Potency Of Cocaine Research Articles

Interactions between iboga agents and methamphetamine sensitization: studies of locomotion and stereotypy in rats.

The phenomenon of sensitization has been theoretically implicated in mediating various aspects of drug addiction. Recent dose-response studies demonstrated that pretreatment with the putative antiaddictive agent, ibogaine (IBO), and a synthetic iboga alkaloid congener, 18-methoxycoronaridine (18-MC), increase the potency of cocaine to elicit behavioral sensitization, an effect proposed to contribute, in part, to their ability to attenuate drug self-administration. As abuse of the methylated amphetamine derivative, methamphetamine (METH), is a growing public health concern, the present study determined the interactions between IBO and 18-MC and the expression of METH-induced behavioral sensitization. The effects of pretreatment with 18-MC (40 mg/kg, IP, 19 h earlier) on the expression of METH-induced locomotion (0, 0.25, 0.5, 1 and 2 mg/kg, IP) and the effects of pretreatment with either IBO or 18-MC on the expression of METH-induced stereotypy (2 and 4 mg/kg, IP) were assessed in rats treated chronically with either METH (4 mg/kg daily for 7 days) or saline. Compared to vehicle-pretreated controls, 18-MC produced an overall enhancement in METH-induced locomotion in rats treated chronically, but not acutely, with METH. In addition, both iboga agents increased the stereotypic response to METH. Iboga agents augment both the locomotor and stereotypic effects of METH in a manner consistent with previous reports for cocaine. Thus, it appears that iboga agents interact in a similar manner with the neural mechanisms mediating motor hyperactivity induced by the chronic administration of stimulant drugs.

Pharmacological and behavioral characterization of cocaine-kindled seizures in mice.

Convulsions associated with cocaine toxicity are a serious aspect of cocaine-related emergency room incidents. Seizures can result from a single high dose of cocaine, and evidence is accumulating that correlates repetitive administration of sub-convulsive doses of cocaine with a decreased seizure threshold, a phenomenon known as pharmacological kindling. A murine model of cocaine kindling has not been characterized. To determine the necessary and sufficient conditions for generating increased sensitivity to the convulsive and lethal effects of cocaine and to characterize some of the basic pharmacological and behavioral consequences of this phenomenon in mice. Male, Swiss-Webster mice were given repeated injections of cocaine. Daily administration of 60 mg/kg cocaine produced robust kindling; significant leftward shifts in the dose-effect curves for seizures were observed in cocaine-kindled mice. Cocaine kindling was enduring as these left shifts persisted for at least 20 days, indicating possible permanent synaptic changes. Induction of convulsions per se, utilizing 75 mg/kg cocaine, was not sufficient to engender kindling with a non-optimal dose (40 mg/kg). However, administration of a non-kindling dose of cocaine (40 mg/kg) for as few as four occasions produced increased seizure sensitivity to a 60-mg/kg cocaine challenge. The lethal potencies of cocaine and methamphetamine were significantly increased in cocaine-kindled mice. The baseline locomotor activity of kindled mice was not different from that of non-kindled mice. However, challenge doses of cocaine revealed significant differences in the vertically directed activity of kindled versus non-kindled mice. Overall, this study provides a description of important parameters for a model of cocaine kindling in mice that may be useful for the elucidation of mechanisms responsible for the long-term changes in sensitivity to cocaine and the discovery of novel pharmacological treatments.

Sensitivity of binding of high-affinity dopamine receptor radioligands to increased synaptic dopamine.

PET and SPECT studies have documented that D2 radioligands of moderate affinity, but not radioligands of high affinity, are sensitive to pharmacological challenges that alter synaptic dopamine levels. The objective of this work was to determine whether the brain kinetics of high-affinity radioligands for dopamine D1 ([(3)H]SCH 23390) and D2 ([(123)I]epidepride) receptors were altered by a prolonged elevation of synaptic dopamine induced by the potent cocaine analog RTI-55. Mice were injected intravenously with radioligands either 30 min after or 4 h before intraperitoneal administration of RTI-55 (2 mg/kg). In separate experiments, the pharmacological effects of RTI-55 were assessed biochemically by measuring uptake of dopamine in synaptosomes prepared from RTI-treated mice and behaviorally by monitoring locomotor activity. Consistent with the expected elevation of synaptic dopamine, RTI-55 induced a long-lasting decrement in dopamine uptake measured ex vivo, and a prolonged increase in locomotor activity. RTI-55 injected prior to the radioligands induced a significant (P < 0.05) increase in striatal concentration of [(123)I]epidepride at 15 min, relative to saline-treated controls, but there were no differences between the two groups at later time-points. For [(3)H]SCH 23390, both initial striatal uptake and subsequent clearance were slightly increased by preadministration of RTI-55. Administration of RTI-55 4 h after the radioligands (i.e., when it was presumed that a state of near equilibrium binding of the radioligands had been reached), was associated with a significant reduction of striatal radioactivity for both radiotracers. Our results are consistent with increased competition between dopamine and radioligand for binding to both D1 and D2 receptors after treatment with RTI-55. We suggest that the magnitude of the competition is reduced by failure of the receptor binding of high-affinity radioligands to rapidly attain equilibrium.

Cocaine-seeking produced by experimenter-administered drug injections: dose-effect relationships in rats.

Relapse to drug taking is a major obstacle to the effective treatment of cocaine abuse. Animal studies have determined that various drugs are able to reinstate extinguished drug-taking behavior. This study was designed to determine whether there is specificity in the ability of drugs to lead to cocaine-seeking and to compare potency and efficacy of a variety of drug primes. Another objective was to compare the effect of drugs with a primary dopaminergic mechanism with those having a secondary effect on dopaminergic substrates. Following acquisition of cocaine self-administration, the ability of injections of cocaine (5.0-20.0 mg/kg), amphetamine (0.30-3.0 mg/kg), methylphenidate (2.0-20.0 mg/kg), nicotine (0.0375-0.60 mg/kg), caffeine (1.25-20.0 mg/kg), morphine (0.10-10.0 mg/kg) or delta 9THC (0.3-3.0 mg/kg) to reinstate extinguished drug taking was measured. Tests were conducted in a single day and were comprised of three phases. The first phase consisted of a 60-min period of cocaine self-administration. During phase 2, the cocaine solution was replaced with saline and responding was extinguished during the next 3-h period. During phase 3, in which saline again was the only solution available for self-administration, responding was monitored for 3-8 h following an injection of a drug prime. Reinstatement was produced by experimenter-administered injections of cocaine, amphetamine, methylphenidate and caffeine but not nicotine, morphine or delta 9THC. The potency and efficacy of cocaine, methylphenidate and caffeine were comparable, whereas amphetamine was more potent and efficacious. Cocaine seeking occurred primarily during the first hour following the injection. These findings suggest that cocaine seeking is only produced following administration of specific drugs. It is suggested that effective drug primes are those that produce a discriminative stimulus that generalizes to the stimulus produced by the reinforcing effects of cocaine.

Pretreatment with the putative anti-addictive drug, ibogaine, increases the potency of cocaine to elicit locomotor responding: a study with acute and chronic cocaine-treated rats.

Results of single-dose studies suggest that the effects of pretreatment with the putative anti-addictive compound, ibogaine, on drug-induced locomotor behavior depends on the previous drug history of the animal. To compare the effects of ibogaine pretreatment on the dose-locomotor response function for cocaine in rats treated chronically with either saline or cocaine. Rats were chronically treated with either cocaine (15 mg/kg, IP, once daily for 5 days, followed by 2 week withdrawal) or saline. Ibogaine (40 mg/kg, IP) or vehicle was administered and 19 h later, a cocaine dose-locomotor response test was conducted (0, 5, 10, 20 and 40 mg/kg, IP). Chronic cocaine administration augmented the locomotor response to cocaine in chronic cocaine-treated rats, compared to acutely treated controls. Ibogaine pretreatment enhanced the locomotor effects of cocaine in both chronic and acute cocaine groups. Furthermore, due to the shape of the dose-response curve, in chronic cocaine but not in acute cocaine rats, ibogaine pretreatment enhanced the locomotor response to 5 and 10 mg/kg cocaine while decreasing the locomotor response to 40 mg/kg cocaine. These data demonstrate definitively that ibogaine can enhance sensitivity to the locomotor stimulant effects of cocaine, an effect which depends, in part, on the previous cocaine history of the animal.

PET examination of three potent cocaine derivatives as specific radioligands for the serotonin transporter

Several positron emission tomography (PET) radioligands based on the aryl tropane structure have been used for studies on monoamine reuptake sites. RTI-364, RTI-330, and RTI-357 (3-β-(4′- n-propyl-, 4′-iso-propyl-, and 4′-iso-propenyl-phenyl)nortropane-2-β-carboxylic acid methyl ester) are three recently synthesized cocaine analogues with higher affinity for the serotonin (5-HTT) than the dopamine transporter (DAT). Unlabelled RTI-364 and RTI-330 were prepared in a two-step synthesis. The key step was the addition of the appropriate propyl Grignard reagent to anhydroecgonine methyl ester. RTI-357 was prepared in a three-step synthesis with a palladium-catalyzed coupling reaction of β-CIT and isopropenylzinc bromide as key step. Hydrolysis of the ester functions gave the carboxylic acid analogues of RTI-364, RTI-330, and RTI-357, which were labelled with 11C using [ 11C]methyl iodide in dimethyl formamide (DMF) and tetrabutylammonium hydroxide (TBAH) as base. All three compounds entered the monkey brain in a high degree (∼5–10%). There was a low uptake of [ 11C]RTI-364 in serotonin-rich brain areas, whereas [ 11C]RTI-330 and [ 11C]RTI-357 showed a marked uptake of radioactivity in the thalamus and the brainstem, regions known to contain serotonin transporters. Transient equilibrium was reached at 15 and 40 min for [ 11C]RTI-330 and [ 11C]RTI-357, respectively. After pretreatment with citalopram, the ratio of radioactivity in the thalamus and the brainstem to the cerebellum were markedly reduced for [ 11C]RTI-357 but not for [ 11C]RTI-330. The results indicate that [ 11C]RTI-357 is a potential PET radioligand for quantitation of the serotonin reuptake site.

Modulation of dopamine transporter activity by nicotinic acetylcholine receptors and membrane depolarization in rat pheochromocytoma PC12 cells.

To elucidate the regulation of the rat dopamine transporter (rDAT), we established several PC12 variants overexpressing the rDAT. Treating these cells with a nicotinic agonist (1,1-dimethyl-4-phenylpiperazinium iodide, 30 microM) depolarized the plasma membrane potential from -31 +/- 2 to 43 +/- 5 mV and inhibited rDAT activity significantly in a calcium- and protein kinase C-independent manner. Membrane depolarization by a high external K+ concentration or two K+ channel blockers (tetraethylammonium hydroxide and BaCl2) also resulted in a marked inhibition of rDAT activity. Such inhibition of dopamine uptake is due to a reduction in Vmax, with no marked effect on the Km for dopamine. The potency of cocaine in inhibiting dopamine uptake was not significantly altered, whereas that of amphetamine was slightly enhanced by membrane depolarization. Removing extracellular Ca2+ or blocking the voltage-sensitive L-type calcium channels using nifedipine did not exert any significant effect on the inhibition of rDAT activity by depolarization. These data confirm that calcium influx on depolarization is not required for inhibition of the rDAT. Collectively, our data suggest that rDAT activity can be altered by a neurotransmitter that modulates the membrane potential, thus suggesting an exquisite mechanism for the fine-tuning of dopamine levels in the synapse.

Cocaine and alcohol interactions in the rat: Effect on cocaine pharmacokinetics and pharmacodynamics

The effect of alcohol coadministration on cocaine pharmacokinetics and pharmacodynamics was investigated in awake, freely moving rats. Cocaine plasma and brain extracellular fluid (ECF) concentration–time profiles were characterized after intraperitoneal (ip) administration of 30 mg/kg cocaine to rats that were pretreated with either normal saline or alcohol at 5 g/kg in a balanced crossover experimental design. The neurochemical response to cocaine administration, measured as the change in dopamine concentration in the nucleus accumbens (N ACC) and the change in the mean arterial blood pressure were monitored simultaneously. Intragastric alcohol administration significantly increased cocaine systemic bioavailability after ip administration from 0.550 ± 0.044 to 0.754 ± 0.071. Also, the absorption rate constant increased from 0.199 ± 0.045 to 0.276 ± 0.059 min-1 due to alcohol coadministration; however, this increase was not significant. Alcohol inhibition of cocaine metabolism caused an increase in cocaine elimination half-life from 26.3 ± 3.6 to 40.0 ± 8.1 min. Also, cocaine tissue distribution was enhanced by alcohol, resulting in a significant increase in cocaine volume of distribution. Analysis of the brain cocaine concentration–neurochemical effect relationship by the sigmoid-Emax pharmacodynamic model showed that Emax increased from 850 ± 200 to 1550 ± 640% of baseline due to alcohol coadministration, whereas EC50 decreased from 3400 ± 580 to 2000 ± 650 ng/mL, indicating higher cocaine potency in the presence of alcohol. The estimates of the indirect inhibitory pharma-codynamic model used to examine the plasma cocaine concentration–change in blood pressure relationship were not significantly different after the two treatments. These results indicate that alcohol significantly alters cocaine absorption, distribution, and elimination, resulting in higher and prolonged cocaine plasma concentration. Alcohol coad-ministration also potentiates the neurochemical response to cocaine administration.

Altered cocaine potency in the nucleus accumbens following 7-day withdrawal from intermittent but not continuous treatment: voltammetric assessment of dopamine uptake in the rat.

Using in vitro fast scan cyclic voltammetry, we measured cocaine potency for inhibiting dopamine uptake/clearance in accumbens slices 7 days after withdrawal from chronic cocaine pretreatments. Rats were pretreated with 40 mg/kg per day for 14 days, either via continuous osmotic minipumps or by once-daily injections. The cocaine potency was subsequently assessed for endogenous and exogenous dopamine applied via single-pulse electrical stimulation and caged-dopamine photolysis, respectively. Under baseline conditions, no differences in either endogenous or exogenous dopamine kinetics were observed in the two cocaine pretreatment groups. In contrast, the potency of bath-applied cocaine for inhibiting endogenous dopamine uptake was enhanced in the intermittent injection group with no change in the continuous infusion group. The selective increase in the cocaine potency following injections was also demonstrable for clearance of photo-applied DA. The enhanced cocaine potency in the accumbens slices following 7 days of withdrawal is consistent with the residual sensitization to cocaine-induced locomotion following daily cocaine injections. Behavioral tolerance following continuous infusion, on the other hand, may be mediated via a mechanism distinct from altered dopamine uptake.

Tyrosine-533 of rat dopamine transporter: involvement in interactions with 1-methyl-4-phenylpyridinium and cocaine

To improve our understanding of structure–function relationships for neurotransmitter transporters, we performed site-directed mutagenesis of the rat dopamine transporter (DAT) and assessed the functions of the mutants in transiently-expressing COS cells. Tyrosine-533 of rat DAT lies in the 11th transmembrane region, where the corresponding amino acid of human DAT is phenylalanine. Alanine substitution of tyrosine-533 (Y533A) conferred an increased affinity for 1-methyl-4-phenylpyridinium (MPP +). Phenylalanine substitution of tyrosine-533 (Y533F) increased the velocity of MPP + uptake but decreased DAT's affinity for MPP +. Cocaine's potency in inhibiting dopamine uptake was unchanged with Y533A, but increased with Y533F. Differences in the uptake kinetics and inhibitory potency of cocaine between rat and human DATs were similar to the differences observed between the wild-type and Y533F mutants DATs. Tyrosine-533 may be important for the DAT function and for species differences in transporter functions, including differential sensitivities to cocaine and 1-methyl-1,2,3,6-tetrahydropyridine (MPTP) in humans and rats.

Cocaine conditioning and cocaine sensitization: what is the relationship?

With repeated cocaine use, cocaine conditioned behavior develops to associated stimuli, and in addition, sensitization can occur to the unconditioned stimulant effects of cocaine. To investigate the relationship between the conditioned and unconditioned behavioral effects of repeated cocaine use, two groups of rats ( n=7) were given five daily paired cocaine treatments (10 mg/kg i.p.) immediately before a 20-min placement in an open-field environment. Other groups received either saline before testing or cocaine unpaired which was administered 30 min after testing in the homecage. When tested in the open-field with saline for conditioned effects, the two cocaine paired groups selectively exhibited substantial and equivalent cocaine conditioned responses. One of these groups was subjected to an extinction procedure which was effective in eliminating the cocaine conditioned responses. Subsequently, all the rats which had received cocaine in the first phase of the experiment, paired and unpaired, along with a subset of saline animals were given a cocaine (10 mg/kg i.p.) challenge test. The paired cocaine animals exhibited an earlier onset of the cocaine induced behavioral response (sensitization) as compared with the saline and the unpaired cocaine animals. Critically, the sensitization effects were unaffected by extinction, and in addition, the conditioned response did not contribute to the sensitization effect. It is suggested that the cocaine drug response occludes the cocaine conditioned response. Subsequent to this sensitization test, the animals were retested for conditioning. In this test, the paired cocaine animals which had not been subjected to the extinction procedure still exhibited a conditioned cocaine response, whereas, the paired cocaine group subjected to extinction was indistinguishable from saline controls. Although the present results show that Pavlovian conditioned responses to exteroceptive contextual cues do not directly contribute to cocaine induced behavioral sensitization effects, the sensitization effects were context-specific, and therefore, were tied to associative processes. It is suggested that context specificity is mediated by a compound stimulus complex comprised of exteroceptive stimuli and interoceptive cocaine drug cues. Furthermore, these exteroceptive and interoceptive cues associated with cocaine effectively expedite the onset of cocaine effects, and thereby, facilitate the addictive potency of cocaine.

Self-administration of cocaine and heroin combinations by rhesus monkeys responding under a progressive-ratio schedule.

The present study examined the reinforcing effects of cocaine and heroin, alone and combined, in rhesus monkeys (n = 4) responding under a progressive-ratio (PR) schedule. The PR schedule consisted of five components, each made up of four trials (i.e., 20 trials total), with each trial in a component having the same response requirement. The initial response requirement was fixed-ratio (FR) 120, which doubled across components to a maximum of FR1920. A trial ended with an injection or the expiration of a 15-min limited hold and the inter-trial interval was 30 min. Cocaine dose-response functions (13-400 micrograms/kg per injection) for injections/ session were monophasic, i.e., increased with dose until responding reached an asymptote or a peak. Heroin dose-response functions (1.6-100 micrograms/kg per injection) for injections/session were biphasic functions. i.e., increased to a peak and then decreased, whereas heroin dose-response functions for response rate were monophasic and reached an asymptote. When cocaine (1.6-200 micrograms/kg per injection) was combined with heroin (0.4-6.4 micrograms/kg per injection), low doses of cocaine (3.2-25 micrograms/kg per injection) and heroin (0.4-1.6 micrograms/kg per injection) that did not maintain behavior when tested alone did so when tested in combination. Combination with heroin resulted in a leftward shift in the cocaine dose-response functions, indicating that heroin increased the potency of cocaine as a reinforcer. This heroin-induced increase in cocaine's reinforcing potency may be a contributing factor to abuse of cocaine and heroin combinations (i.e., "speedballs") in humans. However, maximum injections/session for cocaine combined with heroin were not different from cocaine alone, suggesting that the reinforcing efficacy of combinations of cocaine and heroin were not higher than that of cocaine alone.

Differential potencies of cocaine and its metabolites, cocaethylene and benzoylecgonine, in suppressing the functional expression of somatostatin and neuropeptide Y producing neurons in cultures of fetal cortical cells

Using aggregate cultures derived from 17-day-old fetal rat cortex, we addressed the question: Does cocaine alter the functional expression of neuropeptide Y (NPY) and somatostatin (SRIF) neurons and, if so, are cocaethylene (CE) and benzoylecgonine (BZE) as active as cocaine? NPY/SRIF production in response to brain-derived neurotrophic factor (BDNF) or phorbol-12-myristate-13-acetate (PMA) was used as a functional criterion. A 5-day exposure to cocaine did not affect basal or stimulated (BDNF or PMA) production of NPY but it markedly suppressed BDNF- or PMA-stimulated production of SRIF. Exposure to CE led to a drastic suppression of basal as well as stimulated (BDNF or PMA) production of both NPY and SRIF. These effects of cocaine and CE were concentration dependent (1–100 μM). BZE did not alter any of these functional parameters. Next, we evaluated the fate of cocaine, CE, and BZE in the culture medium. Cocaine was converted to BZE, whereas BZE was not converted to cocaine. CE was converted to cocaine and BZE, with substantial amounts of cocaine and CE remaining in the medium after 72 hr (≈ 20% each). In summary, cocaine, CE, and BZE exhibited differential potencies in suppressing the expression of cultured NPY and SRIF neurons: CE was more potent than cocaine and BZE was inactive. SRIF neurons were more susceptible than NPY neurons to the effects of cocaine. The higher potency of CE may be due to a property of the compound and/or to CE serving as a source for a slow, continuous formation of cocaine by the brain cells themselves.

Synthesis and ligand binding studies of 4'-iodobenzoyl esters of tropanes and piperidines at the dopamine transporter.

Four analogs and two homologs of cocaine, designed as potent cocaine antagonists, were synthesized. The SN2 reaction between ecgonine methyl ester (13) or appropriately substituted piperidinol (19, 21) and appropriately substituted 4-iodobenzoyl chloride gave 4-iodobenzoyl esters of tropanes and piperidines (5-8). 2'-Hydroxycocaine (9) was obtained from 2'-acetoxycocaine (12) by selective transesterification with MeOH saturated with dry HCl gas. 2'-Acetoxycocaine (12) was synthesized from acetylsalicyloyl chloride (23) and ecgonine methyl ester (13). The binding affinities of these compounds were determined at the dopamine transporter for the displacement of [3H]WIN-35428. An iodo group substitution at the 4'-position of cocaine decreased dopamine transporter binding potency, while a hydroxy or acetoxy group at the 2'-position exhibited increased binding potency for the dopamine transporter compared to cocaine (10- and 3.58-fold, respectively). 2'-Hydroxylation also enhanced the bidning potency of 4'-iodococaine (5) by 10-fold. Replacement of the tropane ring with piperidine led to poor binding affinities.

Ethanol effects on self-administration of alfentanil, cocaine, and nomifensine in rhesus monkeys

A common form of polydrug use is that of cocaine and ethanol. The identification of an ethanol-cocaine combination product, cocaethylene, with properties in common with cocaine, has led to speculation that this metabolite may contribute to the co-abuse of cocaine and ethanol. In order to determine whether ethanol pretreatments selectively altered cocaine's reinforcing potency, ethanol pretreatments were given to monkeys trained to press levers and receive IV infusions of several doses of cocaine or alfentanil. In addition, nomifensine, a drug which has a mechanism of action similar to cocaine's, was evaluated in the presence and absence of ethanol in monkeys with the cocaine baseline history. Ethanol, in doses ranging from 100 to 1780 mg/kg, given 10 min before the 130-min session, had no effect on responding maintained by alfentanil. These doses also had no significant effect on cocaine-maintained responding, although the potency of cocaine as a reinforcer was increased following administration of 1000 mg/kg ethanol in two of the four subjects. The potency of nomifensine as a reinforcer was significantly increased by 1000 mg/kg ethanol, but again, this enhancement was limited to the same two subjects. These data indicate that, in this paradigm, cocaethylene did not selectively modify cocaine's reinforcing potency, but there appear to be individual differences with respect to ethanol's ability to stimulate rates of drug-maintained responding.

Cocaine and lidocaine in combination are synergistic convulsants

The abuse of cocaine has dramatically increased in the recent decade. Cocaine obtained on the illegal market is rarely found in pure form. Most often it is adulterated with various substances, especially other local anesthetics. Lidocaine is one of the most common local anesthetics employed for adulteration of illicit cocaine. Toxicity due to the simultaneous ingestion of cocaine and lidocaine has been reported. Acute toxicity to cocaine and other local anesthetics is manifested in central nervous system aberrations, such as seizures and convulsions. This study investigated the convulsant potency of cocaine and lidocaine alone and in combination. Rats were administered intravenous injections of 5 mg/kg or 20 mg/kg of cocaine or lidocaine alone and in combination in equal proportion. Seizure activity and intensity were evaluated. The plasma concentration and brain content of each agent was also determined at the time of toxicity. The administration of 5 mg/kg of each drug alone did not yield seizure activity. However, the concomitant administration of 5 mg/kg of both cocaine and lidocaine produced a seizure response nearly equal to that produced after administration of 20 mg/kg of cocaine alone. Diazepam pre-treatment successfully antagonized the seizures induced by cocaine and lidocaine and raised the seizure threshold dose for the combination treatment by approximately four fold. The results suggest that cocaine and lidocaine interact synergistically to increase seizure activity and that the nature of this response occurs in part through a depression of inhibitory neuronal transmission.

Binding domains for blockers and substrates on the cloned human dopamine transporter studied by protection against N-Ethylmaleimide-induced reduction of 2β-carbomethoxy-3β-(4-fluorophenyl)[ 3H]tropane ([ 3H]WIN 35,428) binding

Binding sites for 2β-carbomethoxy-3β-(4-fluorophenyl)[ 3H]tropane ([ 3H]WIN 35,428) on the human dopamine transporter expressed in C6 glioma cells were alkylated with N-ethylmaleimide (NEM), and the protective potency of the blockers cocaine, N[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine (BTCP), and benztropine, and of the substrates dopamine, d-amphetamine, and norepinephrine was measured. In general, the protective potency was lower (at least 4–5 times) than the potency in inhibiting [ 3H]WIN 35,428 binding with the compounds present under the same experimental conditions used for the NEM alkylation. However, the disparity was substantially greater for all substrates tested (23- to 44-fold) than for the blockers (4- to 11-fold), especially cocaine (5-fold) and BTCP (4-fold). Benztropine took an intermediate place (11-fold) between cocaine (5-fold) and BTCP (4-fold), on the one hand, and dopamine (23-fold), on the other hand. [ 3H]WIN 35,428 binding was best described by a one-site model under the present conditions. The results are discussed in terms of models involving blocker-induced conformational changes and overlapping nonidentical binding domains for blockers and substrates.

Highly potent cocaine analogs cause long-lasting increases in locomotor activity

Three cocaine analogs were compared with cocaine for the capacity to affect: (1) dopamine transporter binding and function; and (2) locomotor activity. RTI-55 (3β-[4-iodophenyl]tropane-2β-carboxylic acid methyl ester tartrate), RTI-121 (3β-[4-iodophenyl]tropan-2β-carboxylic acid isopropyl ester hydrochloride) and RTI-130 (3β-[4-chlorophenyl-2β-[1,2,4-oxadiazol-3-phenyl-5-yl]tropane hydrochloride) competed for [ 3H]WIN 35428 binding in rat striatum in vitro, with IC 50 values at least 50-fold less than that of cocaine. These analogs inhibited [ 3H]dopamine transport into rat striatal synaptosomes, with IC 50 values again less (at least 100-fold) than that for cocaine. Intravenous RTI-55, RTI-121 or RTI-130 injection effected dose-related increases in locomotor activity in mice, with estimated relative potencies at least 10-fold greater than that of cocaine. These increases were long lasting: whereas increased activity ceased within 2 h after cocaine administration, increased locomotion was observed at least 10 h after RTI-55, RTI-121, or RTI-130 administration. Parallel line analysis indicated that the slopes of the ascending portion of the RTI-121 and RTI-130 dose-response curves differed from that of cocaine, suggesting the involvement of mechanisms different from that of cocaine.

Effects of intermittent and continuous cocaine administration on dopamine release and uptake regulation in the striatum: in vitro voltammetric assessment.

Chronic daily injections of cocaine induce behavioral sensitization to subsequent cocaine challenge, while continuous infusion induces tolerance. Following a 7-day withdrawal period, we examined the effects of these two dosing regimens on: (1) baseline dopamine efflux and uptake following single-pulse electrical stimulation, (2) inhibition of uptake by cocaine; and (3) inhibition of efflux by autoreceptor activation. Cocaine (40 mg/kg per day) was administered to rats for 14 days either continuously by osmotic minipumps or intermittently by once-a-day injections. Minipumps containing saline were implanted in the control group. After 7 days of withdrawal, dopamine kinetics in the caudate was examined using in vitro fast-scan cyclic voltammetry. This technique provides very rapid measurements of dopamine in the extracellular space. Thus, when combined with endogenous dopamine efflux evoked by single-pulse, electrical stimulations, it was possible directly to measure the release and uptake components of the efflux. In the absence of pharmacological agents, no group differences were found in the amount of baseline dopamine released or in the uptake kinetics; the potency of bath-applied cocaine (0.03-60 microM) in inhibiting the uptake was also unaltered in either group. In contrast, the potency of quinpirole (an autoreceptor agonist, 5-250 nM) was significantly decreased and increased in the cocaine injection and pump groups, respectively. Thus, the cocaine administration regimen which produces sensitization results in a functional subsensitivity of release-modulating autoreceptors, while the tolerance-producing regimen results in autoreceptor supersensitivity.

Acute sensitivity vs. context-specific sensitization to cocaine as a function of genotype

Individual variability in the acute and chronic effects of psychomotor stimulants is due, in part, to genetic factors. The purpose of this series of studies was to utilize a behavioral model of sensitization, namely increased locomotor activity, to assess individual variability in sensitization to the chronic effects of cocaine and its relationship to the acute stimulant effects of cocaine. Because the degree of sensitization is proportional to the training dose, genetic differences in acute sensitivity to cocaine were assessed and incorporated into the sensitization paradigm. Acute sensitivity and context-dependent sensitization were determined in six inbred mouse strains. Large quantitative and qualitative differences were found in the acute potency and efficacy of cocaine to stimulate locomotor activity. The ED 50, was higher in the strains in which cocaine was most efficacious. Context-specific sensitization was determined via chronic administration of equiactive doses of cocaine (ED 50) specifically paired with the test apparatus or with the home colony. Sensitization was time, environment, and genotype dependent. The differences in the number of trials required to show sensitization were unrelated to the acute locomotor stimulant effects of cocaine. These findings suggest that acute cocaine-induced locomotor activity and context-specific sensitization reflect different pharmacological properties of cocaine.