Dopamine receptor antagonists prevent expression, but not development, of morphine sensitization

Heteromerization of dopaminergic receptors in the brain: Pharmacological implications

Dopamine exerts its physiological effects through two subtypes of receptors, i.e. the receptors of the D₁ family (D_1R and D_5R) and the D₂ family (D_2R, D_3R, and D_4R), which differ in their pattern of distribution, affinity, and signaling. The D₁-like subfamily (D_1R and D_5R) are coupled to Gα_s/olf proteins to activate adenylyl cyclase whereas the D₂-like receptors are coupled to Gα_i/o subunits and suppress the activity of adenylyl cyclase. Dopamine receptors are capable of forming homodimers, heterodimers, and higher-order oligomeric complexes, resulting in a change in the individual protomers' recognition, signaling, and pharmacology. Heteromerization has the potential to modify the canonical pharmacological features of individual monomeric units such as ligand affinity, activation, signaling, and cellular trafficking through allosteric interactions, reviving the field and introducing a new pharmacological target. Since heteromers are expressed and formed in a tissue-specific manner, they could provide the framework to design selective and effective drug candidates, such as brain-penetrant heterobivalent drugs and interfering peptides, with limited side effects. Therefore, heteromerization could be a promising area of pharmacology research, as it could contribute to the development of novel pharmacological interventions for dopamine dysregulated brain disorders such as addiction, schizophrenia, cognition, Parkinson's disease, and other motor-related disorders. This review is articulated based on the three criteria established by the International Union of Basic and Clinical Pharmacology for GPCR heterodimers (IUPHAR): evidence of co-localization and physical interactions in native or primary tissue, presence of a new physiological and functional property than the individual protomers, and loss of interaction and functional fingerprints upon heterodimer disruption.

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.

Heteromers of μ opioid and dopamine D1 receptors modulate opioid-induced locomotor sensitization in a dopamine-independent manner

BACKGROUND AND PURPOSE

Exposure to opiates induces locomotor sensitization in rodents, which has been proposed to correspond to the compulsive drug-seeking behaviour. Numerous studies have demonstrated that locomotor sensitization can occur in a dopamine transmission-independent manner; however, the underlying mechanisms are unclear.

EXPERIMENTAL APPROACH

Co-immunoprecipitation, BRET and cross-antagonism assays were used to demonstrate the existence of receptor heterodimers. Function of heterodimers was evaluated by behavioural studies of locomotor sensitization.

KEY RESULTS

The dopamine D₁ receptor antagonist SCH23390 antagonized the signalling initiated by stimulation of μ opioid receptors with agonists in transfected cells expressing two receptors and in striatal tissues from wild-type but not D₁ receptor knockout (KO) mice, suggesting that SCH23390 modified μ receptor function via receptor heteromers, as the ability of an antagonist of one of the receptors to inhibit signals originated by stimulation of the partner receptor was a characteristic of receptor heteromers. The existence of μ receptor-D₁ receptor heterodimers was further supported by biochemical and biophysical assays. In vivo, when dopamine release was absent (by destruction of the dopaminergic projection from the ventral tegmental area to the striatum), SCH23390 still significantly inhibited μ receptor agonist-induced behavioural responses in rats. Additionally, we demonstrated that D₁ or μ receptor KO mice and thus unable to form μ receptor-D₁ receptor heterodimers, failed to show locomotor sensitization to morphine.

CONCLUSION AND IMPLICATIONS

Our results suggest that μ receptor-D₁ receptor heterodimers may be involved in the dopamine-independent expression of locomotor sensitization to opiates.

Huperzine A inhibits immediate addictive behavior but not behavioral sensitization following repeated morphine administration in rats

Acetylcholinesterase inhibitors are regarded as promising therapeutic agents to treat addiction. The current study aimed to examine the effects of huperzine A, a cholinesterase inhibitor, on behavioral sensitization induced by repeated morphine administration and relapse induced by contextual conditioning. The present study also assessed whether the state-dependency hypothesis may explain the results. Adult rats were divided into four groups (n=8) and intraperitoneally injected with 0.2, 0.3 or 0.4 mg/kg huperzine A or saline (1 ml/kg, control), for 5 days. The effect of repeated huperzine A administration alone on locomotor activity was assessed. For the experiments that analyzed the development of morphine-induced sensitization, 40 rats were divided into five groups (n=8): Saline+Saline, Saline+Morphine, 0.2, 0.3 and 0.4 mg/kg huperzine A+Morphine. Following a withdrawal period of 7 days, all animals were administered saline or morphine, as appropriate. To test the state-dependency hypothesis, the rats in the Saline+Morphine group were injected with saline and morphine, while the other three groups were administered different doses of huperzine A and morphine. To examine the effect of huperzine A on the expression of morphine-induced sensitization, the rats in huperzine A+Morphine groups were injected with appropriate concentrations of huperzine A, and morphine. The current results indicated that the administration of huperzine A alone did not affect locomotor activity, while higher doses of huperzine A inhibited the addictive behavior induced by morphine at the development phase. Additionally, huperzine A administration during the expression phase of morphine sensitization did not inhibit the relapse induced by administration of saline. Furthermore, 0.4 mg/kg huperzine A inhibited the expression of morphine-induced behavioral sensitization. Therefore, the results of the current study do not support the state-dependency hypothesis.

Striatal dopamine D1 and D2 receptors are differentially regulated following buprenorphine or methadone treatment

RATIONALE

Chronic administration of morphine induces adaptations in neurotransmission system such as the dopamine pathway, and these modifications could be influenced by the drug administration pattern. Methadone and buprenorphine are the two main opioid substitution therapies, and despite their protracted use in humans, no study has investigated their ability to regulate dopamine system after chronic exposure/withdrawal.

OBJECTIVES

We evaluated the consequences of two administration patterns of methadone and buprenorphine on striatal dopamine D1 (D1R) and D2 (D2R) receptor levels.

METHODS

Mice were treated with escalating doses of methadone or buprenorphine for 5 days either once daily (binge) or three times a day (TTD). D1R and D2R density in striatum was measured by autoradiography using [(3)H]-SCH23390 and [(3)H]-raclopride, respectively, at 1 (WD1), 14 (WD14), and 35 (WD35) days after the last opioid injection.

RESULTS

A downregulation of D1R was observed upon TTD administration of buprenorphine and binge methadone treatment while an increase of those receptor levels was detected both with binge buprenorphine and TTD methadone treatments. Concerning the D2R, we rather measured an early or late downregulation with both agonists and administration patterns.

CONCLUSIONS

Our results demonstrated that methadone and buprenorphine were able to differentially regulate dopamine receptor density depending on the withdrawal period and the administration pattern.

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.

Nucleus accumbens dopamine D₁ receptors regulate the expression of ethanol-induced behavioural sensitization

Repeated ethanol administration may induce behavioural sensitization, defined as a progressive potentiation of locomotor stimulant effects. This process is associated with neuroadaptations in the mesolimbic pathway and the nucleus accumbens. The aim of the present study was to analyse dopamine D₁ receptor (D₁R) participation in locomotor response to an agonist and an antagonist of the D₁R in mice with different levels of sensitization to ethanol. In three separate experiments, mice received administrations of 2.2 g/kg ethanol or saline every other day for 10 d. According to their locomotor response on the last day, ethanol-treated animals were classified into two groups: sensitized or non-sensitized. After the treatment, mice were challenged with 4 or 8 mg/kg SKF-38393 (i.p.), a D₁R agonist (expt 1); or with 0.01 or 0.1 mg/kg SCH-23390 (i.p.), a D₁R antagonist, followed by 2.2 g/kg ethanol (i.p.) administration (expt 2). In expt 3, mice were challenged with intra-accumbens (intra-NAc) SKF-38393 (1 μg/side, in 0.2 μl), and with intra-NAc SCH-23390 (3 μg/side, in 0.2 μl) followed by 2.2 g/kg ethanol (i.p.). Although the i.p. administration of SKF-38393 did not affect the locomotion of mice, the intra-NAc administration of SKF-38393 significantly increased the locomotor activity in sensitized mice, suggesting that sensitized mice present functionally hyperresponsive D₁Rs in the NAc. Both i.p. and intra-NAc administration of SCH-23390 blocked the expression of ethanol sensitization, suggesting that the activation of NAc D₁Rs seems to be essential for the expression of ethanol sensitization.

Dissociable roles of mGlu5 and dopamine receptors in the rewarding and sensitizing properties of morphine and cocaine

RATIONALE

Drugs of abuse are initially used because of their rewarding properties. As a result of repeated drug exposure, sensitization to certain behavioral effects of drugs occurs, which may facilitate the development of addiction. Recent studies have implicated the metabotropic glutamate receptor 5 (mGlu5 receptor) in drug reward, but its role in sensitization is unclear. Stimulation of dopamine receptors plays an important role in drug reward, but not in the sensitizing properties of cocaine and morphine.

OBJECTIVE

This study aims to evaluate the role of mGlu5 and dopamine receptors in the development of cocaine- and morphine-induced conditioned place preference (CPP) and psychomotor sensitization.

MATERIALS AND METHODS

Rats were treated with the mGlu5 receptor antagonist MTEP (0, 1, 3, and 10 mg/kg, i.p.) or the dopamine receptor antagonist α-flupenthixol (0, 0.125, 0.25, and 0.5 mg/kg, i.p.) during place conditioning with either morphine (3 mg/kg, s.c.) or cocaine (15 mg/kg, i.p.). Furthermore, MTEP (1 mg/kg, i.p.) or α-flupenthixol (0.5 mg/kg, i.p.) was co-administered during cocaine (30 mg/kg, i.p.) or morphine (3.0 mg/kg, s.c.) pretreatment and psychomotor sensitization was tested 3 weeks post-treatment.

RESULTS

MTEP attenuated the development of morphine- but not cocaine-induced CPP. In contrast, MTEP suppressed the development of cocaine- but not morphine-induced psychomotor sensitization. α-Flupenthixol blocked the development of both cocaine- and morphine-induced CPP but did not affect the development of sensitization to either drug.

CONCLUSION

Dopamine receptor stimulation mediates cocaine and morphine reward but not sensitization. In contrast, the role of mGlu5 receptors in reward and sensitization is drug-specific.

Repeated administrations of dopamine receptor agents affect lithium-induced state-dependent learning in mice

The influence of repeated administration of dopamine receptor agents on the effect of lithium on lithium-induced state-dependent learning was examined in mice. Immediate post-training intraperitoneal (i.p.) administrations of lithium (10 and 20 m/kg) decreased the step-down latency of a single-trial inhibitory avoidance task. This was fully or partly reversed by pre-test administration of the same doses of the drug, with maximum response at the dose of 10 mg/kg, suggesting state-dependent learning was induced by lithium. Here, it has also been shown that repeated intracerebroventricular administrations of a mixed D1/D2 dopamine receptors agonist apomorphine (once daily injections of 0.5 microg/mouse for three consecutive days followed by five days of no drug treatment) increased the effect of lower doses of pre-test lithium (1.25, 2.5 and 5 mg/kg, i.p.) on the reinstatement of the step-down latency decreased by post-training lithium (10 mg/kg). On the contrary, not only repeated administrations of the dopamine D1 receptor antagonist SCH 23390 (0.5 and 1 microg/mouse) but also the dopamine D2 receptor antagonist sulpiride (0.3 and 1 microg/mouse) disrupted the state-dependent learning induced by lithium. These results suggest that state-dependent learning induced by lithium may be altered by repeated pretreatment of dopamine receptor agents.

A differential role for the adenosine A2A receptor in opiate reinforcement vs opiate-seeking behavior

The adenosine A(2A) receptor is specifically enriched in the medium spiny neurons that make up the 'indirect' output pathway from the ventral striatum, a structure known to have a crucial, integrative role in processes such as reward, motivation, and drug-seeking behavior. In the present study we investigated the impact of adenosine A(2A) receptor deletion on behavioral responses to morphine in a number of reward-related paradigms. The acute, rewarding effects of morphine were evaluated using the conditioned place preference paradigm. Operant self-administration of morphine on both fixed and progressive ratio schedules as well as cue-induced drug-seeking was assessed. In addition, the acute locomotor response to morphine as well as sensitization to morphine was evaluated. Decreased morphine self-administration and breakpoint in A(2A) knockout mice was observed. These data support a decrease in motivation to consume the drug, perhaps reflecting diminished rewarding effects of morphine in A(2A) knockout mice. In support of this finding, a place preference to morphine was not observed in A(2A) knockout mice but was present in wild-type mice. In contrast, robust cue-induced morphine-seeking behavior was exhibited by both A(2A) knockout and wild-type mice after a period of withdrawal. The acute locomotor response to morphine in the A(2A) knockout was similar to wild-type mice, yet A(2A) knockout mice did not display tolerance to chronic morphine under the present paradigm. Both genotypes display locomotor sensitization to morphine, implying a lack of a role for the A(2A) receptor in the drug-induced plasticity necessary for the development or expression of sensitization. Collectively, these data suggest a differential role for adenosine A(2A) receptors in opiate reinforcement compared to opiate-seeking.

Delayed, context- and dopamine D1 receptor-dependent activation of ERK in morphine-sensitized mice

Exposure to cues previously associated with intake of substances of abuse can promote drug related responses. In this study, we have examined the effect of exposure to a drug-associated context on the expression of morphine psychomotor sensitization. We show that sensitization is markedly increased in mice examined 4 weeks after the last morphine injection. In addition, this incubation period confers to the environment paired with morphine the ability to increase ERK phosphorylation in the shell (but not the core) of the nucleus accumbens. Using transgenic mice with enhanced green fluorescent protein (EGFP) expression under the control of the dopamine D1 receptor (D1R) (Drd1a-EGFP) or D2 receptor promoter (Drd2-EGFP) we show that context-dependent ERK phosphorylation is restricted to D1R-expressing medium spiny neurons. Furthermore, this effect depends on D1R activation. These data show that, following repeated morphine injections, a drug-free period induces context-dependent phosphorylation of ERK in a specific population of neurons within the nucleus accumbens shell. This activation is associated to enhanced psychomotor sensitization and may be implicated in context-elicited drug seeking induced by repeated exposure to drugs of abuse.

Drugs of abuse specifically sensitize noradrenergic and serotonergic neurons via a non-dopaminergic mechanism

A challenge in drug dependence is to delineate long-term neurochemical modifications induced by drugs of abuse. Repeated d-amphetamine was recently shown to disrupt a mutual regulatory link between noradrenergic and serotonergic neurons, thus inducing long-term increased responses to d-amphetamine and para-chloroamphetamine, respectively. We show here that such a sensitization of noradrenergic and serotonergic neurons also occurs following repeated treatment with cocaine, morphine, or alcohol, three compounds belonging to main groups of addictive substances. In all cases, this sensitization is prevented by alpha 1b-adrenergic and 5-HT2A receptors blockade, indicating the critical role of these receptors on long-term effects of drugs of abuse. However, repeated treatments with two non-addictive antidepressants, venlafaxine, and clorimipramine, which nevertheless inhibit noradrenergic and serotonergic reuptake, do not induce noradrenergic and serotonergic neurons sensitization. Similarly, this sensitization does not occur following repeated treatments with a specific inhibitor of dopamine (DA) reuptake, GBR12783. Moreover, we show that the effects of SCH23390, a D1 receptor antagonist known to inhibit development of d-amphetamine behavioral sensitization, are due to its 5-HT2C receptor agonist property. SCH23390 blocks amphetamine-induced release of norepinephrine and RS102221, a 5-HT2C antagonist, can reverse this inhibition as well as inhibition of noradrenergic sensitization and development of behavioral sensitization induced by repeated d-amphetamine. We propose that noradrenergic/serotonergic uncoupling is a common neurochemical consequence of repeated consumption of drugs of abuse, unrelated with DA release. Our data also suggest that compounds able to restore the link between noradrenergic and serotonergic modulatory systems could represent important therapeutic targets for investigation.

Morphine-induced behavioral sensitization increased the mRNA expression of NMDA receptor subunits in the rat amygdala

This study was designed to evaluate the effect of repeated morphine treatment on rat behavioral responses. In the genetic section, the mRNA expression of NMDA receptor subunits (NR1 and NR2A) was measured in certain areas of the male rat brain (striatum, prefrontal cortex, hippocampus, hypothalamus and amygdala). In the behavioral section, the effect of repeated morphine treatment on animal models such as locomotion, oral stereotypy, and state-dependent memory in a passive avoidance test was evaluated in the presence or absence of MK801 (NMDA receptor antagonist). Our results showed that chronic morphine treatment, followed by a 7-day (but not 24-hour) washout period, potentiated the effect of test doses of morphine, which is referred to as behavioral sensitization. Meanwhile, pretreatment of animals with MK801 (0.1 and 0.25 mg/kg), 30 min before a test dose of morphine (5 mg/kg), failed to attenuate the locomotion and oral stereotypy in the behavioral sensitization state. Interestingly, a higher dose of MK801 (0.25 mg/kg) decreased memory retrieval induced by morphine (2.5 mg/kg) in state-dependent memory. This effect may be due to the intrinsic motor enhancer property of higher doses of MK801, rather than the blockade of NMDA receptors. It can be concluded that MK801 does not affect morphine-induced behavioral sensitization in the expression phase. In the genetic section of the study, results of quantitative real-time RT-PCR clearly indicated that morphine sensitization increased the expression of NMDA receptor subunits mRNA in the amygdala (NR1 by 104% and NR2A by 85%), while the other areas of the brain were unaffected. Maenwhile, no change in the mRNA levels was observed in non-sensitized animals (chronic morphine treatment followed by a 24-hour washout period). In summary, the present study indicates that repeated morphine treatment followed by long-term (7-day washout) induces behavioral sensitization and causes a delayed increase in mRNA levels of NMDA receptor subunits in the rat amygdala. Meanwhile, it has previously been reported that the amygdala is involved in behavioral sensitization. Thus, it can be concluded that the increase in NMDA receptor expression is associated with morphine-induced behavioral sensitization.

Uncoupling between noradrenergic and serotonergic neurons as a molecular basis of stable changes in behavior induced by repeated drugs of abuse

A challenge in drug dependence is to delineate long-term behavioral and neurochemical modifications induced by drugs of abuse. In rodents, drugs of abuse induce locomotor hyperactivity, and repeating injections enhance this response. This effect, called behavioral sensitization, persists many months after the last administration, thus mimicking long-term sensitivity to drugs observed in human addicts. Although addictive properties of drugs of abuse are generally considered to be mediated by an increased release of dopamine in the ventral striatum, recent pharmacological and genetic experiments indicate an implication of alpha1b-adrenergic receptors in behavioral and rewarding responses to psychostimulants and opiates. Later on, it was shown that not only noradrenergic but also serotonergic systems, through 5-HT(2A) receptors, were controlling behavioral effects of drugs of abuse. More recently, experiments performed in animals knockout for alpha1b-adrenergic or 5-HT(2A) receptors indicated that noradrenergic and serotonergic neurons, besides their activating effects, inhibit each other by means of the stimulation of alpha1b-adrenergic and 5-HT(2A) receptors and that this mutual inhibition vanishes in wild type mice with repeated injections of psychostimulants, opiates or alcohol. Uncoupling induced by repeated treatments with drugs of abuse installs a stable sensitization of noradrenergic and serotonergic neurons, thus explaining an increased reactivity of dopaminergic neurons and behavioral sensitization. We propose that noradrenergic/serotonergic uncoupling is a common stable neurochemical consequence of repeated drugs of abuse which may also occur during chronic stressful situations and facilitate the onset of mental illness. Drug consumption would facilitate an artificial re-coupling of these neurons, thus bringing a temporary relief.

Activation of the cAMP/PKA/DARPP-32 signaling pathway is required for morphine psychomotor stimulation but not for morphine reward

Activation of the cAMP/PKA pathway in the dopaminoceptive neurons of the striatum has been proposed to mediate the actions of various classes of drugs of abuse. Here, we show that, in the mouse nucleus accumbens and dorsal striatum, acute administration of morphine resulted in an increase in the state of phosphorylation of the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) at Thr34, without affecting phosphorylation at Thr75. The ability of morphine to stimulate Thr34 phosphorylation was prevented by blockade of dopamine D1 receptors. DARPP-32 knockout mice and T34A DARPP-32 mutant mice displayed a lower hyperlocomotor response to a single injection of morphine than wild-type controls. In contrast, in T75A DARPP-32 mutant mice, morphine-induced psychomotor activation was indistinguishable from that of wild-type littermates. In spite of their reduced response to the acute hyperlocomotor effect of morphine, DARPP-32 knockout mice and T34A DARPP-32 mutant mice were able to develop behavioral sensitization to morphine comparable to that of wild-type controls and to display morphine conditioned place preference. These results demonstrate that dopamine D1 receptor-mediated activation of the cAMP/DARPP-32 cascade in striatal medium spiny neurons is involved in the psychomotor action, but not in the rewarding properties, of morphine.

Morphine-induced antinociception in the formalin test: sensitization and interactions with D1 and D2 dopamine receptors and nitric oxide agents

In this study, the effects of dopamine receptor antagonists and nitric oxide agents on morphine-induced sensitization in the formalin test in mice have been investigated. Repeated daily intraperitoneal administration of morphine (30 mg/kg for 3 days) followed by a 11-day wash out period increased morphine-induced antinociception in the formalin test, which may be due to sensitization. The antinociceptive response to higher doses of morphine (6 and 9 mg/kg) but not 3 mg/kg was significantly increased in sensitized animals compared with control groups. Pretreatment of animals with an opioid receptor antagonist, naloxone (4 mg/kg), during repeated administration of morphine, attenuated the morphine-induced sensitization. In the second part of the study, the animals received SCH23390 (D1 receptor antagonist), sulpiride (D2 receptor antagonist), L-Arg (nitric oxide precursor) and NG-nitro-L-Arg methylester (nitric oxide synthase inhibitor) during repeated morphine administration, to evaluate the role of dopamine receptor antagonists and nitric oxide agents in this phenomenon. Pretreatment of animals with NG-nitro-L-Arg methylester (20 mg/kg) and sulpiride (100 mg/kg) during morphine sensitization decreased the antinociceptive response to higher doses of morphine in the formalin test. It is concluded that D2 dopamine receptor and nitric oxide mechanisms may be involved at least partly in morphine-induced sensitization in the formalin test.

Extracellular glutamate and GABA in the ventral tegmental area of alcohol-preferring AA and alcohol-avoiding ANA rats treated repeatedly with morphine

Glutamate and gamma-amino-butyric acid (GABA) have been implicated in neuronal plasticity related to behavioral sensitization. In the present study, we examined morphine-induced changes in the extracellular concentrations of glutamate and GABA in the ventral tegmental area in alcohol-preferring Alko Alcohol (AA) and alcohol-avoiding Alko Non-Alcohol (ANA) rats that have previously been shown to differ in morphine-induced sensitization. The rats were given escalating doses (5-20 mg/kg) of morphine every other day for five days. This treatment produced behavioral sensitization to locomotor effects of morphine in AA, but not in ANA rats, when challenged with an additional injection of morphine (10 mg/kg) 10 days later. Morphine also increased the levels of glutamate in the ventral tegmental area only in AA rats, while no significant changes were found in the extracellular concentrations of GABA between the lines. Challenging the morphine-treated AA rats with ethanol (1.5 g/kg) did not modify the levels of glutamate or GABA. No changes in the concentrations of glutamate or GABA were seen in saline-treated AA and ANA rats after morphine challenge. These results render increased glutamate transmission in the ventral tegmental area a potential contributor to the higher susceptibility of AA rats to morphine-induced behavioral and neurochemical effects relative to ANA rats.

Comparison of the effects of mGluR1 and mGluR5 antagonists on the expression of behavioral sensitization to the locomotor effect of morphine and the morphine withdrawal jumping in mice

The aim of the present study was to compare the influence of group I metabotropic glutamate receptor (mGluR) antagonists (mGluR1 and mGluR5) on the expression of sensitization to the locomotor effect of morphine. We also tested how these compounds affect the morphine withdrawal jumps in mice. In our study, the mGluR1 antagonist EMQMCM [3-ethyl-2-methyl-quinolin-6-yl-(4-methoxy-cyclohexyl)-methanone methanesulfonate] and the mGluR5 antagonist MTEP ([(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine) were used. Sensitization was induced by five intraperitoneal (i.p.) injections of morphine at the dose of 10 mg/kg, every 3 days. Morphine dependence was induced by subcutaneous (s.c.) implantation of pellets containing 37.5 mg of morphine base for three days. Our data indicate that pretreatment with EMQMCM (5, 10, 20 mg/kg) and MTEP (5, 10 mg/kg) on the challenge day, inhibited the expression of sensitization to the locomotor effect of morphine in mice. Antagonists of both subtypes of the group I mGlurs given alone, did not modify the acute locomotor effect of morphine. On the other hand, EMQMCM did not attenuate the morphine withdrawal jumps precipitated by naloxone (4 mg/kg). The results suggest that both subtypes of the group I mGluRs (mGluR1 and mGluR5) take part in the expression of morphine sensitization processes but mGluR1 is not involved in the expression of morphine withdrawal jumps in mice. These findings may have implications for the treatment of opiate addiction in future.

GABAB receptor stimulation accentuates the locomotor effects of morphine in mice bred for extreme sensitivity to the stimulant effects of ethanol

Mice selectively bred for divergent sensitivity to the locomotor stimulant effects of ethanol (FAST and SLOW) also differ in their locomotor response to morphine. The GABA(B) receptor has been implicated in the mediation of locomotor stimulation to both ethanol and morphine, and a reduction in ethanol-induced stimulation has been found with the GABA(B) receptor agonist baclofen in FAST mice. We hypothesized that GABA(B) receptor activation would also attenuate the locomotor stimulant responses to morphine in these mice. In order to test this hypothesis, baclofen was administered to FAST-1 and FAST-2 mice 15 min prior to morphine, and activity was recorded for 30 min. Baclofen attenuated stimulation to 32 mg/kg morphine in FAST-1 mice, but only at a dose that also reduced saline activity. There was no stimulant response to 32 mg/kg morphine in FAST-2 mice, or to 16 mg/kg or 48 mg/kg morphine in FAST-1 mice, but the combination of baclofen with these morphine doses accentuated locomotor activity. Therefore, it appears that GABA(B) receptor activation is not a common mechanism for the locomotor stimulant responses to ethanol and morphine in FAST mice; however, these data suggest that GABA(B) receptor activation may instead enhance some of the behavioral effects of morphine.

Inhibitory effects of paeonol on morphine-induced locomotor sensitization and conditioned place preference in mice

The inhibitory effects of paeonol, a major compound of Paeoniae radix, on the development of locomotor sensitization, conditioned place preference (CPP) and dopamine receptor supersensitivity induced by the repeated administration of morphine were investigated through behavioral experiments. A single administration of morphine produces hyperlocomotion. Repeated administration of morphine develops sensitization (reverse tolerance), a progressive enhancement of locomotion, which is used as a model for studying the drug-induced drug-seeking behaviors, and CPP, which is used as a model for studying drug reinforcement. Paeonol inhibited morphine-induced hyperlocomotion, sensitization and CPP. In addition, paeonol inhibited the development of postsynaptic dopamine receptors supersensitivity, which may be an underlying common mechanism that mediates the morphine-induced dopaminergic behaviors such as sensitization and CPP. Apomorphine (a dopamine agonist)-induced climbing behaviors also were inhibited by a single direct administration of paeonol. These results provide evidence that paeonol exerts anti-dopaminergic activity, and it is suggested that paeonol may be useful for the prevention and therapy of these adverse actions of morphine.

Influence of morphine- or apomorphine-induced sensitization on histamine state-dependent learning in the step-down passive avoidance test

Effects of morphine- or apomorphine-induced sensitization on histamine state-dependent memory of passive avoidance task were examined in mice. Pre-training intracerebroventricular (i.c.v.) administration of histamine (20 microg/mouse) decreased the learning of a one-trial passive avoidance task. Pre-test administration of histamine (10 and 20 microg/mouse) reversed amnesia induced by pre-training of histamine, with maximum response at 20 microg/mouse. Pre-training histamine-induced amnesia was also reversed in morphine- or apomorphine-sensitized mice that had previously received once daily injections of morphine (20 and 30 mg/kg) or apomorphine (0.5 and 1 mg/kg) for 3 days. The reversion of histamine-induced amnesia in morphine-sensitized mice was decreased by once daily administration of naloxone (0.5 and 1 mg/kg), SCH 23390 (0.05 and 0.1 mg/kg) or sulpiride (25, 50 and 100 mg/kg) prior to injection of morphine (30 mg/kg/day, 3 days). Furthermore, once daily administration of sulpiride (50 and 100 mg/kg) but not SCH 23390 (0.01, 0.05 and 0.1 mg/kg) prior to apomorphine (1 mg/kg, for 3 days) decreased the reversion of pre-training histamine-induced amnesia by apomorphine. The results suggest that apomorphine or morphine sensitization affects the impairment of memory induced by histamine and thus it is postulated that opioid and dopamine receptors may play an important role in this effect.

Influence of Morphine and Dopamine Receptor Sensitization on Locomotor Activity in Mice

In the present study, the influence of morphine- and dopamine receptor antagonists-induced sensitization on morphine-induced locomotion in mice was investigated. Morphine (30, 40 and 50 mg/kg) increased, while lower doses of the opioid (10 and 20 mg/kg) decreased locomotor activity of mice. Subchronic repeated pretreatment of animals with morphine showed an increase in locomotion induced by the opioid. Clozapine reduced locomotor activity induced by morphine in both the naïve and subchronic morphine-treated animals. Subchronic pretreatment of clozapine also caused an increase in the locomotion induced by morphine. Sulpiride also decreased locomotion induced by morphine and its subchronic administration of the drug caused an increase in morphine- or apomorphine-induced locomotion. Co-administration of clozapine with sulpiride did not elicit potentiation in inhibiting the morphine effect. The D2 receptor mRNA expression was also increased by repeated morphine administration. It may be concluded that morphine-induced sensitization may be due to increase in D2 receptor mRNA expression. Sulpiride and clozapine may induce sensitization and also inhibit morphine-induced locomotion through their dopamine receptor blocking properties.

The influence of central administration of dopaminergic and cholinergic agents on morphine-induced amnesia in morphine-sensitized mice

In the present study, effects of intracerebroventricular (i.c.v.) injections of dopaminergic and cholinergic agents on morphine-induced amnesia in morphine-sensitized mice were investigated by using a one-trial passive avoidance task. Amnesia induced by pre-training morphine was significantly reversed in morphine-sensitized mice, which had previously received once daily injections of morphine (20 and 30 mg/kg, s.c.) for 3 days. Three daily injections of SKF 38393 (1, 2 and 4 g/mouse, i.c.v.) or SCH 23390 (0.25, 0.5, 0.75 and 1 g/mouse, i.c.v.) before morphine, and during morphine-sensitization, decreased and increased the amnesia induced by pre-training morphine respectively. Three daily injections of quinpirole (0.3, 1 and 3 g/mouse, i.c.v.) or sulpiride (0.03, 0.1, 0.3 and 1 g/mouse, i.c.v.) before morphine, also decreased and increased the amnesia induced by pre-training morphine respectively. Morphine-sensitized mice received similar injections of cholinergic agents. Three daily injections of physostigmine (1, 3 and 5 g/mouse, i.c.v.) or atropine (1, 4 and 7 g/mouse, i.c.v.) before morphine, and during morphine-sensitization, decreased and increased the amnesia induced by pre-training morphine respectively. Three daily injections of nicotine (0.75, 1 and 2 g/mouse, i.c.v.) or mecamylamine (1, 3 and 6 g/mouse, i.c.v.) before morphine, also decreased and increased the amnesia induced by pre-training morphine respectively. The results suggest that morphine sensitization affects the impairment of memory formation and thus it is postulated that central dopaminergic and cholinergic systems may play an important role in this effect.

Morphine State-Dependent Learning Sensitization and Interaction with Nitric Oxide

In the present study, the effects of nitric oxide (NO) precursor L-arginine and L-NAME, a potent inhibitor of NO synthase (NOS), on the expression of sensitization of morphine were investigated. Pre-training administration of morphine (5 mg/kg) impaired memory retrieval compared to pre-training saline-treated animals. Amnesia due to pre-training morphine (5 mg/kg) was restored by pre-test morphine (5 mg/kg). The retrieval impairment was also inhibited in mice which had received once-daily injections of morphine (20 and 30 mg/kg, s.c.) for 3 days, followed by 5 days of no drug treatment before training (in order to induce morphine sensitization). Administration of L-arginine (60 mg/kg/day - 3 days) or L-NAME (20 mg/kg/day - 3 days) before training did not alter morphine state dependency. During acquisition of sensitization, administration of L-arginine (60 mg/kg) 20 min before morphine (10 mg/kg/day, for 3 days) increased, while injection of L-NAME (20 mg/kg) 20 min before morphine (30 mg/kg/day, for 3 days) decreased morphine state dependency. It is concluded that NO is involved in the morphine-induced sensitization.

Sensitization to the rewarding effects of morphine depends on dopamine

The influence of dopamine (DA) on sensitization to the rewarding effects of morphine was evaluated. The effects of pre-treatment with saline or morphine plus naloxone, CGS 10746B, haloperidol, SCH 23390 and raclopride, on the place conditioning induced by 2 mg/kg morphine were evaluated. This dose was ineffective in saline pre-treated animals but induced a clear conditioned place preference in mice pre-treated with morphine, CGS 10746B or haloperidol. Conversely, animals pre-treated with morphine plus naloxone, CGS 10746B, SCH 23390, raclopride and the high dose of haloperidol did not acquire place preference. Our results demonstrated that DA release and subsequent DA D1 and D2 receptor activation is essential for the development of sensitization to the rewarding effects of morphine.

5-HT2A and α1b-adrenergic receptors entirely mediate dopamine release, locomotor response and behavioural sensitization to opiates and psychostimulants

Addictive properties of drugs of misuse are generally considered to be mediated by an increased release of dopamine (DA) in the ventral striatum. However, recent experiments indicated an implication of alpha1b-adrenergic receptors in behavioural responses to psychostimulants and opiates. We show now that DA release induced in the ventral striatum by morphine (20 mg/kg) is completely blocked by prazosin (1 mg/kg), an alpha1-adrenergic antagonist. However, morphine-induced increases in DA release in the ventral striatum were found to be similar in mice deleted for the alpha1b-adrenergic receptor (alpha1b-AR KO) and in wild-type (WT) mice, suggesting the presence of a compensatory mechanism. This acute morphine-evoked DA release was completely blocked in alpha1b-AR KO mice by SR46349B (1 mg/kg), a 5-HT2A antagonist. SR46349B also completely blocked, in alpha1b-AR KO mice, the locomotor response and the development of behavioural sensitization to morphine (20 mg/kg) and D-amphetamine (2 mg/kg). Accordingly, the concomitant blockade of 5-HT2A and alpha1b-adrenergic receptors in WT mice entirely blocked acute locomotor responses but also the development of behavioural sensitization to morphine, D-amphetamine or cocaine (10 mg/kg). We observed, nevertheless, that inhibitory effects of each antagonist on locomotor responses to morphine or D-amphetamine were more than additive (160%) in naïve WT mice but not in those sensitized to either drug. Because of these latter data and the possible compensation by 5-HT2A receptors for the genetic deletion of alpha1b-adrenergic receptors, we postulate the existence of a functional link between these receptors, which vanishes during the development of behavioural sensitization.

Effects of carbamazepine on morphine-induced behavioral sensitization in mice

Effects of carbamazepine on behavioral sensitization to morphine in mice has been investigated. Mice treated daily for 7 days with morphine (10 mg/kg) induced behavioral sensitization. Carbamazepine (10, 20, 40 mg/kg, i.p.) itself dose-dependently inhibited the locomotor activity of mice, but did not affect the acute morphine induced hyperactivity. Chronic treatment with carbamazepine had no effect on the development of morphine behavioral sensitization. Co-administration of carbamazepine 30 min prior to morphine had no significant effect on the development of behavioral sensitization. After the behavioral sensitization has been established, carbamazepine (10, 20, 40mg/kg, i.p.) did not affect the expression of morphine sensitization. However, carbamazepine (10, 20, 40mg/kg, i.p.) dose-dependently potentiated the transfer of morphine sensitization. The data of the present study implies that carbamazepine may influence the maintenance process of behavioral sensitization, which results in the enhancement of the transfer of behavioral sensitization. In clinic, the present results suggest that chronic use of carbamazepine might have abuse potential in opioid abusers.

Morphine state-dependent learning: sensitization and interactions with dopamine receptors

In the present study, the effects of morphine sensitization on impairment of memory formation and the state-dependent learning by morphine have been investigated in mice. Pretraining administration of morphine (0.5, 2.5 and 5 mg/kg) dose dependently decreased the learning of a one-trial passive avoidance task. Pretest administration of morphine (0.5, 2.5 and 5 mg/kg) induced state-dependent retrieval of the memory acquired under pretraining morphine influence. Pretraining or pretest administration of naloxone (0.25, 0.5 and 1 mg/kg) reversed both responses to morphine (5 mg/kg). Amnesia induced by pretraining morphine was significantly reversed in morphine-sensitized mice which had previously received once daily injections of morphine [20 and 30 mg/kg, subcutaneously (s.c.)] for 3 days. Morphine sensitization tended to reverse but did not significantly affect morphine state-dependent memory. The inhibition of morphine-induced amnesia in morphine-sensitized mice was decreased by once daily administration of naloxone (0.5, 1 and 2 mg/kg) 30 min prior to injection of morphine (20 mg/kg/day x 3 days). Three-days administration of 1-phenyl-7,8-dihydroxy-2,3,4,5-tetrahydro-1H-3-benzazepine HCL (SKF 38393; 8, 16 and 32 mg/kg) or SCH 23390; R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine HCL (0.01, 0.05 and 0.1 mg/kg) before morphine (for 3 days) and during morphine-sensitization, decreased and increased, the amnesia induced by pretraining morphine, respectively. Similar administration of quinpirole (0.5, 1 and 2 mg/kg) or sulpiride (25, 50 and 100 mg/kg) before morphine also decreased and increased the amnesia induced by pretraining morphine, respectively. The results suggest that morphine sensitization affects the impairment of memory formation, but not the facilitation of retrieval induced by morphine and thus it is postulated that dopamine receptors may play an important role in this effect.

Effects of dopamine antagonists on alfentanil–induced locomotor activity in horses

OBJECTIVE

To determine whether specific dopamine receptor antagonists block alfentanil-induced locomotor stimulation in horses.

STUDY DESIGN

Randomized, prospective, crossover experiment.

ANIMALS

Eight adult horses (462-604 kg).

METHODS

Doses of dopamine-1 (D1) (NNC 01-0756) and dopamine-2 (D2) antagonists (eticlopride) were selected in a pilot study prior to a three part, blinded, cross-over study. In part 1, horses received 7.5 micro g kg-1 eticlopride, 5 micro g kg-1 NNC 01-0756 or an equal volume of saline. In part 2, they received both antagonists and in part 3, acepromazine at 0.05 mg kg-1. Locomotor activity was assessed by counting the steps taken by a marked forefoot per 2 minutes. The D antagonist was injected IV after a 20-minute control period. The horses were observed for 10 minutes before alfentanil (20 micro g kg-1) was injected IV. Locomotor activity was then monitored for 60 minutes. Statistical analysis was performed on step counts following alfentanil normalised by subtracting the mean control step count from each value recorded after alfentanil. Data were analysed using Friedman tests and Tukey-Kramer comparisons.

RESULTS

Alfentanil increased locomotor activity for 10 minutes. NNC 01-0756 tended to reduce locomotor activity between 0 and 10 minutes (p = 0.261), but neither D antagonist suppressed it significantly. The combination of D antagonists induced more step counts than saline or acepromazine (p = 0.0265) in the 20-40-minute period and more than saline, acepromazine or eticlopride between 40 and 60 minutes (p = 0.0003).

CONCLUSIONS

Neither D1 nor D2 antagonists inhibited alfentanil-induced locomotor activity. Both drugs appeared to cause locomotor stimulation of their own.

CLINICAL RELEVANCE

D1 and D2 antagonism did not reduce opioid-induced excitement in horses and is not suitable for reducing the incidence of this unwanted side-effect of opioids.

Morphine conditioned place preference is attenuated by perinatal lead exposure

The purpose of this investigation was to determine if perinatal lead exposure alters the conditioned reinforcing properties of morphine when offspring were tested as adults. Dams were gavaged daily with 0- (sodium acetate) or 16-mg lead (as lead acetate) for 30 days prior to breeding with nonexposed males. Administration continued through gestation and lactation and was discontinued at weaning (postnatal day [PND] 21). At PND 70 animals were tested in a conditioned place preference (CPP) preparation using 0.00, 0.60, 1.25, 2.50, or 5.00 mg/kg i.p. morphine as the unconditioned stimulus (US). Relative to controls, attenuation of CPP was evident in animals exposed to 16-mg lead at 1.25 and 2.50 mg/kg morphine. Analysis of blood lead concentration revealed that by the end of testing residue levels in metal-exposed animals had returned to control levels. However, data from littermates sacrificed well beyond the current testing period revealed that brain lead residues remained elevated in animals exposed to lead, even though the metal had gained clearance from blood. The present data suggest that early lead exposure may have an enduring impact on the reinforcing properties of morphine.

Elevated levels of GluR1 in the midbrain: a trigger for sensitization to drugs of abuse?

In laboratory animals, repeated administration of drugs of abuse, such as cocaine, morphine or alcohol, causes sensitization (reverse tolerance) to their stimulant and rewarding effects. Neuroadaptations underlying sensitization could be related to those that contribute to addictive behaviors. An increased understanding of the molecular mechanisms of sensitization could lead to improved treatments for addiction. Here, we review evidence that the ability of drugs of abuse to elevate levels of the GluR1 subunit of AMPA glutamate receptors in the ventral tegmental area (VTA) of the midbrain is crucial for the development of sensitization. Even transient increases in GluR1 levels within VTA neurons can trigger complex cascades of other molecular adaptations in these neurons and, within larger neural circuits, can cause enduring changes in the responses of the brain to drugs of abuse. However, there is ongoing debate over whether elevated levels of GluR1 in the VTA are a primary cause, or secondary effect, of the neurobiological underpinnings of sensitization.

Effects of DA D1 and D2 antagonists on the sensitisation to the motor effects of morphine in mice

Acute morphine administration produces hyperactivity in mice and repeated treatment induces an enhancement of this effect. In this experiment, we study the sensitisation to the hyperactivity induced by intermittent morphine administration (40 mg/kg) and the effects of dopamine (DA) antagonists on this phenomenon. Animals received three injections, separated by 48 h, and after each injection, their activity was registered between 30 and 60 min. In Experiment 1, animals were divided into two groups, which received saline and morphine (S-S-M) or only morphine (M-M-M). In Experiment 2, animals were divided into 12 groups. Half, which was designed to study the effects of DA antagonists on the acquisition of morphine sensitisation, received morphine plus 0.125, 0.25, or 0.5 mg/kg SCH 23390 or raclopride in the two first administrations and only morphine in the third (M+SCH-M+SCH-M; M+R-M+R-M). The other groups, designed to study the effects of DA antagonists on the expression of morphine sensitisation, received morphine in the two first administrations and morphine plus DA antagonists in the last injection (M-M-M+SCH; M-M-M+R). Intermittent morphine administration produces greater hyperactivity than acute morphine. DA D1 antagonists reduce acquisition and block expression of sensitisation, while DA D2 antagonists only affect expression with the intermediate and high dose. These results support the implication of DA in the behavioural sensitisation of morphine in mice.

The effects of developmental cadmium exposure on morphine sensitization and challenge with selective D(1) and D(2) antagonists

The purpose of this investigation was to determine the effects of developmental (perinatal) cadmium exposure on the development and expression of behavioral sensitization to morphine. Adult female rats were maintained ad libitum on diets containing 0, 25, or 50 ppm added cadmium (administered as cadmium chloride) for 30 days prior to breeding with nonexposed males. This exposure regimen continued throughout the gestational period and for 15 days postnatally during lactation, at which time regular rat chow was provided. On postnatal day (PND) 21, male pups from the respective litters were weaned and placed on an unadulterated food supply (no added cadmium) and tap water for the remainder of the study. Beginning on PND 70, animals from each exposure condition (0, 25, 50 ppm exposure conditions) received, for 21 consecutive days, either vehicle (distilled water) or 10 mg/kg morphine sulfate injections (ip) prior to being monitored for locomotor activity during 80-min test sessions. Following this 21-day period of morphine sensitization training, dose-effect profiles were determined for each exposure condition with successive daily challenges of 0, 10, and 20 mg/kg morphine. Subsequently, different doses of the D(1) antagonist SCH 23390 (0.01, 0.056, and 0.10 mg/kg) and the D(2) antagonist eticlopride (0.01 and 0.056 mg/kg) were presented prior to administration of the training dose of morphine (10 mg/kg). The results of the investigation revealed that developmental cadmium exposure attenuated the development/expression of morphine sensitization. Furthermore, it was found that the suppressive effects of the D(2) antagonist eticlopride were decreased by early cadmium exposure.

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.

Cortical alpha 1-adrenergic regulation of acute and sensitized morphine locomotor effects

The role of alpha1-adrenergic transmission was tested on locomotor effects of acute or repeated morphine (5 mg/kg, i.p.) administration. Prazosin, an alpha1-adrenergic antagonist, administered 30 min before morphine, either systemically (0.5 mg/kg, i.p.) or locally and bilaterally into the prefrontal cortex (200 pmol/side) reduced the stimulatory influence of morphine on locomotion. The progressive increase of the locomotor response induced by repeated morphine injections was blocked by a prazosin pretreatment but not the behavioral sensitization on the test day. These data suggest that blockade of cortical alpha1-adrenergic receptors reduces the expression of acute and sensitized locomotor responses to morphine, but does not prevent the induction of behavioral sensitization.

Effects of morphine withdrawal on catecholaminergic neurons on heart right ventricle; implication of dopamine receptors

The purpose of our study was to examine the effects of D1-and D2-dopamine receptors blockade on the changes in the ventricular content of catecholamines in rats withdrawn from morphine. Rats were given morphine by subcutaneous (sc) implantation of morphine pellets for 5 days. On the eighth day, morphine withdrawal was induced by sc administration of naloxone (1 mg/kg), and rats were killed 30 min later. Pretreatment with SCH 23390 (dopamine D1, D5 receptor antagonist) 15 min prior to naloxone administration suppressed some the behavioural signs of morphine withdrawal, whereas eticlopride (dopamine D2, D3, D4 receptor antagonist) did not. In addition, biochemical analysis indicate that SCH 23390 completely abolished the withdrawal-induced increase in noradrenaline and dopamine turnover in the right ventricle. By contrast, eticlopride did not block the hyperactivity of catecholaminergic neurons in the heart during morphine withdrawal. These data suggest that the hyperactivity of catecholaminergic neurons in the heart during morphine withdrawal is dependent upon D1 dopamine receptor activation. In addition, our results exclude the involvement of D2 dopamine receptors.Key words: morphine withdrawal, right ventricle, catecholaminergic activity.

Parametric evaluation of the development of sensitization to the effects of morphine on locomotor activity

Animals repeatedly administered drugs of abuse often become more sensitive to their effects. It has been proposed that this behavioral sensitization may serve as a useful model for changes that may underlie the etiology and maintenance of drug-seeking behavior. This study was designed to determine systematically some of the conditions of drug exposure under which sensitization occurs to morphine-induced stimulation of locomotor activity. Groups of rats (n=8 per group) were exposed to a regimen of intermittent morphine or saline injections for 1--4 days and tested at later time points with morphine or saline. The amount of behavioral sensitization observed was related to the number of drug exposures, but not to the dose of morphine used during drug exposures. Sensitization to morphine persisted for as long as 3 months and was completely blocked when naltrexone was administered with the test dose of morphine after the final morphine exposure. Administration of naltrexone with morphine during the exposure regimen did not alter the development of behavioral sensitization. These results indicate a robust behavioral sensitization to morphine that appears to be influenced in an orderly manner within a narrow window of the drug exposure conditions.

Differential effects of adult and perinatal lead exposure on morphine-induced locomotor activity in rats

The effects of adult and perinatal lead treatment on the development of locomotor sensitization produced with repeated morphine administration was investigated. In Experiment 1, adult male rats received a diet containing 250 ppm lead acetate or a control diet for 43 days. Animals then received 10 mg/kg morphine sulfate or water vehicle (ip) and locomotor activity was monitored for 14 consecutive days. While both control and lead-exposed animals demonstrated a locomotor sensitization to morphine, the magnitude of the increased locomotor response was reduced in lead-treated animals. Subsequent analysis of blood-lead in the adult lead-exposed animals indicated residue levels ranging between 20 and 30 microg/dl. In Experiment 2, adult female rats were treated daily with 0, 8, or 16 mg lead via gavage for 30 days before breeding with non-exposed males. Lead exposure in dams continued through gestation and until pups were weaned at postnatal day (PND) 21. At PND 60, male offspring received morphine or vehicle challenges identical to those described in Experiment 1. Animals perinatally exposed to dams receiving 16 mg lead daily demonstrated an enhanced behavioral response to morphine relative to control animals. Analysis of offspring blood indicated lead levels below detectable limits (<1 microg/dl) for all animals. The results suggest exposure to lead at environmentally relevant levels produces long-lasting changes in drug-induced behavior, and the developmental period in which lead exposure occurs is a significant contributor to the manifestation of these effects.

Ventral pallidal injections of a mu antagonist block the development of behavioral sensitization to systemic morphine

Acute activation of opioid receptors in the ventral pallidum increases motor behaviors in rats. The present study was designed to investigate the possibility that the ventral pallidum influences motor responses induced by chronic opiate treatments and to examine the receptors that may be involved in such an effect. For five consecutive days, ambulations were quantified after rats received once-daily intraperitoneal (i.p.) injections of morphine (10 mg/kg) or saline following bilateral intra-ventral pallidal injections of either saline (0.5 microl/hemisphere), the mu antagonist CTOP (2. 1 microg/0.5microl/hemisphere), or the D1 antagonist SCH23390 (0.25 microg/0.5microl/hemisphere). Behavioral sensitization to an acute morphine challenge (10 mg/kg i.p.) was assessed 72 h after terminating the repeated treatment regimen. Rats who repeatedly received the intra-ventral pallidal saline + i.p. morphine exhibited increases in ambulations during the chronic treatment protocol and this effect was greatly enhanced (i.e., sensitized) following the post withdrawal acute morphine challenge. Rats repeatedly treated with intra-ventral pallidal CTOP + i.p. morphine did not display a motor response either during the chronic treatment regime or to the acute morphine challenge; an effect not seen when CTOP was injected into brain structures located dorsal to the ventral pallidum. The rats repeatedly treated with intra-ventral pallidal injections of SCH23390 + i.p. morphine demonstrated a motor response during the chronic protocol but the magnitude of this response was not significantly enhanced by the acute morphine challenge. These results demonstrate that: 1) mu opioid and D1-like dopamine receptors in the ventral pallidum influence the increase in locomotion that occurs during repeated morphine treatments; and 2) mu opioid (but not D1) receptors in the ventral pallidum are important in the postwithdrawal sensitized response to morphine. Such observations indicate that the ventral pallidum plays a critical role in morphine-induced behavioral sensitization.

Lack of specific effects of selective D(1) and D(2) dopamine antagonists vs. risperidone on morphine-induced hyperactivity

In the present study, three different dopamine antagonists were challenged in order to counteract hyperactivity induced by 50 mg/kg of morphine. A wide range of doses of morphine (50, 25, 12.5, 6.25, or 3.12 mg/kg) were evaluated on spontaneous locomotor activity. A significant increase was observed only with the two higher doses tested (25 and 50 mg/kg). No decrease was found with any of the doses used at any period of time. After analyzing doses of SCH 23390 (0.5, 0.1, and 0.05 mg/kg), raclopride (0.5, 0.25, and 0.125 mg/kg) and risperidone (0.1, 0.05, and 0.025 mg/kg) administered alone, only the 0.5 mg/kg dose of SCH 23390 decreased locomotor activity. The three compounds counteracted morphine-induced hyperactivity, but with SCH 23390 it was only achieved with the dose of 0.5 mg/kg, which also decreased spontaneous locomotor activity and induced catalepsy. On the other hand, raclopride and risperidone neutralized morphine-induced hyperactivity at doses that did not affect locomotor activity, although the former induced catalepsy when administered with morphine. It is concluded that although the blockade of D(1) and D(2) DA receptors decreases morphine-induced hyperactivity, this action is not specific, contrary to the action of risperidone, which counteracts this hyperactivity without any other motor effects.

Sensitization to daily morphine injections in rats with unilateral lesions of the substantia nigra

Morphine indirectly enhances dopaminergic activity in the nigrostriatal system, and repeated administration of morphine progressively increases the locomotor activity of rats. We used the rotational behavior model to determine if daily morphine produces an increase in turning and produces cross-sensitization to d-amphetamine and cocaine. Rats with unilateral nigrostriatal lesions received daily injections of saline or morphine (10 mg/kg). Repeated morphine administration produced a progressive increase in turning over 13 days. Next, a morphine dose-response curve (1.0-30 mg/kg) was determined. Both the saline and morphine-treated groups showed dose-dependent increases in turning, but, the peak effect in the morphine group was higher than that in the saline group, indicating sensitization to morphine. The morphine-treated group did not show cross-sensitization to either d-amphetamine (0.1-3 mg/kg) or cocaine (1.0-30 mg/kg); in fact, it showed less cocaine-induced turning than the saline group. Seventy-one days after saline or morphine injections began, the morphine group was still significantly more sensitive to turning induced by 10 mg/kg morphine than the saline group was (200 vs. 750). Therefore, repeated daily injections of morphine produce a progressive sensitization to turning induced by morphine in the absence of cross-sensitization to turning induced by psychomotor stimulants.

Morphine alters the structure of neurons in the nucleus accumbens and neocortex of rats

Rats were given repeated injections of 10 mg/kg of morphine and were then left undisturbed for 24-25 days before their brains were processed for Golgi-Cox staining. Prior exposure to morphine decreased the complexity of dendritic branching and the number of dendritic spines on medium spiny neurons in the shell of the nucleus accumbens and on pyramidal cells in the prefrontal and parietal cortex. It is suggested that some of the long-term behavioral consequences of repeated exposure to morphine may be due to its ability to reorganize patterns of synaptic connectivity in the forebrain.

Reciprocal changes in dopamine responsiveness in the nucleus accumbens shell and core and in the dorsal caudate-putamen in rats sensitized to morphine

In this study, we describe a model of opiate sensitization characterized by a brief schedule of treatment with repeated morphine administrations. In this model, we investigated the changes produced by repeated morphine treatment on dopamine transmission at the level of the two major terminal dopaminergic areas, the dorsolateral caudate-putamen and the nucleus accumbens in its two subdivisions, the shell and the core. Rats were treated twice a day for three days with increasing doses of morphine (10, 20 and 40 mg/kg, s.c.) or with saline. After 15 days of withdrawal, rats were challenged with 1 and 5 mg/kg (s.c.) of morphine, and dopamine transmission was monitored by microdialysis. In this model, we show that repeated morphine produces a strong behavioral sensitization accompanied by increased stimulation of dopamine transmission in the core of the nucleus accumbens and in the caudate-putamen, and by a decreased stimulation of dopamine transmission in the shell of the nucleus accumbens, as compared to control rats. Moreover, we administered to these animals amphetamine (0.5 mg/kg, s.c.) and cocaine (10 mg/kg, i.p.) to assess whether cross-sensitization occurs between opiates and psychostimulants in conditions independent of the context. In the present study, we did not observe either behavioral or biochemical sensitization to amphetamine and to cocaine in rats sensitized to morphine. These results suggest that rats behaviorally sensitized to morphine show opposite changes in the stimulant effect of morphine in the nucleus accumbens shell and core and in the dorsal caudate-putamen. Moreover, this study suggests that sensitization of the dopamine system to a given agent does not necessarily extend to drugs of abuse of different pharmacological classes.