The dopamine receptor agonist 7-OH-DPAT modulates the acquisition and expression of morphine-induced place preference

Insights into the mechanisms underlying opioid use disorder and potential treatment strategies

Opioid use disorder is a worldwide societal problem and public health burden. Strategies for treating opioid use disorder can be divided into those that target the opioid receptor system and those that target non-opioid receptor systems, including the dopamine and glutamate receptor systems. Currently, the clinical drugs used to treat opioid use disorder include the opioid receptor agonists methadone and buprenorphine, which are limited by their abuse liability, and the opioid receptor antagonist naltrexone, which is limited by poor compliance. Therefore, the development of effective medications with lower abuse liability and better potential for compliance is urgently needed. Based on recent advances in the understanding of the neurobiological mechanisms underlying opioid use disorder, potential treatment strategies and targets have emerged. This review focuses on the progress made in identifying potential targets and developing medications to treat opioid use disorder, including progress made by our laboratory, and provides insights for future medication development. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Influence of Selective Dopamine Agonist Ropinirole on Conditioned Place Preference and Somatic Signs of Morphine Withdrawal in Rats

The underlying mechanism of dependence and rewarding effects of morphine is imperative to understand. The primary aim of this study was to investigate whether ropinirole D2/3 agonist affects the rewarding and reinforcing properties of morphine-induced conditioned place preference (CPP) and withdrawal syndromes in rats. On day one, the animals were randomly divided to conduct the pre-test. The morphine (10 mg/kg, i.p.) and/or saline was administered on alternate days in an 8-day CPP session. On day 10, 15 min prior to the post-conditioning test (expression), a single dose of ropinirole (1, 2, and 5 mg/kg, i.p.) was given to rats. In extinction session, ropinirole was injected daily, and CPP was extinguished by repeated testing, with intervals of 3 days. Finally, reinstatement was assessed by administering ropinirole (1, 2, and 5 mg/kg) 15 min before the morphine injection. Morphine dependence was developed by administering increasing doses of morphine (10–50 mg/kg, i.p.). To assess withdrawal symptoms, ropinirole (1, 2, and 5 mg/kg) was injected 15 min before naloxone (2 mg/kg, s.c.) administration. The present study confirms that ropinirole attenuates expression and reinstatement of CPP, while it precipitates the extinction of morphine-induced CPP. Naloxone-precipitated morphine withdrawal symptoms, including wet dog shakes and weight loss, were attenuated although jumping was increased by a single ropinirole injection. Thus, ropinirole was influential in attenuating expression, reducing drug seeking and weakening reinstatement via the dopaminergic system. These findings show that ropinirole might affect neuro-adaptive changes related to dependence.

The effect of microinjection of CART 55-102 into the nucleus accumbens shell on morphine-induced conditioned place preference in rats: Involvement of the NMDA receptor

The addictive properties of opioids may be mediated to some extent by cocaine-and amphetamine-regulated transcript (CART) in the reward pathway. Moreover, some claims CART interacts with the glutamate system. Here, we evaluated whether intra-nucleus accumbens (NAc) shell infusions of CART induces Conditioned Place Preference (CPP) or Conditioned Place Aversion (CPA) and affects morphine reward. We also measured NR1 subunit expressions of the N-methyl-d-aspartate (NMDA) receptor in various parts of the reward pathway (NAc, prefrontal cortex and hippocampus) after conditioning tests. Animals with bilateral intra-NAc shell cannulas were place-conditioned with several doses of subcutaneous morphine prior to intra-NAc shell infusions of artificial cerebrospinal fluid (aCSF). Immunohistochemistry (IHC) showed a dose-dependent increase in the NR1 expression in all examined parts. When rats were conditioned with intra-NAc shell infusions of CART, CPP and CPA induced with 2.5 and 5 μg/side respectively and IHC showed NR1elevation with 2.5 and reduction with 5 μg/side in all areas. Sub-rewarding dose of CART administration (1.25 μg/side) prior to sub-rewarding dose of morphine (2.5 mg/kg) induced CPP and NR1 increased in all examined tissues in IHC. However, infusion of an aversive dose of CART (5 μg/side) prior to the rewarding dose of morphine (5 mg/kg) produced neither CPP nor CPA and NR1 in the NAc and hippocampus decreased significantly. It seems that the rewarding or aversive effects of intra-NAc shell CART and its facilitating or inhibiting effects on morphine reward are dose-dependent. Additionally, NMDA may be closely involved in the affective properties of opioids and CART in the reward pathway.

The Pathophysiology of Tics; An Evolving Story

Background:

Tics, defined as quick, rapid, sudden, recurrent, non-rhythmic motor movements or vocalizations are required components of Tourette Syndrome (TS) - a complex disorder characterized by the presence of fluctuating, chronic motor and vocal tics, and the presence of co-existing neuropsychological problems. Despite many advances, the underlying pathophysiology of tics/TS remains unknown.

Objective:

To address a variety of controversies surrounding the pathophysiology of TS. More specifically: 1) the configuration of circuits likely involved; 2) the role of inhibitory influences on motor control; 3) the classification of tics as either goal-directed or habitual behaviors; 4) the potential anatomical site of origin, e.g. cortex, striatum, thalamus, cerebellum, or other(s); and 5) the role of specific neurotransmitters (dopamine, glutamate, GABA, and others) as possible mechanisms (Abstract figure).

Methods:

Existing evidence from current clinical, basic science, and animal model studies are reviewed to provide: 1) an expanded understanding of individual components and the complex integration of the Cortico-Basal Ganglia-Thalamo-Cortical (CBGTC) circuit - the pathway involved with motor control; and 2) scientific data directly addressing each of the aforementioned controversies regarding pathways, inhibition, classification, anatomy, and neurotransmitters.

Conclusion:

Until a definitive pathophysiological mechanism is identified, one functional approach is to consider that a disruption anywhere within CBGTC circuitry, or a brain region inputting to the motor circuit, can lead to an aberrant message arriving at the primary motor cortex and enabling a tic. Pharmacologic modulation may be therapeutically beneficial, even though it might not be directed toward the primary abnormality.

Controversies Surrounding the Pathophysiology of Tics

Tics are sudden, rapid, recurrent, nonrhythmic motor movements or vocalizations (phonic productions) that are commonly present in children and are required symptoms for the diagnosis of Tourette syndrome. Despite their frequency, the underlying pathophysiology of tics/Tourette syndrome remains unknown. In this review, we discuss a variety of controversies surrounding the pathophysiology of tics, including the following: Are tics voluntary or involuntary? What is the role of the premonitory urge? Are tics due to excess excitatory or deficient inhibition? Is it time to adopt the contemporary version of the cortico-basal ganglia-thalamocortical (CBGTC) circuit? and Do we know the primary abnormal neurotransmitter in Tourette syndrome? Data from convergent clinical and animal model studies support complex interactions among the various CBGTC sites and neurotransmitters. Advances are being made; however, numerous pathophysiologic questions persist.

Merging the Pathophysiology and Pharmacotherapy of Tics

Background

Anatomically, cortical-basal ganglia-thalamo-cortical (CBGTC) circuits have an essential role in the expression of tics. At the biochemical level, the proper conveyance of messages through these circuits requires several functionally integrated neurotransmitter systems. In this manuscript, evidence supporting proposed pathophysiological abnormalities, both anatomical and chemical is reviewed. In addition, the results of standard and emerging tic-suppressing therapies affecting nine separate neurotransmitter systems are discussed. The goal of this review is to integrate our current understanding of the pathophysiology of Tourette syndrome (TS) with present and proposed pharmacotherapies for tic suppression.

Methods

For this manuscript, literature searches were conducted for both current basic science and clinical information in PubMed, Google-Scholar, and other scholarly journals to September 2018.

Results

The precise primary site of abnormality for tics remains undetermined. Although many pathophysiologic hypotheses favor a specific abnormality of the cortex, striatum, or globus pallidus, others recognize essential influences from regions such as the thalamus, cerebellum, brainstem, and ventral striatum. Some prefer an alteration within direct and indirect pathways, whereas others believe this fails to recognize the multiple interactions within and between CBGTC circuits. Although research and clinical evidence supports involvement of the dopaminergic system, additional data emphasizes the potential roles for several other neurotransmitter systems.

Discussion

A greater understanding of the primary neurochemical defect in TS would be extremely valuable for the development of new tic-suppressing therapies. Nevertheless, recognizing the varied and complex interactions that exist in a multi-neurotransmitter system, successful therapy may not require direct targeting of the primary abnormality.

Dopamine D4 receptor stimulation prevents nigrostriatal dopamine pathway activation by morphine: relevance for drug addiction

Morphine is one of the most effective drugs used for pain management, but it is also highly addictive. Morphine elicits acute and long-term adaptive changes at cellular and molecular level in the brain, which play a critical role in the development of tolerance, dependence and addiction. Previous studies indicated that the dopamine D₄ receptor (D₄ R) activation counteracts morphine-induced adaptive changes of the μ opioid receptor (MOR) signaling in the striosomes of the caudate putamen (CPu), as well as the induction of several Fos family transcription factors. Thus, it has been suggested that D₄ R could play an important role avoiding some of the addictive effects of morphine. Here, using different drugs administration paradigms, it is determined that the D₄ R agonist PD168,077 prevents morphine-induced activation of the nigrostriatal dopamine pathway and morphological changes of substantia nigra pars compacta (SNc) dopamine neurons, leading to a restoration of dopamine levels and metabolism in the CPu. Results from receptor autoradiography indicate that D₄ R activation modulates MOR function in the substantia nigra pars reticulata (SNr) and the striosomes of the CPu, suggesting that these regions are critically involved in the modulation of SNc dopamine neuronal function through a functional D₄ R/MOR interaction. In addition, D₄ R activation counteracts the rewarding effects of morphine, as well as the development of hyperlocomotion and physical dependence without any effect on its analgesic properties. These results provide a novel role of D₄ R agonist as a pharmacological strategy to prevent the adverse effects of morphine in the treatment of pain.

The hippocampal NMDA receptors may be involved in acquisition, but not expression of ACPA-induced place preference

Numerous studies have investigated the functional interactions between the endocannabinoid and glutamate systems in the hippocampus. The present study was made to test whether N-methyl-D-aspartate (NMDA) receptors of the CA1 region of the dorsal hippocampus (CA1) are implicated in ACPA (a selective cannabinoid CB1 receptor agonist)-induced place preference. Using a 3-day schedule of conditioning, it was found that intraperitoneal (i.p.) administration of ACPA (0.02mg/kg) caused a significant conditioned place preference (CPP) in male albino NMRI mice. Intra-CA1 microinjection of the NMDA or D-[1]-2-amino-7-Phosphonoheptanoic acid (D-AP7, NMDA receptor antagonist), failed to induce CPP or CPA (condition place aversion), while NMDA (0.5μg/mouse) potentiated the ACPA (0.01mg/kg)-induced CPP; and D-AP7 (a specific NMDA receptor antagonist; 0.5 and 1μg/mouse) reversed the ACPA (0.02mg/kg)-induced CPP. Moreover, microinjection of different doses of glutamatergic agents on the testing day did not alter the expression of ACPA-induced place preference. None of the treatments, with the exception of ACPA (0.04mg/kg), had an effect on locomotor activity. In conclusion, these observations provide evidence that glutamate NMDA receptors of the CA1 may be involved in the potentiation of ACPA rewarding properties in the acquisition, but not expression, of CPP in mice.

CP-154,526 Modifies CREB Phosphorylation and Thioredoxin-1 Expression in the Dentate Gyrus following Morphine-Induced Conditioned Place Preference

Corticotropin-releasing factor (CRF) acts as neuro-regulator of the behavioral and emotional integration of environmental and endogenous stimuli associated with drug dependence. Thioredoxin-1 (Trx-1) is a functional protein controlling the redox status of several proteins, which is involved in addictive processes. In the present study, we have evaluated the role of CRF1 receptor (CRF1R) in the rewarding properties of morphine by using the conditioned place preference (CPP) paradigm. We also investigate the effects of the CRF1R antagonist, CP-154,526, on the morphine CPP-induced activation of CRF neurons, CREB phosphorylation and Trx expression in paraventricular nucleus (PVN) and dentate gyrus (DG) of the mice brain. CP-154,526 abolished the acquisition of morphine CPP and the increase of CRF/pCREB positive neurons in PVN. Moreover, this CRF1R antagonist prevented morphine-induced CRF-immunoreactive fibers in DG, as well as the increase in pCREB expression in both the PVN and DG. In addition, morphine exposure induced an increase in Trx-1 expression in DG without any alterations in PVN. We also observed that the majority of pCREB positive neurons in DG co-expressed Trx-1, suggesting that Trx-1 could activate CREB in the DG, a brain region involved in memory consolidation. Altogether, these results support the idea that CRF1R antagonist blocked Trx-1 expression and pCREB/Trx-1 co-localization, indicating a critical role of CRF, through CRF1R, in molecular changes involved in morphine associated behaviors.

Early Cannabinoid Exposure as a Source of Vulnerability to Opiate Addiction: A Model in Laboratory Rodents

Recent findings have identified an endogenous brain system mediating the actions of cannabis sativa preparations. This system includes the brain cannabinoid receptor (CB-1) and its endogenous ligands anandamide and 2-arachidonoyl-glycerol. The endogenous cannabinoid system is not only present in the adult brain, but is also active at early stages of brain development. Studies developed at our laboratory have revealed that maternal exposure to psychoactive cannabinoid results in neuro-developmental alterations. A model is proposed in which early Δ9-tetrahydrocannabinol (THC) exposure during critical developmental periods results in permanent alterations in brain function by either the stimulation of CB-1 receptors present during the development, or by the alterations in maternal glucocorticoid secretion. Those alterations will be revealed in adulthood after challenges either with drugs (i.e. opiates) or with environmental stressors (i.e. novelty). They will include a modified pattern of neuro-chemical, endocrine, and behavioral responses that might lead ultimately to inadaptation and vulnerability to opiate abuse.

The dopamine D3 receptor antagonist YQA14 that inhibits the expression and drug-primed reactivation of morphine-induced conditioned place preference in rats

Increasing evidence suggests that the mesolimbic dopamine system plays a critical role in opioid addiction. However, there is currently no standard drug treatment for opioid addiction. Growing preclinical evidence indicates that the dopamine D3 receptor antagonists are the potential anti-addiction pharmacotherapeutic agents based on in animal models of multiple drug addiction. In this study, we investigated the inhibitory effects of YQA14, a novel dopamine D3 receptor antagonist with a high affinity and selectivity for dopamine D3 receptor, using morphine-induced conditioned place preference (CPP) in rats. The results suggested that YQA14 (6.25-25mg/kg; intraperitoneal, i.p.) decreased the expression of morphine (10mg/kg, s.c.)-induced CPP in a dose-related manner but did not influence the acquisition of morphine-induced CPP. At a 25mg/kg dose of YQA14, it also notably inhibited the reactivation of morphine-priming CPP. These findings suggest that YQA14 is a potential agent for anti-opioid addiction which warrants further study and development.

Assessing learned associations between conditioned cocaine reward and environmental stimuli in the Wistar Kyoto rat

Clinical studies demonstrate that anxiety disorders increase the risk of substance use disorder. However, few studies have directly assessed anxiety as a vulnerability factor in processing of rewarding stimuli. The Wistar–Kyoto (WKY) rat has been proposed as a model of anxiety vulnerability because it exhibits extreme behavioral inhibition in novel and social environments; yet, it displays paradoxical rapid active avoidance learning that is resistant to extinction. The present study was designed to characterize the acquisition and persistence of cocaine conditioned place preference (CPP) in WKY rats. In the first of a series of three experiments, adult male WKY and Sprague Dawley (SD) rats were given six pairings of cocaine (3, 5, 10, 15 mg/kg) or saline on alternating days. SD rats developed cocaine-induced CPP to each of the four doses of cocaine tested. In contrast, WKY rats demonstrated CPP when conditioned with 3, 5, and 10 mg/kg, but displayed no preference to the 15 mg/kg dose. Next, separate groups of rats were subject to an extended CPP paradigm, which included acquisition, extinction and reinstatement phases. Rats were conditioned with cocaine and saline on alternating days using either a 6/6 (as above) or 4/4 conditioning regimen. Both SD and WKY rats acquired a lasting CPP with the 6/6 conditioning regimen. Results from the 4/4 conditioning regimen show that SD, but not WKY, rats acquired CPP. Preference scores for SD rats during the cocaine primed reinstatement test were significantly different from pretest scores indicating reinstatement of CPP in this group. Paradoxically, WKY rats demonstrated a latent sensitization to the conditioned rewarding effects of cocaine during the drug-primed reinstatement test. Taken together, WKY rats appear to be more sensitive to high doses of cocaine and need more experience with the drug to acquire a preference than SD rats.

Functional interaction between morphine and central amygdala cannabinoid CB1 receptors in the acquisition and expression of conditioned place preference

The present study was done to determine whether cannabinoid CB1 receptors of the central amygdala (CeA) are implicated in morphine-induced place preference. Using a 3-day schedule of conditioning, it was found that subcutaneous (s.c.) administration of morphine (2, 4 and 6 mg/kg) caused a significant dose-dependent conditioned place preference (CPP) in male Wistar rats. Intra-CeA microinjection of the cannabinoid CB1 receptor agonist arachidonylcyclopropylamide (ACPA; 0.5, 2.5 and 5 ng/rat) dose-dependently potentiated the morphine (2mg/kg)-induced CPP. Furthermore, the administration of ACPA (5 ng/rat, intra-CeA) alone induced a significant CPP. It should be considered that the higher dose of ACPA (5 ng/rat, intra-CeA) in combination with morphine decreased locomotor activity on the testing phase. On the other hand, intra-CeA microinjection of the cannabinoid CB1 receptor antagonist AM251 (120 ng/rat) alone induced a significant conditioned place aversion (CPA). Moreover, intra-CeA microinjection of AM251 (90 and 120 ng/rat) inhibited the morphine-induced place preference with a significant interaction. Intra-CeA microinjection of AM251 reversed the effect of ACPA on morphine response. Interestingly, microinjection of ACPA (2.5 and 5 ng/rat) or AM251 (60-120 ng/rat) into the CeA increased or decreased the expression of morphine (6 mg/kg)-induced place preference respectively. These observations provide evidence that cannabinoid CB1 receptors of the CeA are involved in mediating reward and these receptors are also implicated in the acquisition and expression of morphine-induced CPP.

Maternal deprivation and handling modify the effect of the dopamine D3 receptor agonist, BP 897 on morphine-conditioned place preference in rats

RATIONALE

Maternal deprivation and handling can lead to a vulnerability to opiate dependence. However, the involvement of the dopamine D3 receptors has not been investigated.

OBJECTIVES

This study analysed the effects of a selective partial D3 receptor agonist, BP 897, on morphine-conditioned place preference (CPP) in deprived and handled rats.

MATERIALS AND METHODS

The effects of BP 897 were studied on the expression and the extinction of morphine CPP. Quantitative autoradiography of D2, D3 receptors and immunoautoradiography of dopamine transporter were performed in some saline- and morphine-treated rats 24 h after the place preference test.

RESULTS

Morphine (5 mg/kg) induced a more prolonged morphine CPP in deprived and handled rats than in control animals. BP 897 (0.5 or 2 mg/kg) enhanced the expression of morphine conditioning in control rats. Same doses did not change morphine conditioning in deprived rats. BP 897 (2 mg/kg) suppressed morphine CPP in handled rats. An increase in basal D2 receptor density in the mesencephalon of handled rats, which was suppressed after morphine CPP, was observed. A decrease in D2 receptor levels in morphine-treated deprived rats occurred in the nucleus accumbens.

CONCLUSIONS

This study shows that maternal deprivation and handling induced a prolonged morphine CPP, and different changes of D2/D3 receptor functioning revealed after morphine CPP. Early manipulations of infant-mother relationships may have different consequences on the balance of opioidergic and dopaminergic neurotransmission and may be of interest to reveal pharmacological properties of dopamine receptor partial agonists or antagonists potentially useful for therapeutic applications.

Place conditioning in a two- or three-conditioning compartment apparatus: a comparative study with morphine and U-50,488

We have comparatively studied the effects of two opioids in the rat place conditioning paradigm in identical experimental conditions (including double drug/saline conditioning daily sessions for 3 days), with the only exception of using either a two- or three-conditioning compartment apparatus. Morphine-induced place preference appeared to be similar with two- and three-conditioning compartments, but U-50,488-induced place aversion was consistently more prominent when a two-conditioning compartment apparatus was used. It is suggested that, when the results of conditioning are being tested, the presence of neutral environments decreases the sensitivity of the procedure to quantify place aversions.

Involvement of glucose and ATP-sensitive potassium (K+) channels on morphine-induced conditioned place preference

In the present study, the effects of glucose and ATP-sensitive K+ channel compounds on the acquisition of morphine-induced place preference in male mice were investigated. Subcutaneous administration of different doses of morphine (2.5-7.5 mg/kg) produced a dose-dependent conditioned place preference. With a 3-day conditioning schedule, it was found that glucose (100, 200, 500 and 1000 mg/kg), diazoxide (15, 30 and 60 mg/kg) or glibenclamide (3, 6 and 12 mg/kg) did not produce significant place preference or place aversion. Intraperitoneal administration of the glucose (1000 mg/kg) or glibenclamide (6 and 12 mg/kg) with a lower dose of morphine (0.5 mg/kg) elicited the significant conditioned place preference. The response of glibenclamide (6 mg/kg) was reversed by diazoxide (15, 30 and 60 mg/kg). Drug injections had no effects on locomotor activity during the test sessions. It is concluded that glucose and the ATP-sensitive K+ channel may play an active role in morphine reward.

Morphine-induced place preference: Involvement of cholinergic receptors of the ventral tegmental area

In the present study, the effects of intra-ventral tegmental area injections of cholinergic agents on morphine-induced conditioned place preference were investigated by using an unbiased 3-day schedule of place conditioning design in rats. The conditioning treatments with subcutaneous injections of morphine (0.5-7.5 mg/kg) induced a significant dose-dependent conditioned place preference for the drug-associated place. Intra-ventral tegmental area injection of an anticholinesterase, physostigmine (2.5 and 5 microg/rat) or nicotinic acetylcholine receptor agonist, nicotine (0.5 and 1 microg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant conditioned place preference. Furthermore, intra-ventral tegmental area administration of muscarinic acetylcholine receptor antagonist, atropine (1-4 microg/rat) or nicotinic acetylcholine receptor antagonist, mecamylamine (5 and 7.5 microg/rat) dose-dependently inhibited the morphine (5 mg/kg)-induced place preference. Atropine or mecamylamine reversed the effect of physostigmine or nicotine on morphine response respectively. The injection of physostigmine, but not atropine, nicotine or mecamylamine, into the ventral tegmental area alone produced a significant place aversion. Moreover, intra-ventral tegmental area administration of the higher doses of physostigmine or atropine, but not nicotine or mecamylamine decreased the locomotor activity. We conclude that muscarinic and nicotinic acetylcholine receptors in the ventral tegmental area may critically mediate the rewarding effects of morphine.

Differential ability of D1 and D2 dopamine receptor agonists to induce and modulate expression and reinstatement of cocaine place preference in rats

RATIONALE

D1-Like agonists are self-administered by drug-naive animals, whereas D2-like agonists reinstate cocaine-seeking behavior, but the rewarding and reinstating effects of D1- and D2-like agonists in pavlovian-based conditioned place preference are equivocal.

OBJECTIVE

To compare the ability of D1 and D2 agonists to produce conditioned place preference with their modulation of expression and reinstatement of an established cocaine place preference.

METHODS

Using an unbiased procedure, we measured the place preference induced by the D1 receptor agonist SKF 81297 and the D2/D3 receptor agonist quinpirole in drug-naive or cocaine-exposed rats. The rewarding effects of the D1 agonists SKF 82958, ABT-431, A-77636, and the D2/D3 receptor agonist 7-OH-DPAT were also tested. Additionally, we tested the ability of SKF 81297 and quinpirole to modulate expression and reinstatement of an established cocaine place preference.

RESULTS

The D1 receptor agonists SKF 81297, SKF 82958, and ABT-431 produced dose-dependent conditioned place preferences, whereas A-77636 produced only place aversion, and the D2/D3 agonists quinpirole and 7-OH-DPAT were without effect in drug naive rats. In cocaine-treated rats, SKF-81297-induced place preference was reduced, whereas quinpirole-induced place preference was revealed. Pretreatment using either D1 or D2/D3 agonists blocked expression of an established cocaine place preference, but only the D1 agonist SKF 81297 and cocaine dose-dependently reinstated an extinguished cocaine place preference, whereas the D2/D3 agonist quinpirole induced place aversion but failed to alter cocaine-induced reinstatement.

CONCLUSIONS

D1, but not D2/D3, agonists mediate rewarding effects and reinstatement of cocaine place preference, but the reinstating effects differ markedly from self-administration paradigms.

Morphine-induced place preference: Involvement of the central amygdala NMDA receptors

In the present study, the effects of bilateral injections of N-methyl-d-aspartate (NMDA) receptor agonist and/or antagonist into the central amygdala (CeA) on the acquisition and expression of morphine-induced conditioned place preference (CPP) were investigated in male Wistar rats. Animals that received 3 daily subcutaneous (s.c.) injections of morphine (1-9 mg/kg) or saline (1.0 ml/kg) indicated a significant preference for compartment paired with morphine in a dose dependent manner. Intra-CeA administration of the NMDA (0.01, 0.1 or 1 microg/rat) with an ineffective dose of morphine (1 mg/kg, s.c.) elicited a significant CPP. Administration of the non-competitive NMDA receptor antagonist, MK-801 (0.1, 0.3 or 0.5 microg/rat), into the central amygdala dose-dependently inhibited the morphine (6 mg/kg, s.c.)-induced place preference. Furthermore, intra-CeA administration of MK-801 (0.25, 0.5 or 1 microg/rat) reduced the response induced by NMDA (1 microg/rat, intra-CeA) plus morphine (1 mg/kg, s.c.). Neither NMDA nor MK-801 alone produce a significant place preference or place aversion. Moreover, intra-CeA injection of NMDA but not MK-801 before testing significantly increased the expression of morphine (6 mg/kg, s.c.)-induced place preference. NMDA or MK-801 injections into the CeA had no effects on locomotor activity on the testing sessions. These results suggest that the NMDA receptor mechanisms in the central amygdala may be involved in the acquisition and expression of morphine-induced place preference.

GABA(A) receptors of hippocampal CA1 regions are involved in the acquisition and expression of morphine-induced place preference

In the present study, the effects of bilateral intra-hippocampal CA1 (intra-CA1) injections of GABA(A) receptor agonist and/or antagonist on the acquisition and expression of morphine-induced place preference in male Wistar rats have been investigated. The conditioning treatments with subcutaneous (s.c.) injections of different doses of morphine (0.5-7.5 mg/kg) induced a conditioned place preference (CPP) for the drug-associated place in a dose-dependent manner. Intra-CA1 administration of the GABA(A) receptor agonist, muscimol (0.25, 0.5 and 1 microg/rat) significantly inhibited the morphine (5 mg/kg, s.c.)-induced CPP. Intra-CA1 injections of different doses of the GABA(A) receptor antagonist, bicuculline (0.25, 0.5 and 1 microg/rat), in combination with an ineffective dose of morphine (0.5 mg/kg, s.c.) elicited a significant CPP. However, muscimol or bicuculline by themselves did not elicit any effect on place conditioning. Furthermore, the muscimol-induced inhibition of morphine response was reversed by bicuculline (1 microg/rat, intra-CA1) administration. On the other hand, the bilateral intra-CA1 injections of muscimol (0.25, 0.5 and 1 microg/rat) or bicuculline (0.5, 1 and 2 microg/rat) significantly decreased the expression of morphine-induced CPP. Intra-CA1 administration of different doses of muscimol or bicuculline had no effect on locomotor activity in the testing phase. Our data indicated that the GABA(A) receptors of the hippocampal CA1 regions may play an important role in the acquisition and expression of morphine-induced place preference.

Intra-hippocampal inhibition of protein kinase AII attenuates morphine-induced conditioned place preference

Morphine and other drugs of abuse modulate protein kinase A (PKA) signaling within the mesolimbic reward pathway. Using a balanced conditioned place preference (CPP) paradigm, we studied the possible involvement of protein kinase AII (PKA II) on the acquisition, expression and consolidation of morphine place conditioning in male Wistar rats. Subcutaneous administration of various doses of morphine sulfate (1-9 mg/kg) induced CPP in a dose-dependent manner. H-89, a selective PKA II inhibitor, was administered into CA1 region of the hippocampus at 1, 2.5 and 5 microM/rat. Using a 3-day schedule of conditioning, it was found that the H-89 did not produce a significant place preference or place aversion. H-89 (1, 2.5 and 5 microM/rat) significantly reduced the time spent by rats in the morphine compartment when given immediately after each conditioning session (consolidation), whereas it had no effect when administered before morphine during the conditioning phase (acquisition) or before testing for place preference in the absence of morphine (expression). It is concluded that the PKA II may play an active role in the consolidation of reward-related memory of morphine in CA1 region of the hippocampus.

Different roles of dopamine receptor subtypes in footshock stress-induced enhancement of morphine conditioned place preference

The present study investigated the involvement of dopamine mechanism in the effect of intermittent footshock stress on the morphine-induced place preference. A single intermittent footshock session significantly enhanced the place preference induced by 3.0mg/kg morphine. This enhancing effect was inhibited by selective D(1) receptor antagonist SCH23390 and selective D(2) receptor antagonist sulpiride pretreatment 20min before footshock session, suggesting dopamine D(1) and D(2) receptors are required for the development of intermittent footshock stress-induced enhancement of morphine-associated place preference. However, different from D(1) and D(2) receptors this enhancing effect was blocked by stimulation of dopamine D(3) receptor with selective D(3) receptor agonist 7-OH-DPAT pretreatment 20min before footshock session which suggest dopamine D(3) receptor play a negative mediation effect on the intermittent footshock stress-induced this enhancement. These results indicate that dopamine D(1), D(2), and D(3) receptor subtypes play different roles in footshock stress-induced enhancement of morphine conditioned place preference.

Histaminergic receptors of medial septum and conditioned place preference: D1 dopamine receptor mechanism

In the present study, the effects of intra-medial septum injections of histamine and/or the histamine H1 or H2 receptor antagonists on the acquisition of conditioned place preference (CPP) in male Wistar rats have been investigated. Our data showed that the conditioning treatments with intra-medial septum injection of different doses of histamine (0.5-15 microg/rat) induced a significant CPP for the drug-associated place. Using a 3-day schedule of conditioning, it was found that the histamine H1 receptor antagonist, pyrilamine (10 and 15 microg/rat, intra-medial septum) also induced a significant place preference. In addition, pyrilamine inhibited the histamine (7.5 microg/rat)-induced place preference. Intra-medial septum administration of the histamine H2 receptor antagonist, ranitidine (5-15 microg/rat) alone or in combination with histamine did not produce a significant place preference or place aversion. On the other hand, intra-medial septum administration of the dopamine D1 receptor antagonist, SCH 233390 (0.5, 0.75 and 1 microg/rat) inhibited the histamine (7.5 microg/rat) or pyrilamine (15 microg/rat)-induced place preference in a dose-dependent manner, but no effect was observed for the dopamine D2 receptor antagonist, sulpiride on the histamine or pyrilamine response. The administration of histamine (2.5-15 microg/rat) or pyrilamine (10 and 15 microg/rat) during acquisition increased locomotor activity of the animals on the testing days. The results suggest that histaminergic receptors of the medial septum may be involved in CPP and thus it is postulated that dopamine D1 receptors may play an important role in this effect.

The structural evolution of dopamine D3 receptor ligands: structure-activity relationships and selected neuropharmacological aspects

"Evolution consists largely of molecular tinkering."-Following the famous concept of the molecular geneticist and medicine Nobel laureate François Jacob, in this review we describe the structural evolution of dopamine D3 receptor ligands from the natural agonist dopamine (DA) to highly potent and subtype selective new agents by bioisosteric tinkering with well-established and privileged or novel and fancy chemical functionalities and scaffolds. Some of the more than 200 ligands presented herein have already achieved therapeutic or scientific value up to now, some will most likely achieve it in the future. Hence, great importance is not only attached to the relationship between structure and activity of the ligands, but also to their utility as pharmacological tools in animal models or as therapeutics in patients with neurological diseases or other disorders.

Repeated administration of dopaminergic agents in the nucleus accumbens and morphine-induced place preference

In the present study, the effects of repeated intra nucleus accumbens (intra-NAc) injections of dopamine receptor agents on morphine-induced conditioned place preference (CPP) in rats were investigated by using an unbiased 3-days schedule of place conditioning design. The animals receiving once daily subcutaneous (s.c.) injections of morphine (0.5-7.5mg/kg) or saline (1.0 ml/kg, s.c.) showed a significant place preference in a dose-dependent manner. The maximum response was observed with 5mg/kg of the opioid. Three days intra-NAc injections of apomorphine (0.5 and 1 microg/rat) followed by 5 days free of the drug, increased or decreased, respectively CPP induced by the lower dose of morphine (0.5mg/kg, s.c.). Morphine-induced CPP was also significantly increased in the animals that had previously received the 3-days intra-NAc injections of SKF 38393 (4 and 8 microg/rat) or quinpirole (2 and 4 microg/rat, intra-NAc). The CPP induced by a higher dose of morphine (5mg/kg, s.c.) was significantly decreased in the animals that had previously received the 3-days SCH 23390 (0.005 and 0.01 microg/rat; intra-NAc). On the other hand, the CPP induced by morphine (5mg/kg, s.c.) was significantly increased in the animals that had previously received the 3-days sulpiride administration (5 microg/rat, intra-NAc). The 3-days administration of apomorphine, SKF 38393 or quinpirole, but not SCH 23390 and sulpiride reduced the locomotor activity in the test session. It is concluded that repeated injections of dopamine receptors agents followed by 5 days free of the drugs in the NAc can affect morphine reward.

Adenylyl cyclase type 5 (AC5) is an essential mediator of morphine action

Opioid drugs produce their pharmacological effects by activating inhibitory guanine nucleotide-binding regulatory protein-linked mu, delta, and kappa opioid receptors. One major effector for these receptors is adenylyl cyclase, which is inhibited upon receptor activation. However, little is known about which of the ten known forms of adenylyl cyclase are involved in mediating opioid actions. Here we show that all of the major behavioral effects of morphine, including locomotor activation, analgesia, tolerance, reward, and physical dependence and withdrawal symptoms, are attenuated in mice lacking adenylyl cyclase type 5 (AC5), a form of adenylyl cyclase that is highly enriched in striatum. Furthermore, the behavioral effects of selective mu or delta opioid receptor agonists are lost in AC5-/- mice, whereas the behavioral effects of selective kappa opioid receptor agonists are unaffected. These behavioral data are consistent with the observation that the ability of a mu or delta opioid receptor agonist to suppress adenylyl cyclase activity was absent in striatum of AC5-/- mice. Together, these results establish AC5 as an important component of mu and delta opioid receptor signal transduction mechanisms in vivo and provide further support for the importance of the cAMP pathway as a critical mediator of opioid action.

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.

Different effects of GABAergic receptors located in the ventral tegmental area on the expression of morphine-induced conditioned place preference in rat

In the present study, an unbiased conditioned place preference paradigm was used to study the effects of intra-ventral tegmental area injections of Gama-amino-butyric acid (GABA)-A and B (GABA(A) and GABA(B)) receptor agonists and antagonists on the expression of morphine-induced conditioned place preference (CPP) in rats. Subcutaneous (s.c.) injections of morphine sulfate (5 mg/kg) induced CPP. Intra-ventral tegmental area administration of the GABA(A) receptor agonist, muscimol (6 microg/rat) reduced the expression of morphine-induced CPP. Muscimol (25 microg/rat) increased the expression of CPP induced by morphine. A reduction of the expression of morphine-induced CPP was observed on intra-ventral tegmental area injection of GABA(A) receptor antagonist bicuculline (25 microg/rat). Bicuculline (10 microg/rat) increased the expression of CPP induced by morphine. Baclofen (12 microg/rat) increased where as (19 and 25 microg/rat) reduced the expression of morphine-induced CPP. Injection of CGP38345 (10, 19, 25 and 50 microg/rat) into the ventral tegmental area significantly reduced the expression of CPP induced by morphine. It is concluded that GABA(A) and GABA(B) receptor subtypes within the ventral tegmental area may have different effects on the expression of morphine-induced CPP.

The role of central dopamine D3 receptors in drug addiction: a review of pharmacological evidence

The cDNA for the dopamine D3 receptor was isolated and characterized in 1990. Subsequent studies have indicated that D3 receptors, as well as D3 receptor mRNA, are primarily localized in limbic regions in mammals. This finding led to the postulate that D3 receptors may be involved in drug dependence and addiction. However, this hypothesis has been difficult to test due to the lack of compounds with high selectivity for central D3 receptors. The interpretation of results from studies using mixed D2/D3 agonists and/or antagonists is problematic because these agents have low selectivity for D3 over D2 receptors and it is likely that their actions are primarily related to D2 receptor antagonism and possibly interaction with other neurotransmitter receptors. Currently, with the synthesis and characterization of new highly selective D3 receptor antagonists such as SB-277011-A this difficulty has been surmounted. The purpose of the present article is to review, for the first time, the effects of various putative D3 receptor selective compounds in animal models of drug dependence and addiction. The results obtained with highly selective D3 receptor antagonists such as SB-277011-A, SB-414796, and NGB-2904 indicate that central D3 receptors may play an important role in drug-induced reward, drug-taking, and cue-, drug-, and stress-induced reinstatement of drug-seeking behavior. Provided these results can be extrapolated to human drug addicts, they suggest that selective DA D3 receptor antagonists may prove effective as potential pharmacotherapeutic agents to manage drug dependence and addiction.

Role of the cholinergic system in the rat basolateral amygdala on morphine-induced conditioned place preference

The effects of intra-basolateral amygdala (intra-BLA) injections of physostigmine, atropine, nicotine and/or mecamylamine on morphine-induced conditioned place preference (CPP) in rats was investigated by using an unbiased 3-day schedule of place conditioning design. Animals that received 3 daily injections of morphine (0.5-10 mg/kg) subcutaneously (s.c.) or saline (1.0 ml/kg, s.c.) showed a significant preference for compartment paired with morphine. The maximum response was observed with 7.5 mg/kg of the opioid. Administration of the anticholinesterase drug, physostigmine (1, 3 and 5 microg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant CPP. Injections of antimuscarinic receptor agent, atropine (1, 4 and 7 microg/rat) dose-dependently inhibited the morphine (7.5 mg/kg)-induced place preference. The injections of nicotine (0.75, 1 and 2 microg/rat) potentiated the morphine (0.5 mg/kg)-induced place preference, while the nicotinic receptor antagonist, mecamylamine (1, 3 and 6 microg/rat) dose-dependently inhibited the morphine (7.5 mg/kg)-induced place preference. Furthermore, administration of atropine (7 microg/rat) but not mecamylamine (6 microg/rat) reduced the response induced by different doses of physostigmine plus morphine. Moreover, mecamylamine (6 microg/rat) but not atropine (7 microg/rat) reduced the response induced by different doses of nicotine plus morphine. It is concluded that the muscarinic and nicotinic receptor mechanisms in the BLA may be involved in the acquisition of morphine-induced place preference.

Ethanol-induced conditioned place preference is expressed through a ventral tegmental area dependent mechanism

The authors examined the role of the ventral tegmental area (VTA) and nucleus accumbens (NAc) in the expression of ethanol-induced conditioned place preference (CPP). After cannulas were implanted, male DBA/2J mice underwent an unbiased Pavlovian-conditioning procedure for ethanol-induced CPP. Before preference testing, the mice were injected intra-VTA (Experiments 1 and 3) or intra-NAc (Experiment 2) with the nonselective opioid antagonist methylnaloxonium (0-ng, 375-ng, or 750-ng total infusion; Experiments 1 and 2) or the gamma aminobutyric acid (GABA(B)) agonist baclofen (0-ng, 25-ng, or 50-ng total infusion; Experiment 3). Intra-VTA methylnaloxonium or baclofen decreased ethanol-induced CPP, whereas intra-NAc methylnaloxonium had no effect. These findings indicate that the conditioned rewarding effect of ethanol is expressed through a VTA-dependent mechanism that involves both opioid and GABA(B) receptors.

NMDA or AMPA/kainate receptor blockade prevents acquisition of conditioned place preference induced by D(2/3) dopamine receptor stimulation in rats

RATIONALE

Recent experiments from this laboratory demonstrated synergistic effects of AMPA/kainate receptor blockade and D(2/3) dopamine (DA) receptor stimulation on brain stimulation reward and locomotor activity.

OBJECTIVES

Using place conditioning, this study explored further the interaction between DA and glutamate (Glu) using the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, the AMPA/kainate receptor antagonist NBQX, and the D(2/3) DA receptor agonist 7-OH-DPAT.

METHODS

Effects of these compounds, alone and combined, were measured in male Sprague--Dawley rats using an unbiased two-compartment place conditioning procedure.

RESULTS

7-OH-DPAT (0.03--5.0 mg kg(-1), s.c.) administered immediately prior to conditioning was ineffective; when administered 15 min prior to conditioning, only the highest dose (5.0 mg kg(-1), s.c.) induced conditioned place preference (CPP). Acquisition of 7-OH-DPAT-induced CPP was blocked by MK-801 (0.06 or 0.13 mg kg(-1), i.p.) or NBQX (0.5 microg) microinjected into the nucleus accumbens (NAS) shell subregion. Intra-NAS shell administration of 7-OH-DPAT (5.0 microg) or NBQX (0.5 microg), alone or combined, failed to induce place conditioning, and this lack of effect was not due to state dependency. Administration of MK-801 or 7-OH-DPAT (5.0 mg kg(-1)) during the conditioning phase acutely increased horizontal activity, but neither compound, alone or combined, induced conditioned locomotor effects.

CONCLUSIONS

Acquisition of place conditioning induced by systemic administration of 7-OH-DPAT is blocked by systemic NMDA receptor antagonism by MK-801 or by the AMPA/kainate receptor antagonist NBQX microinjected into the NAS shell subregion.

Repeated administration of dopaminergic agents in the dorsal hippocampus and morphine-induced place preference

The aim of the present experiments was to investigate whether repeated intra-hippocampal CA1 (intra-CA1) administration of dopaminergic agents can affect morphine-induced conditioned place preference (CPP). Effects of repeated intra-CA1 injections of dopamine (DA) receptor agonists and antagonists on morphine-induced CPP in rats were investigated using an unbiased 3-day schedule of place conditioning. Animals receiving once-daily subcutaneous (s.c.) injections of morphine (1-9 mg/kg) or saline (1.0 ml/kg, s.c.) showed a significant place preference in a dose-dependent manner: the maximum response was observed with 3 mg/kg morphine. Three days' intra-CA1 injections of apomorphine (0.25-1 microg/rat) followed by 5 days free of the drug, significantly decreased morphine CPP (1 and 3 mg/kg, s.c.). Moreover, pre-treatment with the highest dose of apomorphine (1 microg/rat) altered the effect of morphine to an aversive response. The morphine (1 and 3 mg/kg) CPP was also significantly decreased in animals that previously received three intra-CA1 injections of SKF 38393 (2-9 microg/rat), quinpirole (1-3 microg/rat) or sulpiride (1-3 microg/rat), and significantly increased in animals that had previously received three intra-CA1 injections of SCH 23390 (0.02 microg/rat). The 3-day pre-treatment with apomorphine, SKF 38393 or quinpirole reduced locomotor activity in the test session, while SCH 23390 and sulpiride did not have any influence on locomotor activity. It is concluded that repeated injections of DA receptor agents in the dorsal hippocampus, followed by 5 days free of the drugs, can affect morphine reward.

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.

GABA(A) receptors in the basolateral amygdala are involved in mediating morphine reward

In the present study, the effects of intra-basolateral amygdala (BLA) injection of GABA(A) receptor agonist and antagonist on morphine-induced conditioned place preference (CPP) in male Wistar rats have been investigated. Subcutaneous (s.c.) administration of different doses of morphine sulfate (1-9 mg/kg) produced a dose-dependent CPP. Using a 3-day schedule of conditioning, it was found that the GABA(A) receptor agonist, muscimol (0.125, 0.25 and 0.5 microg/rat) or the GABA(A) receptor antagonist, bicuculline (0.125, 0.25 and 0.5 microg/rat), did not produce a significant place preference or place aversion. Intra-BLA administration of muscimol (0.25 and 0.5 microg/rat) decreased the acquisition of CPP induced by morphine (6 mg/kg). On the other hand, intra-BLA injection of bicuculline (0.25 and 0.5 microg/rat) in combination with an ineffective dose of morphine (1 mg/kg) elicited a significant CPP. The response of different doses of muscimol was attenuated by bicuculline (0.125 and 0.25 microg/rat). Furthermore, intra-BLA administration of bicuculline but not muscimol before testing significantly decreased the expression of morphine (6 mg/kg)-induced place preference. The administration of the higher doses of bicuculline (0.25 and 0.5 microg/rat) during acquisition and the higher dose of muscimol (2 microg/rat) on the test day decreased the locomotor activity of the animals on the testing phase. It can be concluded that GABA(A) receptors in the amygdala are involved in morphine reward.

The role of alpha-adrenoceptor mechanism(s) in morphine-induced conditioned place preference in female mice

It has been shown that the alpha-adrenergic system is involved in some effects of opioids, including analgesia and reward. Gender differences also exist between males and females in response to alpha-adrenergic agents. This study was designed to determine the effects of alpha-adrenoceptor agonists and antagonists on the acquisition or expression of morphine-induced conditioned place preference (CPP) in female mice. The experiments showed that subcutaneous injections of morphine (0.5-8 mg/kg) induced CPP in a dose-dependent manner in mice. Intrapritoneal administration of the alpha-1-adrenoceptor agonist, phenylephrine (0.03, 0.1 and 0.3 mg/kg), and alpha-2 adrenoceptor agonist, clonidine (0.0001, 0.0005 and 0.001 mg/kg), as well as alpha-1-adrenoceptor antagonist, prazosin (0.01, 0.05 and 0.1 mg/kg) or alpha-2 adrenoceptor antagonist, yohimbine (0.005, 0.01 and 0.05 mg/kg) did not induce motivational effects and also did not alter locomotor activity in the animals. In the second set of experiments, the drugs were used before testing on Day 5, to test their effects on the expression of morphine-induced CPP. Intrapritoneal administration of phenylephrine and clonidine decreased the expression of morphine-induced CPP. In contrast, after application of prazosin or yohimbine, the expression of morphine-induced CPP was increased. Administration of lower (0.03 mg/kg) and higher doses of phenylephrine (0.1 and 0.3 mg/kg) during acquisition of morphine CPP decreased and increased the morphine CPP, respectively. Similarly, the administration of prazosin and clonidine decreased while yohimbine increased the morphine CPP. It may be concluded that alpha-adrenoceptor mechanism(s) influence morphine-induced CPP in female mice.

Morphine-induced alterations of immune status are blocked by the dopamine D2-like receptor agonist 7-OH-DPAT

Morphine administration produces profound effects on the immune system, including reductions in natural killer cell activity, mitogen-induced lymphocyte proliferation, and cytokine production. Although it has been established that the activation of central nervous system (CNS) micro-opioid receptors by morphine induces immunomodulation, little is known about the neural mechanisms underlying such processes. Interestingly, it has been shown that the dopamine (DA) D2-like receptor agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) blocks the effect of morphine on a number of behaviors that are mediated by central dopamine pathways. The present study examined whether dopamine is involved in the immunomodulatory effects of morphine. In separate experiments, 7-OH-DPAT was administered either systemically (subcutaneous, s.c.) or centrally (intracerebroventricularly, i.c.v.) prior to morphine treatment in male Lewis rats. The results demonstrate that both systemic and central administration of 7-OH-DPAT attenuate the suppressive effect of morphine on several measures of immune status. Overall, these findings provide the first evidence that CNS dopaminergic mechanisms are directly involved in morphine-induced immunomodulation.

Involvement of dopamine receptors of the dorsal hippocampus on the acquisition and expression of morphine-induced place preference in rats

In the present study, the effects of bilateral intrahippocampal CA1 injections of dopamine receptor agonists and antagonists on the acquisition and expression of morphine-induced place preference were examined in male Wistar rats. Subcutaneous administration of different doses of morphine sulphate (0.5-10 mg/kg) produced a conditioned place preference (CPP) dose-dependently. Using a 3-day schedule of conditioning, it was found that dopamine D1 receptor agonist, SKF 38393 (0.01-1 microg/rat), dopamine D1 receptor antagonist, SCH 23390 (0.25-1 microg/rat), dopamine D(2/3) receptor agonist, quinpirole (0.3-3 microg/rat) or dopamine D2 receptor antagonist, sulpiride (0.04-5 microg/rat) did not produce significant place preference. The administration of SKF 38393 (1 microg/rat) significantly potentiated the acquisition of morphine (0.5 and 2.5 mg/kg)-induced place preference. This potentiation was reversed by SCH 23390 (1 microg/rat) pretreatment. Quinpirole injection (0.3 microg/rat) induced CPP in combination with the lower doses of morphine but decreased the response of the higher doses of morphine. These responses of quinpirole were reversed by sulpiride (5 microg/rat) pretreatment. SCH 23390 or sulpiride reduced the acquisition of morphine (7.5 mg/kg)-induced place preference. The administration of sulpiride, but not other drugs, during acquisition showed an increase in the locomotor activity on the testing days. SKF 38393, SCH 23390 or sulpiride, but not quinpirole when used before testing, reduced the expression of morphine-induced place preference. Sulpiride, but not other drugs, increased locomotion when used before testing. It is concluded that dorsal hippocampal dopamine receptors may play an active role in morphine reward.

Effects of a dopamine D3 receptor ligand, BP 897, on acquisition and expression of food-, morphine-, and cocaine-induced conditioned place preference, and food-seeking behavior in rats

The present study addressed the role of dopaminergic D(3) receptors (D(3)R) in motivational processes in rats. The effects of the selective D(3)R partial agonist, BP 897 (0.25-1 mg/kg, i.p.), on the establishment and the expression of conditioned place preference (CPP) supported by food, morphine (4 mg/kg, s.c.), or cocaine (2 mg/kg, s.c.) were investigated using an unbiased, one-compartment, place-conditioning procedure. When administered alone, BP 897 (0.05-2 mg/kg, i.p.) did not support CPP; on the contrary, conditioned place avoidance (CPA) was observed at 1 mg/kg, suggesting that this dose of BP 897 could be perceived as aversive. When given before each cocaine injection during the conditioning phase, BP 897 (1 mg/kg) prevented the establishment of CPP, and a single administration of BP 897 (0.5 and 1 mg/kg) before the test session impaired the expression of cocaine CPP. In contrast, neither the establishment nor the expression of food- and morphine-CPP were significantly altered by BP 897 (up to 1 mg/kg), whereas the full but less selective D(3)/D(2)R agonists, 7-OH-DPAT (0.5-2 mug/kg, s.c.) and quinelorane (1 mug/kg, s.c.), prevented the acquisition of food CPP. In a within-session extinction schedule of lever pressing for food, BP 897 (0.06-2 mg/kg) was ineffective in potentiating response reinstatement induced by the noncontingent delivery of two food pellets, in contrast with quinelorane and 7-OH-DPAT where previous studies showed to be efficient in this respect (Duarte et al, 2003). These results indicate that BP 897 has no positive appetitive value on its own, and that a moderate degree of stimulation of D(3)R is not sufficient to modulate food-primed food-seeking behavior or alter incentive motivation for food, morphine, and/or their associated cues. However, D(3)R are likely involved in the perception of the rewarding value of cocaine and cocaine-paired cues. This suggests that the appetitive effects of cocaine are subserved by mechanisms different, at least in part, from those of morphine and food, and that D(3)R play a role only in the former.

Sulpiride injections into the medial septum reverse the influence of intra-medial septum injection of L-arginine on expression of place conditioning-induced by morphine in rats

Effects of intra-medial septum injections of L-arginine, a precursor of nitric oxide, N(G)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase, and sulpiride, a selective antagonist of dopamine D2 receptor on morphine-induced conditioned place preference (CPP) in male Wistar rats were examined. Using a 3-day schedule of conditioning, morphine (0.5-7.5 mg/kg, s.c.) produced a significant place preference in a dose-dependent manner. The maximum response was observed with 5.0 mg/kg of opioid. Sulpiride (0.3, 1.0 and 3.0 microg/rat), but not L-arginine (0.3, 1.0 and 3.0 microg/rat) or L-NAME (0.3, 1.0 and 3.0 microg/rat), in combination with morphine (5.0 mg/kg), during conditioning, significantly altered morphine-induced CPP. Single doses (0.3, 1.0 and 3.0 microg/rat) of either L-arginine or L-NAME, during conditioning, did not induce CPP. Sulpiride at 0.3-3.0 microg/rat, intra-medial septum, during conditioning, produced a significant conditioned place aversion. Intra-medial septum injections of L-arginine but not L-NAME or sulpiride, 1-2 min before testing, increased the expression of morphine-induced CPP. The administration of sulpiride (0.3, 1.0 and 3.0 microg/rat), but not L-NAME (0.3, 1.0 and 3.0 microg/rat), 1-2 min before the injection of L-arginine (0.3 microg/rat) on day of test, significantly attenuated the response to L-arginine. L-Arginine (0.3-3.0 microg/rat), during conditioning, showed a statistically significant increase in locomotor activity compared with that to control group. Moreover, sulpiride decreased locomotion by itself or in combination with morphine during conditioning and on the test day of morphine CPP. It can be concluded that L-arginine, a precursor of nitric oxide, in the rat median septum may play a role in expression of morphine conditioning due to dopamine release in this area.

Involvement of dopamine D1 receptors of the central amygdala on the acquisition and expression of morphine-induced place preference in rat

In the present study, the effects of intra-central amygdala (CeA) injection of dopamine D1 receptor agonist and antagonist on morphine-induced conditioned place preference (CPP) were investigated in male Wistar rats. Our data showed that subcutaneous (s.c.) injection of morphine sulphate (0.5-10 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. Intra-CeA administration of the dopamine D1 receptor agonist, SKF 38393 (2 and 4 micro g/rat) with an ineffective dose of morphine (0.5 mg/kg), elicited a significant conditioned place preference. On the other hand, a single dose of SKF 38393 (2 micro g/rat, intra-CeA) in combination with the lower doses (0.5 and 2.5 mg/kg), but not with the higher doses of morphine potentiated morphine-induced CPP. Furthermore, intra-CeA administration of the dopamine D1 receptor antagonist, SCH 23390 (0.5-1 micro g/rat) decreased the acquisition of conditioned place preference induced by morphine (7.5 mg/kg). The response of SKF 38393 was decreased by SCH 23390 (0.75 micro g/rat). SKF 38393 or SCH 23390 by themselves did not elicit any effect on place conditioning. On the other hand, intra-CeA administration of SKF 38393 or SCH 23390 significantly decreased the expression of morphine (7.5 mg/kg)-induced place preference. SKF 38393 or SCH 23390 injections into the CeA had no effects on the locomotor activity on the test sessions. The results indicate that the dopamine D1 receptors in the CeA may be involved in the acquisition and expression of morphine-induced place preference.

Dopamine D3 receptor ligands show place conditioning effect but do not influence cocaine-induced place preference

The importance of dopamine D3 receptors in reward related processes, especially in cocaine addiction, has been investigated extensively. However, in the reported studies a combination of different experimental conditions and different ligands have been used which renders the interpretation and comparison of the diverse results extremely difficult. Here, we report one comparative study investigating a wide range of dopamine D3 receptor ligands in one model of cocaine abuse: the place conditioning paradigm in rats. Of the antagonists tested, the moderately D3 selective nafadotride and the more selective SB-277011 did not produce any place conditioning effect while U-99194A caused place-preference. The most D3 selective agonist PD-128907, the less selective 7-OH-DPAT and the moderately selective partial agonist BP-897 all caused significant place aversion. None of the compounds influenced the cocaine-induced place preference. Results suggest the D3-preferring agonists could affect the reward mechanisms of the brain, however, modulation of D3 receptor function does not appear to be a significant mechanism for modifying the place conditioning effect of cocaine.

Cross-tolerance between morphine- and nicotine-induced conditioned place preference in mice

The acquisition of morphine and nicotine conditioned place preference (CPP) and cross-tolerance between the response of two drugs was studied in mice. A biased CPP paradigm was used to study the effect of the agents. Morphine (5 mg/kg) and nicotine (1 mg/kg) induced CPP. Naloxone (0.5, 1 and 2 mg/kg), but not mecamylamine (0.025, 0.05 and 0.1 mg/kg), induced conditioned place aversion (CPA). Both antagonists reversed CPP induced by morphine and nicotine. Administration of one daily dose of morphine (12.5, 25 or 50 mg/kg) for 3 days or nicotine (0.5, 1 or 2 mg/kg) three times a day for 12 days, in order to develop tolerance to the drugs, reduced the conditioning induced by morphine (5 mg/kg) or nicotine (1 mg/kg). CPA-induced by naloxone was reduced in animals, which were rendered tolerant to morphine (50 mg/kg) or nicotine (2 mg/kg). Mecamylamine, however, which did not induce any response in the nontolerant mice, elicited CPP in the tolerant animals. It is concluded that there may be a cross-tolerance between morphine- and nicotine-induced CPP.