Antagonist of N-methyl-D-aspartate receptors partially prevent the development of cocaine sensitization

Availability of N-Methyl-d-Aspartate Receptor Coagonists Affects Cocaine-Induced Conditioned Place Preference and Locomotor Sensitization: Implications for Comorbid Schizophrenia and Substance Abuse

Schizophrenia is associated with high prevalence of substance abuse. Recent research suggests that dysregulation of N-methyl-d-aspartate receptor (NMDAR) function may play a role in the pathophysiology of both schizophrenia and drug addiction, and thus, may account for this high comorbidity. Our laboratory has developed two transgenic mouse lines that exhibit contrasting NMDAR activity based on the availability of the glycine modulatory site (GMS) agonists d-serine and glycine. Glycine transporter 1 knockdowns (GlyT1(+/-)) exhibit NMDAR hyperfunction, whereas serine racemase knockouts (SR(-/-)) exhibit NMDAR hypofunction. We characterized the behavior of these lines in a cocaine-induced (20 mg/kg) conditioned place preference (CPP) and locomotor sensitization paradigm. Compared with wild-type mice, GlyT1(+/-) mice displayed hastened extinction of CPP and robust cocaine-induced reinstatement. SR(-/-) mice appeared to immediately "forget" the learned preference, because they did not exhibit cocaine-induced reinstatement and also displayed attenuated locomotor sensitization. Treatment of GlyT1(+/-) mice with gavestinel (10 mg/kg on day 1; 5 mg/kg on days 2-17), a GMS antagonist, attenuated cocaine-induced CPP and caused them to immediately "forget" the learned preference. Treatment of SR(-/-) mice with d-serine (300 mg/kg on day 1; 150 mg/kg on days 2-17) to normalize brain levels caused them to avoid the cocaine-paired side of the chamber during extinction. These results highlight NMDAR dysfunction as a possible neural mechanism underlying comorbid schizophrenia and substance abuse. Also, these findings suggest drugs that directly or indirectly activate the NMDAR GMS could be an effective treatment of cocaine abuse.

The competitive NMDA receptor antagonist CPP disrupts cocaine-induced conditioned place preference, but spares behavioral sensitization

Recently, the notion that memory and addiction share similar neural substrates has become widely accepted. N-methyl-d-aspartate receptors (NMDAR) are the cornerstones of synaptic models of memory. The present study examined the effect of the competitive NMDAR antagonist CPP on the induction of behavioral sensitization and conditioned place preference to cocaine. Conditioned place preference is an associative memory model of drug seeking, while sensitization is a non-associative model of the transition from casual to compulsive use. There were three principal findings: (1) co-administration of CPP and cocaine altered the acute response to cocaine, suggesting a direct interaction between the two drugs; (2) NMDAR antagonism had no effect on behavioral sensitization; and (3) NMDAR antagonism abolished conditioned place preference. A review of prior evidence supporting a role for NMDARs in sensitization suggests that NMDAR antagonists directly interfere with cocaine's psychostimulant effects, and this interaction could be misinterpreted as a disruption of sensitization. Finally, we suggest that addiction recruits multiple kinds of plasticity, with sensitization recruiting NMDAR-independent mechanisms.

Altered acquisition and extinction of amphetamine-paired context conditioning in genetic mouse models of altered NMDA receptor function

Repeated intermittent exposure to amphetamine (AMPH) results in the development of persistent behavioral and neurological changes. When drug exposure is paired with a specific environment, contextual cues can control conditioned responses, context-specific sensitization, and alterations in dendritic morphology in the nucleus accumbens (NAc). Intact N-methyl-D-aspartate (NMDA) glutamate receptor signaling is thought to be required for associative learning. The acquisition of context-specific behavioral sensitization to AMPH and extinction of conditioned hyperactivity have been investigated in two genetically modified mouse strains: the serine racemase homozygous knockout (SR-/-) and glycine transporter 1 heterozygous mutant (GlyT1-/+). These strains have reciprocally altered NMDA receptor co-agonists, D-serine and glycine, levels that result in decreased (SR-/-) or increased (GlyT1-/+) NMDA receptor signaling. AMPH-induced changes in dendritic morphology in the NAc were also examined. SR-/- mice showed reduced expression of context-specific sensitization and conditioned hyperactivity. However, the conditioned hyperactivity in these mice is completely resistant to extinction. Extinction reversed AMPH-induced increased in NAc spine density in wild-type but not SR-/- mice. GlyT1 -/+ mice showed a more rapid acquisition of sensitization, but no alteration in the extinction of conditioned hyperactivity. The SR-/- data demonstrate that a genetic model of NMDA receptor hypofunction displays a reduced ability to extinguish conditioned responses to drug-associated stimuli. Findings also demonstrate that the morphological changes in the NAc encode conditioned responses that are sensitive to extinction and reduced NMDA receptor activity. NMDA receptor hypofunction may contribute to the comorbidity of substance abuse in schizophrenia.

Genetic NMDA receptor deficiency disrupts acute and chronic effects of cocaine but not amphetamine

NMDA receptor-mediated glutamate transmission is required for several forms of neuronal plasticity. Its role in the neuronal responses to addictive drugs is an ongoing subject of investigation. We report here that the acute locomotor-stimulating effect of cocaine is absent in NMDA receptor-deficient mice (NR1-KD). In contrast, their acute responses to amphetamine and to direct dopamine receptor agonists are not significantly altered. The striking attenuation of cocaine's acute effects is not likely explained by alterations in the dopaminergic system of NR1-KD mice, since most parameters of pre- and postsynaptic dopamine function are unchanged. Consistent with the behavioral findings, cocaine induces less c-Fos expression in the striatum of these mice, while amphetamine-induced c-Fos expression is intact. Furthermore, chronic cocaine-induced sensitization and conditioned place preference are attenuated and develop more slowly in mutant animals, but amphetamine's effects are not altered significantly. Our results highlight the importance of NMDA receptor-mediated glutamatergic transmission specifically in cocaine actions, and support a hypothesis that cocaine and amphetamine elicit their effects through differential actions on signaling pathways.

Lesions of the medial prefrontal cortex prevent the induction but not expression of morphine-induced behavioral sensitization in mice

Repetitive exposure to morphine induces behavioral sensitization, which is supposed to involve in the process of addiction to drugs. As the underlying neuropharmacological mechanisms and anatomical substrates are considerably different between different drugs and different phases of behavioral sensitization, this study was designed to investigate the roles of the medial prefrontal cortex in the induction and expression of morphine-induced behavioral sensitization in mice. In experiments 1 and 2, mice were dosed with morphine (10 mg/kg, i.p.) once daily for 7 continuous days after induction of sham or kainic acid lesions of the medial prefrontal cortex. Locomotor activity was measured on days 1 and 7 to test acute morphine-induced hyperactivity and the induction of behavioral sensitization. In experiment 3, mice were subjected to surgery after behavioral sensitization was achieved. The mice were challenged with morphine 7 days later to evaluate the expression of behavioral sensitization. The results showed that lesions of the medial prefrontal cortex (mPFC) blocked acute morphine-induced hyperactivity and the induction of behavioral sensitization, but the lesions had no effect on the expression of behavioral sensitization. These results provide the first direct evidence that the mPFC may be involved in the induction, but not the expression, of morphine-induced behavioral sensitization.

Expression of mutant NMDA receptors in dopamine D1 receptor-containing cells prevents cocaine sensitization and decreases cocaine preference

The interaction of dopamine and glutamate in limbic brain regions mediates behaviors associated with psychostimulants, which act in part to increase dopamine signaling at both D1 receptors (D1Rs) and D2 receptors. Many addictive behaviors are a result of learned associations, and NMDA receptor activation has been shown to be important for these behaviors. We hypothesized that if NMDA receptor activation in dopamine receptor-containing cells is required for the addictive properties of psychostimulants, then mice with reduced NMDA receptor activity in D1R-containing cells would have attenuated long-term behavioral changes to these drugs. We generated a mouse line in which D1R-containing cells express an NR1 NMDA receptor subunit containing a mutation in the pore that reduces calcium flux. Mice expressing the mutant NMDA receptors in D1R-containing cells have normal basal activity and display similar increases in locomotor activity when treated with acute amphetamine or cocaine. However, the mutant mice fail to display locomotor sensitization to repeated cocaine administration. In addition, these mice also have a decreased ability to form a conditioned place preference to cocaine. These data suggest that intact NMDA receptor signaling in D1R-containing cells is required for the manifestation of behaviors associated with repeated drug exposure.

Dopamine D1 receptors mediate CREB phosphorylation via phosphorylation of the NMDA receptor at Ser897-NR1

Addictive drugs such as amphetamine and cocaine stimulate the dopaminergic system, activate dopamine receptors and induce gene expression throughout the striatum. The signal transduction pathway leading from dopamine receptor stimulation at the synapse to gene expression in the nucleus has not been fully elucidated. Here, we present evidence that D1 receptor stimulation leads to phosphorylation of the transcription factor Ca2+ and cyclic AMP response element binding protein (CREB) in the nucleus by means of NMDA receptor-mediated Ca2+ signaling. Stimulation of D1 receptors induces the phosphorylation of Ser897 on the NR1 subunit by protein kinase A (PKA). This phosphorylation event is crucial for D1 receptor-mediated CREB phosphorylation. Dopamine cannot induce CRE-mediated gene expression in neurons transfected with a phosphorylation-deficient NR1 construct. Moreover, stimulation of D1 receptors or increase in cyclic AMP levels leads to an increase in cytosolic Ca2+ in the presence of glutamate, but not in the absence of glutamate, indicating the ability of dopamine and cyclic AMP to facilitate NMDA channel activity. The recruitment of the NMDA receptor signal transduction pathway by D1 receptors may provide a general mechanism for gene regulation that is fundamental for mechanisms of drug addiction and long-term memory.

Effects of agmatine on the escalation of intravenous cocaine and fentanyl self-administration in rats

Escalation of drug intake reliably occurs when animals are allowed extended self-administration access. As a form of plasticity, escalation of drug intake may be accompanied by neuroadaptive changes that are related to the transition from controlled use to addiction. The purpose of the present experiment was to examine the effects of agmatine (decarboxylated L-arginine) on the escalation of intravenous (iv) fentanyl and cocaine self-administration in rats. Subjects were allowed 12 h of daily access to fentanyl (2.5 microg/kg) or cocaine (0.2 mg/kg) under a fixed-ratio (FR) 1 schedule of reinforcement for 30 days. Animals self-administering fentanyl were distributed into three groups: (1) low-dose agmatine (10 mg/kg) throughout self-administration; (2) high-dose agmatine (30 mg/kg) throughout self-administration; and (3) high-dose agmatine after significant escalation (Day 18) of drug intake had occurred. Animals in a fourth group were pretreated with a high dose of agmatine throughout 30 days of cocaine self-administration. Both doses of agmatine, when given throughout self-administration, significantly decreased the escalation of responding that occurred for fentanyl but not cocaine. In the group that received agmatine after significant escalation had occurred, fentanyl-maintained responding was not significantly altered. These data indicate that agmatine attenuates the escalation of fentanyl self-administration if administered before the escalation begins and may mediate neuroadaptive events related to chronic opioid self-administration.

Effects of cocaine administration on VTA cell activity in response to prefrontal cortex stimulation

The repeated use of psychostimulants in humans has been associated with progressive enhancement of anxiety, panic attacks, and eventually paranoid psychosis. The appearance of such behaviors has been termed behavioral sensitization, which forms part of the basic pathological mechanisms involved in drug addiction. Psychostimulants act via a circuit involving the ventral tegmental area (VTA), prefrontal cortex (PFC), and nucleus accumbens. The PFC sends glutamatergic projections that activate dopaminergic neurons in the VTA. These projections provide an extremely important excitatory drive necessary for the development of sensitization. The effects of cocaine administration on the response of dopaminergic VTA cells to activation of the PFC have not been reported. Here the effects of acute cocaine administration on VTA cell response to PFC stimulation are examined. Statistical analysis of the changes in spontaneous activity and evoked response revealed a significant decrease in spontaneous activity at 1.0 mg/kg i.v. after cocaine treatment compared to baseline levels. The net effect was an increase in signal-to-noise ratio. Treatment with MK-801 at a dose of 2 mg/kg showed that the excitatory response was, at least partially, NMDA-mediated. Prazosin pretreatment (0.5 mg/kg i.p.) did not prevent a significant decrease in spontaneous activity brought about by cocaine (15 mg/kg, i.p.). Nonetheless, prazosin alone induced a significant decrease in the response to PFC stimulation when compared to baseline. In addition, iontophoretic application of norepinephrine (NE) onto VTA cells revealed that NE potentiated (19.2%), enhanced (26.9%), or suppressed (46.2%) the glutamate-evoked response in VTA cells. The results suggest that a possible role of cocaine in the process of sensitization might be to amplify the PFC-induced excitation at the VTA. Since the iontophoretic release of NE in almost half of the sampled cells produced similar effects to those of cocaine it may suggest a possible NE-mediated mechanism for cocaine actions.

The neurotensin receptor antagonist, SR48692, attenuates the expression of amphetamine-induced behavioural sensitisation in mice

The effects of acute administration of the neurotensin receptor antagonist, SR48692 (2-[[1-(7-chloroquinolin-4-yl)-5-(2,6-dimethoxyphenyl)-1H-pyrazol-3-carbonyl]amino]adamantane-2-carboxylic acid), on amphetamine-induced behavioural sensitisation were studied with the locomotor activity of mice in an open-field as an experimental parameter. The animals were repeatedly pretreated with saline or amphetamine (2.0 mg/kg, i.p. once a day, every other day for 13 days) and 2, 9 and 16 days after the last injection they received an acute i.p. administration of saline or 0.3 mg/kg SR48692 15 min before a challenge i.p. injection of 2.0 mg/kg amphetamine. Locomotor activity of the amphetamine-challenged mice was significantly higher in amphetamine-pretreated animals than in saline-pretreated mice on days 9 and 16 after withdrawal. SR48692 prevented the expression of this behavioural sensitisation. In addition, in saline-pretreated mice, the first two challenge injections of amphetamine sufficed to induce a sensitized locomotor response to the third challenge injection of the drug. SR48692 administration before amphetamine challenge injections prevented the development of this challenge injection-induced sensitisation in saline-pretreated mice but not in amphetamine-pretreated animals. In order to determine the effects of SR48692 on the expression of amphetamine-induced behavioural sensitisation in the absence of this challenge injection-induced sensitisation, the experiment was redone with a single challenge test 9 days after pretreatment. Once again, SR48692 prevented the expression of amphetamine-induced behavioural sensitisation. These results suggest that neurotensinergic transmission has a critical role in both the initiation and expression of locomotor sensitisation to amphetamine.

The role of MK-801 in sensitization to stimulants

Behavioral responses to stimulants can be progressively and persistently enhanced by their repeated administration. This phenomenon, called behavioral sensitization, may underlie substance abuse, psychosis, recurrence in bipolar disorder, or other psychiatric problems. A growing body of work has implicated excitatory amino acid systems in behavioral sensitization. Most of the evidence for a role of excitatory amino acids has come from experiments demonstrating prevention of sensitization by excitatory amino acid antagonists, especially the noncompetitive NMDA receptor antagonist MK-801. Results of studies with MK-801 have varied, however, leading to conflicting interpretations of its relationship to behavioral sensitization. This paper critically discusses the design of experiments that have used MK-801, and interprets data from these experiments in terms of the two leading explanations for the role of MK-801: 1) that sensitization is an example of the family of plastic events that require excitatory amino acid transmission or 2) that interoceptive cues associated with MK-801 lead to state-dependent learning that modifies sensitization because, in essence, the animal does not recognize the stimulant as the same drug if it is given in close association with MK-801. Based on conflicting reports on effects of MK-801, we propose 1) strategies for distinguishing components of MK-801's effects on responses to stimulants, 2) a model that is a hybrid of the two interpretations of its effects on sensitization, and 3) experimental strategies for testing this model.

NMDA glutamate receptor role in the development of context-dependent and independent sensitization of the induction of stereotypy by amphetamine or apomorphine

We have been studying sensitization of psychostimulant-induced stereotyped behavior in mice using both a context-dependent and a context-independent paradigm. In the present study, we tested whether N-methyl-D-aspartate (NMDA) receptor antagonists prevent development of sensitization in either of these models. Male CF-1 mice were pretreated with 20 mg/kg (+)3-(2-carboxypiperazine-4yl)-propyl-1-phosphonic acid (CPP), 0.1 mg/kg (+)5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohopten-5, 10-imine maleate (MK-801, dizocilpine maleate), or 25 mg/kg 7-nitroindazole 30 min before a single dose (context-dependent paradigm) or each of three daily doses (context-independent paradigm) of 14 mg/kg amphetamine or 40 mg/kg apomorphine. Two days following this pretreatment, mice were injected with 7 mg/kg amphetamine or 3 mg/kg apomorphine. The stereotyped behavioral response was enhanced in mice pretreated with amphetamine or apomorphine alone, indicating that sensitization had developed. Both CPP and MK-801 prevented the development of sensitization in the context-dependent model but not in the context-independent paradigm. 7-Nitroindazole did not attenuate development of sensitization in either model. The results suggest that activation of glutamatergic receptors is important in some sensitization paradigms but not others, indicating that glutamate can be important but is not always required for the development of sensitization.

Effect of melatonin on cocaine-induced behavioral sensitization

Melatonin, a pineal hormone and a potent free radical scavenger with neuroprotective actions, has been reported to act as an inhibitor of nitric oxide synthase (NOS). We have earlier shown that inhibitors of NOS (N(omega)-nitro-L-arginine methyl ester [L-NAME], 7-nitroindazole [7-NI]) block cocaine-induced behavioral sensitization. In the present study, the effects of melatonin on cocaine behavior were studied. A single injection of melatonin markedly augmented cocaine-induced locomotor activity. Rats injected daily with melatonin prior to cocaine injections failed to elicit cocaine sensitization. These behavioral effects of melatonin do not completely mimic those of other NOS inhibitors, suggesting that the effects of melatonin on cocaine behavior are mediated by both NOS-dependent as well as NOS-independent mechanisms.

Both glutamate receptor antagonists and prefrontal cortex lesions prevent induction of cocaine sensitization and associated neuroadaptations

Behavioral sensitization to psychomotor stimulants is accompanied by a number of alterations in the mesoaccumbens dopamine (DA) system, including DA autoreceptor subsensitivity in the ventral tegmental area (VTA) and DA D1 receptor supersensitivity in the nucleus accumbens (NAc). We investigated the role of excitatory amino acid (EAA) transmission in the induction of cocaine sensitization and these accompanying DA receptor alterations. To do so, we used three glutamate receptor antagonists, the noncompetitive NMDA receptor antagonist MK-801 (0.1 mg/kg), the competitive NMDA receptor antagonist CGS 19755 (10.0 mg/kg), and the AMPA receptor antagonist NBQX (12.5 mg/kg). Rats received daily double injections of either one of these antagonists or saline with either cocaine (15.0 mg/kg) or saline for 5 days. Cocaine sensitization was defined as an increase in horizontal locomotor activity in response to cocaine challenge (7.5 mg/kg) on the third day of withdrawal. All three antagonists prevented the induction of cocaine sensitization. Extracellular single cell recordings revealed that these antagonists also prevented the induction of DA autoreceptor subsensitivity in the VTA and DA D1 receptor supersensitivity in the NAc. To determine whether the relevant glutamate receptors were under regulation by medial prefrontal cortex (mPFC) EAA efferents, we next lesioned the mPFC bilaterally with ibotenic acid at least 7 days before repeated cocaine treatment began. These lesions also prevented the induction of cocaine sensitization and the associated neuroadaptations. Our findings indicate that glutamate transmission from mPFC to the mesoaccumbens DA system is critical for the induction of cocaine sensitization and its cellular correlates.

The expression of cocaine sensitization is not prevented by MK-801 or ibotenic acid lesions of the medial prefrontal cortex

Previous work has established that the development of cocaine sensitization is prevented by co-administration of the non-competitive NMDA receptor antagonist MK-801 or by prior ibotenic acid lesions of the medial prefrontal cortex (PFC). The present study examined the effect of these treatments on the expression of cocaine sensitization. Rats were treated with 15 mg/kg cocaine for 5 days and then challenged with cocaine 3 days later to establish the presence of sensitization. The next day, rats received 0.1 mg/kg MK-801 30 min before cocaine challenge. This dose of MK-801, which is sufficient to prevent the development of cocaine sensitization, did not prevent its expression. Rather, it augmented the response of sensitized rats to cocaine challenge and produced a non-significant trend towards augmentation of the acute response to cocaine in saline-pretreated rats. For PFC lesion experiments, rats were sensitized to cocaine and then received either ibotenic acid or sham lesions of the PFC. One week later, all rats were challenged with cocaine. Sham lesioned and ibotenic acid lesioned rats exhibited the same degree of sensitization. Thus, neither NMDA receptor transmission nor PFC projections appear necessary for the expression of cocaine sensitization.

Sensitization to the locomotor stimulant effect of cocaine modifies the binding of [3H]MK-801 to brain regions and spinal cord of the mouse

The effects of chronic administration of cocaine and its subsequent withdrawal on the binding of [3H]MK-801, an antagonist of the N-methyl-D-aspartate (NMDA) receptor, to brain regions and spinal cord of the mouse was determined. Chronic administration of cocaine (10 mg/kg, SC) twice a day for 7 days sensitized male Swiss-Weber mice to its locomotor activity, as evidenced by a greater locomotor activity in comparison with vehicle-injected controls. In nonwithdrawing mice, the binding of [3H]MK-801 was increased in the cerebellum and spinal cord but was decreased in the cortex and hypothalamus. On the other hand, during withdrawal from cocaine, a significant decrease in the binding of [3H]MK-801 was observed only in the cortex, but the changes in the cerebellum, spinal cord and hypothalamus were normalized. It is concluded that, in the mouse, sensitization to the locomotor activating action of cocaine is associated with differential changes in the NMDA receptors of the central nervous system.

Early withdrawal from repeated cocaine administration upregulates muscarinic and dopaminergic D2-like receptors in rat neostriatum

The present results show an increase in locomotor activity 24 h following repeated cocaine administration only with the higher dose (10 mg/kg, i.p., daily for 1 week) compared to controls (administered with saline). Binding assays were done and the ligands used were [3H]N-methylscopolamine ([3H]-NMS), [3H]-SCH 23390, and [3H]-spiroperidol to determine muscarinic (M1- and M2-like), D1 and D2 receptors, respectively. Scatchard analyses revealed alterations in Bmax not only for muscarinic, but also for D2-like receptors that were significantly increased. On the other hand, no alterations were detected on D1-like receptors densities and dissociation constant values. However, the Kd value was significantly increased for D2 receptors. The changes in muscarinic receptors were observed predominantly on M2-like, which presented an increase of 84% with the 10 mg/kg, i.p., dose only. On D2-like receptors, increases of 63 and 54% were demonstrated with the doses of 5 and 10 mg/kg, i.p.. The preferential effects of cocaine on muscarinic and D2-like receptors were also demonstrated in vitro where decreases in [3H]-NMS and [3H]-spiroperidol binding were observed. The results indicate that the effects of cocaine on muscarinic and dopaminergic postsynaptic receptors are functions of dose, duration of treatment, and time of drug withdrawal.

Does the noncompetitive NMDA receptor antagonist dizocilpine (MK801) really block behavioural sensitization associated with repeated drug administration?

The term 'behavioural sensitization' refers to the progressively augmented behavioural response that is produced by many drugs of abuse upon their repeated administration. From most of the available data, it has been concluded that co-administration of an N-methyl-D-aspartate (NMDA) receptor antagonist [such as dizocilpine (MK801)] together with the sensitizing drug can block the development of behavioural sensitization. However, the picture might not be that simple. Recent experimental evidence suggests that instead of blocking sensitization, co-administration of dizocilpine enhances the effect of the the sensitizing drug or has more complex effects on the development of sensitization. In this article, Thomas Tzschentke and Werner Schmidt present these two different views and emphasize that the conclusions that can be drawn from sensitization experiments about the effects of dizocilpine and related drugs on behavioural plasticity crucially depend on how, when and under what conditions a test for sensitization is conducted.

Effects of cyclazocine on cocaine self-administration in rats

Cyclazocine is a kappa-opioid receptor agonist and mu-opioid receptor antagonist that was studied in the 1960s as a potential treatment for heroin addicts. Based on the evidence that opioid mechanisms modulate the reinforcing effects of cocaine, it has been suggested that cyclazocine be reconsidered for use in treating cocaine dependence. In the present study, the effects of orally administered (+/-)-cyclazocine, (+)-cyclazocine and (-)-cyclazocine on intravenous cocaine self-administration were assessed in rats. (+/-)-Cyclazocine produced a dose-related (2-8 mg/kg) decrease in cocaine intake without affecting bar-press responding for water. Neither enantiomer significantly altered responding for either cocaine or water. The efficacy of orally administered (+/-)-cyclazocine on cocaine self-administration was comparable to that previously observed using the intraperitoneal route. Distinct actions of the enantiomers of cyclazocine that might contribute to the unique efficacy of the racemate are discussed. Although the mechanistic basis for the results are not entirely understood, the data suggest that (+/-)-cyclazocine should be considered as a potential treatment for cocaine dependence.

Glutamatergic involvement in psychomotor stimulant action

The sympathomimetic psychomotor stimulants, including cocaine, amphetamines, and the phenylethylamine amphetamine-like derivatives, exert actions in mammalian systems that implicate involvement of the excitatory neurotransmitter, glutamate and its receptors. Despite evidence that psychomotor stimulants do not directly stimulate glutamate receptors, blockade of acute lethal, convulsive, circulatory, thermoregulatory, locomotor and stereotypical responses, as well as interference with slowly developing behavioral sensitization and brain monoaminergic neurotoxicities, can be achieved by receptor antagonists at both N-methyl-D-aspartate and AMPA/kainate glutamate receptor subtypes. Alterations in glutamatergic neurobiology, including elevations in extracellular glutamate levels, changes in glutamate receptor properties and glutamatergic neuronal degeneration, have also been attributed to psychomotor stimulant administration. Blockade of glutamate receptors offers therapeutic options in management of psychomotor stimulant toxicity.

The role of excitatory amino acids in behavioral sensitization to psychomotor stimulants

Behavioral sensitization refers to the progressive augmentation of behavioral responses to psychomotor stimulants that develops during their repeated administration and persists even after long periods of withdrawal. It provides an animal model for the intensification of drug craving believed to underlie addiction in humans. Mechanistic similarities between sensitization and other forms of neuronal plasticity were first suggested on the basis of the ability of N-methyl-D-aspartate (NMDA) receptor antagonists to prevent the development of sensitization [Karler, R., Calder, L. D., Chaudhry, I. A. and Turkanis, S. A. (1989) Blockade of "reverse tolerance" to cocaine and amphetamine by MK-801. Life Sci., 45, 599-606]. This article will review the large number of subsequent studies addressing: (1) the roles of NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and metabotropic glutamate receptors in the development and expression of behavioral sensitization, (2) excitatory amino acids (EAAs) and the role of conditioning in sensitization, (3) controversies regarding EAA involvement in behavioral sensitization based on studies with MK-801, (4) the effects of acute and repeated stimulant administration on EAA neurochemistry and EAA receptor expression, and (5) the neuroanatomy of EAA involvement in sensitization. To summarize, NMDA, AMPA metabotropic glutamate receptors all participate in the development of sensitization, while maintenance of the sensitized state involves alterations in neurochemical measures of EAA transmission as well as in the expression and sensitivity of AMPA and NMDA receptors. While behavioral sensitization likely involves complex neuronal circuits, with EAAs participating at several points within this circuitry, EAA projections originating in prefrontal cortex may play a particularly important role in the development of sensitization, perhaps via their regulatory effects on midbrain dopamine neurons. The review concludes by critically evaluating various hypotheses to account for EAA involvement in the development of behavioral sensitization, and considering the question of whether EAA receptors are involved in mediating the rewarding effects of psychomotor stimulants and sensitization of such rewarding effects.

Effects of acute and repeated administration of N-methyl-D-aspartate (NMDA) into the ventral tegmental area: locomotor activating effects of NMDA and cocaine

Repeated, intermittent administration of psychostimulants produces an enhancement of the subsequent behavioral effects of these drugs. This behavioral sensitization has been implicated in maintenance of and relapse to drug-taking. As a result, there has been great interest in elucidating the mechanisms underlying both the development and expression of sensitization. An accumulation of data from studies of stimulant-induced locomotor activity has implicated excitatory amino acids in the development of behavioral sensitization. In the present study, N-methyl-D-aspartate (NMDA) (0.6, 1.25 or 2.5 microg) infused bilaterally into the ventral tegmental area (VTA) produced dose-dependent locomotor activation. The locomotor activating effect of NMDA was increased following repeated NMDA administration (two exposures to intra-VTA NMDA), suggesting sensitization. However, repeated intra-VTA NMDA failed to sensitize rats to the locomotor activating effects of systemically administered cocaine (5.0, 10.0 or 20.0 mg/kg). These findings are consistent with the notion that repeated activation of NMDA receptors is sufficient for the development of behavioral sensitization to NMDA. Other neuroadaptations produced by repeated psychostimulant administration are required in order for the development of sensitization to the behavioral effects of those drugs.

Repeated amphetamine administration alters the expression of mRNA for AMPA receptor subunits in rat nucleus accumbens and prefrontal cortex

Recent evidence suggests that behavioral sensitization to amphetamine is associated with alterations in excitatory amino acid (EAA) transmission in perikarya (ventral tegmental area) and terminal regions (nucleus accumbens [NAc]) of the mesoaccumbens dopamine system. The present study determined whether repeated amphetamine administration alters expression of mRNAs for AMPA receptor subunits. We studied the NAc, because it is the site of expression of amphetamine sensitization, and the prefrontal cortex (PFC), because it is the origin of EAA projections that regulate the mesoaccumbens dopamine system. Rats were treated for 5 days with 5 mg/kg/day amphetamine sulfate or vehicle (controls) and perfused 3 or 14 days after the last injection. We used a novel in situ hybridization method that allows quantification of mRNA levels [Lu et al. (1996) J. Neurosci. Methods, 65:69-76]. Repeated amphetamine administration decreased levels of GluR1 and GluR2 but not GluR3 mRNAs in both core and shell subregions of the NAc at the 14 day withdrawal time; no changes were observed after 3 days of withdrawal. In contrast, levels of GluR1 mRNA in the PFC were increased at 3 but not 14 days of withdrawal, while GluR2 and 3 mRNAs were unchanged. Levels of GluR4 mRNA were very low in both NAc and PFC. Functional properties of heteromeric AMPA receptors are determined by subunit composition. Thus, the observed changes in mRNAs for AMPA receptor subunits may result in altered AMPA transmission in NAc and PFC. This, in turn, may influence the responsiveness of the mesoaccumbens DA system to psychomotor stimulants and potentially contribute to behavioral sensitization.

Ibotenic acid lesions of prefrontal cortex do not prevent expression of behavioral sensitization to amphetamine

We have shown previously that ibotenic acid lesions of the prefrontal cortex, performed prior to repeated amphetamine administration, do not affect sensitization of stereotyped behaviors but do prevent sensitization of post-stereotypy locomotor hyperactivity [Wolf et al., Neuroscience, 69 (1995) 417-439]. This could reflect an effect of the lesion on either development or expression of locomotor sensitization. To test the latter possibility, rats were treated with repeated amphetamine injections and tested to establish behavioral sensitization. Then, half received ibotenic acid lesions of prefrontal cortex and half received sham lesions. A second amphetamine challenge, 7 days later, demonstrated that the lesion failed to prevent expression of sensitization. Together with previous results, this suggests that intrinsic neurons of prefrontal cortex, most likely those sending excitatory amino acid-containing projections to the ventral tegmental area, are required for the development but not the expression of behavioral sensitization to amphetamine.