Withdrawal from ingested diazepam produces a pentylenetetrazol-like stimulus in rats

An interoceptive stimulus produced by withdrawal from chronic diazepam is similar to that produced by the anxiogenic drug pentylenetetrazol (PTZ). Because the PTZ stimulus can be measured objectively in animals, its use was proposed as a bioassay for diazepam-withdrawal stimuli. In previous studies, diazepam was given chronically by the intraperitoneal route. The purpose of the present study was to determine whether withdrawal from diazepam given in the diet would similarly produce a PTZ-like subjective effect. Administering drug through the diet has the advantage that it eliminates the necessity of giving injections every 6–8 hr and more closely parallels human chronic dosing. Rats were trained with food-reward in a two-lever operant task. Presses on one lever were reinforced after injections of PTZ, 20 mg/kg, and on the other lever after saline. After rats had acquired the PTZ discrimination, training was halted, and they were given diazepam, 240 mg/kg/day, in a nutritionally complete liquid diet. Withdrawal was precipitated after 3 or 4 days of chronic diazepam with the benzodiazepine receptor blocker Ro 15–1788, either after a bolus dose of diazepam, given intraperitoneally, or during continued dietary diazepam. During precipitated withdrawal, the rats selected the PTZ lever, indicating the presence of a PTZ-like stimulus, and this stimulus was blocked by phenobarbital, 80 mg/kg. The percentage of rats selecting the PTZ lever depended on the dose of Ro 15–1788. By 3.5 days after cessation of chronic diazepam, Ro 15–1788 no longer produced a PTZ-like stimulus and by 11 days, rats recovered baseline discrimination performance, as indicated by saline-lever selection after saline injections and PTZ-lever selection after PTZ. These data indicate that a subjective effect of withdrawal similar to that obtained after parenteral diazepam can be produced after chronic diazepam given daily in the diet.     

Pentylenetetrazole-like stimulus is produced in rats during withdrawal from ingested chlordiazepoxide

The present study was undertaken to determine whether withdrawal from chlordiazepoxide administered via a liquid diet would produce a pentylenetetrazole (PTZ)-like stimulus. Rats were trained with food reward in a two-lever operant task. Presses on one lever were reinforced after injections of PTZ (20 mg/kg, i.p.) and on the other lever after saline (1 ml/kg, i.p.). After rats had acquired the PTZ discrimination, training was halted, and chlordiazepoxide (240 mg/kg/day) was administered via a nutritionally balanced liquid diet to three groups of rats for 3, 4, or 6 days. Upon termination of chronic administration, withdrawal was precipitated with the benzodiazepine receptor blocker flumazenil (Ro 15-1788) given intraperitoneally. During precipitated withdrawal, the rats selected the PTZ-appropriate lever, indicating the presence of a PTZ-like stimulus, and this stimulus was blocked by phenobarbital (80 mg/kg, i.p.). The percentage of rats selecting the PTZ-appropriate lever depended on the duration of chlordiazepoxide treatment and dose of flumazenil. At 10 days after the last chlordiazepoxide dose, the rats had recovered baseline discrimination, as indicated by their selection of the saline appropriate lever following saline injections and the PTZ-appropriate lever following PTZ. These data indicate that a subjective effect of withdrawal similar to that produced by the anxiogenic drug PTZ is present during withdrawal from oral chlordiazepoxide.     

The pentylenetetrazol-like interoceptive stimulus produced by ethanol withdrawal is potentiated by bicuculline and picrotoxinin

We investigated whether the interoceptive discriminative stimulus (IDS) arising from ethanol withdrawal was related to decreased activity of the gamma-aminobutyric acid (GABA) system by determining whether the sensitivity of rats to the GABA antagonists was altered by chronic treatment with ethanol. Rats were trained to obtain food reward by responding on one lever following pentylenetetrazol (PTZ) and the other lever following saline. Whereas all of the trained rats selected the PTZ-appropriate lever after PTZ, no more than 50% of them selected this lever following an optimum dose of bicuculline or picrotoxinin. After either saline or diazepam (5 mg/kg), all of the rats selected the saline-appropriate lever. Ethanol (0.24 mol/kg/day) was then administered to the rats for 4 days via a nutritionally balanced liquid diet. Between 48 and 96 hours postethanol, 30% of the rats selected the PTZ-appropriate lever following saline, whereas selection of this lever was increased to 80% following either bicuculline or picrotoxinin. Thus, further antagonism of GABAergic activity increased the subjective effect of ethanol withdrawal. These data support the hypothesis that the PTZ-like IDS produced during withdrawal from ethanol is related to an ethanol-induced deficit in the activity of the GABA-benzodiazepine receptor-coupled chloride channel.     

Enhancement of a diazepam withdrawal symptom by bicuculline and yohimbine

The role of the GABA system in producing a pentylenetetrazol-like interoceptive discriminative stimulus during withdrawal from diazepam was investigated in rats by determining the sensitivity of this system to GABAergic drugs before and after chronic treatment with diazepam. Food-restricted rats were trained to obtain a reward of food by responding on one lever following an injection of pentylenetetrazol (PTZ; 20 mg/kg) and the other lever following an injection of saline (1 ml/kg). After rats had acquired this discrimination, the effectiveness of Ro 15–1788, bicuculline and yohimbine to substitute for pentylenetetrazol was determined. Prior to chronic treatment with diazepam, rats selected the appropriate lever for saline after Ro 15–1788 and the appropriate lever for pentylenetetrazol after bicuculline (0.04–2.5mg/kg) or yohimbine (0.16–5.0mg/kg). Although the selection of the appropriate lever for pentylenetetrazol was dose-dependent, full substitution for pentylenetetrazol was not obtained with either drug as larger doses of bicuculline produced convulsions while the rats began to select the appropriate lever for saline after larger doses of yohimbine (bell-shaped curve). Diazepam blocked the pentylenetetrazol-like interoceptive discriminative stimulus for bicuculline. The rats were then injected with diazepam (80mg/kg/8 hr) for 24 days. Upon termination of the administration of diazepam, the animals were tested for lever-selection following the administration of saline, Ro 15–1788 (10mg/kg), bicuculline (0.32, 0.64 and 1.25mg/kg) or yohimbine (0.16, 0.64 and 2.5mg/kg). After saline, 33% of the rats selected the appropriate lever for pentylenetetrazol whereas selection of this lever was enhanced after Ro 15–1788, bicuculline or yohimbine. Thus, either displacement of diazepam or further reduction of GABAergic activity increased the subjective effect of the withdrawal from diazepam. These data support the hypothesis that the chronic administration of diazepam produces a reduction in the activity of the GABA/benzodiazepine complex, which is manifested during withdrawal as a pentylenetetrazol-like stimulus.     

Protracted withdrawal: sensitization of the anxiogenic response to cocaine in rats concurrently treated with ethanol.

Rats were trained to respond on one lever following an injection of saline and the alternate lever after the anxiogenic drug pentylenetetrazol (PTZ 20 mg/kg), according to a fixed ratio (FR10) schedule of food reinforcement. The trained animals were then administered dependence-producing regimens of either cocaine (20 mg/kg, [IP], three times daily for 7 days) or ethanol (mixed 4.5% w/v with sweetened liquid diet given for 5 days). Separate groups of trained rats were given either subthreshold regimens of cocaine (20 mg/kg, IP, three times daily for 5 days), ethanol (2.25% w/v of the diet given for 5 days), or both. Additional groups were matched for control groups. After discontinuation of these regimens, rats were administered test injections of either saline or cocaine, and tested for elicitation of the PTZ-stimulus at selected intervals of withdrawal. After a saline injection, maximum elicitation of the PTZ-stimulus was observed 12 hours following chronic treatment with the higher dose of ethanol, and 120 hours following longer treatment with cocaine. During those periods of withdrawal when a saline injection failed to produce a PTZ-like stimulus, a test injection of cocaine (10 mg/kg) elicited the PTZ-stimulus in the ethanol withdrawn rats, although only partially eliciting the PTZ-stimulus in the cocaine withdrawn group. In the pair-fed controls, or rats withdrawn from the smaller dosage of either ethanol or cocaine, the test dose of saline or cocaine did not elicit the PTZ-stimulus; only 30% of rats selected the PTZ-appropriate level at the highest dose of cocaine tested (10 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)     

Sex differences in nicotine substitution to a pentylenetetrazol discriminative stimulus during ethanol withdrawal in rats

Rationale: Nicotine and ethanol are frequently co-abused in men and women, but few studies compare common stimulus effects produced by these substances between males and females. Objectives: This study compared the anxiety-like behavior induced by nicotine prior to and during ethanol withdrawal in intact male, sham-operated female, and ovariectomized (OVX) rats. Methods: Using an animal model of anxiety, the pentylenetetrazol (PTZ) drug-discrimination assay, rats were trained to discriminate PTZ (16 mg/kg, i.p.) from saline and were subjected to the following tests: (1) PTZ-lever selection at 12 h after termination of ethanol diet (4.5% for 10 days); (2) dose–response tests for nicotine (0.08– 1.3 mg/kg) prior to ethanol and 1.5, 6, and 7 days after ethanol withdrawal. Results: (1) During acute ethanol withdrawal (12 h), more male rats (43.4%) responded on the PTZ lever than OVX (29%) or sham female (15.3%) rats. (2) For nicotine dose–response tests, more male rats (70%) selected the PTZ lever than OVX (37.5%) or sham female (50%) rats prior to ethanol. At 1.5 days, nicotine fully generalized to the PTZ stimulus in male (100%) and OVX (90%), but only partially in sham female (50%) rats. At 6 days and 7 days after ethanol withdrawal, the PTZ-lever selection decreased, but more male rats (78%) tended to respond on a PTZ lever than OVX (63.6%) or sham female rats (62.5%). Conclusions: Acute nicotine produces anxiety-like behavior similar to that of PTZ in male and female rats, and this effect of nicotine is intensified during ethanol withdrawal in male and OVX rats, but not in sham female rats. Received: 20 July 1999 / Final version: 29 December 1999     

Effects of methoxyflurane and flurothyl in mice trained to discriminate pentylenetetrazol from saline

A procedure is described for training mice to discriminate 30mg/kg pentylenetetrazol (PTZ) from saline when lever pressing was maintained under a fixed-ratio 20 schedule of milk presentation. To define the pharmacological profile of the PTZ stimulus in mice, generalization testing was conducted with oxazepam and Ro 15-4513 (sarmazenil). Consistent with data obtained by others in rats, oxazepam (1mg/kg) blocked the PTZ stimulus whereas Ro 15-4513 substituted for PTZ, but only at a dose (2mg/kg) that also decreased rates of responding. The effects of both a depressant and excitatory vapor in this model were also determined. The volatile anesthetic methoxyflurane (1000-2000 ppm) blocked the discriminative stimulus effects of PTZ in a concentration-dependent manner, while the convulsant vapor flurothyl (900 ppm) produced greater than 90% PTZ-lever responding without disrupting rates of responding. The PTZ-like discriminative stimulus effects of flurothyl were dose-dependently blocked by oxazepam (0.03-1.0mg/kg). As has been shown in numerous previous studies in rats, PTZ could be established as a discriminative stimulus in mice. PTZ discrimination could be blocked by a benzodiazepine agonist and shares some properties with a benzodiazepine inverse agonist. Substitution and antagonism studies can also be performed with vapors, illustrating the utility of this model for comparing their behavioral effects to those of more widely studied drugs.     

Anxiety-like subjective effect of ethanol antagonist RO 15-4513 demonstrated in pentylenetetrazol discrimination

Ro 15-4513, a benzodiazepine-receptor ligand which antagonizes ethanol, was tested in the pentylenetetrazol discrimination, a bioassay for anxiogenic drugs. Rats were trained with food reward to discriminate pentylenetetrazol (PTZ) from saline in a two-lever operant task. In lever-selection tests, rats selected the PTZ lever both after PTZ and after Ro 15-4513. The PTZ-like stimulus produced by Ro 15-4513 was blocked by diazepam and by the benzodiazepine receptor blocker Ro 15-1788. Substitution for the anxiogenic drug PTZ, and blockade by the anxiolytic diazepam, support the hypothesis that Ro 15-4513 is anxiogenic; blockade by Ro 15-1788 suggests that the PTZ-like stimulus produced by Ro 15-4513 occurs through its action at the benzodiazepine receptor.     

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

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

Discriminative stimulus properties of pentylenetetrazol and bemegride: Some generalization and antagonism tests

In an operant procedure of lever pressing on a FR 10 schedule of food reinforcement, male hooded rats were trained to respond with a lever on one side of a food cup following a drug injection, and to respond with a lever on the alternate side following a 1ml/kg saline injection. All of 14 subjects learned to discriminate reliably between the effects of 20 mg/kg pentylene-tetrazol (PTZ) and saline. Seven of eight rats learned to discriminate between the effects of bemegride (5 mg/kg) and saline. None of 14 rats learned to discriminate between 5 mg/kg PTZ and saline. The bemegride discriminative stimulus generalized to PTZ (20 mg/kg) and was antagonized by chlordiazepoxide (10 mg/kg). Chlordiazepoxide, diazepam, flurazepam, clobazam, and meprobamate were all effective antagonists of PTZ in a dose-dependent manner. Bemegride and cocaine generalized to the PTZ discriminative stimulus in a dose-dependent manner, but d-amphetamine, methylphenidate, and nicotine did not. Since bemegride and PTZ are convulsants at higher doses, the discriminative stimulus properties of these drugs might be based on a subtle convulsive brain state. The anxiolytic properties of benzodiazepines and meprobamate suggest that the discriminative stimulus produced by these convulsants is related to an anxiety-inducing action.     

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.     

Opioid modulation of the discriminative stimulus produced by pentylenetetrazol

Rats were trained to detect the stimulus properties of pentylenetetrazol (PTZ), 16 mg/kg, and prototypic drugs for mu, kappa and sigma opioid receptors were tested for their ability to block or substitute for PTZ. Only the sigma agonist, phencyclidine, showed any capacity for blocking the PTZ stimulus. Drugs with selective kappa or sigma actions did not substitute for PTZ. However, morphine, fentanyl and Mr 2034 did substitute for the PTZ stimulus. This substitution was found to be centrally mediated in that quaternary morphine did not produce a PTZ-like stimulus. In contrast to the substitution of these drugs for PTZ, in rats trained to detect the stimulus properties of fentanyl, no substitution of PTZ for the fentanyl stimulus occurred. In tests of the capacity of various drugs to block the PTZ-like stimulus of mu agonists, the stimulus produced by morphine or fentanyl was blocked by naloxone, diazepam and haloperidol, but not by scopolamine. These results demonstrate that drugs with mu agonist properties show a one-way substitution for the discriminative stimulus produced by PTZ. The observation that haloperidol blocked the PTZ-like stimulus of mu agonists suggests the possible involvement of dopaminergic mechanisms in the mediation of the effect.     

Adolescent nicotine exposure produces less affective measures of withdrawal relative to adult nicotine exposure in male rats

Vulnerability to nicotine addiction is significantly increased in individuals who begin smoking during adolescence; however, the underlying mechanisms of this phenomenon remain unclear. This study examined the motivational effects of nicotine withdrawal in adolescent (PND 27-42) and adult (PND 60-75) rats using the conditioned place aversion paradigm. Male Wistar rats were tested for their initial preference for either of two distinct compartments of our conditioning apparatus. Rats were then implanted with subcutaneous (sc) pumps that produce equivalent blood plasma levels of nicotine for 14 days. Conditioning was conducted over the last 8 days of nicotine exposure. Rats received the nicotinic antagonist mecamylamine (1.5 or 3.0 mg/kg, sc) to precipitate withdrawal in their initially preferred compartment, and on alternate days they received saline in their non-preferred compartment. Following conditioning, rats were re-tested for their preference for each compartment. A subsequent study was conducted to examine potential developmental differences in learning place aversion produced by another aversive stimulus, lithium chloride (LiCl). Rats received LiCl (0, 10, 30, or 100 mg/kg, sc) in their initially preferred side using similar conditioning procedures. Adults displayed robust place aversion produced by nicotine withdrawal. This effect was lower in adolescent rats even in a group of young rats that received 7 additional days of nicotine exposure prior to conditioning. This developmental difference was specific to nicotine withdrawal since there were no differences between adolescents and adults in learning place aversion with LiCl. Our findings demonstrating reduced effects of nicotine withdrawal constitute a powerful basis for the increased vulnerability to nicotine dependence during adolescence.     

Sensitivity of pentylenetetrazol discrimination increased by a stimulus fading technique

The interoceptive stimulus produced by pentylenetetrazol (PTZ) is pharmacologically similar to anxiety and is used in a behavioral assay for anxiety-related stimuli (the PTZ model of anxiety). The stimulus fading technique was tested as a method to increase the sensitivity of this assay. Rats were trained with food-reward to press one lever after injection of PTZ and an alternate lever after saline. Rats initially learned the discrimination at a PTZ dose of 20 mg/kg. They were then trained with sequentially lower doses until they reliably discriminated a PTZ dose of 10 mg/kg. Substitution test with other doses and drugs showed that, after the fading procedure, dose-response curves were shifted to lower doses for PTZ, Ro 5-3663, and nicotine. Similarly, the dose of diazepam required to block the low dose of PTZ was lower than that required to block the higher dose of PTZ. These results indicated that the sensitivity of the discrimination was enhanced in rats trained to discriminate a lower dose of PTZ. Doses of nikethamide, cocaine, and yohimbine that did not substitute for the higher dose of PTZ also did not substitute for the lower dose. These data suggest that rats can be trained to discriminate a low dose of PTZ by the stimulus fading technique. Moreover, they suggest that this training method does not compromise the specificity of the discrimination.     

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.     

Interoceptive stimuli produced by cocaine are blocked during diazepam withdrawal

Rats were trained to select one lever after an intraperitoneal (ip) injection of cocaine (10 mg/kg), and an alternate lever after saline injection under a FR10 schedule of food reinforcement. Following chronic administration of cocaine (20 mg/kg/8hr), but not diazepam (80 mg/kg/8hr) for 7 days, the dose-effect curve for the generalization of cocaine shifted twofold to the right. Subsequently, in the rats receiving chronic diazepam, RO 15-1788, a drug known to precipitate diazepam withdrawal, failed to substitute for cocaine: however, the discriminability of cocaine (5 mg/kg) was reduced from 83% to 13% selection of cocaine-appropriate lever after 72 hr following the last diazepam, injection. These data suggest that chronic administration of cocaine, but not diazepam, produced tolerance to the discriminative stimulus properties of cocaine, and following chronic diazepam administration and withdrawal, the stimulus properties of cocaine are antagonized.     

An analysis of response to nicotine infusion using an automated radiotelemetry system

Previous studies from our laboratory have demonstrated that chronic nicotine infusion evokes tolerance to nicotine injected IP several hours after withdrawal from chronic infusion. This method may introduce problems related to withdrawal reactions and to stress associated with handling of the animals. The studies reported here measured tolerance to nicotine in mice using an automated radiotelemetry system. DBA/2 mice were infused intravenously with saline for 4 days followed by infusion of a 4 mg/kg per h dose of nicotine for 7 days. After the nicotine treatment, the mice were infused with saline for 7 days. The nicotine was infused continuously or in four 1 mg/kg pulses, two 2 mg/kg pulses or one 4 mg/kg pulse each hour. Home cage activity and body temperature were measured throughout the treatment periods using a radiotelemetry system. Nicotine infusion produced an abrupt decrease in body temperature and activity, but this effect was totally reversed within 12 h in the continuously infused and four infusions/h treatment groups. Mice that received one or two infusions/h also showed a rapid response to nicotine that was reversed as treatment proceeded, but nicotine continued to produce a measurable effect for several days. After nicotine withdrawal, temperature and activity returned to predrug infusion values in all of the groups except those infused once per hour. This group showed depressed activity for a minimum of 3 days after nicotine treatment stopped. Thus, the kinetics of nicotine administration affected the intensity of response during continued treatment as well as activity after cessation of chronic treatment.     

Cessation of chronic nicotine administration enhances wet-dog shake responses to 5-HT2 receptor stimulation in rats

RATIONALE: The involvement of central serotonergic systems has been hypothesized clinically to contribute to nicotine withdrawal symptoms. However, involvement of the serotonin2 (5-HT(2)) receptor system in nicotine withdrawal is not clear. OBJECTIVES: The changes in wet-dog shake responses induced by (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a selective 5-HT(2) receptor agonist, following nicotine cessation was investigated in rats. METHODS: DOI (1 mg/kg SC) was administered 24 h after the final treatment of saline or nicotine (0.5 mg/kg per day SC) for 7 or 21 days. RESULTS: Cessation of nicotine administration for 7 or 21 days increased DOI-induced wet-dog shake responses. A single administration of nicotine (0.5 mg/kg SC) had no effect on DOI-induced wet-dog shakes. The enhancement by the cessation of nicotine treatment for 7 days was abolished by coadministration of nicotine. Mecamylamine (3 mg/kg IP), a nicotinic receptor antagonist, precipitated DOI-induced wet-dog shake responses in rats chronically treated with nicotine but not with saline. CONCLUSIONS: These findings suggest that cessation of chronic nicotine produced increased sensitivity to 5-HT(2) receptor systems, and that the 5-HT(2) receptor systems may be involved in the nicotine withdrawal symptoms.     

Antagonism of a nicotine plus midazolam discriminative cue in rats

Rats were trained to discriminate nicotine (0.4mg/kg s.c.), midazolam (0.2mg/kg s.c.) or the combination of these drugs from saline (n = 10). The rats were trained to 95% accuracy in a two-bar operant procedure with a tandem schedule of food reinforcement. Testing with the individual drugs in the mixture-trained group showed that nicotine (85% drug-appropriate responding) was a more salient component than midazolam (47%) in the compound stimulus. The rats were tested with benzodiazepine and nicotine antagonists individually and in combination (mecamylamine 0.2-1.6mg/kg s.c.; flumazenil 2.5-20mg/kg i.p.). Results for the mixture-trained animals showed that flumazenil had no effect on its own, however mecamylamine on its own produced a significant but incomplete block in doses of 0.4-1.6mg/kg. The greater salience of the nicotine component of the cue would explain the block by mecamylamine but not flumazenil. The antagonists in combination produced greater blockade than mecamylamine on its own. The selectivity of the antagonist actions on the different cue components was also demonstrated. The results suggest that in drug discrimination experiments, "false negative" results may be obtained with antagonists when a training drug produces a stimulus with more than one component.     

Glycyl-glutamine inhibits nicotine conditioned place preference and withdrawal

Glycyl-glutamine (Gly-Gln) is an inhibitory dipeptide synthesized from beta-endorphin(1-31). Previously, we showed that Gly-Gln inhibits morphine conditioned place preference, tolerance, dependence and withdrawal. In this study, we tested whether Gly-Gln's inhibitory activity extends to other rewarding drugs, specifically nicotine. Rats were conditioned with nicotine (0.6 mg/kg, s.c.) for four days and tested on day five. Glycyl-glutamine (100 nmol i.c.v.) inhibited acquisition and expression of a nicotine place preference significantly. Cyclo(Gly-Gln) (100 nmol i.c.v. or 25 mg/kg i.p.), a cyclic Gly-Gln derivative, blocked expression of nicotine place preference but Gly-d-Gln (100 nmol i.c.v.) was ineffective. To study nicotine withdrawal, rats were treated with nicotine (9 mg/kg/day) for seven days and conditioned place aversion was induced with mecamylamine (1 mg/kg, s.c.). Glycyl-glutamine blocked acquisition of place aversion to mecamylamine but not U50,488, a kappa opioid receptor agonist. Glycyl-glutamine thus inhibits the rewarding effects of nicotine and attenuates withdrawal in nicotine dependent rats.