The nicotinic antagonist mecamylamine precipitates nicotine abstinence syndrome in the rat

Recently, a rodent model of nicotine abstinence syndrome has been developed based on observing the frequency of spontaneous behavioral signs following termination of continuous subcutaneous infusion of nicotine tartrate. In the present study, the nicotinic antagonist mecamylamine precipitated an abstinence syndrome in nicotine-dependent rats. Twelve rats were each infused for 7 days with 9 mg/kg per day nicotine tartrate in saline via Alzet osmotic minipumps; another 12 rats were shamoperated and remained nicotine-naive. Six rats from each group received 1 mg/kg mecamylamine in saline SC immediately before a 30-min observation, while the remaining six rats from each group received saline alone. Nicotine-infused rats receiving mecamylamine exhibited significantly more (P<0.01), overall abstinence signs than all other groups. In terms of categories of signs, they displayed significantly more gasps/writhes, teeth chatter/chews, shakes/tremors and ptosis. In a second experiment utilizing only nicotine-naive rats, a far higher dose of mecamylamine (5 mg/kg sc) induced a quasi-nicotine abstinence syndrome. The results provide further validation for this rodent model of nicotine abstinence syndrome.     

The nitric oxide synthesis inhibitor nitro-L-arginine (L-NNA) attenuates nicotine abstinence syndrome in the rat

Nitric oxide synthesis contributes to opiate tolerance and dependence. Nicotine dependence and abstinence syndrome in the rat appear to involve opiate mechanisms. Therefore, it was postulated that nitric oxide synthase (NOS) activity might be essential for the expression of nicotine abstinence syndrome. Twenty-one rats were rendered dependent by SC infusion of 9 mg/kg per day nicotine tartrate via Alzet osmotic minipump. Rats were pretreated SC with vehicle alone, or with 18 or 30 mg/kg of the NOS inhibitor L-NNA (nitro-L-arginine). Thirty minutes later, rats were challenged by 1 mg/kg of the nicotinic antagonist mecamylamine SC and observed for 30 additional minutes. Rats pretreated with vehicle displayed a total of 68.7 ± 8.0 mecamylamine-precipitated abstinence signs (mean ± SEM), while those receiving 18 or 30 mg/kg L-NNA had 12.7 ± 2.0 and 5.1 ± 1.7 signs, respectively. All three groups differed significantly from one another according to Dunn’s post-hoc procedure. Rats pretreated with L-NNA combined with an excess of the NOS substrate L-arginine had significantly more mecamylamine-precipitated abstinence signs than rats receiving L-NNA combined with D-arginine. Also, D-NNA, which does not selectively bind to NOS, was significantly less effective than L-NNA in preventing mecamylamine-precipitated abstinence syndrome. Additional studies determined the effect of L-NNA on spontaneous nicotine abstinence syndrome. Rats were assessed for abstinence signs at 17 and 20 h after termination of nicotine infusion. They received injections of 9, 18, or 30 mg/kg L-NNA SC or vehicle alone immediately before the 20-h observation; all rats were observed for 30 min. Signs at 20 h (post-injection) as a percentage of signs at 17 h (pre-injection) declined significantly as a function of L-NNA dose. Once again, this effect was attenuated significantly more by co-administration of L-arginine than by D-arginine. The overall pattern of results suggests that nitric oxide synthesis is critical to the expression of nicotine abstinence syndrome. Received: 20 January 1998/Final version: 8 April 1998     

Discriminative stimulus properties of the nicotinic agonist cytisine

Cytisine binds with high affinity and specificity to neuronal nicotinic receptors but its physiological and behavioural effects are complex and differ from those of nicotine. The present study explores the behavioural aspects further by comparing the discriminative stimulus effects of cytisine with those of nicotine. Two groups of rats were trained to discriminate cytisine (2 mg/kg SC) or nicotine (0.2 mg/kg SC) from saline in a two-lever operant conditioning procedure with food reinforcers presented on a tandem VI FR schedule. A third group of rats was trained to discriminate cytisine (3 mg/kg SC). Rats acquired these discriminations within 50 training sessions. The stimulus effects of both cytisine and nicotine appeared within 4 min of SC injection. In generalization tests, rats trained with either cytisine or nicotine showed steep dose-response curves (generalization gradients) for their respective training drug. However, rats trained with cytisine showed full dose-related, generalization to nicotine (93%), whereas rats trained with nicotine exhibited only partial generalization to cytisine (54%). Rats trained with either cytisine or nicotine exhibited similar, partial generalization (76–77%) to (+)-amphetamine. The nicotine antagonist mecamylamine blocked the discriminative stimulus effects of both cytisine and nicotine; it was confirmed that the block of nicotine (0.2 mg/kg) was complete, whereas the block of cytisine (2 and 3 mg/kg) was incomplete in two separate experiments. Overall, the results showed that cytisine, like nicotine, can serve as a robust discriminative stimulus but, in contrast to its relatively high affinity in binding experiments, cytisine was much less potent than nicotine in the behavioural studies. Although the stimulus effects of the two drugs were very similar, there were some subtle differences such as the asymmetrical cross-generalizations between them and possible small differences in susceptibility to antagonism by mecamyl-amine. These effects were interpreted either in terms of a putative partial agonist effect of cytisine, or by assuming that nicotine produces a compound stimulus. Such a stimulus would be mediated through two or more subtypes of nicotinic receptor, and cytisine would act at some, but not all, of these receptor subtypes. Received: 17 June 1996/Final version: 6 September 1996     

Different methods of assessing nicotine-induced antinociception may engage different neural mechanisms

Two methods were used to assess nicotine-in-duced antinociception: tail withdrawal from a hot water bath and hind paw withdrawal from a hotplate. Nicotine doses which produced 75–80% maximum response were 0.75 mg/kg (free base) for tail withdrawal and 0.35 mg/kg for paw withdrawal. The peripheral blocker chlorisondamine (0.1 mg/kg, SC) and the central antagonist, mecamylamine (1 mg/kg, SC) were each effective in blocking nicotine-induced increases in tail withdrawal latencies, suggesting that this effect of nicotine depends on either the action of nicotine at peripheral receptors or the functional integrity of those receptors. In contrast, nicotine-induced increases in paw withdrawal latencies were blocked by mecamylamine but not by chlorisondamine, even at other agonist/antagonist dose combinations. The results indicate that these two effects of nicotine involve at least partially separate pathways and may reflect a different mix of the antinociceptive and motor depressing effects of nicotine.     

Examining the clinical efficacy of bupropion and nortriptyline as smoking cessation agents in a rodent model of nicotine withdrawal

Rationale At present, there is a lack of an established animal model to demonstrate the clinical efficacy of smoking cessation agents in the laboratory. The aim of this study was to compare the effects of the antidepressants bupropion and nortriptyline, clinically proven smoking cessation aids, within a rodent model of a nicotine withdrawal based on somatic measures. Materials and methods Male hooded Lister rats were chronically exposed to nicotine (3.16 mg kg¹ day¹) for 7 days via SC implanted ALZET osmotic minipumps. Animals were acutely pre-treated with bupropion (10, 30 or 60 mg/kg, IP) or nortriptyline (1.5, 4.7 and 15 mg/kg, IP), and nicotine withdrawal was precipitated by mecamylamine (1 mg/kg). Results Precipitation of nicotine withdrawal led to an increase in somatic signs including body shakes, chews, eye blinks, foot licks, head shakes and ptosis. Bupropion dose-dependently decreased the total abstinence scores and reduced the occurrence of some individual somatic signs. Pre-treatment with 60 mg/kg bupropion did not result in a significant increase in total abstinence scores or individual somatic signs scores after mecamylamine challenge, compared to the mecamylamine control group, suggesting nicotine withdrawal is fully attenuated at this dose. Similarly, the highest dose of nortriptyline reduced total abstinence scores and some individual somatic signs to the level of the mecamylamine control group. However, nortriptyline was only effective at alleviating somatic measures of withdrawal at doses which also suppressed locomotor activity. Conclusion In concurrence with clinical findings proposing alleviation of withdrawal states as a possible mechanism of bupropion and nortriptyline’s smoking cessation action, both drugs were found to ameliorate somatic signs of nicotine withdrawal in rodents.     

Naloxone precipitates nicotine abstinence syndrome in the rat

Recently, a rodent model of nicotine abstinence syndrome has been developed based on continuous subcutaneous infusion of nicotine tartrate and observing the frequency of spontaneous behavioral signs following termination of infusion. The observed signs closely resemble those commonly seen in rat opiate abstinence syndrome, raising the possibility that there is an endogenous opioid component in nicotine dependence. The present study demonstrates that the opiate antagonist naloxone can precipitate an abstinence syndrome in nicotine-dependent rats. Fourteen rats were infused for 7 days with 9 mg/kg/day nicotine tartrate in saline via an Alzet osmotic minipump. Fourteen rats were sham-operated and remained nicotine-naive. Half of each group received 4.5 mg/kg naloxone SC immediately before a blind 15-min observation, while the other half received saline alone. ANOVA revealed significant nicotine infusion, naloxone injection and interaction effects. Post-hoc analysis showed that the nicotine-infused rats injected with naloxone had significantly more signs than all other groups (P<0.01). In a second experiment, 2 mg/kg morphine sulfate SC produced a significant (P<0.01) 91.2% reduction of spontaneous abstinence signs observed 21 h after termination of nicotine infusion. These results are consistent with the hypothesized endogenous opioid component in nicotine dependence and abstinence syndrome.     

Bupropion attenuates nicotine abstinence syndrome in the rat

Rationale Bupropion reduces discomfort and craving associated with smoking cessation. This study determined whether a rat model of nicotine dependence could detect such nicotine abstinence-alleviating effects. Objectives Experiments determined whether the abstinence-alleviating effects of bupropion were detectable by (1) behavioral abstinence signs precipitated by the nicotinic antagonist mecamylamine, (2) place aversion conditioned to mecamylamine-precipitated nicotine abstinence, and (3) spontaneous behavioral abstinence signs after abrupt nicotine withdrawal. Methods In experiments 1 and 2, nicotine-dependent rats were coinfused for 7 days with 3.15 mg/kg/day nicotine and 20 mg/kg/day bupropion or with nicotine alone. They were then challenged with 1 mg/kg mecamylamine and observed for behavioral abstinence signs (experiment 1) or place aversion conditioned to precipitated abstinence (experiment 2). In experiment 3, rats were nicotine-infused for 7 days as above. A day after termination of nicotine infusion, rats were observed for spontaneous nicotine abstinence signs before and after injection with saline or bupropion. Results In experiment 1, rats coinfused with nicotine and bupropion had significantly fewer mecamylamine-precipitated abstinence signs than rats infused with nicotine alone but similar numbers to rats infused with saline alone. In experiment 2, bupropion pretreatment significantly reduced the aversiveness of mecamylamine-precipitated nicotine abstinence. In experiment 3, a single bupropion injection dose-dependently alleviated spontaneous nicotine abstinence syndrome. Conclusions These results suggest that these rat models of nicotine dependence and abstinence syndrome may be useful in detecting nicotine abstinence-alleviating effects of potential medications for smoking cessation. The effects of acute bupropion administration raise interesting questions regarding bupropion's mechanism of action.     

Mecamylamine does not precipitate withdrawal in cigarette smokers

Mecamylamine is an antihypertensive that acts via nicotinic antagonism and has been suggested as an aid in smoking cessation. Nicotine dependent patients may not accept mecamylamine if it precipitates withdrawal, as it does in nicotine dependent rats. This study examined mecamylamine’s effects using procedures designed to measure precipitated withdrawal symptoms in humans. Ten cigarette smokers (mean of 37.5 cigarettes/day) and ten non tobacco-using subjects participated in three 6-h sessions. After a 2-h baseline period in which smokers smoked one cigarette every 30 min, oral mecamylamine (0, 10, or 20 mg randomly ordered across sessions) was administered (double-blind). No smoking was allowed for the remainder of the session. Mecamylamine reduced blood pressure and increased heart rate relative to placebo in both the smokers and the non-tobacco users. No reliable direct subjective effects of mecamylamine were observed. Smokers’ subjective reports of cigarette craving and tobacco withdrawal increased, and DSST performance was disrupted over the last 4 h of each session. Effects were independent of dose (placebo versus active). These results suggest that up to 20 mg mecamylamine will not precipitate nicotine withdrawal and that this medication would be acceptable for use in smoking cessation. Received: 20 April 1996/Final version: 3 June 1996     

Discrimination and self-administration of nicotine by inbred strains of mice

These studies aim to characterize the discriminative stimulus effects of nicotine in two inbred strains of mice that differ in many pharmacological responses, and to investigate the feasibility of IV self-administration studies with nicotine in one of the strains. For discrimination studies, three groups of C57BL/6 and one group of DBA/2 mice were trained in a two-lever operant conditioning paradigm with a tandem VI-30″ FR-10 schedule of food reinforcement. After 40 training sessions, accuracy reached 57.5, 77.5 and 90.0% in C57BL/6 mice trained with (–)-nicotine (SC) in doses of 0.4, 0.8 and 1.6 mg/kg, respectively (n = 8). DBA/2 mice trained with 0.8 mg/kg nicotine attained similar (73.3%) accuracy (n = 9). Results from extinction tests showed that all groups of mice yielded orderly dose-response curves for nicotine (0.03–1.6 mg/kg), but stimulus control remained notably weaker for the mice trained with 0.4 mg/kg nicotine than for any other group. Overall rates of responding in the undrugged state were lower for DBA/2 than for C57BL/6 mice; DBA/2 mice were also slightly less sensitive than C57BL/6 mice to the response rate-reducing effect of nicotine. The nicotine antagonist mecamylamine (1.5 mg/kg SC) blocked the discriminative stimulus effect of the training dose of nicotine in all groups. The results of the IV self-administration study suggest that nicotine (0.1 mg/kg) can serve as a positive reinforcer in drug–naive C57BL/6J mice (n = 13). Behaviour maintained by 0.1 mg/kg nicotine injections was significantly greater than behaviour maintained by vehicle injections, and it was maintained under an intermittent schedule of reinforcement (FR4). The methods described provide possible approaches for genetic analyses of strain differences in sensitivity to the discriminative and reinforcing stimulus properties of nicotine. Received: 11 April 1998/Final version: 28 June 1998     

The influence of nicotine pretreatment on mesoaccumbens dopamine overflow and locomotor responses to D-amphetamine

Pretreatment with psychostimulant drugs causes sensitisation of their effects on locomotor activity and dopamine (DA) overflow in the nucleus accumbens (NAcc) and there is evidence for similarities in the mechanisms involved. This study used in vivo microdialysis in conscious freely moving rats to investigate the extent to which pretreatment with nicotine causes sensitisation to D-amphetamine. Pretreatment with nicotine (0.4 mg/kg SC daily for 5 days) caused sensitisation of the locomotor responses to D-amphetamine (0.1–0.5 mg/kg SC) but not cocaine (15 mg/kg IP). Nicotine pretreatment did not influence the increase in DA overflow into dialysis probes, located in the core of the NAcc, evoked by systemic injections of D-amphetamine or cocaine (15 mg/kg IP) but decreased the overflow evoked by the administration of D-amphetamine (1 × 10^–6M) through the dialysis probe. The results provide further evidence for a dissociation between the expression of sensitised locomotor responses to psychostimulant drugs and sensitisation of their stimulatory effects on DA overflow in the core of the NAcc. The results suggest that the sensitisation of the effects of nicotine on DA overflow in this subdivision of the NAcc may be pharmacologically specific to nicotinic drugs. Received: 23 July 1997/Final version: 19 March 1998     

Temporal effects of nicotine nasal spray and gum on nicotine withdrawal symptoms

Nicotine nasal spray and nicotine gum have been found to be effective in relieving nicotine withdrawal symptoms. In this randomized single-blind study, 91 cigarette smokers were randomly assigned to a single 1 mg dose of active nicotine nasal spray (n = 29), active 4 mg nicotine gum (n = 31), saline placebo nasal spray (n = 16) or placebo gum (n = 15). Following overnight abstinence, subjects repeatedly completed visual analog scales for assessing nicotine withdrawal symptoms over 30 min preceding (time -30 min to time 0) and 120 min following a single dose of study medication. This sequence was performed 3 times during the day. Nicotine withdrawal symptoms were assessed on a 41-point visual analog scale (1 = no withdrawal, 41 = extreme withdrawal). At the initial session only, blood samples for serum nicotine levels were taken at baseline, then at 5, 10, 30 and 120 min following study drug administration. The mean (± SD) age of the subjects was 38.6 (±10.1) years, 48% were females, smoking rate was 24.5 (±7.8) cigarettes per day, and years of smoking was 19.9 (±10.0). A single 1 mg dose of nicotine nasal spray provided more immediate relief for craving for a cigarette compared to a single 4 mg dose of nicotine gum. Serum venous nicotine levels for the active nicotine nasal spray and nicotine gum were comparable at 5 and 10 min while the levels were higher for nicotine gum at 30 and 120 min. Changes in withdrawal symptoms were not found to be related to serum venous nicotine levels. Our findings provide a rationale for the as needed use of nicotine nasal spray to control withdrawal symptoms, possibly in combination with other medications with longer acting effects. Received: 18 February 1998/Final version: 1 May 1998     

Activation of acetylcholine receptors and 5-HT2 receptors have additive effects in the suppression of neocortical high-voltage spindles in aged rats

We investigated if activation of the muscarinic or nicotinic acetylcholine receptors and serotonin (5-hydroxytryptamine; 5-HT) subtype 2 receptors would have additive or synergistic effects on the suppression of thalamocortically generated rhythmic neocortical high-voltage spindles (HVSs) in aged rats. The 5-HT₂ receptor antagonist, ketanserin, at a moderate dose (5 mg/kg) prevented the ability of a muscarinic acetylcholine receptor agonist, (oxotremorine 0.1 mg/kg), and a nicotinic acetylcholine receptor agonist (nicotine 0.1 mg/kg), to decrease HVSs. At a higher dose (20 mg/kg), ketanserin completely blocked the decrease in HVSs produced by moderate doses of muscarinic acetylcholine receptor agonists (pilocarpine 1 mg/kg and oxotremorine 0.1 mg/kg), and by a high dose of nicotine (0.3 mg/kg), though not that produced by high doses of pilocarpine (3 mg/kg) and oxotremorine (0.9 mg/kg). The ability of a 5-HT₂ receptor agonist, (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.1–1.0 mg/kg), to suppress HVSs was non-significantly modulated by the nicotinic acetylcholine receptor antagonist, mecamylamine (1–15 mg/kg), and the muscarinic acetylcholine receptor antagonist, scopolamine (0.03–0.3 mg/kg). The effects of the drugs on behavioral activity could be separated from their effects on HVSs. The results suggest that activation of the muscarinic or nicotinic acetylcholine receptors plus 5-HT₂ receptors has additive effects in the suppression of thalamocortical oscillations in aged rats. Received: 2 November 1996 /Final version: 7 February 1997     

Detection of visual signals by rats: effects of chlordiazepoxide and cholinergic and adrenergic drugs on sustained attention

Central cholinergic and adrenergic pathways support the attentional processes necessary for detecting and reporting temporally unpredictable stimuli. To assess the functional effects of pharmacological manipulations of these pathways, male Long-Evans rats performed a two-choice, discrete-trial signal-detection task in which food was provided for pressing one lever after presentation of a signal (a 300-ms light flash), and for pressing a second lever at the end of a trial lacking a signal. Seven signal intensities were presented during each 1-h session in a pseudo-random order across three 100-trial blocks. After acquisition of a stable performance baseline, the acute effects of chlordiazepoxide (0, 3, 5, 8 mg/kg IP), pilocarpine (0, 1.0, 1.8, 3.0 mg/kg SC), scopolamine 0, 0.030, 0.056, 0.100 mg/kg SC), nicotine (0, 0.08, 0.25, 0.75 mg/kg SC), mecamylamine (0, 1.8, 3.0, 5.6 mg/kg IP), clonidine (0, 0.003, 0.010, 0.030 mg/kg SC), and idazoxan (0, 1, 3, 10 mg/kg SC) were assessed. Five measures of performance were analyzed: response failures; the proportion of “hits” [P(hit): the proportion of correct responses on signal trials]; the proportion of “false alarms” [P(fa): the proportion of incorrect responses on non-signal trials]; and response times (RT) for hits and for correct rejections. All drugs which slowed responding affected RT for hits and correct rejections equivalently, suggesting little or no influence of motor slowing on choice accuracy. Chlordiazepoxide reduced P(hit) at low signal intensities only, without affecting P(fa) or RT, consistent with sensory impairment (reduced visual sensitivity). All other drugs except nicotine reduced P(hit) at high signal intensities preferentially, suggesting a non-visual source of the impairment. Scopolamine, mecamylamine and clonidine affected both P(hit) and P(fa); pilocarpine and idazoxan reduced P(hit) without affecting P(fa). Nicotine at 0.75 mg/kg decreased P(hit) in the first block of trials; at 0.08 mg/kg it increased P(hit) in the second block; no dose affected P(fa). RTs were increased by pilocarpine, scopolamine, mecamylamine and clonidine, but not by nicotine or idazoxan. The data suggest that drugs which reduce cholinergic or adrenergic tone (scopolamine, mecamylamine and clonidine) impair sustained attention by decreasing the detection of signals and by increasing the false alarm rate, whereas drugs which elevate cholinergic or adrenergic tone (pilocarpine, nicotine and idazoxan) decrease attention by impairing detection of signals without affecting the false alarm rate. In contrast, the GABA-facilitating drug chlordiazepoxide appeared to affect visual thresholds rather than attention. Received: 16 October 1996 /Final version: 16 May 1997     

<Emphasis Type="Italic">Hypericum perforatum</Emphasis> attenuates nicotine withdrawal signs in mice

Rationale Hypericum perforatum is used as a natural antidepressant, and other antidepressants have been marketed to aid in smoking cessation. Objective We investigated the effects of an extract of Hypericum perforatum (Ph-50) on withdrawal signs produced by nicotine abstinence in mice. Methods Nicotine (2 mg/kg, four injections daily) was administered for 14 days to mice. Different doses of Ph-50 (125–500 mg/kg) were administered orally immediately after the last nicotine injection. In another experiment, Ph-50 (500 mg/kg) was orally administered in combination with nicotine, i) starting from day 8 until the end of the nicotine treatment period, or ii) during nicotine treatment and after nicotine withdrawal, or iii) immediately after the last nicotine injection. On withdrawal from nicotine, all animals were evaluated for locomotor activity and abstinence signs. Results The locomotor activity reduction induced by nicotine withdrawal was abolished by Ph-50, which also significantly and dose-dependently reduced the total nicotine abstinence score when injected after nicotine withdrawal. Conclusions These data show that treatment with Hypericum perforatum attenuates nicotine withdrawal signs in mice. Further studies are necessary to test the possibility that it may be used for smoking cessation treatment in humans.     

Mecamylamine blocks the development of tolerance to nicotine in rats: implications for the mechanisms of tolerance

 Chronic injections of nicotine in rats produce upregulation of nicotinic cholinergic receptors. It has been proposed that this upregulation is a reflection of receptor desensitization and is the basis of functional tolerance. Mecamylamine, a non-competitive antagonist that blocks activation of nicotinic receptors, does not prevent upregulation produced by nicotine injections. This suggests that receptor activation is not a prerequisite for nicotine-induced receptor upregulation. Therefore, the present experiments tested whether mecamylamine would also fail to prevent the development of tolerance to nicotine. Six daily pairings of mecamylamine (1 mg/kg SC) with nicotine did block the development of tolerance to nicotine-induced antinociception (0.35 mg/kg) and to the ability of nicotine to suppress milk intake (0.66 mg/kg). In another experiment, six daily injections of mecamylamine, when given alone, did not alter the effects of a subsequent, acute injection of nicotine (0.35 mg/kg) in inducing antinociception in rats. There was no evidence that after six pairings of mecamylamine with nicotine, the cues associated with mecamylamine delivery took on conditioned antagonistic properties. These findings suggest that, unlike the receptor upregulation that results from either continuous or repeated nicotine administration, the tolerance following a short series of intermittent nicotine injections is dependent on receptor activation. Received: 20 May 1998 / Final version: 23 July 1998     

Effects of central and peripheral nicotinic blockade on human nicotine discrimination

Nicotine produces interoceptive stimulus effects in humans, which may be critical in understanding tobacco use. It has not yet clearly been demonstrated that discrimination of nicotine, or any drug, in humans is due to its central effects. We compared effects of mecamylamine (10 mg PO), a central and peripheral nicotine antagonist, on nicotine discrimination with those of trimethaphan (10–40 μg/kg per min IV), a peripheral nicotine antagonist only, and placebo. Smokers (n = 6) were first trained to reliably discriminate 0 versus 20 μg/kg nicotine by nasal spray and then tested on generalization of this discrimination across a range of nicotine doses (0, 3, 6, 12, 20 μg/kg) following antagonist/placebo pretreatment. Nicotine self-administration was also assessed after generalization testing by having participants intermittently choose between nicotine versus placebo spray. Compared with responding following placebo pre-treatment, discrimination of the highest dose of nicotine was significantly attenuated following mecamylamine but not trimethaphan. Similar results were observed for some subjective responses to nicotine. Mecamylamine also tended to increase nicotine self-administration. Consistent with previous animal studies, these results suggest that discriminative stimulus effects of nicotine in humans are mediated at least in part by its central effects. Received: 15 April 1998/Final version: 23 July 1998     

Cue properties of oral and transdermal nicotine in the rat

In a standard two-lever drug discrimination paradigm, rats were trained to discriminate nicotine 0.5 mg/kg PO from saline. Injections occurred 15 min before the session. Subjects reached the training criterion in a mean of 38 sessions. Nicotine PO, SC, and IP generated similar dose-effect curves (ED₅₀=0.073 mg/kg PO, 0.076 mg/kg SC, 0.090 mg/kg IP); the dose-effect curve for transdermal (TD) administration fell approximately 1 log unit to the right (ED₅₀=1.34 mg/kg). The percentage of rats choosing the nicotine-appropriate lever peaked at 15 min and gradually decreased to 50% or less by 180 min for nicotine PO and TD, a time-decay function similar to that previously shown for SC administration. The nicotinic cholinergic agonist cytisine (0.5–8.0 mg/kg) PO and TD produced up to 56% nicotine-appropriate responding, while the muscarinic cholinergic agonist arecoline (1.0–4.0 mg/kg) PO and TD produced only saline-appropriate responding. The nicotine cue did not generalize to the cholinergic antagonist mecamylamine (0.125–0.5 mg/kg) PO or TD; mecamylamine 0.5 mg/kg PO but not TD completely blocked the PO and TD nicotine cues. These results show that an approximately equal cue occurs with PO, IP, and SC administration, and that the TD cue is considerably weaker. The significance of the procedure as an animal analog of human transdermal nicotine intake is discussed.     

大鼠尼古丁依赖──戒断综合征模型的建立

本文介绍一种较为便利的大鼠尼古丁戒断综合征模型的建立方法。♂大鼠每日ｓｃ尼古丁６次，剂量从０．５ｍｇ·ｋｇ－１逐渐递增至４ｍｇ·ｋｇ－３，共给药９ｄ。未次给药后６０ｍｉｎ，ｓｃ　１ｍｇ　·ｋｇ－１美加明激发，观察记录大鼠行为和症状变化２０ｍｉｎ，实验表明，大鼠出现下列明显戒断症状：咀嚼／错齿，扭体／喘气，颤抖／震颤，眼睑下垂及舔脚、抓挠和打哈欠等。而且这些症状可被ｓｃ０．５ｍｇ·ｋｇ－１尼古丁所明显减轻。该模型的建立将为进一步研究尼古丁成瘾提供有效的帮助。     

Removal of continuous nicotine infusion produces somatic but not behavioral signs of withdrawal in mice

The introduction of transgenic and knockout mice has shaped new interest in developing novel and modified behavioral methods for mice that evaluate the various manifestations of nicotine withdrawal syndromes. This study assessed the disruption of operant baselines during drug withdrawal, an established rat model of nicotine dependence, in mice. Subjects were trained to lever press for food reinforcement during daily operant sessions. After stable operant baselines were established, mice were implanted with osmotic minipumps containing 0 (saline), 6, 12, 24, or 48 mg/kg/day nicotine base. Operant responding was assessed for disruptions in daily sessions throughout the experiment. Somatic signs of withdrawal were assessed after the operant session on day 7, following administration of mecamylamine (1 mg/kg), and on days 12, 13, and 14, following spontaneous removal of nicotine. Spontaneous removal of nicotine increased somatic signs of withdrawal but did not disrupt operant responding. Mecamylamine failed to produce signs of precipitated withdrawal in either procedure. This study demonstrated nicotine dependence in mice during spontaneous removal of nicotine. Moreover, since signs of behavioral withdrawal (i.e. disruptions in operant response rates) were not observed, these findings suggest the importance of considering differences in the apparent manifestations of withdrawal syndromes while evaluating nicotine dependence.     

Behavioural and biochemical adaptations to nicotine in rats: influence of MK801, an NMDA receptor antagonist

Chronic exposure of rats to nicotine can result in sensitization to the stimulant effects of nicotine on locomotor activity. At a biochemical level, chronic exposure to nicotine increases the number of CNS nicotinic binding sites, and this has been suggested as the basis for sensitization to nicotine. The present experiment was conducted to examine the effects of MK801, an NMDA receptor antagonist, on sensitization to nicotine. In addition, the hypothesis that MK801 may block behavioural sensitization by preventing the up-regulation of nicotinic receptors was tested by measuring receptor numbers in the same individuals using quantitative autoradiography with [³H]-cytisine and [³H]-MK801. Male Sprague-Dawley rats were chronically treated with nicotine (0.4 mg/kg SC) or saline daily for 7 days. Over the next 2 days, in a counterbalanced order, rats were challenged with nicotine (0.4 mg/kg SC) or saline and locomotor activity was monitored. In saline-pretreated rats, nicotine produced a small increase in activity. Nicotine-pretreated rats exhibited higher levels of activity following a nicotine challenge. This sensitized response was attenuated in rats administered MK801 (0.3 mg/kg IP) 30 min before each daily nicotine injection. Rats pretreated with MK801 alone showed activity scores no different from saline pretreated control groups. Biochemical studies revealed increased [³H]-cytisine binding following chronic nicotine treatment; however, receptor increases were significantly attenuated by MK801 pretreatment. Binding of [³H]-MK801 remained unchanged across the four groups. The results suggest that MK801 prevents behavioural sensitization to nicotine via the prevention of receptor up-regulation. Although the findings support the notion that receptor up-regulation may be the basis for the increased responsiveness to nicotine, other interpretations are possible. Received: 23 January 1997/Final version: 8 May 1997