Reinforcing and subject-rated effects of methylphenidate and d-amphetamine in non-drug-abusing humans.

The reinforcing effects of methylphenidate (20-40 mg), d-amphetamine (10-20 mg), and placebo were assessed in eight healthy, non-sleep-deprived, non-drug-abusing outpatient volunteers. A modified progressive-ratio schedule was used to assess drug reinforcement in which a sampling session always preceded a self-administration session. During sampling sessions, volunteers received a drug dose to acquaint them with the drug effects. Drug doses were administered in eight identical capsules (i.e., each capsule contained 12.5% of the total dose). During self-administration sessions, which generally were conducted the next day, volunteers were given eight opportunities to work on a computer and could earn all, or some, of the capsules that were administered the previous day. To earn the first capsule, volunteers had to click a computer mouse 50 times. The number of clicks required to earn each additional capsule doubled (i.e., 100, 200, 400, 800, 1,600, 3,200, and 6,400 clicks). The dependent measure on this task was the break point (i.e., the last ratio completed). To characterize more fully the behavioral effects of methylphenidate and d-amphetamine, a battery of subject-rated drug-effect questionnaires, performance tasks, and physiologic measures was also used. Both doses of d-amphetamine increased the break point significantly above placebo levels, whereas only the high dose of methylphenidate did so. Break-point values for the doses of methylphenidate and d-amphetamine that maintained the greatest responding did not differ significantly. Methylphenidate and d-amphetamine produced some stimulantlike subject-rated drug effects (e.g., increased ratings of "drug liking"). These data suggest that methylphenidate, like d-amphetamine, can function as a reinforcer under a modified progressive-ratio schedule and, by inference, has at least some abuse potential in healthy, non-sleep-deprived, non-drug-abusing volunteers.

Discriminative stimulus characteristics of BMY 14802 in the pigeon.

Pigeons were trained to discriminate intramuscular injections of 5.6 mg/kg BMY 14802, a drug that has relatively high affinity for sigma binding sites, from saline in a two-key operant procedure. Many compounds that displace sigma binding failed to produce BMY 14802-like discriminative stimulus effects; these included (+)-SKF 10,047, (+)3-PPP, DTG and MR 2035; the typical antipsychotic haloperidol; the putative antipsychotics tiospirone, cinuperone and rimcazole; and the uncompetitive NMDA antagonist phencyclidine. In addition, MR 2035 and tiosperone failed to antagonize the discriminative stimulus effects of BMY 14802. The selective D2 antagonist eticlopride and the norepinephrine uptake blocker and antidepressant desmethylimipramine also failed to evoke substantial BMY 14802-appropriate responding. In contrast to sigma ligands and other reference compounds, the 5-HT1A agonists buspirone, 8-OH-DPAT and spiroxatrine dose-dependently produced BMY 14802-like discriminative stimulus effects. The limited-efficacy 5-hydroxytryptamine (HT)1A agonist NAN 190 did not produce BMY 14802-like discriminative effects; however, it did competitively antagonize the stimulus effects of BMY 14802 and the BMY 14802-like stimulus effects of (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin. Other serotonergic compounds failed to produce substantial BMY 14802-appropriate responding; such as 5-HT1 agonist I-5-HTP; 5-HT1A/1B agonist RU24969; 5-HT1B/1C agonist m-CPP; 5-HT1C/2 agonist quipazine; 5-HT1C/2 antagonists, metergoline and the atypical antipsychotic clozapine; and 5-HT3 antagonist ondansetron. Also, metergoline, ondansetron and pirenpirone failed to antagonize the stimulus effects of BMY 14802. These results indicate that the discriminative stimulus effects of BMY 14802 are serotonergically mediated primarily by 5-HT1A receptors rather than by sigma sites.

[99mTc]TRODAT-1: a novel technetium-99m complex as a dopamine transporter imaging agent.

Technetium-99m is the most commonly used radionuclide in routine nuclear medicine imaging procedures. Development of 99mTc-labeled receptor-specific imaging agents for studying the central nervous system is potentially useful for evaluation of brain function in normal and disease states. A novel 99mTc-labeled tropane derivative, [99mTc]TRODAT-1, which is useful as a potential CNS dopamine transporter imaging agent, was evaluated and characterized. After i. v. injection into rats, [99mTc]TRODAT-1 displayed specific brain uptake in the rat striatal region (striatum-cerebellum/cerebellum ratio 1.8 at 60 min), where dopamine neurons are concentrated. The specific striatal uptake could be blocked by pretreating rats with a dose of competing dopamine transporter ligand, beta-CIT (or RTI-55, i.v., 1 mg/kg). However, the specific striatal uptake of [99mTc]TRODAT-1 was not affected by co-injection of excess free ligand (TRODAT-1, up to 200 microg per rat) or by pretreating the rats with haloperidol (i.v., 1 mg/kg). The specific uptake in striatal regions of rats that had prior 6-hydroxydopamine lesion in the substantia nigra area showed a dramatic reduction. The radioactive material recovered from the rat striatal homogenates at 60 min after i.v. injection of [99mTc]TRODAT-1 showed primarily the original compound (>95%), a good indication of in vivo stability in brain tissue. Similar and comparable organ distribution patterns and brain regional uptakes of [99mTc]TRODAT-1 were obtained for male and female rats. Ex vivo autoradiography results of rat brain sections further confirmed the high uptake and retention of [99mTc]TRODAT-1 in the striatal region. In vitro binding studies measuring the affinity to dopamine transporters for the free ligand, TRODAT-1, and a nonradioactive rhenium derivative, Re-TRODAT-1, showed Ki values of 9.7 nM and 14.1 nM, respectively. Behavioral studies in rats using the free ligand, TRODAT-1 and Re-TRODAT-1 indicated that, unlike other tropane derivatives, they displayed no effect on locomotor activity, suggesting low toxicity. These results strongly support the conclusions that this novel 99mTc radioligand binds selectively to dopamine transporters in the brain and that is is potentially useful for in vivo assessment of the loss of dopamine neurons in Parkinson's and other neurodegenerative diseases.

IPT: a novel iodinated ligand for the CNS dopamine transporter.

An iodinated cocaine derivative, N-(3'-iodopropen-2'-yl)-2 beta-carbomethoxy-3 beta-(4-chlorophenyl) tropane (IPT), was evaluated as a probe for in vitro and in vivo labeling of dopamine (DA) and serotonin (5-HT) transporters in Sprague-Dawley rat brain. Saturation analysis of [125I]IPT in rat striatal homogenates, in two different buffer solutions, Tris-HCl and phosphate, demonstrated a one-site binding with affinities (Kd) of 0.25 +/- 0.02 and 0.16 +/- 0.02 nM and densities (Bmax) of 939 +/- 161 and 1,982 +/- 137 fmol/mg protein, respectively. Competition by known DA transporter ligands showed a rank order of RTI-55 > IPT > GBR12909 > mazindol > (-)cocaine. Binding to 5-HT transporter sites was evaluated in rat cortical homogenates. Saturation experiment results showed a single site with a Kd value of 1.2 +/- 0.2 nM and a Bmax value of 100 +/- 20 fmol/mg protein. The rank order of potency of several monoamine uptake inhibitors (paroxetine > fluoxetine > mazindol > R-nisoxetine > GBR12909) suggests that [125I] IPT labels 5-HT transporters in rat cortical homogenates. Both ex vivo and in vitro autoradiographic studies revealed high densities of [125I]IPT binding sites in the caudate nucleus, putamen, olfactory tubercle and nucleus accumbens, areas known to be rich in dopaminergic innervation. Moderate accumulation of activity was also observed in the substantia nigra. The dorsal raphe, a region with a high density of 5-HT innervation, was labeled using in vitro autoradiography with [125I]IPT, but the labeling using ex vivo autoradiography was less prominent at 30 min postinjection and not noticeable at 60 min postinjection.(ABSTRACT TRUNCATED AT 250 WORDS)

Variations in the behavioral responses to apomorphine in different strains of rats.

The present experiments compared patterns of locomotor activity during repeated acclimation sessions and determinations of locomotion and stereotypy elicited by administration of the direct dopamine receptor agonist apomorphine in five inbred strains of rats: the results suggest that each strain can be differentiated phenotypically according to these behavioral responses. Brown Norway rats demonstrated the greatest locomotion during acclimation sessions. Low doses of apomorphine (0.1 and 0.32mg/kg) produced a flat body posture in Lewis animals. A higher dose of apomorphine (1.0mg/kg) markedly increased locomotion in Fisher rats. Buffalo animals showed licking during control sessions and the greatest increase in gnawing at higher doses of apomorphine. DA rats were less responsive than the other strains of apomorphine. Between-strains autoradiographic determination of dopamine receptor densities revealed several differences in D1 receptors labeled by (3)H-SCH 23390 and D2/D3 receptors labeled by (125)I-NCQ 298 in the caudate-putamen and nucleus accumbens. However, the heterogeneity of dopamine receptor densities was not sufficient to explain the strain-specific behavioral responses. These experiments demonstrate variations in behavioral and neurochemical characteristics of inbred strains of rats which could be used to model genetically determined differences in dopamine-mediated behavioral responses.

Serotonergic properties of cocaine: effects on a 5-HT2 receptor-mediated behavior and on extracellular concentrations of serotonin and dopamine.

The present study examined the ability of cocaine to produce behavioral and neuropharmacological effects through serotonin (5-HT) systems. Pretreatment with fluoxetine or cocaine potentiated the head-shake response to the 5-HT precursor, 5-hydroxytryptophan (5-HTP; 75 mg/kg), a behavior mediated by the activation of 5-HT2 receptors. This effect was antagonized by the selective 5-HT2 receptor antagonist ketanserin (1 mg/kg). In contrast, pretreatment with the selective norepinephrine uptake inhibitor desipramine (10 mg/kg) or the selective dopamine (DA) uptake inhibitor GBR 12909 (32 mg/kg) failed to potentiate the head-shake response. The effects of cocaine on extracellular concentrations of DA and 5-HT in the nucleus accumbens were examined using in vivo microdialysis in a separate group of anesthetized rats. Cocaine (10 mg/kg) increased the extracellular concentrations of DA and 5-HT by 300-350% over baseline levels. Cocaine's ability to increase the head-shake response and to increase extracellular concentrations of 5-HT may be due to its ability to block 5-HT uptake.

Anatomical differentiation within the nucleus accumbens of the locomotor stimulatory actions of selective dopamine agonists and d-amphetamine.

The effects of local injections of dopamine receptor agonists into various areas within the nucleus accumbens or the medial caudate-putamen on the generation of locomotor activity were examined. Combinations of 0.32 microgram/side of the dopamine receptor agonists SKF 38393 (D1) and quinpirole (D2) produced increases in locomotor activity that varied according to the rostral-caudal placement of the cannulae within the nucleus accumbens. The greatest levels of locomotion were generated by injections into a region in the caudal-central nucleus accumbens, with lower levels of activity elicited by injections into more rostral or caudal regions. A similar pattern of responses was produced by administration of the indirect dopamine agonist d-amphetamine. These results indicate that there is marked heterogeneity in the response of discrete sub-regions of the nucleus accumbens to dopamine receptor stimulation and that this heterogeneity is functionally expressed in the mediation of the locomotor effects of dopaminergic agonists.

Receptor mechanisms of opioid drug discrimination.

Receptor theory of opioid action has provided an extremely useful interpretive framework for the discriminative stimulus effects of opioids. By and large, receptor theory has been applied to opioid actions as they are measured in in vitro and reflex systems. It is clear, however, that it can also assist in interpreting data from experiments addressing operantly conditioned behavior, and provide a link between these data and those obtained using other procedures. The current paper describes the criteria that can be used to determine whether a drug effect is receptor mediated and applies these criteria to the effects of mu and kappa opioids in drug-discrimination studies. Criteria for distinguishing between drug effects occurring through one, as opposed to two, receptor systems are described and again applied to the discriminative stimulus effects of mu and kappa opioids. The potential difficulties that can be caused by postreceptor variability and the presence of multiple receptor systems are noted, since they can modify the effects predicted from simple receptor theory, and are likely to play an important role when studies of opioid action are made in the whole animal. In discrimination studies, complicating variables include dose of the training drug, subject species, nature of the training drug, and context of the discrimination. Finally, the ability of receptor theory to guide future investigation of the phenomenon of partial generalization is explored.