllycaconitine (MLA, 1.0-10 mg/kg) had no effect. The mice showed strong CTA to 2.0 mg/kg of nicotine and marginally to 0.6 and 1.2 mg/kg of nicotine. DHbetaE (3.0-5.6 mg/kg) attenuated the CTA while MLA (1.0-10 mg/kg) had no effect. These studies show that nicotine has discriminative and aversive stimulus properties in C57BL/6J mice and that the effects are mediated primarily by receptors sensitive to DHbetaE; there was no evidence for the involvement of alpha7 nicotinic receptors.

Blockade of nicotine self-administration with nicotinic antagonists in rats

The reinforcing properties of a variety of drugs abused by humans have been investigated using the technique of intravenous self-administration in the rat. To examine the effect of nicotine dose on nicotine self-administration, Wistar rats were allowed to self-administer various doses of nicotine using a within-subjects Latin square design. An inverted U-shaped dose response curve was obtained, with the highest rates of responding at the 0.03 mg/kg/inf dose. With 1-h daily nicotine self-administration sessions, rats did not appear dependent on nicotine 24 h later, as indicated by the absence of somatic signs of withdrawal after subcutaneous injection of a nicotinic acetylcholine receptor antagonist, mecamylamine (0.57 mg/kg). In another set of studies, pretreatment with subcutaneous mecamylamine or dihydro-beta-erythroidine, two nicotinic acetylcholine receptor antagonists, resulted in significant dose-dependent reductions in nicotine self-administration, at two nicotine doses (0.03 and 0.06 mg/kg/inf). These results indicate that nicotine is an effective reinforcer in Wistar rats under the present parameters, and that these reinforcing effects are mediated by activation of nicotinic acetylcholine receptors.

Dramatic decreases in brain reward function during nicotine withdrawal

Tobacco smoking is a worldwide public health problem. In the United States alone, over 400,000 deaths and $50 billion in medical costs annually are directly attributed to smoking. Accumulated evidence indicates that nicotine is the component of tobacco smoke that leads to addiction, but the means by which nicotine produces addiction remain unclear. Nicotine is less effective as a positive reinforcer than other drugs of abuse in non-dependent animals. Nevertheless, nicotine-withdrawal symptoms, including depressed mood, anxiety, irritability and craving in dependent subjects may contribute to the addictive liability of nicotine. We show here that spontaneous nicotine withdrawal in rats resulted in a significant decrease in brain reward function, as measured by elevations in brain reward thresholds, which persisted for four days. Further, systemic injections of a competitive nicotinic-receptor antagonist led to a dose-dependent increase in brain reward thresholds in chronic nicotine-treated rats. The decreased function in brain reward systems during nicotine withdrawal is comparable in magnitude and duration to that of other major drugs of abuse, and may constitute an important motivational factor that contributes to craving, relapse and continued tobacco consumption in humans.

Nicotinic antagonist administration into the ventral hippocampus and spatial working memory in rats

Nicotinic acetylcholine receptors are important for maintaining optimal memory performance. In order to more fully characterize the involvement of nicotinic systems in memory, the contributions of nicotinic acetylcholine receptor subtypes were investigated. This study targeted the alpha 7 and alpha 4 beta 2 nicotinic receptors in the ventral hippocampus, an area known to be important for spatial working memory. Antagonists of alpha 7 and alpha 4 beta 2 receptors were locally infused into the ventral hippocampus of rats and the effects on memory were examined with the radial-arm maze. The subtype-specific competitive antagonists infused into separate groups of rats were methyllycaconitine citrate (an alpha 7 antagonist) and dihydro-beta-erythroidine hydrobromide (an alpha 4 beta 2 antagonist). Their effects on radial-arm maze performance were contrasted with the non-specific competitive antagonist, D-tubocurarine chloride. Significant deficits in radial-arm maze choice accuracy performance were found at 78.7 micrograms/side for methyllycaconitine and at 106.9 micrograms/side for dihydro-beta-erythroidine. Increased response latency was also seen at these doses. Tubocurarine induced seizures at doses previously reported to have no effect. Wet dog shakes were seen in most rats at 0.1 microgram/side with tubocurarine, 26.3 micrograms/side with methyllycaconitine and 106.9 micrograms/side with dihydro-beta-erythroidine. This study suggests that both alpha 7 and alpha 4 beta 2 nicotinic acetylcholine receptor subtypes are involved in working memory formation and that the hippocampus is a critical site for nicotinic cholinergic involvement in memory function, though the high doses of antagonists needed to produce the memory impairment may have had less than completely specific effects.

Contribution of CNS nicotine metabolites to the neuropharmacological effects of nicotine and tobacco smoking

Nicotine, the principal alkaloid in tobacco products, is generally accepted to be the active pharmacological agent responsible for CNS effects resulting from tobacco use. Arguments are presented in this commentary which take issue with this popular dogma, by providing evidence that nicotine metabolites may also be responsible for the CNS effects commonly attributed to nicotine. CNS effects attributed to nicotine include reinforcing effects, mood elevation, arousal, locomotor stimulant effects, and learning and memory enhancement. The reinforcing and locomotor stimulant effects of nicotine have been suggested to be the result of activation of CNS dopaminergic systems, and nicotine-induced modulation of dopaminergic neurotransmission has been studied in detail. Nicotine acts at a family of nicotinic receptor subtypes composed of multiple subunits; however, the exact composition of the subunits in native nicotinic receptors and the functional significance of the receptor subtype diversity are currently unknown. This nicotinic subtype diversity increases the complexity of the potential mechanisms of action of nicotine and its metabolites. Although peripheral metabolism of nicotine has been studied extensively, metabolism in the CNS has not been investigated to any great extent. Recently, studies from our laboratory have demonstrated that several nicotine metabolites are present in the CNS after acute nicotine administration. Moreover, nicotine metabolites are pharmacologically active in neurochemical and behavioral assays. Thus, CNS effects resulting from nicotine exposure may not be due solely to nicotine, but may result, at least in part, from the actions of nicotine metabolites.