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

Nicotine sensitization (Part 2): Time spent in the centre of an open field sensitizes to repeated nicotine into the drug-free state in female rats

RATIONALE

Nicotine is initially anxiogenic and becomes anxiolytic after prolonged exposure. The mechanisms that facilitate the shift in anxiety-like behaviour produced by nicotine are unclear.

OBJECTIVE

We investigated the change in time spent in the centre of an open field (as a measure anxiety-like behaviour) produced by three intermittent injections of nicotine as part of experiments of locomotor sensitization to nicotine.

METHODS

Rats were injected with nicotine (0.4 mg/kg) or saline and immediately placed in the open field arena for 1 h on two consecutive days and again 9 days later.

RESULTS

When given saline, time spent in the centre of the arena did not change, whereas repeated nicotine injections increased in time spent in the centre beyond the increase produced by an acute injection of nicotine. Repeated nicotine (and not acute nicotine) also increased time in the centre in a drug-free state when tested 24 h after the last injection.

CONCLUSION

Repeated nicotine sensitized the time spent in the centre of an open field with the long-lasting sensitization of this measure of anxiety-like behaviour evident in a drug-free state, in contrast to locomotor sensitization which does not persist in the drug-free state. The results suggest independence of the mechanisms of sensitization that underlie locomotor and anxiolytic effects.

Female rats self-administer heroin by vapor inhalation

Over the last two decades the United States has experienced a significant increase in the medical and non-medical use of opioid drugs, resulting in record numbers of opioid-related overdoses and deaths. There was an initial increase in non-medical use of prescription opioids around 2002, followed later by increased heroin use and then most recently fentanyl. Inhalation is a common route of administration for opioids, with a documented history spanning back to Mediterranean antiquity and up through modern use with e-cigarette devices. Unfortunately, preclinical studies using inhalation as the route of administration remain relatively few. This study was conducted to determine the efficacy of e-cigarette vapor inhalation of heroin in rats. Non-contingent exposure to heroin or methadone vapor produced anti-nociceptive efficacy in male and female rats. Female rats were trained to self-administer heroin vapor; the most-preferring half of the distribution obtained more vapor reinforcers when the concentration of heroin was reduced in the vapor vehicle and when pre-treated with the opioid receptor antagonist naloxone. The anti-nociceptive effect of heroin self-administered by vapor was identical in magnitude to that produced by intravenous self-administration. Finally, anxiety-like behavior increased 24-48 h after last heroin vapor access, consistent with withdrawal signs observed after intravenous self-administration. In sum, these studies show that rewarding and anti-nociceptive effects of heroin are produced in rats by vapor inhalation using e-cigarette technology. Importantly, self-administration models by this route can be deployed to determine health effects of inhaled heroin or other opioids.

Influence of nicotine on doxorubicin and cyclophosphamide combination treatment-induced spatial cognitive impairment and anxiety-like behavior in rats

In the present study, we examined the effects of nicotine on cognitive impairment, anxiety-like behavior, and hippocampal cell proliferation in rats treated with a combination of doxorubicin and cyclophosphamide. Combined treatment with doxorubicin and cyclophosphamide produced cognitive impairment and anxiety-like behavior in rats. Nicotine treatment reversed the inhibition of novel location recognition induced by the combination treatment. This effect of nicotine was blocked by methyllycaconitine, a selective α7 nicotinic acetylcholine receptor (nAChR) antagonist, and dihydro-β-erythroidine, a selective α4β2 nAChR antagonist. In addition, nicotine normalized the amount of spontaneous alternation seen during the Y-maze task, which had been reduced by the combination treatment. This effect of nicotine was inhibited by dihydro-β-erythroidine. In comparison, nicotine did not affect the anxiety-like behavior induced by the combination treatment. Furthermore, the combination treatment reduced the number of proliferating cells in the subgranular zone of the hippocampal dentate gyrus, and this was also prevented by nicotine. Finally, the combination of doxorubicin and cyclophosphamide significantly reduced hippocampal α7 nAChR mRNA expression. These results suggest that nicotine inhibits doxorubicin and cyclophosphamide-induced cognitive impairment via α7 nAChR and α4β2 nAChR, and also enhances hippocampal neurogenesis.

Cholinergic agonist reverses H1-induced memory deficit in mice

This study investigated the effects of bilateral intraamygdalar microinjections of PNU-282987, a nicotinic cholinergic agonist, on anxiety and the reversal of amnesia induced by chlorpheniramine (CPA), an H1 histaminergic antagonist, in mice subjected to the elevated plusmaze (EPM). Two experiments were performed with seventy-nine adult male Swiss mice. The isolated microinjections of PNU-282987 did not produce effects on emotional memory; however, the combined microinjections of PNU-282987 and CPA were able to reverse the deficit in memory induced by CPA (ANOVA, p<0.05). Taken together, these results suggest that intraamygdalar injections of PNU-282987 did not induce effects on anxiety and emotional memory per se; however, concurrent microinjections of PNU-282987 and CPA-reverse amnesia induced-CPA which is suggestive of an interaction between the histaminergic and cholinergic systems in the modulation of emotion memory acquisition in mice.

Extending the Role of Associative Learning Processes in Nicotine Addiction

Compulsive smoking is a worldwide public health problem. Although research has confirmed the importance of associative learning processes in nicotine addiction, therapies targeting nicotine-associated cues still have a high relapse rate. Most theories conceptualize nicotine as an ‘outcome’ that reinforces behaviors and/or changes the affective value of stimuli. Albeit important, this view does not capture the complexity of associative processes involved in nicotine addiction. For example, nicotine serves as a conditional stimulus acquiring new appetitive/affective properties when paired with a non-drug reward. Also, nicotine functions as an occasion setter that participates in higher-order associative processes that likely permit a more pervasive influence of conditioned cues that are resistant to typically cue-exposure therapy techniques. Finally, nicotine appears to amplify the salience of other stimuli that have some incentive value resulting in enhanced nicotine selfadministration and conditioned reinforcement processes. Future smoking intervention strategies should take into consideration these additional associative learning processes.

Erythrina mulungu alkaloids are potent inhibitors of neuronal nicotinic receptor currents in mammalian cells

Crude extracts and three isolated alkaloids from Erythrina mulungu plants have shown anxiolytic effects in different animal models. We investigated whether these alkaloids could affect nicotinic acetylcholine receptors and if they are selective for different central nervous system (CNS) subtypes. Screening experiments were performed using a single concentration of the alkaloid co-applied with acetylcholine in whole cell patch-clamp recordings in three different cell models: (i) PC12 cells natively expressing α3* nicotinic acetylcholine receptors; (ii) cultured hippocampal neurons natively expressing α7* nicotinic acetylcholine receptors; and (iii) HEK 293 cells heterologoulsy expressing α4β2 nicotinic acetylcholine receptors. For all three receptors, the percent inhibition of acetylcholine-activated currents by (+)-11á-hydroxyerysotrine was the lowest, whereas (+)-erythravine and (+)-11á-hydroxyerythravine inhibited the currents to a greater extent. For the latter two substances, we obtained concentration-response curves with a pre-application protocol for the α7* and α4β2 nicotinic acetylcholine receptors. The IC₅₀ obtained with (+)-erythravine and (+)-11á-hydroxyerythravine were 6 µM and 5 µM for the α7* receptors, and 13 nM and 4 nM for the α4β2 receptors, respectively. Our data suggest that these Erythrina alkaloids may exert their behavioral effects through inhibition of CNS nicotinic acetylcholine receptors, particularly the α4β2 subtype.

α4β2 nicotinic acetylcholine receptor partial agonists with low intrinsic efficacy have antidepressant-like properties

Previous studies have suggested that treatment with antagonists or partial agonists of nicotinic acetylcholine receptors containing the β2-subunit (β2 nAChRs) results in antidepressant-like effects. In this study, we tested three novel compounds with different affinity and functional efficacy at α4β2 nAChRs, which were synthesized as part of nAChR discovery projects at Pfizer, in the tail-suspension, forced-swim, and novelty-suppressed feeding tests of antidepressant efficacy. All compounds tested reduced immobility in the forced-swim test and one of the compounds also reduced immobility in the tail-suspension test. All the compounds appeared to affect food intake on their own, with two compounds reducing feeding significantly in the home cage, precluding a clear interpretation of the results in the novelty-suppressed feeding test. None of the compounds altered locomotor activity at the doses and time points used here. Therefore, a subset of these compounds has pharmacological and behavioral properties that demonstrate the potential of nicotinic compounds as a treatment of mood disorders. Further development of nicotinic-based antidepressants should focus on increasing nAChR subtype selectivity to obtain consistent antidepressant properties with an acceptable side-effect profile.

TC-5214 (S-(+)-mecamylamine): a neuronal nicotinic receptor modulator with antidepressant activity

Both clinical and preclinical data support a potential therapeutic benefit of modulating the activity of CNS neuronal nicotinic receptors (NNRs) to treat depression and anxiety disorders. Based on the notion that the depressive states involve hypercholinergic tone, we have examined the potential palliative role of NNR antagonism in these disorders, using TC-5214 (S-(+) enantiomer of mecamylamine), a noncompetitive NNR antagonist. TC-5214 demonstrated positive effects in a number of animal models of depression and anxiety. TC-5214 was active in the forced swim test in rats (minimum effective dose (MED)=3 mg/kg i.p.), a classical depression model. It was also active in the behavioral despair test in mice (0.1-3.0 mg/kg i.p.), another model of depression. In the social interaction paradigm in rats, a model of generalized anxiety disorder (GAD), TC-5214 was active at a dose of 0.05 mg/kg s.c. In the light/dark chamber paradigm in rats, a model of GAD and phobia, TC-5214 was also active at a dose of 0.05 mg/kg s.c. Although TC-5214 shows modest selectivity among NNR subtypes, the antidepressant and anxiolytic effects seen in these studies are likely attributable to antagonist effects at the alpha4beta2 NNRs. This is supported by the observation of similar effects with alpha4beta2-selective partial agonists such as cytisine and with alpha4beta2-selective antagonists such as TC-2216. TC-5214 was well tolerated in acute and chronic toxicity studies in mice, rats, and dogs, showed no mutagenicity and displayed safety pharmacology, pharmacokinetic and metabolic profiles appropriate for therapeutic development. Overall, the results support a novel nicotinic cholinergic antagonist mechanism for antidepressant and anxiolytic effects and highlight the potential of NNR antagonists such as TC-5214 as therapeutics for the treatment of anxiety and depression.

Effects of nicotine and a cannabinoid receptor agonist on negative contrast: distinction between anxiety and disappointment?

RATIONALE

Animals trained to lick for a sucrose solution of a given incentive value that subsequently encounter an incentive downshift (i.e. 32-4% sucrose) display an exaggerated decrease in the amount consumed, relative to unshifted controls. This change has been classified as a successive negative contrast (SNC) effect. The emotional component to this robust behavioural change is dynamic and changes from post-shift day (PSD) 1 to 2. Anxiolytics block SNC, but the possible link between anxiety and SNC needs further exploration. Both nicotine and a cannabinoid receptor agonist have been reported to change anxiety and both have actions on the reward process, but their effects on SNC have not been investigated.

OBJECTIVES

To determine: (1) whether exposure to SNC evokes an anxiogenic response; (2) whether an anxiolytic dose of nicotine has the same effects on SNC as those of chlordiazepoxide; (3) the effects of a low (anxiolytic) and a high (anxiogenic) dose of the cannabinoid receptor agonist CP 55,940 on SNC.

METHODS

Two groups of animals were given access to high (32%) or low (4%) sucrose solutions for 5 min per day for 10 days. On PSD 1 and 2, the shifted group had access to a devalued incentive (from 32 to 4% sucrose) and the unshifted group remained at 4% sucrose. The volumes (ml) of sucrose solution consumed were measured pre-shift and on PSD 1 and 2. In experiment 1, immediately after SNC testing on PSD 1 and 2, the rats were tested in the social interaction and elevated plus-maze tests of anxiety. In experiment 2, the effects of chlordiazepoxide (5 and 7.5 mg/kg) and nicotine (0.1 mg/kg) were examined on PSD 1 and 2. In experiment 3, the effects of CP 55,940 (5 and 40 microg/kg) were examined on PSD 1 and 2.

RESULTS

There were no anxiogenic effects of shift in either test of anxiety on either test day. However, on PSD 1, the shifted group had significantly higher locomotor activity and spent a higher percentage of time on the open arms, perhaps reflecting search strategies. Nicotine was without significant effect on SNC on either test day. On PSD 1, chlordiazepoxide (5 mg/kg) and CP 55,940 (5 and 40 microg/kg, IP) blocked SNC. On PSD 2, both doses of chlordiazepoxide and the low, anxiolytic dose of CP 55,940 (5 microg/kg) blocked SNC, the high dose of CP 55,940 was without effect.

CONCLUSIONS

The pattern of results allows for the separation between effects on anxiety and SNC. The block of contrast on PSD 1 was independent of changes in anxiety, since both anxiolytic and anxiogenic drug doses were effective. It is suggested that this may provide an animal model of disappointment in which the cannabinoid system plays an important role. An anxiolytic action would seem to be a necessary, but not a sufficient, action to block SNC on PSD 2.

Timing of conditioned responding in a nicotine locomotor conditioning preparation: manipulations of the temporal arrangement between context cues and drug administration

Using a locomotor conditioning preparation, we examined whether manipulating time between exposure to distinct environmental cues and nicotine administration affected conditioned responding. Rats that received nicotine (0.42 mg/kg base) immediately before placement in an environment for 30 min on eight separate occasions displayed hyperactivity relative to controls in a subsequent injection/drug-free test. This conditioned hyperactivity was weaker if nicotine was administered 15 min before environment exposure. Conditioning was not evidenced when nicotine was administered 15 min after placement or upon removal from the environment. In a follow-up experiment, rats received 45 min in the environment; nicotine was administered 15 min after placement. This group showed conditioning that was localized to the last two-thirds of a 45 min test indicating that a 15 min delay did not prevent conditioning given 30 min of environment/nicotine overlap. This apparent timing of conditioned responding was not due to increasing environment exposure to 45 min. Further, a state-dependent environmental familiarization account of locomotor hyperactivity during testing was eliminated by the finding that rats displayed temporally specific increases in activity on the test day despite the fact that the context was previously experienced without drug for 15 min on eight consecutive days.

Chronic nicotine treatment leads to induction of tyrosine hydroxylase in locus ceruleus neurons: the role of transcriptional activation

Chronic nicotine treatment (two daily subcutaneous injections administered approximately 12 h apart for 14 days) is associated with long-term inductions of tyrosine hydroxylase (TH) protein and TH mRNA in locus ceruleus (LC) neurons. These increases persist for at least 3 days after the final nicotine injection in LC cell bodies and for at least 7 to 10 days in LC nerve terminal regions. We tested whether this long-term response is due to sustained stimulation of TH gene transcription rate. A semiquantitative reverse transcription-polymerase chain reaction assay was developed to assess changes in the levels of TH RNA primary transcripts; these changes are an indirect measurement of changes in TH gene transcription rate. TH RNA primary transcript levels increase rapidly in the LC after a single nicotine administration and return to basal levels by 24 h. A similar rapid and transient induction of LC TH RNA primary transcripts occurs after chronic nicotine administration. In contrast, TH RNA primary transcript levels remain elevated for a sustained period of time (at least 1 day) in the adrenal medulla after chronic nicotine administration. Similar rapid, but transient changes in LC TH RNA primary transcript levels are observed after repeated immobilization stress. These results suggest that TH gene transcription rate in the LC is stimulated rapidly after each nicotine injection; however, in contrast to the adrenal medulla, there is no sustained transcriptional response elicited by chronic nicotine treatment or repeated immobilization stress in the LC, suggesting that post-transcriptional mechanisms may also play a role in these long-term responses.

Unconditioned and conditioned anxiogenic effects of the cannabinoid receptor agonist CP 55,940 in the social interaction test

In spite of the addictive properties of cannabinoids, under certain circumstances, they can evoke strong anxiogenic and aversive responses in humans and in animal tests of anxiety. Effects of different doses of CP 55,940 (10, 20, and 40 microg/kg) were tested in the low-light, familiar (LF) apparatus test condition of the social interaction test. The 40-microg/kg dose of CP 55,940 significantly decreased the time spent in social interaction, indicating an anxiogenic effect. This dose also had an independent effect of reducing locomotor activity. In rats tested undrugged 24 h after testing with 40 microg/kg, there was a significant anxiogenic effect, indicating conditioned anxiety. The group of rats injected with 40 microg/kg immediately after the social interaction test showed an unexpected significant anxiolytic effect when tested undrugged 24 h later. In an additional experiment, rats were tested in the high-light, familiar (HF) apparatus test condition after 10 or 40 microg/kg, and only those that were tested after 40 microg/kg showed an anxiogenic effect on the test day and a conditioned anxiogenic effect when tested undrugged 24 h later. Once again, those injected with 40 microg/kg after the social interaction test displayed an anxiolytic effect when tested undrugged 24 h later. We provide the first evidence for unconditioned and conditioned anxiogenic-like responses to a cannabinoid agonist in the social interaction test.

Corticotropin releasing factor antagonist, alpha-helical CRF(9-41), reverses nicotine-induced conditioned, but not unconditioned, anxiety

RATIONALE

Unconditioned anxiogenic effects of nicotine have been observed in the social interaction (SI) test 5 min after injection of a low dose and both 5 min and 30 min after injection of a high dose. Conditioned anxiety has also been observed 24 h after testing in the SI with a high dose of nicotine.

OBJECTIVES

In order to determine whether these three anxiogenic effects shared a common mechanism, we investigated the role of corticotropin releasing factor (CRF). We therefore examined whether the CRF antagonist alpha-helical CRF(9-41) could block these three anxiogenic effects of nicotine.

METHODS

To test the unconditioned anxiogenic effects, pairs of male rats were tested in SI 5 min after s.c. vehicle or nicotine (0.1 mg/kg) or 30 min after s.c. vehicle or nicotine (0.45 mg/kg), and 30 min after i.c.v. artificial cerebrospinal fluid (aCSF) or alpha-helical CRF(9-41). To test conditioned anxiety, rats were exposed to the SI test on day 1, 5 min after vehicle or nicotine (0.1 mg/kg). On day 2, they were re-tested in SI 30 min after i.c.v. aCSF or alpha-helical CRF(9-41) (5 microg).

RESULTS

alpha-Helical CRF(9-41) did not block the unconditioned anxiogenic effect of either dose of nicotine. Nicotine (0.1 mg/kg, 5 min) elicited a conditioned anxiogenic response that was significantly reversed by alpha-helical CRF(9-41). The CRF antagonist alone had no effect.

CONCLUSIONS

CRF is an important mediator of the conditioned anxiety to nicotine, but may not play a role in mediating the acute anxiogenic effects.