Conditioned anxiety to nicotine

Abstract Rationale. Despite its reinforcing properties nicotine has also been reported to produce anxiety in humans and anxiogenic effects in animal tests of anxiety. Objective. The aims of this study were three-fold: (a) to investigate whether anxiety can be conditioned to cues associated with an acute anxiogenic dose of nicotine, (b) to investigate whether the conditioned anxiety is specific to a particular test of anxiety, and (c) to investigate whether nicotine pre-exposure influences the development of a conditioned anxiogenic effect. Methods. An anxiogenic dose of nicotine was administered to rats either before or after experience with the social interaction (SI) test. The retention of a conditioned anxiogenic response was examined when the rats were re-tested undrugged in the SI test 24 h later. To test whether conditioned anxiety was test specific, rats that had been tested in the elevated plus-maze with an anxiogenic dose of nicotine were retested undrugged in the SI test 24 h later, and vice versa. We then examined the effects of 4 days or 4 weeks pre-exposure to nicotine on the development of a conditioned anxiogenic response in the SI test. Results. Rats injected with nicotine (0.45 mg/kg s.c.) 5 min before the social interaction test spent significantly less time in SI, indicating an unconditioned anxiogenic effect than did vehicle-injected controls or rats injected with nicotine after the test. After 24 h when all groups were tested undrugged only those previously tested in SI after nicotine injection showed a significant conditioned anxiogenic effect. This conditioned anxiety was test specific. Rats injected with nicotine before the SI test did not show an anxiogenic response when tested 24 h later undrugged in the plus-maze, and vice versa. Furthermore, although 4 days exposure to nicotine (0.45 mg/kg s.c.) did not prevent the development of a conditioned anxiogenic response, 4 weeks self-administration of nicotine (total dose, 0.45 mg/kg i.v) in an operant chamber did not affect the acute anxiogenic response to nicotine in the SI test, but it did prevent the development of conditioned anxiety. Conclusions. The present findings suggest that anxiety can be conditioned following exposure to an anxiogenic dose of nicotine, and that this anxiety is specific to the contextual cues associated with the SI test. Electronic Publication     

Social isolation modifies nicotine's effects in animal tests of anxiety

1. These experiments determined whether the housing conditions of rats influenced the effects of nicotine in two animal tests of anxiety, social interaction and elevated plus-maze tests. 2. In animals housed singly for 7 days, (-)nicotine (0.025 mg kg(-1) s.c.) was ineffective, but 0.05, 0.1 and 0.25 mg kg(-1) (s.c.) significantly increased the time spent in social interaction, without changing locomotor activity, thus indicating anxiolytic actions. (-)Nicotine (0.45 mg kg(-1) s.c.) significantly reduced social interaction, indicating an anxiogenic effect. 3. However, in group-housed animals, (-)nicotine (0.025 mg kg(-1) s.c.) had a significant anxiolytic effect in the social interaction test, but 0.01, 0.05, 0.1, 0.25 and 0.45 mg kg(-1) were ineffective. (-)Nicotine (1 mg kg(-1)) reduced motor activity and social interaction in the group-housed animals. 4. In the elevated plus-maze, the time-course and the dose-response curve to nicotine were investigated. In both singly- and group-housed rats, (-) nicotine (0.1 - 0.45 mg kg(-1) s.c.) decreased the per cent entries into, and per cent time spent on, the open arms, indicating anxiogenic effects. 5. The housing condition influenced the time course, with significant effects at 5 and 30 min after injection in group-housed rats, and significant effects at 30 and 60 min in singly-housed rats. 6. In the social interaction test there was no difference in the scores of the first and last rats removed from group cages, whereas the order of removal from the cages did affect the scores in the elevated plus-maze. 7. These results provide further evidence that the two animal tests model distinct states of anxiety, and show how social isolation powerfully modifies both anxiolytic and anxiogenic effects of nicotine.     

Time-course of changes in the social interaction test of anxiety following acute and chronic administration of nicotine

The purpose of these experiments was to explore the hypothesis that the effects of nicotine on anxiety depend on the time since administration and the duration of treatment. In the social interaction test of anxiety, acute nicotine administration (0.1 mg/kg, subcutaneously) decreased social interaction when rats were tested 5 min after injection, but increased it when they were tested 30 min after injection. Social interaction was also decreased 1 h post-injection, but levels returned to baseline between 3 and 30 h. As these changes were independent of any changes in locomotor activity, nicotine seemed to be having both anxiogenic and anxiolytic effects at different times after injection. An anxiolytic effect was also observed 30 min after the second nicotine injection, and the anxiogenic effect observed 5 min after injection remained after 4 days of nicotine administration. However, after 7 days of nicotine treatment, tolerance was observed to both these effects. When rats were tested 72 h after the last of 7 or 14 days of nicotine treatment, an anxiogenic withdrawal response was observed. Thus, an oppositional mechanism may underlie tolerance to the anxiolytic effects, whereas there is as yet no evidence for this type of mechanism mediating tolerance to the anxiogenic effects.     

Tolerance to nicotine's effects in the elevated plus-maze and increased anxiety during withdrawal

In the elevated plus-maze test of anxiety, nicotine (0.1 mg/kg sc; 30 min after injection) had a significant anxiogenic effect, shown by specific decreases in the percentage of time spent on the open arms and in the percentage of open-arm entries. Tolerance developed to this anxiogenic effect after 7 days of nicotine treatment (0.1 mg/kg/day). Five minutes after an acute injection, nicotine (0.1 mg/kg) was ineffective, but after 7 days of treatment a significant anxiolytic effect, shown by specific increases in the percentage of time spent on the open arms and in the percentage of open-arm entries, emerged. After 14 days of nicotine treatment, tolerance developed to this anxiolytic effect. There was a complete dissociation between the effects of nicotine on the measures of anxiety, and on the locomotor activity as measured by closed-arm entries. No changes in closed-arm entries were found after acute administration of nicotine, but rats tested 30 min after their 7th injection made significantly fewer, and those tested 5 min after their 14th injection made significantly more, entries than their respective controls. Rats that were tested after 24 h withdrawal from six daily nicotine injections showed a significant anxiogenic effect. A low dose of nicotine (5 ng) injected into the dorsal hippocampus was without effect in vehicle pretreated rats, but it was able to reverse the anxiogenic effect found after 24 h of withdrawal from 6 days of nicotine treatment.     

The dorsal raphé nucleus is a crucial structure mediating nicotine's anxiolytic effects and the development of tolerance and withdrawal responses

RATIONALE: Smokers frequently report that they obtain anxiety-reducing (anxiolytic) effects from smoking, and this may be one factor which contributes to nicotine dependence. OBJECTIVE: The aim of this study was to investigate the role of the dorsal raphé nucleus (DRN) in mediating the acute anxiolytic effect of nicotine, the development of tolerance to this effect and the anxiogenic response observed on withdrawal from chronic nicotine. METHODS: The social interaction test of anxiety was used to investigate the effects of a range of doses of (-)-nicotine (2.5-4000 ng) following DRN infusion, and whether co-administration of the specific 5-HT1A receptor antagonist WAY 100635 could antagonise the anxiolytic action of nicotine. We then examined the effects of intra-DRN nicotine (2.5-7 ng) following six daily injections of subcutaneous (s.c.) (-)-nicotine (0.1 mg/kg). Finally, we examined whether s.c. or intra-DRN (-)-nicotine could antagonise the anxiogenic response seen 72 h after the termination of 7 days of nicotine treatment. RESULTS: Acute nicotine administration into the DRN produced dose-related effects: low doses (2.5-10 ng) induced an anxiolytic effect, intermediate doses were behaviourally silent (100-1000 ng), and an anxiogenic effect was seen following administration of a high dose (4 micrograms). The anxiolytic effect of (-)-nicotine (5 ng) was reversed by co-administration of a behaviourally inactive dose of WAY 100635 (200 ng). Following 6 days of treatment with s.c. 0.1 mg/kg per day (-)-nicotine, tolerance developed to its anxiolytic action in the DRN. Rats withdrawn for 72 h following this chronic treatment showed an anxiogenic response which was reversed by (-)-nicotine injected s.c. (0.1 mg/kg) or into the DRN (5 ng). CONCLUSIONS: The present findings therefore suggest that the DRN plays an important role in mediating the acute effects of nicotine on anxiety, as measured in the social interaction test, and that the anxiolytic effect is mediated by activation of somatodendritic 5-HT1A autoreceptors. The DRN is also concerned with mediating the development of tolerance to nicotine's anxiolytic effects and because there is an anxiogenic response 72 h after withdrawal from chronic nicotine, this suggests that an oppositional, compensatory mechanism is mediating the tolerance.     

Nicotine self-administration and withdrawal: modulation of anxiety in the social interaction test in rats

RATIONALE: Most smokers report smoking has an anxiolytic effect, which may contribute to nicotine dependence. OBJECTIVE: To examine effects in the social interaction test (SI) of anxiety after 4 weeks' self-administered nicotine (15 infusions of 0.03 mg/kg, totalling 0.45 mg/kg per day), and after 24 and 72 h of withdrawal. The effect of exposure to the operant chamber on withdrawal responses was also examined. METHODS: Animals were trained to self-administer saline or nicotine and after 4 weeks they were tested in SI after their daily self-administration session. Animals were retested after 24 and 72 h withdrawal, when they were either taken directly from the home cage or were tested 5 min after a 30-min exposure to the operant chamber. RESULTS: Compared with the saline control group, the animals that had been self-administering nicotine for 4 weeks showed decreased social interaction with no decrease in locomotor activity, indicating a significant anxiogenic effect of the nicotine infusions. There was no change in social interaction after 24 and 72 h withdrawal from chronic nicotine, regardless of whether or not the rats were exposed to the operant chamber just prior to being tested. CONCLUSIONS: Nicotine self-administration is not maintained because of its anxiolytic effect, but despite, or because of, its anxiogenic effect. There was no evidence of an anxiogenic response after either 24 or 72 h of withdrawal and thus increased anxiety on withdrawal from nicotine does not seem to contribute to nicotine self-administration.     

Anxiety conditioned to nicotine in the elevated plus-maze is time dependent

Conditioning to the anxiogenic effects of nicotine has previously been demonstrated in the social interaction test and there was no generalization of conditioning between the social interaction and elevated plus-maze tests. Because the two tests generate distinct states of anxiety, the conditioning could have occurred to the cues associated with the test environment and/or to those associated with the type of anxiety generated by the test. The elevated plus-maze permits separation of these two factors, because quite distinct states of anxiety are generated on trials 1 and 2, whereas the apparatus cues remain the same. Rats that had been tested on day 1 in the plus-maze, 5 min after nicotine (0.45 mg/kg), showed a conditioned anxiogenic response when tested undrugged on day 2. This was shown by significantly lower percentages of open-arm entries and percentage of time spent on the open arms, compared with control groups. Thus, conditioning to apparatus cues is sufficient to mediate a conditioned anxiogenic effect. The importance of the timing of the nicotine-associated cues was demonstrated by the failure to obtain conditioned anxiogenic effects when rats were exposed to the plus-maze on day 1, 30 min after nicotine (0.45 or 0.1 mg/kg).     

Different effects of chronic nicotine treatment regimens on body weight and tolerance in the rat

The effect of different chronic nicotine administration regimens on body weight and the development of tolerance was examined in female rats. Groups of animals were either treated with nicotine via a subcutaneous continuous release pellet or via two injections each day of either a high (5.6 mg/kg) or low (0.8 mg/kg) dose. Both the Pellet and Low injection groups showed a progressive weight loss during nicotine treatment followed by a weight gain upon cessation of treatment, but the time course and size of these weight changes were quite distinct. In contrast, the High injection group gained weight during the 17 days of nicotine treatment. Tolerance, as measured by locomotor activity following an acute injection of nicotine 1 week after cessation of chronic nicotine treatment, was evident only in the Low injection group. This study demonstrates that the regimen in which nicotine is administered is an important factor in determining the behavioral effects produced by chronic nicotine treatment.     

Diazepam withdrawal responses measured in the social interaction test of anxiety and their reversal by baclofen

After 21 days of treatment with diazepam (0.5 or 2 mg/kg/day) rats were tolerant to the effects of diazepam to increase social interaction in the low light unfamiliar test condition of the social interaction test of anxiety. When they were tested 24 h after the last of 21 injections they showed significant decreases in social interaction, indicating an anxiogenic withdrawal response. However, the social interaction scores of rats tested 48 h after withdrawal from diazepam treatment were no longer different from those of the control group. The decreased social interaction, indicating increased anxiety, detected 24 h after withdrawal of diazepam (21 daily injections of 0.5 or 2 mg/kg), could be reversed by the usual daily diazepam dose (0.5 or 2 mg/kg, respectively) or by baclofen (0.5 or 1 mg/kg). Baclofen (2 mg/kg) was sedative in both control treated and diazepam-dependent rats, but was ineffective at reversing the decrease in social interaction seen after diazepam withdrawal. Possible sites of action mediating these effects of baclofen are discussed, and it is suggested that either post-synaptic GABA_B sites in the hippocampus are involved or that the reversal of the decreased social interaction detected on withdrawal of diazepam treatment is due to a baclofen-mediated inhibition of 5-HT release in the hippocampus.     

Mecamylamine prevents tolerance but enhances whole brain [3H]epibatidine binding in response to repeated nicotine administration in rats

RATIONALE: Chronic administration of nicotine in rats results in upregulation of neuronal nicotinic receptors. Upregulation has been proposed to reflect receptor desensitization, which may underlie functional tolerance to nicotine's effects. However, evidence indicates that tolerance and upregulation do not always parallel each other, suggesting that either upregulation does not always reflect desensitization, or mechanisms other than receptor desensitization account for tolerance to nicotine. OBJECTIVES: The present studies examined tolerance to nicotine-induced antinociception and changes in receptor binding after two regimens of intermittent nicotine injections in rats. The role of receptor activation in upregulation and tolerance was also examined by co-administering nicotine with the non-competitive antagonist, mecamylamine. METHODS: Sprague-Dawley rats were administered a short (once-daily, s.c. for 6 days (0.35 mg/kg)) or long (twice-daily for 11 days (0.66 mg/kg)) series of injections and tolerance to nicotine-induced antinociception and [3H]epibatidine binding in whole brain were measured. RESULTS: The short series of injections resulted in tolerance to nicotine-induced antinociception, but failed to increase [3H]epibatidine binding. In contrast, the long series of injections resulted in both tolerance and increased receptor binding. Once-daily pairings of mecamylamine (1 mg/kg, s.c.) with nicotine (0.35 mg/kg) for 6 days blocked the development of tolerance, indicating receptor activation is necessary for tolerance to occur. Pairing mecamylamine with nicotine (0.66 mg/kg) twice daily for 11 days blocked tolerance but produced a greater increase in [3H]epibatidine binding than nicotine alone. CONCLUSIONS: A dissociation of tolerance from receptor upregulation was observed in the present study. The finding that receptor activation may be necessary for tolerance but not upregulation is discussed within the context of possible mechanisms controlling tolerance to nicotine.     

Subchronic treatment of rats with nicotine: effects on tolerance and on [3H]acetylcholine and [3H]nicotine binding in the brain

The effects of subchronic nicotine treatment on the development of tolerance and on two nicotinic ligand binding sites were investigated. After subcutaneous injection of nicotine (0.45 mg nicotine base/kg) or saline twice a day for 14 days the body weight of rats was significantly lower than that of control animals. A significant tolerance to the acute effects of nicotine on locomotor activity and body temperature was observed after the treatment period. Nicotine treatment also resulted in a significant increase in [3H]acetylcholine (3H-ACh) binding in the midbrain (48.3% increase in comparison with controls) and hippocampus (38.3% increase), whereas the binding of [3H]nicotine (3H-NIC) was unaffected in all brain areas investigated. These results indicate that subchronic s.c. injections of nicotine can differentially affect the binding of two different nicotinic ligands in the brain. It is also concluded that the development of tolerance to the acute effects of nicotine on locomotor activity and temperature is not directly dependent upon changes in binding of [3H]nicotine to the brain.     

Do different mechanisms underlie two anxiogenic effects of systemic nicotine?

Alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs) and 5-hydroxytryptamine 1A (5-HT1A) receptors have been implicated in the anxiogenic effects of centrally administered nicotine, but the receptors that mediate the anxiogenic effects of systemic nicotine are not known. This study explored whether competitive nAChR antagonists [dihydro-beta-erythroidine (DHbetaE), 4 mg/kg, and methyllycaconitine (MLA), 5 mg/kg], and a 5-HT1A receptor antagonist (WAY 100635, 0.5 and 1 mg/kg) could block the effects of two anxiogenic doses of nicotine in the social interaction test of anxiety. The anxiogenic effect of 0.1 mg/kg nicotine, given 5 min before the test, was blocked by DHbetaE and WAY 100635, establishing roles for alpha4beta2 nAChRs and 5-HT1A receptors. None of the antagonists could block the effect of 0.45 mg/kg nicotine, given 30 min before the test, precluding firm conclusions about the mechanisms underlying this anxiogenic effect. However, there was evidence for a role of alpha7 nAChRs in mediating an endogenous anxiogenic tone, since MLA itself had an anxiolytic effect that was blocked by both doses of nicotine. Thus, both alpha7 and alpha4beta2 nAChRs might have a role in mediating the anxiogenic effects of nicotine.     

Flumazenil prevents the development of chlordiazepoxide withdrawal in rats tested in the social interaction test of anxiety

Rats were chronically treated with chlordiazepoxide (CDP 10 mg/kg/day) or vehicle for 27 days. Twenty-four hours after their last dose, they received flumazenil (4 mg/kg) or vehicle and were tested in the social interaction test, in a low-light, familiar arena. CDP withdrawal significantly reduced the time spent in social interaction compared with controls, indicating an anxiogenic withdrawal response. This was completely reversed by flumazenil. A second group received CDP for 27 days and, in addition, received a single dose of flumazenil (4 mg/kg) 6 days before testing. Flumazenil prevented the development of the anxiogenic withdrawal response in these rats.     

The effect of treatment regimen on the development of tolerance to the sedative and anxiolytic effects of diazepam

Rationale: Chronic treatment with benzodiazepines results in tolerance to their sedative and anxiolytic effects and there is considerable evidence that different mechanisms underlie the development of tolerance to different behavioural effects. Objective: The purpose of the present experiment was to compare the behavioural effects of chronic treatment with diazepam (15 mg/kg per day) given as daily subcutaneous injections or by osmotic minipump. Both regimens resulted in continual receptor occupancy, but the daily injections also provided a period of higher brain concentrations. Methods: Rats were tested in the holeboard, which provides measures of exploration and locomotor activity, and in the elevated plus-maze and social interaction tests of anxiety. For those in the subcutaneous injection group the tests were 2 h after injection, when brain concentrations were highest. Results: Despite a higher brain concentration in the injected group, both groups showed tolerance to diazepam’s sedative effects, after 7 days of treatment. In contrast, in the elevated plus-maze, there was tolerance to the anxiolytic effects in the pump group after 14 days, but a persisting anxiolytic effect in the injected group at 14 and 28 days. Whilst higher brain concentrations could explain this result in the plus-maze, they cannot account for the pattern observed in the social interaction test, where the injection group showed a significant anxiogenic effect at 28 days. Conclusions: Whereas the mechanism underlying tolerance to the sedative effects of diazepam was insensitive to the different treatment regimens, the results suggest that different adaptive mechanisms were triggered in the two tests of anxiety with a differential sensitivity to the treatment regimen. The adaptive mechanism predominating in the social interaction test was favoured by the injection regimen which produced intermittent peak concentrations. This mechanism seems to be an oppositional one, leading to an anxiogenic response, which was manifest despite high brain concentrations of diazepam at the time of testing. Received: 12 July 1998 / Final version: 9 March 1999     

The use of flumazenil in prevention of Diazepam dependence in the rat

Rats were treated with diazepam (4 mg/kg per day) or its vehicle for 21 days. Twenty-four hours after their last dose they were tested in the social interaction test of anxiety. Diazepam withdrawal significantly reduced the time spent in social interaction compared with controls, indicating an anxiogenic withdrawal response. The administration of a single dose of flumazenil (4 mg/kg) during chronic diazepam treatment, 7 or 14 days before testing, prevented the development of this withdrawal response. The clinical implications of these findings are discussed.     

Repeated administration of nicotine attenuates the development of morphine tolerance and dependence in mice

Clinical use of morphine in pain management is a controversial issue. Both nicotine and morphine are widely abused. So, investigating the interaction between nicotinic and opioid receptors is of great interest to both basic mechanistic and clinical view. We investigated the influence of repeated administration of nicotine on the development of morphine tolerance and dependence. Adult male albino mice were rendered dependent on morphine by subcutaneous (s.c.) injections three times daily for 3 days. Repeated intraperitoneal (i.p.) injection of nicotine (0.001-2 mg/kg) or saline (1 ml/kg) was performed 15 min prior to each morphine injection. Maximal possible effect (MPE%) of morphine (50 mg/kg; s.c.) was used on the fourth day as an index for the development of tolerance. Likewise, to assess the occurrence of dependence in drug-treated mice, naloxone (5 mg/kg; i.p.) was injected 2 h after the last dose of morphine. Repeated nicotine administration significantly attenuated the development of tolerance in a dose-dependent manner whereas it significantly decreased withdrawal jumping behavior in a biphasic profile (V-shape) manner. Furthermore, the central nicotinic receptor antagonist mecamylamine (0.01-0.1 mg/kg; i.p.) neither the peripheral nicotinic receptor antagonist hexamethonium (0.01 and 0.1 mg/kg; i.p.) nor the muscarinic receptor antagonist atropine (2.5-10 mg/kg; i.p.), dose-dependently antagonized both the inhibition of withdrawal jumping as well as increase in MPE% which was produced by repeated nicotine administration (0.1 mg/kg; i.p.). On the other hand, 3 days of solely nicotine treatment resulted in significant jumping behavior precipitated by naloxone after single morphine injection on the test day. The data suggests that the inhibitory effect of nicotine on the morphine tolerance and dependence is mediated by central nicotinic receptors and there is a cross-dependence between nicotine and morphine.     

Nicotine antagonizes caffeine- but not pentylenetetrazole-induced anxiogenic effect in mice

Rationale Nicotine and caffeine are widely consumed licit psychoactive drugs worldwide. Epidemiological studies showed that they were generally used concurrently. Although some studies in experimental animals indicate clear pharmacological interactions between them, no studies have shown a specific interaction on anxiety responses. Objectives The present study investigates the effects of nicotine on anxiety induced by caffeine and another anxiogenic drug, pentylenetetrazole, in mice. The elevated plus-maze (EPM) test was used to evaluate the effects of drugs on anxiety. Methods Adult male Swiss Webster mice (25–32 g) were given nicotine (0.05–0.25 mg/kg s.c.) or saline 10 min before caffeine (70 mg/kg i.p.) or pentylenetetrazole (15 and 30 mg/kg i.p.) injections. After 15 min, mice were evaluated for their open- and closed-arm time and entries on the EPM for a 10-min session. Locomotor activity was recorded for individual groups by using the same treatment protocol with the EPM test. Results Nicotine (0.05–0.25 mg/kg) itself did not produce any significant effect in the EPM test, whereas caffeine (70 mg/kg) and pentylenetetrazole (30 mg/kg) produced an anxiogenic effect, apparent with decreases in open-arm time and entry. Nicotine (0.25 mg/kg) pretreatment blocked the caffeine- but not pentylenetetrazole-induced anxiety. Administration of each drug and their combinations did not produce any effect on locomotor activity. Conclusions Our results suggest that the antagonistic effect of nicotine on caffeine-induced anxiety is specific to caffeine, instead of a non-specific anxiolytic effect. Thus, it may extend the current findings on the interaction between nicotine and caffeine.     

Effects of co‐administration of bupropion and nicotine or d‐amphetamine on the elevated plus maze test in mice

Objectives A variety of abused drugs, including psychostimulants, can modulate the expression of anxiety. Although the effect of nicotine and d ‐amphetamine on anxiety‐related behaviour in animal models has been investigated, the mechanisms underlying the anxiogenic or anxiolytic actions of these drugs have not been clarified. Bupropion is an antidepressant drug which may alleviate some symptoms of nicotine withdrawal, although its effects on anxiety are not clear. We have investigated the effect of nicotine and d ‐amphetamine on anxiety in the elevated plus maze test in mice. Methods We examined the influence of acute administration of nicotine (0.1 mg/kg, s.c.) and d ‐amphetamine (2 mg/kg, i.p.) on anxiety level. We then evaluated the anxiety‐related response after subchronic injection of both psychostimulants, including crossover effects. For this purpose, nicotine (0.1 mg/kg, s.c.) was administered daily for six days, and on the seventh day mice were challenged with nicotine (0.1 mg/kg, s.c.) or d ‐amphetamine (2 mg/kg, i.p.). A distinct group of mice was pretreated with d ‐amphetamine (2 mg/kg, i.p., 8 days), and subjected to d ‐amphetamine (2 mg/kg, i.p.) or nicotine (0.1 mg/kg, s.c.) challenge on the ninth day. Moreover, we investigated acute and subchronic effects of co‐administration of bupropion (5, 10 and 20 mg/kg; i.p.) and nicotine or d ‐amphetamine. Key findings We observed that acute anxiogenic effects of nicotine and d ‐amphetamine as well as the development of tolerance and cross‐tolerance to their effects were blunted by a pretreatment with a nonactive dose of bupropion (5 mg/kg, i.p.). Conclusions These results demonstrated that similar neural mechanisms were involved in the regulation of nicotine and d ‐amphetamine anxiety‐like behaviour in mice. The results have provided new findings to support the use of bupropion in the treatment of nicotine and/or amphetamine addiction.     

Low but not high doses of buspirone reduce the anxiogenic effects of diazepam withdrawal

After 21 days of treatment with diazepam (2 mg/kg/day IP) rats were tested 24 h after the last injection in the social interaction and elevated plus-maze tests of anxiety. Compared with control-treated rats, they showed significant decreases in social interaction, in the % numbers of entries onto open arms of the plus-maze and in the % of time spent on the open arms, indicating an anxiogenic response on withdrawal from diazepam. Buspirone (200 µg/kg SC) significantly increased social interaction in diazepam withdrawn rats and in the plus-maze also this dose significantly reversed the anxiogenic effects of diazepam withdrawal. Buspirone (400 µg/kg SC) was without effect in the plus-maze, but buspirone (800 µg/kg SC) significantly decreased the % of time spent on open arms in control-treated rats, indicating an anxiogenic effect. In the social interaction test buspirone (800 µg/kg SC) was without significant effect. The contrasting effects of the 200 and 800 µg/kg doses are discussed in terms of the pre- and post-synaptic actions of buspirone. The findings are consistent with earlier proposals that the increased anxiety during benzodiazepine withdrawal is at least partly caused by an increased release of hippocampal 5-HT.     

Anxiogenic effects of nicotine in the dorsal hippocampus are mediated by 5-HT1A and not by muscarinic M1 receptors

After direct administration into the dorsal hippocampus nicotine decreased the time spent in social interaction, without changing locomotor activity, indicating an anxiogenic effect. The possibility that post-synaptic M1 muscarinic receptors mediated this effect was examined by determining whether dorsal hippocampal administration of a specific M1 receptor agonist (McN-A-343) had anxiogenic effects, and whether the anxiogenic effect of nicotine could be reversed by co-administration of the M1 receptor antagonist, pirenzepine. McN-A-343 (0.3, 1.6, 3.2, 15.8 nmol) was without effect on social interaction, and pirenzepine (0.7 and 2.4 nmol) injection into the dorsal hippocampus failed to reverse the decrease in social interaction caused by nicotine (6.3 nmol) injection into this area. However, the decrease in social interaction after nicotine (50 nmol) was completely reversed by the specific 5-HT1A receptor antagonist, WAY 100635 (0.4 nmol) after co-administration of both drugs into the dorsal hippocampus. Thus, the anxiogenic effect of nicotine in this brain region seems to be mediated by 5-HT1A, but not M1, receptors. In contrast to the effect of nicotine in naive animals, those retested after a second injection of 50 nmol did not show a significant anxiogenic effect. The theoretical implications of this are discussed and from a practical point of view this suggests caution in the retesting of animals after central injections.