Sex differences in the discriminative stimulus characteristics of a morphine occasion setter in rats.

The current study investigated whether the stimulus effects of morphine can function as a positive and negative feature in a Pavlovian occasion setting drug discrimination preparation in male and female rats. Sprague-Dawley rats were assigned to a feature positive (FP) or feature negative (FN) training group and all received intermixed morphine (3.2 mg/kg, IP) or saline injections 15 min before 20-min daily training sessions. For FP rats, on morphine sessions, each of eight 15-s white noise (WN) presentations was followed by 4-s access to sucrose (0.01 ml, 26% w/v); on saline sessions, sucrose was withheld. FN rats learned the reverse contingency. FP discrimination was acquired somewhat sooner than FN discrimination, and females, but not males, became sensitized to the locomotor effects of morphine, which did not influence conditioned responding. Rats then entered dose generalization testing. There was no sex difference in dose generalization for FN groups (ED50 1.26 for males and 1.57 for females). Yet for FP rats, the dose response curve for females was shifted to the right compared to males (ED50 0.54 for males and 1.94 for females). FP females exhibited enhanced responding at a dose higher than that of their original training. These findings reveal the need to reassess our notions of drug stimuli that guide appropriate associative behaviours from the perspective of sex differences.

The discriminative stimulus effects of the neurotensin NTS1 receptor agonist PD149163 in rats: stimulus generalization testing with dopamine D1 and D2 receptor ligands.

Brain-penetrant neurotensin NTS1 receptor agonists produce antipsychotic drug-like effects in animal models, including inhibition of conditioned avoidance responding and reversal of psychostimulant-induced hyperactivity and stereotypy. Allosteric interactions between NTS1 receptors and dopamine D2 receptors may account for some of these antipsychotic effects. In order to determine the role that dopamine receptors may play in the behavioral effects produced by activation of NTS1 receptors, a drug discrimination approach was used in rats to evaluate the potential mediation of NTS1 receptor agonist stimulus effects by dopamine D1 and D2 receptors. Rats were trained to discriminate either the NTS1 receptor agonist PD149163, the D1 receptor agonist SKF81297, or the D2 receptor agonist quinpirole from vehicle in a two choice drug discrimination task. Full stimulus generalization occurred from PD149163 to the typical antipsychotic drug and D2 receptor-preferring antagonist haloperidol. However, stimulus generalization did not occur from SKF81297 or quinpirole to PD149163. The discriminative cue for SKF91297 and quinpirole was fully blocked the D1 receptor antagonist SCH23390 and the D2/3 receptor antagonist raclopride, respectively. Cross generalization did not occur between SKF91297 and quinpirole. Based on these findings, the stimulus effects of PD149163 may be mediated, in part, through D2 receptor antagonism, but this may only be evident when PD149163 is used as the training drug.

Salvinorin B derivatives, EOM-Sal B and MOM-Sal B, produce stimulus generalization in male Sprague-Dawley rats trained to discriminate salvinorin A.

Salvinorin A, the main active component of Salvia divinorum, is a potent and selective κ opioid receptor agonist. Synthetic derivatives of this substance may be useful in the development of medicinal treatments for pain, mood disorders, and drug dependence. Such developments require extensive preclinical screening of these compounds. The drug discrimination assay is a valuable method for exploring potential similarities between novel compounds and known drugs of abuse with respect to their interoceptive stimulus properties, and can be used to investigate the potency of salvinorin A and its derivatives in vivo. This study used drug discrimination methods to compare two synthetic derivatives of salvinorin B, the ethoxymethyl ether (EOM-Sal B) and methoxymethyl ether (MOM-Sal B) with salvinorin A. Male Sprague-Dawley rats were trained to discriminate 2.0 mg/kg of salvinorin A from its vehicle (75% dimethylsulfoxide/25% water) in a fixed ratio 20 food-reinforced drug discrimination procedure, and were tested for stimulus generalization with EOM-Sal B and MOM-Sal B. For comparison, substitution tests were also conducted with a µ agonist, morphine, a dissociative hallucinogen, ketamine, and two serotonergic hallucinogens, D-lysergic diethylamide (LSD) and 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane. Time-course tests were also conducted with salvinorin A and EOM-Sal B. Both EOM-Sal B and MOM-Sal B substituted fully for salvinorin A and displayed greater potency than salvinorin A. EOM-Sal B was discriminated at longer postinjection intervals than salvinorin A. Morphine and 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane failed to substitute for salvinorin A, although ketamine and LSD produced significant drug-appropriate responding. The current findings are consistent with previous reports that salvinorin A produces detectable stimulus effects that are distinct from those of other drug classes and, for the first time, establish that synthetic derivatives of this substance produce similar discriminative stimulus effects. The unexpected partial substitution with LSD and ketamine indicate that further preclinical studies of these novel κ opioid receptor agonists may be warranted.

Dissimilar cannabinoid substitution patterns in mice trained to discriminate Δ(9)-tetrahydrocannabinol or methanandamide from vehicle.

Δ(9)-Tetrahydrocannabinol (THC) discrimination in rodents is a behavioral assay that has been used to probe differences among classes of cannabinoids in rats. The purpose of this study was to determine whether traditional and anandamide-like cannabinoids were distinguishable in cannabinoid discrimination procedures in mice. Male mice were trained to discriminate 30 mg/kg THC or 70 mg/kg methanandamide from vehicle in a two-lever milk-reinforced drug discrimination procedure. After acquisition, agonist tests with THC, methanandamide, CP 55940, and anandamide were conducted, as were antagonism tests with rimonabant. Substitution (agonism) and antagonism tests were also carried out in female mice trained to discriminate THC. THC and CP 55940 fully substituted in THC-trained mice of both sexes. Further, THC substitution was rimonabant reversible. In contrast, mice injected with methanandamide or anandamide failed to respond substantially on the THC lever, even up to doses that decreased overall responding. In methanandamide-trained mice, methanandamide fully generalized to the methanandamide training dose. Rimonabant did not reverse this generalization. Although THC, CP 55940, and anandamide also increased responding on the methanandamide lever, the magnitude of substitution was less than for methanandamide. These results suggest incomplete overlap in the underlying mechanisms mediating endocannabinoid pharmacology and marijuana intoxication. Further, they suggest that methanandamide discrimination may involve a non-CB(1) receptor mechanism that is particularly prominent at higher doses.

The metabolites N-desmethylclozapine and N-desmethylolanzapine produce cross-tolerance to the discriminative stimulus of the atypical antipsychotic clozapine in C57BL/6 mice.

It has been previously shown that cross-tolerance to the discriminative stimulus properties of clozapine can be demonstrated with the drug discrimination paradigm. This study examined the ability of N-desmethylclozapine and N-desmethylolanzapine (metabolites of the atypical antipsychotic drugs clozapine and olanzapine, respectively) to induce cross-tolerance to the discriminative stimulus effects of clozapine. After C57BL/6 mice were trained to reliably discriminate 2.5 mg/kg clozapine from vehicle, a clozapine generalization curve was generated. Next, training was suspended and the mice received a maintenance dosing regimen in which they were injected twice daily with 10 mg/kg N-desmethylclozapine for 10 days. Then a second clozapine generalization curve was generated. This was followed by a 10-day washout period during which the mice did not receive drug injections or discrimination training. Finally, a third clozapine generalization curve was generated. These same procedures were followed for N-desmethylolanzapine (10 mg/kg twice daily during maintenance dosing). Both N-desmethylclozapine and N-desmethylolanzapine produced significant rightward shifts in the clozapine generalization curve indicating cross-tolerance between N-desmethylclozapine and clozapine and between N-desmethylolanzapine and clozapine. After a washout period with no training or drug administration this cross-tolerance effect was lost for both metabolites. This cross-tolerance drug discrimination procedure demonstrated in-vivo similarities between these two metabolites and clozapine and suggests that common underlying pharmacological mechanisms were responsible for the cross-tolerance that was observed. These findings also demonstrated that this procedure may be useful for identifying drugs with therapeutic efficacy similar to the atypical antipsychotic clozapine under repeated dosing conditions.

Acetylcholinesterase inhibitors partially generalize to nicotine discriminative stimulus effect in rats.

Acetylcholinesterase inhibitors (AChE-Is), galantamine, physostigmine and tacrine, enhance central levels of synaptic acetylcholine. Galantamine and physostigmine, but not tacrine, exhibit allosteric potentiation ligand (APL) properties on nicotinic acetylcholine receptors. The purpose of this study was to investigate whether AChE-Is with nicotinic acetylcholine receptor-positive allosteric modulator properties generalize to the discriminative stimulus effect of nicotine in rats. A two-lever operant conditioning paradigm was used in which rats were trained to discriminate nicotine (0.2 mg/kg/ml intraperitoneally) from saline. After training, generalization tests were carried out with galantamine (1, 3 or 5 mg/kg intraperitoneally), physostigmine (0.05, 0.1 or 0.2 mg/kg subcutaneously) or tacrine (0.625, 1.25 or 2.5 mg/kg subcutaneously) given as a single presession administration. Galantamine, physostigmine or tacrine produced a partial generalization of 62.2%, 55.1% or 43.6% , respectively, on the nicotine-appropriate lever compared with 100% partial generalization induced by the nicotine-training dose. High doses of galantamine, physostigmine or tacrine produced small but significant decreases in operant response rate, suggesting a possible underestimation of the degree of generalization. Our study showed that these three AChE-Is partially generalized to the nicotine stimulus without a significant distinction between AChE-Is with or without APL properties. These findings suggest that the APL property is not necessary for inducing nicotine-like effects in vivo in this behavioural paradigm.

Clozapine and N-methyl-D-aspartate have positive modulatory actions on their respective discriminative stimulus properties in C57BL/6 mice.

The impairment of N-Methyl-D-Aspartate receptors is thought to contribute to negative symptoms and cognitive deficits. In vitro studies suggest that atypical antipsychotic drugs like clozapine may help to alleviate these deficits by enhancing glutamatergic function. The present study examined the in vivo interaction of clozapine with N-Methyl D-aspartate by training one group of C57BL/6 mice to discrimination 2.5 mg/kg clozapine from vehicle and another group to discriminate 30 mg/kg N-Methyl D-aspartate from vehicle in a two-lever drug discrimination task. Cross-generalization testing revealed that N-Methyl D-aspartate (3-56 mg/kg) failed to substitute for clozapine in the clozapine-trained mice, while clozapine (0.625 mg/kg) produced partial substitution in the N-Methyl D-aspartate-trained mice. Interestingly, administration of a low, non-generalizing dose of each training drug in combination with the full range of doses of the alternate training drug produced full and dose-dependent substitution in both clozapine- and N-Methyl D-aspartate-trained mice. The α(1) antagonist prazosin fully and dose-dependently substituted for both clozapine and N-Methyl D-aspartate. These results suggest that the shared discriminative stimulus properties between clozapine and N-Methyl D-aspartate may be mediated through indirect mechanisms, possibly in part through α(1) adrenergic antagonism.

The selective dopamine uptake inhibitor, D-84, suppresses cocaine self-administration, but does not occasion cocaine-like levels of generalization.

A successful replacement pharmacotherapy for treating cocaine dependency would likely reduce cocaine's abuse, support a low abuse liability, overlap cocaine's subjective effects, and have a long duration of action. Inhibitors with varying selectivity at the dopamine transporter (DAT) have approximated these properties. The objective of the present study was to characterize the behavioural effects of an extremely selective DAT inhibitor, (+) trans-4-(2-Benzhydryloxyethyl)-1-(4-fluorobenzyl) piperadin-3-ol (D-84), a 3-hydroxy substituted piperidine derivative of GBR-12935, for its cocaine-like discriminative stimulus effects, its effects on cocaine self-administration, and for its own self-administration. During cocaine discrimination tests, cocaine occasioned the 10 mg/kg cocaine training stimulus with an ED(50) value of 3.13 (1.54-6.34) mg/kg, and reduced response rates with an ED(50) value of 20.39 (7.24-57.44) mg/kg. D-84 incompletely generalized to the cocaine stimulus occasioning a maximal 76% cocaine-lever responding, while reducing response rates with lower potency than cocaine (ED(50)=30.94 (12.34-77.60) mg/kg). Pretreatment with D-84 (9.6-30.4 mg/kg) significantly (P<0.05) reduced cocaine intake at 17.1 mg/kg D-84 when cocaine was self-administered at 0.5 mg/kg/infusion, and at 30.4 mg/kg D-84 when cocaine was self-administered at 0.1, 0.5 .and 1.0 mg/kg/infusion. During self-administration tests with D-84 (0.1-1 mg/kg/infusion), numbers of infusions significantly exceeded vehicle levels at 0.3 mg/kg/infusion. These results show that D-84 pretreatment can decrease cocaine intake especially when high doses of cocaine are being self-administered. This observation, combined with its incomplete generalization to the cocaine discriminative stimulus and its reported long duration of action, provides a profile consistent with a potential replacement therapy for treating cocaine-abusing patients.

Reduced sensitivity to the locomotor-stimulant effects of cocaine is associated with increased sensitivity to its discriminative stimulus properties.

Outbred Long-Evans rats exhibit wide variation in their locomotor response to cocaine. Here, we investigated the relationship between these individual differences and interoceptive effects of cocaine in low cocaine responder (LCR) and high cocaine responder (HCR) phenotypes. Rats were trained to discriminate cocaine (10.0 mg/kg, intraperitoneally) from saline by repeated pairings of injections with one of two response levers. In subsequent tests for stimulus generalization to other cocaine doses (1.25-15.0 mg/kg), LCRs exhibited partial-to-full generalization at 1.85 and 2.5 mg/kg cocaine, respectively, whereas HCRs did not. When the selective 5-HT reuptake inhibitor fluoxetine (5.0 mg/kg) was coadministered with saline or different cocaine doses, we observed similar upward shifts in dose-response in both phenotypes. In contrast, coadministration of the 5-HT2A/2C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 0.3 mg/kg) led to partial substitution of DOI for cocaine and enhancement of the stimulus properties of 1.25 mg/kg cocaine in LCRs only. Finally, a retest of cocaine-induced locomotion after discrimination testing revealed marked behavioral sensitization in LCRs and modest changes in behavior in HCRs. Taken together, these results suggest that initial sensitivity to the locomotor-stimulant effects of cocaine is inversely related to its interoceptive properties and that differences in 5-HT systems may contribute to the phenotypic differences observed.

Effect of 8-hydroxy-2-(N,N-di-n-propylamino)tetralin and MDMA on the discriminative stimulus effects of the classical hallucinogen DOM in rats.

Co-administration of the 5-HT1A serotonin receptor agonist (+/-)8-hydroxy-2-(N,N-di-n-propylamino)tetralin [(+/-)8-OH DPAT] enhances the discriminative stimulus effects of the classical hallucinogen 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) in rats. In the present investigation, using Sprague-Dawley rats trained to discriminate DOM (1.0 mg/kg) from saline vehicle under a VI-15 s schedule of reinforcement, it was shown that the stimulus-enhancing actions of 8-OH DPAT are related more to its R(+)-isomer than to its S(-)-enantiomer, and that the (+/-)- and R(+)8-OH DPAT-induced effects are antagonized by the 5-HT1A receptor antagonist NAN-190. (+/-)8-OH DPAT and its isomers substitute in rats trained to discriminate the designer drug N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA; methylenedioxymethamphetamine) from vehicle indicating some similarity of effect. On this basis, it was hypothesized that MDMA might be capable of enhancing the DOM stimulus. Co-administration of MDMA with low (i.e., 0.1 and 0.3 mg/kg) doses of DOM resulted in greater DOM-appropriate responding than engendered by administration of DOM alone. As such, the present findings are the first to demonstrate an MDMA-induced enhancing effect on the discriminative stimulus actions of a classical hallucinogen. The results also suggest that a 5-HT1A serotonin receptor mechanism might contribute to this phenomenon.

Effects of the nicotinic receptor partial agonists varenicline and cytisine on the discriminative stimulus effects of nicotine in rats.

The nicotinic partial agonist varenicline (VCL) is a recently approved medication for the treatment of tobacco dependence, yet very little preclinical research on this drug has been published. The present experiment examined the nicotinic partial agonist properties of VCL and its parent compound, cytisine (CYT), in a nicotine discrimination assay. Rats were trained to discriminate nicotine (0.4 mg/kg, s.c.) from saline using a two-lever discrimination procedure, followed by generalization and antagonism tests with VCL and CYT. Antagonism was examined across a range of nicotine doses. In generalization tests, VCL produced a maximum of 63% responding on the nicotine-appropriate lever, indicating partial generalization. In antagonism tests, VCL decreased the % responding on the nicotine-appropriate lever at 0.2 and 0.4 mg/kg nicotine, indicating antagonism of nicotine's discriminative stimulus effects. No dose of VCL produced significant effects on response rate. The two highest doses of CYT weakly substituted for nicotine, producing a maximum of 23% nicotine-appropriate responding. CYT produced a weak antagonism of the discrimination of moderate nicotine doses, but not of the training dose. These results demonstrate that VCL and CYT partially generalize to and partially antagonize nicotine's discriminative stimulus effects, consistent with a partial agonist mechanism of action.

Constructing quality profiles for taste compounds in rats: a novel paradigm.

We developed a novel behavioral method, adapted from the work by Morrison (1967), for the assessment of taste quality in rats. Four groups of rats were trained to discriminate a standard stimulus (either NaCl, sucrose, quinine, or citric acid, which are widely thought to represent the four basic human taste qualities of salty, sweet, bitter, and sour, respectively) from the remaining three compounds (each at multiple concentrations). Animals were then tested for generalization to the standard stimuli when test compounds were presented and a quality profile was constructed. Rats generalized novel concentrations of standard stimuli completely to their training concentrations and generalized their responses to mixtures of NaCl and sucrose on the basis of the relative concentrations of the stimulus components. In general, the sugars (at high concentrations), denatonium, tartaric acid, and sodium gluconate generalized to sucrose, quinine, citric acid, and NaCl, respectively. Monosodium glutamate generalized to a mixture of sucrose and NaCl. KCl produced a complex generalization profile with notable quinine and citric acid components. Our procedure supplements the current use of the conditioned taste aversion generalization procedure which has some procedural and interpretive limitations. Although our procedure involves the use of a complex stimulus delivery and response measurement apparatus and requires substantial initial conditioning of animals, once trained, a single cohort of animals can be tested for its response to a substantial number of test stimuli over the course of many months without any ostensible loss of stimulus control.

The role of comparison in perceptual learning: effects of concurrent exposure to similar stimuli on the perceptual effectiveness of their unique features.

In three experiments, rats were given concurrent exposure to a compound flavor (AX) and to one of the elements of the compound (X). The perceptual effectiveness of A was then assessed by a test involving generalization of a conditioned aversion. Comparison was made with a preexposure procedure in which the compound and the common element were presented on separate trials, either in alternation or in separate blocks of trials. The effectiveness of the unique cue was less after blocked preexposure than after either of the other procedures; concurrent preexposure did not produce a greater effect than did alternating preexposure. These results challenge the suggestion that concurrent preexposure engages a special comparison process that will facilitate this form of perceptual learning.

Discriminative stimulus effects of tiagabine and related GABAergic drugs in rats.

RATIONALE: Tiagabine is an anticonvulsant drug which may also have sleep-enhancing properties. It acts by inhibiting reuptake at the gamma-aminobutyric acid (GABA) transporter (GAT-1). OBJECTIVES: The aim of the study was to determine whether tiagabine acted as a discriminative stimulus and, if so, whether other GABAergic compounds would generalise to it. MATERIALS AND METHODS: Rats were trained to discriminate tiagabine (30 mg/kg p.o.) from vehicle, and generalisation to drugs that modulate GABA was assessed. RESULTS: Gaboxadol (5-20 mg/kg p.o.), a selective extrasynaptic GABA A agonist, generalised to tiagabine, although the extent of the generalisation was inconclusive. Indiplon (1 mg/kg p.o.), a benzodiazepine-like hypnotic, also partially generalised to tiagabine, although zolpidem and S-zopiclone did not. Baclofen, a GABA B receptor agonist, and gabapentin, which increases synaptic GABA, did not generalise to tiagabine. (+)-Bicuculline (3 mg/kg i.p.), a GABA A receptor antagonist, blocked the tiagabine cue, but the less brain-penetrant salt form, bicuculline methochloride, had no effect. CONCLUSIONS: These data suggest that tiagabine generates a discriminative stimulus in rats, and provides a central GABA-mediated cue, but is distinct from the other GABAergic compounds tested.

The conditional stimulus effects of nicotine vary as a function of training dose.

Although past research has shown that the interoceptive effects of nicotine serve as a conditional stimulus using sucrose as the unconditioned stimulus, very little is known about the importance of dose. Accordingly, rats were assigned to 0.1, 0.2, or 0.4 mg nicotine base/kg as the training dose. Sucrose (4-s access) was delivered 36 times on nicotine sessions; sucrose was withheld on intermixed saline sessions. The discrimination was acquired for all groups, as measured by more photobeam breaks in the dipper receptacle before the first sucrose delivery on nicotine sessions, compared with a similar interval on saline sessions. Thirty nicotine sessions without sucrose deliveries (extinction) decreased conditioned responding with the 0.4 mg/kg dose maintaining higher responding than the lower doses. After reestablishing discrimination performance, rats were tested with their training dose at various injection-to-placement intervals. Conditioned responding diminished with longer intervals; 0.4 mg/kg nicotine-evoked conditioned responding at longer intervals. Subsequent generalization testing with nicotine or saline at the 5-min training interval found that conditioned responding was evoked by lower test doses in the 0.1 mg/kg group than in the other groups. Combined, this research demonstrates that the nicotine conditional stimulus shows some variation with training dose.

Participation of the dorsal hippocampus in stimulus discrimination with scopolamine.

Stimulus discrimination is the capacity of an organism to differentiate between stimuli and emit associated responses. The administration of the muscarinic antagonist scopolamine can be used as a stimulus by mammals in a discrimination task. The present study analyzes the contribution of the hippocampus in scopolamine discrimination and generalization. Male Wistar rats, weighing 250-300 g at the beginning of the experiment, were trained to discriminate between scopolamine (1.0 mg/kg, i.p.) and saline administration using a two-lever operant task; rats had to respond differentially to each lever depending on the preceding drug or saline administration. Once stimulus control was attained, rats were tested with different scopolamine doses (0.0, 0.056, 0.091, 0.16, 0.31 and 1.0 mg/kg, i.p.) in order to obtain generalization curves. After generalization the rats were randomly assigned to hippocampal CA1 lesion or control groups. Hippocampus impairment produced a transient decrease in the capacity to discriminate between scopolamine and saline conditions; nonetheless, scopolamine correct responses were rapidly recovered after a few sessions and even maintained after 90 days. Correct responses for saline condition were never recovered. The generalization curve obtained after hippocampus lesion showed a response gradient severely flattened. Results suggest that the hippocampus participates as a neural system supporting the sensitivity to detect discrete changes in stimulus properties and relational memory, more than on the capacity to recall for simple associative responses.

Psilocybin-induced stimulus control in the rat.

Although psilocybin has been trained in the rat as a discriminative stimulus, little is known of the pharmacological receptors essential for stimulus control. In the present investigation rats were trained with psilocybin and tests were then conducted employing a series of other hallucinogens and presumed antagonists. An intermediate degree of antagonism of psilocybin was observed following treatment with the 5-HT(2A) receptor antagonist, M100907. In contrast, no significant antagonism was observed following treatment with the 5-HT(1A/7) receptor antagonist, WAY-100635, or the DA D(2) antagonist, remoxipride. Psilocybin generalized fully to DOM, LSD, psilocin, and, in the presence of WAY-100635, DMT while partial generalization was seen to 2C-T-7 and mescaline. LSD and MDMA partially generalized to psilocybin and these effects were completely blocked by M-100907; no generalization of PCP to psilocybin was seen. The present data suggest that psilocybin induces a compound stimulus in which activity at the 5-HT(2A) receptor plays a prominent but incomplete role. In addition, psilocybin differs from closely related hallucinogens such as 5-MeO-DMT in that agonism at 5-HT(1A) receptors appears to play no role in psilocybin-induced stimulus control.

The 5-HT3 receptor partial agonist MD-354 (meta-chlorophenylguanidine) enhances the discriminative stimulus actions of (+)amphetamine in rats.

The effect of the 5-HT3 receptor partial agonist MD-354 (meta-chlorophenylguanidine) was examined on the discriminative stimulus produced by (+)amphetamine. Using male Sprague-Dawley rats trained to discriminate 1.0 mg/kg (i.p.) of (+)amphetamine from saline vehicle (VI 15-s schedule of reinforcement) in a two-lever operant procedure for appetitive reward, tests of stimulus generalization (substitution) and antagonism showed that MD-354 neither substituted for, nor antagonized, the amphetamine stimulus at the doses evaluated. Administration of (+)amphetamine doses in combination with a fixed (i.e., 1.0 mg/kg) dose of MD-354 shifted the (+)amphetamine dose-response curve to the left such that, following 0.3 mg/kg of (+)amphetamine, stimulus generalization occurred. Furthermore, MD-354 doses of 0.1, 0.3 and 1.0 mg/kg, but not doses of 0.01, 0.5, 1.5 or 3.0 mg/kg (i.p.), administered in combination with the ED(50) dose (0.33 mg/kg) of (+)amphetamine resulted in stimulus generalization (i.e., >80% drug-appropriate responding). It is concluded that even though MD-354 lacks amphetamine-like central stimulant actions of its own it can modulate the discriminative stimulus effects of (+)amphetamine in rats.

Cycloheximide: no ordinary bitter stimulus.

Cycloheximide (CyX), a toxic antibiotic with a unique chemical structure generated by the actinomycete, Streptomyces griseus, has emerged as a primary focus of studies on mammalian bitter taste. Rats and mice avoid it at concentrations well below the thresholds for most bitter stimuli and T2R G-protein-coupled receptors specific for CyX with appropriate sensitivity are identified for those species. Like mouse and rat, golden hamsters, Mesocricetus auratus, also detected and rejected micromolar levels of CyX, although 1mM CyX failed to activate the hamster chorda tympani nerve. Hamsters showed an initial tolerance for 500microM CyX, but after that, avoidance of CyX dramatically increased, plasticity not reported for rat or mouse. As the hamster lineage branches well before division of the mouse-rat lineage in evolutionary time, differences between hamster and mouse-rat reactions to CyX are not surprising. Furthermore, unlike hamster LiCl-induced learned aversions, the induced CyX aversion neither specifically nor robustly generalized to other non-ionic bitter stimuli; and unlike adverse reactions to other chemosensory stimuli, aversions to CyX were not mollified by adding a sweetener. Thus, CyX is unlike other bitter stimuli. The gene for the high-affinity CyX receptor is a member of a cluster of five orthologous T2R genes that are likely rodent-specific; this "CyX clade" is found in the mouse, rat and probably hamster, but not in the human or rabbit genome. The rodent CyX-T2R interaction may be one of multiple lineage-specific stimulus-receptor interactions reflecting a response to a particular environmental toxin. The combination of T2R multiplicity, species divergence and gene duplication results in diverse ligands for multiple species-specific T2R receptors, which confounds definition of 'bitter' stimuli across species.

Behavioral and neuropharmacological characterization of nicotine as a conditional stimulus.

In rats, the pharmacological (interoceptive) effects of 0.4 mg/kg nicotine can serve as a conditional stimulus in a Pavlovian conditioning task. Nicotine administration is paired with intermittent access to a liquid sucrose unconditional stimulus; sucrose is withheld on saline sessions. An increase in sucrose receptacle entries (goal tracking) on nicotine sessions indicates conditioning. Rats were trained on a nicotine dose ((-)-1-Methyl-2-(3-pyridyl)pyrrolidine; 0.1, 0.2, or 0.4 mg base/kg, s.c.). Generalization was examined using 0.025, 0.05, 0.1, 0.2, and 0.4 mg/kg nicotine and saline. Some behavioral effects of nicotine have been attributed to dopamine and glutamate. Accordingly, potential blockade of the nicotine cue via the dopamine system was examined by administering (R)-(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390; 0.005, 0.01, and 0.03 mg/kg), 3-Chloro-5-ethyl-N-[[(2S)-1-ethyl-2-pyrrolidinyl)methyl]-6-hydroxy-2-methoxy-benzamide hydrochloride (eticlopride; 0.01, 0.03, 0.1, and 0.3 mg/kg), or N-[(1-Butyl-2-pyrrolidinyl)methyl]-4-cyano-1-methoxy-2-naphthalenecarboxamide (nafadotride; 0.03, 0.1, 0.3, 1, and 3 mg/kg) before nicotine. 2-Methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP; 0.3, 1, and 3 mg/kg) and (5S,10R)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801; 0.01, 0.03, 0.1, and 0.2 mg/kg; dizocilpine) were used to examine possible glutamatergic components. Substitution tests were conducted with MPEP and nafadotride. Differential conditioned responding was acquired in the 3 groups. Conditioned responding generally decreased as the nicotine test dose moved away from the training dose; responding increased when 0.4 mg/kg trained rats were tested with 0.2 mg/kg. SCH-23390, eticlopride, nafadotride, and MPEP decreased conditioned responding on nicotine at doses that also decreased chamber activity. In contrast, MK-801 decreased goal tracking on nicotine without decreasing chamber activity, indicating a role for N-methyl-D-aspartate receptors in expression of nicotine-evoked conditioned responding.