Nicotine ameliorates NMDA receptor antagonist-induced deficits in contextual fear conditioning through high-affinity nicotinic acetylcholine receptors in the hippocampus

NMDA glutamate receptors (NMDARs) and nicotinic acetylcholine receptors (nAChRs) are both involved in learning and synaptic plasticity. Increasing evidence suggests processes mediated by these receptors may interact to modulate learning; however, little is known about the neural substrates involved in these interactive processes. The present studies investigated the effects of nicotine on MK-801 hydrogen maleate (MK-801) and DL-2-Amino-5-phosphonovaleric acid (APV)-induced disruption of contextual fear conditioning in male C57BL/6J mice, using direct drug infusion and selective nAChR antagonists to define the brain regions and the nAChR subtypes involved. Mice treated with MK-801 showed a deficit in contextual fear conditioning that was ameliorated by nicotine. Direct drug infusion demonstrated that the NMDAR antagonists disrupted hippocampal function and that nicotine acted in the dorsal hippocampus to ameliorate the deficit in learning. The high-affinity nAChR antagonist Dihydro-β-erythroidine hydrobromide (DhβE) blocked the effects of nicotine on MK-801-induced deficits while the α7 nAChR antagonist methyllycaconitine citrate salt hydrate (MLA) did not. These results suggest that NMDARs and nAChRs may mediate similar hippocampal processes involved in contextual fear conditioning. Furthermore, these results may have implications for developing effective therapeutics for the cognitive deficits associated with schizophrenia because a large subset of patients with schizophrenia exhibit cognitive deficits that may be related to NMDAR dysfunction and smoke at much higher rates than the healthy population, which may be an attempt to ameliorate cognitive deficits.     

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

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

Genetic background influences the effects of withdrawal from chronic nicotine on learning and high-affinity nicotinic acetylcholine receptor binding in the dorsal and ventral hippocampus

Rationale

The effects of nicotine on cognitive processes may play an important role in nicotine addiction. Nicotine withdrawal impairs hippocampus-dependent learning and genetic factors influence this effect. However, the neural changes that contribute to these impairments are unknown. Chronic nicotine upregulates hippocampal nicotinic acetycholine receptors (nAChRs), which may contribute to cognitive deficits when nicotine administration ceases. If nAChR upregulation underlies withdrawal deficits in learning, then strains of mice exhibiting withdrawal deficits in hippocampus-dependent learning should also show upregulation of hippocampal nAChRs.

Objectives

Here, we examined the effects of nicotine withdrawal on fear conditioning and [³H]epibatidine binding in the dorsal and ventral hippocampus in two inbred mouse strains and their F1 hybrids.

Methods

Male C57BL/6NTac, 129S6/SvEvTac, and B6129SF1/Tac mice were administered chronic nicotine (18 mg/kg/day) for 12 days through osmotic pumps and then were trained and tested in fear conditioning 24 h after cessation of nicotine treatment.

Results

Nicotine withdrawal impaired hippocampus-dependent contextual conditioning in C57BL/6NTac mice but not 129S6/SvEvTac or B6129SF1/Tac mice; no changes were observed in hippocampus-independent cued fear conditioning. Upregulated [³H]epibatidine binding was found in the dorsal, but not ventral, hippocampus of C57BL/6NTac mice and in the ventral hippocampus of B6129SF1/Tac mice after chronic nicotine.

Conclusions

Upregulation of high-affinity binding sites in the dorsal hippocampus of C57BL/6NTac mice, the only strain that exhibited nAChR upregulation in this region and withdrawal deficits in contextual conditioning, suggests that upregulation of high-affinity binding sites in the dorsal hippocampus mediates, in part, nicotine withdrawal deficits in contextual conditioning and genetic background modulates these effects.     

β2 subunit-containing nicotinic receptors mediate the enhancing effect of nicotine on trace cued fear conditioning in C57BL/6 mice

Rationale Previous research indicates that acute nicotine administration enhances the acquisition of contextual fear conditioning and trace cued fear conditioning. Pharmacological inhibition of α4β2 nicotinic acetylcholine receptors (nAChRs), but not α7 nAChRs, blocked the enhancing effect of nicotine on contextual fear conditioning. Similarly, genetic deletion of the β2 nAChR subunit but not the α7 nAChR subunit blocked the enhancing effect of nicotine on contextual fear conditioning. Objectives In the present study, nAChR subunit knockout mice were used to compare the involvement of β2 subunit-containing nAChRs and α7 subunit-containing nAChRs in the effects of nicotine on hippocampus-dependent trace cued fear conditioning and contextual fear conditioning. Methods β2 nAChR subunit knockout mice, α7 nAChR subunit knockout mice, and their wild-type littermates received either nicotine or saline 5 minutes before training and testing. Mice were trained using five conditioned stimulus (CS; 30 s, 85 dB white noise)—trace (30 s)—unconditioned stimulus (US; 2 s footshock) pairings. Freezing to the context and freezing to the CS were assessed 24 h later. Results Both contextual and trace cued fear conditioning were enhanced by nicotine administration in wild-type littermates and in α7 nAChR subunit knockout mice. In contrast, neither contextual fear conditioning nor trace cued fear conditioning was enhanced by nicotine administration in β2 nAChR subunit knockout mice. Conclusions These results suggest that β2 subunit-containing nAChRs but not α7 nAChR subunit-containing nAChRs are critically involved in the enhancing effect of nicotine on contextual and trace cued fear conditioning.     

Bupropion dose-dependently reverses nicotine withdrawal deficits in contextual fear conditioning

Bupropion, a norepinephrine and dopamine reuptake inhibitor and nicotinic acetylcholine receptor antagonist, facilitates smoking cessation and reduces some symptoms of nicotine withdrawal. However, the effects of bupropion on nicotine withdrawal-associated deficits in learning remain unclear. The present study investigated whether bupropion has effects on contextual and cued fear conditioning following withdrawal from chronic nicotine or when administered alone. Bupropion was administered alone for a range of doses (2.5, 5, 10, 20 or 40 mg/kg), and dose-dependent impairments in contextual and cued fear conditioning were observed (20 or 40 mg/kg). Follow-up studies investigated if bupropion disrupted acquisition or expression of fear conditioning. Bupropion (40 mg/kg) administration on training day only produced deficits in contextual fear conditioning. Alternatively, bupropion (20 or 40 mg/kg) administration during testing dose-dependently produced deficits in contextual and cued fear conditioning. To test the effect of bupropion on nicotine withdrawal, mice were withdrawn from 12 days of chronic nicotine (6.3 mg/kg/day) or saline treatment. Withdrawal from chronic nicotine disrupted contextual fear conditioning; however, 5 mg/kg bupropion reversed this deficit. Overall, these results indicate that a low dose of bupropion can reverse nicotine withdrawal deficits in contextual fear conditioning, but that high doses of bupropion produce deficits in fear conditioning.     

The effects of DHBE and MLA on nicotine-induced enhancement of contextual fear conditioning in C57BL/6 mice

Rationale Previous research indicates that nicotine administration enhances hippocampus-dependent forms of learning, including contextual fear conditioning. This effect is blocked by mecamylamine, a noncompetitive, broad-spectrum nicotinic receptor antagonist. Objectives The present study extends previous research by further characterizing the nicotinic acetylcholinergic receptor (nAChR) subtypes through which nicotine acts to enhance contextual fear conditioning. Methods C57BL/6J mice were trained with two conditioned stimulus (CS; 30 s, 85-dB white noise)–unconditioned stimulus (US; 2 s, 0.57-mA foot shock) pairings and tested 24 h later for contextual and cued fear conditioning. The effects of the α7 nAChR antagonist methyllycaconitine (MLA; 1.00, 10.00, and 20.00 mg/kg) and the effects of the α4β2 nAChR antagonist dihydro-beta-erythroidine (DHBE; 1.00, 3.00, and 6.00 mg/kg) on cued and contextual fear conditioning and on the enhancement of contextual fear conditioning by nicotine (0.25 mg/kg) were examined. Results We demonstrate that DHBE (all doses) administration attenuates the enhancing effect of nicotine on contextual fear conditioning, and MLA administration has no significant effect on the enhancement of contextual fear conditioning by nicotine. Conclusions The data suggest that non-α7 nAChRs (most likely α4β2 nAChRs) underlie the enhancement of contextual fear conditioning by nicotine.     

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 (0.25 mg/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 (5 mg/kg), but not the heteromeric (β2 or β4 subunit-containing) nAChR antagonist dihydro-β-erythroidine (DHβE, 3 mg/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.     

Interactive effects of ethanol and nicotine on learning in C57BL/6J mice depend on both dose and duration of treatment

Objective and rationale Alcohol and nicotine are commonly co-abused; one possible explanation for co-abuse is that each drug ameliorates the aversive effects of the other. Both drugs have dose-dependent effects on learning and memory. Thus, this study examined the interactive effects of acute ethanol and acute, chronic, or withdrawal from chronic nicotine on fear conditioning in C57BL/6J mice. Materials and methods Conditioning consisted of auditory conditioned stimulus-foot-shock unconditioned stimulus pairings. For acute studies, saline or ethanol, then saline or nicotine was administered before training, and saline or nicotine was also administered before testing. For chronic and withdrawal studies, saline or nicotine was administered chronically, and ethanol or saline was administered before training. Results Acute nicotine (0.09 mg/kg) reversed ethanol-induced deficits (1.0 and 1.5 g/kg) in contextual and cued fear conditioning, whereas a low dose of ethanol (0.25 g/kg) reversed nicotine (6.3 mg kg⁻¹ day⁻¹) withdrawal-induced deficits in contextual conditioning. Tolerance developed for the effects of nicotine on ethanol-induced deficits in conditioning and cross-tolerance between chronic nicotine and acute ethanol was seen for the enhancing effects of ethanol on conditioning. Conclusions The complex and sometimes polar actions of ethanol and nicotine on behavior may contribute to co-abuse of these drugs. Specifically, smoking may initially reduce the aversive effects of ethanol, but tolerance develops for this effect. In addition, low doses of alcohol may lessen nicotine withdrawal symptoms.     

Nicotine withdrawal disrupts new contextual learning

Interactions between nicotine and learning could contribute to nicotine addiction. Although previous research indicates that nicotine withdrawal disrupts contextual learning, the effects of nicotine withdrawal on contextual memories acquired before withdrawal are unknown. The present study investigated whether nicotine withdrawal disrupted recall of prior contextual memories by examining the effects of nicotine withdrawal on recall of nicotine conditioned place preference (CPP) and contextual fear conditioning. C57BL/6J mice trained in CPP exhibited a significant preference for an initially non-preferred chamber that was paired with 0.35 mg/kg nicotine. Following CPP, mice were implanted with mini-osmotic pumps containing 6.3 mg/kg/d nicotine or saline. Pumps were removed twelve days later and nicotine CPP was retested 24 h later. Mice withdrawn from chronic nicotine exhibited CPP, suggesting that older drug-context associations are not disrupted by nicotine withdrawal. One hour later, the same mice were trained in contextual and cued fear conditioning; nicotine withdrawal disrupted contextual but not cued fear conditioning. A subsequent experiment demonstrated that nicotine withdrawal did not disrupt recall of contextual or cued fear conditioning when acquisition occurred before nicotine withdrawal. These data suggest that nicotine withdrawal disrupts new contextual learning, but does not alter contextual learning that occurred before withdrawal.     

Characterizing functional α6β2 nicotinic acetylcholine receptors in vitro: mutant β2 subunits improve membrane expression, and fluorescent proteins reveal responsive cells

α6* nicotinic acetylcholine receptors (nAChRs) are highly expressed in mesostriatal and nigrostriatal dopaminergic systems, and participate in motor control, reward, and learning and memory. In vitro functional expression of α6* nAChRs is essential for full pharmacological characterization of these receptors and for drug screening, but has been challenging. We expressed eGFP-tagged-α6 and β2 nAChR subunits in Neuro-2a cells, leading to functional channels. Inward currents were elicited with 300 μM ACh in 26% (5/19) of cells with evenly expressed α6-eGFP in cytoplasm and periphery. We dramatically increased chances of detecting functional α6-eGFPβ2 nAChRs by (i) introducing two endoplasmic reticulum (ER) export-enhancing mutations into β2 subunits, and (ii) choosing cells with abundant Sec24D-mCherry-labeled ER exit sites. Both manipulations also modestly increased α6-eGFPβ2 nAChR current amplitude. α6-eGFPβ2 nAChRs were also activated by nicotine and by TC-2403. The α6-eGFPβ2 currents were desensitized by 1 μM nicotine, blocked by α-conotoxin MII, partially inhibited by dihydro-β-erythroidine, and potentiated by extracellular Ca²⁺. Single-channel recordings showed that α6-eGFPβ2 nAChRs had similar single-channel conductance to, but longer open time than, α4-eGFPβ2 nAChRs. These methods provide avenues for developing cell lines expressing subtypes of α6* nAChRs for both pharmacological study and drug screening.     

Ganglionic nicotinic acetylcholine receptor activation by the novel agonist ABT-418

ABT-418 was functionally characterized as a neuronal nicotinic acetylcholine receptor (nAChR) channel agonist using preparations that contain nAChRs characteristic of the ganglionic subtypes. In PC12 cells, ABT-418, like (?)nicotine, activated an inward current that decayed within seconds in the continued presence of agonist. ABT-418 was 4-fold less potent than (?)nicotine (EC₅₀ = 214 ± 30 μM and 52 ± 4 μM, respectively) while the efficacy of ABT-418 was not significantly different from (?)nicotine when the peak response amplitude was measured. Responses to 300 μM ABT-418 were reversibly inhibited 81 ± 3% by 10 μM mecamylamine, 38 ± 1% by 10 μM dihydro-β-erythroidine, and 82 ± 2% by 100 μM dihydro-β-erythroidine. These nAChR antagonists affected the response to (?)nicotine similarly. Furthermore, responses to maximal concentrations of ABT-418 (3 mM) and (?)nicotine (1 mM) were not additive, consistent with ABT-418 and (?)nicotine acting through the same receptor(s). However, the Hill coefficient for ABT-418 (1.18 ± 0.20) was smaller than that for (?)nicotine (1.77 ± 0.18), and high concentrations of ABT-418 appeared to elicit a more rapidly decaying response than did (?)nicotine. In the rat superior cervical sympathetic ganglion also, ABT-418 was 2.5-fold less potent than (?)nicotine in blocking nicotinic transmission, presumably through nicotinic receptor desensitization. These studies provide the most direct evidence that ABT-418 activates nicotinic cholinergic channels, and suggest that ABT-418 would have reduced potency compared to (?)nicotine in peripheral ganglia, consistent with the reduced side effect liability of this novel nAChR agonist. © Wiley-Liss, Inc.     

α7β2 nicotinic acetylcholine receptors assemble, function, and are activated primarily via their α7-α7 interfaces

We investigated assembly and function of nicotinic acetylcholine receptors (nAChRs) composed of α7 and β2 subunits. We measured optical and electrophysiological properties of wild-type and mutant subunits expressed in cell lines and Xenopus laevis oocytes. Laser scanning confocal microscopy indicated that fluorescently tagged α7 and β2 subunits colocalize. F?rster resonance energy transfer between fluorescently tagged subunits strongly suggested that α7 and β2 subunits coassemble. Total internal reflection fluorescence microscopy revealed that assemblies localized to filopodia-like processes of SH-EP1 cells. Gain-of-function α7 and β2 subunits confirmed that these subunits coassemble within functional receptors. Moreover, α7β2 nAChRs composed of wild-type subunits or fluorescently tagged subunits had pharmacological properties similar to those of α7 nAChRs, although amplitudes of α7β2 nAChR-mediated, agonist-evoked currents were generally ~2-fold lower than those for α7 nAChRs. It is noteworthy that α7β2 nAChRs displayed sensitivity to low concentrations of the antagonist dihydro-β-erythroidine that was not observed for α7 nAChRs at comparable concentrations. In addition, cysteine mutants revealed that the α7-β2 subunit interface does not bind ligand in a functionally productive manner, partly explaining lower α7β2 nAChR current amplitudes and challenges in identifying the function of native α7β2 nAChRs. On the basis of our findings, we have constructed a model predicting receptor function that is based on stoichiometry and position of β2 subunits within the α7β2 nAChRs.     

Elevation of BACE in an Aβ rat model of Alzheimer's disease: exacerbation by chronic stress and prevention by nicotine

In Alzheimer's disease (AD), progressive accumulation of β-amyloid (Aβ) peptides impairs nicotinic acetylcholine receptor (nAChR) function by a mechanism that may involve α7 and α4β2-nAChR subtypes. Additionally, the beta-site amyloid precursor protein (APP)-cleaving enzyme (BACE), the rate-limiting enzyme in the pathogenic Aβ production pathway, is expressed at high levels in hippocampal and cortical regions of AD brains. We measured hippocampal area CA1 protein levels of BACE and α7- and α4β2-nAChR subunits using an Aβ rat model of AD (14-d osmotic pump i.c.v. infusion of 300 pmol/d Aβ peptides) in the presence and absence of chronic stress and/or chronic nicotine treatment. There was a significant increase in the levels of BACE in Aβ-infused rats, which were markedly intensified by chronic (4-6 wk) stress, but were normalized in Aβ rats chronically treated with nicotine (1 mg/kg b.i.d.). The levels of the three subunits α7, α4 and β2 were significantly decreased in Aβ rats, but these were also normalized in Aβ rats chronically treated with nicotine. Chronic stress did not further aggravate the reduction of nAChRs in Aβ-infused rats. The increased BACE levels and decreased nAChR levels, which are established hallmarks of AD, provide additional support for the validity of the Aβ i.c.v.-infused rat as a model of AD.     

Atomoxetine reverses nicotine withdrawal-associated deficits in contextual fear conditioning.

Recent evidence suggests that the cognitive symptoms of nicotine withdrawal and the cognitive symptoms of attention deficit hyperactivity disorder (ADHD) may share neural correlates. Thus, therapeutics that ameliorate ADHD symptoms may also ameliorate nicotine-withdrawal symptoms. The present research tested this hypothesis in an animal model of nicotine withdrawal-associated cognitive deficits using atomoxetine, a norepinephrine reuptake inhibitor that is approved by the FDA to treat the symptoms of ADHD. C57BL/6 mice were prepared with osmotic minipumps that administered 6.3 mg/kg/day of nicotine or saline, and the minipumps were removed after 12 days of continuous treatment. Twenty-four hours later, mice were trained in delay fear conditioning using two paired presentations of an auditory conditioned stimulus (CS) with a footshock unconditioned stimulus. Testing for freezing in response to the training context and for freezing in response to the CS occurred the next day. Nicotine-withdrawn mice and their saline-treated counterparts received either saline or atomoxetine before training and the context test. Consistent with previous research, the results indicate that mice withdrawn from chronic nicotine demonstrated lower levels of contextual fear conditioning than mice that were not withdrawn from chronic nicotine. Atomoxetine dose-dependently reversed the deficit, suggesting that nicotine withdrawal may be associated with changes in noradrenergic function, acetylcholinergic function, and/or with changes in cell signaling cascades that are activated by both nicotine and norepinephrine. These data suggest that atomoxetine may be efficacious for treating nicotine withdrawal-associated cognitive deficits that promote relapse in abstinent smokers.     

Genetic matters: Thirty years of progress using mouse models in nicotinic research

This research update summarizes thirty years of studies on genetic influences on responses to the acute or chronic administration of nicotine. Early studies established that various inbred mice are differentially sensitive to the effects of the drug. Classical genetic analyses confirmed that nicotine effects on locomotion, body temperature and seizures are heritable. A significant inverse correlation between the locomotor and hypothermic effects and the density of nicotine binding sites suggested that differential expression α4β2-neuronal nicotinic acetylcholine receptor (nAChR) mediated some of this genetic variability. Subsequent studies with α4 and β2 nAChR null (decreased sensitivity) and gain of function mutants (increased sensitivity) supports the role of the α4β2*nAChR subtype. However, null mutant mice still respond to nicotine, indicating that other nAChR subtypes also mediate these responses. Mice differing in initial sensitivity to nicotine also differ in tolerance development following chronic treatment: those mice that are initially more sensitive to nicotine develop tolerance at lower treatment doses than less sensitive mice, indicating that tolerance is an adaptive response to the effects of nicotine. In contrast, the sensitivity of mice to pre-pulse inhibition of acoustic startle response is correlated with the expression of α7-nAChR. While genetic variability in nAChR expression and function is an important factor contributing to differences in response to nicotine, the observations that altered activity of opioid, glutamate, and cannabinoid receptors among others also change nicotine sensitivity reinforces the proposal that the genetics of nicotine response is more complex than differences in nAChRs.     

Long-term exposure to nicotine markedly reduces kynurenic acid in rat brain — In vitro and ex vivo evidence

Kynurenic acid (KYNA) is a recognized broad-spectrum antagonist of excitatory amino acid receptors with a particularly high affinity for the glycine co-agonist site of the N-methyl-d-aspartate (NMDA) receptor complex. KYNA is also a putative endogenous neuroprotectant. Recent studies show that KYNA strongly blocks α7 subtype of nicotinic acetylcholine receptors (nAChRs). The present studies were aimed at assessing effects of acute and chronic nicotine exposure on KYNA production in rat brain slices in vitro and ex vivo. In brain slices, nicotine significantly increased KYNA formation at 10 mM but not at 1 or 5 mM. Different nAChR antagonists (dihydro-β-erythroidine, methyllycaconitine and mecamylamine) failed to block the influence exerted by nicotine on KYNA synthesis in cortical slices in vitro. Effects of acute (1 mg/kg, i.p.), subchronic (10-day) and chronic (30-day) administration of nicotine in drinking water (100 μg/ml) on KYNA brain content were evaluated ex vivo. Acute treatment with nicotine (1 mg/kg i.p.) did not affect KYNA level in rat brain. The subchronic exposure to nicotine in drinking water significantly increased KYNA by 43%, while chronic exposure to nicotine resulted in a reduction in KYNA by 47%. Co-administration of mecamylamine with nicotine in drinking water for 30 days reversed the effect exerted by nicotine on KYNA concentration in the cerebral cortex. The present results provide evidence for the hypothesis of reciprocal interaction between the nicotinic cholinergic system and the kynurenine pathway in the brain.     

Multiple CNS nicotinic receptors mediate l-dopa-induced dyskinesias: Studies with parkinsonian nicotinic receptor knockout mice

Accumulating evidence supports the idea that drugs acting at nicotinic acetylcholine receptors (nAChRs) may be beneficial for Parkinson's disease, a neurodegenerative movement disorder characterized by a loss of nigrostriatal dopaminergic neurons. Nicotine administration to parkinsonian animals protects against nigrostriatal damage. In addition, nicotine and nAChR drugs improve l-dopa-induced dyskinesias, a debilitating side effect of l-dopa therapy which remains the gold-standard treatment for Parkinson's disease. Nicotine exerts its antidyskinetic effect by interacting with multiple nAChRs. One approach to identify the subtypes specifically involved in l-dopa-induced dyskinesias is through the use of nAChR subunit null mutant mice. Previous work with β2 and α6 nAChR knockout mice has shown that α6β2* nAChRs were necessary for the development/maintenance of l-dopa-induced abnormal involuntary movements (AIMs). The present results in parkinsonian α4 nAChR knockout mice indicate that α4β2* nAChRs also play an essential role since nicotine did not reduce l-dopa-induced AIMs in such mice. Combined analyses of the data from α4 and α6 knockout mice suggest that the α6α4β2β3 subtype may be critical. In contrast to the studies with α4 and α6 knockout mice, nicotine treatment did reduce l-dopa-induced AIMs in parkinsonian α7 nAChR knockout mice. However, α7 nAChR subunit deletion alone increased baseline AIMs, suggesting that α7 receptors exert an inhibitory influence on l-dopa-induced AIMs. In conclusion, α6β2*, α4β2* and α7 nAChRs all modulate l-dopa-induced AIMs, although their mode of regulation varies. Thus drugs targeting one or multiple nAChRs may be optimal for reducing l-dopa-induced dyskinesias in Parkinson's disease.     

Recent advances in understanding nicotinic receptor signaling mechanisms that regulate drug self-administration behavior

Tobacco smoking is one of the leading causes of disease and premature death in the United States. Nicotine is considered the major reinforcing component in tobacco smoke responsible for tobacco addiction. Nicotine acts in the brain through the neuronal nicotinic acetylcholine receptors (nAChRs). The predominant nAChR subtypes in mammalian brain are those containing α4 and β2 subunits. The α4β2 nAChRs, particularly those located in the mesoaccumbens dopamine pathway, play a key role in regulating the reinforcing properties of nicotine. Considering that twelve mammalian nAChR subunits have been cloned, it is likely that nAChRs containing subunits in addition to, or other than, α4 and β2 also play a role in the tobacco smoking habit. Consistent with this possibility, human genome-wide association studies have shown that genetic variation in the CHRNA5–CHRNA3–CHRNB4 gene cluster located in chromosome region 15q25, which encode the α5, α3 and β4 nAChR subunits, respectively, increases vulnerability to tobacco addiction and smoking-related diseases. Most recently, α5-containing nAChRs located in the habenulo-interpeduncular tract were shown to limit intravenous nicotine self-administration behavior in rats and mice, suggesting that deficits in α5-containing nAChR signaling in the habenulo-interpeduncular tract increases vulnerability to the motivational properties of nicotine. Finally, evidence suggests that nAChRs may also play a prominent role in controlling consumption of addictive drugs other than nicotine, including cocaine, alcohol, opiates and cannabinoids. The aim of the present review is to discuss recent preclinical findings concerning the identity of the nAChR subtypes that regulate self-administration of nicotine and other drugs of abuse.