Discriminative stimulus effects of mecamylamine and nicotine in rhesus monkeys: Central and peripheral mechanisms

Mecamylamine is a non-competitive nicotinic acetylcholine receptor (nAChR) antagonist that has been prescribed for hypertension and as an off-label smoking cessation aid. Here, we examined pharmacological mechanisms underlying the interoceptive effects (i.e., discriminative stimulus effects) of mecamylamine (5.6 mg/kg s.c.) and compared the effects of nAChR antagonists in this discrimination assay to their capacity to block a nicotine discriminative stimulus (1.78 mg/kg s.c.) in rhesus monkeys. Central (pempidine) and peripherally restricted nAChR antagonists (pentolinium and chlorisondamine) dose-dependently substituted for the mecamylamine discriminative stimulus in the following rank order potency (pentolinium > pempidine > chlorisondamine > mecamylamine). In contrast, at equi-effective doses based on substitution for mecamylamine, only mecamylamine antagonized the discriminative stimulus effects of nicotine, i.e., pentolinium, chlorisondamine, and pempidine did not. NMDA receptor antagonists produced dose-dependent substitution for mecamylamine with the following rank order potency (MK-801 > phencyclidine > ketamine). In contrast, behaviorally active doses of smoking cessation aids including nAChR agonists (nicotine, varenicline, and cytisine), the smoking cessation aid and antidepressant bupropion, and the benzodiazepine midazolam did not substitute for the discriminative stimulus effects of mecamylamine. These data suggest that peripheral nAChRs and NMDA receptors may contribute to the interoceptive stimulus effects produced by mecamylamine. Based on the current results, the therapeutic use of mecamylamine (i.e., for smoking or to alleviate green tobacco sickness) should be weighed against the potential for mecamylamine to produce interoceptive effects that overlap with another class of abused drugs (i.e., NMDA receptor agonists).

Amino acid modulation of dopamine in the nucleus accumbens mediates sex differences in nicotine withdrawal

The aversive effect of nicotine withdrawal is greater in female versus male rats, and we postulate that this sex difference is mediated in the nucleus accumbens (NAc). Nicotine withdrawal induces decreases in NAc dopamine and increases in acetylcholine (ACh) levels in male rats. To our knowledge, these neurochemical markers of nicotine withdrawal have not been compared in female versus male rats. Given the role of amino acids in modulating NAc dopaminergic and cholinergic transmission, concomitant measures of gamma-aminobutyric acid (GABA) and glutamate levels were also compared across sex. Rats received continuous nicotine exposure for 14 days, and then NAc dialysate was collected during baseline and following administration of the nicotinic receptor antagonist mecamylamine to precipitate withdrawal. Chronic nicotine exposure was associated with larger increases in baseline dopamine, GABA and glutamate levels in the NAc of female versus male rats, whereas baseline ACh was only increased in male rats. During withdrawal, both sexes displayed equivalent increases in NAc ACh levels. As expected, male rats displayed decreases in dopamine, coupled with increases in GABA and decreases in glutamate levels, suggesting the possibility of increased inhibitory tone in the NAc during withdrawal. Relative to males, female rats displayed larger decreases in NAc dopamine and related increases in GABAergic transmission. As female rats also showed elevated glutamate levels that persist during withdrawal, it is suggested that sex differences may arise from increased glutamatergic drive of inhibitory tone in the NAc. The findings provide a potential mechanism whereby the aversive effects of nicotine withdrawal are enhanced in female rats.

Effects of continuous nicotine treatment and subsequent termination on cocaine versus food choice in male rhesus monkeys

One complicating factor in cocaine addiction may be concurrent exposure and potential dependence on nicotine. The aim of the present study was to determine the effects of continuous nicotine treatment and subsequent termination on cocaine versus food choice in rhesus monkeys (Macaca mulatta). For comparison, we also determined effects of the nicotinic receptor antagonist mecamylamine on cocaine versus food choice during continuous saline and nicotine treatment. Rhesus monkeys (N = 3) responded under a concurrent schedule of food pellet (1 g) and intravenous cocaine (0-0.1 mg/kg/injection) availability. Saline and ascending nicotine doses (0.1-1.0 mg/kg/hr, intravenous) were continuously infused for 7-day treatment periods and separated by 24-hr saline treatment periods. Acute effects of mecamylamine (0.32-1.8 mg/kg, intramuscular, 15 min pretreatment) were determined during continuous saline and 0.32-mg/kg/hr nicotine treatments. During saline treatment, cocaine maintained a dose-dependent increase in cocaine choice. Nicotine treatment did not alter cocaine versus food choice. In contrast, preference of 0.032 mg/kg/injection cocaine was attenuated 24 hr following termination of 0.32-mg/kg/hr nicotine treatment, despite no somatic abstinence signs being observed. Acute mecamylamine enhanced cocaine choice during saline treatment and mainly suppressed rates of behavior during nicotine treatment. Overall, continuous nicotine exposure, up to 1 mg/kg/hr, does not enhance cocaine choice and does not produce nicotine dependence, as demonstrated by the lack of abstinence signs.

Clinical pharmacokinetics of the nicotinic channel modulator dexmecamylamine (TC-5214) in subjects with various degrees of renal impairment

BACKGROUND AND OBJECTIVE

Dexmecamylamine (TC-5214) is a nicotinic channel modulator that was evaluated as a potential adjunct treatment to an antidepressant for patients with major depressive disorder. Dexmecamylamine is almost completely eliminated via the kidneys, with more than 90 % of a given dose excreted unchanged in urine. The aim of this study was to assess the single-dose pharmacokinetics of dexmecamylamine in subjects with various degrees of renal impairment and subjects undergoing hemodialysis.

METHODS

A single-dose, open-label, parallel-group study was conducted at two study centers in the USA. There were four treatment groups with eight subjects in each, receiving a single dose of dexmecamylamine 8 mg (subjects with normal renal function and mild renal impairment) or TC-5412 2 mg [subjects with moderate renal impairment and end-stage renal disease (ESRD)]. The pharmacokinetics of dexmecamylamine in plasma, urine, and dialysate were evaluated using non-compartmental analysis.

RESULTS

The plasma pharmacokinetics of dexmecamylamine were influenced by renal function. The increase in dose-normalized area under the plasma concentration-time curve (AUC) was statistically significant with an approximately doubled exposure in subjects with moderate renal impairment compared with subjects with normal renal function. The maximum plasma concentration was not impacted by renal function. Plasma clearance of dexmecamylamine in ESRD subjects appeared negligible, with flat plasma concentration-time profiles. Hemodialysis had a relatively modest effect on reduction of dexmecamylamine plasma concentrations. There was no discernable relationship between renal clearance and urinary pH.

CONCLUSION

Renal impairment increased the AUC, prolonged the elimination half-life, and decreased the clearance of dexmecamylamine following administration as a single oral dose. It is likely that renal function would need to be taken into account when setting the dose. Dexmecamylamine administration should be avoided or the dose significantly reduced in patients with severe renal impairment and ESRD.

The effects of oxotremorine, epibatidine, atropine, mecamylamine and naloxone in the tail-flick, hot-plate, and formalin tests in the naked mole-rat (Heterocephalus glaber)

The naked mole-rat (Heterocephalus glaber) is a promising animal model for the study of pain mechanisms, therefore a thorough characterization of this species is essential. The aim of the present study was to establish the naked mole-rat as a model for studying the cholinergic receptor system in antinociception by investigating the involvement of muscarinic, nicotinic and opioid receptors in nociceptive tests in this species. The effects of systemic administration of the muscarinic receptor agonist oxotremorine and the nicotinic receptor agonist epibatidine were investigated in the tail-flick, the hot-plate, and the formalin tests. The effects of co-administration of the muscarinic receptor antagonist atropine, the nicotinic receptor antagonist mecamylamine, and the opioid receptor antagonist naloxone were also investigated. Oxotremorine and epibatidine induced a significant, dose-dependent antinociceptive effect in the tail-flick, hot-plate, and formalin tests, respectively. The effects of oxotremorine and epibatidine were blocked by atropine and mecamylamine, respectively. In all three nociceptive tests, naloxone in combination with oxotremorine or epibatidine enhanced the antinociceptive effects of the drugs. The present study demonstrated that stimulation of muscarinic and nicotinic receptors produces antinociceptive effects in the naked-mole rat. The reversal effect of atropine and mecamylamine suggests that this effect is mediated by cholinergic receptors. As naloxone increases the antinociceptive effects of cholinergic agonists, it is suggested that the cholinergic antinociception acts via a gateway facilitated by opioid receptor blockage; however, the precise interaction between these receptor systems needs further investigation.

[Influence of agonist and antagonist Nalpha7-cholinoreceptors on active avoidance learning and gonadal hormones levels in ovariectomized female rats]

The aim of this work was to study the influence of co-administration of agonist and antagonist of Nalpha7-cholinoreceptors with low dose of 17beta-estradiol on dynamics of acquisition and retention in active avoidance test, on the behavior in the "open field" test, as well as tropic and gonadal hormones levels in ovariectomiczed female rats. Antagonist of Nalpha7-cholinoreceptors - RJR-2403 (1.0 mg/kg, i. p.) and antagonist of Nalpha7-cholinoreceptors - mecamylamine (1.0 mg/kg, i. p.) were administered chronically (14 days) isolated or in combination with low dose of 17beta-estradiol (0.5 m/rat, s. c.) to ovariectomized rats. It was revealed that RJR-2403 in combination with low dose of 17beta -estradiol completely restored impairments in mechanisms of dynamics of active avoidance performance and increased exploratory and grooming behavior in the "open field" test in ovariectomized rats. Moreover, RJR-2403 treatment decreased follitropine and lutropine levels and increased estradiol level in the blood serum in ovariectomized rats. Administration ofmecamylamine alone or in combination with low dose of 17beta-estradiol in ovariectomiczed rats failed to influence on dynamics of passive avoidance learning and failed to modify behavior in the "open field" test. The results of the present study suggest positive effect of Nalpha7-cholinoreceptors stimulation on background of low dose for 17beta-estradiol on active avoidance learning and tropic and gonadal hormones levels at estrogen deficiency.

Topical mecamylamine for diabetic macular edema

PURPOSE

Stimulation of nicotinic acetylcholine (nACh) receptors on vascular endothelial cells promotes angiogenesis and vascular permeability in animal models. The safety and bioactivity of topical mecamylamine, an antagonist of nACh receptors, was tested in patients with diabetic macular edema.

DESIGN

A multicenter phase I/II clinical trial.

METHODS

Twenty-three patients with chronic diabetic macular edema received 1% mecamylamine topically twice daily for 12 weeks, the primary end point. Patients underwent safety assessments, measurement of best-corrected visual acuity (BCVA), and measurement of foveal thickness using optical coherence tomography at baseline, 1, 4, 8, 12, and 16 weeks.

RESULTS

Mecamylamine drops were well tolerated and there were no drug-related safety problems. Mean improvement in BCVA at 1, 4, 8, 12, and 16 weeks was 2.8, 1.9, 2.4, 0.8, and 3.1 letters, respectively. There was little change in mean excess foveal thickness. There was substantial heterogeneity in response, because 8 patients showed convincing improvement in BCVA, foveal thickness, or both, 9 patients showed equivocal or no substantial changes, and 4 patients showed worsening. Five patients showed a substantial improvement in BCVA, foveal thickness, or both between their last visit while receiving mecamylamine and 1 month after stopping mecamylamine.

CONCLUSIONS

This study suggested that administration of topical mecamylamine, a nonspecific nACh receptor blocker, may have heterogeneous effects in patients with diabetic macular edema. Variable expression of nACh receptor subtypes on endothelial cells that have different effects on permeability would provide an explanation for these results and should be investigated, because more specific nACh receptor blockers may dissociate antipermeability and propermeability effects.

Tobacco smoke exposure induces nicotine dependence in rats

RATIONALE

Tobacco smoke contains nicotine and many other compounds that act in concert on the brain reward system. Therefore, animal models are needed that allow the investigation of chronic exposure to the full spectrum of tobacco smoke constituents.

OBJECTIVES

The aim of these studies was to investigate if exposure to tobacco smoke leads to nicotine dependence in rats.

METHODS

The intracranial self-stimulation procedure was used to assess the negative affective aspects of nicotine withdrawal. Somatic signs were recorded from a checklist of nicotine abstinence signs. Nicotine self-administration sessions were conducted to investigate if tobacco smoke exposure affects the motivation to self-administer nicotine. Nicotinic receptor autoradiography was used to investigate if exposure to tobacco smoke affects central alpha7 nicotinic acetylcholine receptor (nAChR) and non-alpha7 nAChR levels (primarily alpha4beta2 nAChRs).

RESULTS

The nAChR antagonist mecamylamine dose-dependently elevated the brain reward thresholds of the rats exposed to tobacco smoke and did not affect the brain reward thresholds of the untreated control rats. Furthermore, mecamylamine induced more somatic withdrawal signs in the smoke-exposed rats than in the control rats. Nicotine self-administration was decreased 1 day after the last tobacco smoke exposure sessions and was returned to control levels 5 days later. Tobacco smoke exposure increased the alpha7 nAChR density in the CA2/3 area and the stratum oriens and increased the non-alpha7 nAChR density in the dentate gyrus.

CONCLUSION

Tobacco smoke exposure leads to nicotine dependence as indicated by precipitated affective and somatic withdrawal signs and induces an upregulation of nAChRs in the hippocampus.

Nicotinic and dopamine D2 receptors mediate nicotine-induced changes in ventral tegmental area neurotensin system

Neuropeptides have been implicated in the psychopathology of stimulants of abuse. Neurotensin is a neuropeptide associated with the regulation of the nigrostriatal and mesolimbic dopamine pathways. In addition, the ventral tegmental area, a midbrain region implicated in the rewarding effects of most, if not all, addictive drugs, appears to be a particularly critical target for nicotine action. Because neurotensin has been linked with both mesolimbic and mesocortical dopamine function, we examined the impact of nicotine treatment on central nervous neurotensin systems by measuring changes in neurotensin tissue content because it has been shown that such changes reflect alterations in release and activity of this peptide system. Male Sprague-Dawley rats received multiple administrations of (+/-) nicotine 4.0 mg/kg/day (0.8 mg/kg, i.p.; 5 x 2-h intervals) in the presence or absence of selective dopamine receptor antagonists (dopamine D(1); SCH 23390 or dopamine D(2); eticlopride) or two doses of the non-selective nicotinic acetylcholine receptor antagonist (mecamylamine; 3.0 and 6.0 mg/kg, s.c.). The nicotine treatment significantly decreased neurotensin-like immunoreactivity content in the ventral tegmental area, as well as related regions such as prefrontal cortex, substantia nigra, and anterior striatal region 12-18 h after drug treatment, but not the nucleus accumbens. The nicotine-mediated decrease in the neurotensin-like immunoreactivity of the ventral tegmental area was selectively blocked by a specific dopamine D(2), but not a dopamine D(1), receptor antagonist, while mecamylamine attenuated at the low (3.0 mg/kg) and completely blocked at high (6.0 mg/kg) dose this nicotine effect. These findings with previous studies, suggest that nicotine-mediated dopamine release activates D(2) receptors which in turn increases neurotensin release, turnover and acutely reduces tissue levels in the ventral tegmental area and other limbic and basal ganglia structures.

Thalamic microinjection of nicotine reverses sevoflurane-induced loss of righting reflex in the rat

BACKGROUND

Neuronal nicotinic acetylcholine receptors are both potently inhibited by anesthetics and densely expressed in the thalamus. Brain imaging shows that thalamic activity suppression accompanies anesthetic-induced unconsciousness. Therefore, anesthetic-induced unconsciousness may involve direct antagonism of thalamic nicotinic receptors. The authors test this by separately attempting to block or enhance anesthetic-induced loss of righting in rats using intrathalamic microinjections of nicotine or its antagonist.

METHODS

Rats were implanted with a cannula aimed at the thalamus or control locations. A week later, loss of righting was induced using sevoflurane (1.4 +/- 0.2%). A dose-parameter study (n = 35) first identified an optimal intrathalamic nicotine dose associated with arousal. Subsequently, this dose was used to pinpoint the thalamic site mediating the arousal response (n = 107). Finally, sevoflurane righting dose and response specificity were assessed after blocking nicotinic channels with intrathalamic mecamylamine pretreatment (n = 8) before nicotine challenge.

RESULTS

Nicotine (150 microg/0.5 microl over 1 min) was the optimal arousal dose, because lower doses (75 microg) were ineffective and higher doses (300 microg) often caused seizures. Nicotine temporarily restored righting and mobility in animals when microinjections involved the central medial thalamus (P < 0.0001, chi-square). Righting occurred despite continued sevoflurane administration. Intrathalamic mecamylamine pretreatment did not lower the sevoflurane dose associated with loss of righting, but prevented the nicotine arousal response.

CONCLUSIONS

The reversal of unconsciousness found here with intrathalamic microinfusion of nicotine suggests that suppression of the midline thalamic cholinergic arousal system is part of the mechanism by which anesthetics produce unconsciousness.

A randomized, controlled trial to assess the efficacy and safety of a transdermal delivery system of nicotine/mecamylamine in cigarette smokers

AIMS

To determine the efficacy and safety of nicotine transdermal therapy co-administered with the nicotine antagonist, mecamylamine, compared to a nicotine transdermal patch alone (21 mg nicotine + 6 mg mecamylamine, 21 mg nicotine + 3 mg mecamylamine, and 21 mg nicotine + 0 mg mecamylamine).

DESIGN

Multi-center (n = 4), double-blind, randomized, parallel group, repeat-dose study.

SETTING

Clinical laboratory.

PARTICIPANTS

A total of 540 subjects were enrolled into the study-135 from each of four sites; 180 patients in each of three treatment arms.

INTERVENTION

Treatment was administered for the first 6 weeks of the 8-week study. Patients were instructed to continue smoking for the first 2 weeks of treatment.

MEASUREMENTS

The primary efficacy parameter was 4-week continuous abstinence after the quit date, confirmed with an expired carbon monoxide of < 10 parts per million.

FINDINGS

Analysis of the 4-week continuous abstinence for the intent-to-treat population showed overall rates of 29% (nicotine + 6 mg mecamylamine), 29% (nicotine + 3 mg mecamylamine) and 23% (nicotine only) using the slip definition which allows smoking in the first 2 weeks after the quit date. Statistical analyses revealed no significant treatment differences. Analyses using the strict definition (no smoking after the quit date) yielded similar non-significant group differences (29%, 27%, 26%).

CONCLUSION

If adding mecamylamine to nicotine replacement therapy (NRT) improves the chances of success at stopping smoking, the results of this study suggest that the effect is very small.

Morphine-induced place preference: Involvement of cholinergic receptors of the ventral tegmental area

In the present study, the effects of intra-ventral tegmental area injections of cholinergic agents on morphine-induced conditioned place preference were investigated by using an unbiased 3-day schedule of place conditioning design in rats. The conditioning treatments with subcutaneous injections of morphine (0.5-7.5 mg/kg) induced a significant dose-dependent conditioned place preference for the drug-associated place. Intra-ventral tegmental area injection of an anticholinesterase, physostigmine (2.5 and 5 microg/rat) or nicotinic acetylcholine receptor agonist, nicotine (0.5 and 1 microg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant conditioned place preference. Furthermore, intra-ventral tegmental area administration of muscarinic acetylcholine receptor antagonist, atropine (1-4 microg/rat) or nicotinic acetylcholine receptor antagonist, mecamylamine (5 and 7.5 microg/rat) dose-dependently inhibited the morphine (5 mg/kg)-induced place preference. Atropine or mecamylamine reversed the effect of physostigmine or nicotine on morphine response respectively. The injection of physostigmine, but not atropine, nicotine or mecamylamine, into the ventral tegmental area alone produced a significant place aversion. Moreover, intra-ventral tegmental area administration of the higher doses of physostigmine or atropine, but not nicotine or mecamylamine decreased the locomotor activity. We conclude that muscarinic and nicotinic acetylcholine receptors in the ventral tegmental area may critically mediate the rewarding effects of morphine.

Evaluation of muscarinic and nicotinic receptor antagonists on attention and working memory

Cholinergic receptor antagonists are commonly used to model attentional and mnemonic impairments associated with neuropsychiatric disorders such as Alzheimer's disease. However, few studies have systematically assessed the effects of these drugs following manipulations that affect attention or working memory within the same task. In the present experiment, rats were trained to discriminate visual signals from "blank" trials when no signal was presented. This task was modified to include retention intervals on some trials to tax working memory. During standard task performance, rats received systemic injections of the muscarinic receptor antagonist, scopolamine, or of the nicotinic receptor antagonist, mecamylamine. A second experiment tested the effects on this task of co-administering doses of scopolamine and mecamylamine that, when administered alone, did not significantly affect task performance. Scopolamine (0.3 and 1.0 mg/kg) decreased detection of 500 ms signals but did not affect accurate identification of non-signals. Scopolamine did not differentially affect performance across the retention interval. Elevated omission rates were associated with high doses of scopolamine or mecamylamine. Combination drug treatment was associated with decreased signal detection and elevated omission rates. Collectively, the data suggest that muscarinic and nicotinic receptor antagonists do not exclusively impair working memory.

Cholinergic modulation of Pavlovian fear conditioning in rats: differential effects of intrahippocampal infusion of mecamylamine and methyllycaconitine

The cholinergic system has consistently been implicated in Pavlovian fear conditioning. Considerable work has been done to localize specific nicotinic receptor subtypes in the hippocampus and determine their functional importance; however, the specific function of many of these subtypes has yet to be determined. An alpha7 nicotinic antagonist methyllycaconitine (MLA) (35 microg), and a broad spectrum non-alpha7 nicotinic antagonist mecamylamine (35 microg) was injected directly into the dorsal hippocampus or overlying cortex either 15 min pre-, 1 min post-, or 6h post-fear conditioning. One week after conditioning, retention of contextual and cue (tone) conditioning were assessed. A significant impairment in retention of contextual fear was observed when mecamylamine was injected 15 min pre- and 1 min post-conditioning. No significant impairment was observed when mecamylamine was injected 6h post-conditioning. Likewise, a significant impairment in retention of contextual fear was observed when MLA was injected 1 min post-conditioning; however, in contrast, MLA did not show any significant impairments when injected 15 min pre-conditioning, but did show a significant impairment when injected 6h post-conditioning. There were no significant impairments observed when either drug was injected into overlying cortex. No significant impairments were observed in cue conditioning for either drug. In general, specific temporal dynamics involved in nicotinic receptor function were found relative to time of receptor dysfunction. The results indicate that the greatest deficits in long-term retention (1 week) of contextual fear are produced by central infusion of MLA minutes to hours post-conditioning or mecamylamine within minutes of conditioning.

Interaction of lobeline and nicotinic receptor ligands with the discriminative stimulus properties of cocaine and amphetamine

Lobeline has high affinity for nicotinic acetylcholine receptors and inhibits the function of vesicular and plasmalemmal monoamine transporters. Moreover, lobeline has been shown to alter the neurochemical and behavioral effects of psychostimulants. The present study determined the effect of lobeline and drugs selective for nicotinic receptors on the discriminative stimulus properties of low doses of cocaine (1.6 or 5.0 mg/kg) or d-amphetamine (0.3 mg/kg) in rats, using a standard two-lever drug discrimination procedure with food reinforcement. Nicotine substituted for both amphetamine and cocaine. The nicotinic receptor antagonists mecamylamine and hexamethonium did not substitute for or block the cocaine or amphetamine stimulus. In contrast, lobeline substituted for cocaine, but did not substitute for amphetamine. In antagonism tests, lobeline doses that did not substitute for cocaine decreased responding on the cocaine-paired levers. Surprisingly, lobeline did not alter the discriminative stimulus properties of amphetamine. This research further supports the supposition that nicotine, cocaine and amphetamine produce similar, but distinct subjective states. Furthermore, the present findings suggest that lobeline has a complex mechanism of action to disrupt the behavioral effects of drugs of abuse.

A preliminary study of the effects of cigarette smoking on prepulse inhibition in schizophrenia: involvement of nicotinic receptor mechanisms

BACKGROUND

Schizophrenics exhibit deficits in prepulse inhibition (PPI) of the startle response, and have high rates of cigarette smoking. We evaluated the effects of cigarette smoking on PPI deficits in schizophrenia, and the role of nicotinic acetylcholine receptors (nAChRs) in mediating cigarette smoking-related PPI enhancement.

METHODS

PPI was assessed at baseline, after overnight abstinence, and after smoking reinstatement during three separate test weeks in nicotine-dependent schizophrenia (n=15) and control (n=14) smokers pre-treated with the nAChR antagonist mecamylamine (MEC; 0.0, 5.0 or 10.0 mg/day).

RESULTS

PPI was comparable between schizophrenia and control smokers after ad lib cigarette smoking. Overnight smoking abstinence significantly reduced PPI, while smoking reinstatement reversed abstinence-induced worsening of PPI deficits in schizophrenia. However, acute abstinence and reinstatement did not alter PPI in controls. PPI enhancement by smoking reinstatement in schizophrenia was dose-dependently blocked by MEC, whereas MEC had no effect on PPI in control smokers.

CONCLUSIONS

These results suggest that: 1) Non-deprived smokers with schizophrenia have comparable levels of PPI to non-deprived smoking controls; 2) In schizophrenia, PPI is impaired by smoking abstinence and improved by acute smoking reinstatement, and; 3) enhancement of PPI by cigarette smoking in schizophrenia is mediated by stimulation of central nAChRs. Our findings may contribute to understanding the increased vulnerability to nicotine dependence in schizophrenia, with implications for treatment of PPI deficits in this disorder.

Neuropsychological deficits in nonsmokers with schizophrenia: effects of a nicotinic antagonist

BACKGROUND

Biochemical, physiological and genetic evidence suggests dysregulation of central nicotinic acetylcholine receptor (nAChR) systems in schizophrenia, which may contribute to neuropsychological dysfunction and the high rates of smoking in this disorder. To evaluate the effects of nAChR blockade on neuropsychological performance in schizophrenia without the confounding effects of cigarette smoking, we compared neuropsychological performance in schizophrenia and healthy control nonsmokers after pre-treatment with the centrally-acting nAChR antagonist mecamylamine (MEC).

METHODS

Using a within-subjects, counterbalanced design, schizophrenia (n = 14) and control (n = 15) nonsmokers were pre-treated for 3 days with MEC (0.0, 5.0, and 10.0 mg/day). Subjects performed repeated neuropsychological assessments including visuospatial working memory (VSWM), Continuous Performance Test (CPT), Wisconsin Card Sorting Test (WCST), Word Serial Position Test (WSPT) and Stroop Color Word Test (SCWT) during three sequential test sessions per week over three test weeks.

RESULTS

We found significant main effects of schizophrenia diagnosis on: VSWM 30 and 60 delays (p's < 0.01), CPT (% Hit Rate, Reaction Time, Variability Index; p < 0.01 for all outcomes), WCST (p < 0.01 for all outcomes) and Word Serial Position Test (p < 0.01). However, there were no main effects of repeated test administration (Session) or MEC dose on any of these outcomes, and no significant 3-way (DiagnosisxSessionxMEC dose) interactions.

CONCLUSIONS

Our results suggest that there are a broad range of neuropsychological deficits in nonsmokers with schizophrenia. Furthermore, pretreatment with a centrally-acting nAChR antagonist did not alter neuropsychological performance in either nonsmoking patients with schizophrenia or controls.

Nicotinic mechanisms contribute to soman-induced symptoms and lethality

The symptoms and lethality of intoxication with the acetylcholinesterase inactivator soman are attributed primarily to excessive activation of muscarinic acetylcholine receptors; nicotinic activation is considered of less importance, a notion that may rely on studies that have used nicotinic antagonists at low doses. In this study pretreatment with the centrally acting nicotinic antagonist mecamylamine, 20mg/kg, but not 2mg/kg, prolonged survival in mice exposed to soman, 250 microg/kg (1.5 LD(50)), from 14+/-3 to 135+/-38 min (mean+/-S.E.M.; surviving animals were killed 240 min after soman administration). Pretreatment with the muscarinic blocker scopolamine, 2 or 20mg/kg (but not 0.5mg/kg) prolonged survival significantly (mean for both groups: 91 min), but the animals responded to soman with immobility, irregular respiration, fasciculation, and short episodes of convulsive crawling. These symptoms were absent in animals pretreated with scopolamine plus mecamylamine, both drugs 20mg/kg, a suggestion that they were caused by activation of nicotinic receptors. All animals pretreated with scopolamine and mecamylamine (both drugs 20 mg/kg) survived the full 240 min observation period. Administration of mecamylamine, 5 mg/kg, 5 min after soman exposure to scopolamine-pretreated animals reduced fasciculation and respiratory irregularity and prolonged survival compared to scopolamine alone, but mecamylamine, 20 mg/kg, given 10 min after soman exposure shortened survival (18+/-1 min). These results suggest that nicotinic activation plays an important part in soman-induced symptomatology and lethality but also that nicotinic antagonists given in large doses after soman exposure may have untoward effects.

Low-dose mecamylamine improves learning of rats in the radial-arm maze repeated acquisition procedure

The nicotinic antagonist mecamylamine has been widely shown to cause cognitive impairment. However, these effects are mainly seen with high doses. There have been scattered findings that low doses of mecamylamine can have the opposite effect. This may be due to opposite effects of low doses of mecamylamine. In the current study, an extensive dose-effect function of mecamylamine was characterized in the low-dose range. Adult female Sprague-Dawley rats were trained on a repeated acquisition procedure on an automated 8-arm radial maze. Three of the eight arms were designated as correct for any particular session. Five trials per session were run. The number of errors per trial to find the three correct arms was determined. The rats were trained on the repeated acquisition procedure for at least 18 sessions at which time they showed reliable learning each session. Then, the effect of low doses of mecamylamine between 0 and 1 mg/kg were assessed in a repeated measures counterbalanced design. This dose range of mecamylamine did not affect performance on the first trial when the rats were naïve to the array to be learned. On trials 2-5 a significant (p<.025) quadratic dose-effect function was seen over this dose range. The most substantial effect was seen with 0.125 mg/kg of mecamylamine, which caused a significant (p<.05) improvement relative to the saline control condition. The effect diminished with increasing mecamylamine doses and with the 1 mg/kg dose choice accuracy was back to control levels. This study showed that low doses of mecamylamine can effectively improve learning. A U-shaped dose-effect curve was documented. This suggests possible low-dose nicotinic antagonist lines of treatment for cognitive impairment.

A Placebo Controlled, Double-Blind Study of Mecamylamine Treatment for Cocaine Dependence in Patients Enrolled in an Opiate Replacement Program

A placebo controlled, double-blind trial of mecamylamine treatment of cocaine dependence was performed in methadone or LAAM maintained subjects who met DSM-IV criteria for cocaine dependence. After an eight-week placebo run-in screening period, 35 subjects were randomly assigned to receive either mecamylamine (6 mg/day) or placebo transdermal patches for a 16-week treatment period. Outcome measures included quantitative urine benzoylecognine (BE) levels, self-report of cocaine use, cocaine craving, global impression scores, mood, retention, and safety. Mecamylamine was well tolerated, and study retention did not differ by treatment group. Evidence for cocaine use, based on urine BE levels and cocaine abstinence rates, did not differ by treatment group. Self reported cocaine use, cocaine craving, and global impression scores showed moderate improvement in both groups, with a significantly greater reduction in cocaine craving (p < 0.05) and self-rated severity of cocaine dependence (p < 0.05) in the placebo group. This pilot study does not support the effectiveness of mecamylamine for the treatment of cocaine dependence in methadone or LAAM maintained patients.

Mecamylamine (Targacept)

Targacept Inc, under license from the University of South Florida, is developing a low-dose, liquid gel capsule formulation of the nicotinic acetylcholine antagonist mecamylamine, as a potential treatment for various neuropsychiatric disorders.

The influence of mecamylamine on trigeminal and olfactory chemoreception of nicotine

Nicotine presented to the nasal cavity at low concentrations evokes 'odorous' sensations, and at higher concentrations 'burning' and 'stinging' sensations. A study in smokers and nonsmokers provided evidence of a relationship between the experience with the pharmacological action of S-(-)-nicotine and the perceived pleasantness/unpleasantness following nasal stimulation with S-(-)-nicotine. Mecamylamine, a nicotinic acetylcholine-receptor-(nAch-R) antagonist, was able to block painful responses following chemical stimulation of the human tongue and to block responses from the rat's ethmoidal nerve. The aim of our study in humans was to investigate the effects of mecamylamine on the olfactory and the trigeminal chemoreception of nicotine enantiomers. In order to achieve this aim, we determined-before and after mecamylamine-(1) detection thresholds, trigeminal thresholds, and intensity estimates (stimulus intensity) and (2) recorded the negative mucosal potential (NMP) following nasal stimulation with nicotine in a placebo-controlled double blind study (n = 15). CO(2) was used as a trigeminal and H(2)S as an olfactory control stimulus. Mecamylamine significantly increased trigeminal thresholds of S-(-)-nicotine and reduced intensity estimates and NMPs following stimulation with nicotine enantiomers, whereas mecamylamine did not influence NMPs and trigeminal intensity estimates following stimulation with CO(2). In contrast, mecamylamine did neither influence detection thresholds nor olfactory intensity estimates following stimulation with olfactory nicotine concentrations. These results demonstrate that the trigeminal nasal chemoreception of nicotine enantiomers, in contrast to CO(2), is mediated by nAch-Receptors and give evidence that the olfactory chemoreception of nicotine is independent from peripheral nAch-Receptors.

Dynorphin A (2-13) improves mecamylamine-induced learning impairment accompanied by reversal of reductions in acetylcholine release in rats

Accumulating evidence indicates that the endogenous opioid peptides dynorphin A (1-17) and synthetic dynorphin A (1-13) interact not only with opioid receptors but also with as yet poorly characterized non-opioid binding sites. Dynorphin A (1-13) improved impairments of learning and memory via not only kappa-opioid receptor-mediated, but also 'non-opioid' mechanisms. In the present study, the effects of des-tyrosine(1) dynorphin A (2-13) as a non-opioid metabolite of dynorphin A, and dynorphin A (1-13) on mecamylamine-induced impairment of the acquisition of learning in rats were investigated using a step-through type passive avoidance task. Further, hippocampal acetylcholine release was examined using in vivo microdialysis. Mecamylamine significantly shortened the step-through latency when given 30 min before the acquisition trial. Not only dynorphin A (1-13) but also dynorphin A (2-13) attenuated the mecamylamine-induced impairment of the acquisition of learning. The effect of dynorphin A (2-13) was not blocked by pre-treatment with nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist. Dynorphin A (2-13) completely abolished the decrease in the extracellular acetylcholine concentration induced by mecamylamine and this effect was not blocked by nor-BNI. Taken together with our previous findings, the present results may indicate that dynorphin A (2-13) improves impairment of learning and/or memory in 'non-opioid' mechanisms and dynorphin A (1-13) ameliorates impairment of the acquisition of learning via not only kappa-opioid receptor-mediated mechanisms but also 'non-opioid' mechanisms, by regulating the release of extracellular acetylcholine.

Investigation of the mechanisms involved in the central effects of glucagon-like peptide-1 on ethanol-induced gastric mucosal lesions

The aim of this study was to investigate the effects of intracerebroventricularly injected glucagon-like peptide-1 (GLP-1) on ethanol-induced gastric mucosal damage and to elucidate the mechanisms involved. Absolute ethanol was administered through an orogastric cannula 5 min before GLP-1 (1 microg/10 microl) injection. One hour later, the rats were decapitated, their stomachs were removed and scored for mucosal damage. GLP-1 inhibited the ethanol-induced gastric mucosal damage by 92%. Centrally injected atropine sulphate, a muscarinic receptor antagonist (5 microg/10 microl), prevented the gastroprotective effect of GLP-1, while mecamylamine, a nicotinic receptor antagonist (25 microg/10 microl), was ineffective. Peripherally injected atropine methyl nitrate (1 mg/kg) did not change the effect of GLP-1, but mecamylamine (5 mg/kg) blocked it. Cysteamine, a somatostatin depletor (280 mg/kg, s.c.), did not affect the protective activity of GLP-1, while inhibition of nitric oxide (NO) synthesis by L-NAME (3 mg/kg, i.v.) significantly abolished the protective effect of GLP-1 on ethanol-induced gastric mucosal lesions. We conclude that central muscarinic and peripheral nicotinic cholinergic receptors and NO, but not somatostatin, contribute to the protective effect of intracerebroventricularly injected GLP-1 on ethanol-induced gastric mucosal damage.

The involvement of central cholinergic system in the pressor effect of intracerebroventricularly injected U-46619, a thromboxane A2 analog, in conscious normotensive rats

The aim of this study was to determine the involvement of the central cholinergic system in the rise in blood pressure evoked by the thromboxane A2 (TxA2) analog, U-46619, given centrally. Intracerebroventricular (i.c.v.) injections of U-46619 (0.5, 1.0 and 2.0 microg) caused dose- and time-related increases in blood pressure and decreased heart rate in awake rats. U-46619 (1 microg; i.c.v.) also produced an approximately 65% increase in posterior hypothalamic extracellular acetylcholine and choline levels. Pretreatment with SQ-29548 (8 microg; i.c.v.), selective TxA2 receptor antagonist, completely inhibited both the cardiovascular responses and the increase in acetylcholine and choline levels to subsequent injection of U-46619 (1 microg; i.c.v.). Atropine (10 microg; i.c.v.), nonselective muscarinic receptor antagonist, pretreatment did not affect the cardiovascular responses observed after U-46619 (1 microg; i.c.v.). Pretreatment with the nonselective nicotinic receptor antagonist, mecamylamine (50 microg; i.c.v.) attenuated the pressor effect of U-46619 (1 microg; i.c.v.). Higher doses of mecamylamine (75 and 100 microg; i.c.v.) pretreatments did not change the magnitude of the blockade of pressor response to U-46619; however, they abolished the bradycardic effect of U-46619 dose-dependently. Interestingly, pretreatment of rats with methyllycaconitine (10 microg; i.c.v.) or alpha-bungarotoxin (10 microg; i.c.v.), selective antagonists of alpha7 subtype of nicotinic acetylcholine receptors (alpha7nAChRs), partially abolished the pressor response to i.c.v. injection of U-46619 (1 microg). Similar to the mecamylamine data, the use of higher doses of methyllycaconitine (25 and 50 microg; i.c.v.) produced the same magnitude of blockade that was observed after the 10 microg methyllycaconitine pretreatment, but it completely abolished the bradycardic effect of U-46619 (1 microg; i.c.v.) at the dose of 25 microg. The present results show that central administration of U-46619 produces pressor and bradycardic effect and increase in hypothalamic acetylcholine and choline levels by activating central TxA2 receptors. The activation of central nicotinic receptors, predominantly alpha7nAChRs, partially mediates the cardiovascular responses to i.c.v. injection of U-46619.

ABT-594 (a nicotinic acetylcholine agonist): anti-allodynia in a rat chemotherapy-induced pain model

ABT-594 ((R)-5-(2-azetidinylmethoxy)-2-chloropyridine) represents a novel class of broad-spectrum analgesics whose primary mechanism of action is activation of the neuronal nicotinic acetylcholine receptors. The present study characterized the effects of ABT-594 in a rat chemotherapy-induced neuropathic pain model, where it attenuated mechanical allodynia with an ED50 = 40 nmol/kg (i.p.). This anti-allodynic effect was not blocked by systemic (i.p.) pretreatment with naloxone but was blocked completely with mecamylamine. Pretreatment with chlorisondamine (0.2-5 micromol/kg, i.p.) only partially blocked the effects of ABT-594 at the higher doses tested. In contrast, central (i.c.v.) pretreatment with chlorisondamine completely blocked ABT-594's anti-allodynic effect. Taken together, the data demonstrate that ABT-594 has a potent anti-allodynic effect in the rat vincristine model and that, in addition to its strong central site of action, ABT-594's effects are partially mediated by peripheral nicotinic acetylcholine receptors in this animal model of chemotherapy-induced neuropathic pain.

Contractile effect of acetylcholine on isolated ampullar segment of Fallopian tubes

The Fallopian tubes are scarcely innervated with cholinergic nerve fibers. Acetylcholine released from these nerves contracts smooth muscles of the tubes. The aim of our study was to investigate the effect of acetylcholine on the Fallopian tubes using selective antagonists in different hormonal settings. We have investigated effects of acetylcholine on isolated ampulla of Fallopian tubes taken from 83 patients during abdominal hysterectomy with adnexectomy. Twenty-eight patients were in follicular, 36 were in luteal phase of menstrual cycle, and 19 patients were in menopause. Selective and non-selective muscarinic and nicotinic receptor blockers were used for investigation of the effects. Acetylcholine (from 1.8 to 658.6 microM/l) produced concentration-dependent tonic contraction of ampulla taken from the patients in follicular phase, luteal phase and menopause. The nicotinic receptor blocker mecamylamine (6.5 microM/l) and local anesthetic lidocaine (230.8 microM/l) did not affect the effect of acetylcholine. While M(1)- and M(2)-selective muscarinic receptor blockers pirenzepine (1.6 microM/l) and methoctramine (0.9 microM/l) did not show specific effect, atropine (0.01 microM/l) and selective M(3)-receptor blocker p-fluoro-hexahydro-sila-difenidol (pFHHSiD, 0.2 microM/l) effectively blocked contractions caused by acetylcholine (maximal pA(2) values 9.74 and 7.54, respectively). The affinity of pFHHSiD for muscarinic receptors was highest in the follicular phase. The results of our study suggest existence of functional M(3) muscarinic receptors in ampulla of Fallopian tubes, located on the smooth muscle cells.

Local perfusion of nicotine differentially modulates somatodendritic dopamine release in the rat ventral tegmental area after nicotine preexposure

We examined the effects of nicotine perfusion into the ventral tegmental area (VTA) on extracellular dopamine (DA) levels in rats using in vivo microdialysis. Local perfusion with nicotine for 80 min (10-100 microM) modestly increased (approximately 105-131% of basal) the extracellular DA levels in the VTA of rats that had been pretreated with saline for 5 days. In animals that had been pretreated with nicotine for 5 days (0.3 mg/kg, s.c.), perfusion with nicotine for 80 min (10-100 microM) dose-dependently increased the extracellular DA levels in the VTA of rats and did so to a greater extent than in saline-pretreated animals (125-171% of basal). Co-perfusion through the dialysis probe with 100 microM mecamylamine, a nonselective nicotinic acetylcholine receptor (nAChR) antagonist, or 100 microM dihydro-beta-erythroidine, a high affinity and competitive nAChR antagonist, attenuated the enhancement of extracellular DA levels produced by 100 microM nicotine alone. These results suggest that local nicotine challenge potentiated the somatodendritic DA release after nicotine preexposure by stimulation of high-affinity nAChRs in the VTA.

Extinguishing the rewarding value of smoke cues: pharmacological and behavioral treatments

The present study examined several pharmacological and behavioral treatments designed to promote extinction of the responses to rewarding cigarette smoke cues. Pharmacological treatments comprised nicotine skin patches (21 mg/24 hr) and the nicotinic acetylcholine receptor antagonist mecamylamine (10 mg/day), administered separately or in combination. Behavioral manipulations included switching to denicotinized cigarettes, to cigarettes having different menthol flavor, or to ventilated-filter (low tar and nicotine) cigarettes. Smokers were assigned to the various treatments for 2 weeks before they quit smoking. During weekly test sessions, subjects rated the rewarding effects of their usual brands of cigarettes or cigarettes with different menthol content (mentholated vs. nonmentholated). Over the 2-week treatment period, all pharmacological treatments reduced ratings of reward for the usual-brand test cigarettes. Switching to smoking denicotinized cigarettes for 2 weeks similarly decreased rewarding effects of the usual-brand test cigarettes. Subjects also strongly preferred cigarettes with the same menthol content to which they were accustomed. However, manipulating the menthol content of the cigarettes smoked during the 2 weeks of treatment had different effects, depending on whether smokers habitually smoked mentholated or nonmentholated cigarettes. For menthol smokers, removal of the menthol cue hampered extinction of reward ratings for the usual-brand (mentholated) test cigarette. For nonmenthol smokers, addition of the menthol cue did not affect the progress of extinction of nonmenthol smoke cues. These findings demonstrate the importance of sensory cues in determining subjective reward and show that the reward value of these cues can be altered by removal of nicotine from tobacco or by pharmacological manipulations that interfere with the reinforcing effects of nicotine.

Differential effects of M1 muscarinic receptor blockade and nicotinic receptor blockade in the dorsomedial striatum on response reversal learning

The present studies determined whether blockade of M(1)-like muscarinic or nicotinic cholinergic receptors in the dorsomedial striatum affects acquisition or reversal learning of a response discrimination. Testing occurred in a modified cross-maze across two consecutive sessions. In the acquisition phase, a rat learned to turn to the left or to the right. In the reversal learning phase, a rat learned to turn in the opposite direction as required during acquisition. Experiment 1 investigated the effects of the M(1)-like muscarinic receptor antagonist, pirenzepine infused into the dorsomedial striatum on acquisition and reversal learning. Experiment 2 examined the effects of the nicotinic cholinergic antagonist, mecamylamine injected into the dorsomedial striatum on acquisition and reversal learning. Bilateral injections of pirenzepine at 10 microg, but not 1 microg, selectively impaired reversal learning. Analysis of the errors indicated that pirenzepine treatment did not impair the initial shift, but increased reversions back to the original response choice following the initial shift. Bilateral injections of mecamylamine, 6 or 18 microg, did not affect acquisition or reversal learning. The results suggest that activation of M(1) muscarinic cholinergic receptors, but not nicotinic cholinergic receptors, in the dorsomedial striatum is important for facilitating the flexible shifting of response patterns.

Nicotine withdrawal in adolescent and adult rats

Previous research with animal models has demonstrated that adolescent rats display heightened sensitivity to the reinforcing and stimulant effects of nicotine relative to adult rats. Little work has focused on the response of adolescent rats to measures of nicotine withdrawal. To test the hypothesis that adolescent rats may be differentially sensitive to withdrawal relative to their adult counterparts, the present study was designed to compare precipitated withdrawal in adolescent and adult rats following chronic nicotine administration. Adult and adolescent rats were prepared with subcutaneous osmotic minipumps that delivered either saline or nicotine (9 mg/kg per day, salt; N =12 per group). All rats were challenged with the nicotinic receptor antagonist mecamylamine (1.5 mg/kg) on day 7 of chronic nicotine treatment. Twenty minutes after the injection, overt somatic signs of withdrawal (i.e., eye blinks, writhes, body shakes, teeth chatter, gasps, and ptosis) were recorded for 10 min. Adult rats were observed on postnatal day 73-77, and adolescent rats were tested on postnatal day 36-40. The results revealed a robust increase in mecamylamine-induced withdrawal signs in adult rats receiving chronic nicotine relative to adult rats receiving saline. In contrast, mecamylamine did not precipitate withdrawal signs in adolescent rats receiving chronic nicotine. These results indicate that there is decreased sensitivity to the somatic aspects of nicotine withdrawal in adolescent rats that may maximize the reinforcing effects of nicotine during adolescence by minimizing the aversive effects of abstinence.

Neurogenic vasodilation of dural blood vessels is not mediated by cholinergic transmission in the anaesthetised rat

Dural vessel dilation induced by activation of trigeminal sensory fibres may be responsible for some component of the migraine attack. The presence in some patients with migraine and cluster headache of clinical features, such as lacrimation, suggests cranial parasympathetic activation and poses the question as to whether neurogenic meningeal dilatation has a cholinergic component. Rats were prepared in order to record on-line the diameter of a middle meningeal artery branch through a closed cranial window using an intravital microscope coupled to a video dimension analyser. Acetylcholine (1 microg, intravenously, i.v.) was administered before and after muscarinic receptor inhibition (n=5) with scopolamine (2 mg/kg, i.v.) or nicotinic receptor inhibition (n=6) with mecamylamine (4 mg/kg, i.v.). Further vasodilation was induced by electrical stimulation of the cranial window surface before and after muscarinic receptor inhibition with i.v. scopolamine (n=8). The mean dural vessel percentage increase caused by acetylcholine stimulation was significantly different before and after muscarinic receptor inhibition (P=0.045). Moreover, there was no difference between the post receptor inhibition values and those obtained after vehicle infusion (P=0.431). In contrast, no difference was detected in the effect of acetylcholine before and after nicotinic receptor inhibition (P=0.688). In the second experiment, where the effect of muscarinic receptor inhibition on the neurogenic dilation model was assessed, no significant difference was demonstrated (P=0.538). Cholinergic dilation of the rat dural arteries is mediated by muscarinic receptors, but this mechanism does not play a significant role in the rat dural vessel dilation induced by closed cranial window electrical stimulation.

Nicotine enhances responding with conditioned reinforcement

RATIONALE

The mesolimbic dopamine system has been implicated in the primary reinforcing properties of drugs of abuse as well as in enhanced responding with conditioned reinforcement produced by psychomotor stimulant drugs. Despite clinical observations that nicotine self-administration (i.e. smoking) depends strongly upon conditioned reinforcement (i.e. cues support smoking behavior), little is known about whether nicotine directly affects motivational processes.

OBJECTIVE

In these experiments, we investigated whether acute nicotine would influence responding with conditioned reinforcement and the degree to which pretreatment with the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine would modify any nicotine-induced behavioral effects.

METHODS

After subjects had been trained to associate an initially neutral stimulus with water reward, they received acute nicotine (43,25-350 micro g/kg SC; -5 min) or saline injections and were tested on the acquisition of a new response for conditioned reinforcement paradigm. In separate experiments, the effect of pretreatment with the nicotinic acetylcholine receptor antagonist mecamylamine (300 or 1000 micro g/kg SC; -20 min) alone, or in combination with nicotine (350 micro g/kg SC; -5 min), on conditioned reinforcement was also examined.

RESULTS

Acute nicotine injection produced a selective enhancement of responding with conditioned reinforcement (i.e. on the CR lever), without producing non-selective increases in overall responding. The effect of nicotine (350 micro g/kg SC; -5 min) was selectively blocked by mecamylamine (300 micro g/kg).

CONCLUSIONS

These findings demonstrate that acute exposure to nicotine augments the control over behavior by a conditioned reinforcer, suggesting that nicotine may enhance motivational processes.

Mecamylamine attenuates the subjective stimulant-like effects of alcohol in social drinkers

BACKGROUND

Recent studies have implicated central nicotinic cholinergic receptor systems in the reinforcing properties of alcohol. In laboratory animals, mecamylamine, a central nicotinic receptor antagonist, reduces the consumption of and preference for alcohol. This study investigated the effect of mecamylamine on the subjective responses to alcohol in humans. It was hypothesized that mecamylamine (7.5 and 15 mg) would attenuate the stimulant-like subjective effects of alcohol (0.8 g/kg) and decrease the self-reported desire to consume additional alcohol beverages.

METHODS

Fourteen male and 13 female nonsmokers participated in 6 laboratory sessions. During each session, subjects received, in randomized order under double-blinded conditions, a capsule containing mecamylamine (7.5 or 15 mg) or placebo followed by a beverage containing alcohol (0.8 g/kg) or placebo. Physiologic and subjective-effect measures were taken at 30-min intervals for 2 hr after beverage consumption.

RESULTS

Mecamylamine attenuated the stimulant and euphoric effects of alcohol and reduced the self-reported desire to consume additional alcohol beverages. This effect was most pronounced in men, even though women exhibited greater physiologic reactions to mecamylamine.

CONCLUSIONS

These findings suggest that nicotinic cholinergic receptors are involved in mediating some of the stimulant-like effects of alcohol.

Mecamylamine (Inversine): an old antihypertensive with new research directions

Mecamylamine (Inversine), the first orally available antihypertensive agent, is now rarely used. Although celebrated in the 1950s, mecamylamine fell out of favour because of its widespread ganglionic side effects at antihypertensive doses (30-90 mg/day). However, recent studies suggest that mecamylamine is very effective at relatively low doses (2.5-5 mg b.i.d.) for blocking the physiological effects of nicotine and improving abstinence rates in smoking cessation studies, particularly for women. When these lower doses of mecamylamine are given, patients do not experience the severity of side effects that made the drug unpopular for the treatment of hypertension. Tobacco smoking is a strong risk factor for cardiovascular morbidity, including accelerated atherosclerosis and increased risk of heart attacks. Though currently untested, the available evidence suggests that low-dose mecamylamine therapy might reduce blood pressure variability and atherogenetic lipid profile in smokers. With this in mind, mecamylamine should be an important research tool in the field of hypertension research, particularly in recalcitrant smokers with mild to moderate hypertension.

New medications for nicotine dependence treatment

For several years, nicotine replacement therapy (nicotine gum, patches, and nasal spray) has been the mainstay for the treatment of nicotine dependence. The nicotine vapor inhaler is a new pharmacological adjunct shown to be effective in placebo-controlled trials. It delivers a vaporized form of nicotine to the oral mucosa. Bupropion sustained release (SR) is the first non-nicotine pharmacological treatment approved for smoking cessation and is thought to be effective because of its dopaminergic activity on the pleasure and reward pathways in the mesolimbic system and nucleus accumbens. Though few studies have been reported, there is pharmacological rationale to use combined pharmacotherapies for the treatment of nicotine dependence. While there are a limited number of reported studies with mixed findings using higher than the standard nicotine patch dose, use of higher doses of nicotine patch therapy (i.e., more than one patch at a time) may be appropriate for smokers who previously failed single dose patch therapy or in those whose nicotine withdrawal symptoms were not adequately relieved with standard therapy. The concept of therapeutic drug monitoring can be applied to nicotine replacement therapy. A new product, a sublingual nicotine tablet, has shown efficacy in a double-blind placebo-controlled trial and will likely be approved in the future. The anti-hypertensive, mecamylamine, has been found to have efficacy for smoking cessation in a small trial. Nicotine and mecamylamine both occupy receptors that would otherwise be acted upon by nicotine from cigarettes, thus, when administered in combination, would be expected to occupy more receptors than either drug alone, thereby attenuating smoking reward and facilitating extinction of the smoking behavior. Pivotal trials of this combination are underway. Remaining questions include: (1) what is the optimal dose and duration of treatment using nicotine replacement therapy? (2) What is the optimal duration of treatment using bupropion? (3) What are the best combination treatments and which smokers are best suited for combination treatment? (4) Will other similar pharmacological agents with dopaminergic/noradrenergic activity have efficacy similar to bupropion?

Nicotine and bupropion share a similar discriminative stimulus effect

Bupropion is a weakly potent central nervous system (CNS) stimulant that is marketed both as an antidepressant and as an anti-smoking aid. The mechanism(s) by which it produces its effects is not well understood. In the present study, the effect of bupropion was examined in rats trained to discriminate the stimulus effect of 0.60 mg/kg of (-)-nicotine from saline in a two-lever drug discrimination task. In tests of stimulus generalization (substitution), the nicotine (ED(50)=0.17 mg/kg) stimulus completely generalized to bupropion (ED(50)=5.50 mg/kg). In addition, interaction studies were conducted that evaluated the effect of 3.0 mg/kg of bupropion, a dose that when given alone produced saline-appropriate responding, in combination with various doses of nicotine. This application resulted in an enhancement of the potency of nicotine (ED(50)=0.05 mg/kg), as indicated by a leftward shift of the nicotine dose-effect function. In tests of stimulus antagonism, various doses of bupropion were administered prior to the training dose of nicotine and were found to be ineffective as antagonists of the nicotine stimulus. In contrast, the nicotinic acetylcholine receptor (nicotine receptor) antagonist mecamylamine (AD(50)=0.40 mg/kg) completely blocked the stimulus effect of nicotine. Mecamylamine did not attenuate the stimulus generalization of bupropion. The results demonstrated that bupropion can produce a nicotine-like response in nicotine-trained animals, but it does so via a mechanism of action that is unlike that of nicotine. It is speculated that bupropion may be somewhat effective as an anti-smoking treatment in people who are motivated to quit smoking because low doses of bupropion produce a nicotine-like effect(s) that serve as a suitable substitute for nicotine.

Open field habituation learning is improved by nicotine and attenuated by mecamylamine administered posttrial into the nucleus accumbens

Using the paradigm of habituation learning in the open field, we tested the effects of unilateral microinjections of the agonist nicotine (8.0, 40.0, and 80.0 microg) and the nicotine receptor antagonist mecamylamine (0.1, 1.0, 10.0 microg) into the core of the nucleus accumbens. When injected posttrial, that is, immediately after the first exposure to the open field, nicotine dose-dependently enhanced behavioral habituation during the test on the following day, indicating a facilitation of memory, whereas mecamylamine impaired habituation at the highest dose, but not at the two lower doses. When injected 5 h after the learning trial, nicotine (40 microg) and mecamylamine (10 microg) impaired habituation on the subsequent day. A control experiment did not provide evidence for possible proactive effects of mecamylamine. These findings are discussed with respect to the possible behavioral functions of cholinergic, and especially nicotinic, mechanisms in the nucleus accumbens. They may also be relevant for understanding cholinergic-linked psychopathologies such as Alzheimer's disease, since the nucleus accumbens is one of the sites where cholinergic neurons are lost in this neurodegenerative disease.

Mechanisms involved in the water intake-related pressor response in the rat

OBJECTIVE

In this study we aimed to characterize and clarify the mechanisms involved in the acute blood pressure increase observed concomitantly with water intake in moderately dehydrated rats.

DESIGN

Short-term water deprivation was employed as a model to induce controlled water intake to study concomitant cardiovascular responses in the rat.

METHODS

Male Wistar rats were deprived of water for 18-24 h before the experiments and were allowed to drink for 20 s periods during the experimental session. During these periods water intake was accompanied by steady arterial pressure increases. This pressor response was unaffected by topical anesthesia of the oral cavity. Direct administration of water into the stomach did not cause pressor responses. The pressor response was not affected by bilateral adrenal demedullation or by pretreatment with diazepam, homatropine methyl bromide, d(CH2)5 Tyr(Me)AVP, losartan or RX821002. The pressor response was significantly reduced by ganglionic blockade with mecamylamine or pretreatment with the alpha1-adrenoceptor antagonist, prazosin.

CONCLUSIONS

Our results show that: (1) short-term dehydration can be used as a model to study cardiovascular responses associated with water intake in rats; and (2) the sympathetic nervous system and vascular smooth muscle alpha1-adrenoceptors are involved in the pressor response to water intake by dehydrated rats.

Intravenously administered lidocaine in therapeutic doses increases the intraspinal release of acetylcholine in rats

The local anesthetic lidocaine suppresses different pain conditions when administered systemically. Part of the antinociceptive effect appears to be mediated via receptor mechanisms. We have previously shown that muscarinic and nicotinic agonists that produce antinociception increase the intraspinal release of acetylcholine. In the present study it was hypothesized that systemically administered lidocaine is acting through the same mechanisms as cholinergic agonists and affects the intraspinal release of acetylcholine. Microdialysis probes were placed in anesthetized rats for sampling of acetylcholine. Ten and 30 mg/kg lidocaine injected intravenously significantly increased the intraspinal release of acetylcholine. The effect of lidocaine could be reduced by pretreatment with intraspinally administered atropine or mecamylamine. Our results suggest that the antinociceptive effect produced by systemically administered lidocaine is mediated through an action on muscarinic and nicotinic receptors.

Platelet monoamine oxidase, smoking cessation, and tobacco withdrawal symptoms

Previous studies have found that constituents in tobacco inhibit both forms of the enzyme monoamine oxidase (MAO-A and MAO-B). This enzyme is important in the breakdown of the amine neurotransmitters, including dopamine, which is thought to mediate the reinforcing effects of nicotine and contribute to tobacco dependence. To further examine the relationship between cigarette smoking, smoking cessation and MAO, we measured platelet MAO-B activity in 16 smokers before and after being switched to smoking denicotinized cigarettes; in a subset of six subjects who subsequently quit-smoking, MAO-B activity was also measured at 1 and 4 weeks following cessation. Smoking cessation treatment was provided in an open-label format, and included nicotine skin patch treatment in conjunction with oral mecamylamine (a nicotinic antagonist) and neostigmine (a peripherally acting acetylcholinesterase inhibitor, administered to counteract constipation experienced from mecamylamine). Results showed that smoking behavior, indexed by expired air carbon monoxide levels, was negatively correlated with platelet MAO-B activity prior to smoking cessation. Moreover, MAO-B activity significantly increased by approximately 100% at 4 weeks after quitting smoking. However, little or no recovery occurred within the first week of abstinence, suggesting that the constituents in tobacco responsible for MAO inhibition may have half-lives of several days. Thus, if relapse to smoking is due in part to withdrawal from the MAO-inhibiting effects of tobacco, this effect likely occurs more than 1 week after quitting. Additionally, low baseline MAO-B activity significantly predicted the intensity of withdrawal symptoms reported upon switching to the denicotinized cigarettes as well as after smoking cessation. These results support the view that MAO inhibition from non-nicotine constituents in cigarette smoke is relevant to tobacco dependence and that continued investigation of the potential use of MAO inhibitors in smoking cessation treatment is warranted.

Nicotine induced seizures blocked by mecamylamine and its stereoisomers

Recent genetic research has shown that certain forms of epilepsy may arise from mutations in the genes encoding for the alpha7 and alpha4 neuronal nicotinic acetylcholine receptor (nAChR) ion channels. These receptors are also involved with the induction of nicotine-induced seizures. (+/-)-Mecamylamine (Inversine), a classic nAChR antagonist, potently inhibits nicotine-induced seizures. The purpose of the present study was to assess the inhibitory effects of (+/-)-mecamylamine and its stereoisomers on nicotine-induced seizures in male Sprague-Dawley rats. Rats received saline, (+/-)-mecamylamine, R-(-)-mecamylamine, or S-(+)-mecamylamine (s.c.) at doses of 0.1, 0.3, or 1.0 mg/kg 15 minutes prior to nicotine injection, 3.6 mg/kg (s.c.), an optimal dose for seizure induction. Rats were observed for 30 minutes with seizure latency, duration, and severity as primary measures and locomotor activity recorded as a secondary measure at 5-minute intervals. The results indicate that mecamylamine and each of its stereoisomers block nicotine-induced seizures in a dose-related manner and suggest that the S-(+/-)- mecamylamine isomer has inhibitory properties more similar to the racemic than to the R-(-)-mecamylamine isomer. The results of this study may be clinically important for the future design of novel anti-seizure medications.

The effects of physostigmine and cholinergic receptor ligands on novelty-induced neophobia

The aim of the study was to analyse in a well-established model of neophobia the effects of peripheral and central (ICV) administration of a prototypical and easily penetrating to the brain acetylcholinesterase inhibitor (AChE-I)--physostigmine, hemicholinium, a selective blocker of the high affinity choline uptake sites, as well as muscarinic and nicotinic receptor ligands. Thus, an attempt was made to address the question whether anxiolytic-like effects of AChE-I, reported in the clinic, are directly related to the anti-emotional action. The effects of peripherally and centrally administrated cholinergic ligands on novelty-induced decrease in exploratory behaviour were examined in rats. It was found that in a limited dose-range physostigmine and nicotine given peripherally or ICV selectively disinhibited rat exploration in the open field, whereas scopolamine stimulated animal motor activity and increased thigmotaxis. Locomotor effects of physostigmine and nicotine appeared at the higher doses and could be easily separated from their anti-neophobic action. The rat's exploratory behaviour tended to be attenuated by central administration of hemicholinium (a choline uptake blocker), and it was significantly inhibited by mecamylamine (a nicotinic receptor antagonist), and pirenzepine (a selective M1 receptor antagonist). Gallamine, a selective M2 receptor antagonist, did not influence on animal novelty-induced anxiety-related behaviour. It is concluded that AChE-I can selectively affect brain emotional processes evoked by neophobia-related stimuli. Probably both nicotinic and M1 cholinergic receptors mediate such an action of AChE-I.

Effects of Uncaria tomentosa total alkaloid and its components on experimental amnesia in mice: elucidation using the passive avoidance test

The effects of Uncaria tomentosa total alkaloid and its oxindole alkaloid components, uncarine E, uncarine C, mitraphylline, rhynchophylline and isorhynchophylline, on the impairment of retention performance caused by amnesic drugs were investigated using a step-down-type passive avoidance test in mice. In this test, the retention performance of animals treated with the amnesic and test drugs before training was assessed 24 h after training. Uncaria tomentosa total alkaloid (10-20 mg kg(-1), i.p.) and the alkaloid components (10-40 mg kg(-1), i.p.), as well as the muscarinic receptor agonist oxotremorine (0.01 mg kg(-1), i.p.), significantly attenuated the deficit in retention performance induced by the muscarinic receptor antagonist scopolamine (3 mg kg(-1), i.p.). The effective doses of uncarine C and mitraphylline were larger than those of other alkaloid components. Uncarine E (20 mg kg(-1), i.p.) also blocked the impairment of passive avoidance performance caused by the nicotinic receptor antagonist mecamylamine (15 mg kg(-1), i.p.) and the N-methyl-D-aspartate (NMDA) receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 7.5 mg kg(-1), i.p.), but it failed to affect the deficit caused by the benzodiazepine receptor agonist diazepam (2 mg kg(-1), i.p.). Rhynchophylline significantly reduced the mecamylamine-induced deficit in passive avoidance behaviour, but it failed to attenuate the effects of CPP and diazepam. These results suggest that Uncaria tomentosa total alkaloids exert a beneficial effect on memory impairment induced by the dysfunction of cholinergic systems in the brain and that the effect of the total alkaloids is partly attributed to the oxindole alkaloids tested. Moreover, these findings raised the possibility that the glutamatergic systems are implicated in the anti-amnesic effect of uncarine E.

Chronic, but not acute, nicotine exposure attenuates ethanol withdrawal-induced hippocampal damage in vitro

BACKGROUND

Long-term ethanol use and long-term tobacco use frequently occur together, which suggests concurrent dependence on ethanol and nicotine. Consequences of this form of polydrug dependence are not well understood, however. Previous evidence suggests detrimental effects of long-term ethanol and beneficial effects of nicotine exposure on neuronal viability. Thus, the present study was designed to use an organotypic hippocampal slice culture model to examine the ability of chronic and acute nicotine exposure to reduce neurotoxicity associated with withdrawal from long-term ethanol exposure.

METHODS AND RESULTS

Twenty-four hours of withdrawal after continuous 10 day ethanol exposure (50 or 100 mM in culture medium) resulted in cytotoxicity in hippocampal slice explants obtained from neonatal rat, most notably in pyramidal cell layers of the CA1 region. Exposure of slices to the N-methyl-D-aspartate receptor blocker MK-801 during ethanol withdrawal significantly reduced this toxicity. Exposure of slices to nicotine (0.1-10.0 microM) during the 24 hr withdrawal period did not reduce hippocampal damage. However, treatment of slices with nicotine (0.1-10.0 microM) during 10 days of ethanol exposure was associated with significant reductions in subsequent withdrawal-induced cytotoxicity, an effect reduced by mecamylamine coexposure with nicotine and ethanol.

CONCLUSIONS

These findings indicate the development of marked hippocampal neurotoxicity during withdrawal from long-term ethanol exposure that is mediated, in part, by overactivation of N-methyl-D-aspartate receptors. Furthermore, these data suggest that one consequence of concurrent dependence on ethanol and nicotine may be reduced neurological damage associated with ethanol withdrawal.

The antinociceptive effects of alpha7 nicotinic agonists in an acute pain model

Nicotinic receptors have been found to play a role in modulating pain transmission in the CNS. Activation of cholinergic pathways by nicotine and nicotinic agonists has been shown to elicit antinociceptive effects in a variety of species and pain tests. The involvement of alpha(7) nicotinic receptors in nicotinic analgesia was assessed after spinal (i.t.) and intraventricular (i.c.v.) administration in mice. Dose-dependent antinociceptive effects were seen with the alpha(7) agonist choline after spinal and supraspinal injection using the tail-flick test. Furthermore, alpha(7) antagonists MLA and alpha-BGTX significantly blocked the effects of choline. Dihydro-beta-erythroidine and mecamylamine failed to block choline-induced antinociception. These results strongly support the involvement of alpha(7) subunits in choline's antinociceptive effects. DMXB and 4-OH-DMXB, partial alpha(7) agonists, failed to elicit a significant antinociceptive effect. However, they blocked choline-induced antinociception in a dose-dependent manner following i.t. injection. This antagonism is probably related to their partial agonistic properties of the alpha(7) receptors. These studies suggest that activation of alpha(7) receptors in the CNS elicits antinociceptive effects in an acute thermal pain model.

Spinal mechanisms underlying A-85380-induced effects on acute thermal pain

Systemic administration of nicotinic receptor (nAChR) agonists is antinociceptive in models of acute pain whereas their intrathecal (i. t.) administration has been reported to be antinociceptive, nociceptive or without effect. It has been hypothesized that the action induced is dependent upon the subtype and location of the nAChR activated. In addition, there is considerable evidence that nAChR ligand-induced antinociception is mediated by other neurotransmitter systems via descending pathways from the brainstem to the spinal cord. The present study investigated the effects of i. t. and systemic administration of A-85380, a novel nAChR agonist, in the paw withdrawal model of acute thermal pain in the rat. Given i.t. , A-85380 (1 and 10 nmol/rat) decreased the latency to paw withdrawal by 2-4 s. This pronociception was accompanied by a spontaneous flinching behavior. Both of these effects were differentially blocked by i.t. pretreatment with the nAChR antagonists mecamylamine (10 nmol)>MLA (100 nmol)>DHbetaE (50% with 1000 nmol) but not by alpha-bungarotoxin (0% at 0.63 nmol). Given systemically, A-85380 (0.56 micromol/kg, i.p.) induced antinociception as indicated by an increased latency to paw withdrawal, an effect differentially altered by i.t. pretreatment with monoaminergic antagonists (100 nmol/rat). While mecamylamine and prazosin had no effect, scopolamine, methysergide and MDL 72222 partially antagonized and idazoxan completely antagonized A-85380-induced antinociception. Finally, as measured by in vivo microdialysis, levels of 5-HT, but not NE, in the i.t. space of the lumber region of the spinal cord were significantly increased following the systemic administration of A-85380. Together these data suggest that the nociceptive properties of spinally administered nAChR agents are not mediated by either an alpha(4)beta(2) or an alpha(7) subtype nAChR, whereas the antinociceptive properties of systemically-administered nAChR agents are mediated by descending noradrenergic, serotonergic and muscarinic inhibitory pathways.

Mecamylamine blockade of both positive and negative effects of IV nicotine in human volunteers

The ganglionic blocker mecamylamine blocks the positive reinforcing effects of IV nicotine, but has been shown to increase cigarette smoking behavior under some conditions. The effects of mecamylamine on subjective and physiologic responses to IV nicotine were evaluated in seven healthy male volunteer cigarette smokers who provided informed consent and resided on a clinical pharmacology research unit. On four separate days, each subject was given a different oral dose of mecamylamine (placebo, 5, 10, or 20 mg). One hour later subjects received the first of four doses of IV nicotine (placebo, 0.75, 1.5, and 3.0 mg); the remaining injections were given at 1-h intervals. Both the positive effects following 0.75 mg and negative effects following 3.0 mg of nicotine were significantly reversed by mecamylamine. Thus, the mecamylamine-induced increase in smoking may be due both to competitive blockade of nicotinic receptors and nicotine's reversal of aversive effects.

Receptor subtypes mediating depressor responses to microinjections of nicotine into medial NTS of the rat

Microinjections (50 nl) of nicotine (0.01-10 microM) into the nucleus of the solitary tract (NTS) of adult, urethan-anesthetized, artificially ventilated, male Wistar rats, elicited decreases in blood pressure and heart rate. Prior microinjections of alpha-bungarotoxin (alpha-BT) and alpha-conotoxin ImI (specific toxins for nicotinic receptors containing alpha7 subunits) elicited a 20-38% reduction in nicotine responses. Similarly, prior microinjections of hexamethonium, mecamylamine, and alpha-conotoxin AuIB (specific blockers or toxin for nicotinic receptors containing alpha3beta4 subunits) elicited a 47-79% reduction in nicotine responses. Nicotine responses were completely blocked by prior sequential microinjections of alpha-BT and mecamylamine into the NTS. Complete blockade of excitatory amino acid receptors (EAARs) in the NTS did not attenuate the responses to nicotine. It was concluded that 1) the predominant type of nicotinic receptor in the NTS contains alpha3beta4 subunits, 2) a smaller proportion contains alpha7 subunits, 3) the presynaptic nicotinic receptors in the NTS do not contribute to nicotine-induced responses, and 4) EAARs in the NTS are not involved in mediating responses to nicotine.

Nicotine-mecamylamine interactions

OBJECTIVES

Mecamylamine blocks nicotinic cholinergic receptors and has been proposed, in combination with nicotine, as a novel therapy to aid smoking cessation. The objective of this study was to characterize the pharmacokinetic and pharmacodynamic interactions between transdermal mecamylamine and intravenous nicotine.

STUDY DESIGN

Twelve cigarette smokers were studied while receiving transdermal mecamylamine 6 mg/24 h and placebo patches for 7 days each. On the fifth day of patch use, subjects received a combined infusion of deuterium-labeled nicotine and cotinine, with measurement of disposition kinetics of nicotine and cotinine, and cardiovascular and plasma catecholamine responses to nicotine. Half of the subjects were studied under alkaline urine conditions and the other half under acidic urine conditions.

RESULTS

Steady-state plasma mecamylamine concentrations were twice as high (mean, 12.2 versus 6.3 ng/mL), consistent with lower renal clearance (2.1 versus 5.8 mL/min/kg) during alkaline compared with acidic urine conditions. Mecamylamine did not significantly affect the clearances of nicotine or cotinine. Mecamylamine significantly reduced the volume of distribution and inhibited the cardioacceleratory and epinephrine-releasing effects of nicotine.

CONCLUSIONS

Mecamylamine has little effect on the clearance of nicotine and is not expected to affect steady-state levels during transdermal nicotine dosing. The reduction of the volume of distribution of nicotine by mecamylamine suggests that part of the antagonism of nicotinic central nervous system effects by mecamylamine may be due to a pharmacokinetic interaction-most likely decreased transport of nicotine into the brain or decreased binding to nicotine receptors. Mecamylamine may decrease the potential adverse cardiovascular effects of coadministered nicotine.