Differential reinforcing effects of cocaine and GBR-12909: biochemical evidence for divergent neuroadaptive changes in the mesolimbic dopaminergic system

Effects of repeated GBR 12909 administration on brain stimulation reward

Male rats were trained at three separate currents to bar press for intracranial self-stimulation. On days 1 and 15, all subjects were given 1-(2-bis(4-fluorophenyl)-methoxy)-ethyl-4-(3-phenylpropyl) piperazine, also known as GBR 12909 (10 mg/kg, i.p.), prior to test session. Between these days, the paired Chronic-before group was injected (every other day) with GBR 12909 prior to intracranial self-stimulation, while unpaired, Chronic-after group was given the drug just after the end of the session. A third group (Control) received saline injections (i.p.) 20 min following the session. Although GBR 12909 was found to be reward enhancing, neither sensitization nor tolerance developed to the rewarding and performance/motor effects regardless of the injection regimen. In addition, the rewarding effects of intracranial self-stimulation were found to be independent of both current and environment-specific pairing. The present data obtained for GBR 12909 agree with previous observations of the effects of repeated administration of drugs of abuse on intracranial self-stimulation.

Progression of changes in dopamine transporter binding site density as a result of cocaine self-administration in rhesus monkeys

The present study examined the time course of alterations in levels of dopamine transporter (DAT) binding sites that accompany cocaine self-administration using quantitative in vitro receptor autoradiography with [(3)H]WIN 35,428. The density of dopamine transporter binding sites in the striatum of rhesus monkeys with 5 d, 3.3 months, or 1.5 years of cocaine self-administration experience was compared with DAT levels in cocaine-naive control monkeys. Animals in the long-term (1.5 years) exposure group self-administered cocaine at 0.03 mg/kg per injection, whereas the initial (5 d) and chronic (3.3 months) treatment groups were each divided into lower dose (0.03 mg/kg per injection) and higher dose (0.3 mg/kg per injection) groups. Initial cocaine exposure led to moderate decreases in [(3)H]WIN 35,428 binding sites, with significant changes in the dorsolateral caudate (-25%) and central putamen (-19%) at the lower dose. Longer exposure, in contrast, resulted in elevated levels of striatal binding sites. The increases were most pronounced in the ventral striatum at the level of the nucleus accumbens shell. At the lower dose of the chronic phase, for example, significant increases of 21-42% were measured at the caudal level of the ventral caudate, ventral putamen, olfactory tubercle, and accumbens core and shell. Systematic variation of cocaine dose and drug exposure time demonstrated the importance of these factors in determining the intensity of increased DAT levels. With self-administration of higher doses especially, increases were more intense and included dorsal portions of the striatum so that every region at the caudal level exhibited a significant increase in DAT binding sites (20-54%). The similarity of these findings to previous studies in human cocaine addicts strongly suggest that the increased density of dopamine transporters observed in studies of human drug abusers are the result of the neurobiological effects of cocaine, ruling out confounds such as polydrug abuse, preexisting differences in DAT levels, or comorbid psychiatric conditions.

Subtle differences in the discriminative stimulus effects of cocaine and GBR-12909

1. In addition to inhibiting the dopamine transporter, cocaine affects a variety of other neurotransmitter systems. In the present study, the involvement of both dopaminergic and the nondopaminergic systems in the behavioral effects of cocaine was studied using an intravenous drug discrimination procedure. 2. One group (Group 1) of rats were trained to discriminate cocaine (1 mg/kg, i.v.) from saline, while a second group (Group 2) of rats were trained to discriminate the same dose of cocaine from both GBR-12909 (1 mg/kg i.v.), a dopamine-selective uptake inhibitor, and saline. 3. Following training, substitution tests with different doses of cocaine and several drugs pharmacologically related to cocaine were conducted. When cocaine dose was varied, there was a dose-dependent generalization to the cocaine-training stimulus in both groups of rats. Conversely, GBR-12909 and GBR-12935, another dopamine-selective uptake inhibitor, generalized to the cocaine-training stimulus in Group 1, but there was minimal or no generalization in Group 2 4. The norepinephrine-selective uptake inhibitors, desipramine and nisoxetine, and the serotonin-selective uptake inhibitor, zimeldine, produced little or no generalization to the cocaine-training stimulus in either group of rats. The sodium channel blocker, dimethocaine which has a relatively high affinity for the dopamine transporter fully generalized to the cocaine stimulus in both groups of rats, while procaine which has a low affinity for the dopamine transporters only partially generalized to the cocaine-training stimulus in both groups of rats 5. Finally, lidocaine, which has negligible affinity for the dopamine transporter, did not generalize to the cocaine-training stimulus in either group of rats. The findings suggest similarities as well as subtle, but important, differences between the discriminative stimulus effects of cocaine and the dopamine uptake inhibitors, GBR-12909 and GBR12935.

Nicotine as an Addictive Substance: A Critical Examination of the Basic Concepts and Empirical Evidence

The present review is a critical analysis of the concepts behind and the empirical data supporting the view that tobacco use represents an addiction to nicotine. It deals with general aspects of the notion of addiction, while concentrating on specific problems associated with incorporating nicotine into current frameworks. The notion of addiction suffers from unprecedented definitional difficulties. The definitions offered by various authorities are very different, even contradictory. Definitions that reasonably include nicotine are so broad and vague that they allow many trivial things, such as salt, sugar, and watching television, to be considered addictive. Definitions that exclude the trivia also exclude nicotine. The addiction hypothesis, in general, is strongly shaped by views that certain drugs bring about a molecular level subversion of rationality. The main human evidence for this is verbal reports of smokers who say that they can't quit. On the other hand, the existence of many millions of successful quitters suggests that most people can quit. Some smokers don't quit, but whether they can't is another matter. The addiction hypothesis would be greatly strengthened by the demonstration that any drug of abuse produces special changes in the brain. It has yet to be shown that any drug produces changes in the brain different from those produced by many innocuous substances and events. The effects of nicotine on the brain are similar to those of sugar, salt, exercise, and other harmless substances and events. Apart from numerous conceptual and definitional inadequacies with the addiction concept in general, the notion that nicotine is addictive lacks reasonable empirical support. Nicotine does not have the properties of reference drugs of abuse. There are so many findings that conflict so starkly with the view that nicotine is addictive that it increasingly appears that adhering to the nicotine addiction thesis is only defensible on extra-scientific grounds.

A comparison of cocaine, GBR 12909, and phentermine self-administration by rhesus monkeys on a progressive-ratio schedule

The dopamine reuptake inhibitor GBR 12909 and the dopamine releaser phentermine may have potential for the treatment of cocaine abuse in humans. Pre-session treatment with either drug can decrease cocaine-maintained responding in rhesus monkeys while not affecting food-maintained responding. Both drugs are self-administered, but in some reports the patterns of responding they maintain differ from typical cocaine-reinforced responding. This study compared self-administration of cocaine (1--100 microg/kg/inj), GBR 12909 (3--100 microg/kg/inj), and phentermine (10--170 microg/kg/inj) in rhesus monkeys on a progressive-ratio schedule. Individual unit doses of each drug were available across several consecutive sessions. Cocaine self-administration was typical: the average number of ratios completed per session was a bitonic (increasing/decreasing) function of unit dose. Phentermine self-administration was variable across subjects (two of four monkeys self-administered reliably); one subject exhibited clear signs of behavioral toxicity. Self-administration of GBR 12909 was similarly variable across subjects. In the two subjects that self-administered GBR 12909 reliably, self-administration of small to mid-sized unit doses was enhanced following exposure to large unit doses. These data indicate that differences in self-administration of these drugs can be observed under progressive ratio procedures. Further, the data add to existing evidence suggesting that phentermine and GBR 12909 have at least moderate potential to be abused by humans.

Cocaine self-administration in rats differentially alters mRNA levels of the monoamine transporters and striatal neuropeptides

The potential neuroadaptations to cocaine self-administration (SA) were evaluated using quantitative in situ hybridisation histochemistry. Levels of mRNAs of the monoamine transporters, i.e. the primary molecular targets of cocaine, and the striatal neuropeptides substance P and enkephalin, which predominantly exist in different populations of dopaminoceptive striatal neurons, were quantified in rats which had reached different stages of acquisition of cocaine SA. Thus, animals were killed 1 h after completing a self-administration session (i) early in or after acquisition of cocaine SA (ii) after various regimes of chronic cocaine SA, and (iii) a 10-day period of withdrawal from chronic cocaine intake. Control mRNA levels of all molecules under study were those quantified in animals receiving i.v. saline yoked to rats self-administering cocaine (1.5 or 0.75 mg/kg per infusion, depending on the experiment). Monoamine transporter expression was differentially altered by cocaine; dopamine transporter mRNA levels in the ventral tegmental area, but not in the substantia nigra, were increased following withdrawal from cocaine, suggesting a role for the upregulated mesolimbic dopamine transporter in the mechanisms underlying relapse to cocaine taking. By contrast, serotonin transporter mRNA in the dorsal raphé and noradrenaline transporter mRNA in the locus coeruleus remained unaltered under all experimental conditions. In addition, the expression of the striatal neuropeptides was also differentially altered; substance P mRNA levels were transiently increased in the shell of the nucleus accumbens by prolonged cocaine self-administration, but enkephalin mRNA levels in the dorsal and ventral striatum remained unaltered under all conditions.

A comparison of the behavioral effects of the repeated administration of PTT, 2beta-propanoyl-3beta-(4-tolyl)tropane and cocaine

In the present study the effects of the repeated administration of the novel tropane analog PTT (2beta-propanoyl-3beta(4-tolyl)tropane) on spontaneous locomotor activity were compared to those of cocaine. Previous reports describing the in vivo effects of PTT have focused solely on its acute effects following a single administration. In Experiment 1, PTT (1.0, 3.0 mg/kg), cocaine (30 mg/kg), or vehicle were administered intraperitoneally to male Sprague-Dawley rats daily for 10 consecutive days and locomotor activity was assessed. In Experiment 2, the locomotor effects of PTT (1.0 mg/kg) and cocaine (15 mg/kg) were assessed following 5 days of drug exposure to either PTT (1.0, 3.0 mg/kg) or cocaine (30 mg/kg) and 18 days of withdrawal. In both paradigms, PTT (1.0, 3.0 mg/kg) and cocaine (30 mg/kg) produced marked increases in locomotor activity acutely and an augmented response to drug challenge following repeated exposure, indicative of behavioral sensitization. These data indicate that, despite differences in the pharmacological profiles of PTT and cocaine, both drugs produce behavioral sensitization. These data are consistent with previous reports in the literature describing the effects of the repeated administration of other dopamine reuptake inhibitors and suggest that the development of behavioral sensitization is a uniform characteristic of this class of drugs.

Alteration of the behavioral effects of nicotine by chronic caffeine exposure

The prevalence of tobacco smoking and coffee drinking place nicotine and caffeine among the most used licit drugs in many societies and their consumption is often characterised by concurrent use. The pharmacological basis for any putative interaction between these drugs remains unclear. Some epidemiological reports support anecdotal evidence, which suggests that smokers consume caffeine to enhance the effects of nicotine. This paper reviews various aspects of the pharmacology of caffeine and nicotine, in humans and experimental animals, important for the understanding of the interactions between these drugs. In particular, recent experiments are reviewed in which chronic exposure to caffeine in the drinking water of rats facilitated acquisition of self-adminstration behavior, enhanced nicotine-induced increases in dopamine levels in the shell of the nucleus accumbens and altered the dopaminergic component of a nicotine discrimination. These studies provide evidence that the rewarding and subjective properties of nicotine can be changed by chronic caffeine exposure and indicate that caffeine exposure may be an important environmental factor in shaping and maintaining tobacco smoking.

Cocaine upregulates the dopamine transporter in fetal rhesus monkey brain

Cocaine is a highly addictive drug that binds to the dopamine transporter (DAT), inhibits the reuptake of dopamine, and initiates multiple actions within midbrain dopaminergic systems. Using the rhesus monkey, we have investigated the consequences of in utero cocaine exposure on the expression of DAT in the fetal brain. By using the selective DAT ligand [125I]RTI-121 and tyrosine hydroxylase (TH) immunocytochemistry, we found that DAT binding sites are highly developed by day 70 of gestation and show a distribution pattern similar to TH. The rank order of specific 3beta-(4-[125I]iodophenyl)tropane-2beta-carboxylic acid isopropyl ester ([125I]RTI-121) binding densities was substantia nigra-ventral tegmental area > putamen > caudate > lateral hypothalamus > accumbens > linear/interfascicular nuclei >/= globus pallidus > prefrontal cortex. Furthermore, we observed that DAT mRNA was differentially expressed within fetal midbrain dopamine neurons with the highest levels detected in the ventral tier of the substantia nigra pars compacta, and the lowest levels in the ventral tegmental area and the linear/interfascicular nuclei. In utero cocaine exposure between days 22 and 70 significantly increased DAT mRNA expression, and the density of [125I]RTI-121 binding sites within midbrain dopamine neurons in the 70-d-old fetus. This increased DAT expression is accompanied by other presynaptic and postsynaptic neuronal changes, which collectively suggest that midbrain dopamine neurons are hypoactive after prolonged cocaine exposure, a state that may be a contributing factor in the development of attention deficit disorders observed in subjects exposed prenatally to cocaine.

Design, synthesis, and biological evaluation of novel non-piperazine analogues of 1-[2-(diphenylmethoxy)ethyl]- and 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazines as dopamine transporter inhibitors

A series of novel diamine, amine-amide, and piperazinone analogues of N-[2-(bisarylmethoxy)ethyl]-N'-(phenylpropyl)piperazines, GBR 12909 and 12935, were synthesized and evaluated as inhibitors of presynaptic monoamine neurotransmitter transporters. The primary objective of the study was to determine the structural requirements for selectivity of ligand binding and potency for neurotransmitter reuptake inhibition. In general, the target compounds retained transporter affinity; however, structural variations produced significant effects on reuptake inhibition and transporter selectivity. For example, analogues prepared by replacing the piperazine ring in the GBR structure with an N, N'-dimethylpropyldiamine moiety displayed enhanced selectivity for binding and reuptake inhibition at the norepinephrine (NE) transporter site (e.g. 4 and 5). Congeners in which the amide nitrogen atom was attached to the aralkyl moiety of the GBR molecule showed moderate affinity (K(i) = 51-61 nM) and selectivity for the dopamine transporter (DAT) site. In contrast, introduction of a carbonyl group adjacent to either nitrogen atom of the piperazine ring (e.g. 25 and 27) was not well tolerated. From the compounds prepared, analogue 16 was selected for further evaluation. With this congener, locomotor activity induced by cocaine at a dose of 20 mg/kg was attenuated with an AD(50) (dose attenuating cocaine-induced stimulation by 50%) of 60.0 +/- 3.6 mg/kg.

Cocaine Addiction: Neurobiology and Related Current Research in Pharmacotherapy

In this article, recent research studies in the field of cocaine addiction are reviewed, with an eye toward emergent options for treatment innovation. Particular attention is paid to the neurobiology and specific neurotransmitter and receptor mechanisms involved in cocaine abuse, dependence, and other unique phenomena of addiction such as sensitization, craving, compulsive drug use, and withdrawal. The vicissitudes in the dopamine theory of brain reward mechanisms, dopaminergic effects of cocaine, and emerging roles of GABA, serotonin, glutamate, and nitric oxide in cocaine addiction and its sequelae are discussed. Neuroanatomic findings elicited with imaging studies using PET and functional MRI are summarized. These findings support the role of specific brain regions within the dopaminergic system such as the ventral tegmentum and nucleus accumbens in the induction of the cocaine "high" and craving, respectively. Research approaches to the problem of developing effective pharmacotherapeutic options to render cocaine ineffective and modalities under study, such as dopamine uptake inhibitors and immunotherapy, are also discussed in the context of a variety of practical problems faced by these experimental therapies. Pharmacotherapeutic strategies and new directions in this research, such as the adaptive changes of the opioid system in cocaine addiction, are reviewed. Potential areas for further study are brought forth for further debate and possible clinical evaluation.

Doses of GBR12909 that suppress cocaine self-administration in non-human primates substantially occupy dopamine transporters as measured by [11C] WIN35,428 PET scans

GBR12909 (GBR) is a high-affinity, selective, and long-acting inhibitor of dopamine (DA) uptake that produces a persistent and noncompetitive blockade of DA transporters and substantially reduces cocaine-induced increases in extracellular DA in the nucleus accumbens of rats. Prior studies showed that intravenous infusion of GBR to Rhesus monkeys selectively reduced (1 mg/kg) and eliminated (3 mg/kg) cocaine self-administration. This study tested the hypothesis that doses of GBR that reduce cocaine self-administration in nonhuman primates produce significant occupation of DA transporters. DA transporters were quantitated in two baboons using [11C]WIN35,428 and positron emission tomography (PET). Each baboon underwent paired control/blocked PET scans (performed on three separate study days, 3-4 weeks apart). On the first scan the baboon received saline (3 ml/kg) 90 minutes before the injection of the radiotracer. GBR (1 mg/kg i.v.) was infused 90 minutes before the second [11C]WIN 35,428 study. The same experimental design was repeated with GBR doses of 3 and 10 mg/kg, respectively. Doses of 1 (n = 2), 3 mg/kg (n = 2), and 10 mg/kg (n = 2) reduced binding potential by 26, 53, and 72%, respectively. GBR was well tolerated in all baboons. These results demonstrate that doses of GBR that suppress cocaine self-administration in nonhuman primates also produce high occupancy of the DA transporter. These data strongly suggest that occupancy for the DA transporter by GBR explains its ability to attenuate cocaine-induced increases in extracellular DA and to suppress cocaine self-administration. Moreover, these data suggest that experimental human studies of orally administered GBR to test the DA hypothesis of cocaine addiction should use doses that produce at least 70% occupancy of the DA transporter.

Differential binding of tropane-based photoaffinity ligands on the dopamine transporter

Benztropine and its analogs are tropane ring-containing dopamine uptake inhibitors that produce behavioral effects markedly different from cocaine and other dopamine transporter blockers. We investigated the benztropine binding site on dopamine transporters by covalently attaching a benztropine-based photoaffinity ligand, [125I]N-[n-butyl-4-(4"'-azido-3"'-iodophenyl)]-4', 4"-difluoro-3alpha-(diphenylmethoxy)tropane ([125I]GA II 34), to the protein, followed by proteolytic and immunological peptide mapping. The maps were compared with those obtained for dopamine transporters photoaffinity labeled with a GBR 12935 analog, [125I]1-[2-(diphenylmethoxy)ethyl]-4-[2-(4-azido-3-iodophenyl)ethy l]p iperazine ([125I]DEEP), and a cocaine analog, [125I]3beta-(p-chlorophenyl)tropane-2beta-carboxylic acid, 4'-azido-3'-iodophenylethyl ester ([125I]RTI 82), which have been shown previously to interact with different regions of the primary sequence of the protein. [125I]GA II 34 became incorporated in a membrane-bound, 14 kDa fragment predicted to contain transmembrane domains 1 and 2. This is the same region of the protein that binds [125I]DEEP, whereas the binding site for [125I]RTI 82 occurs closer to the C terminal in a domain containing transmembrane helices 4-7. Thus, although benztropine and cocaine both contain tropane rings, their binding sites are distinct, suggesting that dopamine transport inhibition may occur by different mechanisms. These results support previously derived structure-activity relationships suggesting that benztropine and cocaine analogs bind to different domains on the dopamine transporter. These differing molecular interactions may lead to the distinctive behavioral profiles of these compounds in animal models of drug abuse and indicate promise for the development of benztropine-based molecules for cocaine substitution therapies.

Reduced glutamate immunolabeling in the nucleus accumbens following extended withdrawal from self-administered cocaine

Alterations in the density of GABA and glutamate immunolabeling within nerve terminals in the shell region of the nucleus accumbens were assessed in rats withdrawn from intravenous cocaine exposure. Four groups of rats were used: one group self-administered cocaine (0.42 mg/kg/infusion) in daily 3-h sessions for approximately 2 weeks, two additional groups received either saline or cocaine in a noncontingent fashion, and a fourth comprised a drug-naive, age-matched control group. Immunogold electron microscopy was used to quantify presynaptic terminal GABA and glutamate density within the vesicular and mitochondrial pools approximately 18 days following the last drug or saline exposure in the treatment groups. A significant 27.7% decrease in vesicular glutamate density within asymmetrical nerve terminals was observed in animals that self-administered cocaine as compared to controls. This group also showed an 18.6% decrease in vesicular nerve terminal glutamate immunolabeling as compared to animals that were administered a similar total dose of cocaine in a response-independent fashion. No significant changes in the density of nerve terminal GABA vesicular immunolabeling were observed in any groups. For both transmitters, no differences were detected in the density of immunolabeling within the presynaptic mitochondrial (i.e., metabolic) pool. These results demonstrate that glutamate density is suppressed in the shell region of the nucleus accumbens following withdrawal from 2 weeks of cocaine exposure. The findings also suggest that the motivational aspects that accompany self-administration may participate in this reduction.

Modifications of dopamine D1 receptor complex in rats self-administering cocaine

Cocaine is spontaneously and experimentally self-administered and, when given repeatedly, it induces a stable form of sensitization to a previously assessed minimum active dose. In the present study, triads of rats chronically implanted with a jugular catheter were treated as follows: one animal was trained to self-inject cocaine, while the other two passively received either cocaine or saline whenever the self-administering rat completed the response requirement. After 30 days of stable responding, the animals were sacrificed and dopamine D1 receptor density and adenylyl cyclase activity were measured in different brain areas. Animals receiving cocaine (both self-administering and yoked) showed a down-regulation of dopamine D1 receptor number and of dopamine stimulated adenylyl cyclase activity in the nucleus accumbens, as compared to saline rats. In the olfactory tubercle, dopamine stimulated adenylyl cyclase activity appeared selectively and significantly down-regulated in self-administering animals.