Repeated and chronic morphine administration causes differential long-lasting changes in dopaminergic neurotransmission in rat striatum without changing its delta- and kappa-opioid receptor regulation

Long-term administration of morphine specifically alters the level of protein expression in different brain regions and affects the redox state

We investigated the changes in redox state and protein expression in selected parts of the rat brain induced by a 4 week administration of morphine (10 mg/kg/day). We found a significant reduction in lipid peroxidation that mostly persisted for 1 week after morphine withdrawal. Morphine treatment led to a significant increase in complex II in the cerebral cortex (Crt), which was accompanied by increased protein carbonylation, in contrast to the other brain regions studied. Glutathione levels were altered differently in the different brain regions after morphine treatment. Using label-free quantitative proteomic analysis, we found some specific changes in protein expression profiles in the Crt, hippocampus, striatum, and cerebellum on the day after morphine withdrawal and 1 week later. A common feature was the upregulation of anti-apoptotic proteins and dysregulation of the extracellular matrix. Our results indicate that the tested protocol of morphine administration has no significant toxic effect on the rat brain. On the contrary, it led to a decrease in lipid peroxidation and activation of anti-apoptotic proteins. Furthermore, our data suggest that long-term treatment with morphine acts specifically on different brain regions and that a 1 week drug withdrawal is not sufficient to normalize cellular redox state and protein levels.

HIV-1 Tat and morphine interactions dynamically shift striatal monoamine levels and exploratory behaviors over time

Despite the advent of combination anti-retroviral therapy (cART), nearly half of people infected with HIV treated with cART still exhibit HIV-associated neurocognitive disorders (HAND). HAND can be worsened by co-morbid opioid use disorder. The basal ganglia are particularly vulnerable to HIV-1 and exhibit higher viral loads and more severe pathology, which can be exacerbated by co-exposure to opioids. Evidence suggests that dopaminergic neurotransmission is disrupted by HIV exposure, however, little is known about whether co-exposure to opioids may alter neurotransmitter levels in the striatum and if this in turn influences behavior. Therefore, we assayed motor, anxiety-like, novelty-seeking, exploratory, and social behaviors, and levels of monoamines and their metabolites following 2 weeks and 2 months of Tat and/or morphine exposure in transgenic mice. Morphine decreased dopamine levels, but significantly elevated norepinephrine, the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and the serotonin metabolite 5-hydroxyindoleacetic acid, which typically correlated with increased locomotor behavior. The combination of Tat and morphine altered dopamine, DOPAC, and HVA concentrations differently depending on the neurotransmitter/metabolite and duration of exposure but did not affect the numbers of tyrosine hydroxylase-positive neurons in the mesencephalon. Tat exposure increased the latency to interact with novel conspecifics, but not other novel objects, suggesting the viral protein inhibits exploratory behavior initiation in a context-dependent manner. By contrast, and consistent with prior findings that opioid misuse can increase novelty-seeking behavior, morphine exposure increased the time spent exploring a novel environment. Finally, Tat and morphine interacted to affect locomotor activity in a time-dependent manner, while grip strength and rotarod performance were unaffected. Together, our results provide novel insight into the unique effects of HIV-1 Tat and morphine on monoamine neurochemistry that may underlie their divergent effects on motor and exploratory behavior.

Naloxone precipitated withdrawal increases dopamine release in the dorsal striatum of opioid dependent men

Dopamine (DA) neurotransmission is critical in the neurobiology of reward and aversion, but its contribution to the aversive state of opioid withdrawal remains unknown in humans. To address this, we used updated voxelwise methods and retrospectively analyzed a [¹¹C]raclopride-PET dataset to measure D_2/3 receptor availability and relative cerebral blood flow (R1) in male opioid use disorder (OUD) participants (n = 10) during placebo and acute opioid withdrawal conditions. We found that acute withdrawal precipitated by the opioid antagonist naloxone significantly increased dorsal striatal DA release in OUD participants (p_FWE < 0.05). Net changes in striatal DA were significantly correlated with a subjective index of withdrawal aversion such that greater DA increases were associated with more aversive responses (r(8) = 0.82, p < 0.005). Withdrawal also affected brain function, as indexed by increases in relative cerebral blood flow in the insula and putamen (p_FWE < 0.05). Our findings are different from preclinical studies that have primarily reported decreases in ventral striatal DA during naloxone precipitated withdrawal, whereas this effect was not significant in OUD participants (p = 0.79). In sum, we provide evidence for the contribution of increases in dorsal striatal DA to the aversive state of naloxone precipitated withdrawal in humans.

Restraint Stress Potentiated Morphine Sensitization: Involvement of Dopamine Receptors within the Nucleus Accumbens

Sensitization to psychostimulant drugs, as well as morphine, subjected to cross-sensitization with stress. The development of morphine sensitization is associated with enhancements in dopamine overflow in the Nucleus accumbens (NAc). This study aimed to examine the role of accumbal D1/D2-like dopamine receptors in restraint stress (RS) induced sensitization to morphine antinociceptive effects. Adult male Wistar rats weighing 220-250 g underwent stereotaxic surgery. Two stainless steel guide cannulae were bilaterally implanted, 1 mm above the NAc injection site. Different solutions of SCH-23390, as a D1-like receptor antagonist or sulpiride, as a D2-like receptor antagonist, were microinjected into the NAc five min before exposure to RS. Restraint stress lasted for 3 h, 10 min after RS termination; animals received a subcutaneous injection of morphine (1 mg/kg) for 3 consecutive days. The procedure was followed by a 5-day drug and/or stress-free period. After that, on the 9th day, the nociceptive response was evaluated by the tail-flick test. The results revealed that intra-NAc administration of D1/D2-like dopamine receptor antagonists, SCH-23390 or sulpiride, respectively, blocked morphine sensitization-induced by RS and morphine co-administration in rats for three consecutive days. This work provides new insight into the determinant role of accumbal dopamine receptors in morphine sensitization produced by RS-morphine co-administration.

Allostatic Changes in the cAMP System Drive Opioid-Induced Adaptation in Striatal Dopamine Signaling

Opioids are powerful addictive agents that alter dopaminergic influence on reward signaling in medium spiny neurons (MSNs) of the nucleus accumbens. Repeated opioid exposure triggers adaptive changes, shifting reward valuation to the allostatic state underlying tolerance. However, the cellular substrates and molecular logic underlying such allostatic changes are not well understood. Here, we report that the plasticity of dopamine-induced cyclic AMP (cAMP) signaling in MSNs serves as a cellular substrate for drug-induced allostatic adjustments. By recording cAMP responses to optically evoked dopamine in brain slices from mice subjected to various opioid exposure paradigms, we define profound neuronal-type-specific adaptations. We find that opioid exposure pivots the initial hyper-responsiveness of D1-MSNs toward D2-MSN dominance as dependence escalates. Presynaptic dopamine transporters and postsynaptic phosphodiesterases critically enable cell-specific adjustments of cAMP that control the balance between opponent D1-MSN and D2-MSN channels. We propose a quantitative model of opioid-induced allostatic adjustments in cAMP signal strength that balances circuit activity.

Muntean et al. examine how opioid exposure influences cyclic AMP (cAMP) responses to dopamine in striatal medium spiny neurons (MSNs). They describe allostatic adaptations in the processing of dopaminergic signals by D1-MSN and D2-MSN populations as opioid administration progresses from acute exposure to chronic use, and they define molecular elements contributing to the process.

Pediatric Clinical Endpoint and Pharmacodynamic Biomarkers: Limitations and Opportunities

Medical research in children typically lags behind that of adult research in both quantity and quality. The conduct of rigorous clinical trials in children can raise ethical concerns because of children's status as a 'vulnerable' population. Moreover, carrying out studies in pediatrics also requires logistical considerations that rarely occur with adult clinical trials. Due to the relatively smaller number of pediatric studies to support evidence-based medicine, the practice of medicine in children is far more reliant upon expert opinion than in adult medicine. Children are at risk of not receiving the same level of benefits from precision medicine research, which has flourished with new technologies capable of generating large amounts of data quickly at an individual level. Although progress has been made in pediatric pharmacokinetics, which has led to safer and more effective dosing, gaps in knowledge still exists when it comes to characterization of pediatric disease and differences in pharmacodynamic response between children and adults. This review highlights three specific therapeutic areas where biomarker development can enhance precision medicine in children: asthma, type 2 diabetes mellitus, and pain. These 'case studies' are meant to update the reader on biomarkers used currently in the diagnosis and treatment of these conditions, and their shortcomings within a pediatric context. Current research on surrogate endpoints and pharmacodynamic biomarkers in the above therapeutic areas will also be described. These cases highlight the current lack in pediatric specific surrogate endpoints and pharmacodynamic biomarkers, as well as the research presently being conducted to address these deficiencies. We finally briefly highlight other therapeutic areas where further research in pediatric surrogate endpoints and pharmacodynamic biomarkers can be impactful to the care of children.

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.

Dopamine D1 receptor agonist treatment attenuates extinction of morphine conditioned place preference while increasing dendritic complexity in the nucleus accumbens core

The dopamine D1 receptor (D1R) has a role in opioid reward and conditioned place preference (CPP), but its role in CPP extinction is undetermined. We examined the effect of D1R agonist SKF81297 on the extinction of opioid CPP and associated dendritic morphology in the nucleus accumbens (NAc), a region involved with reward integration and its extinction. During the acquisition of morphine CPP, mice received morphine and saline on alternate days; injections were given immediately before each of eight daily conditioning sessions. Mice subsequently underwent six days of extinction training designed to diminish the previously learned association. Mice were treated with either 0.5mg/kg SKF81297, 0.8mg/kg SKF81297, or saline immediately after each extinction session. There was a dose-dependent effect, with the highest dose of SKF81297 attenuating extinction, as mice treated with this dose had significantly higher CPP scores than controls. Analysis of medium spiny neuron morphology revealed that in the NAc core, but not in the shell, dendritic arbors were significantly more complex in the morphine conditioned, SKF81297-treated mice compared to controls. In separate experiments using mice conditioned with only saline, SKF81297 administration after extinction sessions had no effect on CPP and produced differing effects on dendritic morphology. At the doses used in our experiments, SKF81297 appears to maintain previously learned opioid conditioned behavior, even in the face of new information. The D1R agonist's differential, rather than unidirectional, effects on dendritic morphology in the NAc core suggests that it may be involved in encoding reward information depending on previously learned behavior.

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.

The association between heroin expenditure and dopamine transporter availability--a single-photon emission computed tomography study

One of the consequences of heroin dependency is a huge expenditure on drugs. This underlying economic expense may be a grave burden for heroin users and may lead to criminal behavior, which is a huge cost to society. The neuropsychological mechanism related to heroin purchase remains unclear. Based on recent findings and the established dopamine hypothesis of addiction, we speculated that expenditure on heroin and central dopamine activity may be associated. A total of 21 heroin users were enrolled in this study. The annual expenditure on heroin was assessed, and the availability of the dopamine transporter (DAT) was assessed by single-photon emission computed tomography (SPECT) using [(99m)TC]TRODAT-1. Parametric and nonparametric correlation analyses indicated that annual expenditure on heroin was significantly and negatively correlated with the availability of striatal DAT. After adjustment for potential confounders, the predictive power of DAT availability was significant. Striatal dopamine function may be associated with opioid purchasing behavior among heroin users, and the cycle of spiraling dysfunction in the dopamine reward system could play a role in this association.

Swimming reduces the severity of physical and psychological dependence and voluntary morphine consumption in morphine dependent rats

Previous studies have indicated that voluntary exercise decreases the severity of the anxiogenic-like behaviors in both morphine-dependent and withdrawn rats. This study examined the effects of regular swimming exercise during the development of dependency and spontaneous morphine withdrawal on the anxiety-depression profile and voluntary morphine consumption in morphine dependent rats. The rats were chronically treated with bi-daily doses (10 mg/kg, at 12h intervals) of morphine over a period of 14 days. The exercising rats were allowed to swim (45 min/d, five days per a week, for 14 or 21 days) during the development of morphine dependence and withdrawal. Then, rats were tested for the severity of morphine dependence, the elevated plus-maze (EPM), sucrose preference test (SPT) and voluntary morphine consumption using a two-bottle choice paradigm in animal models of craving. The results showed that withdrawal signs were decreased in swimmer morphine dependent rats than sedentary rats (P<0.05). Also, the swimmer morphine-dependent and withdrawn rats exhibited an increase in EPM open arm time and entries (P<0.05), higher levels of sucrose preference (P<0.001) than sedentary rats. Voluntary consumption of oral morphine was less in the swimmer morphine-withdrawn rats than the sedentary groups during four periods of the intake of drug (P<0.01). We conclude that regular swimming exercise reduces the severity of morphine dependence and voluntary morphine consumption with reducing anxiety and depression in morphine-dependent and withdrawn rats. Thus, swimming exercise may be a potential method to ameliorate some of the deleterious behavioral consequences of morphine dependence.

Availability of dopamine and serotonin transporters in opioid-dependent users--a two-isotope SPECT study

RATIONALE AND OBJECTIVE

The aims of this study were to examine the differences between 32 opioid-dependent users treated with a very low dose of methadone or undergoing methadone-free abstinence and 32 controls.

METHODS

SPECT analysis using [(99m)Tc] TRODAT-1 to assess striatal dopamine transporter (DAT) availability and [(123)I] ADAM to assess midbrain serotonin transporter (SERT) availability were performed.

RESULTS

Lower striatal DAT and midbrain SERT availabilities were noted in low-dose methadone users. History of metamphatamine use was associated with the lower striatal DAT. The striatal DAT of methadone-free abstainers was also lower than controls. The midbrain SERT availability tended to be higher in the methadone-free abstainers than the low-dose methadone users. The severity of depressive symptoms was negatively correlated with midbrain SERT availability in the opioid users.

CONCLUSION

The availability of striatal DAT tended to be, and the availability of midbrain SERT was, lower in the opioid users. History of metamphatamine use may confound the difference in straital DAT between controls and opioid users, as midbrain SERT and depressive symptoms are also associated with opioid use and abstinence.

Decreased striatal dopamine transporters in codeine-containing cough syrup abusers

BACKGROUND

In recent years, codeine-containing cough syrups (CCS) have been reported as substances of abuse, especially in adolescents. Chronic CCS abuse can induce physical and psychological dependence. However, the potential brain impairment caused by chronic CCS abuse has not been reported previously. The present study was designed to determine if the striatal dopamine transporter (DAT) levels measured by (99m)Tc-TRODAT-1 single photon emission computed tomography (SPECT) brain scans were altered in CCS-dependent subjects and to explore the CCS-caused brain impairment.

METHODS

SPECT brain scans were acquired on 22 CCS-dependent subjects and 27 healthy age matched controls. The volume (V) and weight (W) of bilateral corpus striatum as well as the (99m)Tc-TRODAT-1 uptake ratio of corpus striatum/the whole brain (Ra) were calculated using mathematical models.

RESULTS

It was displayed that DAT availability of striatum was significantly decreased and the V, W and Ra were greatly reduced in the CCS-dependent group compared to controls (for each comparison p<0.01).

CONCLUSION

Taken together, these results suggest that chronic CCS abuse may cause serious damage to the brain and the neuroimaging findings further illustrate the mechanism of CCS dependence.

Chronic and intermittent morphine treatment differently regulates opioid and dopamine systems: a role in locomotor sensitization

RATIONALE

Behavioral sensitization induced by repeated morphine administrations may depend on patterns of administration. However, neurobiological mechanisms involved in this sensitization are largely unknown.

OBJECTIVES

We compared the effects of intermittent (20 mg/kg, once daily for 7 days) and chronic (escalating doses from 5 to 40 mg/kg, three times a day for 5 days) morphine treatments in mice on locomotor activity. We also quantified, by autoradiography, mu opioid receptor (MOR) in ventral tegmental area (VTA), dopamine D1 (D1R) and D2 (D2R) receptors in striatum.

RESULTS

Whereas the intermittent treatment led to a long-term sensitization to locomotor effects of morphine [until withdrawal day (WD) 14], the chronic treatment induced a tolerance (WD1) followed by a transient sensitization (WD14). Binding studies demonstrated a decrease of MOR in VTA at WD1 for the chronic treatment. In contrast, striatal D1R level was decreased at WD1, and increased at WD14 for the chronic treatment. For the D2R, we observed a decrease from WD1 to WD14 for the intermittent treatment and an increase at WD1 followed by a decrease at WD14 for the chronic treatment.

CONCLUSIONS

These results demonstrate that chronic and intermittent morphine treatments could induce different behavioral adaptations that could be explained in part by distinct changes occurring in dopamine and opioid systems.

Morphine sensitization in the pentylenetetrazole-induced clonic seizure threshold in mice: role of nitric oxide and μ receptors

Behavioral sensitization occurs after repeated administration of μ-opioid receptor agonists following a drug-free period. It seems that the changes in dopaminergic systems induced by μ-opioid receptor agonists play a crucial role in behavioral sensitization to opioids. Nitric oxide also plays a role in some behavioral effects of morphine, including sensitization to the locomotor-stimulating effect. This study investigated whether morphine sensitization appears in seizure threshold and the possible role of μ-opioid receptor and nitric oxide in this sensitization. Sensitization was produced by daily injections of morphine (0.1, 0.5, 1, 5, 15, or 30 mg/kg), followed by a 10-day washout period. Then the challenge test was performed using morphine (0.1, 0.5, 1, 5, 15, or 30 mg/kg) in different groups. To assess clonic seizure threshold, pentylenetetrazole (PTZ) was administered intravenously. Subcutaneous administration of morphine (0.1 and 0.5 mg/kg) induced sensitization in PTZ-induced clonic seizures in mice. Intraperitoneal administration of L-NAME (20 mg/kg), a nonselective inhibitor of nitric oxide synthase, or naltrexone (10 mg/kg), an opioid receptor antagonist, along with morphine inhibited morphine-induced sensitization in PTZ-induced seizure threshold. In conclusion, at low doses, morphine induces sensitization in PTZ-induced clonic seizures in mice probably as a result of the interaction with μ-receptors and nitric oxide.

Extracellular matrix plasticity and GABAergic inhibition of prefrontal cortex pyramidal cells facilitates relapse to heroin seeking

Successful treatment of drug addiction is hampered by high relapse rates during periods of abstinence. Neuroadaptation in the medial prefrontal cortex (mPFC) is thought to have a crucial role in vulnerability to relapse to drug seeking, but the molecular and cellular mechanisms remain largely unknown. To identify protein changes that contribute to relapse susceptibility, we investigated synaptic membrane fractions from the mPFC of rats that underwent 21 days of forced abstinence following heroin self-administration. Quantitative proteomics revealed that long-term abstinence from heroin self-administration was associated with reduced levels of extracellular matrix (ECM) proteins. After extinction of heroin self-administration, downregulation of ECM proteins was also present in the mPFC, as well as nucleus accumbens (NAc), and these adaptations were partially restored following cue-induced reinstatement of heroin seeking. In the mPFC, these ECM proteins are condensed in the perineuronal nets that exclusively surround GABAergic interneurons, indicating that ECM adaptation might alter the activity of GABAergic interneurons. In support of this, we observed an increase in the inhibitory GABAergic synaptic inputs received by the mPFC pyramidal cells after the re-exposure to heroin-conditioned cues. Recovering levels of ECM constituents by metalloproteinase inhibitor treatment (FN-439; i.c.v.) prior to a reinstatement test attenuated subsequent heroin seeking, suggesting that the reduced synaptic ECM levels during heroin abstinence enhanced sensitivity to respond to heroin-conditioned cues. We provide evidence for a novel neuroadaptive mechanism, in which heroin self-administration-induced adaptation of the ECM increased relapse vulnerability, potentially by augmenting the responsivity of mPFC GABAergic interneurons to heroin-associated stimuli.

Neural mechanisms of reproduction in females as a predisposing factor for drug addiction

There is an increasing awareness that adolescent females differ from males in their response to drugs of abuse and consequently in their vulnerability to addiction. One possible component of this vulnerability to drug addiction is the neurobiological impact that reproductive physiology and behaviors have on the mesolimbic dopamine system, a key neural pathway mediating drug addiction. In this review, we examine animal models that address the impact of ovarian cyclicity, sexual affiliation, sexual behavior, and maternal care on the long-term plasticity of the mesolimbic dopamine system. The thesis is that this plasticity in synaptic neurotransmission stemming from an individual's normal life history contributes to the pathological impact of drugs of abuse on the neurobiology of this system. Hormones released during reproductive cycles have only transient effects on these dopamine systems, whereas reproductive behaviors produce a persistent sensitization of dopamine release and post-synaptic neuronal responsiveness. Puberty itself may not represent a neurobiological risk factor for drug abuse, but attendant behavioral experiences may have a negative impact on females engaging in drug use.

Strain differences between Lewis and Fischer 344 rats in the modulation of dopaminergic receptors after morphine self-administration and during extinction

The Lewis (LEW) and Fischer 344 (F344) rat strains have been used as a model to study genetic vulnerability to drug addiction and they differ in their dopaminergic systems. We have studied the variation in the D1-like and D2-like receptors in distinct brain regions of LEW and F344 rats that self-administered morphine (1 mg/kg) for 15 days and also after different extinction periods (3, 7 and 15 days). Under basal conditions, binding to D1-like receptors in the olfactory tubercle and substantia nigra, and to D2-like receptors in the Pyriform cortex and hippocampal-CA1 was lower in LEW rats than in F344 rats. Conversely, the LEW rats exhibited stronger D2-like binding in the caudate-putamen. In most brain regions there was a decrease in D1-like binding in LEW rats after self-administration while the F344 animals displayed an increment. Additionally, D2 receptors of LEW rats were down-regulated after self-administration in the caudate-putamen and in the nucleus accumbens (shell and core divisions). Binding to D1-like receptors increased in both strains in the early phases of extinction, while in the later stages a differential regulation was observed between both strains. During the early phases of extinction only F344 rats showed alterations in D2-like receptor binding, however in the latter phases a specific modulation occurred in both strains. These differences in basal D1-like and D2-like receptor binding, and their differential modulation after self-administration and during extinction, may be reflected in the greater vulnerability to opiate addiction shown by LEW strain.

Changes in accumbal and pallidal pCREB and deltaFosB in morphine-sensitized rats: correlations with receptor-evoked electrophysiological measures in the ventral pallidum

Activation of mu-opioid receptors in the ventral pallidum (VP) is important for the induction of behavioral sensitization to morphine in rats. The present study was designed to ascertain if neurons within the VP demonstrate sensitization at a time when morphine-induced behavioral sensitization occurred (ie 3 or 14 days after five once-daily injections of 10 mg/kg i.p. morphine) in rats. Western blotting was used to evaluate transcription factors altered by opiates, CREB and deltaFosB. CREB levels did not change in the VP, but there was a significant decrease in levels of its active, phosphorylated form (pCREB) at both 3- and 14-days withdrawal. DeltaFosB levels were elevated following a 3-day withdrawal, but returned to normal by 14 days. This profile also was obtained from nucleus accumbens tissue. In a separate group of similarly treated rats, in vivo electrophysiological recordings of VP neuronal responses to microiontophoretically applied ligands were carried out after 14-days withdrawal. The firing rate effects of local applications of morphine were diminished in rats withdrawn from i.p. morphine. Repeated i.p. morphine did not alter GABA-mediated suppression of firing, or the rate enhancing effects of the D1 dopamine receptor agonist SKF82958 or glutamate. However, VP neurons from rats withdrawn from repeated i.p. morphine showed a higher propensity to enter a state of depolarization inactivation to locally applied glutamate. Overall, these findings reveal that decreased pCREB in brain regions such as the VP accompanies persistent behavioral sensitization to morphine and that this biochemical alteration may influence the excitability of neurons in this brain region.

Intermittent administration of morphine alters protein expression in rat nucleus accumbens

Repeated exposure to drugs of abuse causes time-dependent neuroadaptive changes in the mesocorticolimbic system of the brain that are considered to underlie the expression of major behavioral characteristics of drug addiction. We used a 2-D gel-based proteomics approach to examine morphine-induced temporal changes in protein expression and/or PTM in the nucleus accumbens (NAc) of morphine-sensitized rats. Rats were pretreated with saline [1 mL/kg subcutaneously (s.c.)] or morphine (10 mg/kg, s.c.) once daily for 14 days and the animals were decapitated 1 day later. The NAc was extracted and proteins resolved by 2-DE. Several protein functional groups were found to be regulated in the morphine-treated group, representing cytoskeletal proteins, proteins involved in neurotransmission, enzymes involved in energy metabolism and protein degradation, and a protein that regulates translation.

Glutamate-mediated neuroplasticity in a limbic input to the hypothalamus

Emotionally-salient stressors are processed by cortical and limbic circuits that provide important regulatory input to the hypothalamic-pituitary-adrenal (HPA) axis. However, exposure to chronic or severe stress may cause disregulation of the axis and a variety of physiological and psychological symptoms. The mechanisms that underlie stress-induced alterations in HPA axis function are not well characterized, but one possibility is that severe stress causes plastic changes in limbic inputs to the hypothalamus. We examined plasticity within the bed nucleus of stria terminalis (BNST) and the hypothalamic paraventricular nucleus (PVN) with a stimulating electrode in the BNST and a recording electrode in the PVN. High-frequency BNST stimulation produced long-lasting suppression of evoked field potentials recorded from the PVN, and this effect was blocked by administration of MK-801. Accordingly, rapid glutamate-mediated neuroplasticity in the BNST to PVN neurocircuitry may contribute to plasticity in limbic regulation of the HPA axis.

Neuroimaging studies of brain corpus striatum changes among heroin-dependent patients treated with herbal medicine, U'finer capsule

Chronic exposure to heroin is associated with structural changes in dopaminergic (DA) neurones. The present study examined the effects of a new herbal medicine, U'finer capsule, on the brain corpus striatum and DA systems, comparing pre- and post- treatment in 36 heroin-dependent patients. Neuroimaging studies were performed by using single photon emission computed tomography (SPECT) with 99mTc-TRODAT-1 as radiotracer. The results show that U'finer significantly repaired the damaged bilateral corpus striatum, restoring it to a 'panda eye' shape, analogous in size and shape to that of the healthy volunteers. DA transporter (DAT) function in the bilateral corpus striatum was restored to a normal state after recovery from neurotoxic insult. These findings suggest that U'finer is a reliable herbal medicine in the treatment of heroin dependency.

Morphine causes a delayed increase in glutamate receptor functioning in the nucleus accumbens core

Enhanced excitatory neurotransmission in the mesocorticolimbic system may contribute to the persistence of addiction behaviour. Here, we demonstrated that glutamate-, N-methyl-D-aspartate (NMDA)- and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-induced [3H]-gamma-aminobutyric acid (GABA) release from superfused rat nucleus accumbens core slices is profoundly enhanced 3 weeks, but not 3 days, after a single s.c. morphine injection. This delayed increase in glutamate receptor functioning is associated with enhanced gene transcript levels of ionotropic NMDA and AMPA/kainate receptor subunits. These data reveal that morphine may progressively enhance glutamate neurotransmission within the nucleus accumbens core subsequent to drug exposure.

The development of tolerance to locomotor effects of morphine and the effect of various opioid receptor antagonists in rats chronically treated with morphine

Behavioural measures are considered to be highly sensitive indices of opioid withdrawal. Opioids, depending on dose and time protocols may induce both reduction and enhancement of locomotor activity and chronic opioid treatment results in tolerance and sensitisation to these effects. In the present study the locomotor activity as experimental model was used to assess the development of tolerance to subcutaneous morphine challenge at different time points following morphine withdrawal in rats exposed to gradually increasing subcutaneous doses of morphine for 11 days. Tolerance developed to the inhibitory action of morphine (10 mg/kg) was observed even 8 weeks after morphine withdrawal, while tolerance to its locomotor activity enhancing effect (3 mg/kg) was detected 18 h after withdrawal, but not 3 weeks later. In the other series of experiments the locomotor activity of animals exposed to chronic morphine treatment was tested 18 h after spontaneous or subcutaneously administrated opioid antagonists precipitated withdrawal. Spontaneous withdrawal resulted in a moderate decrease of locomotion. Both the non-selective antagonist naloxone in low, mu opioid-receptor selective doses and the delta opioid-receptor selective naltrindole induced marked reduction of locomotor activity. The results provide further evidence that both mu and delta opioid-receptors might be affected during chronic morphine treatment.

Chronic opiate treatment enhances both cocaine-reinforced and cocaine-seeking behaviors following opiate withdrawal

After chronic exposure to psychostimulants or opiates, self-administration or conditioned place preference with either class is increased (sensitized). Cross-sensitization of conditioned place preference, i.e., enhancement of psychostimulant-induced preferences after exposure to opiates, has also been described, but increases in cocaine self-administration after morphine pretreatment have not been reported. The present study evaluated effects of chronic morphine treatment on cocaine reinforcement. Opiate dependence was established in Wistar rats by administration of morphine as a constant infusion that was gradually increased to a dose of 50mg/kg per day over a 1-week period. Immediately after discontinuation of chronic morphine treatment, animals were allowed to acquire cocaine self-administration under a simple fixed-ratio schedule (FR-1), and were subsequently advanced to a progressive ratio schedule. Acquisition of cocaine self-administration under the FR-1 did not differ in saline- and morphine-pretreated animals. For cocaine self-administration under a progressive ratio schedule measured at 5 or more days after the onset of opiate withdrawal, chronic pretreatment with morphine increased the number of ratios completed, augmented final response requirements, and produced a more stable pattern of cocaine self-administration. Responding was also increased in morphine-pretreated animals during an initial extinction session. These results show that chronic opiate treatment can enhance both cocaine-reinforced and cocaine-seeking behaviors following opiate withdrawal. A similar effect may occur in human patients who discontinue methadone or other forms of replacement therapy for opiate abuse, and may contribute to relapse involving use of cocaine or other psychostimulants.

Sexual experience alters D1 receptor-mediated cyclic AMP production in the nucleus accumbens of female Syrian hamsters

Drugs of abuse produce long-term changes in dopamine neurotransmission and receptor-effected intracellular signaling. Similar changes in neuronal activity are mediated by motivated behaviors. To explore cellular mechanisms underlying these neuroadaptations following sexual experience, cyclic AMP accumulation following stimulation of D1 dopamine receptors, G-proteins, and adenylate cyclase was compared in the nucleus accumbens and caudate nucleus of sexually naive and experienced female hamsters following sexual behavior. Direct stimulation of adenylate cyclase with forskolin or indirectly by activation of G-proteins with Gpp(NH)p produced dose-dependent increases in the formation of cyclic AMP in the nucleus accumbens and caudate nucleus, with no effects of sexual experience on these measures. Specific D1 receptor stimulation increased Gpp(NH)p-induced adenylate cyclase activity in the nucleus accumbens and caudate nucleus of all animals. Interestingly, this stimulation was further enhanced only in membranes from the nucleus accumbens, but not from the caudate nucleus, of sexually experienced hamsters compared to the response of naive females. These results demonstrate that sexual behavior experience can sensitize mesolimbic dopamine pathways and that this sensitization occurs through an increase in D1 receptor-mediated signaling.

Long-term opiate effects on amphetamine-induced dopamine release in the nucleus accumbens core and conditioned place preference

Withdrawal following chronic exposure to opiates or other drugs of abuse, administered as frequent doses, or a chronic infusion can cause reductions in mesolimbic dopamine (DA) transmission. However, mesolimbic DA transmission can be enhanced by opiates or psychostimulants administered intermittently as a single daily injection. Both enhanced and attenuated responsiveness of the mesolimbic DA system may have important implications for substance abuse disorders. Previous studies have shown that procedures that use electrical stimulation or drug treatments to augment neurotransmitter release are more effective for demonstrating declines in mesolimbic DA transmission that persist for extended periods following opiate withdrawal. The present study evaluated the effects of pretreatment with noncontingent morphine on amphetamine-induced DA release in the nucleus accumbens core and conditioned place preference (CPP). Morphine pretreatment was administered as a constant infusion, which was gradually increased to a dose of 50 mg/kg/day over a 1-week period in Wistar rats. At 10 days after cessation of morphine pretreatment, baseline dialysate DA levels in the nucleus accumbens core were unchanged, but amphetamine-induced increases in DA were attenuated by greater than 50% in morphine-pretreated animals. Morphine pretreatment did not modify locomotor activity during conditioning sessions, expressed as absolute values or change in activity counts between saline and morphine injections. Place preference, conditioned by two morphine pairings at 10 and 11 days after the onset of opiate withdrawal, was enhanced by opiate pretreatment between 12 and 33 days after the onset of withdrawal. In conclusion, morphine pretreatment delivered as a constant infusion can have pronounced and long-lasting effects on DA release and CPP, which may have important implications for drug-seeking behavior and treatment of substance abuse disorders.

Repeated morphine injections induce behavioural sensitization in Roman high- but not in Roman low-avoidance rats

The selective breeding of Roman high- (RHA) and low-avoidance (RLA) rats for, respectively, rapid vs poor active avoidance acquisition has resulted in two phenotypes that differ in their behavioural and neurochemical responses to addictive drugs, including morphine. To compare the ability of these lines to develop behavioural sensitization to morphine, female RHA and RLA rats were treated twice daily with either saline or escalating doses of morphine (5, 10, and 20 mg/kg, s.c. on the 1st, 2nd, and 3rd day of treatment, respectively), and were challenged with morphine (0.5 or 2 mg/kg, s.c.) 1 day before and 3 weeks after repeated morphine administration. The locomotor activation produced by either challenge dose of morphine was more pronounced in RHA rats repeatedly treated with morphine vs the respective saline-treated controls, whereas no significant change in locomotor activity was observed in RLA rats. The results show that behavioral sensitization to morphine was induced in RHA but not in RLA rats.

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.

Differential behavioural sensitization to intermittent morphine treatment in alcohol-preferring AA and alcohol-avoiding ANA rats: role of mesolimbic dopamine

Alcohol-preferring AA (Alko Alcohol) and alcohol-avoiding ANA (Alko Non-Alcohol) rats have well-documented differences in their voluntary ethanol consumption and brain opioidergic systems. The aim of the present study was to investigate whether these rat lines differ in their susceptibility to morphine-induced behavioural and neurochemical sensitization. The rats were given 15 injections of morphine (10 mg/kg, s.c.) or saline every other day. Locomotor activity and release of dopamine in the nucleus accumbens were monitored after a challenge with additional morphine injections (10 mg/kg) 1 and 5 weeks after withdrawal from the repeated treatment. Morphine increased locomotion more in the previously morphine-treated rats than in the saline-treated controls. Furthermore, AA rats were more sensitive to this effect of morphine than ANA rats. Accumbal morphine-induced dopamine release was significantly higher in the morphine-treated AA than ANA rats after the first challenge injection 1 week from withdrawal, but no differences were observed after the second challenge. The brain and plasma concentrations of morphine were similar among the lines suggesting that the differences in the effects of morphine cannot be explained in terms of differential pharmacokinetics of morphine in these lines. These data show that AA rats are more susceptible to morphine-induced behavioural sensitization than ANA rats. Furthermore, it suggests that mesolimbic dopamine has at best only a transient role in the expression of opioid-induced behavioural sensitization. The relationship between the mechanisms underlying the differential sensitivity of these rat lines to the effects of repeated morphine and voluntary ethanol drinking remains to be determined.

Understanding polydrug use: review of heroin and cocaine co-use

The use of cocaine by heroin-dependent individuals, or by patients in methadone or buprenorphine maintenance treatment, is substantial and has negative consequences on health, social adjustment and outcome of opioid-addiction treatment. The pharmacological reasons for cocaine use in opioid-dependent individuals, however, are poorly understood and little is known about the patterns of heroin and cocaine co-use. We reviewed anecdotal evidence suggesting that cocaine is co-used with opioid drugs in a variety of different patterns, to achieve different goals. Clinical and preclinical experimental evidence indicates that the simultaneous administration of cocaine and heroin (i.e. 'speedball') does not induce a novel set of subjective effects, nor is it more reinforcing than either drug alone, especially when the doses of heroin and cocaine are high. There is mixed evidence that the subjective effects of cocaine are enhanced in individuals dependent on opioids, although it is clear that cocaine can alleviate the severity of symptoms of withdrawal from opioids. We also reviewed preclinical studies investigating possible neurobiological interactions between opioids and cocaine, but the results of these studies have been difficult to interpret mainly because the neurochemical mechanisms mediating the motivational effects of cocaine are modified by dependence on, and withdrawal from, opioid drugs. Our analysis encourages further systematic investigation of cocaine use patterns among opioid-dependent individuals and in laboratory animals. Once clearly identified, pharmacological and neuroanatomical methods can be employed in self-administering laboratory animals to uncover the neurobiological correlates of specific patterns of co-use.

Psychostimulant-induced behavioral sensitization depends on nicotinic receptor activation

Animal studies have shown that nicotine and psychostimulant drugs (amphetamine and cocaine) share the property of inducing long-lasting behavioral and neurochemical sensitization, which is thought to contribute to their addictive properties. Neuroplasticity subserving learning and memory mechanisms is considered to be involved in psychostimulant-induced sensitization and addiction behavior. Because nicotinic receptors in the brain play a role in the storage of drug-related information underlying reinforcement learning, we evaluated the possibility that activation of central nicotinic receptors may underlie psychostimulant-induced sensitization. Repeated exposure of rats to nicotine profoundly enhanced the psychomotor effects of nicotine and amphetamine 3 weeks after nicotine pretreatment. Moreover, the nicotinic receptor antagonist mecamylamine completely blocked the induction, but not the long-term expression, of behavioral sensitization to amphetamine in amphetamine-pretreated rats. Mecamylamine also prevented the development of cocaine-induced behavioral sensitization. Behavioral sensitization induced by nicotine, amphetamine, or cocaine was associated with an increase in the electrically evoked release of [(3)H]dopamine from nucleus accumbens slices. Coadministration of mecamylamine during pretreatment with nicotine, amphetamine, or cocaine prevented the development of this long-term hyperreactivity of nucleus accumbens dopamine neurons. Similarly, the high-affinity non-alpha7 subtype nicotinic receptor antagonist dihydro-beta-erythroidine prevented the development of amphetamine-induced behavioral and neurochemical sensitization. These data indicate that nicotinic receptor activation (by endogenously released acetylcholine) is a common denominator initiating neuroplasticity involved in the development of amphetamine, as well as cocaine-induced sensitization.

Long-term lithium administration abolishes the resistance to stress in rats sensitized to morphine

Morphine sensitized rats appear protected from the sequelae of an unavoidable stress: when exposed to stress (after a 7-day morphine wash-out) and then tested for escape, they perform like naive animals. This protection appears similar to that induced by chronic imipramine treatment, as it is antagonized by the inhibition of D(1)-dopamine receptors before exposure to unavoidable stress. Repeated unavoidable stress induces in rats a condition characterized by hyporeactivity to noxious stimuli and reverted by long-term antidepressant treatments, and this state is regarded as an experimental model of depression. The resistance to stress in morphine sensitized rats could be considered as the behavioral counterpart of the sensitivity to stress in control rats, i.e. as a model of mania. The aim of the present study was to validate such a putative model by studying whether the resistance to stress induced by morphine sensitization would respond to a long-term administration of lithium, the reference antimanic drug. Long-term lithium treatment induces in rats a condition of hyporeactivity to noxious stimuli, accompanied by decreased levels of dopamine in the nucleus accumbens shell. In morphine sensitized rats chronic lithium abolished the resistance to stress, but it did not modify the D(1)-dopamine receptor mediated response to morphine, nor did it modify the levels of extraneuronal dopamine in the nucleus accumbens shell. Thus, lithium treatment abolished the resistance to stress in morphine sensitized rats, conferring predictive validity to the paradigm. Moreover, it did so through a mechanism which appeared to be independent of D(1)-dopamine receptor activity.

Relapse to drug and alcohol use: a matter of sensitization

Repeated exposure of rats to cocaine, amphetamine, opiates, nicotine and alcohol causes a very long-lasting (months) increase in the behavioral effects of these addictive drugs and drug-associated environmental stimuli (sensitization). This hypersensitivity is associated with persistent changes in the reactivity of neurons of the motivational (mesocorticolim-bic) system in the brain. Using an animal model for relapse, recent studies in our laboratory show that relapse to drug-seeking behavior (following extinction of intravenous cocaine or heroin self-administration) depends on the occurrence of sensitization. Accordingly, sensitization and conditioning seem to be more important for the persistence of drug and alcohol addiction then the occurrence of withdrawal phenomena. Biochemical research on the molecular and cellular basis of behavioral sensitization and behavioral studies on readjustment of stimulus responsiveness in rats, is of great importance for the development of an adequate pharmacotherapy of addiction.

Role of endogenous adenosine in the expression of opiate withdrawal in rats

Samples of extracellular fluid from striatum and nucleus accumbens of anaesthetised rats undergoing opiate withdrawal were collected using microdialysis and then analysed for adenosine and its metabolites using high performance liquid chromatography (HPLC) and ultraviolet (UV) detection. Although the amount of adenosine present in the dialysate from either brain region was below the limit of detection by 90 min after probe placement, the metabolites could still be detected. Samples of dialysates collected from the nucleus accumbens contained significantly higher concentrations of hypoxanthine and inosine following naloxone challenge. The data are compatible with the hypothesis that endogenous adenosine might be involved in the expression of the opiate abstinence syndrome.

Presynaptic dopaminergic function in the nucleus accumbens following chronic opiate treatment and precipitated withdrawal

Naloxone treatment at three days following implantation of pellets containing morphine base increased uptake of tritiated dopamine by the nucleus accumbens but did not alter efflux of tritiated dopamine by the nucleus accumbens or tritiated norepinephrine by the hippocampus. At six days following placement of pellets containing morphine base, withdrawal score was increased after treatment with either saline or naloxone, indicating that animals were undergoing spontaneous opiate withdrawal. Fractional efflux of tritiated dopamine was decreased at this time point following intermittent stimulation with 317 and 1000 microM 4-aminopyridine, for striatal slices obtained from animals pretreated with either saline or naloxone. For the nucleus accumbens at six days after placement of morphine pellets, similar decreases in the efflux of tritiated dopamine were only observed in slices obtained from naloxone treated animals. Fractional dopamine efflux was also diminished after in vitro exposure to rising concentrations of 4-aminopyridine, amphetamine, or cocaine for tissue obtained from the nucleus accumbens, but not for slices from the striatum at six days following morphine pellet implantation. In conclusion, deficits in dopamine efflux by the nucleus accumbens occur at a time when animals are undergoing spontaneous opiate withdrawal at six days following morphine pellet implantation, but do not occur at an earlier time point when withdrawal is precipitated by naloxone treatment. These deficits are apparent for brain slices obtained from the striatum or nucleus accumbens after exposure to rising concentrations of different in vitro treatments, with tissue obtained from the nucleus accumbens being more sensitive than the striatum to dopamine efflux produced by a wider range of treatments.

Drug-induced reinstatement of heroin- and cocaine-seeking behaviour following long-term extinction is associated with expression of behavioural sensitization

The present study was designed to evaluate the relationship between reinstatement of drug-seeking behaviour following long-term extinction of intravenous (i.v.) drug self-administration (an animal model for craving) and long-term behavioural sensitization. Rats were allowed to self-administer heroin (50 microg/kg per inj., 14 daily sessions), cocaine (500 microg/kg per inj., 10 daily sessions) or saline. Following a 3-week extinction period, reinstatement tests were performed to evaluate priming effects of amphetamine, cocaine and heroin on nonreinforced drug-seeking behaviour. In addition, the occurrence of long-term behavioural sensitization in rats with a history of heroin or cocaine self-administration was determined. Heroin-seeking behaviour was reinstated by heroin (0.25 mg/kg), amphetamine (1.0 mg/kg) and cocaine (10 mg/kg). In addition, animals with a history of heroin self-administration displayed locomotor sensitization to both heroin and amphetamine. Cocaine-seeking behaviour was reinstated by cocaine and amphetamine, but not by heroin. Interestingly, locomotor sensitization to amphetamine, but not heroin, was observed in animals with a history of cocaine self-administration. In other words, the induction of drug-seeking behaviour following a prolonged drug-free period was found to be associated with the expression of long-term behavioural sensitization. These data provide experimental evidence for a role of behavioural sensitization in the incentive motivation underlying drug-seeking behaviour. If drug hyperresponsiveness would indeed be a crucial factor in drug-induced craving in human addicts, pharmacological readjustment of the neuroadaptations underlying drug sensitization may prevent relapse to drug use long after detoxification.

Selegiline prevents long-term changes in dopamine efflux and stress immobility during the second and third weeks of abstinence following opiate withdrawal

Selegiline is an irreversible inhibitor of monoamine oxidase B with trophic and neuroprotective effects. Because of evidence for decreased dopaminergic function during the withdrawal syndromes associated with opiates and other medications with potential for abuse, we investigated effects of treatment with selegiline on in vitro measures of dopamine efflux following opiate withdrawal. Treatment with 2.0 mg/kg/day of selegiline did not modify the severity of opiate withdrawal, as assessed by weight loss over the first 3 days of abstinence. Opiate withdrawal increased immobility in response to a forced warm water swim test performed during the second and third weeks of abstinence following the onset of withdrawal. Brain slices obtained from the nucleus accumbens of opiate-withdrawn animals immediately following swim stress testing displayed diminished efflux of tritiated dopamine after two in vitro exposures to cocaine or amphetamine. Cocaine increases neurotransmitter efflux through blockade of dopamine reuptake, while amphetamine augments efflux by stimulating release of dopamine from intracellular storage vesicles. Although slices from opiate withdrawal subjects showed decreases in efflux after in vitro treatment with these agents, no differences were observed after exposure to 4-aminopyridine, which increases neurotransmitter release by prolonging action potential duration. These findings indicate mechanisms of action that are specific for catecholamine neurotransmitter systems are important for demonstrating long-term changes in dopaminergic function following opiate withdrawal. Selegiline prevented decreases in the efflux of tritiated dopamine in slices obtained from opiate-withdrawn subjects. In addition, selegiline decreased withdrawal-induced immobility during warm water swim testing. In conclusion, treatment with selegiline can prevent long-term changes in stress-induced immobility and deficits in presynaptic dopaminergic function that occur following the opiate withdrawal syndrome.

Different effects of opiate withdrawal on dopamine turnover, uptake, and release in the striatum and nucleus accumbens

The purpose of these experiments was to further characterize changes in dopaminergic function that follow withdrawal from chronic opiate treatment. Withdrawal after treatment to a maximum dose of 120 mg/kg of morphine did not alter dopamine concentrations in the substantia nigra, ventral tegmental area, striatum, or nucleus accumbens; but did decrease concentrations of DOPAC and the ratio of DOPAC to dopamine in the lateral striatum and nucleus accumbens. Uptake of tritiated dopamine was diminished for withdrawn slices obtained from the striatum with no effect observed for tissue from the nucleus accumbens. Deficits of in vitro release of tritiated dopamine also occurred following withdrawal, with the nucleus accumbens being sensitive to dependence produced by a lower dose of morphine. In conclusion, opiate withdrawal produces a complex pattern of effects on dopaminergic function that is specific for the striatum and nucleus accumbens.

Involvement of cAMP-dependent protein kinase in the nucleus accumbens in cocaine self-administration and relapse of cocaine-seeking behavior

cAMP-dependent protein kinase (PKA) in the nucleus accumbens (NAc) has been implicated in cocaine addiction because (1) cocaine reinforcement is mediated by dopamine receptors that modulate cAMP formation, and (2) repeated exposure to cocaine upregulates the cAMP system in NAc neurons. This study tested PKA involvement in cocaine self-administration and relapse of cocaine-seeking behavior by infusing cAMP analogs that activate or inhibit PKA into the NAc of rats. Bilateral intra-NAc infusions of the PKA inhibitor Rp-cAMPS reduced baseline cocaine self-administration, shifted the dose-response curve for cocaine self-administration to the left, and induced relapse of cocaine-seeking behavior after extinction from cocaine self-administration, consistent with an enhancement of cocaine effects in each paradigm. In contrast, pretreatment with intra-NAc infusions of a PKA activator, Sp-cAMPS or dibutyryl cAMP, increased baseline cocaine self-administration during the second hour of testing and shifted the dose-response curve to the right, consistent with an antagonist-like action. After extinction from cocaine self-administration, similar infusions of Sp-cAMPS induced generalized responding at both drug-paired and inactive levers. As an index of PKA activity in vivo, NAc infusions of Rp-cAMPS reduced basal levels of dopamine-regulated phosphoprotein-32 phosphorylation and blocked amphetamine-induced increases in cAMP response element-binding protein (CREB) phosphorylation. Conversely, NAc infusions of Sp-cAMPS increased phosphorylation of CREB. Together, these results suggest that sustained upregulation of the cAMP system in the NAc after repeated cocaine exposure could underlie tolerance to cocaine reinforcement, whereas acute inhibition of this system may contribute to drug craving and relapse in addicted subjects.

Does dizocilpine (MK-801) inhibit the development of morphine-induced behavioural sensitization in rats?

Intermittent morphine pretreatment (10 mg/kg/day for 14 days) induced long-lasting (one month post-treatment) sensitization to the locomotor effects of morphine and amphetamine in rats. Co-administration of the non-competitive NMDA-receptor antagonist dizocilpine (MK-801) (0.1 mg/kg) with morphine did not prevent the development of long-term behavioural sensitization. However, this dose of MK-801 did cause long-term sensitization to its own locomotor effects. Co-administration of 0.25 mg/kg MK-801 with morphine caused death in 60% of the animals. In the animals that survived MK-801 plus morphine pretreatment, neither short-term (3 days) nor long-term morphine-induced sensitization was observed. MK-801 alone (0.25 mg/kg/day for 14 days) induced short-term cross-sensitization to morphine. Thus, the development of long-term morphine-induced locomotor sensitization could only be prevented by a dose of MK-801 that yields a lethal combination with morphine. In addition, MK-801 induced sensitization to its own locomotor effects and cross-sensitization to morphine. These findings seriously question whether MK-801 can be used to study the development of morphine-induced behavioural sensitization.

Morphine-6-glucuronide-induced locomotor stimulation in mice: role of opioid receptors

Morphine-6beta-glucuronide is a major metabolite of morphine with potent analgesic actions. To explore the importance of this opiate when administered as a drug by its own or in morphine action, we studied the locomotor activity response to morphine and morphine-6-glucuronide in drug-naive C57 BL/6JBom mice. The effects of administration of the two opiates on a battery of 7 different locomotor activities were studied and compared to saline controls. A dose of 20 micromol/kg morphine-6-glucuronide resulted in more locomotion than the same dose of morphine, while at higher doses (up to 120 micromol/kg), similar increases for most locomotor behaviours were recorded for both drugs. Pretreatment with naltrindole indicated that the delta-receptors play an equivalent but minor role in mediating both morphine-6-glucuronide and morphine hyperlocomotion. Administration of high naltrexone doses (10 mg/kg) completely abolished the locomotor stimulation induced by both opiates. However, at intermediate naltrexone doses of 0.25 and 0.5 mg/kg, morphine-induced behaviours was completely inhibited while morphine-6-glucuronide induced behaviours demonstrated partial resistance to naltrexone inhibition. The mu1-specific receptor antagonist naloxonazine caused 75% reduction of morphine induced behaviours and only 50% inhibition of morphine-6-glucuronide induced behaviors. Taken together our observations indicated general similarity but also marked differences between morphine and morphine-6-glucuronide with respect to opiate receptors mediating the locomotor stimulatory effect.

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.

Opiate modulation of striatal dopamine and hippocampal norepinephrine release following morphine withdrawal

When opiates are abruptly withdrawn after chronic treatment, increases in hippocampal noradrenergic function are observed which are accompanied by decreases in striatal dopamine release. The latter effects have to shown to persist for several weeks following the onset of opiate withdrawal. We examined the long-term effects of opiate withdrawal on 4-aminopyridine and potassium stimulated release of striatal dopamine and hippocampal norepinephrine. Tissue samples were obtained either from rats that had been exposed to opiate withdrawal following a seven day morphine infusion or sham treated control subjects. At 48 hours after the onset of withdrawal (cessation of morphine infusions), slices were loaded with [3H] neurotransmitter, washed extensively, and exposed to different drug treatments. 4-aminopyridine induced concentration related increases in striatal dopamine release, which was 36% calcium independent. Similar values for fractional release of striatal dopamine were obtained in morphine withdrawn and control subjects, for both potassium and 4-aminopyridine induced release. In addition, thresholds for 4-aminopyridine or potassium induced release of striatal dopamine did not differ between control and morphine withdrawn subjects. Treatment with 1.0 microM morphine sulfate potentiated potassium evoked release of norepinephrine to an equal extent in both morphine withdrawn and sham treated hippocampal tissue. Exposure to a threshold concentration of potassium (8.0 mM), stimulated increased release of hippocampal norepinephrine in a significantly greater fraction of tissue samples obtained from morphine withdrawn animals. Although these results do not support changes in striatal dopamine release following opiate withdrawal, opiate mechanisms appear to be important determinants of in vitro hippocampal norepinephrine release.

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

Protracted changes in basal "steady-state" opioid peptide gene expression in the brain may represent adaptations underlying the behavioral effects of drugs of abuse, observed long after drug exposure. Here, we have studied the long-term effects of two distinct regimens of morphine administration ("intermittent" vs "chronic" morphine treatment) on behavioral sensitization and "steady-state" striatal preprodynorphin and preproenkephalin gene expression in rats. Opioid peptide gene expression was investigated using in situ hybridization at three rostrocaudal levels (rostral, intermediate and caudal) of the caudate-putamen and the nucleus accumbens. Behavioral studies showed that the intermittent morphine treatment resulted in a significantly greater enhancement of morphine-induced locomotion than the chronic morphine treatment three weeks after cessation of opiate exposure. The intermittent morphine treatment resulted in an initial decrease of preprodynorphin gene expression of about 5-10% in the caudate-putamen and the nucleus accumbens at the rostral and intermediate levels one day after the last morphine administration. In contrast, a protracted increase of preprodynorphin gene expression of about 20% throughout the caudate-putamen and of about 6% in intermediate sections of the nucleus accumbens was observed 21 days after cessation of intermittent morphine treatment. Although the chronic morphine treatment induced a decrease of preprodynorphin messenger RNA levels one day after the last administration, no significant changes were observed three weeks after cessation of chronic morphine treatment. No long-term changes were observed in preproenkephalin gene expression after either morphine treatment. Since the intermittent morphine administration induced long-term behavioral sensitization much more effectively than the chronic morphine treatment, we tentatively suggest that the protracted increase of preprodynorphin gene expression may play a facilitative role in the long-term character of opiate-induced behavioral sensitization.