Delayed occurrence of enhanced striatal preprodynorphin gene expression in behaviorally sensitized rats: differential long-term effects of intermittent and chronic morphine administration

A review of the kappa opioid receptor system in opioid use

The kappa opioid receptor (KOR) system is implicated in dysphoria and as an "anti-reward system" during withdrawal from opioids. However, no clear consensus has been made in the field, as mixed findings have been reported regarding the relationship between the KOR system and opioid use. This review summarizes the studies to date on the KOR system and opioids. A systematic scoping review was reported following PRISMA guidelines and conducted based on the published protocol. Comprehensive searches of several databases were done in the following databases: MEDLINE, Embase, PsycINFO, Web of Science, Scopus, and Cochrane. We included preclinical and clinical studies that tested the administration of KOR agonists/antagonists or dynorphin and/or measured dynorphin levels or KOR expression during opioid intoxication or withdrawal from opioids. One hundred studies were included in the final analysis. Preclinical administration of KOR agonists decreased drug-seeking/taking behaviors and opioid withdrawal symptoms. KOR antagonists showed mixed findings, depending on the agent and/or type of withdrawal symptom. Administration of dynorphins attenuated opioid withdrawal symptoms both in preclinical and clinical studies. In the limited number of available studies, dynorphin levels were found to increase in cerebrospinal fluid (CSF) and peripheral blood lymphocytes (PBL) of opioid use disorder subjects (OUD). In animals, dynorphin levels and/or KOR expression showed mixed findings during opioid use. The KOR/dynorphin system appears to have a multifaceted and complex nature rather than simply functioning as an anti-reward system. Future research in well-controlled study settings is necessary to better understand the clinical role of the KOR system in opioid use.

Altered Accumbal Dopamine Terminal Dynamics Following Chronic Heroin Self-Administration

Administration of heroin results in the engagement of multiple brain regions and the rewarding and addictive effects are mediated, at least partially, through activation of the mesolimbic dopamine system. However, less is known about dopamine system function following chronic exposure to heroin. Withdrawal from chronic heroin exposure is likely to drive a state of low dopamine in the nucleus accumbens (NAc), as previously observed during withdrawal from other drug classes. Thus, we aimed to investigate alterations in NAc dopamine terminal function following chronic heroin self-administration to identify a mechanism for dopaminergic adaptations. Adult male Long Evans rats were trained to self-administer heroin (0.05 mg/kg/inf, IV) and then placed on a long access (FR1, 6-h, unlimited inf, 0.05 mg/kg/inf) protocol to induce escalation of intake. Following heroin self-administration, rats had decreased basal extracellular levels of dopamine and blunted dopamine response following a heroin challenge (0.1 mg/kg/inf, IV) in the NAc compared to saline controls. FSCV revealed that heroin-exposed rats exhibited reduced stimulated dopamine release during tonic-like, single-pulse stimulations, but increased phasic-like dopamine release during multi-pulse stimulation trains (5 pulses, 5–100 Hz) in addition to an altered dynamic range of release stimulation intensities when compared to controls. Further, we found that presynaptic D3 autoreceptor and kappa-opioid receptor agonist responsivity were increased following heroin self-administration. These results reveal a marked low dopamine state following heroin exposure and suggest the combination of altered dopamine release dynamics may contribute to increased heroin seeking.

The Mechanisms Involved in Morphine Addiction: An Overview

Opioid use disorder is classified as a chronic recurrent disease of the central nervous system (CNS) which leads to personality disorders, co-morbidities and premature death. It develops as a result of long-term administration of various abused substances, along with morphine. The pharmacological action of morphine is associated with its stimulation of opioid receptors. Opioid receptors are a group of G protein-coupled receptors and activation of these receptors by ligands induces significant molecular changes inside the cell, such as an inhibition of adenylate cyclase activity, activation of potassium channels and reductions of calcium conductance. Recent data indicate that other signalling pathways also may be involved in morphine activity. Among these are phospholipase C, mitogen-activated kinases (MAP kinases) or β-arrestin. The present review focuses on major mechanisms which currently are considered as essential in morphine activity and dependence and may be important for further studies.

Dynorphin and κ-Opioid Receptor Dysregulation in the Dopaminergic Reward System of Human Alcoholics

Molecular changes induced by excessive alcohol consumption may underlie formation of dysphoric state during acute and protracted alcohol withdrawal which leads to craving and relapse. A main molecular addiction hypothesis is that the upregulation of the dynorphin (DYN)/κ-opioid receptor (KOR) system in the nucleus accumbens (NAc) of alcohol-dependent individuals causes the imbalance in activity of D1- and D2 dopamine receptor (DR) expressing neural circuits that results in dysphoria. We here analyzed post-mortem NAc samples of human alcoholics to assess changes in prodynorphin (PDYN) and KOR (OPRK1) gene expression and co-expression (transcriptionally coordinated) patterns. To address alterations in D1- and D2-receptor circuits, we studied the regulatory interactions between these pathways and the DYN/KOR system. No significant differences in PDYN and OPRK1 gene expression levels between alcoholics and controls were evident. However, PDYN and OPRK1 showed transcriptionally coordinated pattern that was significantly different between alcoholics and controls. A downregulation of DRD1 but not DRD2 expression was seen in alcoholics. Expression of DRD1 and DRD2 strongly correlated with that of PDYN and OPRK1 suggesting high levels of transcriptional coordination between these gene clusters. The differences in expression and co-expression patterns were not due to the decline in neuronal proportion in alcoholic brain and thereby represent transcriptional phenomena. Dysregulation of DYN/KOR system and dopamine signaling through both alterations in co-expression patterns of opioid genes and decreased DRD1 gene expression may contribute to imbalance in the activity of D1- and D2-containing pathways which may lead to the negative affective state in human alcoholics.

Effects of the adenosinergic system on the expression and acquisition of sensitization to conditioned place preference in morphine-conditioned rats

In the presented study, we attempt to investigate if the sensitization to conditioned place preference (CPP) induced by low doses of morphine was developed in rats which have been previously conditioned with morphine. The experiments were performed in the CPP test. Firstly, it has been demonstrated that administration of ineffective dose of morphine on the 9th day induces the increase in time spent of rats at a morphine-paired compartment, confirming that sensitization to CPP has been developed in these animals. Secondly, it has been shown that stimulation of A1 receptor significantly inhibits the expression of morphine-induced of sensitization, and blockade of these receptors produces the opposite effect. Finally, it has been indicated that both stimulation and blockade of A1 and/or A2A receptors inhibit the acquisition of sensitization to CPP. The obtained results have strongly supported the significance of adenosinergic system in both expression and acquisition of studied sensitization. These results seem to be important for the identification of connections in the central nervous system which can help finding new strategies to attenuate rewarding action of morphine.

Long-term antagonism of κ opioid receptors prevents escalation of and increased motivation for heroin intake

The abuse of opioid drugs, both illicit and prescription, is a persistent problem in the United States, accounting for >1.2 million users who require treatment each year. Current treatments rely on suppressing immediate withdrawal symptoms and replacing illicit drug use with long-acting opiate drugs. However, the mechanisms that lead to preventing opiate dependence are still poorly understood. We hypothesized that κ opioid receptor (KOR) activation during chronic opioid intake contributes to negative affective states associated with withdrawal and the motivation to take increasing amounts of heroin. Using a 12 h long-access model of heroin self-administration, rats showed escalation of heroin intake over several weeks. This was prevented by a single high dose (30 mg/kg) of the long-acting KOR antagonist norbinaltorphimine (nor-BNI), paralleled by reduced motivation to respond for heroin on a progressive-ratio schedule of reinforcement, a measure of compulsive-like responding. Systemic nor-BNI also significantly decreased heroin withdrawal-associated anxiety-like behavior. Immunohistochemical analysis showed prodynorphin content increased in the nucleus accumbens core in all heroin-exposed rats, but selectively increased in the nucleus accumbens shell in long-access rats. Local infusion of nor-BNI (4 μg/side) into accumbens core altered the initial intake of heroin but not the rate of escalation, while local injection into accumbens shell selectively suppressed increases in heroin intake over time without altering initial intake. These data suggest that dynorphin activity in the nucleus accumbens mediates the increasing motivation for heroin taking and compulsive-like responding for heroin, suggesting that KOR antagonists may be promising targets for the treatment of opioid addiction.

Salvinorin A analogs and other κ-opioid receptor compounds as treatments for cocaine abuse

Acute activation of kappa-opioid receptors produces anti-addictive effects by regulating dopamine levels in the brain. Unfortunately, classic kappa-opioid agonists have undesired side effects such as sedation, aversion, and depression, which restrict their clinical use. Salvinorin A (Sal A), a novel kappa-opioid receptor agonist extracted from the plant Salvia divinorum, has been identified as a potential therapy for drug abuse and addiction. Here, we review the preclinical effects of Sal A in comparison with traditional kappa-opioid agonists and several new analogs. Sal A retains the anti-addictive properties of traditional kappa-opioid receptor agonists with several improvements including reduced side effects. However, the rapid metabolism of Sal A makes it undesirable for clinical development. In an effort to improve the pharmacokinetics and tolerability of this compound, kappa-opioid receptor agonists based on the structure of Sal A have been synthesized. While work in this field is still in progress, several analogs with improved pharmacokinetic profiles have been shown to have anti-addictive effects. While in its infancy, it is clear that these compounds hold promise for the future development of anti-addictive therapeutics.

κ-opioid receptor/dynorphin system: genetic and pharmacotherapeutic implications for addiction

Addictions to cocaine or heroin/prescription opioids [short-acting μ-opioid receptor (MOPr) agonists] involve relapsing cycles, with experimentation/escalating use, withdrawal/abstinence, and relapse/re-escalation. κ-Opioid receptors (KOPr; encoded by OPRK1), and their endogenous agonists, the dynorphins (encoded by PDYN), have counter-modulatory effects on reward caused by cocaine or MOPr agonist exposure, and exhibit plasticity in addictive-like states. KOPr/dynorphin activation is implicated in depression/anxiety, often comorbid with addictions. In this opinion article we propose that particular stages of the addiction cycle are differentially affected by KOPr/dynorphin systems. Vulnerability and resilience can be due to pre-existing (e.g., genetic) factors, or epigenetic modifications of the OPRK1 or PDYN genes during the addiction cycle. Pharmacotherapeutic approaches limiting changes in KOPr/dynorphin tone, especially with KOPr partial agonists, may hold potential for the treatment of specific drug addictions and psychiatric comorbidity.

Episodic withdrawal promotes psychomotor sensitization to morphine

The relative intermittency or continuity of drug delivery is a major determinant of addictive liability, and also influences the impact of drug exposure on brain function and behavior. Events that occur during the offset of drug action (ie, acute withdrawal) may have an important role in the consequences of intermittent drug exposure. We assessed whether recurrent episodes of acute withdrawal contribute to the development of psychomotor sensitization in rodents during daily morphine exposure. The acoustic startle reflex--a measure of anxiety induced by opiate withdrawal-was used to resolve and quantify discrete withdrawal episodes, and pharmacological interventions were used to manipulate withdrawal severity. Startle potentiation was observed during spontaneous withdrawal from a single morphine exposure, and individual differences in initial withdrawal severity positively predicted the subsequent development of sensitization. Manipulations that reduce or exacerbate withdrawal severity also produced parallel changes in the degree of sensitization. These results demonstrate that the episodic experience of withdrawal during daily drug exposure has a novel role in promoting the development of psychomotor sensitization--a prominent model of drug-induced neurobehavioral plasticity. Episodic withdrawal may have a pervasive role in many effects of intermittent drug exposure and contribute to the development of addiction.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Effects of prolonged treatment with the opiate tramadol on prodynorphin gene expression in rat CNS

A low abuse liability is reported for tramadol, an analgesic drug centrally acting through either opioid or nonopioid mechanisms. In this paper, we evaluated the effects of the repeated administration (7 d) of different doses of tramadol (10, 20, and 80 mg/kg, intraperitoneally) on the opioid precursor prodynorphin biosynthesis, in comparison with morphine (10 mg/kg, intraperitoneally), in the rat central nervous system (CNS). Northern analysis showed that morphine and tramadol produced different effects. While morphine caused a downregulation of prodynorphin mRNA levels in all investigated areas (hypothalamus, hippocampus, and striatum), tramadol did not cause any significant change in the striatum, and did not decrease prodynorphin biosynthesis in the hypothalamus and in the hippocampus, at nontoxic doses (10 and 20 mg/kg). The highest dose of tramadol (80 mg/kg) decreased prodynorphin mRNA levels in the hypothalamus and the hippocampus but not in the striatum. These data give some information on tramadol effects at molecular level in the CNS. They indicate that the alterations of prodynorphin gene expression caused by tramadol and morphine show a different pattern that may be related to the different abuse potential of the two analgesic drugs.

Prenatal cannabis exposure increases heroin seeking with allostatic changes in limbic enkephalin systems in adulthood

BACKGROUND

Prenatal cannabis exposure is a growing concern with little known about the long-term consequences on behavior and neural systems relevant for reward and emotional processing.

METHODS

We used an animal model to study the effects of prenatal exposure to Delta(9)-tetrahydrocannabinol (THC) on heroin self-administration behavior and opioid neural systems in adult males (postnatal day 62). Rats were exposed to THC (.15 mg/kg) or vehicle from gestational day 5 to postnatal day 2.

RESULTS

Both pretreatment groups showed similar heroin intake, but THC-exposed rats exhibited shorter latency to the first active lever press, responded more for low heroin doses, and had higher heroin-seeking during mild stress and drug extinction. THC exposure reduced preproenkephalin (PENK) mRNA expression in the nucleus accumbens during early development, but was elevated in adulthood; no adult striatal changes on preprodynorphin mRNA or PENK in caudate-putamen. PENK mRNA was also increased in the central and medial amygdala in adult THC-exposed animals. THC animals had reduced heroin-induced locomotor activity and nucleus accumbens mu opioid receptor coupling.

CONCLUSIONS

This study demonstrates enduring effects of prenatal THC exposure into adulthood that is evident on heroin-seeking behavior during extinction and allostatic changes in mesocorticolimbic PENK systems relevant to drug motivation/reward and stress response.

Disruption of the CRF/CRF1 Receptor Stress System Exacerbates the Somatic Signs of Opiate Withdrawal

Escape from the extremely stressful opiate withdrawal syndrome may motivate opiate seeking and taking. The corticotropin-releasing factor receptor-1 (CRF1) pathway mediates behavioral and endocrine responses to stress. Here, we report that genetic inactivation (CRF1-/-) as well as pharmacological antagonism of the CRF/CRF1 receptor pathway increased and prolonged the somatic expression of opiate withdrawal. Opiate-withdrawn CRF1-/- mice also showed aberrant CRF and dynorphin expression in the paraventricular nucleus of the hypothalamus (PVN) and the striatum, indicating profound impairments in stress-responsive brain circuitry. Intake of nonstressful amounts of corticosterone effectively reduced the exaggerated somatic reactions of CRF1-/- mice to opiate withdrawal. Exogenous corticosterone also restored "wild-type-like" patterns of CRF and dynorphin gene expression in the PVN and the striatum of opiate-withdrawn CRF1-/- mice, respectively. The present findings unravel a key role for the hypothalamus-pituitary-adrenal (HPA) system and brain extra-hypothalamic CRF/CRF1 receptor circuitry in somatic, molecular, and endocrine alterations induced by opiate withdrawal.

The psychogenetically selected Roman high- and low-avoidance rat lines: A model to study the individual vulnerability to drug addiction

The Roman high- (RHA) and low-avoidance (RLA) rat lines were selected for, respectively, rapid vs poor acquisition of two-way active avoidance in the shuttle-box. Here, we review experimental evidence indicating that, compared with their RLA counterparts, RHA rats display a robust sensation/novelty seeking profile, a marked preference and intake of natural or drug rewards, and more pronounced behavioral and neurochemical responses to the acute administration of morphine and psychostimulants. Moreover, we show that (i) the repeated administration of morphine and cocaine elicits behavioral sensitization in RHA, but not RLA, rats, (ii) in sensitized RHA rats, acute morphine and cocaine cause a larger increment in dopamine output in the core, and an attenuated dopaminergic response in the shell of the nucleus accumbens, as compared with RHA rats repeatedly treated with saline, and (iii) such neurochemical changes are not observed in the mesoaccumbens dopaminergic system of the sensitization-resistant RLA line. Behavioral sensitization plays a key role in several cardinal features of addiction, including drug craving, compulsive drug seeking and propensity to relapse following detoxification. Comparative studies in the Roman lines may therefore represent a valid approach to evaluate the contribution of the genotype on the neural substrates of drug sensitization and addiction.

Amphetamine triggers an increase in met-enkephalin simultaneously in brain areas and immune cells

We analyzed effects of amphetamine on proenkephalin-derived peptides in brain areas and immune cells in rats. Acute, as well as a repeated amphetamine treatment, decreased the concanavalin-A-induced lymphocyte proliferation, concomitantly with an increase of free met-enkephalin in nucleus accumbens, prefrontal cortex, spleen, thymus and splenic macrophages. Proenkephalin protein increased in prefrontal cortex, thymus (32 kDa isoform), nucleus accumbens and spleen (44 kDa isoform), while proenkephalin mRNA levels decreased in brain stem. The influence of met-ENK in key brain areas for sensitization and in immune organs is consistent with the idea that changes on met-ENK could underlie amphetamine's effects on brain and IS.

The psychogenetically selected Roman rat lines differ in the susceptibility to develop amphetamine sensitization

The mesolimbic dopamine system is considered to play a pivotal role in the locomotor activation produced by psychostimulants and in the augmentation of this effect observed upon repeated drug administration, a process denominated behavioral sensitization. The selective breeding of Roman high- (RHA) and low-avoidance (RLA) rats, respectively, for rapid versus poor active avoidance acquisition has resulted in two phenotypes that differ in the functional properties of the mesolimbic dopamine system and in their behavioral and neurochemical responses to addictive drugs, including psychostimulants and opiates. Hence, the present study was designed to compare the ability of these lines to develop behavioral sensitization to psychostimulants. To this aim, the acute effects of amphetamine (0.125 or 0.25 mg/kg, s.c.) on locomotion were assessed in RHA and RLA rats prior to and subsequent to 10 daily doses of either amphetamine (1 mg/kg, s.c.) or saline (1 ml/kg, s.c.). In the RHA line, the locomotor activation produced by either challenge dose of amphetamine was more pronounced in rats that had been repeatedly treated with amphetamine versus the respective saline treated controls. In contrast, no significant change in locomotor activity was observed in RLA rats. Likewise, repeated amphetamine caused an increased frequency of sniffing, rearing, licking/gnawing, and grooming versus the control, repeated saline, group only in the RHA line. The results show that the repeated treatment regimen used in this study induced behavioral sensitization to amphetamine in RHA rats, but not in their RLA counterparts, and underscore the utility of these lines for studying the influence of neural substrates and genetic make up on the individual vulnerability to addiction.

Tolerance develops in spinal cord, but not in brain with chronic [Dmt1]DALDA treatment

Previously, we reported that H-2',6'-dimethyltyrosine [Dmt(1)]-d-Arg-Phe-Lys-NH(2) (DALDA), an analogue of the naturally occurring opioid peptide dermorphin, is a highly potent and selective mu receptor agonist with low cross-tolerance to morphine. In the present study, we investigated the effect of treating mice chronically with [Dmt(1)]DALDA. The AD(50) of [Dmt(1)]DALDA (s.c.) increased eight-fold in animals given this drug chronically; in contrast, the AD(50) increased two-fold in mice chronically treated with morphine. The AD(50) of morphine (s.c.) in these [Dmt(1)]DALDA-treated animals was increased more than 120 times, while that of the more selective mu agonist [d-Ala(2)-MePhe(4)-Gly-ol(5)]enkephalin (DAMGO) given intrathecally was increased more than 240 times. However, the AD(50) of DAMGO given intracerebroventricularly was essentially the same in animals treated chronically with [Dmt(1)]DALDA as in naive animals. The dose of naloxone required to precipitate withdrawal in [Dmt(1)]DALDA-treated animals was 20 times lower than that in morphine-tolerant animals. Using real-time quantitative PCR, we found that expression of the mu opioid receptor, delta opioid receptor, preproenkephalin and preprodynorphin genes was upregulated in the brain by [Dmt(1)]DALDA treatment. No significant changes in expression of opioid receptor or opioid peptide genes were detected in the spinal cord of [Dmt(1)]DALDA-treated mice, nor in the brain or spinal cord of morphine-treated mice. We conclude that a high degree of tolerance to [Dmt(1)]DALDA develops in the spinal cord but not brain, and cannot be accounted for by changes in expression of opioid receptors or opioid peptides in these tissues.

Mu opioid receptor signaling in morphine sensitization

We used a previously reported model of morphine sensitization that elicited a complex behavioral syndrome involving stereotyped and non stereotyped activity. To identify the mechanism of these long-lasting processes, we checked the density of mu opioid receptors, receptor-G-protein coupling and the cyclic AMP (cAMP) cascade. In morphine-sensitized animals mu opioid receptor autoradiography revealed a significant increase in the caudate putamen (30% versus controls), nucleus accumbens shell (16%), prefrontal and frontal cortex (26%), medial thalamus (43%), hypothalamus (200%) and central gray (89%). Concerning morphine's activation of G proteins in the brain, investigated in the guanylyl 5'-[gamma-(35)S]thio]triphosphate ([(35)S]GTPgammaS) binding assay, a significant increase in net [(35)S]GTPgammaS binding was seen in the caudate putamen (39%) and hypothalamus (27%). In the caudate putamen this was due to an increase in the amount of activated G proteins, and in the hypothalamus to a greater affinity of G proteins for guanosine triphosphate (GTP). The main second messenger system linked to the opioid receptor is the cAMP pathway. In the striatum basal cAMP levels were significantly elevated in sensitized animals (70% versus controls) and [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) significantly inhibited forskolin-stimulated cAMP production in control (30%) but not in sensitized rats. In the hypothalamus no significant changes were observed in basal cAMP levels and DAMGO inhibition. These cellular events induced by morphine pre-exposure could underlie the neuroadaptive processes involved in morphine sensitization.

Chronic morphine treatment modulates the extracellular levels of endogenous enkephalins in rat brain structures involved in opiate dependence: a microdialysis study

The endogenous opioid system is often assumed to play a role in vulnerability to drug abuse. However, controversial results have been reported regarding the levels of enkephalins or preproenkephalin in neurons of rodent brains after opiate administration. The present study was performed to determine the extracellular levels of enkephalins and its physiological antagonist cholecystokinin (CCK), using in vivo microdialysis in freely moving rats after morphine-induced physical dependence or positive place conditioning. A large increase (340%) of Met-enkephalin was observed in the periaqueductal gray matter, a structure involved in morphine withdrawal syndrome, in morphine-dependent rats. No change in CCK immunoreactivity occurred in these conditions. Moreover, using the conditioning place preference paradigm, we observed for the first time opposite changes of enkephalin outflow in the nucleus accumbens (NAc). Thus, an increase in enkephalin levels was observed in rats placed in the drug-associated compartment and a decrease in the saline-paired side. These changes in opioid peptides in the NAc may reflect an "emotional state" of the animals in relation to the expectation of drug reward (reinforcing effects of morphine). Moreover, the lack of regulation in CCK outflow suggests that CCK-opioid interactions in morphine dependence involve probably post-receptor events.

Recent advances in the biology of addiction

Recent research into the biologic basis of drug addiction continues to offer considerable promise for understanding how neurochemistry, pharmacology, and molecular biology relate to the reinforcing effects of abused drugs. One area of research is the development and pharmacologic and neurochemical characterization of cocaine and opiate polydrug abuse, a growing subset of the drug abuse population. Considerable advances have also been made in understanding how chronic and persistent drug use induces biochemical and molecular biologic adaptations in brain regions related to drug reinforcement.

Drugs of abuse and brain gene expression

Addictive drugs like cocaine, ethanol, and morphine activate signal transduction pathways that regulate brain gene expression. Such regulation is modulated by the presence of certain transcription factor proteins present in a given neuron. This article summarizes the effects of several addictive drugs on transcriptional processes contributing to the development of a drug-dependent state. The characterization of drug-induced changes in gene expression shows promise for improving our understanding of drug-addiction phenomena and cellular modes of cocaine, ethanol, and morphine action.

Stressor- or drug-induced sensitization of the corticosterone response is not critically involved in the long-term expression of behavioural sensitization to amphetamine

Repeated exposure to drugs of abuse induces long-lasting behavioural sensitization, which is thought to play a role in the persistence of drug-seeking behaviour. Recently, we showed that repeated exposure of rats to cocaine resulted in a long-lasting (weeks) sensitization of the hypothalamus-pituitary-adrenal axis, i.e. hypersecretion of adrenocorticotropic hormone and of the glucocorticoid corticosterone. Moreover, we found that the administration of a glucocorticoid receptor antagonist abolished the expression of psychostimulant-induced behavioural sensitization. In the present study we tested whether stressor- or drug-induced long-term hypersecretion of corticosterone is associated with the long-term expression of behavioural sensitization to psychostimulant drugs. To that end, groups of male Wistar rats were exposed once to interleukin-1beta or to footshocks, treatments that are known to induce long-term sensitization of the hypothalamus-pituitary-adrenal axis, or were treated with amphetamine or morphine, according to protocols known to induce long-lasting behavioural (locomotor) sensitization. Three weeks later, the groups and their controls were challenged with amphetamine or vehicle. Previous exposure to interleukin-1beta or footshocks enhanced adrenocorticotropic hormone and corticosterone responses, but did not affect the long-term locomotor sensitization to amphetamine. Prior amphetamine treatment enhanced the locomotor response and the adrenocorticotropic hormone and corticosterone responses to amphetamine. Prior morphine treatment resulted in long-term locomotor sensitization, whereas the adrenocorticotropic hormone and corticosterone responses to amphetamine were decreased. From these findings and the absence of within-group correlation between corticosterone and locomotor responses in interleukin-1beta and morphine-pretreated rats, we conclude that there is no correlation between sensitization of the corticosterone response and behavioural sensitization to amphetamine. Apparently, sensitization of the corticosterone response is not a prerequisite for the long-term expression of behavioural sensitization, which suggests that drug-induced long-term behavioural sensitization may involve corticosteroid receptor-dependent (central) mechanisms that occur independent of hypothalamus-pituitary-adrenal axis responsiveness.

Drug addiction as dopamine-dependent associative learning disorder

Natural rewards preferentially stimulate dopamine transmission in the nucleus accumbens shell. This effect undergoes adaptive changes (one-trial habituation, inhibition by appetitive stimuli) that are consistent with a role of nucleus accumbens shell dopamine in associative reward-related learning. Experimental studies with a variety of paradigms confirm this role. A role in associative stimulus-reward learning can provide an explanation for the extinction-like impairment of primary reinforcement that led Wise to propose the 'anhedonia hypothesis'. Addictive drugs share with natural rewards the property of stimulating dopamine transmission preferentially in the nucleus accumbens shell. This response, however, in contrast to that to natural rewards, is not subjected to one-trial habituation. Resistance to habituation allows drugs to activate dopamine transmission in the shell non-decrementally upon repeated self-administration. It is hypothesized that this process abnormally strengthens stimulus-drug associations thus resulting in the attribution of excessive motivational value to discrete stimuli or contexts predictive of drug availability. Addiction is therefore the expression of the excessive control over behaviour acquired by drug-related stimuli as a result of abnormal associative learning following repeated stimulation of dopamine transmission in the nucleus accumbens shell.

Drug addiction as a disorder of associative learning. Role of nucleus accumbens shell/extended amygdala dopamine

Conventional reinforcers phasically stimulate dopamine transmission in the nucleus accumbens shell. This property undergoes one-trial habituation consistent with a role of nucleus accumbens shell dopamine in associative learning. Experimental studies with place- and taste-conditioning paradigms confirm this role. Addictive drugs share with conventional reinforcers the property of stimulating dopamine transmission in the nucleus accumbens shell. This response, however, undergoes one-trial habituation in the case of conventional reinforcers but not of drugs. Resistance to habituation allows drugs to repetitively activate dopamine transmission in the shell upon repeated self-administration. This process abnormally facilitates associative learning, leading to the attribution of excessive motivational value to discrete stimuli or contexts predictive of drug availability. Addiction is therefore the expression of the excessive control over behavior acquired by drug-related stimuli as a result of abnormal strenghtening of stimulus-drug contingencies by nondecremental drug-induced stimulation of dopamine transmission in the nucleus accumbens shell.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

Intermittent morphine administration induces a long-lasting synergistic effect of corticosterone on dopamine D1 receptor functioning in rat striatal GABA neurons

Glucocorticoid receptor (GR)-mediated facilitation of striatal dopaminergic (DA) neurotransmission has been proposed to play a role in behavioral sensitization induced by intermittent exposure to drugs of abuse or stressors. Searching for possible common neuronal substrates acted upon by drugs of abuse and corticosterone, we addressed the question as to whether such a facilitatory effect is apparent (i.e., persists) in primary cultures of rat striatum subsequent to intermittent (prenatal) morphine administration. As previously observed in striatal slices of morphine-treated rats, intermittent morphine exposure in vivo caused a long-lasting increase in DA D1 receptor-stimulated adenylyl cyclase activity, that appeared to persist in primary cultures of rat striatal gamma-aminobutyric acid (GABA) neurons. Subsequent in vitro exposure of these striatal neurons to corticosterone or dexamethasone, simultaneously activating GR and mineralocorticoid receptors (MR), about doubled this adaptive effect of previous in vivo morphine administration. The selective MR agonist aldosterone was ineffective in this respect. Prior in vivo morphine treatment also enhanced the stimulatory in vitro effect of corticotropin releasing hormone (CRH) on adenylyl cyclase in cultured GABA neurons. However, the enhanced CRH receptor functioning was not potentiated by in vitro corticosterone exposure. Activation of GR by corticosterone did not facilitate the increase in D1 receptor efficacy induced by sustained activation of muscarinic receptors in cultured striatal neurons. These data indicate that previous intermittent morphine administration induces a long-lasting synergistic effect of corticosterone on enhanced striatal DA neurotransmission at the level of postsynaptic D1 receptors. Moreover, the induction of this neuroadaptation seems to display opioid receptor selectivity and its long-term expression may be confined to D1 receptors. Since exposure to drugs of abuse or stressors not only increase striatal DA release but also plasma corticosterone levels, we hypothesize that this adaptive phenomenon in DA-sensitive GABA neurons is involved in the expression of morphine-induced long-term behavioral sensitization to drugs of abuse and stressors.