Discriminative Stimulus Properties Research Articles

Early preclinical studies of discriminable sedative and hallucinogenic drug effects

One important technique in behavioral pharmacology is to train laboratory animals to discriminate between a psychoactive drug effect and a nondrug condition. Tests with different drugs have identified several categories of drugs that have different discriminable effects. The two authors describe and discuss the early research on discriminable effects of sedative and hallucinogenic drugs and their acquaintance with each other at Yale University prior to their early and frequent publications on discriminable drug effects. Herb Barry studied sedative drugs primarily and Jim Appel studied hallucinogenic drugs. Sedative drugs include ethyl alcohol, barbiturates, and benzodiazepines. Their discriminable effects are largely attributable to the activation of an inhibitory neurotransmitter, gamma-amino butyric acid. Alcohol has the most pervasive effect in accordance with the high dose required to alter behavior. Hallucinogenic drugs include lysergic acid diethylamide and mescaline. They increase the activity of the neurotransmitter 5-hydroxytryptamine and, perhaps, dopamine in the central nervous system (CNS). In spite of their relatively low concentrations in the brain, both of these neurotransmitters have many important behavioral effects. Various sedative drugs cause a discriminable decrease in the function of the CNS. Different types of sedatives can be discriminated from each other. Indole and phenylethylamine hallucinogens have potent discriminative stimulus properties, which are related to the actions of biogenic amine neurotransmitters in the CNS.

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

Though interoceptive properties of antidepressants have been described, discriminative stimulus (DS) properties of mirtazapine, which does not affect monoamine reuptake, remain uncharacterized. 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. Using a two-lever, fixed-ratio 10 schedule, rats were trained to discriminate mirtazapine (2.5 mg/kg, i.p.) from saline. 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. 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.

The role of M1 muscarinic cholinergic receptors in the discriminative stimulus properties of N-desmethylclozapine and the atypical antipsychotic drug clozapine in rats

The discriminative stimulus properties of clozapine (CLZ) have been studied for decades because it remains the prototype for atypical antipsychotic drug effects and yet is unique in many ways, including increased efficacy in treatment-resistant schizophrenia and in reducing suicidality. Recent studies have indicated that the active CLZ metabolite N-desmethylclozapine (NDMC) may play a role in mediating the cognitive efficacy of CLZ and may also have atypical antipsychotic properties. The present study sought to determine if NDMC has discriminative stimulus properties similar to that of its parent drug CLZ. Rats were trained to discriminate 1.25 mg/kg CLZ from vehicle in a two-choice drug discrimination task. Although NDMC (2.5-20.0 mg/kg) failed to substitute for CLZ, the combination of NDMC (5.0 and 10.0 mg/kg) with a low dose (0.3125 mg/kg) of CLZ produced full substitution (>80% CLZ-appropriate responding) for the 1.25 mg/kg CLZ training dose. Co-administration of the M1-preferring receptor antagonist trihexyphenidyl (6.0 mg/kg) with a 5.0 mg/kg dose of NDMC produced partial substitution (>60% to <80% CLZ-appropriate responding) for CLZ, while administration of trihexyphenidyl alone (0.3-12.0 mg/kg) failed to substitute for CLZ. These findings suggest that NDMC produces discriminative stimulus effects that are different from those elicited by its parent drug CLZ. This difference may be due to the agonist properties of NDMC at M(1) muscarinic cholinergic receptors.

Ethanol-induced alterations of c-Fos immunoreactivity in specific limbic brain regions following ethanol discrimination training

The discriminative stimulus properties of ethanol are functionally regulated by ionotropic GABA A and NMDA receptors in specific limbic brain regions including the nucleus accumbens, amygdala, and hippocampus, as determined by microinjection studies. The purpose of the present work was to further investigate potential neural substrates of ethanol's discriminative stimulus effects by examining if ethanol discrimination learning produces changes in brain regional response to ethanol. To accomplish this goal, immunohistochemistry was used to assess the effects of ethanol (2 g/kg) on c-Fos immunoreactivity (Fos-IR). Comparisons in ethanol-induced Fos-IR were made between a group of rats that was trained to discriminate the stimulus properties of ethanol (2 g/kg, IG) from water (IG) and a drug/behavior-matched control group that did not receive differential reinforcement for lever selection, which precluded acquisition of discriminative stimulus control by ethanol. In some brain regions discrimination training had no effect on ethanol-induced Fos-IR changes (caudate putamen, bed nucleus of the stria terminalis, and CA1 region of the hippocampus). In contrast, discrimination training altered the pattern of ethanol-induced Fos-IR in the nucleus accumbens (core), medial septum, and the hippocampus (dentate and CA3). These results indicate that having behavior under the stimulus control of ethanol can change ethanol-induced Fos-IR in some brain regions. This suggests that learning about the subjective properties of ethanol produces adaptive changes in how the brain responds to acute ethanol exposure.

Discriminative stimulus properties of naloxone in Long–Evans rats: assessment with the conditioned taste aversion baseline of drug discrimination learning

The characterization of the discriminative stimulus properties of naloxone has focused primarily on its actions at the mu opioid receptor, although naloxone also displays an affinity for delta and kappa receptor subtypes. The present study extends this characterization of the naloxone cue by investigating if relatively specific antagonists for the mu (naltrexone: 0.10-0.56 mg/kg), delta (naltrindole: 1-18 mg/kg), and kappa (MR2266: 1.8-10 mg/kg) opioid receptor subtypes will substitute for naloxone in animals trained to discriminate naloxone from its vehicle. The temporal nature of the naloxone cue was examined by varying pretreatment time points (15, 30, 45, 60 min). Finally, various doses of naltrexone methobromide (1-18 mg/kg) were assessed to determine peripheral mediation of the cue. Female Long-Evans rats (N = 30) received an injection of naloxone (1 mg/kg; i.p.) 15 min prior to a pairing of saccharin (20-min access) and the emetic LiCl (1.8 mEq; i.p.; n = 16, group NL) or vehicle (n = 14, group NW); on other days, they were injected with saline prior to saccharin alone. Substitution tests with compounds with various receptor affinities and selective CNS and PNS actions were then assessed. Only naloxone and naltrexone produced dose-dependent decreases in saccharin consumption. Naloxone administered at 15 and 30 min before saccharin produced decreases in consumption similar to that displayed on training days. Naltrexone methobromide substituted only at the highest dose tested (18 mg/kg). Naloxone's stimulus effects appear to be mediated centrally via activity at the mu opioid receptor.

Effects of priming injections of MDMA and cocaine on reinstatement of MDMA- and cocaine-seeking in rats

Exposure to self-administered drugs is sufficient to produce drug-seeking in animal models. In many cases priming injections of drugs that share discriminative stimulus properties with the self-administered drug also can lead to drug-seeking, suggesting that exposure might precipitate relapse. The present investigation examined the ability of MDMA or cocaine priming injections to reinstate extinguished drug-seeking in rats. Priming injections of cocaine (0–20.0 mg/kg) and MDMA (0.0–10.0 mg/kg) reinstated extinguished drug-taking for both the cocaine- and MDMA-trained rats. In a separate group of cocaine-trained rats that received repeated exposure to 10.0 mg/kg MDMA, the initial exposure to MDMA (10.0 mg/kg, IP) failed to reinstate extinguished responding but MDMA became an effective prime for reinstatement of extinguished cocaine-taking behavior with repeated exposure. Effects of MDMA in MDMA-trained rats was greater than the effect in cocaine-trained rats suggesting that extensive experience with MDMA self-administration might have sensitized rats to this effect. These findings show that extinguished MDMA self-administration, like self-administration of other drugs of abuse, can be reinstated by exposure to psychostimulants thereby precipitating relapse.

Differential Involvement of the Basolateral Amygdala and Mediodorsal Thalamus in Instrumental Action Selection

Although it has been shown that the basolateral amygdala (BLA) and the mediodorsal thalamus (MD) are critical for goal-directed instrumental performance, much remains unknown about the respective contributions of these structures to action selection. The current study assessed the effects of post-training BLA and MD lesions on several tests of instrumental action selection. We found that MD damage disrupted the influence of pavlovian cues over action selection but left intact rats' ability to select actions based on either the expected value or the discriminative stimulus properties of the outcome. In contrast, BLA lesions impaired performance on all three tests of action selection. Because both lesion types disrupted the influence of cues that signal reward over instrumental performance, we then investigated the involvement of these structures in pavlovian contingency learning using a task in which the predictive status of one of two cues is degraded by delivering its outcome noncontingently during the intertrial interval. As expected, the sham group selectively suppressed their conditioned approach performance to the cue that no longer signaled its outcome but continued to respond to the control stimulus. In contrast, both lesioned groups were impaired on this task. Interestingly, whereas the MD group displayed a nonspecific reduction in responding to both cues, the BLA group continued to show high levels of responding to both cues as if their performance was completely insensitive to this contingency manipulation. These findings demonstrate that the BLA and MD make important yet distinct contributions to instrumental action selection.

Mice lacking fatty acid amide hydrolase have facilitated acquisition of anandamide discriminative stimulus,

Anandamide (AEA), an endocannabinoid (eCB), shares some, but not all, pharmacological properties with THC. This pharmacological dissociation is in part due to the rapid catabolism of AEA by fatty acid amide hydrolase (FAAH). With inhibitors of FAAH, studies have demonstrated that AEA shares THC's discriminative stimulus (DS) properties providing the leading evidence for the involvement of the eCB system in the subjective effects of marijuana. However, this remains to be fully elucidated without demonstrating that these DS effects are cross symmetrical. Thus, the current study investigated whether FAAH (−/−) mice, those lacking the ability to breakdown AEA, could be trained to discriminate AEA (6 mg/kg) versus VEH in a 2‐lever discrimination. AEA was successfully trained and dose dependently generalized. THC fully and dose dependently substituted whereas oleamide, another FAA, failed to substitute. THC's and AEA's DS effects were blocked by SR141716. This study demonstrates that: AEA's DS can be trained in FAAH (−/−) mice, THC and AEA cross generalize, and these effects are CB1 receptor mediated. Oleamide's lack of AEA‐like DS effects provides further support for the specificity of this model. Taken together, this research demonstrates that THC or AEA activation of the eCB system results in similar DS effects. Research supported by NIH grant DA‐09789.

The hallucinogen derived from Salvia divinorum, salvinorin A, has κ-opioid agonist discriminative stimulus effects in rats

Data from clinical and preclinical studies converge implicating the plant-derived hallucinogen salvinorin A as an important pharmacologic tool; this psychoactive compound may expand scientific understandings on mammalian κ-opioid receptor systems. Human salvinorin A effects, consistent with κ-opioid receptor agonism, include antinociception, sedation, dysphoria and distorted perceptions. The experiments reported here measured salvinorin A (1–3 mg/kg, i.p.) discriminative stimulus properties in male Sprague–Dawley rats conditioned to recognize the discriminative stimulus cue generated by the well characterized κ-opioid agonist U-69593 (0.56 mg/kg, i.p.). At three distinct active doses, salvinorin A fully substituted for U-69593 without altering response rates. The lever choice pattern in U-69593 trained animals reverted to vehicle lever responding when a kappa selective antagonist compound, nor-BNI (4.5 nM, i.c.v.) was administered 1 h prior to salvinorin A, yet nor-BNI alone failed to impact the rate or pattern of subject responses. These findings confirm and extend results published after similar drug discrimination tests were performed in rhesus monkeys. The discussion section of this article highlights public concern over salvinorin A misuse and emphasizes several potential pharmacotherapeutic applications for salvinorin A or analogue compounds.

Modafinil evokes striatal [ 3H]dopamine release and alters the subjective properties of stimulants

Modafinil is a mild psychostimulant used for the treatment of sleep and arousal-related disorders, and has been considered a pharmacotherapy for cocaine and amphetamine dependence; however, modafinil's mechanism of action is largely unclear. The present study investigated modafinil using drug discrimination and slice superfusion techniques. Rats were trained to discriminate cocaine (1.6 or 5 mg/kg) or amphetamine (0.3 mg/kg) from saline injection for food reinforcement. Modafinil (64–128 mg/kg) substituted partially for both cocaine doses and amphetamine. Pretreatment with a lower modafinil dose (32 mg/kg) augmented the discriminative stimulus properties of cocaine (1.6 mg/kg dose group) and amphetamine. In neurochemical experiments, modafinil (100–300 μM) evoked [ 3H]overflow from rat striatal slices preloaded with [ 3H]dopamine in a concentration-dependent manner; however, modafinil was less potent and efficacious than amphetamine and nicotine. The dopamine transporter inhibitor nomifensine (10 μM) blocked modafinil-evoked [ 3H]overflow, and concentrations of modafinil (< 100 μM) that did not have intrinsic activity attenuated amphetamine (1 and 3 μM)-evoked [ 3H]overflow. Modafinil-evoked [ 3H]overflow was not altered by the nicotinic acetylcholine receptor antagonist mecamylamine, and modafinil did not alter nicotine-evoked [ 3H]overflow, indicating that nicotinic acetylcholine receptors likely are not important for modafinil's mechanism of action. The present results indicate that modafinil evokes dopamine release from striatal neurons and is a psychostimulant that is pharmacologically similar to, but much less potent and efficacious than, amphetamine.

Selective reinstatement of instrumental performance depends on the discriminative stimulus properties of the mediating outcome

We conducted three experiments to investigate the associative structure underlying the reinstatement of instrumental performance after extinction. In each experiment, rats were initially rewarded on two responses with different outcomes. At test, both responses were extinguished in order to assess the impact of a single noncontingent outcome delivery on response selection. Experiment 1 found evidence of outcome-selective reinstatement (i.e., more responses were performed on the lever that was trained with the reinstating outcome than on the other lever). Experiment 2 demonstrated that the outcome's capacity to reinstate performance was not affected by a reduction in its motivational value. Experiment 3 found evidence that the reinstating outcome selectively retrieved the response it signaled rather than the response it followed during training. Together, these findings are consistent with the view that instrumental reinstatement depends on the discriminative stimulus properties of the reinstating outcome.

Discriminative stimulus properties of the selective norepinephrine reuptake inhibitor, reboxetine, in rats: a characterization with α/β-adrenoceptor subtype selective ligands, antidepressants, and antagonists at neuropeptide receptors

Although little information is available concerning discriminative stimulus (DS) properties of antidepressants, rats can be trained to recognize the selective norepinephrine (NE) reuptake inhibitor, reboxetine (2.5 mg/kg i.p.). By analogy to reboxetine (effective dose50, 1.1), 'full' (80%) substitution dose50 was obtained with the NE reuptake inhibitors, nisoxetine (4.9), nomifensine (0.5) and BW1555,U88 (1.0). Full substitution was also attained with the NE/serotonin (5-HT) reuptake inhibitors, S33005 (0.3), venlafaxine (4.8) and duloxetine (26.8), and the tricyclics, imipramine (2.5) and clomipramine (2.9). In contrast, the 5-HT reuptake inhibitors, citalopram, sertraline and paroxetine (all >2.5), and the 5-HT reuptake inhibitors/5-HT2 receptor antagonists, nefazodone and trazodone (both >10.0), did not substitute for reboxetine. The 'atypical' antidepressants, mirtazapine (>10.0) and mianserin (>2.5), similarly failed to substitute. DS properties of reboxetine were dose-dependently blocked by the alpha1-adrenoceptor (AR) antagonists, prazosin (inhibitory dose50, 0.3) and WB4101 (0.5), but resistant to the alpha2-AR antagonists, atipamezole (>0.63), idazoxan (>2.5) and RX821,002 (>0.08), and to the beta1-AR and beta2-AR antagonists, betaxolol (>2.5) and ICI118,551 (>10.0). Interestingly, the neurokinin-1 receptor antagonist, GR205,171, stereospecifically substituted for reboxetine (1.1) compared to its less active isomer, GR226,206 (>10.0). The corticotrophin-releasing factor-1 antagonists, DMP695 (>40), CP154,526 (>10.0) and SN003 (>40.0), and the melanin-concentrating hormone-1 antagonist, SNAP-7941 (>40.0), failed to substitute for reboxetine. In conclusion, DS properties of reboxetine are mimicked by antidepressants recognizing NE transporters, and require functionally intact alpha1-ARs for their expression. The neurokinin-1 antagonist, GR205,171, mimics the interoceptive properties of reboxetine, possibly reflecting its elevation of extracellular levels of NE in corticolimbic structures.

5-Hydroxytryptamine (Serotonin)2AReceptors in Rat Anterior Cingulate Cortex Mediate the Discriminative Stimulus Properties ofd-Lysergic Acid Diethylamide

d-Lysergic acid diethylamide (LSD), an indoleamine hallucinogen, produces profound alterations in mood, thought, and perception in humans. The brain site(s) that mediates the effects of LSD is currently unknown. In this study, we combine the drug discrimination paradigm with intracerebral microinjections to investigate the anatomical localization of the discriminative stimulus of LSD in rats. Based on our previous findings, we targeted the anterior cingulate cortex (ACC) to test its involvement in mediating the discriminative stimulus properties of LSD. Rats were trained to discriminate systemically administered LSD (0.085 mg/kg s.c.) from saline. Following acquisition of the discrimination, bilateral cannulae were implanted into the ACC (AP, +1.2 mm; ML, +/-1.0 mm; DV, -2.0 mm relative to bregma). Rats were tested for their ability to discriminate varying doses of locally infused LSD (0.1875, 0.375, and 0.75 microg/side) or artificial cerebrospinal fluid (n = 3-7). LSD locally infused into ACC dose-dependently substituted for systemically administered LSD, with 0.75 microg/side LSD substituting completely (89% correct). Systemic administration of the selective 5-hydroxytryptamine (serotonin) (5-HT)(2A) receptor antagonist R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol (M100907; 0.4 mg/kg) blocked the discriminative cue of LSD (0.375 microg/side) infused into ACC (from 68 to 16% drug lever responding). Furthermore, M100907 (0.5 microg/microl/side) locally infused into ACC completely blocked the stimulus effects of systemic LSD (0.04 mg/kg; from 80 to 12% on the LSD lever). Taken together, these data indicate that 5-HT(2A) receptors in the ACC are a primary target mediating the discriminative stimulus properties of LSD.

Discriminative stimulus properties of S32504, a novel D3/D2 receptor agonist and antiparkinson agent, in rats: attenuation by the antipsychotics, aripiprazole, bifeprunox, N-desmethylclozapine, and by selective antagonists at dopamine D2 but not D3 receptors

Drug-discrimination studies have proven instructive in the characterization of psychotropic agents, a procedure applied herein to the novel antiparkinson agent, S32504. This highly selective agonist at dopamine D(3) and (less potently) D(2) receptors displays potent antiparkinson, neuroprotective and antidepressant properties (Millan et al., J Pharmacol Exp Ther 309:936-950, 2004a; Millan et al., J Pharmacol Exp Ther 309:903-920, 2004b; Millan et al., J Pharmacol Exp Ther 309:921-935, 2004c). To generate a discriminative stimulus (DS) with S32504 and undertake substitution/antagonism studies with diverse antiparkinson and antipsychotic agents. Using a two-lever, fixed-ratio 10 schedule, rats were trained to recognize S32504 (0.04 mg/kg, s.c.) from saline. S32504 displayed dose-dependent and stereospecific substitution in comparison to its less active racemic form, (+/-) S31411, and to its inactive (-) distomer, S32601. Apomorphine, and the selective D(3)/D(2) receptor agonists, ropinirole, PD128,907, 7-OH-DPAT and CGS15855A, fully (=80%) substituted for S32504, whereas D(4) and D(1)/D(5) receptor agonists were ineffective. The selective D(3) vs D(2) receptor partial agonist, BP897, did not substitute for S32504 and the selective D(3) receptor antagonists, S33084, SB277,011, GR218,231, PNU99194A and S14297, did not block its DS properties. By contrast, S32504 lever selection was blocked by the preferential D(2) vs D(3) receptor antagonists, L741,626 and S23199, and by the D(2)/D(3) antagonists, raclopride and haloperidol. The D(2)/D(3) receptor partial agonists and antipsychotics, aripiprazole, bifeprunox, N-desmethylclozapine and preclamol did not substitute for S32504: indeed, they dose-dependently attenuated its DS properties. The antiparkinson agent, S32504, displays DS properties principally mediated by high-efficacy activation of D(2) receptors Antipsychotics known to act as partial agonists at D(2)/D(3) receptors attenuate DS properties of S32504, actions reflecting their low efficacy at these sites.

MGlu 5 receptors are involved in the discriminative stimulus effects of self-administered ethanol in rats

Recent work has identified a role for metabotropic glutamate receptor subtype 5 (mGlu 5) in the discriminative stimulus properties of investigator-administered ethanol. The purpose of this study was to determine if mGlu 5 receptors modulate the discriminative stimulus properties of self-administered ethanol. Results show that the mGlu 5 receptor antagonist 6-Methyl-2-(phenylethynyl)pyridine (MPEP; 10 mg/kg) inhibited the discriminative stimulus properties of consumed ethanol during a self-administration test session. Further, 10 mg/kg MPEP increased and 1 mg/kg MPEP decreased the amount of self-administered ethanol required to produce full substitution. These results indicate that mGlu 5 receptors are involved in the expression of the discriminative stimulus properties of self-administered ethanol.

Review of the Pharmacology and Clinical Profile of Bupropion, an Antidepressant and Tobacco Use Cessation Agent

Bupropion hydrochloride ((+/-)-2-tert-butylamino)-3'-chloropropiophenone x HCl) is a nonselective inhibitor of the dopamine transporter (DAT) and the norepinephrine transporter (NET) and is also an antagonist at neuronal nicotinic acetylcholine receptors (nAChRs). In animal models used commonly to screen for antidepressant activity, bupropion shows a positive response. Also using animal models, bupropion has been shown to attenuate nicotine-induced unconditioned behaviors, to share or enhance discriminative stimulus properties of nicotine and to have a complex effect on nicotine self-administration, i.e., low doses augmenting nicotine self-administration and high doses attenuating self-administration. Current studies show that bupropion facilitates the acquisition of nicotine conditioned place preference in rats, further suggesting that bupropion enhances the rewarding properties of nicotine. Bupropion has been shown to attenuate the expression of nicotine withdrawal symptoms in both animal models and human subjects. With respect to relapse, current studies show that bupropion attenuates nicotine-induced reinstatement in rats, but large individual differences are apparent. Clinically, bupropion is used as a treatment for two indications, as an antidepressant, the indication for which it was developed, and as a tobacco use cessation agent. In clinical trials, bupropion is being tested as a candidate treatment for psychostimulant drug abuse, attention-deficit hyperactivity disorder (ADHD) and obesity. Bupropion is available in three bioequivalent oral formulations, immediate release (IR), sustained release (SR), and extended release (XL). Extensive hepatic metabolism of bupropion produces three pharmacologically active metabolites, which may contribute to its clinical profile.

Discriminative stimulus properties of the selective and highly potent α 2-adrenoceptor agonist, S18616, in rats: Mediation by the α 2A subtype, and blockade by the atypical antidepressants, mirtazapine and mianserin

The novel spiroimidazoline, S18616, a potent and efficacious agonist at α 2-adrenoceptors (ARs), shows >100-fold selectivity versus α 1-ARs, imidazoline receptors and all other sites examined. Herein, we characterized its discriminative stimulus (DS) properties in rats trained to recognise S18616 (0.01 mg/kg, s.c.) from saline. S18616 dose-dependently (0.0063–0.01) and “fully” (≥80% “S18616” lever selection) substituted for itself. Full substitution was also acquired for the agonist, UK14,304 (0.04–0.16), while the partial agonist, clonidine (0.01–0.08), yielded sub-maximal substitution (67%). Guanfacine (0.16–1.25) and guanabenz (0.00063–0.04), preferential agonists at α 2A-ARs, revealed full substitution for S18616. In contrast, the α 1-AR agonists, cirazoline and ST587 (both 0.04–0.63), did not substitute. The α 2-AR antagonists, RX821,002, atipamezole (both 0.0025–0.04) and idazoxan (0.04–0.63) blocked the S18616 DS, whereas the α 1-AR antagonists, prazosin (0.16–0.63) and WB4101 (0.04–0.63), were inactive. Prazosin is also a preferential antagonist at α 2B/2C- versus α 2A-ARs and a further preferential α 2B/2C-AR antagonist, BRL41,992 (0.63–2.5), was likewise ineffective. In contrast, the α 2A-AR antagonist, BRL44,408 (0.04–0.16), dose-dependently abolished the S18616 DS. Finally, the “atypical” antidepressants, mirtazapine (0.16–10.0) and mianserin (0.63–10.0), which behave as antagonists at α 2A-ARs, dose-dependently blocked the S18616 DS. In conclusion, S18616 elicits a robust DS in rats that principally reflects engagement of α 2A-ARs. This novel procedure should prove useful in the characterisation of psychoactive drugs which interact with α 2-ARs.

Effects of ethanol on nicotine-induced conditioned place preference in C57BL/6J mice

It has been shown that small doses of ethanol (≤ 1.0 g/kg) may antagonize the discriminative stimulus properties of nicotine. The aim of the present study was to evaluate whether ethanol could antagonize nicotine's rewarding effects in the conditioned place preference procedure. For comparison, effects of ethanol on nicotine-induced seizures were assessed. Male C57BL/6J mice were used in all experiments. Lower doses of nicotine (0.3 and 0.6 mg/kg, s.c.) induced significant conditioned place preference, while higher doses (0.9 and 1.2 mg/kg) induced neither conditioned place preference nor conditioned place aversion. In the following experiments, ethanol (0.5 or 1.0 g/kg, i.p.) was administered 5 min before 0.3 mg/kg nicotine. Ethanol did not antagonize nicotine-induced conditioned place preference. Contrary to our hypothesis, a non-significant ( p = 0.07) enhancement of nicotine-induced place preference conditioning was observed in mice pre-treated with 1.0 g/kg ethanol. Both doses of ethanol (0.5 and 1.0 g/kg) suppressed seizures elicited by a high dose of nicotine (6.0 mg/kg). Ethanol totally eliminated clonic–tonic component of nicotine-induced seizures. Maximal blood ethanol levels after i.p. administration of 0.5 or 1.0 g/kg ethanol exceeded 60 and 115 mg%, respectively. The present results may indicate that the rewarding and seizure-inducing effects of nicotine are differentially modulated by clinically relevant concentrations of ethanol in mice.

Effects of dopamine D1 receptor agonists in rats trained to discriminate dihydrexidine

The full D1 receptor agonist dihydrexidine (DHX) [(+/-)-trans-10,11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenanthridine hydrochloride] is under clinical development (DAR-100) for Parkinson's disease and schizophrenia. Despite the clinical development of DHX, very little is known about its discriminative stimulus properties in rats. To more fully characterize the discriminative stimulus properties of DHX, we trained rats to discriminate DHX (3 mg/kg, i.p.) from vehicle. Substitution tests in rats discriminating DHX (3 mg/kg, i.p.) from vehicle were performed with structurally distinct D1 receptor agonists with both partial and full intrinsic efficacy. In addition, the peripherally restricted D1 agonist, fenoldopam, was evaluated. SKF 75670A, ABT-431, dinapsoline, SKF 81297, and SKF 82958 all fully substituted in a dose-dependent manner. The rank order of potency for substitution was SKF 82958<ABT-431<SKF 75670A<or=dinapsoline<SKF 81297<DHX. Fenoldopam (10 and 30 mg/kg) did not substitute and was without effect on rates of responding. DHX produces prominent dopamine D1 receptor agonist effects in vivo and is likely to produce subjective effects in humans similar to other D1 receptor agonists.

Nicotine-induced conditioned taste aversion in the rat: Effects of ethanol

It has been shown that small doses of ethanol antagonise the discriminative stimulus properties of nicotine in the rat. The aim of the present study was to evaluate whether ethanol could antagonise the aversive stimulus effects of nicotine. Wistar rats were trained to associate nicotine injections with a novel tasting fluid (0.1% saccharin) in the conditioned taste aversion procedure. Nicotine (0.3 mg/kg, s.c.) was injected 5 min after the end of a 20-min exposure to the saccharin solution. Ethanol (0.25–0.5 g/kg, i.p.) was administered 5 or 50 min before nicotine. In general, ethanol did not inhibit nicotine-induced conditioned taste aversion. Contrary to the findings in drug discrimination studies, a slight but significant enhancement of nicotine-induced taste aversion conditioning was observed after ethanol pre-treatment. Blood ethanol levels were measured in a separate group of rats. Maximal blood ethanol levels after i.p. administration of 0.25 or 0.5 g/kg ethanol exceeded 20 and 80 mg%, respectively. Concluding, the present results may indicate that ethanol does not attenuate nicotine-induced conditioned taste aversion in the rat.