Anxiolytics antagonize yohimbine's discriminative stimulus properties

Neurobiology of Stress-Induced Nicotine Relapse

Tobacco smoking is the leading cause of preventable death and disease. Although there are some FAD-approved medicines for controlling smoking, the relapse rate remains very high. Among the factors that could induce nicotine relapse, stress might be the most important one. In the last decades, preclinical studies have generated many new findings that lead to a better understanding of stress-induced relapse of nicotine-seeking. Several molecules such as α3β4 nicotinic acetylcholine receptor, α2-adrenergic receptors, cannabinoid receptor 1, trace amine-associated receptor 1, and neuropeptide systems (corticotropin-releasing factor and its receptors, dynorphine and kappa opioid receptor) have been linked to stress-induced nicotine relapse. In this review, we discuss recent advances in the neurobiology, treatment targets, and potential therapeutics of stress-induced nicotine relapse. We also discuss some factors that may influence stress-induced nicotine relapse and that should be considered in future studies. In the final section, a perspective on some research directions is provided. Further investigation on the neurobiology of stress-induced nicotine relapse will shed light on the development of new medicines for controlling smoking and will help us understand the interactions between the stress and reward systems in the brain.

Discriminative stimulus properties of the atypical antidepressant, mirtazapine, in rats: a pharmacological characterization

RATIONALE

Though interoceptive properties of antidepressants have been described, discriminative stimulus (DS) properties of mirtazapine, which does not affect monoamine reuptake, remain uncharacterized.

OBJECTIVES

The objectives of the study are to train rats to recognize a mirtazapine DS, then perform substitution studies with other antidepressants and drugs acting at sites occupied by mirtazapine.

MATERIALS AND METHODS

Using a two-lever, fixed-ratio 10 schedule, rats were trained to discriminate mirtazapine (2.5 mg/kg, i.p.) from saline.

RESULTS

Sessions, 63 +/- 8, were necessary to reach the criterion for 14 rats that all subsequently recognized (100%) mirtazapine at the training dose. Mirtazapine blocks serotonin (5-HT)(2C) receptors, and the 5-HT(2C) antagonists, SB242,084, SB243,213 and S32006, revealed dose-dependent and full (> or =80%) substitution at doses of 2.5, 2.5, and 0.63 mg/kg, respectively. By contrast, the 5-HT(2A) antagonists, MDL100,907 and SR46349-B, the 5-HT(2B) antagonist, SB204,741, and the 5-HT(3) antagonist, ondansetron, showed no significant substitution. Though mirtazapine indirectly recruits 5-HT(1A) receptors, the 5-HT(1A) agonists, buspirone and 8-OH-DPAT, did not substitute. Mirtazapine blocks alpha(2)-adrenoceptors, but several alpha(2)-adrenoceptor antagonists (yohimbine, RX821,002 and atipamezole) failed to substitute. Despite blockade by mirtazapine of histamine H(1) receptors, no substitution was seen with the selective H(1) antagonist, pyrilamine. Finally, the selective noradrenaline reuptake inhibitor, reboxetine (0.16), fully substituted for mirtazapine, whereas the 5-HT/noradrenaline reuptake inhibitors, duloxetine and S33005, several 5-HT reuptake inhibitors (citalopram, fluvoxamine, and paroxetine) and the dopamine reuptake inhibitors, bupropion and GBR12,935, did not substitute.

CONCLUSION

Mirtazapine elicits a DS in rats for which selective antagonists at 5-HT(2C) receptors display dose-dependent substitution, whereas drugs acting at other sites recognized by mirtazapine are ineffective.

Interactions between Delta(9)-tetrahydrocannabinol and mu opioid receptor agonists in rhesus monkeys: discrimination and antinociception

RATIONALE

Opioid receptor agonists can enhance some effects of cannabinoid receptor agonists, and cannabinoid receptor agonists can enhance some effects of opioid receptor agonists; however, the generality of these interactions is not established.

OBJECTIVE

This study examined interactions between the discriminative stimulus and antinociceptive effects of mu opioid receptor agonists and Delta(9)-tetrahydrocannabinol (THC) in rhesus monkeys.

RESULTS

Neither heroin nor morphine (intravenous (i.v.) or subcutaneous (s.c.)) altered the discriminative stimulus effects of THC in monkeys (n = 5) discriminating 0.1 mg/kg THC i.v. In contrast, THC (s.c.) markedly attenuated the discriminative stimulus effect of morphine and heroin in nondependent monkeys (n = 4) discriminating 1.78 mg/kg morphine s.c. Doses of THC that attenuated the discriminative stimulus effects of morphine in nondependent monkeys failed to modify the discriminative stimulus effects of morphine in morphine-dependent (5.6 mg/kg/12 h) monkeys (n = 4) discriminating 0.0178 mg/kg naltrexone s.c. THC also failed to modify the discriminative stimulus effects of naltrexone in morphine-dependent monkeys or the effects of midazolam in monkeys (n = 4) discriminating 0.32 mg/kg midazolam s.c. Doses of THC (s.c.) that attenuated the discriminative stimulus effects of morphine in nondependent monkeys enhanced the antinociceptive effects of morphine (s.c.) in nondependent monkeys. While mu receptor agonists did not alter the discriminative stimulus effects of THC, THC altered the effects of mu receptor agonists in a context-dependent manner.

CONCLUSION

That the same doses of THC enhance, attenuate, or do not affect morphine, depending on the condition, suggests that attenuation of morphine by THC can result from perceptual masking rather than common pharmacodynamic mechanisms or pharmacokinetic interactions.

Attenuation of the amphetamine discriminative cue in rats with the atypical antipsychotic olanzapine

Sixteen male Sprague-Dawley rats were trained to discriminate between saline and amphetamine injections (1.0 mg/kg ip) using a standard two-lever (FR10) drug discrimination paradigm. A baseline dose-effect curve was generated for amphetamine administration alone, using doses both above and below the training dose (0.0-2.2 mg/kg ip). Once completed, a single dose of olanzapine (OLZ; 1.5 mg/kg sc) was tested for its ability to attenuate the amphetamine cue. OLZ pretreatment (60 min) successfully interfered with an animal's ability to discriminate amphetamine injections across various doses. The percentage of correct responding on the amphetamine lever and rate of responding were both significantly decreased across some but not all of the amphetamine doses. Therefore, we believe that this preliminary investigation has successfully shown that an OLZ dose of 1.5 mg/kg sc at 60 min can interfere with an animal's ability to detect some subjective cue(s) associated with amphetamine administration.

Discriminative stimulus properties of CGS 10746B: similarity to dopamine D1 receptor antagonists

CGS 10746B is an imidazole-derivative related to the atypical antipsychotic clozapine which produces a decrease in dopamine release without altering dopamine metabolism or occupying D2 receptors. Rats were trained on an appetitively-motivated, two-choice, operant task to discriminate 20.0 mg/kg CGS 10746B from its vehicle. CGS 10746B was highly discriminable, producing rapid acquisition of the discrimination, and its effects were dose-responsive allowing generation of an ED50 value of 6.16 mg/kg. Substitution tests were conducted with other typical and atypical antipsychotic compounds: haloperidol, chlorpromazine, clozapine and SCH 23390. Additional tests examined generalization from the CGS 10746B stimulus properties to the calcium channel blocker isradipine, as well as to the anticholinergics atropine, scopolamine and methylscopolamine, as well as to the serotonergic agonist DOI. Clozapine and SCH 23390 were the only substances to substitute for the CGS 10746B stimulus cue. Results are discussed in terms of potential D1 receptor selectivity of CGS 10746B.

Antiemetic effects of flesinoxan in cats: comparisons with 8-hydroxy-2-(di-n-propylamino)tetralin

The antiemetic effects of flesinoxan were evaluated following s.c. administration in cats. Flesinoxan produced a dose-dependent suppression of motion sickness and also reduced xylazine-induced emesis at higher doses. Flesinoxan had a short latency to onset and may have a brief duration of action. It was slightly more potent that 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), in contrast to their relative potencies on most other in vivo measures. High doses of both agonists produced defensive behavior as a result of 5-HT1A receptor stimulation. (-)-Propranolol, which previously reduced 8-OH-DPAT suppression of feline motion sickness, failed to reduce the antiemetic effect of flesinoxan. The dose of 3 mg/kg of NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine) produced a slight decrease in motion sickness and added to the suppression of motion sickness by low doses of flesinoxan via an uncertain mechanism. It also reduced the antiemetic effect of higher doses of flesinoxan. In contrast, NAN-190 produced additive antiemetic effects when combined with 8-OH-DPAT and little if any reduction. NAN-190 reduced the defensiveness produced by both flesinoxan and 8-OH-DPAT. Phentolamine and sulpiride reduced neither the antiemetic effect nor the defensive behavior produced by flesinoxan, thus ruling out a role for alpha-adrenoceptors and dopamine D2 receptors. Flesinoxan exerted a broad spectrum antiemetic effect by an action at 5-HT1A receptors as does 8-OH-DPAT, but differed in its response to putative 5-HT1A receptor antagonists.

Perceptual masking of the chlordiazepoxide discriminative cue by both caffeine and buspirone

Twelve male Sprague-Dawley rats were trained to discriminate between the interoceptive stimulus attributes of 5 mg/kg chlordiazepoxide (CDP) and saline in a two-lever operant task under a fixed-ratio 10 (FR-10) schedule of food reinforcement. Caffeine, buspirone, and Ro 15-1788 failed to engender complete generalization when tested in combination with saline. In drug interaction test sessions caffeine (56 mg/kg) blocked the discriminative stimulus properties of the training dose of CDP and shifted the CDP discriminative dose-response function to the right. This rightward shift in CDP discriminative function was paralleled by a concomitant downward shift in the rate-of-responding dose-response function. Drug interaction test sessions conducted with 3.2 mg/kg of buspirone in combination with various doses of CDP engendered a downward shift in both the discriminative and rate-of-responding dose-response functions. Because 3.2 mg/kg buspirone in combination with the training dose of CDP resulted in complete response rate suppression, additional combination tests were conducted with 3 mg/kg CDP, a dose which reliably engendered > 90% CDP-appropriate responding, and various doses of buspirone. Similar to the CDP-caffeine interactions, buspirone blocked the cueing properties of 3 mg/kg CDP with a parallel reduction in response rates. Interaction test sessions conducted with Ro 15-1788 and CDP resulted in rightward shifts in both the discriminative and rate functions of CDP. We suggest that the interactions between CDP and both caffeine and buspirone resulted from the perceptual masking of the interoceptive (subjective) effects of CDP, whereas the interaction between Ro 15-1788 and CDP reflect pharmacological antagonism.

Evidence for noradrenergic involvement in mediating the FG 7142 discriminative stimulus

Rats were trained to discriminate the stimulus properties of the benzodiazepine receptor partial inverse agonist beta-carboline-3-carboxylate acid methyl amide (FG 7142) (5.0 mg/kg) or the alpha 2-adrenergic receptor antagonist 17 alpha-hydroxyyohimban-16 alpha-carboxylic acid methyl ester (yohimbine) (3.0 mg/kg) from vehicle in a two-lever, food-motivated operant task. These compounds have in common a beta-carboline structure and anxiogenic behavioral profiles. The yohimbine discriminative stimulus was mimicked by the alpha 2-adrenergic receptor antagonist idazoxan and antagonized by the alpha 2-adrenergic receptor agonist clonidine, indicating that the yohimbine stimulus was mediated through the alpha 2-adrenergic receptor. The anxiogenic beta-carbolines FG 7142, 1,2,3,4-tetrahydro-beta-carboline (THBC), and norharmane, the anxiogenic/convulsant agent pentylenetetrazole (PTZ), and two physiological stressors failed to mimic the yohimbine discriminative stimulus. In contrast, both yohimbine and idazoxan dose responsively mimicked the anxiogenic FG 7142 stimulus. The present results demonstrate that an asymmetrical generalization exists between the discriminative stimuli produced by yohimbine and FG 7142. Furthermore, these data suggest that yohimbine can produce a multicomponent discriminative stimulus, part of which may be anxiogenic in nature. The ability of alpha 2-adrenergic receptor antagonists to mimic the FG 7142 cue suggests that activation of the noradrenergic system may underlie cues produced by benzodiazepine receptor inverse agonists.

Behavioral and neuroendocrine response to psychosocial stress in male rats: the effects of the 5-HT 1A agonist ipsapirone

The effect of the 5-HT 1A agonist ipsapirone on the behavior, plasma catecholamine, and corticosterone levels was studied in male Wistar rats during the psychosocial stress of confrontation with a confined dominant opponent 24 hr after defeat. The effect of the drug was also studied during a predefeat confrontation with the confined (would-be dominant) rat. Blood samples were withdrawn via a permanent heart catheter. The drug (5 mg/kg, ip) or vehicle was given 30 min before transportation to the experimental room. Ipsapirone had no major effects on the plasma hormone concentrations and had no influence upon the behavioral response to the confined rat. At the postdefeat test ipsapirone led to a significant increase of immobility, whereas both rearing and time spent sniffing the cage were diminished. Postdefeat psychosocial stress resulted in an increase of the hormone, particularly catecholamine levels. These responses were further elevated by the drug. The presence of high corticosterone levels in the home cage after postdefeat ipsapirone treatment leads to the hypothesis that postsynaptic 5-HT 1A receptor hypersensitivity develops after the social stress of defeat.

Behavioural physiology of serotonergic and steroid-like anxiolytics as antistress drugs

Pharmacological studies are useful tools to understand the neurobiological basis of behavioural and physiological stress mechanisms. Ipsapirone, a 5-HT1A autoreceptor agonist is a representative of novel anxiolytics without the disadvantages of benzodiazepam-like drugs. Behavioural, physiological and neuroendocrine studies in the rat are reviewed which were aimed to investigate the antistress properties of ipsapirone during reexposure to various conditioned emotional stress situations. It is demonstrated that in certain situations, probably due to a stress-induced sensitisation of postsynaptic 5-HT1A receptors, anxiolytic doses of the drug may show prostress (anxiogenic) behavioural and neuroendocrine effects. Furthermore, brain corticosteroid receptors, probably interacting with the serotonergic transmission, are involved in anxiogenic/prostress processes. In this respect antagonists of the brain mineralocorticoid-like (type I) receptors may be important antistress drugs of the future.

Discriminative stimulus effects of the alpha 2-adrenoceptor antagonist idazoxan

Rats were trained to discriminate a dose of the alpha 2-adrenoceptor antagonist idazoxan (10 mg/kg IP) from saline. The discriminative stimulus produced by idazoxan was dose related and generalised to yohimbine. However, generalisation did not occur with a variety of compounds from other pharmacological categories including the alpha 1-adrenoceptor agonist cirazoline, the alpha 1-adrenoceptor antagonist prazosin, and the alpha 2-adrenoceptor agonist clonidine. The idazoxan stimulus was not antagonised by either prazosin or clonidine, although it was clear that idazoxan antagonised the reductions in response rate produced by clonidine. Dose-related responding on the idazoxan-associated lever was produced by the anxiolytics buspirone and ipsapirone and by their metabolite MJ 13653 (1-PP), which has previously been shown to be an alpha 2-adrenoceptor antagonist. In general, however, high levels of generalisation occurred with these three compounds only at doses which substantially reduced response rates. These results demonstrate that idazoxan can give rise to a discriminative stimulus which is probably mediated through antagonism at alpha 2-adrenoceptors although the failure of clonidine to block the idazoxan stimulus is difficult to explain.

Evidence for perceptual masking of the discriminative morphine stimulus

Morphine-amphetamine and morphine-naltrexone interactions were examined in three groups of White Carneaux pigeons (n = 3), which were trained in a two-choice drug discrimination procedure under a FR-30 schedule of food reinforcement using 3.2 mg/kg morphine and saline as discriminative stimuli. Once stimulus control was acquired by these initial training stimuli, the training doses of morphine were gradually changed to 1.0 mg/kg for group A and to 10 mg/kg for group C. The three groups differed in the minimum dose required for stimulus control and the drugs to which the training stimulus generalized. Stimulus generalization to amphetamine was inversely related to training dose. Amphetamine potentiated the discriminative stimulus properties of morphine. Naltrexone blocked the discriminative stimulus properties of morphine to varying degrees, which appeared to be limited by the training dose and the rate-suppressing effects of naltrexone when administered alone. Challenging the morphine stimulus with amphetamine resulted in a qualitatively similar blockade. This blockade was a direct function of the morphine training dose. It is argued that MS-AMP interactions result in perceptual masking of the MS stimulus, which can be differentiated from pharmacological antagonism by NTX. Two other challenge drugs, ketamine and sodium pentobarbital, did not alter stimulus control by morphine.

Behavioural effects of the alpha 2-adrenoceptor antagonists idazoxan and yohimbine in rats: comparisons with amphetamine

Although yohimbine has long been known to increase arousal, reactivity and anxiety in animals and humans, little is known about the behavioural effects of more selective alpha 2-adrenoceptor antagonists such as idazoxan. In a recent experiment, however, it was found that in rats both yohimbine and idazoxan increased low rates of lever pressing, an effect also produced by amphetamine. The purpose of the present study was to investigate further the effects of yohimbine and idazoxan in comparison with those of d-amphetamine on the operant behaviour of rats. In rats trained to press a lever on a FI 60s schedule to obtain food both yohimbine and idazoxan increased response rates, although the effect of yohimbine was considerably greater than that of idazoxan. Lower doses of d-amphetamine had no consistent effect on overall rates of responding whereas a higher dose suppressed responding. Characteristically, d-amphetamine increased responding during early portions of the intervals and decreased responding during the final portions. Idazoxan and yohimbine tended to increase responding throughout the intervals except immediately after reinforcement. When idazoxan was administered in combination with prazosin FI response rates were markedly decreased. Administration of DSP4 did not alter the response rate-increasing effects of either yohimbine or idazoxan. In rats trained to discriminate d-amphetamine from saline both idazoxan and yohimbine gave rise to responding on the saline associated lever. Combination of idazoxan with d-amphetamine did not antagonise the amphetamine cue but produced substantial reductions in response rates, probably due to toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term effects of yohimbine on behavioral sensitivity to a stressor

Two experiments examined the long-term effects of repeated administration of yohimbine, a suspected anxiogenic drug, on behavioral sensitivity to a conditioned cue for shock. In Experiment 1, rats were trained to bar press following injection of yohimbine (either 1 or 2 mg/kg) or saline. At the end of this training, injections were suspended and the rats were given Pavlovian fear conditioning to establish a light as a shock signal. Rats were then returned to the bar press situation (about 2 weeks after their last injection) and the capacity of the light to disrupt responding was tested. Rats previously treated with yohimbine were less disrupted by the light than were controls. In contrast, Experiment 2 found that previously experiencing the yohimbine-induced state only in the home cage increased subsequent disruption of bar pressing by the light. Yohimbine pretreatment had no effect on acquisition of freezing behavior to the light, nor on bar pressing during testing in the absence of the light, in either experiment. The results show that yohimbine can have long-term effects on behavior in the presence of a stressor, and that the nature of these effects are dependent upon environmental and/or behavioral context in which the yohimbine-induced state was experienced. These findings appear compatible with an internal stimulus view of stress and stress inoculation.

Animal models for the study of anti-anxiety agents: a review

Animal models for the study of anxiolytic agents are reviewed and evaluated according to pharmacological and behavioral criteria. Although there are important exceptions, in general, most early animal models have not provided a reliable basis for identifying compounds with potential anxiolytic action, or for delineating the mechanisms of anxiolytic drug action. The possibility that phylogenetically 'prepared' forms of defensive learning might serve as a basis for the study of anxiolytic agents is introduced.

Tolerance to the depressant effects of diazepam in the drug discrimination paradigm

Seven groups of rats (n = 35) were run in operant drug experiments. All groups were trained on a Fixed Ratio 10 schedule to discriminate diazepam from saline. Two groups (n = 7, n = 6), after extensive drug discrimination training (doses of 2.0 and 3.0 mg/kg diazepam), were submitted to generalization experiments with various doses of the training drug. Two additional groups, (n = 6, n = 8) in the initial phase of drug discrimination, were trained on intermediate and high doses of diazepam (i.e., 5.0 and 10.0 mg/kg). The development of tolerance to the depressant effects of diazepam for these two groups was compared to the low dose sophisticated rats. Of the above-mentioned groups, two groups were given tests after a waiting period in drug discrimination training. In this test the two groups were compared to an additional group (n = 8) in its initial phases of drug discrimination training. The results show that a large number of low doses (i.e., doses below 3.0 mg/kg) is not able to induce any tolerance to the depressant effects of diazepam in this particular paradigm. Intermediate doses of diazepam (i.e., 3.0 mg/kg), administered in a large number, induced some tolerance to the depressant effects, while another intermediate dose (5.0 mg/kg) and a high dose (10.0 mg/kg) rapidly induced a significant tolerance. Once developed, the tolerance persisted for 51 days.

Behavioral analogues of anxiety. Animal models

In the absence of fully characterized biological indexes, anxiety is at present measured as unpleasant effects reported verbally by patients. Because of the subjective nature of the syndrome, animal analogues have been difficult to design, but quests for new anxiolytics and a deeper understanding of the neurobiology of anxiety have fostered the development of several animal models. Usually, animals are exposed to exteroceptive or interoceptive stimuli which can be interpreted as capable of causing anxiety in humans. Then, the animals are observed for responses or behavioral deficits resulting from those stimuli in order to provide an index of anxiety. Behavioral responses that are reliably produced by those stimuli and that are also antagonized by anxiolytic drugs are accepted as analogues of anxiety. Exteroceptive stimuli, useful in this respect, consist of a variety of noxious treatments such as exposure to conflict-situations or unavoidable electric shock, whereas interoceptive stimuli consist of treatment with anxiogenic drugs or electrical stimulation of selected brain areas. Elicitation of unconditioned behavior or changes in the rate of conditioned (learned) responding have been employed as measures of anxiety responses following application of either exteroceptive or interoceptive stimuli. These measures, although useful in detecting anxiolytic drugs, possess several weaknesses. They suffer from difficulties in obtaining quantitative and objective data, they do not differentiate between anxiety and stress or fear, they are unable to measure further deterioration of behavior expected to occur when more potent anxiogenic stimuli are tested and they often present difficulty in differentiating direct motor effects of a number of stimuli are not related to anxiety. More recently, interest in the development of other analogues of anxiety has led to the use of drug-discrimination paradigms. In this approach, interoceptive discriminative stimuli, produced by anxiogenic drugs, are used as analogues of anxiety in animals. As an example of this approach, data are reviewed showing that pentylenetetrazol, an anxiogenic drug in humans, produces interoceptive stimuli which can be readily discriminated by rats. Further, these stimuli can be easily quantified through dose-response analysis. All known anxiogenic drugs generalize to pentylenetetrazol-induced interoceptive discriminative stimuli. Similarly, other anxiety-provoking situations in humans, such as withdrawal from dependence on benzodiazepines, also generalize to the pentylenetetrazol-induced stimuli. Alternatively, all known anxiolytic drugs antagonize these stimuli with a relative potency similar to

Stereoselective antagonism of phencyclidine's discriminative properties by adenosine receptor agonists

Rats trained to discriminate between phencyclidine and saline vehicle were used to test various agents for their ability to mimic or block the phencyclidine cue. ketamine, dexoxadrol, tiletamine, and phencyclidine analogs were found to mimic phencyclidine's behavioral effects. Treatment with the adenosime receptor agonists N6-cyclohexyladenosine and L-phenylisopropyladenosine blocked the discriminative properties of phencyclidine. These results suggest that adenosine receptor agonists might be useful in treating phencyclidine-induced psychosis.