Amfonelic acid: similarity to other dopamine agonists

Rats were trained to discriminate between the stimulus properties of intraperitoneally administered 0.8 mg/kg amfonelic acid and its vehicle in a two-lever, food-motivated operant task. Once trained, rats showed a dose-related decrease in discriminative performance with lower amfonelic acid doses and analysis of the dose-response curve indicated an ED50 of 0.11 mg/kg. Administration of 0.08-0.6 mg/kg d-amphetamine produced a pattern of responding similar to that observed with amfonelic acid, with an ED50 of 0.10 mg/kg and a non-parallel dose-response curve. Likewise, the discriminative stimulus properties of amfonelic acid were shown to generalize to both d,l-cathinone and cocaine but not to apomorphine. The results suggest that amfonelic acid, as well as other non-amphetamine stimulants, acts by a different mechanism of action than does amphetamine and biochemical studies are reviewed to further evidence this observation.     

Effect of altering dopamine or serotonin neurotransmitters upon cathinone discrimination.

Rats were trained to discriminate between the stimulus properties of 0.8 mg/kg l-cathinone and its vehicle in a two-lever, food-motivated operant task. Once trained, rats showed a dose-related decrease in discriminative performance when tested with lower cathinone doses. An analysis of the dose-response curve indicated an ED50 value of 0.23 mg/kg. Pretreatment with CGS 10746B (5-20 mg/kg) resulted in a dose-related decrease in cathinone discrimination with the highest dose blocking cathinone discrimination. In contrast to the ability of this dopamine release inhibitor to decrease cathinone discrimination, pretreatment with three doses of the calcium channel blocker isradipine (2.5-10 mg/kg) or with the 5-HT3 antagonist MDL 72222 (0.1-0.4 mg/kg) had no effect upon cathinone discrimination. The results suggest that cathinone controls differential responding in a discriminative stimulus task by a mechanism involving presynaptic release of dopamine, which may not be regulated by either neuronal calcium influx through L-type calcium channels or by serotonergic neurons.     

Temporal parameters of cathinone, amphetamine and cocaine

Rats were trained to discriminate intraperitoneally administered 0.8 mg/kg 1-cathinone from its vehicle in a two-lever operant procedure. The normal injection-to-session interval was fifteen minutes. When tested in session at 2-180 min postadministration, cathinone discrimination was seen to have a rapid onset (5 minutes) and offset (60 minutes). When the same rats were tested with either 0.8 mg/kg d-amphetamine or 10.0 mg/kg cocaine at the same postinjection time periods, the peak discriminative generalization to each of these other psychostimulants was observed to be later, i.e., an onset of action at 15-30 minutes with a slightly longer duration of action. The results indicate that cathinone exerts discriminative response control within five minutes of intraperitoneal injection and that it has a shorter duration than amphetamine and cocaine.     

Cathinone, cocaine and methamphetamine: similarity of behavioral effects

The discriminative stimulus properties of (+/-)-cathinone were tested by training eight rats to discriminate between the interoceptive cues produced by 0.6 mg/kg (+/-)-cathinone and saline in a food-reinforced, two lever operant task. Doses of cocaine and methamphetamine were observed to transfer to the cathinone cue and all three drugs exhibited decreased discriminative performance with decreasing doses. The ED50 for (+/-)-cathinone, (+/-)-methamphetamine and cocaine were 0.23, 0.17, and 1.97 mg/kg, respectively, and the three curves were shown to be parallel. These data indicate the possibility of a common mechanism/site of action for these three stimulants, presumably by their actions upon central dopaminergic neurons.     

Effect of dopamine agonists and fenfluramine on discriminative behavior in obese and lean Zucker rats

Dopamine agonists and fenfluramine were used as pharmacological probes to investigate the possible difference in sensitivity and time course of drug action in genetically obese Zucker rats and their lean littermates. All rats were trained to discriminate between the stimulus properties of 0.6 mg/kg d-amphetamine and its vehicle in a two-lever, food-motivated operant task. Once trained, both groups of rats showed a dose-related decrease in discriminative performance with lower amphetamine doses. Analysis of the dose-response curves indicated an ED50 for the obese rats of 0.17 mg/kg and for the lean group of 0.14 mg/kg. Administration of 0.3-1.2 mg/kg l-amphetamine and 2.5-10.0 mg/kg cocaine produced a pattern of responding similar to that observed with d-amphetamine. In contrast, 0.08-mg/kg apomorphine produced saline-appropriate responding and 1.5-2.5 mg/kg fenfluramine produced intermediate results in both groups. Time-course experiments indicated that the lean rats maintain errorless discriminative performance through 90 min post-injection, whereas the obese rats discriminate d-amphetamine significantly less at that post-administration time. The results suggest a similar sensitivity to d-amphetamine and other dopaminergic agonists in obese and lean rats with a difference in the time-course of d-amphetamine's action between these two groups.     

Stability of the stimulus properties of drugs over time

Three separate groups of rats were trained to discriminate the stimulus effects of either 600 mg/kg ethanol (n = 5), 0.8 mg/kg d-amphetamine (n = 8) or 1.0 mg/kg 1-(3-trifluoromethylphenyl)piperazine (TFMPP; n = 10). Once criterion performance was attained, each group was tested with various doses of the drug used in their training, thus allowing for calculations of dose-response curves and ED50 values. A second dose-response relationship was established at a later time, averaging over a year later, and this result was compared to the initial curve. In none of the three groups was there substantial change in the sensitivity of the rats to different doses of the drug used in training as indicated by similar ED50 values. These results suggest that the drug discrimination procedure is stable over a period of continuous training and testing.     

Discriminative properties of l-cathinone compared to dl- and d-cathinone

Rats were trained to discriminate between the stimulus properties of 0.6 mg/kg l-cathinone and its vehicle in a two-lever, food-motivated operant task. Once trained, rats showed a dose-dependent decrease in discrimination performance with lower l-cathinone doses and analysis of the dose-response relationship indicated an ED50 of 0.19 mg/kg. Administration of either dl-or d-cathinone produced a pattern of discriminative responding similar to l-cathinone with ED50s of 0.29 and 0.63 mg/kg, respectively. Thus, the potency of the racemeric cathinone lies approximately midway between that of the two isomers. Time-course data indicate that l-cathinone has a peak effect at 15-30 min post-administration with a duration of 180 min. Pretreatment with the serotonic receptor blocker pirenperone did not affect l-cathinone discrimination, whereas pretreatment with 0.2 mg/kg haloperidol, a dopamine receptor blocking agent, attenuated the l-cathinone discrimination. These data suggest that the stimulus properties of l-cathinone are possibly mediated by brain dopaminergic systems.     

Dopaminergic mediation of a behavioral effect of l-cathinone

Ten male rats were trained to discriminate between the stimulus properties of 0.6 mg/kg l-cathinone and saline in a two-lever food-motivated operant task. Once trained, rats showed a dose-dependent increase in discrimination over a dosage range of 0.15-1.2 mg/kg l-cathinone. Analysis of this dose-response relationship indicated an ED50 of 0.27 mg/kg. Pretreatment with 0.2 mg/kg of the specific dopamine blocking drug haloperidol increased this ED50 to 0.47 mg/kg and significantly decreased discriminative performance when co-administered with either 0.15, 0.3, or 0.6 mg/kg l-cathinone. Since the dose-effect curves for cathinone with and without haloperidol pre-treatment were parallel, it is suggested that l-cathinone, the active constituent in khat, produces its discriminative properties, in part, by mediation of dopaminergic neuronal systems.     

Apomorphine increases ethanol discrimination

Rats were trained to discriminate between the stimulus properties of 600 mg/kg ethanol and saline in a two-lever, food-motivated operant task. Once trained, rats showed a dose-related decrease in discriminative performance with lower ethanol doses and analysis of the dose-response curve indicated an ED50 of 372 mg/kg. Pretreatment with 0.16 mg/kg apomorphine produced increased discriminative performance at each ethanol dose and the combination generated a dose-response curve parallel to ethanol administered alone with an ED50 of 232 mg/kg. This significant shift to the left of the ethanol dose-response curve after apomorphine administration is discussed in relation to dopaminergic neuronal systems and the clinical use of apomorphine alcoholics.     

Rats become acutely tolerant to cathine after amphetamine or cathinone administration

The drug discrimination paradigm was used to evaluate in rats the ability of the discriminate response to either 0.8 mg/kgd-amphetamine or 0.8 mg/kgl-cathinone to generalize to 2.4–6.0 mg/kg of the active cathinone metabolited-norpseudoephedrine, also known as cathine. When tested 24 h after vehicle administration, cathine generalized in a dose-related fashion in rats (n=6) trained with cathinone (ED₅₀=3.03 mg/kg) and in rats (n=8) trained with amphetamine (ED₅₀=2.93 mg/kg). In contrast, when cathine was tested 24 h after the administration of either amphetamine or cathinone, it produced significantly decreased discriminative performance. The possibility that this acute tolerance may have been produced by release, and subsequent depletion, of brain dopamine was tested by pretreating rats with the dopamine release inhibitor CGS 10746B. When CGS 10746B was administered prior to cathinone it significantly decreased cathinone discrimination. In addition, acute tolerance to cathine at 24 h after vehicle-cathinone co-administration was reversed when cathine was tested 24 h after CGS 10746B-cathinone co-administration. The results suggest that cathinone-produced discriminative stimulus, as well as the acute tolerance to cathine, may be dopaminergically mediated.     

Discriminative properties of the psychostimulant dl-cathinone in a two lever operant task. Lack of evidence for dopaminergic mediation

In order to analyse further the discriminative stimulus properties of stimulant drugs, rats were trained to discriminate 2.0 mg/kg of dl-cathinone in a two-lever operant task. Dose-related generalization was seen to cathinone itself and to a wide range of stimulant drugs including d-amphetamine, cocaine, methylphenidate, pipradrol and cathine, i.e. (+)norpseudoephedrine. The high degree of specificity of the cathinone cue at the specific training dose studied was shown by the fact that the following nonstimulant drugs failed to generalize at all to cathinone, even in large doses--haloperidol, chlordiazepoxide, fenfluramine and fentanyl. The cathinone cue at 2.0 mg/kg was probably of central origin because phydroxyamphetamine (a polar congener of amphetamine) failed to generalize to cathinone at a dose nearly 50 times the ED50 for amphetamine generalization. Phenylethylamine (PEA; alpha-demethylamphetamine) and deuterated phenylethylamine (alpha, alpha, d2-PEA), a long acting derivative of phenylethylamine which is resistant to metabolism by monoamine oxidase, produced at most partial (60%) generalization to cathinone, even in large doses. alpha-Demethylcathinone failed to generalize at all to cathinone at doses up to 10 times the ED50 for cathinone. Thus, the alpha-methyl groups of both amphetamine and cathinone are important in determining their cue properties. The involvement of dopaminergic systems in the cathinone cue was investigated by examining generalization to apomorphine and antagonism by haloperidol. Apomorphine produced at most 29% generalization to cathinone. Haloperidol, at doses up to 0.3 mg/kg, produced at most 50% antagonism of both the cathinone cue and of the ability of amphetamine to substitute for cathinone. It is suggested that the evidence for dopaminergic mediation of the cue properties of cathinone and of other CNS stimulants is somewhat tenuous, whilst endogenous phenylethylamine may play some part in the mediation of the stimulant cue. Haloperidol, alone and in conjunction with amphetamine or cathinone, produced a remarkable tendency for subjects to emit a greater proportion of their total responses on the inoperative rather than the operative lever than was seen after saline or injections of vehicle. This action of a neuroleptic drug suggests, in accordance with Colpaert, Niemegeers and Janssen (1977), that in drug discrimination antagonism studies involving neuroleptics, and perhaps other drugs, quantal (lever selection) rather than quantitative (percentage of responses on the drug lever) indices may be the procedures of choice.     

Effect of repeated administrations upon cathinone discrimination and conditioned place preference.

1. Eight male rats were trained to discriminate the interoceptive cues produced by 0.8 mg/kg l-cathinone in a two-lever, food-motivated operant task and they were, subsequently, tested for preference to cathinone in a conditioned place preference (CPP)-test. 2. Once trained, the rats were placed on a 10 day regimen of twice-a-day non-contingent administrations of saline followed by a similar regimen of multiple injections of 0.8 mg/kg cathinone. 3. After each series of non-contingent administrations, the rats' ability to discriminate (0.2-0.8 mg/kg) cathinone, as well as their preference for it, was determined. 4. Results indicate that tolerance tends to develop to the effect of cathinone in its ability to control discriminative behavior as indicated by deficits in discriminative performance and a two-fold shift of the dose-response curve to the right. 5. In contrast, preference for cathinone, in the CPP-tests, was not significantly affected by the multiple cathinone administration regimen. 6. The possibility that tolerance to some behavioral effects may occur in habitual users of the cathinone-containing Khat shrub is discussed.     

Discriminative stimulus properties of d-amphetamine in pigeons

Two out of four pigeons were successfully trained in an operant procedure to discriminate between the presence and absence of the effects induced by d-amphetamine (final dose: 1.6 mg/kg). The solvent (saline) or d-amphetamine was administered intramuscularly (IM) 30 min prior to training. Tests with other drugs and dosages indicated that l-amphetamine (ED50 = 0.55 mg/kg) and cocaine (ED50 = 1.05 mg/kg) fully generalized to d-amphetamine (ED50 = 0.35 mg/kg), whereas drugs such as p-hydroxy-amphetamine (1.6 and 3.2 mg/kg), morphine (1.5, 3.0 and, 6.0 mg/kg), and delta 9-THC (0.125, 0.25, and 0.50 mg/kg) failed to do so at the doses tested. Apomorphine (0.25 and 0.50 mg/kg) and LSD-25 (0.04 and 0.08 mg/kg) produced intermediate results. Pretreatment with haloperidol (dose range: 0.04 to 1.28 mg/kg), but not propranolol (10 and 20 mg/kg), attenuated significantly the d-amphetamine (1.6 mg/kg) stimulus effects. The two pigeons emitted predominantly d-amphetamine appropriate responses when the training dose (1.6 mg/kg) of d-amphetamine was tested on different occasions 15, 60, and 120 min after the administrations. One bird emitted mostly vehicle appropriate responses when tested 240 min after the d-amphetamine injection whereas the other bird performed d-amphetamine appropriate responses. Selection of the non-drug associated key occurred in the two birds when testing was carried out 480 min (8 hrs) after the administration of d-amphetamine.     

The effects of opioid receptor antagonism on the discriminative stimulus effects of cocaine and d-amphetamine in the rat

There is evidence that suggests that endogenous opioids can modulate brain dopamine systems and the behavioral effects of drugs that are mediated by those systems. The aim of this study was to examine the role of endogenous opioids in the discriminative stimulus effects of d-amphetamine and cocaine, through the use of receptor antagonists. Two separate groups of rats were trained to discriminate 1.0mg/kg d-amphetamine or 10mg/kg cocaine from saline, in a discrete-trial, avoidance/escape procedure. Dose-response curves for d-amphetamine (0.1-1.0mg/kg, i.p.) and cocaine (1.0-10mg/kg, i.p.) were generated (using a cumulative dosing regimen) in the absence and presence of: the nonselective opioid antagonist naloxone (1.0 and 5.0mg/kg, s.c.), the delta-opioid antagonist naltrindole (10 and 30μg, i.c.; 1mg/kg, i.p.), the mu-opioid antagonist beta-funaltrexamine (30μg, i.c.), and the kappa-opioid antagonist nor-binaltorphimine (norBNI) (10 and 30μg, i.c.). The ED(50)s for d-amphetamine and cocaine alone were 0.2mg/kg (0.16-0.33) and 3.9mg/kg (2.7-5.1), respectively. None of the opioid antagonists produced significant changes in the ED(50) for d-amphetamine. NorBNI significantly increased the potency of cocaine to produce a discriminative stimulus (ED(50) 2.2mg/kg; 1.7-2.6), whereas the other antagonists were without effect. Endogenous mu- and delta-opioids do not appear to be involved in mediating the discriminative stimulus effects of d-amphetamine or cocaine; however, the endogenous kappa-opioids might have a negative modulatory effect on cocaine's interoceptive cue. The lack of effect of norBNI on the discriminative stimulus effects of d-amphetamine suggests novel differences in the mechanisms of action of d-amphetamine and cocaine.     

Direct microinjection of cathinone into the rat brain produces discriminative stimuli.

Rats were trained to discriminate IP administration of 800 micrograms/kg cathinone using a food-motivated, two-lever discrimination procedure. Following training, 800 micrograms/kg cathinone discrimination was produced (generalized) by lower cathinone doses in a dose-responsive manner after IP administration; an ED50 value of 330 micrograms/kg was calculated. Subsequently, guide cannulae were implanted into the lateral ventricle and bilaterally into the nucleus accumbens. After recovery, injections were made via cannulae that extended 0.5 mm past the tip of the guide cannulae. ICV administration of 256 micrograms cathinone/rat produced discriminative responding on the cathinone-appropriate lever to the same degree as did the peripherally administered training dose of cathinone. Decreasing ICV doses produced decreased discriminative performance and allowed the calculation of an ED50 value of 90.5 micrograms. Likewise, administration of 64 micrograms cathinone/nucleus accumbens (for a total of 128 micrograms/rat) substituted for the IP training dose of cathinone. These results evidence the central mediation of the cathinone-induced discriminative stimulus cue and show that administration of cathinone into the nucleus accumbens is sufficient to produce these stimuli. Thus, these data suggest that receptors in the nucleus accumbens are important for the discrimination of this psychostimulant.     

Discriminative stimulus properties of CGS 9896: interactions within the GABA/benzodiazepine receptor complex

Male rats were trained to discriminate the stimulus effects of CGS 9896 (30.0 mg/kg) from its vehicle. Once trained, discriminative performance was observed to be dose-responsive in the 3.75-30.0 mg/kg range and analysis of the dose-response curve generated an ED50 of 6.44 mg/kg. Generalization testing with chlordiazepoxide and pentobarbital produced CGS 9896-appropriate responding, whereas administration of the GABA agonists SL 75 102 resulted in 75% (intermediate) generalization to the CGS 9896 discriminative stimulus. Although full antagonism of the CGS 9896 cue was obtained following administration of Ro15-1788 and pentylenetetrazole, the inverse agonist DMCM failed to provide complete antagonism. These results suggest that the discriminative properties of CGS 9896 are consistent with its activity as a benzodiazepine receptor agonist.     

Effects of corticosterone and its synthesis blockade on the cocaine-induced discriminative stimulus effects in rats

Several studies have argued that the hypothalamo-pituitary-adrenal (HPA) axis is of significance to behavioral effects evoked by drugs of abuse (e.g. cocaine). The role of the HPA axis in the subjective effects of cocaine was investigated in rats trained to discriminate cocaine (10 mg/kg, ip, -15 min) from saline (ip, -15 min) in a two-choice, water-reinforced fixed ratio (FR) 20 drug discrimination paradigm. In substitution tests, neither the exposure to a novel environment nor the social defeat stress, applied to rats after a dose of cocaine (2.5 mg/kg) which induced a ca. 42% drug-appropriate responding, influenced cocaine discrimination. Given alone, corticosterone (20 and 40 mg/kg, sc, -60 min) elicited a ca. 7% drug-appropriate responding. Combined injections of corticosterone and cocaine (0.625-5 mg/kg) did not affect the dose-response curve for cocaine. Surgical adrenalectomy did not modify the effects of cocaine; using a cumulative dosing procedure in the drug discrimination paradigm we found, that the dose-response curves for cocaine in adrenalectomized rats and sham-operated controls practically did not differ. Ketoconazole (an inhibitor of adrenocorticosteroid synthesis; 50 mg/kg, ip) given acutely (60 min) did not affect cocaine discrimination. Given subacutely (24, 16 and 1 h before tests), ketoconazole (50 mg/kg) produced a left-ward shift in the dose-response curve for cocaine and decreased its ED50 value. Another inhibitor of corticosterone secretion, metyrapone (50 mg/kg, sc), given acutely (120 min) did not affect the dose-response curve for cocaine. However, repeated injections (24, 16 and 2 h before tests) of metyrapone (50 mg/kg) with different doses of cocaine resulted in a rightward shift in the dose-response curve for cocaine and an increase in its ED50 value. The obtained results seem to exclude any role of the HPA axis in mediating subjective effects of cocaine, since neither corticosterone and stress nor adrenalectomy modified the discriminative stimulus effects of cocaine in rats. The reduction and potentiation of cocaine discrimination following subacute metyrapone and ketoconazole, respectively, may depend on changes in the levels of intermediate neurosteroids "upstream" from corticosterone in its biosynthesis pathway.     

Discrimination of agonist-antagonist mixtures: experiments with nicotine plus mecamylamine

Discrimination of a mixture of an agonist plus an antagonist has been analysed by training rats to discriminate (-)-nicotine (0.32mg/kg s.c.) from saline; in different groups of rats (n = 8), nicotine was administered either alone or in combination with the non-competitive nicotine antagonist mecamylamine (0.1-0.8mg/kg s.c.). Rats were trained in a two-bar operant conditioning procedure with a tandem schedule of food reinforcement. After 50 sessions, rats trained with nicotine alone had acquired the discrimination with an accuracy of about 85%. In combination, mecamylamine blocked accuracy during acquisition in a dose-related manner. In generalization tests, rats trained with nicotine alone yielded a typical dose-response curve for nicotine (ED(50) = 0.082mg/kg), without depression of response rate. In rats trained with nicotine plus 0.2mg/kg of mecamylamine, the ED(50) for the discriminative effect of nicotine was lowered (ED(50) = 0.036mg/kg), again without depression of response rate. In rats trained with nicotine plus 0.4-0.8mg/kg of mecamylamine, nicotine did not acquire stimulus control over behaviour (flat dose-response relationships), but in these animals, nicotine had a pronounced response rate-decreasing effect. These characteristics of discriminations based on nicotine plus mecamylamine differed substantially from those of previously described discriminations of nicotine plus midazolam, supporting the hypothesis that interactions between the latter drugs were based on a behavioural mechanism (overshadowing) rather than on interactions at the level of receptors.     

Methcathinone: a new and potent amphetamine-like agent

The purpose of the present investigation was to examine the effect of N-monomethylation of phenylisopropylamine derivatives on amphetamine-like activity. In tests of stimulus generalization using rats trained to discriminate 1.0 mg/kg of (+)-amphetamine from saline, the N-monomethyl derivatives of 1-(X-phenyl)-2-aminopropane, where X = 2,4-dimethoxy (2,4-DMA), 3,4-dimethoxy (3,4-DMA), 2,4,5-trimethoxy (2,4,5,-TMA), and 2-methoxy-4,5-methylenedioxy (MMDA-2), did not produce amphetamine-appropriate responding at the doses evaluated. However, the N-monomethyl derivative of cathinone (i.e., methcathinone), like cathinone, resulted in stimulus generalization. Further studies with this agent revealed that (a) in the amphetamine-trained animals, methcathinone (ED50 = 0.37 mg/kg) is more potent than racemic cathinone or racemic amphetamine (ED50 = 0.71 mg/kg in both cases), (b) methcathinone is capable of inducing release of radioactivity from [3H]dopamine-prelabeled tissue of rat caudate nucleus in a manner similar to that observed with cathinone, amphetamine, and methamphetamine, and (c) methcathinone is more potent than cathinone as a locomotor stimulant in mice as determined by their effect on spontaneous activity. The results of the present study provide evidence for a structural analogy between the prototypic psychostimulants amphetamine/methamphetamine and cathinone/methcathinone, and lend further support to the concept that amphetamine and cathinone correspond in their pharmacological effects.     

Effect of neuroleptics and tricyclic antidepressants upon d-amphetamine discrimination

After rats were trained to differentiate between the effects of intraperitqoneal administration of 0.6 mg/kg d-amphetamine and saline, pretreatment with various neuroleptic drugs was observed to significantly inhibit d-amphetamine discrimination. Thus, trifluoperazine, haloperidol, fluphenazine, chlorpromazine and thioridazine, but not clozapine, decreased d-amphetamine-induced control of discriminative performance. The ED50s of the effective neuroleptics for this inhibition were similar to those reported for antagonism of amphetamine-induced stereotypic behavior in the rat and the slopes of the dose-response curves were parallel indicating a common site and mechanism of action, presumably blockade of postsynaptic dopaminergic receptors. In contrast, pretreatment with the tricyclic antidepressant agents, imipramine, nortryptiline and desipramine had no significant effect on the discrimination of a dose of d-amphetamine which produced a low degree of discriminative control. The results are viewed in relation to the "dopamine hypothesis" of schizophrenia and affective disorders and the use of this animal behavioral method for determining brain dopamine interactions is discussed.