Chronic treatment with five different neuroleptics elicits behavioral supersensitivity to opiate infusion into the nucleus accumbens

Opioid-induced rewards, locomotion, and dopamine activation: A proposed model for control by mesopontine and rostromedial tegmental neurons

Opioids, such as morphine or heroin, increase forebrain dopamine (DA) release and locomotion, and support the acquisition of conditioned place preference (CPP) or self-administration. The most sensitive sites for these opioid effects in rodents are in the ventral tegmental area (VTA) and rostromedial tegmental nucleus (RMTg). Opioid inhibition of GABA neurons in these sites is hypothesized to lead to arousing and rewarding effects through disinhibition of VTA DA neurons. We review findings that the laterodorsal tegmental (LDTg) and pedunculopontine tegmental (PPTg) nuclei, which each contain cholinergic, GABAergic, and glutamatergic cells, are important for these effects. LDTg and/or PPTg cholinergic inputs to VTA mediate opioid-induced locomotion and DA activation via VTA M5 muscarinic receptors. LDTg and/or PPTg cholinergic inputs to RMTg also modulate opioid-induced locomotion. Lesions or inhibition of LDTg or PPTg neurons reduce morphine-induced increases in forebrain DA release, acquisition of morphine CPP or self-administration. We propose a circuit model that links VTA and RMTg GABA with LDTg and PPTg neurons critical for DA-dependent opioid effects in drug-naïve rodents.

Contribution of Delta-Opioid Receptors to Pathophysiological Events Explored by Endogenous Enkephalins

Very few discoveries in the neurosciences have triggered clinical speculation and experimentation regarding the etiology of psychiatric illness to the same extent as that following identification of the opiate receptor(s) and subsequent isolation of endogenous morphine-like peptides. There is overwhelming evidence in animals and in human that opioids are involved in behaviorally relevant issues such as the modulation of pain, the response to stress, motivation, addiction, sexuality, food intake, etc., but our knowledge on the possible relation between opioids and mental illness is still very limited.These responses could be explored eitheir by using higlhy selective delta agonist or by emphasizing the effects of phasically secreted endogenous opioid peptides, enkephalin. Both approaches were investigated in particular through protection of enkephalin degradation by dual enkephalinase ihibitors DENKIs such as RB101, PL37 or PL265.

Can antipsychotic treatment contribute to drug addiction in schizophrenia?

Individuals with schizophrenia are at very high risk for drug abuse and addiction. Patients with a coexisting drug problem fare worse than patients who do not use drugs, and are also more difficult to treat. Current hypotheses cannot adequately account for why patients with schizophrenia so often have a co-morbid drug problem. I present here a complementary hypothesis based on evidence showing that chronic exposure to antipsychotic medications can induce supersensitivity within the brain's dopamine systems, and that this in turn can enhance the rewarding and incentive motivational effects of drugs and reward cues. At the neurobiological level, these effects of antipsychotics are potentially linked to antipsychotic-induced increases in the striatal levels of dopamine D2 receptors and D2 receptors in a high-affinity state for dopamine, particularly at postsynaptic sites. Antipsychotic-induced dopamine supersensitivity and enhanced reward function are not inevitable consequences of prolonged antipsychotic treatment. At least two parameters appear to promote these effects; the use of antipsychotics of the typical class, and continuous rather than intermittent antipsychotic exposure, such that silencing of dopaminergic neurotransmission via D2/3 receptors is unremitting. Thus, by inducing forms of neural plasticity that facilitate the ability of drugs and reward cues to gain control over behaviour, some currently used treatment strategies with typical antipsychotics might contribute to compulsive drug seeking and drug taking behaviours in vulnerable schizophrenia patients.

Role of mu- and delta-opioid receptors in the nucleus accumbens in cocaine-seeking behavior

Earlier studies suggest that opioid receptors in the ventral tegmental area, but not the nucleus accumbens (NAc), play a role in relapse to drug-seeking behavior. However, environmental stimuli that elicit relapse also release the endogenous opioid beta-endorphin in the NAc. Using a within-session extinction/reinstatement paradigm in rats that self-administer cocaine, we found that NAc infusions of the mu-opioid receptor (MOR) agonist DAMGO moderately reinstated responding on the cocaine-paired lever at low doses (1.0-3.0 ng/side), whereas the delta-opioid receptor (DOR) agonist DPDPE induced greater responding at higher doses (300-3000 ng/side) that also enhanced inactive lever responding. Using doses of either agonist that induced responding on only the cocaine-paired lever, we found that DAMGO-induced responding was blocked selectively by pretreatment with the MOR antagonist, CTAP, whereas DPDPE-induced responding was selectively blocked by the DOR antagonist, naltrindole. Cocaine-primed reinstatement was blocked by intra-NAc CTAP but not naltrindole, indicating a role for endogenous MOR-acting peptides in cocaine-induced reinstatement of cocaine-seeking behavior. In this regard, intra-NAc infusions of beta-endorphin (100-1000 ng/side) induced marked cocaine-seeking behavior, an effect blocked by intra-NAc pretreatment with the MOR but not DOR antagonist. Conversely, cocaine seeking elicited by the enkephalinase inhibitor thiorphan (1-10 microg/side) was blocked by naltrindole but not CTAP. MOR stimulation in more dorsal caudate-putamen sites was ineffective, whereas DPDPE infusions induced cocaine seeking. Together, these findings establish distinct roles for MOR and DOR in cocaine relapse and suggest that NAc MOR could be an important therapeutic target to neutralize the effects of endogenous beta-endorphin release on cocaine relapse.

Changes in central dopaminergic systems and morphine reward by prenatal and neonatal exposure to bisphenol-A in mice: evidence for the importance of exposure period

Bisphenol-A has been extensively evaluated for toxicity in a variety of tests as the most common environmental endocrine disruptors. In a previous study, we reported that exposure to bisphenol-A affects the development of the central dopaminergic system in the mouse limbic area. The present study was undertaken to investigate the relationship between the developmental toxicity of bisphenol-A and its exposure period. The exposure to bisphenol-A during either organogenesis or lactation, but not implantation and parturition, significantly enhanced the morphine-induced hyperlocomotion and rewarding effects. Furthermore, exposure to bisphenol-A during either organogenesis or lactation also produced an up-regulation of dopamine receptor function to activate G-protein in the mouse limbic forebrain. These results indicate that both organogenesis and lactation are more sensitive to the bisphenol-A-induced developmental neuronal toxicology than any other periods. In conclusion, the present data suggest that the organogenesis and lactation are the most important period to cause the alternation of dopaminergic system by bisphenol-A exposure in the mouse.

Facilitation of enkephalins-induced delta-opioid behavioral responses by chronic amisulpride treatment

The endogenous opioid system is known to have a great influence on the dopaminergic system. Conversely, blockade of the dopaminergic system in D2 receptor knock-out mice triggers an increase in enkephalin supporting the important physiological relationship between both systems. Therefore, the aim of this study was to investigate whether or not chronic treatment with the specific D2 antagonist amisulpride (20mg/kg, i.p., twice daily for 5 days) could lead to a facilitation of behavioral effects of enkephalins, protected from their enzymatic degradation by the dual inhibitor N-[(R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyl dithio]-1-oxopropyl]-l-phenylalanine benzyl ester (RB101) (5mg/kg, i.v.) in mice. RB101 induced an increase in locomotor activity, antidepressant-like effects in the forced swim test, and antinociceptive effects in the hot-plate test. Chronic treatment with amisulpride potentiated the action of RB101 and this effect seemed to be restricted to behavioral responses induced by opioids acting on delta-opioid receptors (locomotor activity and forced swim test). This was confirmed by the use of the selective delta-opioid receptor agonist, (+)-4-[alpha-R*)-alpha-((2S*,5R*)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80; 2.5mg/kg, i.p.), and antagonist, naltrindole (5mg/kg, i.p.). Considering the involvement of delta-opioid receptors in mood regulation, the interaction between amisulpride and RB101 could lead to a new therapeutic approach in the treatment of some mood disorders.

Prenatal and neonatal exposure to bisphenol-A affects the morphine-induced rewarding effect and hyperlocomotion in mice

Bisphenol-A (BPA), one of the most common environmental endocrine disrupters, has been extensively evaluated for toxicity and carcinogenicity. However, little is still known about its action on the CNS. Here we found that prenatal and neonatal exposure to BPA resulted in the enhancement of the rewarding effect and hyperlocomotion induced by morphine in mice. Under these conditions, no change in the G-protein activation by morphine and mu-opioid receptor expression in the lower midbrain was observed by prenatal and neonatal exposure to BPA. These results suggest that chronic exposure to BPA produces the supersensitivity of the morphine-induced rewarding effect and hyperlocomotion without direct changes in mu-opioid receptor function in the lower midbrain. The present data provide further evidence that prenatal and neonatal exposure to BPA can directly influence the development of the central dopaminergic system.

Molecular evidence for the functional role of dopamine D3 receptor in the morphine-induced rewarding effect and hyperlocomotion

The aim of the present study was to investigate the role of dopamine D(3) receptors in the rewarding effect and hyperlocomotion induced by a prototypical mu-opioid receptor agonist morphine using dopamine D(3) receptor knock-out mice. The mu-opioid receptor in the brain determined by the [tylosil-3,5-(3)H(N)]-[D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin binding assay was not significantly changed by a deletion of the dopamine D(3) receptor gene. Furthermore, we found that no significant differences in G-protein activation by morphine in the limbic forebrain and lower midbrain were noted between the two genotypes. These results suggest that the function of the mu-opioid receptor itself was not affected by a deletion of the dopamine D(3) receptor gene. To ascertain the morphine-induced rewarding effect in both genotypes, the conditioned place preference paradigm was performed. Deletion of the dopamine D(3) receptor gene resulted in a remarkable enhancement of the morphine-induced rewarding effect. Furthermore, knock-out mice with deletions of the dopamine D(3) receptor revealed a dramatic potentiation of morphine-induced hyperlocomotion. Under these conditions, a loss of the dopamine D(3) receptor gene had no effect on the basal levels of dopamine and the increased dopamine turnover by morphine in the limbic forebrain. These findings provide further evidence that dopamine D(3) receptor contributes to the postsynaptically negative modulation of the mesolimbic dopaminergic pathway that is associated with the rewarding effect and hyperlocomotion through the stimulation of mu-opioid receptors induced by morphine in the mouse.

Brain reward circuitry: insights from unsensed incentives

The natural incentives that shape behavior reach the central circuitry of motivation trans-synaptically, via the five senses, whereas the laboratory rewards of intracranial stimulation or drug injections activate reward circuitry directly, bypassing peripheral sensory pathways. The unsensed incentives of brain stimulation and intracranial drug injections thus give us tools to identify reward circuit elements within the associational portions of the CNS. Such studies have implicated the mesolimbic dopamine system and several of its afferents and efferents in motivational function. Comparisons of natural and laboratory incentives suggest hypotheses as to why some habits become compulsive and give insights into the roles of reinforcement and of prediction of reinforcement in habit formation.

Involvement of dopamine-dependent and -independent mechanisms in the rewarding effects mediated by delta opioid receptor subtypes in mice

The rewarding effects of the delta 1 opioid receptor agonist [D-Pen2, Pen5]enkephalin (DPDPE) and the delta 2 opioid receptor agonist [D-Ala2]deltorphin II (DELT) on the activity of mesolimbic and nigrostriatal dopamine (DA) neurons were examined in mice. Both DPDPE (15 nmol, i.c.v.) and DELT (5 nmol, i.c.v.) produced a significant place preference in mice. The DPDPE (15 mol, i.c.v.)-induced place preference was abolished by 7-benzylidenenaltrexone (BNTX; 0.5 mg/kg, s.c.), a delta1 opioid receptor antagonist, but not by naltriben (NTB; 0.5 mg/kg, s.c.), a delta 2 opioid receptor antagonist. In contrast, the DELT (5 nmol, i.c.v.)-induced place preference was antagonized by NTB, but not BNTX. I.c.v.. injection of DPDPE, but not DELT, at a dose that produced a significant place preference produced a significant elevation of DA turnover in the mouse limbic forebrain, and this effect of DPDPE was antagonized by BNTX but not by NTB. In addition, i.c.v. injection of DPDPE or DELT not affect DA turnover in the mouse striatum. These results suggest that the rewarding effects produced by the activation of central delta 1, but not delta 2, opioid receptors may be caused through the enhancement of the mesolimbic DA neurotransmission, and confirm our previous hypothesis that the DA-dependent and -independent mechanisms may exist in the rewarding effects produced by the activation of central delta opioid receptor subtypes.

The effects of dopamine D1 and D2 receptor antagonists on the rewarding effects of delta 1 and delta 2 opioid receptor agonists in mice

The effects of the dopamine D1 antagonist SCH23390 and the D2 antagonist sulpiride on the rewarding effects of delta opioid receptor agonists were examined in mice. Both [D-Pen2, Pen5]enkephalin (DPDPE, 1-15 nmol, ICV), a selective delta 1 opioid receptor agonist, and [D-Ala2]deltorphin II (DELT, 0.5-5 nmol, ICV), a selective delta 2 opioid receptor agonist, produced a dose-dependent place preference in mice. The DPDPE (15 nmol, ICV)-induced place preference was abolished by BNTX (0.5 mg/kg, SC), a delta 1 opioid receptor antagonist, but not by NTB (0.5 mg/kg, SC), a delta 2 opioid receptor antagonist. In contrast, the DELT (5 nmol, ICV)-induced place preference was antagonized by NTB, but not BNTX. Pretreatment with SCH23390 (3 micrograms/kg, SC) abolished the DPDPE-induced place preference, but not affect the DELT-induced place preference. Moreover, pretreatment with sulpiride (40 mg/kg, SC) did not modify the place preference induced by DPDPE or DELT. In the present study, we found that the activation of both central delta 1 and delta 2 opioid receptors produced rewarding effects. Furthermore, these results suggest that the rewarding effects of delta 1 opioid receptor agonist may be produced through activation of the central dopaminergic system, especially dopamine D1 receptors, whereas the rewarding effects of delta 2 opioid receptor agonists may be produced by some other mechanism(s).

The role of dopamine in drug abuse viewed from the perspective of its role in motivation

Drugs of abuse share with conventional reinforcers the activation of specific neural pathways in the CNS that are the substrate of their motivational properties. Dopamine is recognized as the transmitter of one such neural pathway, being involved in at least three major aspects of motivation: modulation of motivational state, acquisition (incentive learning) and expression of incentive properties by motivational stimuli. Drugs of abuse of different pharmacological classes stimulate in the low dose range dopamine transmission particularly in the ventral striatum. Apart from psychostimulants, the evidence that stimulation of dopamine transmission by drugs of abuse provides the primary motivational stimulus for drug self-administration is either unconvincing or negative. However, stimulation of dopamine transmission is essential for the activational properties of drugs of abuse and might be instrumental for the acquisition of responding to drug-related incentive stimuli (incentive learning). Dopamine is involved in the induction and in the expression of behavioural sensitization by repeated exposure to various drugs of abuse. Sensitization to the dopamine-stimulant properties of specific drug classes leading to facilitation of incentive learning of drug-related stimuli might account for the strong control over behaviour exerted by these stimuli in the addiction state. Withdrawal from drugs of abuse results in a reduction in basal dopamine transmission in vivo and in reduced responding for conventional reinforcers. Although these changes are likely to be the expression of a state of dependence of the dopamine system their contribution to the motivational state of drug addiction is unclear.

Effects of chronic naloxone administration on vacuous chewing movements and catalepsy in rats treated with long-term haloperidol decanoate

Most antipsychotic medications produce motoric side effects, including parkinsonism and tardive dyskinesia (TD). Correlates of these behaviors in rats (catalepsy and vacuous chewing movements, respectively) were used as a model to assess the usefulness of chronic naloxone administration in symptom reduction. Previous studies have suggested that increased neurotransmission in the endogenous opioid system modulates neuroleptic-induced motoric side effects. Rats were treated with haloperidol decanoate or vehicle for 27 weeks, and withdrawn for 30 weeks. Subsequently, naloxone (0.5 to 2.0 mg/kg SC twice daily) was given for 5 weeks. Long-term haloperidol treatment produced a syndrome of vacuous chewing movements (VCMs) that persisted during the drug withdrawal period. Catalepsy developed rapidly and also persisted. Naloxone treatment had little effect on VCMs but increased catalepsy scores in both haloperidol and vehicle treated groups. Naloxone reduced rearing and grooming in haloperidol rats while increasing these measures in vehicle treated rats. The results indicate that neuroleptic-induced motoric side effects are not reversed by naloxone in rats. Furthermore, they suggest that increased opioid neurotransmission may not underlie the expression of VCMs. This does not rule out the possibility that endogenous opioid system may be involved in the development of VCMs. To the extent that this animal model is valid, naloxone may not be effective in treating TD and neuroleptic-induced parkinsonism in humans.

N-methyl-D-aspartic acid-induced lesions of the nucleus accumbens and/or ventral pallidum fail to attenuate lateral hypothalamic self-stimulation reward

The role of ventral striatum in the maintenance and transmission of a hypothalamic intracranial self-stimulation (ICSS) reward signal was investigated using the rate-frequency multiple-curve shift paradigm. The excitotoxin N-methyl-D-aspartic acid (NMDA) was bilaterally administered into the nucleus accumbens (15 micrograms per side), the ventral pallidum (15 micrograms per side) or the juncture between the two structures (20 micrograms per side) creating three lesion groups. Both the nucleus accumbens (NAC) lesion group and the ventral pallidum (VP) lesion group displayed substantial NMDA-induced damage which was generally restricted to the intended limbic structure. The NMDA lesions in the third group displayed extensive damage to both the NAC and VP, as intended, but also typically diffused into adjacent medial structures. NMDA-induced lesions in all groups caused a suppression in motor/performance activity at all currents tested. Contrary to motor effects, reward efficacy was relatively unaffected for the NAC and VP groups. The lack of reward effects may be due to plasticity of neuronal systems and redundancy of circuit connections. However, this explanation is questionable given the fact that NMDA lesions which encompassed both the NAC and VP had little effect on reward efficacy. The above data suggests that the nucleus accumbens and the ventral pallidum are not critical for ICSS rewards stimulation and that hypothalamic ICSS reward signals are processed downstream from these limbic structures.

Mesoaccumbens dopamine-opiate interactions in the control over behaviour by a conditioned reinforcer

These experiments examined the role of dopamine-opiate interactions in the ventral tegmental area (VTA) and nucleus accumbens in the mediation of reinforcement-related behaviour. It has been shown previously that opiates induce a dopamine-dependent increase in locomotor activity in rats when infused into the VTA, and a dopamine-independent hyperactivity when infused into the nucleus accumbens. The present study investigated the generality and significance of these two findings, by examining dopamine-opiate interactions in the control over behaviour exerted by a conditioned reinforcer (CR), an arbitrary stimulus which gains control by association with primary reinforcement. Rats were trained to associate a light/noise stimulus with sucrose reinforcement, and the efficacy of the CR in controlling behaviour was assessed by measuring its ability to support a new lever pressing response. Responding on one lever (CR lever) produced the CR, responding on the other lever had no programmed consequences. In experiment 1, intra-accumbens infusions of d-amphetamine (10 micrograms), the D1 dopamine receptor agonist SKF-38393 (0.1 microgram), the D2 dopamine receptor agonist LY-171555 (quinpirole; 0.1 microgram) or the opiate receptor agonist [D-Ala2]-methionine enkephalinamide (DALA; 1 microgram) selectively increased responding on the CR lever. Infusion with DALA intra-VTA had no effect. However, pretreatment with DALA intra-VTA (10 x 1 microgram/day) subsequently reduced the selectivity of the response to infusions intra-accumbens with d-amphetamine or SKF-38393, and blocked the response to LY-171555 or DALA. Pretreatment also shifted to the right the dose-response function for DALA intra-accumbens. In experiment 2, intra-accumbens infusions of d-amphetamine, SKF-38393, LY-171555 or DALA again increased responding on the CR lever only. Pretreatment with intra-accumbens d-amphetamine (5 x 1 microgram/day) reduced the selectivity of the response subsequently to d-amphetamine, and blocked the response to SKF-38393, LY-171555 or DALA. In experiment 3, intra-accumbens infusions of the mu-opiate receptor agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (0.003-0.1 microgram), or the delta-opiate receptor agonist [D-Pen2,5]-enkephalin (0.03-1 microgram) enhanced selectively responding on the CR lever. Thus, the dopamine-dependent locomotor-stimulant properties of intra-VTA infusions of opiates are associated with impaired conditioned reinforcer efficacy. Finally, repeated stimulation of the mesoaccumbens dopamine pathway may compromise the dopamine-independence of the opiate system within the nucleus accumbens.

The opiate antagonist naloxone suppresses a rodent model of tardive dyskinesia

The effects of both opiate agonists and the opiate antagonist naloxone were examined in a rodent model of tardive dyskinesia (TD). Chronic (approximately 20 weeks) administration of fluphenazine resulted in the emergence of vacuous chewing mouth movements (VCMs), a response which may be a useful model for this disorder. Fluphenazine-induced VCMs were not affected by a variety of selective opiate agonists administered intracerebroventricularly, but were potently suppressed by subcutaneous administration of the opiate antagonist naloxone. These findings suggest that increased opiate transmission may contribute to the pathogenesis of TD. Further investigation of the role of opiate antagonists in treating this disorder are warranted.

Reward and abuse of opiates

The dependence creating properties of drugs are mediated by structures in the brain. The mesolimbic system seems to play a crucial role in the behaviourally reinforcing effects of opiates and other drugs of abuse. The significance of dopamine in opiate reinforcement is still a matter of debate, in spite of the large number of studies on this subject. Dopamine appears to be involved in conditioning processes and in drug self-administration behaviour only once it has been established. Neuropeptides, centrally active fragments of hormones, may play a role in the individual vulnerability for the development of drug dependence. Administration of a number of wellknown neuropeptides attenuates the acquisition of drug self-administration behaviour. The virtues and flaws of some widely used animal models for drug dependence are discussed.

Localization of drug reward mechanisms by intracranial injections

Intracranial drug injections are useful in localizing brain areas where drugs of abuse initiate their habit-forming actions. However, serious methodological problems accompany such studies. Pharmacological controls are necessary to assess non-receptor-mediated local actions of the drug, anatomical controls are necessary to rule out drug efflux to distal sites of action, and behavioral controls are necessary to separate rewarding from general activating effects of drugs. Five brain sites have been advanced as sites of rewarding opiate actions: the ventral tegmental area (VTA), nucleus accumbens septi (NAS), lateral hypothalamus, periaqueductal gray, and hippocampus. Current evidence appears to confirm two of these--VTA and NAS; evidence is currently incomplete in the case of the hippocampus and is conflicting in the case of the lateral hypothalamus and periaqueductal gray. Two sites have been advanced as sites of rewarding psychomotor stimulant actions: NAS and the frontal cortex; each site seems implicated, but puzzling differences between amphetamine and cocaine findings remain to be resolved. Each of the clearly implicated sites is local to dopamine cell bodies or dopamine terminals that have been implicated in the rewarding effects of brain stimulation, food, and sex.

Morphine and dynorphin(1-13) microinjected into the medial preoptic area and nucleus accumbens: effects on sexual behavior in male rats

The effects on sexual behavior of opiate receptor stimulation within A10 and A14 terminal areas were examined in the following experiments. Morphine (0.01-6 nmol) and dynorphin(1-13) (0.01-3 pmol) were microinjected into the medial preoptic area (MPOA). Morphine (10-100 pmol) and dynorphin (10-100 fmol) injected into the MPOA reduced both the latency to ejaculate and the number of intromissions triggering ejaculation. Morphine (6 nmol) produced a failure to resume copulating following the second ejaculation. Morphine (1-10 nmol) injected into the nucleus accumbens (ACC) shortened the latency to the first intromission and lengthened the second postejaculatory interval. Naloxone (3 mg/kg i.p.) reversed the effects of morphine on intromission latency and attenuated the lowering of ejaculatory threshold.

Effects induced by chronic treatment with selective D1 or D2 antagonists on open-field behavior and colonic temperature

We have studied the behavioral responses in open-field and the changes in body temperature induced after chronic treatment with a selective D1 antagonist, SCH 23390, a selective D2+ antagonist, sulpiride, or a non specific but preferential D2 antagonist, haloperidol. After chronic treatment with SCH 23390 or sulpiride, rats were challenged with SKF 38393, selective D1 agonist, or LY 171555, selective D2 agonist, in order to study the responses of D1 and D2 stimulation. After chronic SCH 23390, an increase of the locomotion and of the number of rears were observed whereas, no changes were induced by chronic sulpiride or haloperidol. Acute treatment with sulpiride blocked the hyperlocomotion induced by chronic SCH 23390. In naive rats acute administration of SKF 38393 or LY 171555 did not produce any change in locomotion or rearing. In rats treated chronically with SCH 23390 this acute administration of LY 171555 induced an increase of the number of squares and of the number of rears. In these animals, acute administration of SKF 38393 also augmented the number of squares crossed. In contrast, chronic sulpiride did not modify behavioral responses obtained after acute SKF 38393 or LY 171555. Colonic temperature was not changed after acute SKF 38393 while acute LY 171555 induced a hypothermia. Chronic sulpiride did not modify the responses of SKF 38393 or LY 171555, but an increase in body temperature was observed after acute SKF 38393 in animals chronically treated with SCH 23390. The present results support a different behavioral expression of D1 and D2 supersensitivity in rats. Furthermore, chronic treatment with a D1 antagonist induced facilitatory effects on D2 behavioral responses; however, these D1-D2 interactions were not observed in body temperature responses.

Similar behavioral and biochemical effects of long-term haloperidol and caerulein treatment in albino mice

Behavioral and biochemical experiments on male albino mice have revealed similar effects after the cessation of repeated (15 days) haloperidol (0.5 mg/kg daily IP) and caerulein (0.1 mg/kg daily SC) treatment. Tolerance developed to the action of muscimol (a GABA-A agonist, 1 mg/kg IP), caerulein (a CCK-8 agonist, 15 micrograms/kg SC) and flumazenil (a benzodiazepine antagonist, 10 mg/kg IP). Muscimol and caerulein were not able to suppress the motor activity of mice after 15 days treatment with haloperidol and caerulein. Flumazenil, which increased motor activity in saline-treated animals, also failed to affect activity after extended haloperidol or caerulein treatment. In contrast, the motor excitation induced by amphetamine (an indirect dopamine agonist, 3 mg/kg IP) was increased after haloperidol or caerulein administration. In radioligand binding studies the density of dopamine-2-receptors in striatum, opioid receptors in mesolimbic structures, and benzodiazepine and GABA-A receptors in brainstem was significantly elevated after long-term haloperidol or caerulein treatment. Simultaneously, the number of CCK-8, benzodiazepine and GABA-A receptors in cerebral cortex was decreased. It is probable that CCK-8-ergic mechanisms are involved closely in the action of repeated haloperidol treatment. CCK-8 seems to modulate the action of haloperidol through altering the sensitivity of dopamine, opioid, GABA-A and benzodiazepine receptors.

Chronic blockade of D2 but not D1 dopamine receptors facilitates behavioural responses to endogenous enkephalins, protected by kelatorphan, administered in the accumbens in rats

It has previously been shown that kelatorphan, (R)-3-(N-hydroxycarboxamido-2-benzyl-propanoyl)-L-alanine, a mixed inhibitor of the catabolism of enkephalins, injected into the nucleus accumbens, induced a dose-dependent hyperlocomotion in rats. In this study, the consequence of chronic treatment with sulpiride, a selective D2 dopamine receptor antagonist, SCH 23390, a selective D1 dopamine receptor antagonist, or haloperidol, a nonspecific but preferential D2 receptor antagonist, on the behavioural response induced by acute administration of kelatorphan into the accumbens, has been investigated in rats. The drug SCH 23390 did not modify the behavioural response to kelatorphan, whereas sulpiride and haloperidol induced an increase which was maximal in the third week after the beginning of treatment, a period corresponding to the appearance of the antipsychotic effect of the neuroleptics. This facilitation was reversed by prior administration of the delta-selective antagonist, ICI 174864. These results suggest that the phasic activity of enkephalinergic neurones of the nucleus accumbens and the associated behavioural hyperactivity are facilitated after chronic blockade of the D2 but not the D1 subtypes of dopamine receptor.

Chronic flupentixol treatment potentiates the reinforcing properties of systemic heroin administration

The behavioral effects of systemic heroin administration were examined in rats subjected to flupentixol impregnation prior to and during behavioral testing. In the first experiment, the dose of heroin required to produce a place preference was determined in two groups of rats, one of which had received chronic flupentixol decanoate (12 mg/kg, sc, every 10 days for 6 weeks) and the other which had received the palm oil vehicle during the same time period. It was found that whereas 60 micrograms/kg of heroin was required to produce a place preference in control rats, only 7.5 micrograms/kg was sufficient to do so in chronic neuroleptic-treated rats. In a second experiment, the locomotor activating effect of heroin was evaluated in two groups of rats that had been subjected to the same chronic regimen as in Experiment 1. Locomotor activity was enhanced in both groups following 120 micrograms/kg heroin, whereas 30 micrograms/kg was ineffective in either group. Finally, it was found that neuroleptic-treated, but not control, rats rapidly learned to self-administer intravenous infusions of an unusually low dose of heroin (4 micrograms) and to discriminate these infusions from vehicle infusions. Together, these data show that chronic dopamine (DA) receptor blockade produces a marked increase in the sensitivity to the reinforcing and discriminative stimulus properties of systemic heroin administration and that this increase is not attributable to heroin-induced locomotor activation. The results are discussed in terms of the role of DA systems in opiate reinforcement processes.

Kelatorphan, a potent enkephalinases inhibitor, and opioid receptor agonists DAGO and DTLET, differentially modulate self-stimulation behaviour depending on the site of administration

Endogenous enkephalins have been found in the perikaryon of the mesolimbic dopaminergic ventral tegmental area and in axonal terminals in the nucleus accumbens. To examine whether endogenous opioid peptides may modulate this mesolimbic system, injections of dopamine receptor agonists and antagonist, the mu-opioid receptor agonists DAGO and morphine, the delta-opioid receptor agonist DTLET and kelatorphan, a new potent inhibitor of multiple enkephalin-degrading enzymes, were performed into the lateral ventricle and into the nucleus accumbens. Intracranial self-stimulation behaviour, obtained through electrodes chronically implanted into the medial forebrain bundle in the posterolateral hypothalamus of the rat, was used as behavioural paradigm. Injections of kelatorphan and DTLET into the lateral ventricle both induced an ICI 174,864-reversible increased self-stimulation behaviour, a similar increase was observed after injection of d-amphetamine, while morphine and DAGO reduced the rate of self-stimulation. In contrast, the administration of kelatorphan or dopamine receptor agonists into the nucleus accumbens reduced the rate of intracranial self-stimulation, while DTLET was without effect, when injected into the same structure. Finally, intra-accumbens injections of DAGO produced a similar behavioural profile to that produced by intraventricular injections of the drugs. Opioids may thus differentially affect intracranial self-stimulation behaviour, as a function of the neuroanatomical locus of administration. Furthermore, these results suggest that kelatorphan may increase self-stimulation behaviour through an action at delta-opioid receptor, while DAGO and morphine may reduce self-stimulation behaviour through an action at mu-opioid receptors.

Opiate reward: sites and substrates

Opiates appear to have rewarding actions at more than one locus in the brain. Studies of the effects of dopaminergic lesions and dopamine receptor blockade indicate that intravenous heroin self-administration depends importantly on a dopaminergic substrate. Mapping of effective injection sites for morphine-conditioned place preference establishes one site of rewarding action near the dopamine cell bodies of the ventral tegmental area (VTA). Studies of the complex interactions of opiates, neuroleptics, and brain stimulation reward confirm that reward-related VTA opioid actions are dopamine-dependent. Opioid injections into the nucleus accumbens (NAS) also facilitate brain stimulation reward and serve as rewards in their own right, though these actions have not yet been localized by identification of negative sites in surrounding regions. The relation of this putative reward site to the dopamine system is not yet clear. Suggestions that the lateral hypothalamus or periaqueductal gray contain opioid reward sites remain to be confirmed. While opioid injections into these sites can be rewarding, these rewarding effects have not been localized to these sites, and opiate injections into each of these areas are reported not to facilitate brain stimulation reward. Intravenous heroin self-administration is not disrupted by kainic acid lesions of the bed nucleus of the lateral hypothalamus. Thus only the VTA and the NAS are firmly established as sites of opiate rewarding actions. Recent reports suggest that the kappa-opioid dynorphin may also have central rewarding actions and central and peripheral aversive actions; the CA3 region of the hippocampus is a possible site of the rewarding action.

Regional neuroleptic microinjections indicate a role for nucleus accumbens in lateral hypothalamic self-stimulation reward

Bilateral microinjections of the neuroleptic, cis-flupenthixol, were made into 56 forebrain targets distributed across various dopamine (DA) terminal fields in the forebrain. Drug effects on medial forebrain bundle (MFB) stimulation-produced reward were assessed with a rate-frequency procedure implemented in a runway paradigm in a discrete-trial fashion. This method generated independent measures of drug-induced changes in the MFB stimulation reward and operant motor/performance capacity. Control experiments were run with the inactive isomer, trans-flupenthixol. Results indicate a major role for accumbens DA in MFB reward, but not for the DA in caudate and medial frontal cortex. Few drug-induced motor/performance deficits were found at any site. In 14 selected subjects, 6-OHDA-induced chronic DA lesions were made at the same site as neuroleptic microinjection. These results confirmed the reward effects of acute DA receptor blockade, but produced a greater associated motor/performance impairment. Both behavioral effects of the lesion recovered within 2 weeks in many, but not all subjects.

Blockade of dopamine receptors reverses the behavioral effects of endogenous enkephalins in the Nucleus caudatus but not in the Nucleus accumbens: differential involvement of delta and mu opioid receptors

We have previously (Daugé et al. 1988) demonstrated that injection of the mu agonist [D-Ala2, MePhe4, Gly-ol5]-enkephalin (DAGO) or the delta agonist [D-Thr2, Leu5]-enkephalyl-Thr6 (DTLET) into the rat Nucleus accumbens (N.Acc.), or Nucleus caudatus (N.Caud.) induced a hypoactivity followed by hyperactivity 150 min later in the case of the mu agonist and a hyperactivity in the case of the delta agonist. Moreover, naloxone reversible delta-type responses were obtained by local infusion of kelatorphan, ([(R)-3(N-hydroxylcarboxamido-2-benzylpropanoyl)-L-alanine]), a complete inhibitor of enkephalin catabolism, suggesting a tonic control of the behavioral activity of rat by the endogenous opioid peptides. In this work, the putative involvement of the dopaminergic system in these behavioral responses was investigated by using the DA antagonist thioproperazine. In the N.Acc., the behavioral effects of kelatorphan or of mu or delta agonists were not altered by thioproperazine-induced blockade of dopamine receptors. In contrast, the hyperactivity produced by DTLET or by kelatorphan in the N.Caud. was reversed by thioproperazine while the time-dependent biphasic effect resulting from DAGO injection remained unaffected by the DA antagonist. This blocking effect of thioproperazine is in agreement with the previously described delta-selective enhancement of the release of newly synthesized DA in the striatum but not in the N.Acc.

Conditioning of rotational behavior after the administration of a single dose of apomorphine in rats with unilateral denervation of the dopaminergic nigrostriatal pathway: relevance to drug addiction

Our aim is to study the relationship of drug activation of the dopamine neurotransmission system and the conditioning of environmental stimuli present at the time of drug administration. We injected a single dose of apomorphine (0.05 mg/kg SC) in rats with the nigrostriatal dopamine pathways unilaterally denervated with 6-hydroxydopamine, which generates rotational behavior contralateral to the lesioned hemisphere. We observed rotational behavior without apomorphine administration when animals were reexposed at different time intervals to the same environment in which they performed turning behavior. The present findings show that this rotational behavior can be conditioned to environmental stimuli in a strong and long-lasting way. In light of the relationship between opioids and the dopaminergic system, similar conditioning could take place in the learning processes implicated in drug addiction.

A comparison of the effects of intra-accumbens injections of amphetamine and morphine on reinstatement of heroin intravenous self-administration behavior

In rats previously trained to self-administer heroin intravenously, the application of morphine directly to the ventral tegmental area (VTA) has been shown to reinstate responding after a period of extinction, suggesting that the activation of mesolimbic dopamine neurons might underlie the priming effects of i.v. injections of opiates and stimulants found previously. In the present experiments rats were trained to self-administer heroin intravenously. Following extinction training, and after a period of at least 30 min of no responding, bilateral microinjections of either 0.5 microliter saline or 10 micrograms/0.5 microliter (+)-amphetamine sulfate were made into the N. accumbens. Amphetamine, but not saline reinstated self-administration behavior for about 1 h. In contrast, bilateral intra-accumbens injections of either 5 or 10 micrograms/0.5 microliter morphine sulfate to these same animals led to only infrequent responses late in the 90 min session. Both drugs increased locomotor activity measured in independent tests. Because the locomotor activity produced by intra-accumbens morphine occurs independent of the mesolimbic dopamine system, unlike that produced by VTA morphine and intra-accumbens amphetamine, and because it does not show sensitization, it is argued that the reinstatement effects of opiates and stimulants on self-administration behavior are mediated by the mesolimbic dopamine system, and may be related to the ability of opiates and stimulant drugs to cause sensitization within that system.

Comparison of the behavioural effects induced by administration in rat nucleus accumbens or nucleus caudatus of selective mu and delta opioid peptides or kelatorphan an inhibitor of enkephalin-degrading-enzymes

The effects of selective agonists for delta opioid receptors: [D-Thr2, Leu5]-enkephalyl-Thr6 (DTLET) and mu receptors: [D-Ala2, MePhe4, Gly-ol5]-enkephalin (DAGO) and of (R)-3-(N-hydroxyl-carboxamido-2-benzylpropanoyl)-L-alanine (kelatorphan), a complete inhibitor of enkephalin degrading enzymes, on the motor activity of rats was examined after their local administration into the nucleus accumbens (NA) or nucleus caudatus (NC). In both structures DTLET dose dependently enhanced locomotor activity as measured in the open-field test. This strong effect was reversed by the selective delta antagonist: ICI 174,864. Contrastingly, DAGO induced hypoactivity followed by hyperactivity 150 min later. This biphasic effect was blocked by systemic injection of naloxone, but not by ICI 174,864. The physiological relevance of these effects was ascertained by the naloxone-reversible stimulatory responses induced by kelatorphan, supporting a role for endogenous enkephalins in the control of behavior through delta receptor stimulation.

Endogenous opiates: 1986

This is the ninth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1986. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic processes; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; activity; sex, pregnancy, and development; and some other behaviors.

The role of reward pathways in the development of drug dependence

In commenting on the discovery of "opiate" receptors, Goldstein (1976) said: "It seemed unlikely, a priori, that such highly stereospecific receptors should have been developed by nature to interact with alkaloids from the opium poppy" (p. 1081). Endogenous opioid peptides and opioid receptor systems have now been identified in invertebrates that are unlikely to have had ancestors exposed to opium poppies (Kavaliers et al., 1983; Kream et al., 1980; Leung and Stefano, 1984; Stefano et al., 1980). Moreover, endogenous opioids play a role in stress-induced feeding in the slug (Kavaliers and Hirst, 1986) just as they play a role in stress-induced feeding in rodents (Lowy et al., 1980; Morley and Levine, 1980). If we are to understand the actions of opiates and other drugs of abuse we must understand them in terms of their abilities to interact with neural systems that evolved in the service of primitive biological functions, long before any serious incidence of addiction itself. The most primitive axes of the biological substrates of behavior are the axes of approach and withdrawal. Addictive drugs appear to be able to activate the mechanisms of approach, which is termed "positive reinforcement" and to inhibit the mechanisms of withdrawal, which is termed "negative reinforcement." Anatomically distinct sets of pathways have evolved to serve these two forms of reward. Activation of the medial forebrain bundle and associated structures serves positive reinforcement and induces forward locomotion. Approach and forward locomotion are the unconditioned responses to positive reinforcing stimuli such as food and sex partners, and approach to environmental objects and positive reinforcement is induced by electrical stimulation of this structure. The locomotor stimulating effects and the positive reinforcing effects of opiates and psychomotor stimulants result from their activation of this mechanism; stimulants activate the mechanism at the level of dopaminergic synapses of the nucleus accumbens, frontal cortex, and perhaps other forebrain structures, while opiates activate the system at two points: at the level of the dopaminergic synapse and at the level of the afferents to the dopaminergic cell bodies. Ethanol, nicotine, caffeine and phencyclidine stimulate both locomotor activity and dopamine turnover, but their sites of interaction with reward pathways have not yet been identified. Benzodiazepines and barbiturates stimulate locomotor activity without stimulating dopamine turnover; they may interact with reward pathways at a synapse efferent to the dopaminergic link in the pathways.(ABSTRACT TRUNCATED AT 400 WORDS)