Alterations in excitatory amino acid-mediated regulation of midbrain dopaminergic neurones induced by chronic psychostimulant administration and stress: relevance to behavioural sensitization and drug addiction

Adolescent stress differentially modifies dopamine and norepinephrine release in the medial prefrontal cortex of adult rats

Adolescent stress (AS) has been associated with higher vulnerability to psychiatric disorders such as schizophrenia, depression, or drug dependence. Moreover, the alteration of brain catecholamine (CAT) transmission in the medial prefrontal cortex (mPFC) has been found to play a major role in the etiology of psychiatric disturbances. We investigated the effect of adolescent stress on CAT transmission in the mPFC of freely moving adult rats because of the importance of this area in the etiology of psychiatric disorders, and because CAT transmission is the target of a relevant group of drugs used in the therapy of depression and psychosis. We assessed basal dopamine (DA) and norepinephrine (NE) extracellular concentrations (output) by brain microdialysis in in the mPFC of adult rats that were exposed to chronic mild stress in adolescence. To ascertain the role of an altered release or reuptake, we stimulated DA and NE output by administering either different doses of amphetamine (0.5 and 1.0 mg / kg s.c.), which by a complex mechanism determines a dose dependent increase in the CAT output, or reboxetine (10 mg/kg i.p.), a selective NE reuptake inhibitor. The results showed the following: (i) basal DA output in AS rats was lower than in controls, while no difference in basal NE output was observed; (ii) amphetamine, dose dependently, stimulated DA and NE output to a greater extent in AS rats than in controls; (iii) reboxetine stimulated NE output to a greater extent in AS rats than in controls, while no difference in stimulated DA output was observed between the two groups. These results show that AS determines enduring effects on DA and NE transmission in the mPFC and might lead to the occurrence of psychiatric disorders or increase the vulnerability to drug addiction.

Repeated chemogenetic activation of dopaminergic neurons induces reversible changes in baseline and amphetamine-induced behaviors

RATIONALE

Repeated chemogenetic stimulation is often employed to study circuit function and behavior. Chronic or repeated agonist administration can result in homeostatic changes, but this has not been extensively studied with designer receptors exclusively activated by designer drugs (DREADDs).

OBJECTIVES

We sought to evaluate the impact of repeated DREADD activation of dopaminergic (DA) neurons on basal behavior, amphetamine response, and spike firing. We hypothesized that repeated DREADD activation would mimic compensatory effects that we observed with genetic manipulations of DA neurons.

METHODS

Excitatory hM3D(Gq) DREADDs were virally expressed in adult TH-Cre and WT mice. In a longitudinal design, clozapine N-oxide (CNO, 1.0 mg/kg) was administered repeatedly. We evaluated basal and CNO- or amphetamine (AMPH)-induced locomotion and stereotypy. DA neuronal activity was assessed using in vivo single-unit recordings.

RESULTS

Acute CNO administration increased locomotion, but basal locomotion decreased after repeated CNO exposure in TH-CrehM3Dq mice relative to littermate controls. Further, after repeated CNO administration, AMPH-induced hyperlocomotion and stereotypy were diminished in TH-CrehM3Dq mice relative to controls. Repeated CNO administration reduced DA neuronal firing in TH-CrehM3Dq mice relative to controls. A two-month CNO washout period rescued the decreases in basal locomotion and AMPH response.

CONCLUSIONS

We found that repeated DREADD activation of DA neurons evokes homeostatic changes that should be factored into the interpretation of chronic DREADD applications and their impact on circuit function and behavior. These effects are likely to also be seen in other neuronal systems and underscore the importance of studying neuroadaptive changes with chronic or repeated DREADD activation.

Repeated exposure to cocaine during adolescence enhances the rewarding threshold for cocaine-conditioned place preference in adulthood

Previous studies have shown that adolescent exposure to cocaine increases drug use in adulthood, albeit incubation of cocaine seeking was found to be attenuated in rats trained to self-administer cocaine during adolescence. We here hypothesize that adolescent exposure to cocaine could alter the rewarding properties of the psychostimulant in adulthood. By employing two of the most widely used animal-experimental-preclinical models to investigate drug addiction, we evaluated whether contingent versus non-contingent cocaine self-administration during adolescence modulates its rewarding threshold in adulthood evaluated by conditioned place preference (CPP). Cocaine self-administration during adolescence increases the rewarding threshold in adulthood; CPP for cocaine was observed at the higher (20 mg/kg), but not at the lower (10 mg/kg), dose employed. Rats exposed to either contingent or non-contingent cocaine during adolescence exhibited the same behavior in the CPP paradigm suggesting that, under our experimental conditions, cocaine rewarding properties are shaped by the psychostimulant itself and not by its motivational effects. From a mechanistic standpoint, the preference for the 20 mg/kg cocaine-paired side in a CPP paradigm appears to depend, at least partially, upon the formation of GluA2-lacking Ca²⁺ -permeable AMPA receptors and the consequent increase of αCaMKII activity in the NAc, both of which are instead reduced when the 10 mg/kg dose was used. In conclusion, contingent or non-contingent cocaine exposure during adolescence desensitizes adult animals to a rewarding dose of cocaine (10 mg/kg) elevating the rewarding threshold necessary (20 mg/kg) to drive conditioned place preference, an effect that may predispose to higher consumption of cocaine during adulthood.

Neural Substrates and Circuits of Drug Addiction

Drug addiction is a chronic relapsing disorder, and a significant amount of research has been devoted to understand the factors that contribute to the development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide an overview of various theories of addiction to drugs of abuse and the neurobiology involved in elements of the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the role of the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the contribution of these pathways and associated circuits to conditioned responses, drug craving, and loss of behavioral control that may underlie drug relapse. By enhancing the understanding of the neurobiological factors that mediate drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.

Systems level neuroplasticity in drug addiction

Drug addiction is a chronic relapsing disorder for which research has been dedicated to understand the various factors that contribute to development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide a broad overview of various theories of addiction, drugs of abuse, and the neurobiology involved across the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the involvement of these pathways and associated circuits in mediating conditioned responses, drug craving, and loss of behavioral control thought to underlie withdrawal and relapse. With a better understanding of the neurobiological factors that underlie drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.

Descending glutamatergic pathways of PFC are involved in acute and chronic action of methylphenidate

Progressive augmentation of behavioral response following repeated psychostimulant administrations is known as behavioral sensitization, and is an indicator of a drug's liability for abuse. It is known that methylphenidate (MPD) (also known as Ritalin), a drug used to treat attention-deficit hyperactivity disorder (ADHD), induces sensitization in animals following repeated injections. It was recently reported that bilateral electric (non-specific) lesion of prefrontal cortex (PFC) prevented MPD elicited behavioral sensitization. Since PFC sends glutamatergic afferents to both ventral tegmental area (VTA) and nucleus accumbens (NAc), sites that are involved in induction and expression of behavioral sensitization respectively and glutamate from PFC is known to modulate dopamine cell activity in VTA and NAc, this study investigated the role of descending glutamate from PFC in MPD elicited behavioral sensitization. Locomotor activity of three groups of rats-control, sham operated and group with specific chemical lesion of glutamate neurons of PFC-was recorded using an open-field assay. On experimental day (ED) 1, the locomotor activity was recorded post a saline injection. The sham and lesion groups underwent respective surgeries on ED 2, and were allowed to recover for 5 days (from ED 3 to ED 7). The post-surgery baseline was recorded on ED 8 following a saline injection. On ED's 9 through 14, 2.5 mg/kg MPD was given, followed by a 4-day washout period (ED 15 -18). All three groups received a rechallenge injection of 2.5 mg/kg on ED 19 and their locomotor activity on various days was analyzed. It was found that ibotenic acid lesion modulated the acute and chronic effects of MPD and hence suggests that PFC glutamatergic afferents are involved in the acute effect of MPD as well as in its chronic effects such as behavioral sensitization to MPD.

Midbrain acetylcholine and glutamate receptors modulate accumbal dopamine release

This study determined the role of ventral tegmental area acetylcholine and glutamate receptors in modulating laterodorsal tegmentum stimulation-evoked dopamine efflux in the nucleus accumbens. Rapid changes in dopamine oxidation current were measured at carbon fiber microelectrodes using fixed potential amperometry in urethane anesthetized male mice. Intraventral tegmental area infusions of the muscarinic acetylcholine receptor antagonist scopolamine, the nicotinic acetylcholine receptor antagonist mecamylamine, or the ionotropic glutamate receptor antagonist kynurenate significantly diminished dopamine efflux in the nucleus accumbens evoked by brief electrical stimulation of the laterodorsal tegmentum. These findings suggest that acetylcholine and ionotropic glutamate receptors influence rapid dopaminergic activity and thus the communication of behaviorally relevant information from ventral tegmental area dopamine cells to forebrain areas.

The neurocircuitry of addiction: an overview

Drug addiction presents as a chronic relapsing disorder characterized by persistent drug-seeking and drug-taking behaviours. Given the significant detrimental effects of this disease both socially and economically, a considerable amount of research has been dedicated to understanding a number of issues in addiction, including behavioural and neuropharmacological factors that contribute to the development, loss of control and persistence of compulsive addictive behaviours. In this review, we will give a broad overview of various theories of addiction, animal models of addiction and relapse, drugs of abuse, and the neurobiology of drug dependence and relapse. Although drugs of abuse possess diverse neuropharmacological profiles, activation of the mesocorticolimbic system, particularly the ventral tegmental area, nucleus accumbens, amygdala and prefrontal cortex via dopaminergic and glutamatergic pathways, constitutes a common pathway by which various drugs of abuse mediate their acute reinforcing effects. However, long-term neuroadaptations in this circuitry likely underlie the transition to drug dependence and cycles of relapse. As further elucidated in more comprehensive reviews of various subtopics on addiction in later sections of this special issue, it is anticipated that continued basic neuroscience research will aid in the development of effective therapeutic interventions for the long-term treatment of drug-dependent individuals.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Identification of an excitatory amino acid-mediated component of the ventral tegmental area local field potential response to medial prefrontal cortex stimulation: effect of acute d-amphetamine

The induction of sensitisation to the behavioural effects of d-amphetamine - a model of drug addiction - involves the potentiation of exctiatory amino acid (EAA)-ergic synapses on dopaminergic neurons in the ventral tegmental area (VTA). Such potentiation has been reported as early as 2 hr post-injection, however earlier time points have not been assessed. Consequently, we examined the effects of systemic d-amphetamine on an EAA-mediated component of the VTA local field potential response to stimulation of the medial prefrontal cortex, an EAAergic afferent critical for sensitisation, over the immediate 2 hr post-injection period. D-amphetamine and saline both depressed the amplitude of this component to a similar extent throughout the recording session. It is concluded that overt aspects of EAA-mediated potentiation appear to be delayed with respect to drug administration, which may have implications for sensitisation's putative role in linking drug-related environmental stimuli and the central effects of the drug.

Local injection of a glutamate uptake inhibitor into the ventral tegmental area produces sensitization to the behavioural effects of d-amphetamine

Circumstantial evidence suggests that sensitization to the behavioral effects of d-amphetamine is mediated by increased glutamate levels in the ventral tegmental area. To test this directly, the present study examined whether increasing glutamate levels in the ventral tegmental area with a glutamate uptake inhibitor is sufficient, in the absence of d-amphetamine administration, to elicit sensitization to a subsequent d-amphetamine challenge. Rats were treated bilaterally once a day for 2 days with either intra-ventral tegmental area L-trans-pyrollidine-2,4-dicarboxylic acid (50 nmol), saline, L-trans-pyrollidine-2,4-dicarboxylic acid coadministered with the competitive N-methyl-d-aspartate antagonist (+/-)-3-(2-carboxy-piperazin-4-yl)-propyl-1-phosphonic acid; CPP, 0.5 nmol), or CPP alone (0.5 nmol; all 1.0 microl/side). Following a 2 day withdrawal period, all rats were administered systemic d-amphetamine (1 mg/kg, i.p.). Repeated intra-ventral tegmental area injection of L-trans-pyrollidine-2,4-dicarboxylic acid sensitized animals to the behavioral effects of a systemic d-amphetamine challenge, an action which was blocked by co-administration of CPP. The results directly implicate ventral tegmental area glutamate in the process of sensitization to d-amphetamine. Furthermore, they demonstrate that inhibition of glutamate uptake produces the neuroadaptations necessary to induce sensitization, adding support to the contention that d-amphetamine sensitizes by modulating glutamate uptake.

Cocaine-induced potentiation of synaptic strength in dopamine neurons: behavioral correlates in GluRA(-/-) mice

Synaptic plasticity in the mesolimbic dopamine (DA) system is thought to contribute to the neural adaptations that mediate behavioral sensitization, a model for core aspects of addiction. Recently, it has been demonstrated that multiple classes of drugs of abuse, as well as acute stress, enhance strength at excitatory synapses on midbrain DA neurons. Here, we show that both the cocaine- and stress-induced synaptic enhancement involves an up-regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. This enhancement requires the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit GluRA as evidenced by its absence in mice lacking this subunit. The cocaine-elicited, but not the stress-elicited, synaptic potentiation in DA neurons was blocked by a D1-like receptor antagonist, indicating that the in vivo triggering mechanisms differ for these forms of experience-dependent synaptic modification. Surprisingly, behavioral sensitization to cocaine was elicited in GluRA(-/-) mice, indicating that potentiation of excitatory synaptic transmission in DA neurons is not necessary for this form of behavioral plasticity. However, GluRA(-/-) mice did not exhibit a conditioned locomotor response when placed in a context previously paired with cocaine, nor did they exhibit conditioned place preference in response to cocaine. We suggest that the drug-induced enhancement of excitatory synaptic transmission in midbrain DA neurons, although not required for behavioral sensitization per se, may contribute to the attribution of incentive value to drug-associated cues.

Environmental context and drug history modulate amphetamine-induced c-fos mRNA expression in the basal ganglia, central extended amygdala, and associated limbic forebrain

The context in which amphetamine is administered modulates its ability to induce both behavioral sensitization and immediate early gene expression. When given in a novel test environment amphetamine produces greater levels of c-fos and arc mRNA expression in many brain regions relative to when it is given in the home cage. The purpose of the current study was to determine if environment and drug history interact to influence amphetamine-induced c-fos mRNA expression. Rats with a unilateral 6-hydroxydopamine lesion were treated for 7 days with saline or 0.5 mg/kg of d-amphetamine (i.v.) in a distinct and relatively novel test environment (Novel), or in their home cage (Home). Following a 10-12-day withdrawal period, a challenge injection of either saline or 0.5 mg/kg d-amphetamine was administered. In situ hybridization histochemistry was used to examine c-fos mRNA expression in several regions of the basal ganglia, the central extended amygdala, and limbic forebrain. In most brain regions amphetamine given in the Novel environment produced greater c-fos mRNA expression than when given it was given at Home, and drug history had no effect on amphetamine-induced c-fos mRNA expression. However, within the subthalamic nucleus, substantia nigra reticulata, and central nucleus of the amygdala prior experience with amphetamine in the Novel but not Home environment enhanced the effect of an amphetamine challenge injection on c-fos mRNA expression. In contrast, there was a decrease in c-fos mRNA expression in amphetamine-pretreated animals, regardless of environmental context, in the ventral portion of the far caudal striatum. Reexposure to an environment previously paired with amphetamine produced a conditioned increase in c-fos mRNA expression in portions of the caudate-putamen, the subthalamic nucleus, the nucleus accumbens shell and a conditioned decrease in c-fos mRNA expression in the central nucleus of the amygdala. We conclude that environmental context and drug history interact to alter the basal ganglia and central extended amygdala circuitry engaged by subsequent exposure to amphetamine, or exposure to an environment previously paired with amphetamine.

Pedunculopontine tegmental stimulation evokes striatal dopamine efflux by activation of acetylcholine and glutamate receptors in the midbrain and pons of the rat

The pedunculopontine tegmental nucleus appears to influence striatal dopamine activity via cholinergic and glutamatergic afferents to dopaminergic cells of the substantia nigra pars compacta. We measured changes in striatal dopamine oxidation current (dopamine efflux) in response to electrical stimulation of the pedunculopontine tegmental nucleus using in vivo electrochemistry in urethane-anaesthetized rats. Pedunculopontine tegmental nucleus stimulation evoked a three-component change in striatal dopamine efflux, consisting of: (i) an initial rapid increase of 2 min duration; followed by (ii) a decrease below prestimulation levels of 9 min duration; then by (iii) a prolonged increase lasting 35 min. Intra-nigral infusions of the ionotropic glutamate receptor antagonist kynurenate (10 microg/ microL) or the nicotinic cholinergic receptor antagonist mecamylamine (5 microg/0.5 microL) selectively attenuated the rapid first component, while systemic injections of the muscarinic cholinergic antagonist scopolamine (5 mg/kg, i.p.) diminished the second and third components. In addition, intra-pedunculopontine tegmental nucleus infusions of the M2 muscarinic antagonist methoctramine (50 microg/ microL) selectively abolished the inhibitory second component, while intranigral infusions of scopolamine (200 microg/ microL) selectively abolished the prolonged third component. Intra-nigral infusions of the metabotropic glutamate receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine (2 microg/ microL) had no effect on pedunculopontine tegmental nucleus-elicited striatal dopamine efflux. These results suggest that the pedunculopontine tegmental nucleus utilizes nicotinic and ionotropic glutamate receptors in the substantia nigra to mediate rapid activation, M2-like muscarinic autoreceptors in the pedunculopontine tegmental nucleus to mediate decreased activation, and muscarinic receptors in the substantia nigra (probably of the M5 subtype) to mediate prolonged activation, of the nigrostriatal dopaminergic system.

Amphetamine and cocaine do not increase Narp expression in rat ventral tegmental area, nucleus accumbens or prefrontal cortex, but Narp may contribute to individual differences in responding to a novel environment

Narp is an immediate early gene product that acts extracellularly to cluster AMPA receptors at excitatory synapses. The present study tested the hypothesis that drugs of abuse alter Narp expression and thereby influence AMPA receptor transmission in addiction-related circuits. Immunohistochemical studies demonstrated the existence of Narp-positive cells in hippocampus, prefrontal cortex (PFC) and nucleus accumbens (NAc), with lower levels of staining in the ventral tegmental area (VTA). To study the effects of psychomotor stimulants, Narp levels were quantified by Western blotting and normalized to actin. There were no differences in Narp levels in any brain region between rats treated with repeated saline injections, a single amphetamine injection (5 mg/kg), repeated amphetamine injections (5 mg/kg x 5 days), or repeated cocaine injections (20 mg/kg twice daily x 7 days). We also examined the possible role of Narp in individual differences in responding to a novel environment, a predictor of behavioural responses to psychomotor stimulant drugs including the propensity to acquire drug self-administration. Narp levels in the PFC, but not other regions, were significantly correlated with locomotor activity in a novel environment. These findings suggest that differential Narp expression in the PFC may be involved in determining individual vulnerability to drugs of abuse, perhaps by influencing the activity of its excitatory projections.

In vivo modulation of ventral tegmental area dopamine and glutamate efflux by local GABA(B) receptors is altered after repeated amphetamine treatment

The activity of dopamine neurons in the ventral tegmental area is modulated by excitatory (glutamatergic) and inhibitory (GABAergic) afferents. GABA, released by intrinsic neurons and by projection neurons originating in the nucleus accumbens and other regions, inhibits dopamine neurons via activation of GABA(A) and GABA(B) receptor subtypes. Using in vivo microdialysis in freely moving rats, we investigated the role of ventral tegmental area GABA(B) receptors in modulating levels of dopamine and glutamate within the ventral tegmental area, both in naive rats and in rats treated repeatedly with saline or amphetamine (5 mg/kg i.p., for 5 days). In naive rats, administration of a potent and selective GABA(B) receptor antagonist (CGP 55845A) into the ventral tegmental area elicited a concentration-dependent increase in dopamine levels, but did not alter glutamate levels. In rats tested 3 days after discontinuing repeated amphetamine administration, 50 microM CGP 55845A increased dopamine levels to a greater extent than in saline controls. This difference was no longer present in rats tested 10-14 days after discontinuing repeated amphetamine injections. CGP 55845A (50 microM) had no effect on glutamate levels in the ventral tegmental area of saline-treated rats. However, it produced a robust increase in glutamate levels in rats tested 3 days, but not 10-14 days, after discontinuing repeated amphetamine injections. These results suggest that somatodendritic dopamine release is normally under strong tonic inhibitory control by GABA(B) receptors. Repeated amphetamine administration enhances GABA(B) receptor transmission in the ventral tegmental area during the early withdrawal period, increasing inhibitory tone on both dopamine and glutamate levels. This is the first demonstration, in an intact animal, that drugs of abuse alter GABA(B) receptor transmission in the ventral tegmental area.

Addiction and the brain: the neurobiology of compulsion and its persistence

People take addictive drugs to elevate mood, but with repeated use these drugs produce serious unwanted effects, which can include tolerance to some drug effects, sensitization to others, and an adapted state - dependence - which sets the stage for withdrawal symptoms when drug use stops. The most serious consequence of repetitive drug taking, however, is addiction: a persistent state in which compulsive drug use escapes control, even when serious negative consequences ensue. Addiction is characterized by a long-lasting risk of relapse, which is often initiated by exposure to drug-related cues. Substantial progress has been made in understanding the molecular and cellular mechanisms of tolerance, dependence and withdrawal, but as yet we understand little of the neural substrates of compulsive drug use and its remarkable persistence. Here we review evidence for the possibility that compulsion and its persistence are based on a pathological usurpation of molecular mechanisms that are normally involved in memory.

Single cocaine exposure in vivo induces long-term potentiation in dopamine neurons

How do drugs of abuse modify neural circuitry and thereby lead to addictive behaviour? As for many forms of experience-dependent plasticity, modifications in glutamatergic synaptic transmission have been suggested to be particularly important. Evidence of such changes in response to in vivo administration of drugs of abuse is lacking, however. Here we show that a single in vivo exposure to cocaine induces long-term potentiation of AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid)-receptor-mediated currents at excitatory synapses onto dopamine cells in the ventral tegmental area. Potentiation is still observed 5 but not 10 days after cocaine exposure and is blocked when an NMDA (N-methyl-d-aspartate) receptor antagonist is administered with cocaine. Furthermore, long-term potentiation at these synapses is occluded and long-term depression is enhanced by in vivo cocaine exposure. These results show that a prominent form of synaptic plasticity can be elicited by a single in vivo exposure to cocaine and therefore may be involved in the early stages of the development of drug addiction.

Cocaine self-administration alters the morphology of dendrites and dendritic spines in the nucleus accumbens and neocortex

We studied the influence of cocaine use on the structure of neurons in brain regions that contribute to its rewarding effects by allowing rats to self-administer cocaine (0.33 mg/infusion) for 1 h a day for 1 month. Control animals were left undisturbed or allowed to work for food for the same period of time. After an additional 1 month drug-free period the brains were processed for Golgi-Cox staining. In rats that self-administered cocaine, but not rats that worked for food, there was a significant increase in dendritic branching and in the density of dendritic spines on medium spiny neurons in the shell of the nucleus accumbens and on pyramidal cells in the prefrontal and parietal (but not occipital) cortex. There was also a 2.6-fold increase in the incidence of spines with multiple heads (branched spines) on medium spiny neurons. Finally, in the prefrontal cortex some of the apical dendrites of pyramidal cells appeared misshaped, having large bulbous structures on their terminal tips. We speculate that cocaine self-administration experience alters patterns of synaptic connectivity within limbocortical circuitry that is thought to contribute to cocaine's incentive motivational effects and may have neuropathological effects in frontal areas involved in decision making and judgment. Together, these two classes of drug-induced neuroadaptations may contribute to the development of addiction.

Enhanced vulnerability to cocaine self-administration is associated with elevated impulse activity of midbrain dopamine neurons

Individual differences in responding to a novel environment predict behavioral and neurochemical responses to psychostimulant drugs. Rats with a high locomotor response to a novel environment (HRs) exhibit enhanced self-administration (SA) behavior, sensitization, and basal or drug-induced dopamine release in the nucleus accumbens compared with rats with a low response to the novel context (LRs). In this study, we determined whether such differences in vulnerability to drug addiction might be related to differences in dopamine (DA) neuron activity. Rats were divided into HRs and LRs according to their response to a novel environment and then tested for acquisition of cocaine SA. HRs rapidly acquired cocaine SA (175 microg/kg per infusion), whereas LRs did not. Differences in cocaine SA were not caused by differences in exploratory behavior or sampling because these behaviors did not differ in HRs and LRs self-administering a saline solution. In a separate experiment, we used extracellular single-unit recordings and found that HRs exhibit higher basal firing rates and bursting activity of DA neurons in the ventral tegmental area and, to a lesser extent, in the substantia nigra pars compacta. The greater activity of midbrain DA cells in HRs was accompanied by reduced sensitivity to the inhibitory effects of a DA D2-class receptor agonist, indicating possible subsensitivity of impulse-regulating DA autoreceptors. These results demonstrate that differences in the basal activity of DA neurons may be critically involved in determining individual vulnerability to drugs of abuse.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Flupenthixol as a potential pharmacotreatment of alcohol and cocaine abuse/dependence

Preclinical and clinical studies suggest that the mesolimbic dopamine system plays a major role in mediating the reinforcing effects of drugs of abuse, including alcohol and psychostimulants, and that pharmacological blockade of dopamine D1 and/or D2 receptors may reduce intake of these drugs, as well as relapse rates. The neuroleptic flupenthixol, which has dopamine D1 and D2 receptor antagonist properties and which may be given intramuscularly in order to improve compliance, has been studied as a possible anti-craving drug in substance abuse disorders. Flupenthixol has been shown to attenuate the discriminative stimulus effects of psychostimulants, as well as their intake in animal models of drug abuse. In addition, the compound was found to reduce alcohol intake in a rat model of alcoholism, but the 'anti-alcohol' effect appeared to be only weakly selective and nonspecific. Clinically, the drug has been studied in alcoholics, cocaine addicts and in patients with comorbid psychiatric disorders. Although the data base is still limited and a number of recent trials have not been completely analyzed, these studies suggest that flupenthixol may be useful in decreasing cocaine consumption. Recent studies in alcoholism, however, have shown disappointing results. A number of pilot studies suggest that probably the most promising area may be the treatment of substance abuse/dependence in patients with comorbid psychiatric disorders. Future studies should focus on dosing issues, the differentiation between short- and long-term effects and the identification of subgroups of patients with particular psychopathology.

Modulation of long-term depression by dopamine in the mesolimbic system

Long-lasting adaptations in the mesolimbic dopamine (DA) system in response to drugs of abuse likely mediate many of the behavioral changes that underlie addiction. Recent work suggests that long-term changes in synaptic strength at excitatory synapses in the two major components of this system, the nucleus accumbens (NAc) and ventral tegmental area, may be particularly important for the development of drug-induced sensitization, a process that may contribute to addiction, as well as for normal response-reinforcement learning. Using whole-cell patch-clamp recording techniques from in vitro slice preparations, we have examined the existence and basic mechanisms of long-term depression (LTD) at excitatory synapses on both GABAergic medium spiny neurons in the NAc and dopaminergic neurons in the midbrain. We find that both sets of synapses express LTD but that their basic triggering mechanisms differ. Furthermore, DA blocks the induction of LTD in the midbrain via activation of D2-like receptors but has minimal effects on LTD in the NAc. The existence of LTD in mesolimbic structures and its modulation by DA represent mechanisms that may contribute to the modifications of neural circuitry that mediate reward-related learning as well as the development of addiction.

Peptide signaling paths related to intoxication, memory and addiction

Abstract Many peptides bind to G protein-coupled receptors and activate intracellular signaling paths for adaptive cellular responses. The components of these paths can be affected by signals from other neurotransmitters to produce overall integrated results not easily predicted from customary a priori considerations. This intracellular cross-talk among signaling paths provides a "filter" through which long-term tonic signals affect short-term phasic signals as they progress toward the nucleus and induce long-term adaptation of gene expression which provide enduring attributes of acquired memories and addictions. Peptides of the PACAP family provide intracellular signaling that involves kinases, scaffolding interactions, Ca2 + mobilization, and gene expression to facilitate development of tolerance to alcohol and development of associative memories. The peptide-induced enhancement of NMDA receptor responses to extracellular glutamate also may increase behavioral sensitization to the low doses of alcohol that occur at the onset of each bout of drinking. Because many gene products participate in each signaling path, each behavioral response to alcohol is a polygenic process of many steps with no single gene product sufficient to interpret fully the adaptive response to alcohol. Different susceptibility of individuals to alcohol addiction may be a cumulative result of small differences among the many signaling components. Understanding this network of signals may help interpret future "magic bullets" proposed to treat addiction.

Amphetamine-induced glutamate efflux in the rat ventral tegmental area is prevented by MK-801, SCH 23390, and ibotenic acid lesions of the prefrontal cortex

We showed previously that amphetamine challenge produces a delayed increase in glutamate efflux in the ventral tegmental area of both naive and chronic amphetamine-treated rats. The present study examined the mechanisms underlying this response. The NMDA receptor antagonist MK-801 (0.1 mg/kg, i.p.) or the D1 dopamine receptor antagonist SCH 23390 (0.1 mg/kg, i.p.), given 30 min before acute amphetamine (5 mg/kg, i.p.), prevented amphetamine-induced glutamate efflux. Neither antagonist by itself altered glutamate efflux. Ibotenic acid lesions of the prefrontal cortex similarly prevented amphetamine-induced glutamate efflux, while producing a trend toward decreased basal glutamate levels (82.8% of sham group). Previous work has shown that the doses of NMDA and D1 receptor antagonists used in this study prevent the induction of behavioral sensitization when coadministered repeatedly with amphetamine, and that identical prefrontal cortex lesions performed before repeated amphetamine prevent the induction of ambulatory sensitization. Thus, treatments that prevent acute amphetamine from elevating glutamate efflux in the ventral tegmental area also prevent repeated amphetamine from eliciting behavioral sensitization. These findings suggest that repeated elevation of glutamate levels during a chronic amphetamine regimen may contribute to the cascade of neuroadaptations within the ventral tegmental area that enables the induction of sensitization.

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.

Withdrawal from repeated amphetamine administration reduces NMDAR1 expression in the rat substantia nigra, nucleus accumbens and medial prefrontal cortex

Glutamate plays a critical role in neuroadaptations induced by drugs of abuse. This study determined whether expression of the NMDAR1 subunit of the NMDA receptor is altered by repeated amphetamine administration. We quantified NMDAR1 mRNA (using in situ hybridization with 35S-labelled oligonucleotide probes) and immunolabelling (using immunocytochemistry with 35S-labelled secondary antibodies) in rat ventral midbrain, nucleus accumbens and prefrontal cortex after 3 or 14 days of withdrawal from five daily injections of saline or amphetamine sulphate (5 mg/kg/day). No changes in NMDAR1 expression were observed after 3 days of withdrawal, whereas significant decreases were observed in all regions after 14 days. NMDAR1 mRNA levels in midbrain were too low for reliable quantification, but immunolabelling was decreased significantly in intermediate and caudal portions of the substantia nigra. This may indicate a reduction in excitatory drive to substantia nigra dopaminergic neurons. In the nucleus accumbens, there were significant decreases in NMDAR1 mRNA levels (74.8 +/- 7. 7% of control, P < 0.05) and immunolabelling (76.7 +/- 4.4%, P < 0. 05). This may account for previously-reported decreases in the electrophysiological responsiveness of nucleus accumbens neurons to NMDA after chronic amphetamine treatment, and contribute to dysregulation of goal-directed behaviour. In prefrontal cortex, there was a significant decrease in NMDAR1 mRNA levels (76.1 +/- 7. 1%, P < 0.05) and a trend towards decreased immunolabelling (89.5 +/- 7.0%). This may indicate decreased neuronal excitability within prefrontal cortex. A resultant decrease in activity of excitatory prefrontal cortical projections to nucleus accumbens or midbrain could synergize with local decreases in NMDAR1 to further reduce neuronal excitability in these latter regions.

Properties and plasticity of excitatory synapses on dopaminergic and GABAergic cells in the ventral tegmental area

Excitatory inputs to the ventral tegmental area (VTA) influence the activity of both dopaminergic (DA) and GABAergic (GABA) cells, yet little is known about the basic properties of excitatory synapses on these two cell types. Using a midbrain slice preparation and whole-cell recording techniques, we found that excitatory synapses on DA and GABA cells display several differences. Synapses on DA cells exhibit a depression in response to repetitive activation, are minimally affected by the GABAB receptor agonist baclofen, and express NMDA receptor-dependent long-term potentiation (LTP). In contrast, synapses on GABA cells exhibit a facilitation in response to repetitive activation, are depressed significantly by baclofen, and do not express LTP. The relative contribution of NMDA and non-NMDA receptors to the synaptic currents recorded from the two cell types is the same as is the depression of synaptic transmission elicited by the application of adenosine, serotonin, or methionine enkephalin (met-enkephalin). The significant differences in the manner in which excitatory synaptic inputs to DA and GABA cells in the VTA can be modulated have potentially important implications for understanding the behavior of VTA neurons during normal behavior and during pathological states such as addiction.

Repeated amphetamine administration alters AMPA receptor subunit expression in rat nucleus accumbens and medial prefrontal cortex

Glutamate is critical for the induction and maintenance of behavioral sensitization and associated neuroadaptations in the mesocorticolimbic dopamine (DA) system. We have shown previously [Lu et al. (1997) Synapse 26:269-280] that repeated amphetamine administration alters AMPA receptor subunit mRNA levels in rat nucleus accumbens (NAc) and medial prefrontal cortex (PFC). The present study determined if amphetamine elicits corresponding changes in AMPA receptor subunit immunolabeling. Rats were injected with amphetamine sulphate (5 mg/kg/day) or saline for 5 days and perfused 3 or 14 days after the last injection. AMPA receptor subunit immunolabeling was quantified using autoradiographic immunocytochemistry. In the NAc, GluR1 and GluR2 immunolabeling were unchanged after 3 days of withdrawal, but both were decreased significantly after 14 days of withdrawal (GluR1, 85.5+/-2.6% of control group, P<0.01; GluR2, 79.2+/-3.2%, P<0.01). Analysis of core and shell subregions at the 14-day withdrawal time indicated that GluR1 immunolabeling decreased significantly in shell, while GluR2 immunolabeling decreased significantly in both core and shell. No changes in GluR2/3, GluR2/4, or GluR4 immunolabeling in NAc were found at either withdrawal time. In the PFC, GluR1 immunolabeling increased after 3 days of withdrawal (115.3+/-7.0%, P<0.01) but returned to control levels after 14 days. The present results correspond well with our previous findings at the mRNA level. These alterations in AMPA receptor expression may account for previously described changes in the electrophysiological responsiveness of NAc and PFC neurons to glutamate and AMPA. Along with alterations in DA function, they may contribute to drug-induced dysregulation of reward-related neurotransmission.

Long-term potentiation at excitatory amino acid synapses on midbrain dopamine neurons

Evidence suggests that a process analogous to long-term potentiation (LTP) may underlie the enhanced behavioural responses attending chronic administration of amphetamine and cocaine in animals (behavioural sensitization). Augmented excitatory amino acid (EAA)-mediated transmission at the level of midbrain dopamine neurons has been implicated as a change critical to the development of sensitization. Here we provide an initial demonstration that EAA synapses on dopamine neurons can undergo plasticity. Tetanic stimulation of the subthalamic nucleus induced a long-lasting increase (39.2 +/- 10.4%) in the amplitude of excitatory postsynaptic potentials recorded in dopamine neurons of the substantia nigra. This LTP, which did not occur in the presence of NMDA antagonists, may constitute the mechanism that lies at the heart of sensitization.