Lisa E. Baker

Oligomers of the amyloid-β protein disrupt working memory: Confirmation with two behavioral procedures

Converging lines of evidence suggest that oligomers of amyloid-beta play a role in the cognitive impairment characteristic of Alzheimers disease, but only three studies have provided experimental evidence of such impairment. To provide additional information about the effects of these oligomers on memory, the present study examined the memory of groups of rats exposed to ICV injections of the culture media (CM) of Chinese Hamster Ovary cells that were (7PA2) and were not (CHO-) transfected with a human mutation of amyloid precursor protein that appears to cause early-onset Alzheimers disease. The 7PA2 CM, which contained concentrations of soluble amyloid-beta oligomers physiologically relevant to those found in human brain, significantly disrupted working memory in rats tested in a radial-arm maze. In contrast, CHO- CM, which did not contain such oligomers, had no effect on memory. The disruptive effects of 7PA2-derived amyloid-beta oligomers, evident 2h after exposure, disappeared within a day. These findings are compared to results from 7PA2 CM tested under a complex procedure thought to measure aspects of executive function. The results confirm the disruptive effects of low-n amyloid-beta oligomers and extend them to a well-established rat model of memory.

The role of monoamine uptake in the discriminative stimulus effects of cocaine and related compounds

The involvement of monoamine neurotransmitter uptake in the discriminative stimulus effects of cocaine was examined in rats (n = 48) trained to discriminate 10 mg/kg of this substance from saline in a two-lever, water-reinforced (FR 20), drug discrimination situation. Compounds that act primarily by inhibiting dopamine (DA) uptake substituted for the cocaine cue; the order of potency was mazindol nomifensine GBR 12909 bupropion, although efficacy was lowest with GBR 12909. Desipramine, which inhibits norepinephrine (NE) uptake, substituted partially for cocaine while two drugs that inhibit serotonin (5-HT) uptake, citalopram and fluoxetine, did not mimic cocaine. When given in combination with cocaine, cis-flupenthixol and SCH 23390 reduced responding on the cocaine-appropriate lever significantly and to a greater extent than either haloperidol, ( |Mp )sulpiride or ( - ) sulpiride; neither ( + )sulpiride nor metergoline had significant effects. Cocaine substitutions with DA uptake inhibitors were also attenuated to varying extents by cis-flupenthixol, SCH 23390 and haloperidol, but not by metergoline. These data, in conjunction with results reported previously, suggest that inhibition of DA uptake is involved to a greater extent than either NE or 5-HT uptake in the discriminative stimulus properties of cocaine and related compounds. Since both the cocaine cue and its substitution by DA uptake inhibitors appear to be blocked most effectively, reliably and potently by compounds that act either non-selectively at DA receptors (cis-flupenthixol) or primarily at D1 receptors (SCH 23390), D1 mechanisms may play a particularly important role in the neuronal substrates of these behavioral effects.

Assessment of the MDA and MDMA Optical Isomers in a Stimulant-Hallucinogen Discrimination

The phenylisopropylamine derivatives 3,4-methylenedioxymethamphetamine (MDMA) and 3,4-methylenedioxyamphetamine (MDA) have been compared to both psychostimulants and hallucinogens in drug discrimination investigations. The stereoisomers of these compounds, in particular those of MDA, appear to produce differential effects. Previous studies have demonstrated that animals trained to discriminate amphetamine from vehicle generalize to the S(+)-isomers but not the R(-)-isomers of MDA and MDMA while animals trained to discriminate LSD from saline generalize to R(-)-MDA and neither isomer of MDMA. However, animals trained to discriminate mescaline from vehicle generalize to both stereoisomers of these phenylisopropylamine derivatives. The present study consisted of two experiments in which a three-choice drug discrimination procedure was employed to compare the stereoisomers of MDA and MDMA to both amphetamine and either mescaline (experiment one) or LSD (experiment two). Sixteen male Sprague-Dawley rats were trained to discriminate S(+)-amphetamine (1.0 mg/kg) and mescaline (12.5 mg/kg) and eight rats were trained to discriminate S(+)-amphetamine (1.0 mg/kg) and LSD (0.08 mg/kg) from saline in three-choice, food reinforced drug discrimination procedures. Substitution tests were administered with the isomers of MDA and MDMA. In the second experiment, substitution tests were also administered with lower doses of each training compound and with the stimulant cocaine and the hallucinogen 2,5-dimethoxy-4-methylphenylisopropylamine (DOM). In both experiments, all of the isomers produced very few responses on the S(+)-amphetamine lever. In the first experiment, R(-)-MDA and R(-)-MDMA produced nearly complete substitution for mescaline. The results of the second experiment revealed partial substitution for LSD with both isomers of MDMA and S(+)-MDA, and nearly complete substitution with R(-)MDA for LSD. The present findings do not support previous reports that S(+)-MDMA and S(+)-MDMA substitute for S(+)-amphetamine. The three-lever drug discrimination procedure may provide a more sensitive behavioral assay in which to examine the discriminative stimulus effects of drugs with compound stimulus properties.

A Three-Choice Discrimination Procedure Dissociates the Discriminative Stimulus Effects of d-Amphetamine and (±)-MDMA in Rats

(+/-)-3,4-Methylenedioxymethamphetamine (MDMA) produces subjective effects in humans that are similar to, but distinguishable from, those of psychostimulants. Drug discrimination studies in nonhumans have yielded inconsistent results regarding the similarities between MDMA and the psychomotor stimulant d-amphetamine. This study successfully used a 3-choice operant procedure to establish MDMA and d-amphetamine as discriminative stimuli in rats. Cocaine produced complete substitution for d-amphetamine, and LSD produced dose-dependent increases in MDMA-appropriate responding with nearly complete substitution (78%) for MDMA. The hallucinogen 2,5-dimethoxy-4-bromoamphetamine only partially substituted for MDMA and severely disrupted response rate. Fenfluramine and both isomers of 3,4-methylenedioxyamphetamine (MDA) all produced complete substitution for MDMA. The serotonin-receptor antagonist pirenpirone only partially blocked MDMA discrimination.

Assessment of the discriminative stimulus effects of the optical isomers of ecstasy (3,4-methylenedioxymethamphetamine; MDMA).

The discriminative stimulus effects of the stereoisomers of 3,4-methylenedioxymethamphetamine (MDMA) were studied in rats trained to discriminate 1.25 mg/kg of (+)-MDMA or 3.5 mg/kg of (−)-MDMA from saline, in a two lever, water-reinforced, drug discrimination situation. The isomers of MDMA and 3,4-methylenedioxyamphetamine (MDA) substituted completely for both training drugs. The stimulants amphetamine and cocaine did not substitute for either MDMA isomer. The hallucinogens (±)-2,5-dimethoxy-4-methylamphetamine (DOM), (+)-lysergic acid diethylamide (LSD), and mescaline failed to substitute completely for (+)-MDMA. Similarly, DOM and mescaline did not substitute for (−)-MDMA; however, LSD did substitute for this isomer at a dose of 0.06 mg/kg but not at higher doses. Substitution tests with 5-HT-releasing agents revealed that fenfluramine substituted partially for (+)-MDMA and completely for (−)-MDMA while p-chloroamphetamine substituted completely for both isomers of MDMA. When given in combination with (+)-or (−)-MDMA, neither the 5-HT2. antagonist pirenpirone nor the less selective 5-HT antagonist metergoline consistently blocked drug-appropriate responding. These results indicate that the stereoisomers of MDMA and MDA have similar discriminative stimulus properties. More importantly, the present findings suggest that 5-HT release may be important: for the discriminative stimulus effects of (+)-and (−)-MDMA. Actions at 5-HT2 receptors, however, do not appear to be critical.

MDMA and learning: effects of acute and neurotoxic exposure in the rat.

In two experiments, the effects of MDMA on the acquisition of lever-press responding of rats were examined under procedures in which water delivery was delayed by 0, 10, or 20 s relative to the response that produced it. In the first study, experimentally naive, water-deprived rats received an intraperitoneal injection of MDMA (0, 1.0, 3.2, or 5.6 mg/kg) prior to one 8-h experimental session. Response acquisition was observed under all conditions at all drug doses. MDMA increased the total number of responses emitted and the total number of water deliveries earned in dose-dependent fashion, but only when reinforcement was immediate. Under conditions of delay, MDMA had no effect on either measure. Under all reinforcement conditions, higher doses of MDMA typically produced an initial reduction in lever pressing, and in that sense interfered with learning. In the second study, rats received an MDMA injection regimen previously shown to be neurotoxic. Control rats received saline solution according to the same injection schedule. Two weeks after completing the regimen, rats were water deprived and exposed to behavioral procedures as described for the first experiment. Although MDMA significantly reduced 5-HT and 5-HIAA levels in the striatum and prefrontal cortex, mean performance of rats exposed to MDMA did not differ from that of rats exposed to vehicle. Twenty-five percent of the rats exposed to MDMA and delayed reinforcement did fail to acquire responding, which suggests that further study of the effects of neurotoxic doses of MDMA on initial response acquisition is warranted.

Discriminative stimulus properties of 1.25 and 5.0 mg/kg doses of clozapine in rats: examination of the role of dopamine, serotonin, and muscarinic receptor mechanisms

Clozapine (CLZ), an atypical antipsychotic drug (APD), produces minimal extrapyramidal side effects (EPS) and has significant advantages for treating both positive and negative symptoms in schizophrenic patients. CLZ has been established as a discriminative cue in the drug discrimination paradigm and in generalization tests the CLZ cue is more selective for atypical, rather than typical, APDs. However, greater selectivity for atypical antipsychotics has been demonstrated with a lower (1.25 mg/kg) CLZ training dose in rats [Psychopharmacology, 149 (2000) 189], rather than the traditional, higher training dose (5.0 mg/kg). It is therefore of interest to evaluate the properties mediating the 1.25 mg/kg CLZ discriminative cue. In the present study, rats were trained to discriminate either 1.25 mg/kg (N=7) or 5.0 mg/kg (N=7) CLZ from vehicle in a two-lever drug discrimination task. The typical antipsychotic haloperidol (0.1-0.4 mg/kg) did not substitute for either CLZ cue, whereas the atypical antipsychotic melperone (0.37-3.0 mg/kg) provided full substitution in both groups (>80% CLZ-appropriate responding). The 5-HT(1A) receptor agonist (+)-8-OH-DPAT (0.04-0.16 mg/kg), and the selective 5-HT(2A) receptor antagonist M100907 (0.03-1.0 mg/kg) did not produce substitution in either group. (+)-8-OH-DPAT combined with haloperidol (0.05 mg/kg) engendered only partial substitution (>60% CLZ-appropriate responding) for both CLZ cues, and M100907 combined with haloperidol (0.05 and 0.1 mg/kg doses) failed to provide substitution in either group. Trihexyphenidyl (0.18-6.0 mg/kg), a muscarinic M(1)-preferring receptor antagonist, engendered full substitution for the 1.25 mg/kg CLZ cue, but only partial substitution for the 5.0 mg/kg CLZ cue. These results provide evidence that antagonism at the muscarinic M(1) receptor is sufficient to provide 1.25 mg/kg CLZ-like discriminative stimulus effects.

Salvinorin A fails to substitute for the discriminative stimulus effects of LSD or ketamine in Sprague–Dawley rats

Salvia divinorum is a small perennial shrub that has gained recent popularity among the drug-using subculture as a legal alternative to hallucinogens. Salvinorin A, the main active compound found in the S.divinorum plant, is an atypical hallucinogen with pharmacological selectivity at kappa opioid (KOP) receptor sites and is a unique non-nitrogenous neoclerodane diterpene which is structurally distinct from other opioid compounds. The novel structure of salvinorin A and its specific binding affinity to KOP receptors provide a unique opportunity to investigate neurochemical mechanisms of hallucination and hallucinogenic compounds. The current investigation assessed the substitution of salvinorin A in 16 male Sprague-Dawley rats trained to discriminate either the prototypical serotonergic hallucinogen, LSD (0.08mg/kg, S.C., n=8) or the dissociative anesthetic and glutamatergic hallucinogen, ketamine (8.0mg/kg, I.P., n=8) from vehicle under a FR 20 schedule of food-reinforced responding. Results indicated that neither LSD nor ketamine discrimination generalized to salvinorin A. These findings are consistent with the growing body of evidence that salvinorin A is pharmacologically distinct from other traditional hallucinogenic compounds.

Common discriminative stimulus properties in rats of muscarinic antagonists, clozapine and the D3 preferring antagonist PNU-99194a: an analysis of possible mechanisms.

Dopamine D 3 receptors have been implicated in the aetiology of schizophrenia and the actions of antipsychotic drugs. The initial studies reported here assessed the involvement of such receptors in the in vivo actions of the atypical antipsychotic clozapine and the putative D 3 ‐preferring antagonist PNU‐99194A in drug discrimination assays. Rats trained to discriminate clozapine consistently generalized to PNU‐99194A in two separate studies. However, four other putative D 3 ‐preferring antagonists (PD 152255, (+)‐S14297, nafadotride and (+)‐AJ 76) did not induce generalization to clozapine. In rats trained to discriminate PNU‐99194A, which has been suggested to induce a stimulus mediated specifically by D 3 antagonism, the D 3 ‐preferring antagonist (+)‐UH 232 and clozapine both induced full generalization. However, the PNU‐99194A‐trained animals also generalized fully to the muscarinic antagonists scopolamine and trihexyphenidyl. A possible explanation for the symmetrical generalization observed between clozapine and PNU‐99194A is that these drugs have common muscarinic antagonist actions, since muscarinic antagonists have been reported to substitute for clozapine in numerous prior studies. However, in vitro receptor binding studies with M 1 −M 5 receptors indicated that (with the possible exception of the M 4 receptor), no muscarinic receptor subtype had high affinity for both clozapine, PNU‐99194A and scopolamine. In addition, other binding studies indicated that whereas clozapine and PNU‐99194A had high affinity for the D 3 receptor, scopolamine did not. It is therefore concluded that: (1) The generalization seen between clozapine, PNU‐99194A and muscarinic antagonists may be mediated by common effects ‘downstream’ from either muscarinic or D 3 receptors; (2) D 3 antagonism does not play a critical role in the clozapine stimulus (since D 3 ‐preferring antagonists did not consistently induce generalization to clozapine); (3) although D 3 antagonism plays a role in the PNU‐91994A stimulus (since the D 3 ‐preferring antagonist (+)‐UH 232 induced full generalization, in accord with results from prior studies with other D 3 ‐preferring antagonists), the PNU‐99194A stimulus also has commonalities with that induced by muscarinic antagonists and clozapine. The in vivo differences observed between PNU‐99194A and other D 3 ‐preferring antagonists should be borne in mind when this agent is used as a tool to study D 3 receptor functioning in vivo. The similarities between the PNU‐99194A and clozapine stimuli suggest tentatively that compounds with a profile like PNU‐99194A may have antipsychotic actions similar to clozapine. Some preclinical data are suggestive of such effects of PNU‐99194A.

Time course analysis of the discriminative stimulus effects of the optical isomers of 3,4-methylenedioxymethamphetamine (MDMA).

The present study examined the discriminative stimulus effects of the MDMA optical isomers administered at different presession injection intervals. In the first experiment, male Sprague-Dawley rats were trained in a two-lever, food-reinforced operant procedure to discriminate either (+)-MDMA (1.25 mg/kg) or (-)-MDMA (3.50 mg kg) at either 20 or 90 min following injection. Animals administered (+)-MDMA or saline 90 min before training sessions failed to attain the discrimination criteria after 73 training sessions, whereas (-)-MDMA successfully established discriminative stimulus control at both the 20 min and the 90 min postinjection intervals. (+)-Amphetamine did not substitute for either isomer, although a significant amount of drug-appropriate responding occurred in animals trained to discriminate (+)-MDMA at 20 min and (-)-MDMA at 90 min. Sch 39166 partially reduced the discrimination of (+)-MDMA at 20 min and (-)-MDMA at 90 min, although this effect was not dose dependent. Sch 39166 had no effect on animals trained to discriminate (-)-MDMA at 20 min. Haloperidol did not alter the discrimination of (+)-MDMA at 20 min but partially reduced the discriminative stimulus control of (-)-MDMA at 20 min and (-)-MDMA at 90 min. Fenfluramine substituted for both isomers of MDMA. Pirenpirone completely blocked the discriminative stimulus effects of (-)-MDMA at 20 min, although (+)-MDMA at 20 min and (-)-MDMA at 90 min were only partly blocked. WAY 100,135 had little effect on drug-appropriate responding; however, the discrimination of (+)-MDMA at 20 min was partly reduced by this 5-HT1A antagonist. In a second experiment, rats trained to discriminate (+)-MDMA (1.5 mg/kg) or (-)-MDMA (3.0 mg/kg) from saline were administered substitution tests with both isomers 20, 60, 90 and 120 min after injection. Results confirmed those of the first experiment that (+)-MDMA appears to have a shorter duration of action than (-)-MDMA. These results are discussed in light of the training doses employed.