The effects of nicotine, varenicline, and cytisine on schedule-controlled responding in mice: differences in α4β2 nicotinic receptor activation

Nicotine, varenicline, and cytisine are pharmacotherapies for tobacco dependence; the extent to which their in vivo effects vary as a function of differences in nicotinic acetylcholine receptor agonism is not clear. Male C57BL/6 J mice responding under a fixed ratio 30 schedule of food delivery were used to establish the potency and time course of nicotine, varenicline, and cytisine; antagonism was examined with the non-competitive, non-selective antagonist mecamylamine and the competitive, α4β2 nicotinic receptor antagonist dihydro-β-erythroidine (DHβE). Intraperitoneal nicotine, varenicline, and cytisine dose-dependently decreased responding; nicotine was more potent (ED₅₀ value = 0.83 mg/kg) than varenicline (ED₅₀ value = 2.51 mg/kg) and cytisine (ED₅₀ value = 2.97 mg/kg). The agonists had a similar time course including a rapid onset (5 min or less) and relatively short duration of action (30 min). Mecamylamine dose-dependently attenuated the rate-decreasing effects of a fixed dose of nicotine (1.78 mg/kg), varenicline (5.6 mg/kg), and cytisine (5.6 mg/kg). Mecamylamine (1 mg/kg) produced parallel rightward shifts in the dose–response curves for nicotine (3.3-fold), varenicline (3.1-fold), and cytisine (2.3-fold). In contrast, DHβE (3.2 mg/kg) produced 2-fold antagonism of nicotine and did not antagonize varenicline or cytisine. The data strongly suggest that nicotinic acetylcholine receptors mediate the effects of the agonists to decrease operant responding in mice. However, α4β2 receptor agonism appears to contribute partially to the rate-decreasing effects of nicotine but not to the rate-decreasing effects of varenicline and cytisine. Differential activation of α4β2 receptors in vivo might contribute to differences in the effectiveness of these smoking cessation aids.     

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-β-erythroidine (DHβE, 0.6–3.0 mg/kg) shifted the dose–response for the discriminative stimulus effect of nicotine to the right; the 7 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. DHβE (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 DHβE; there was no evidence for the involvement of 7 nicotinic receptors.     

Effects of chronic sazetidine-A, a selective α4β2 neuronal nicotinic acetylcholine receptors desensitizing agent on pharmacologically-induced impaired attention in rats

Rationale

Nicotine and nicotinic agonists have been shown to improve attentional function. Nicotinic receptors are easily desensitized, and all nicotinic agonists are also desensitizing agents. Although both receptor activation and desensitization are components of the mechanism that mediates the overall effects of nicotinic agonists, it is not clear how each of the two opposed actions contributes to attentional improvements. Sazetidine-A has high binding affinity at α4β2 nicotinic receptors and causes a relatively brief activation followed by a long-lasting desensitization of the receptors. Acute administration of sazetidine-A has been shown to significantly improve attention by reversing impairments caused by the muscarinic cholinergic antagonist scopolamine and the NMDA glutamate antagonist dizocilpine.

Methods

In the current study, we tested the effects of chronic subcutaneous infusion of sazetidine-A (0, 2, or 6 mg/kg/day) on attention in Sprague-Dawley rats. Furthermore, we investigated the effects of chronic sazetidine-A treatment on attentional impairment induced by an acute administration of 0.02 mg/kg scopolamine.

Results

During the first week period, the 6-mg/kg/day sazetidine-A dose significantly reversed the attentional impairment induced by scopolamine. During weeks 3 and 4, the scopolamine-induced impairment was no longer seen, but sazetidine-A (6 mg/kg/day) significantly improved attentional performance on its own. Chronic sazetidine-A also reduced response latency and response omissions.

Conclusions

This study demonstrated that similar to its acute effects, chronic infusions of sazetidine-A improve attentional performance. The results indicate that the desensitization of α4β2 nicotinic receptors with some activation of these receptors may play an important role in improving effects of sazetidine-A on attention.     

The sensitizing effect of acute nicotine on amphetamine-stimulated behavior and dopamine efflux requires activation of β2 subunit-containing nicotinic acetylcholine receptors and glutamate N-methyl-D-aspartate receptors

Nicotine has been demonstrated to enhance the subsequent use of illicit drugs in animals and humans. We previously demonstrated in female, Holtzman rats that one low dose of nicotine will potentiate locomotor activity and dopamine (DA) efflux in response to a subsequent low dose of d-amphetamine (AMPH) given 1-4 h later. In the present study, we show this also occurs in male rats and characterize the receptors required for the rapid sensitizing effect of nicotine on AMPH-stimulated locomotor behavior and AMPH-induced DA efflux. Pretreatment of male, Holtzman rats with a low dose (0.1 mg/kg, i.p.) of nicotine 2-4 h before a challenge with AMPH (0.32 mg/kg, i.p.) enhanced locomotor behavior as compared to saline pretreatment. Dihydro-β-erythroidine (DHβE), a relatively selective antagonist at β2 subunit-containing (β2?) nicotinic acetylcholine receptors (nAChR), but not methyllycaconitine (MLA), a relatively selective antagonist at α7 nAChRs, blocked the sensitizing effect of nicotine on AMPH-stimulated locomotor activity. Pretreatment with varenicline, a partial agonist selective for β2? nAChRs, blocked the sensitizing effect of nicotine on AMPH-stimulated locomotor behavior. Nicotine pretreatment sensitized AMPH-induced DA overflow in slices from ventral (nucleus accumbens, NAc), but not dorsal striatum as compared to saline-pretreated rats. Nicotine sensitization of the DA overflow was blocked by DHβE. Pretreatment with the glutamate N-methyl-D-aspartate (NMDA) receptor antagonist (+)-MK-801 (0.1 mg/kg, s.c.) 30 min before nicotine blocked sensitization of both locomotion and DA overflow in response to AMPH challenge. These results demonstrate that activation of the β2? nAChRs and NMDA receptors are required for the rapid sensitizing effect of nicotine on AMPH actions. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.     

Ventral hippocampal alpha 7 nicotinic receptor blockade and chronic nicotine effects on memory performance in the radial-arm maze.

Chronic nicotine administration has been shown to significantly improve working memory. Nicotinic involvement in memory function critically involves the ventral hippocampus. Local ventral hippocampal infusions of the nicotinic antagonists mecamylamine, dihydro-beta-erythroidine (DH beta E) and methyllycaconitine (MLA) significantly impair working memory. The impairment caused by hippocampal infusion of the alpha 4 beta 2 antagonist DH beta E is reversed by chronic systemic nicotine. This study determined the interaction of chronic systemic nicotine with acute ventral hippocampal infusions of the alpha 7 antagonist MLA. Adult female Sprague-Dawley rats were trained on an 8-arm radial maze working memory task. Then they underwent ventral hippocampal cannulation and received sc implants of minipumps delivering nicotine (0 or 5 mg/kg/day for 28 days). Acute ventral hippocampal infusions of MLA (0, 4.88, 14.64 and 43.92 microg/side) were given during 3-4 weeks of chronic nicotine. MLA caused a significant dose-related memory impairment. In the rats not receiving nicotine, the 14.64 and 43.92 microg/side MLA doses caused significant memory impairment. Chronic systemic nicotine exposure did not block the MLA-induced memory impairment. Comparing the current results with MLA with previous results with DH beta E, equimolar ventral hippocampal DH beta E more effectively impaired memory than MLA, but the DH beta E-induced impairment was more effectively reversed by chronic systemic nicotine administration.     

Patterns of nicotinic receptor antagonism II: Cardiovascular effects in rats

Background

Tobacco cessation pharmacotherapies currently are limited to nicotine itself, the partial nicotine agonists varenicline and cytisine, and the antidepressant bupropion. Compared with agonists, nicotinic antagonists such as the noncompetitive, nonselective compound mecamylamine, and the competitive, α4β2-preferring antagonist dihydro-β-erythroidine (DHβE) may be a novel approach to the treatment of tobacco smoking as both are effective antagonists of nicotine's central effects. Considering nicotinic acetylcholine receptors mediate critical peripheral effects of acetylcholine, such as cardiovascular effects, it is important to study how nicotinic antagonists would alter the cardiovascular system and the cardiovascular changes induced by nicotine.

Methods

The effects of several nicotinic agonists and antagonists on blood pressure and heart rate were measured in conscious, unrestrained rats following parenteral administration using a telemetry system.

Results

Nicotine and other nicotinic receptor agonists (epibatidine, varenicline, and cytisine) produced similar increases in blood pressure, whereas their effects on heart rate were biphasic. The cardiovascular changes were attenuated by the nonselective nicotine antagonist, mecamylamine, but the peripherally restricted antagonist hexamethonium blocked only the agonist-induced changes in blood pressure. The α7-preferring antagonist, MLA, and the α4β2-preferring antagonist, DHβE, were much less effective in blocking the agonist-induced cardiovascular changes, indicating that nicotine's cardiovascular effects, are due to activation at autonomic ganglia involving nicotinic receptor subtypes other than α4, α7, or β2.

Conclusions

The data indicate that the cardiovascular effects of nicotine and nicotine-like agents are mediated through receptor mechanisms that are distinct from those that mediate the central effects of nicotine.     

Effects of sazetidine-A, a selective α4β2 nicotinic acetylcholine receptor desensitizing agent on alcohol and nicotine self-administration in selectively bred alcohol-preferring (P) rats

Rationale

Manipulations of nicotinic cholinergic receptors have been shown to influence both alcohol and nicotine intake. Sazetidine-A [6-(5(((S)-azetidine-2-yl)methoxy)pyridine-3-yl)hex-5-yn-1-ol] is a novel compound that potently and selectively desensitizes α4β2 nicotinic receptors with only modest receptor activation.

Objectives

The goal of the present study was to examine the effects of sazetidine-A on alcohol and nicotine self-administration in alcohol-preferring (P) rats.

Methods

P rats were given the choice of water or alcohol. Once stable baselines were established, the acute (0, 0.1, 0.3, 1, and 3 mg/kg, s.c.) and chronic (3 mg/kg for 10 days) effects of sazetidine-A on alcohol intake were assessed. Naltrexone (2.5 mg/kg) served as a positive control. The effect of sazetidine-A (3 mg/kg) and naltrexone (4 mg/kg) on saccharin (0.2%) preference was also assessed. In addition, the acute effects of sazetidine-A (3 mg/kg) and naltrexone (4 mg/kg) on alcohol intake after alcohol deprivation were evaluated. In another experiment, the effects of sazetidine-A (0, 1, or 3 mg/kg) on IV nicotine self-administration in P and NP rats were assessed.

Results

Sazetidine-A caused a dose-dependent reduction in alcohol intake. Chronic sazetidine-A also effectively reduced alcohol intake until the seventh day of treatment, when partial tolerance appeared to develop. In the post-deprivation study, sazetidine-A significantly reduced alcohol intake and preference. Sazetidine-A at 3 mg/kg significantly reduced nicotine self-administration in both lines.

Conclusions

Sazetidine-A significantly reduced alcohol and nicotine intake in P rats that self-administer higher levels of both drugs. Sazetidine-A may hold promise for the treatment of alcohol and nicotine addiction.     

Varenicline attenuates nicotine-enhanced brain-stimulation reward by activation of α4β2 nicotinic receptors in rats

Varenicline, a partial α4β2 and full α7 nicotinic receptor agonist, has been shown to inhibit nicotine self-administration and nicotine-induced increases in extracellular dopamine in the nucleus accumbens. In the present study, we investigated whether varenicline inhibits nicotine-enhanced electrical brain-stimulation reward (BSR), and if so, which receptor subtypes are involved. Systemic administration of nicotine (0.25-1.0 mg/kg, i.p.) or varenicline (0.03-3 mg/kg, i.p.) produced biphasic effects, with low doses producing enhancement (e.g., decreased BSR threshold), and high doses inhibiting BSR. Pretreatment with low dose (0.03-1.0 mg/kg) varenicline dose-dependently attenuated nicotine (0.25 or 0.5 mg/kg)-enhanced BSR. The BSR-enhancing effect produced by varenicline was blocked by mecamylamine (a high affinity nicotinic receptor antagonist) or dihydro-β-erythroidine (a relatively selective nicotinic α4-containing receptor antagonist), but not methyllycaconitine (a selective α7 receptor antagonist), suggesting an effect mediated by activation of α4β2 receptors. This suggestion is supported by findings that the α4β2 receptor agonist SIB-1765F produced a dose-dependent enhancement of BSR, while pretreatment with SIB-1765F attenuated nicotine (0.5 mg/kg)-enhanced BSR. In contrast, the selective α7 receptor agonist ARR-17779, altered neither BSR itself nor nicotine-enhanced BSR, at any dose tested. These findings suggest that: 1) varenicline inhibits nicotine-enhanced BSR, supporting its use as a smoking cessation aid; and 2) varenicline-enhanced BSR by itself and varenicline's anti-nicotine effects are mediated by activation of α4β2, but not α7, receptors.     

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.     

Nicotine-induced hypothermia through an indirect dopaminergic mechanism

The effect of nicotine on core body temperature was studied in mice. Intraperitoneal (i.p.) injection of nicotine (0.5, 1 and 2 mg/kg) induced a dose-dependent hypothermia. The response was inhibited by reserpine (5 mg/kg), the centrally active nicotinic receptor antagonist mecamylamine (0.1-1 mg/kg) and the D-2 dopamine receptor antagonist sulpiride (25-100 mg/kg). The β-adrenoceptor antagonist propranolol (5 and 10 mg/kg) and the serotonergic blocker methysergide (5 and 10 mg/kg) did not inhibit but increased the nicotine response. The α-adrenoceptor antagonist phenoxybenzamine, the antimuscarinic agent atropine, the D-1 dopamine receptor antagonist SCH 23390, the peripheral dopamine antagonist domperidone and the peripheral nicotinic antagonist hexamethonium did not alter the nicotine-induced hypothermia. It is concluded that nicotine may cause a fall in core body temperature through a central dopaminergic mechanism.     

Selective antagonism of behavioural effects of nicotine by dihydro-β-erythroidine in rats

The influence of the nicotine antagonist dihydro-β-erythroidine (DHβE) was examined on various behavioural effects of nicotine in rats. Motor activity was recorded in photocell cages whereas discriminative stimulus effects were examined using two-lever drug discrimination procedures with a tandem schedule of food reinforcement (n?=?8 throughout). DHβE (0.1–3.2?mg/kg) failed to antagonise the decreases in motor activity that nicotine (0.4–0.6?mg/kg) produced in experimentally naive rats, whereas mecamylamine (1.5?mg/kg) completely blocked this effect of nicotine. DHβE (0.1–3.2?mg/kg) antagonised the increases in motor activity that nicotine (0.4?mg/kg) produced in rats with extensive previous exposure to both nicotine and the photocell apparatus. In rats trained to discriminate either 0.1 or 0.4?mg/kg nicotine from saline, DHβE (0.1–3.2?mg/kg) blocked the discriminative stimulus effect of nicotine. The block of the discriminative effect could be reversed by increasing the dose of nicotine; DHβE (1.6?mg/kg) shifted the dose-response curve for nicotine discrimination to the right by a factor of 9.4. In addition, nicotine in doses of 0.32–0.64?mg/kg decreased the overall rate of lever pressing but DHβE (1.6?mg/kg) did not influence the dose-response curve for this effect. Thus, DHβE potently blocked the locomotor activating and discriminative stimulus effects of nicotine at doses that did not antagonise its locomotor depressant and operant response rate-reducing effects. This selective blockade supports the involvement of different subtypes of nicotinic receptor in the mediation of diverse behavioural effects. Furthermore, the rightward shift of the dose-response curve for nicotine discrimination suggested a competitive mode of action for DHβE.     

Dual effects of nicotine on dopamine neurons mediated by different nicotinic receptor subtypes

Burst firing of dopaminergic neurons has been found to represent a particularly effective means of increasing dopamine release in terminal areas as well as activating immediate early genes in dopaminoceptive cells. Spontaneous burst firing is largely controlled by the level of activation of NMDA receptors in the ventral tegmental area (VTA) as a consequence of glutamate released from afferents arising mainly in the prefrontal cortex. Nicotine has been found to effectively increase burst firing of dopaminergic cells. This effect of nicotine may be due to an alpha 7 nicotinic receptor-mediated presynaptic facilitation of glutamate release in the VTA. By the use of in-vivo single-cell recordings and immunohistochemistry we here evaluated the role of alpha 7 nicotinic receptors in nicotine-induced burst firing of dopamine cells in the VTA and the subsequent activation of immediate early genes in dopaminoceptive target areas. Nicotine (0.5 mg/kg s.c.) was found to increase firing rate and burst firing of dopaminergic neurons. In the presence of methyllycaconitine (MLA, 6.0 mg/kg i.p.) nicotine only increased firing rate. Moreover, in the presence of dihydro-beta-erythroidine (DH beta E, 1.0 mg/kg i.p.), an antagonist at non-alpha 7 nicotinic receptors, nicotine produced an increase in burst firing without increasing the firing rate. Nicotine also increased Fos-like immunoreactivity in dopamine target areas, an effect that was antagonized with MLA but not with DH beta E. Our data suggest that nicotine's augmenting effect on burst firing is, indeed, due to stimulation of alpha 7 nicotinic receptors whereas other nicotinic receptors seem to induce an increase in firing frequency.     

Nicotine-induced enhancement of responding for conditioned reinforcement in rats: Role of prior nicotine exposure and α4β2 nicotinic receptors

Rationale

Stimuli associated with nicotine can become motivationally significant and may play a role in tobacco dependence. Previous work indicates that nicotine enhances responding for a conditioned reinforcer (CR).

Objectives

These studies examined the effects of prior exposure to nicotine on responding for a CR, persistence of this response, and the role of α4β2 or α7 nicotinic receptor subtypes.

Methods

Water deprived rats were given 13 Pavlovian conditioning sessions where a light/tone conditioned stimulus (CS) was paired with the delivery of water. Then, rats were presented with two levers: one delivered the CS (now a CR), the other was inactive. Experiments examined the effect of nicotine administered prior to Pavlovian conditioning sessions on approach behavior during CS presentations, operant responding for CR in the presence and absence of nicotine, and the persistence of responding for CR. The effects of nicotinic acetylcholine receptor (nAChR) antagonism with mecamylamine and α4β2 or α7 nAChR antagonism with dihydro-beta-erythroidine (DHβE) or methyllycaconitine (MLA) on nicotine-enhanced responding for CR were examined.

Results

Nicotine enhanced approach behavior during CS presentations and potentiated operant responding for CR, an effect sensitized as a result of nicotine exposure during conditioning. Responding for CR and its potentiation by nicotine was stable over multiple tests. Enhanced responding for the CR induced by nicotine was blocked by mecamylamine and DHβE, but not MLA.

Conclusions

Nicotine enhances Pavlovian discriminated approach and shows sensitized nicotine-induced enhancements in responding for CR, an effect depending on α4β2 nAChRs.     

Role of neuronal nicotinic acetylcholine receptors (nAChRs) on learning and memory in zebrafish

Rationale

Neuronal nicotinic acetylcholine receptors (nAChRs) play a modulatory role in cognition, and zebrafish provide a preclinical model to study learning and memory.

Objectives

We investigated the effect of nicotine (NIC) and some new cytisine-derived partial agonists (CC4 and CC26) on spatial memory in zebrafish using a rapid assay on T-maze task. The role of α4/α6β2 and the α7 nAChRs in NIC-induced memory enhancement was evaluated using selective nAChR antagonists.

Results

Low and high doses of NIC, cytisine (CYT), CC4 and CC26 respectively improved and worsened the mean running time, showing an inverted U dose–response function. The effective dose (ED50) (×10^?5 mg/kg) was 0.4 for CC4, 4.5 for CYT, 140 for NIC and 200 for CC26. NIC-induced cognitive enhancement was reduced by the selective nAChR subtype antagonists: methyllycaconitine (MLA) for α7, α-conotoxin (MII) for α6β2, dihydro-β-erythroidine (DhβE) for α4β2, the nonselective antagonist mecamylamine (MEC) and the muscarinic antagonist scopolamine (SCOP), with DhβE being more active than MLA or MII. All the partial agonists blocked the cognitive enhancement. The improvement with the maximal active dose of each partial agonist was blocked by low doses of DhβE (0.001 mg/kg) and MII (0.01 mg/kg). MLA reduced the effects of CC26 and CC4 at doses of 0.01 and 1 mg/kg, respectively, but did not antagonize CYT-induced memory improvement at any of the tested dose. No change in swimming activity was observed.

Conclusions

Our findings demonstrate that zebrafish make a useful model for the rapid screening of the effect of new α4β2 nAChR compounds on spatial memory.     

Activation of α7 nicotinic acetylcholine receptors ameliorates indomethacin-induced small intestinal ulceration in mice

Cholinergic anti-inflammatory actions have been shown to result mainly from the activation of α7 nicotinic acetylcholine receptors. Here, we investigated the possible role of α7 nicotinic acetylcholine receptors in the pathogenesis of indomethacin-induced small intestinal ulceration in mice. Male C57BL/6 mice were given indomethacin (10 mg/kg, s.c.), and sacrificed 24 h later. Nicotine (0.3–3 mg/kg) and PNU-282987 (a selective agonist of α7 nicotinic acetylcholine receptors; 1–10 mg/kg) were administered i.p. twice, at 0.5 h before and 8 h after indomethacin treatment, while methyllycaconitine (a selective antagonist of α7 nicotinic acetylcholine receptors; 10 mg/kg was administered twice, at 0.5 h before each nicotine treatment. Indomethacin caused severe hemorrhagic lesions in the small intestine with marked increases in myeloperoxidase (MPO) activity and inducible nitric oxide synthase (iNOS) expression in the mucosa. Pretreatment with nicotine reduced the severity of intestinal lesions in a dose-dependent manner. The protective effect of nicotine was mimicked by PNU-282987 and significantly attenuated by methyllycaconitine. The increases in MPO activity and iNOS expression induced by indomethacin were also significantly suppressed by nicotine and PNU-282987. Immunohistochemical study showed that the expression of α7 nicotinic acetylcholine receptors was clearly enhanced in the submucosa of the damaged area following indomethacin treatment. These results suggest that the activation of α7 nicotinic acetylcholine receptors ameliorates indomethacin-induced small intestinal ulceration, and that this effect may result from the inhibition of iNOS expression and neutrophil migration.     

Nicotine reverses the enhanced renal vasodilator capacity in endotoxic rats: Role of α7/α4β2 nAChRs and HSP70

BackgroundNicotine alleviates renal inflammation and injury induced by endotoxemia. This study investigated (i) the nicotine modulation of hemodynamic and renal vasodilatory responses to endotoxemia in rats, and (ii) roles of α7 or α4β2-nAChRs and related HSP70/TNFα/iNOS signaling in the interaction.MethodsEndotoxemia was induced by ip lipopolysaccharide (5 mg/kg/day, for 2 days) and changes in systolic blood pressure and vasodilator responsiveness of isolated perfused kidney to acetylcholine or 5′-N-ethylcarboxamidoadenosine (NECA, adenosine receptor agonist) were evaluated.ResultsLipopolysaccharide had no effect on serum creatinine, reduced blood pressure, and increased renal vasodilations induced by acetylcholine or NECA in male and female preparations. Immunohistochemical analyses showed that lipopolysaccharide reduced renal HSP70 expression, but increased α7-nAChRs, α4β2-nAChRs and iNOS expressions. The co-administration of aminoguanidine (iNOS inhibitor), pentoxifylline (TNFα inhibitor), or nicotine attenuated lipopolysaccharide mediation of renal vasodilations and elevations in α7/α4β2-nAChR and iNOS expressions. Nicotine also reversed the downregulating effect of lipopolysaccharide on HSP70 expression. α7-nAChRs (methyllycaconitine citrate, MLA) or α4β2-nAChRs (dihydro-β-erythroidine, DHβE) blockade potentiated the lipopolysaccharide enhancement of renal vasodilations, and abolished the depressant effect of nicotine on lipopolysaccharide responses. A similar abolition of nicotine effects was seen after HSP70 inhibition by quercetin. Alternatively, lipopolysaccharide hypotension was eliminated in rats treated with DHβE/nicotine or quercetin/nicotine regimen in contrast to no effect for nicotine alone or combined with MLA.ConclusionsThese findings establish that nicotine offsets lipopolysaccharide facilitation of renal vasodilations possibly through a crosstalk between HSP70 and nAChRs of the α7 and α4β2 types.     

Discriminative-stimulus effects of NS9283, a nicotinic α4β2* positive allosteric modulator, in nicotine-discriminating rats

Rationale

Neuronal α4β2* nicotinic acetylcholine receptors mediate cognition, pain, and the discriminative and reinforcing effects of nicotine. In addition to traditional orthosteric agonists, α4β2* positive allosteric modulators (PAMs) have recently been identified. With increased subtype selectivity relative to agonists, PAMs administered alone or in combination with low-dose α4β2* agonists may be used as powerful tools for increasing our understanding of α4β2* pharmacology.

Objectives

The present experiments tested the nicotine discriminative-stimulus effects of the α4β2* PAM NS9283 (A-969933) in the presence and absence of low-dose nicotine or nicotinic subtype-selective agonist.

Methods

Rats were trained to discriminate 0.4 mg/kg nicotine from saline in a two-lever drug discrimination paradigm. In subsequent generalization tests, rats were administered nicotine, the α4β2*-preferring agonist ABT-594, and NS9283, alone or in two-drug combinations.

Results

Nicotine and ABT-594 showed dose-dependent nicotine generalization. NS9283 alone resulted in a non-significant increase in nicotine-appropriate lever selection. Combination of non-effective doses of nicotine or ABT-594 with escalating doses of NS9283 resulted in a complete conversion to 100 % nicotine-appropriate choice in the case of nicotine combination and incomplete, though significant, generalization for ABT-594.

Conclusions

The α4β2* PAM NS9283 alone did not produce nicotine-like discriminative effects, but did demonstrate dose-related increases in nicotine lever choice when combined with a non-effective dose of nicotine or the α4β2* agonist ABT-594. This finding provides confirmation of the positive allosteric modulating effect of NS9283 in a functional in vivo paradigm. NS9283 is a potentially valuable tool for studying the role of α4β2* receptors in various nicotinic acetylcholine receptor-related functions.     

Role for α6 nicotinic receptors in l-dopa-induced dyskinesias in parkinsonian mice

l-Dopa-induced dyskinesias are a serious side effect that develops in most Parkinson's disease patients on dopamine replacement therapy. Few treatment options are available to manage dyskinesias; however, recent studies show that nicotine reduces these abnormal involuntary movements (AIMs) in parkinsonian animals by acting at nicotinic acetylcholine receptors (nAChRs). Identification of the nAChR subtypes that mediate this reduction in AIMs is important as it will help in the development of nAChR subtype selective drugs for their treatment. Here we investigate the role of α6β2* nAChRs, a subtype selectively present in the nigrostriatal pathway, using α6 nAChR subunit null mutant (α6(?/?)) mice. Wildtype and α6(?/?) mice were lesioned by unilateral injection of 6-hydroxydopamine (3 μg/μl) into the medial forebrain bundle. They were then given l-dopa (3 mg/kg) plus benserazide (15 mg/kg) 2–3 wk later. l-dopa-induced AIMs developed to a similar extent in α6(?/?) and wildtype mice. However, AIMs in α6(?/?) mice declined to ～50% of that in wildtype mice with continued l-dopa treatment. Nicotine treatment also decreased AIMs by ～50% in wildtype mice, although not in α6(?/?) mice. There were no effects on parkinsonism under any experimental condition. To conclude, the similar declines in l-dopa-induced AIMs in nicotine-treated wildtype mice and in α6(?/?) mice treated with and without nicotine indicate an essential role for α6β2* nAChRs in the maintenance of l-dopa-induced AIMs. These findings suggest that α6β2* nAChR drugs have potential for reducing l-dopa-induced dyskinesias in Parkinson's disease.     

Nicotine potentiation of brain stimulation reward reversed by DHβE and SCH 23390, but not by eticlopride, LY 314582 or MPEP in rats

RATIONALE: Systemic nicotine administration increases dopamine and glutamate levels in reward-related brain areas. Nicotine-induced increases of dopamine in the nucleus accumbens are in part mediated by glutamatergic projections to the ventral tegmental area dopamine neurons. OBJECTIVES: To assess the effects of actions at acetylcholine, dopamine, presynaptic (mGluR(2/3)) and postsynaptic (mGluR(5)) metabotropic glutamate receptors (mGluRs) on the potentiation of brain stimulation reward induced by systemically administered nicotine (0.125-0.5 mg/kg; free base) in rats. METHODS: A discrete-trial current-threshold s stimulation reward procedure (electrodes placed in the posterior lateral hypothalamus) was used to assess the effects of DH beta E (0.5-5 mg/kg), an acetylcholine nicotinic receptor antagonist, SCH 23390 (1.25-5 microg/kg), a dopamine D(1) receptor antagonist, eticlopride (2.5-20 microg/kg), a dopamine D(2) receptor antagonist, LY 314582 (1-20 mg/kg), an mGluR(2/3) agonist, and MPEP (1-9 mg/kg), an mGluR(5) antagonist, on the reward potentiating effects of nicotine (0.25 mg/kg). RESULTS: DH beta E had no effect on reward thresholds when administered alone, but dose-dependently reversed the nicotine-induced potentiation of brain stimulation reward. SCH 23390 (5 microg/kg) elevated thresholds when administered alone, and reversed the nicotine-induced potentiation of brain stimulation reward even at a dose (2.5 microg/kg) that had no effect on reward thresholds. Eticlopride (10-20 microg/kg), LY 314582 (10-20 mg/kg) and MPEP (9 mg/kg) elevated thresholds when administered alone but had no effect on the nicotine-induced potentiation of brain stimulation reward. CONCLUSIONS: These results indicate that nicotinic and dopamine D(1) receptors are involved in the nicotine-induced potentiation of brain stimulation reward, while actions at dopamine D(2), mGlu(2/3) and mGlu(5) receptors did not modulate this effect of nicotine.     

Nicotine attenuates concanavalin A-induced liver injury in mice by regulating the α7-nicotinic acetylcholine receptor in Kupffer cells

Nicotine, a potent parasympathomimetic alkaloid, manifests anti-inflammatory properties by activating nicotinic acetylcholine receptors (nAChRs). In this study, we evaluated the effects of nicotine on concanavalin A (ConA)-induced autoimmune hepatitis. Nicotine (0.5 and 1 mg/kg) was intraperitoneally administered to BALB/c mice and mice were intravenously injected with ConA (15 mg/kg) to induce hepatitis. The results showed that nicotine treatment ameliorated pathological lesions in livers and significantly suppressed the expression of pro-inflammatory cytokines in the livers. Such effects were mediated by inhibiting the nuclear factor-kappa B (NF-κB) signaling in livers. Interestingly, nicotine inhibited the ConA-induced inflammatory response in primary cultured Kupffer cells (KCs) but did not alter the proliferation of splenocytes. The protective effects of nicotine against ConA-induced hepatitis were abolished in KC-depleted mice, indicating the requirement of KCs in this process. Additionally, the expression of α7-nAChR on KCs was dramatically increased by nicotine treatment, and the protective effects of nicotine on ConA-induced liver injury were significantly suppressed by treatment with methyllycaconitine (MLA), a specific α7-nAChR antagonist. Consistently, in primary cultured KCs, the activation of NF-κB signaling was also regulated by nicotine treatment. This study suggests that nicotine increases α7-nAChR-mediated cholinergic activity in KCs resulting in decrease of ConA-induced autoimmune hepatitis through inhibiting NF-κB signaling.