Molecular mechanisms of drug addiction in the mesolimbic dopamine pathway

Molecular mechanisms of drug addiction in the mesolimbic dopamine pathway

seminars in THE NEUROSCIENCES,Vol 5, 1993 : pp 369-376 Molecular mechanisms of drug addiction in the mesolimbic dopamine pathway Eric J. Nestler Stud...

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seminars in THE NEUROSCIENCES,Vol 5, 1993 : pp 369-376

Molecular mechanisms of drug addiction in the mesolimbic dopamine pathway Eric J. Nestler Studies of intracellular messenger proteins have offered a novel approach by which to obtain a more complete understanding of the molecular mechanisms underlying drug addiction. We have identified some common biochemical adaptations induced by chronic morphine, cocaine and, in preliminary studies, alcohol, in the mesolimbic dopamine system. These adaptations include decreased levels of specific G-protein subunits and increased levels of the cyclic A M P second messenger and protein phosphorylation pathway in the nucleus accumbens (NAcc), and increased levels of tyrosine hydroxylase and decreased levels of neurofilament proteins in the ventral tegmental pathway. In addition, inherent differences in these same proteins have been demonstrated between Lewis and Fischer rats, inbred strains that show different behavioral responses to several drugs of abuse, and among individual outbred Sprague-Dawley rats which correlate with individual differences in several drug-related behaviors. These studies promise to reveal mechanisms by which drugs of abuse induce addictive changes in brain function, as well as some of the genetic and environmental factors that contribute to individual vulnerability to drug addiction.

the pathophysiology and individual vulnerability for drug addiction.

Experimental approaches to identify molecular mechanisms of drug addiction A large neuropharmacological literature, described elsewhere in this issue, has established the likely importance of the mesolimbic dopamine system as a critical neural substrate of the reinforcing or psychologically addicting properties of drugs of abuse. The mesolimbic dopamine system consists of dopaminergic neurons in the ventral tegmental area (VTA) and its various projection regions, notably, the nucleus accumbens (NAcc). Identification of specific brain regions implicated in drug reinforcement has led to a large n u m b e r of investigations of possible biochemical changes in these regions associated with addictive phenomena. Most of these studies have focused on neurotransmitters and receptors as the principle targets of drug action. Although these studies have yielded some important information, they are unlikely to result in a complete understanding of the mechanisms underlying drug addiction. This has led an increasing n u m b e r of investigators to study post-receptor intracellular messenger pathways as additional targets of drug action (see ref 1). Most neurotransmitter-receptor systems produce m a n y of their physiological effects in target neurons through a complex cascade of intracellular messengers involving G-proteins, which couple extracellular receptors to intracellular effectors, and the effectors themselves, which include second messengers, protein kinases and protein phosphatases, and phosphoproteins (Figure 1). 2,3 Regulation of these intracellular messenger pathways mediates the effects of neurotransmitter-receptor systems on diverse aspects of neuronal function, including gene expression. Given that many important aspects of drug addiction develop gradually and progressively in response to continued drug exposure, and can persist for a long time after drug withdrawal, it is likely that the

Key words: ventral tegmental area (VTA)/nucleus accumbens (NAcc) / morphine / cocaine / alcohol / cyclic AMP / tyrosine hydroxylase

AN IMPROVED UNDERSTANDING of the neurobiology of drug addiction must include two types of information: (1) mechanisms of pathophysiology-identification of the changes that drugs of abuse produce in the brain that lead to addiction; and (2) mechanisms of individual risk--identification of specific genetic and environmental factors that predispose certain individuals to addiction. This article reviews recent research efforts that have begun to characterize the molecular mechanisms underlying

From the Laboratory of Molecular Psychiatry, Departments of Psychiatry and Pharmacology, Yale University School of Medicine, Connecticut Mental Health Center, 34 Park Street, New Haven, CT 06508, USA 9 Academic Press Ltd 1044-5765/93/050369 + 0858. 00/0 369

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Figure 1. Schematic illustration of intracellular messenger pathways through which extracellular signals produce multiple types of physiological responses, including alterations in gene expression, in target neurons. The figure illustrates the likely role played by these intracellular messengers in mediating the addictive actions of opiates and other drugs of abuse. Modified from ref 1. regulation of neuronal gene expression is of particular relevance to addiction. Over the past several years, we have studied postreceptor signal transduction pathways as possible targets of drugs of abuse. Initial studies focused on the rat locus coeruleus, a brain region implicated in physical aspects of opiate addiction, namely, physical dependence and withdrawal (see ref 1). It was found that whereas acute opiate administration inhibits the cyclic A M P signaling system in the LC, chronic opiates up-regulate the system at every major step in the pathway between receptor and physiological response. Chronic opiates increase levels of the G-protein subunits Gic~ and Goot, adenylate cyclase, cyclic A M P - d e p e n d e n t protein kinase, and multiple phosphoprotein substrates for the protein kinase. Recent studies in collaboration with Dr George K. Aghajanian have demonstrated that up-regulation of the cyclic A M P pathway in the LC represents one mechanism by which opiates induce tolerance, dependence and withdrawal in these neurons. 1,4,5

Chronic opiate treatment produces similar alterations in G-proteins, adenylate cyclase, and cyclic AMP-dependent protein kinase in a number of other regions of the central nervous system, 6-8 including the NAcc. 8 These results suggest that adaptations in G-proteins and the cyclic A M P system may be a common response by many types of neurons to chronic opiates, with such adaptations mediating both physical and psychological aspects of drug addiction depending on the neuronal cell type involved. 1,8 The findings emphasize that the distinction between physical and psychological addiction is arbitrary: both are due to changes in brain function mediated via biochemical adaptations, perhaps even similar adaptations, in specific neuronal cell types that lead to alterations in the functional state of the neurons.

Common actions of morphine and cocaine in t h e V T A - N A c c pathway In recent years, we have identified a series of common chronic actions of morphine and cocaine

Molecular mechanisms of drug addiction

in the mesolimbic dopamine system. In the NAcc, chronic treatment of rats with either drug decreases levels of Giot and GooP and increases levels of adenylate cyclase and cyclic AMP-dependent protein kinase. 8 These biochemical actions of cocaine can be understood within a functional context of known electrophysiological effects of cocaine on NAcc neurons. Chronic cocaine has been shown to make NAcc neurons supersensitive to the inhibitory actions of Dl-dopaminergic agonists. 10 This supersensitivity occurs in the absence of consistent changes in levels of Da receptors per se (see ref 11), suggesting the involvement of post-receptor mechanisms. As D1 receptors are generally thought to exert their effects via activation of the cyclic A M P pathway, the observed increase in adenylate cyclase and cyclic AMP-dependent protein kinase, together with the observed decrease in Gio~ without a change in Gso~ or G/3, could account for D1 receptor supersensitivity observed electrophysiologically. Although the electrophysiological effects of chronic morphine on NAcc neurons have not yet been studied, we would predict similar effects based on our biochemical findings. Additional common actions of chronic morphine and chronic cocaine have been identified in the VTA. Both drugs increase levels of tyrosine hydroxylase 12 and decrease levels of three major neurofilament proteins, NF-200, NF-160 and NF-68,13 in this brain region. Interestingly, the morphine- and cocaine-induced increase in the total amount of tyrosine hydroxylase in the V T A is associated with a decrease in the state of phosphorylation of the enzyme (wi}h no change in total amount) in the NAcc. 12 As phosphorylation of tyrosine hydroxylase increases its catalytic activity, the morphine- and cocaine-induced decrease in tyrosine hyroxylase phosphorylation in the NAcc is probably associated with decreased enzyme activity in this brain region. Indeed, our observed dephosphorylation of tyrosine hyroxylase could account for the reduced levels of in vivo dopamine synthesis observed in response to chronic cocaine in the NAcc (e. g. see ref 14) and for reduced levels of in vivo basal and morphinestimulated dopamine release in this brain region in response to chronic morphine (e.g. see ref 15). As tyrosine hydroxylase present in the NAcc is located within dopaminergic nerve terminals derived from the VTA, the results indicate that this enzyme can be regulated by morphine and cocaine differentially in cell body and nerve terminal regions of the mesolimbic dopamine system. The possible

371 consequences of such differential regulation are discussed in greater detail below. Morphine and cocaine regulation of G-proteins, adenylate cyclase, cyclic AMP-dependent protein kinase, tyrosine hydroxylase and neurofilament proteins in the mesolimbic dopamine system showed temporal, regional and pharmacological specificity. 8,9,12,13 Acute administration of morphine or cocaine had no effect on these various proteins, nor did chronic administration of the drugs influence the proteins significantly in the nigrostriatal dopamine system, a major dopaminergic system in brain not crucially implicated in drug reinforcement. In addition, chronic administration of haloperidol (an antipsychotic drug) or imipramine (an antidepressant drug), or chronic cold or surgical stress, did not alter levels of these proteins in the mesolimbic dopamine system.

Preliminary studies on biochemical actions of ethanol in the mesolimbic dopamine system Based on the similar effects exerted by chronic morphine or cocaine on intracellular messenger proteins in the V T A and NAcc, we have recently begun to study the influence of ethanol on this system. To date, chronic oral self-administration of ethanol has been found to increase levels of tyrosine hydroxylase and decrease levels of neurofilament proteins in the VTA, and to increase levels of cyclic AMP-dependent protein kinase activity in the NAcc. 16,17 The effect of chronic ethanol on each of these proteins resembles that of chronic morphine and chronic cocaine.

Biochemical differences in the VTA-NAcc pathway in inbred genetic rat strains Further evidence to support the idea that morphine, cocaine and ethanol regulation of particular intracellular messenger proteins in the mesolimbic dopamine system is related to drug-related behaviors includes the finding that differences in these intracellular messenger proteins exist in the VTA-NAcc pathway of Lewis and Fischer 344 rats. Lewis rats self-administer opiates, cocaine and alcohol at much higher rates compared to Fischer rats, 18"2~and also develop greater degrees of conditioned place preference to systemic morphine and cocaine. 21 Furthermore, cannabinoids facilitate brain-stimulation reward of

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the mesolimbic dopamine pathway in Lewis but not in Fischer rats. ~2 We have found that the NAcc of Lewis versus Fischer rats contain lower levels of Gio~, higher levels of adenylate cyclase and cyclic A M P dependent protein kinase, and lower levels of tyrosine hydroxylase. 23,~4 In addition, the V T A of Lewis rats contain higher levels of tyrosine hydroxylase and lower levels of neurofilament proteins compared to Fischer rats. 21,23 In each case, levels of these specific intracellular signaling proteins in the V T A - N A c c pathway of the drug-preferring Lewis rats, compared to Fischer rats, resemble morphine-, cocaine- and ethanol-induced changes in these proteins in outbred Sprague-Dawley rats. (Note that the drug-induced decrease in tyrosine hydroxylase activity in the NAcc is functionally equivalent to the strain difference in total enzyme amount.) It should be emphasized that the biochemical differences observed between Lewis and Fischer rats were specific to the mesolimbic dopamine system and were observed in drug-naive animals. The findings raise the exciting possibility that different levels of expression of components of the cyclic A M P system in the V T A - N A c c pathway could contribute to an animal's inherent responsiveness to drugs of abuse. 1

protein kinase compared to the NAcc of H rats. 2~ A tendency for similar L versus H differences in these proteins was observed when groups of rats with the second lowest and second highest locomotor responses were compared; no differences were seen in groups whose locomotor responses were closer to the median. These biochemical differences between L and H rats showed regional specificity, with no significant differences seen in several other regions of brain or spinal cord studied. Differences were also observed between L and H rats in their locomotor responses to acute and repeated cocaine exposure. L rats showed smaller increases in locomotor activity in response to a single, acute injection of cocaine compared to H rats. However, after repeated cocaine exposure, L rats showed a greater augmentation in this locomotor response (i.e. greater locomotor sensitization) than the H rats. 33 These findings are similar to those reported previously for the locomotor responses of L and H rats to acute and repeated amphetamine administration. 25

B i o c h e m i c a l differences in the V T A - N A c c p a t h w a y a m o n g i n d i v i d u a l outbred rats

O u r studies have begun to describe biochemical parameters specific to the mesolimbic dopamine pathway that are shared by several experimental systems: (1) Sprague-Dawley rats treated chronically with morphine, cocaine or ethanol; (2) drug naive Lewis versus Fischer rats; and (3) drug naive L versus H rats. These biochemical parameters, summarized schematically in Figure 2, include higher levels of tyrosine hydroxylase and lower levels of neurofilaments in the V T A and lower levels of certain G-proteins and higher levels of adenylate cyclase and cyclic A M P - d e p e n d e n t protein kinase in the NAcc. Based on our biochemical findings, therefore, we would predict that the drug-treated rats, the Lewis (versus Fischer) rats, and the L rats (versus H rats) share some physiological and behavioral traits mediated by the V T A and NAcc and subserved by these biochemical parameters.

Recent reports have shown that individual differences exist among outbred Sprague-Dawley rats in their acquisition of amphetamine self-administration behavior, and that such differences can be predicted by the animal's locomotor response to novelty. 25 Based on these observations, we determined in a recent study whether such within strain differences in locomotor behavior are also associated with differences in levels of specific intracellular messenger proteins in the mesolimbic dopamine system. 33 Groups of 42 Sprague-Dawley rats were assessed for locomotor activity in a novel environment. The four animals from each group with the lowest locomotor responses were designated L rats, and the four with the highest locomotor responses were designated H rats. It was found that the V T A of L rats exhibited higher levels of tyrosine hydroxylase and lower levels of the neurofilament proteins, NF-200, NF-160 and NF-68, compared to the V T A of H rats. 33 Moreover, the NAcc of L rats exhibited higher levels of cyclic AMP-dependent

F u n c t i o n a l significance of b i o c h e m i c a l parameters studied in the V T A - N A c c

Physiological significance The finding that tyrosine hydroxylase is up-regulated in the V T A and, if anything, down-regulated in the

Molecular mechanisms of drug addiction (A)

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Figure 2. Schematic summary of similar biochemical manifestations of the 'drug-addicted' and 'drug-preferring' state. (A) (Normal state) depicts a control V T A neuron projecting to the NAcc. Shown in the V T A cell are tyrosine hydroxylase (TH), dopamine (DA), presynaptic dopamine receptors (D2) coupled to G-proteins (Gi), and neurofilaments (NFs). Shown in the NAcc are, in addition to T H and DA, dopamine receptors (D 1 and D2) , G-proteins (Gi and Gs), components of the intracellular cyclic A M P system (AC, adenylate cyclase; PKA, cAMP-dependent protein kinase; and possible substrates for the kinase--ion channels and the nuclear transcription factors, CREB, fos and jun), as well as major inputs and outputs of this region (VP, ventral pallidum; HP, hippocampus; AMYG, amygdala; OLF, olfactory cortex; C T X , other cortical regions). (B) (Drug-addicted, drug-preferring state) depicts a V T A neuron projecting to the NAcc after chronic morphine or cocaine, or from a Lewis (drug-preferring) rat versus Fischer (F344) rat. In the drug-addicted or drug-preferring animal, T H levels are increased in the V T A , and decreased in the NAcc (due to either decreased phosphorylation as for morphine and cocaine, or decreased enzyme levels as in Lewis versus Fischer rats). In addition, NF levels are decreased in the V T A in the drug-addicted and drug-preferring state. This decrease in NFs may be associated with alterations in neuronal structure, decreases in axonal caliber, and/or decreases in axonal transport rate in these cells, as indicated in the figure. Such a decrease in axonal transport, demonstrated for chronic morphine, 27 could account for the lack of correspondingly increased levels of T H in dopaminergic terminals in the NAcc. Decreased T H implies decreased dopamine synthesis, and may result in lower dopaminergic transmission to the NAcc. In the NAcc of the drug-addicted or drug-preferring state, Gi is decreased, and adenylate cyclase and cAMP-dependent protein kinase activities are increased, changes that could account for the D 1 receptor supersensitivity observed electrophysiologically. It should be noted that alterations in dopaminergic transmission probably influence m a n y cell types within the NAcc, as well as other nerve terminals in the NAcc. Similarly, altered local dopaminergic transmission in the V T A could also influence other nerve terminals in this brain region. Thus, biochemical alterations in the mesolimbic dopamine system could potentially lead to altered neuronal function in m a n y other brain regions as well. Modified from ref 32.

374 NAcc is consistent with the view that the activity of the dopamine system is regulated differentially in dopaminergic cell bodies and dendrites in the V T A versus dopaminergic nerve terminals in the NAcc and subserves different functions in these two brain regions. Higher levels of tyrosine hydroxylase (and hence higher levels of dopaminergic function) in the V T A would be expected to increase the autoinhibitory influence of dopamine acting on D~ dopamine receptors on these neurons. This effect could lead to subsensitivity of D 2 receptor function and increased spontaneous firing of the neurons, both of which have been observed electrophysiologically. 26 In contrast, lower levels of active tyrosine hydroxylase and of dopaminergic function in the NAcc would be expected to decrease the various pre- and post-synaptic effects of dopamine acting on D 1 and D 2 (and probably other) dopamine receptor subtypes. This relative dopamine deficiency could be the stimulus that leads to DI receptor supersensitivity observed in NAcc neurons in response to chronic cocaine. 1~ The mechanism by which tyrosine hydroxylase is regulated differentially in the V T A versus NAcc is unknown, but could involve neurofilament proteins. A number of studies have shown that lower levels of neurofilament proteins are associated with decreased rates of axonal transport and decreased axonal caliber (see refs 1,13). Lower levels of these proteins could, then, result in decreased transport of tyrosine hydroxylase from cell bodies in the V T A to nerve terminals in the NAcc. At a constant rate of enzyme synthesis, this would tend to lead to a build up of tyrosine hydroxylase in the VTA, with either no change in enzyme levels in the NAcc (as observed in the morphine- and cocaine-treated states) or to an actual decrease in the amount of enzyme in the NAcc (as observed in Lewis versus Fischer rats). We are now investigating this possibility directly by studying axonal transport and caliber in the VTANAcc pathway under these various conditions. Early findings indicate that chronic morphine treatment, as predicted, impairs axoplasmic transport from the V T A to the NAcc. 27 In any event, the finding of drug-induced changes, and inherent Lewis-Fischer and L - H differences, in levels of neurofilament proteins specifically in the V T A provides strong evidence for the view that certain aspects of drug addiction are associated with prominent structural alterations in mesolimbic dopamine neurons. O u r ability to understand the physiological significance of an up-regulated cyclic A M P pathway in the NAcc is limited due to the lack of knowledge

E.J. Nestler concerning the precise mechanisms by which NAcc neurons regulate drug reinforcement. Thus, the NAcc contains multiple neuronal cell types that appear to respond differently to opiates, cocaine and other drugs of abuse. Moreover, these distinct cell types are thought to influence drug reinforcement mechanisms via multiple cortical to subcortical relay pathways, which are now only beginning to be delineated (see other articles in this issue). These considerations make interpretation of any biochemical findings in the NAcc particularly difficult at the present time.

Behavioral significance Establishing which specific behavior(s) is related to the biochemical parameters, and associated physiological properties, is a considerably more difficult task, due to the many behavioral effects of drugs of abuse and the limitations in relating animal behaviors to subjective human responses. Two behaviors to consider, known to be mediated at least in part by the mesolimbic dopamine system, are drug preference and locomotor activity (summarized elsewhere in this issue). A possible role of the biochemical parameters in mediating some aspect of drug preference is suggested by the information that Lewis rats appear to be inherently drugpreferring compared to Fischer rats 18-21 and prior exposure to drugs of abuse seems to increase an animal's preference for those drugs (e.g. see refs 25,28,29). This speculation is supported by the preliminary finding that L rats develop greater degrees of conditioned place preference to cocaine compared to H rats (T.A. Kosten, E.J. Nestler, unpublished observations). In contrast, Piazza et al have reported previously that L rats were less likely to acquire amphetamine self-administration behavior than H rats. 25 This finding, on the surface, argues against the possibility that L rats are inherently drug-preferring. However, drug self-administration is a complex behavioral construct, which can be affected by many variables, including an animal's general activity and exploratory level and the ability to learn the appropriate self-administration response, as well as an animal's inherent sensitivity to the rewarding and aversive effects of a drug, at a given dose, following acute and chronic administration. Thus, the earlier observation 25 that H rats more readily acquire amphetamine self-administration over a 5 day time period could relate particularly

Molecular mechanisms of drug addiction to the higher initial activity/exploratory levels of the animals and/or their greater initial sensitivity to the drug, especially given the very low doses of amphetamine used. Nevertheless, it is still possible that L rats are inherently more prone to changes induced by sustained drug exposure over the longterm. This possibility is supported by our findings 26 and those of Piazza et a125 with drug-induced locomotor activity, which show that L rats, compared to H rats, display smaller responses to acute cocaine or amphetamine treatment, but larger increases (locomotor sensitization) in their responses after repeated drug treatment. These results suggest that L rats may be initially more tolerant (or the H rats more sensitive) to the motor-activating and possibly certain other properties of cocaine (and amphetamine), but are more susceptible to changes with repeated drug exposure. Such an initial tolerance could also explain the differential effects of cocaine on locomotor sensitization in Lewis versus Fischer rats, with Lewis rats and L rats (which resemble one another biochemically) showing some similar responses to cocaine. 2~ In a similar way, chronic treatment of animals with morphine or cocaine would be expected to result in tolerance to at least some of the acute actions of these drugs. M u c h further work is, of course, needed to determine precisely how levels of specific proteins in the V T A and NAcc influence the various behaviors regulated by the mesolimbic dopamine system.

Future directions Clearly, the above interpretations of the data summarized in Figure 2 remain conjectural. However, they define specific hypotheses regarding the functional sequelae of biochemical parameters in the mesolimbic dopamine system that can now be tested by direct experimental means. In particular, future studies will focus on establishing a direct causal link between the various biochemical adaptations and (1) electrophysiological changes induced in V T A and NAcc neurons by chronic drug treatments, as well as inherent electrophysiological differences in the neurons between Lewis versus Fischer and L versus H rats; and (2) behavioral measures of drug reinforcement and other drug-related behaviors. Future studies will also be needed to determine the precise molecular mechanisms by which opiates, cocaine and alcohol alter the levels of intracellular messenger proteins in the mesolimbic dopamine

375 system. Several lines of evidence suggest that druginduced changes in neuronal gene expression may be involved (see refs 1,30,31). Similarly, Lewis-Fischer and L-H rats provide novel experimental systems in which the genetic and environmental factors responsible for the inherent differences in biochemical parameters in the mesolimbic dopamine system and in several drug-related behaviors mediated via this neural pathway can be investigated. Our studies indicate that through the investigation of post-receptor signal transduction mechanisms a more complete understanding will be obtained of the processes by which drugs of abuse produce psychological dependence and of some of the specific factors that predispose certain individuals to drug addiction. Such studies will lead to critical advances in our ability to treat and one day prevent drug addiction.

Acknowledgements This work was supported by U.S.P.H.S. Grants DA07359, DA04060 and DA08227, by the VA-Yale Alcoholism Research Center funded by the Department of Veterans Affairs, and by the Abraham Ribicoff Research Facilities, Connecticut Mental Health Center, State of Connecticut Department of Mental Health.

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21. Guitart X, Beitner-Johnson D, Nestler EJ (1992) Fischer and Lewis rat strains differ in basal levels of neurofilament proteins and in their regulation by chronic morphine. Synapse 12:242-253 22. Gardner EL, LowinsonJH (1991) Marijuana's interactions with brain reward systems: update 1991. Pharmacol Biochem Behav 40:571-580 23. Beitner-Johnson D, Guitart X, Nestler EJ (1991) Dopaminergic brain reward regions of Lewis and Fischer rats display different levels of tyrosine hydroxylase and other morphineand cocaine-regulated phosphoproteins. Brain Res 561: 146-149 24. Guitart X, Kogan JH, Berhow M, Terwilliger RZ, Aghajanian GK, Nestler EJ (1993) Lewis and Fischer rat strains display differences In biochemical, electrophysiological and behavioral parameters: studies in the nucleus accumbens and locus coeruleus of drug naive and morphinetreated animals. Brain Res 611:7-17 25. Piazza PV, DeminiereJ-M, Le Moal M, Simon H (1989) Factors that predict individual vulnerability to amphetamine self-administration Science 1245:1511 - 1513 26. Henry DJ, Greene MA, White FJ (1989) Electrophysiological effects of cocaine in the mesoaccumbens dopamine system: repeated administration, j Pharmacol Exp Ther 251: 833-839 27. Beitner-Johnson D, Nestler EJ (1993) Chronic morphine impairs axonal transport in the rat mesolimbic dopamine system. Neuroreport, in press 28. Lett BT (1989) Repeated exposures intensify rather than diminish the rewarding effects of amphetamine, morphine, and cocaine. Psychopharmacol 98:357-362 29. Horger BA, Shelton K, Schenk S (1990) Preexposure sensitizes rats to the rewarding effects of cocaine Pharmacol Biochem Behav 37:707-711 30. Hope BT, Kosofsky B, Hyman SE, Nestler EJ (1992) Regulation of IEG expression and AP-1 binding by chronic cocaine in the rat nucleus accumbens. Proc Natl Acad Sci USA 89:5764-5768 31. Nestler EJ, Hope BT, Widnell K (1993) Drug addiction: a model for the molecular basis of neural plasticity. Neuron, in press 32. Beitner-Johnson D, Guitart X, Nestler EJ (1992) Common intracellular actions of chronic morphine and cocaine in dopaminergic brain reward regions. Ann NY Acad Sci 684:70-87 33. Miserendino MJD, Guitart X, Terwilliger RZ, Chi S, Nestler EJ (1993) Individual differences in locomotor activity are associated with levels of tyrosine hydroxylase and neurofilament proteins in the ventral tegmental area of Sprague-Dawley rats. Mol Cell Neurosci, in press