Neuroleptics block the positive reinforcing effects of amphetamine but not of morphine as measured by place conditioning

Place preferences induced by electrical stimulation of the external lateral parabrachial subnucleus in a sequential learning task: Place preferences induced by NLPBe stimulation

It is known that electrical stimulation of the external lateral parabrachial nucleus (NLPBe) can sustain concurrent taste and place learning. Place preferences can be learned through different procedures. Previous studies demonstrated that electrical stimulation of the PBNLe can generate aversive and preference place learning using concurrent procedures. In the concurrent procedure, the animals can move freely in the maze, and intracranial electrical stimulation is associated with their voluntary stay in one of the two maze compartments. However, the rewarding properties of most stimuli, whether natural or drugs of abuse, have usually been investigated using the sequential procedure, in which animals are confined while receiving the unconditioned stimulus and then undergo a choice test without stimulation in a later phase. This study examined whether this stimulation can sustain place preference learning in sequential tasks. Results demonstrated that place preferences can also be induced by the electrical stimulation of the NLBe using sequential procedures. These findings suggest that the NLPBe may form part of a brain reward axis that shares certain characteristics with those observed in the processing of natural rewarding agents and especially of drugs of abuse.

Striatal Dopamine Release in Response to Morphine: A [11C]Raclopride Positron Emission Tomography Study in Healthy Men

BACKGROUND

Preclinical and human positron emission tomography studies have produced inconsistent results regarding the effects of opioids on mesolimbic dopamine (DA). Here, we quantify striatal DA release (measured by [11C]raclopride displacement) in response to an intravenous infusion of morphine, and its relationship with morphine-induced subjective effects, in healthy, nondependent opioid-experienced participants.

METHODS

Fifteen healthy male participants were initially included. Sessions were on separate days. On session 1, participants received intravenous morphine (10 mg/70 kg) in the clinic to ensure tolerability. Participants without adverse reactions (n = 10) then received intravenous morphine and placebo (saline) sessions, in counterbalanced order, while undergoing [11C]raclopride positron emission tomography scans. Subjective and physiological responses were assessed. Region-of-interest and voxelwise image analyses were used to assess changes in [11C]raclopride nondisplaceable binding potential.

RESULTS

Morphine produced marked subjective and physiological effects and induced a significant decrease in [11C]raclopride nondisplaceable binding potential, particularly in the nucleus accumbens and globus pallidus, where the change in [11C]raclopride nondisplaceable binding potential was approximately 9%. However, the subjective effects of morphine did not show a simple pattern of correlation with DA release.

CONCLUSIONS

This is, to our knowledge, the first study providing in vivo human evidence that DA transmission in the ventral striatum is affected by morphine. Further studies are required to fully delineate the DA contribution to the reinforcing effects of opioids.

Diazepam blocks 50 kHz ultrasonic vocalizations and stereotypies but not the increase in locomotor activity induced in rats by amphetamine

RATIONALE

We have recently shown that the benzodiazepine diazepam inhibits dopamine release in the NAc and blocks the increased release of dopamine induced by DL-amphetamine. Rewarding stimuli and many drugs of abuse can induce dopamine release in the nucleus accumbens as well as 50-kHz ultrasonic vocalizations (USVs) in rats.

OBJECTIVES

In the present study, we tested the hypothesis that diazepam can also block the increase in locomotor activity and USVs elicited by amphetamine.

METHODS

Fifty-kilohertz USVs, stereotypy, and locomotor behavior were scored in adult male Wistar rats treated with i.p. injections of saline, 3 mg/kg DL-amphetamine, 2 mg/kg diazepam, 0.2 mg/kg haloperidol, or a combination of these drugs.

RESULTS

In agreement with previous studies, amphetamine caused significant increases in the number of USV calls, stereotypies, and locomotor activity. The D2 dopamine receptor antagonist haloperidol blocked the effects of amphetamine on USVs, stereotypy, and locomotor activity. Diazepam blocked the effect of amphetamine on USV and stereotypy, but not on horizontal locomotion.

CONCLUSIONS

These results suggest that diazepam blocks the rewarding effect of amphetamine. This finding is promising for basic research regarding treatments of substance use disorders and evaluation of the impact of benzodiazepines on motivation.

An analysis of the rewarding and aversive associative properties of nicotine in the neonatal quinpirole model: Effects on glial cell line-derived neurotrophic factor (GDNF)

This study analyzed the associative properties of nicotine in a conditioned place preference (CPP) paradigm in adolescent rats neonatally treated with quinpirole (NQ) or saline (NS). NQ produces dopamine D₂ receptor supersensitivity that persists throughout the animal's lifetime, and therefore has relevance towards schizophrenia. In two experiments, rats were ip administered quinpirole (1mg/kg) or saline from postnatal day (P)1-21. After an initial preference test at P42-43, animals were conditioned for eight consecutive days with saline or nicotine (0.6mg/kg free base) in Experiment 1 or saline or nicotine (1.8mg/kg free base) in Experiment 2. In addition, there were NQ and NS groups in each experiment given the antipsychotic haloperidol (0.05mg/kg) or clozapine (2.5mg/kg) before nicotine conditioning. A drug free post-conditioning test was administered at P52. At P53, the nucleus accumbens (NAc) was analyzed for glial cell-line derived neurotrophic factor (GDNF). Results revealed that NQ enhanced nicotine CPP, but blunted the aversive properties of nicotine. Haloperidol was more effective than clozapine at blocking nicotine CPP in Experiment 1, but neither antipsychotic affected nicotine conditioned place aversion in Experiment 2. NQ increased accumbal GDNF which was sensitized in NQ rats conditioned to nicotine in Experiment 1, but the aversive dose of nicotine reduced GDNF in NQ animals in Experiment 2. Both antipsychotics in combination with the aversive dose of nicotine decreased accumbal GDNF. In sum, increased D₂ receptor sensitivity influenced the associative properties and GDNF response to nicotine which has implications towards pharmacological targets for smoking cessation in schizophrenia.

Ghrelin and endocannabinoids participation in morphine-induced effects in the rat nucleus accumbens

RATIONALE AND OBJECTIVES

In addition to dopamine, endocannabinoids are thought to participate in neural reward mechanisms of opioids. Number of recent studies suggests crucial involvement of ghrelin in some addictive drugs effects. Our previous results showed that ghrelin participates in morphine-induced changes in the mesolimbic dopaminergic system associated with reward processing. The goal of the present study was to test whether the growth hormone secretagogue receptor (GHS-R1A) antagonist JMV2959 was able to influence morphine-induced effects on anandamide (N-arachidonoylethanolamine, AEA) and 2-arachidonoylglycerol (2-AG) in the nucleus accumbens shell (NACSh).

METHODS

We used in vivo microdialysis to determine changes in levels of AEA and 2-AG in the NACSh in rats following (i) an acute morphine dose (5, 10 mg/kg s.c.) with and without JMV2959 pretreatment (3, 6 mg/kg i.p.) or (ii) a morphine challenge dose (5 mg/kg s.c.) with and without JMV2959 (3, 6 mg/kg i.p.) pretreatment, administered during abstinence following repeated doses of morphine (5 days, 10-40 mg/kg). Co-administration of ghrelin (40 ug/kg i.p.) was used to verify the ghrelin mechanisms involvement.

RESULTS

Pretreatment with JMV2959 significantly and dose-dependently reversed morphine-induced anandamide increases in the NACSh in both the acute and longer-term models, resulting in a significant AEA decrease. JMV2959 significantly intensified acute morphine-induced decreases in accumbens 2-AG levels and attenuated morphine challenge-induced 2-AG decreases. JMV2959 pretreatment significantly reduced concurrent morphine challenge-induced behavioral sensitization. JMV2959 pretreatment effects were abolished by co-administration of ghrelin.

CONCLUSIONS

Our results indicate significant involvement of ghrelin signaling in morphine-induced endocannabinoid changes in the NACSh.

Rewarding effects of the electrical stimulation of the parabrachial complex: taste or place preference?

The lateral parabrachial complex has been related to various emotional-affective processes. It has been shown that electrical stimulation of the external Lateral Parabrachial (LPBe) nucleus can induce reinforcing effects in place preference and taste discrimination tasks but does not appear to support self-stimulation. This study examined the relative relevance of place and taste stimuli after electrical stimulation of the LPBe nucleus. A learning discrimination task was conducted that simultaneously included both sensory indexes (taste and place) in order to determine the preference of animals for one or the other. After a taste stimulus reversal task, the rewarding effect of stimulation was found to be preferentially associated with place. These results are discussed in the context of the rewarding action and biological constraints induced by different natural and artificial reinforcing agents.

Self-administration of propofol is mediated by dopamine D1 receptors in nucleus accumbens in rats

As a widely used intravenous short-acting anesthetic, propofol is recently indicated by clinical and animal studies for its abuse potential, but the mechanism underlying propofol abuse is largely unknown. This study examined the contribution of dopamine receptor subtype (D1 and D2 receptors) and neuroanatomical locus (i.e. nuclear accumbens) in the maintenance of propofol self-administration in rats. After the acquisition and maintenance of self-administration of propofol (1.7 mg/kg/infusion) under a fixed ratio (FR1) schedule of reinforcement over 14 days, rats were treated by either intraperitoneal injection or intra-nucleus accumbens (NAc) injection of D1 receptor antagonist (SCH23390) or D2 receptor antagonists (spiperone and eticlopride) 10 min prior to the subsequent propofol self-administration. We demonstrated (i) systemic administration of SCH23390 (10, 30, 100 μg/kg, i.p.) dose-dependently decreased the rate of propofol-maintained self-administration, suggesting a critical role of the D1 receptor in mediating propofol self-administration; (ii) the blockade of the propofol self-administration by SCH23390 was specific since spiperone and eticlopride did not affect propofol self-administration and SCH23390 at these doses did not affect food-maintained responding under an FR5 schedule; (iii) intra-accumbenal injection of SCH23390 (2.5 μg/site) but not eticopride (3.0 μg/site) attenuated the propofol self-administration, localizing nuclear accumbal D1 receptors as a critical locus in the reinforcement of propofol. Together, these findings provide the first direct evidence that D1 receptors in nuclear accumbens play an important role in the maintenance of propofol self-administration.

Prefrontal/accumbal catecholamine system processes high motivational salience

Motivational salience regulates the strength of goal seeking, the amount of risk taken, and the energy invested from mild to extreme. Highly motivational experiences promote highly persistent memories. Although this phenomenon is adaptive in normal conditions, experiences with extremely high levels of motivational salience can promote development of memories that can be re-experienced intrusively for long time resulting in maladaptive outcomes. Neural mechanisms mediating motivational salience attribution are, therefore, very important for individual and species survival and for well-being. However, these neural mechanisms could be implicated in attribution of abnormal motivational salience to different stimuli leading to maladaptive compulsive seeking or avoidance. We have offered the first evidence that prefrontal cortical norepinephrine (NE) transmission is a necessary condition for motivational salience attribution to highly salient stimuli, through modulation of dopamine (DA) in the nucleus accumbens (NAc), a brain area involved in all motivated behaviors. Moreover, we have shown that prefrontal-accumbal catecholamine (CA) system determines approach or avoidance responses to both reward- and aversion-related stimuli only when the salience of the unconditioned stimulus (UCS) is high enough to induce sustained CA activation, thus affirming that this system processes motivational salience attribution selectively to highly salient events.

Augmentation of methamphetamine-induced behaviors in transgenic mice lacking the trace amine-associated receptor 1

The trace amine-associated receptor 1 (TAAR1) is a G protein-coupled receptor that is functionally activated by amphetamine-based psychostimulants, including amphetamine, methamphetamine and MDMA. Previous studies have shown that in transgenic mice lacking the TAAR1 gene (TAAR1 knockout; KO) a single injection of amphetamine can produce enhanced behavioral responses compared to responses evoked in wild-type (WT) mice. Further, the psychostimulant effects of cocaine can be diminished by selective activation of TAAR1. These findings suggest that TAAR1 might be implicated in the rewarding properties of psychostimulants. To investigate the role of TAAR1 in the rewarding effects of drugs of abuse, the psychomotor stimulating effects of amphetamine and methamphetamine and the conditioned rewarding effects of methamphetamine and morphine were compared between WT and TAAR1 KO mice. In locomotor activity studies, both single and repeated exposure to d-amphetamine or methamphetamine generated significantly higher levels of total distance traveled in TAAR1 KO mice compared to WT mice. In conditioned place preference (CPP) studies, TAAR1 KO mice acquired methamphetamine-induced CPP earlier than WT mice and retained CPP longer during extinction training. In morphine-induced CPP, both WT and KO genotypes displayed similar levels of CPP. Results from locomotor activity studies suggest that TAAR1 may have a modulatory role in the behavioral sensitization to amphetamine-based psychostimulants. That methamphetamine-but not morphine-induced CPP was augmented in TAAR1 KO mice suggests a selective role of TAAR1 in the conditioned reinforcing effects of methamphetamine. Collectively, these findings provide support for a regulatory role of TAAR1 in methamphetamine signaling.

Opiate versus psychostimulant addiction: the differences do matter

The publication of the psychomotor stimulant theory of addiction in 1987 and the finding that addictive drugs increase dopamine concentrations in the rat mesolimbic system in 1988 have led to a predominance of psychobiological theories that consider addiction to opiates and addiction to psychostimulants as essentially identical phenomena. Indeed, current theories of addiction - hedonic allostasis, incentive sensitization, aberrant learning and frontostriatal dysfunction - all argue for a unitary account of drug addiction. This view is challenged by behavioural, cognitive and neurobiological findings in laboratory animals and humans. Here, we argue that opiate addiction and psychostimulant addiction are behaviourally and neurobiologically distinct and that the differences have important implications for addiction treatment, addiction theories and future research.

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

RATIONALE

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

OBJECTIVE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Chronic nicotine exposure switches the functional role of mesolimbic dopamine transmission in the processing of nicotine's rewarding and aversive effects

The mammalian ventral tegmental area (VTA) and associated mesolimbic dopamine (DA) system are critical neural substrates for processing nicotine's motivational effects. Considerable evidence suggests that the role of DA transmission may be altered as a function of nicotine exposure. Using a combination of in vivo neuronal recording and behavioral conditioning, we report that chronic nicotine exposure induces a functional switch in the role of mesolimbic DA transmission. Thus, in nicotine-naive subjects, blockade of DA transmission potentiates the rewarding effects of sub-reward-threshold doses of nicotine and reverses the motivational valence of nicotine from aversive to rewarding. However, in animals treated chronically with nicotine, DA blockade switches previously sub-reward-threshold or rewarding doses of nicotine into aversion signals. Neuronal VTA recordings similarly revealed a functional switch in this DAergic neuronal circuit resulting in strongly increased sensitivity of the VTA DAergic system to nicotine administration and a tonic reduction in the baseline activity of VTA DAergic neurons. These results demonstrate a functional switch in the role of DAergic transmission during the acute versus chronic phases of nicotine exposure and suggest that mesolimbic DA transmission plays qualitatively distinct roles in the processing of nicotine's motivational effects as a function of drug exposure.

Aripiprazole blocks reinstatement but not expression of morphine conditioned place preference in rats

Aripiprazole is an atypical antipsychotic drug primarily characterized by partial agonist activity at dopamine(DA) D2 receptors and serotonin-1A (5-hydroxytryptamine, 5-HT1A) receptors and minimal side effects.Based on its pharmacological profile, including stabilization of mesocorticolimbic DA activity (a pathway implicated in drug addiction), we investigated the effects of aripiprazole on relapse to morphine seeking in rats. In experiment 1, rats underwent morphine-induced conditioned place preference (CPP) training with alternate injections of morphine (5 mg/kg, s.c.) and saline (1 ml/kg, s.c.) for 8 consecutive days. To examine the effect of aripiprazole on the expression of morphine-induced CPP, rats received aripiprazole (0, 0.03, 0.1,and 0.3 mg/kg, i.p.) 30 min before testing for the expression of CPP. In experiment 2, rats underwent the same CPP training as in experiment 1 and subsequent extinction training. To examine the effect of aripiprazole on reinstatement of morphine-induced CPP, rats received aripiprazole 30 min before testing for reinstatement of CPP. In experiment 3, to assess the effects of aripiprazole on locomotor activity, aripiprazole was administered 30 min before testing for locomotor activity. Aripiprazole significantly decreased the reinstatement of CPP induced by a priming injection of morphine but had no effect on the expression of morphine-induced CPP or locomotor activity. The D2 and 5-HT1A partial agonist and 5-HT2A antagonist properties of aripiprazole likely account for the blockade of relapse to drug seeking. These findings suggest that aripiprazole may have therapeutic value for reducing craving and preventing relapse to drug seeking.

Loss of cocaine locomotor response in Pitx3-deficient mice lacking a nigrostriatal pathway

Both the dorsal and ventral striatum have been demonstrated to have a critical role in reinforcement learning and addiction. Dissecting the specific function of these striatal compartments and their associated nigrostriatal and mesoaccumbens dopamine pathways, however, has proved difficult. Previous studies using lesions to isolate the contribution of nigrostriatal and mesoaccumbens dopamine in mediating the locomotor and reinforcing effects of psychostimulant drugs have yielded inconsistent and inconclusive results. Using a naturally occurring mutant mouse line, aphakia, that lacks a nigrostriatal dopamine pathway but retains an intact mesoaccumbens pathway, we show that the locomotor activating effects of cocaine, including locomotor sensitization, are dependent on an intact nigrostriatal dopamine projection. In contrast, cocaine reinforcement, as measured by conditioned place preference and cocaine sensitization of sucrose preference, is intact in these mice. In light of the well-established role of the nucleus accumbens in mediating the effects of psychostimulants, these data suggest that the nigrostriatal pathway can act as a critical effector mechanism for the nucleus accumbens highlighting the importance of intrastriatal connectivity and providing insight into the functional architecture of the striatum.

Effects of reversible inactivation of the ventral tegmental area on the acquisition and expression of morphine-induced conditioned place preference in the rat

The mesolimbic dopaminergic system that projects from the ventral tegmental area (VTA) to the nucleus accumbens is critical for initiation of opioid reinforcement and reward-related effects of drugs of abuse. In the present study, the effects of reversible inactivation of VTA on acquisition and expression of morphine-induced conditioned place preference (CPP) were investigated in rats. One hundred and eighty-one adult male albino Wistar rats were used in these experiments. Reversible inactivation of VTA was done through the unilateral and bilateral intra-VTA microinjection of 2% lidocaine during the acquisition and expression of morphine (5 mg/kg; s.c.)-induced CPP. Animal displacement, conditioning score and locomotor activity were recorded by Ethovision software. The results showed that bilateral but not unilateral intra-VTA administration of lidocaine significantly decreases the acquisition (P<0.01) and expression (P<0.05) of morphine-induced CPP compared to their respective saline-microinjected groups. Moreover, intra-VTA administration of lidocaine had no effect on locomotor activity in these experiments. Our results further support the idea that VTA may play an important role in the acquisition of morphine-induced CPP. In addition, there is no functional lateralization in the VTA for acquisition and/or expression of morphine-induced CPP in the rat.

Ventral tegmental area neurons in learned appetitive behavior and positive reinforcement

Ventral tegmental area (VTA) neuron firing precedes behaviors elicited by reward-predictive sensory cues and scales with the magnitude and unpredictability of received rewards. These patterns are consistent with roles in the performance of learned appetitive behaviors and in positive reinforcement, respectively. The VTA includes subpopulations of neurons with different afferent connections, neurotransmitter content, and projection targets. Because the VTA and substantia nigra pars compacta are the sole sources of striatal and limbic forebrain dopamine, measurements of dopamine release and manipulations of dopamine function have provided critical evidence supporting a VTA contribution to these functions. However, the VTA also sends GABAergic and glutamatergic projections to the nucleus accumbens and prefrontal cortex. Furthermore, VTA-mediated but dopamine-independent positive reinforcement has been demonstrated. Consequently, identifying the neurotransmitter content and projection target of VTA neurons recorded in vivo will be critical for determining their contribution to learned appetitive behaviors.

Dissociation of food and opiate preference by a genetic mutation in zebrafish

Both natural rewards and addictive substances have the ability to reinforce behaviors. It has been unclear whether identical neural pathways mediate the actions of both. In addition, little is known about these behaviors and the underlying neural mechanisms in a genetically tractable vertebrate, the zebrafish Danio rerio. Using a conditioned place preference paradigm, we demonstrate that wildtype zebrafish exhibit a robust preference for food as well as the opiate drug morphine that can be blocked by the opioid receptor antagonist naloxone. Moreover, we show that the too few mutant, which disrupts a conserved zinc finger-containing gene and exhibits a reduction of selective groups of dopaminergic and serotonergic neurons in the basal diencephalon, displays normal food preference but shows no preference for morphine. Pretreatment with dopamine receptor antagonists abolishes morphine preference in the wildtype. These studies demonstrate that zebrafish display measurable preference behavior for reward and show that the preference for natural reward and addictive drug is dissociable by a single-gene mutation that alters subregions of brain monoamine neurotransmitter systems. Future genetic analysis in zebrafish shall uncover further molecular and cellular mechanisms underlying the formation and function of neural circuitry that regulate opiate and food preference behavior.

Morphine-conditioned single-trial place preference: role of nucleus accumbens shell dopamine receptors in acquisition, but not expression

RATIONALE

A large body of evidence indicates an involvement of the mesolimbic dopamine (DA) pathway innervating the ventral striatum in the motivational effects of drug abuse.

OBJECTIVE

The goal of the study is to clarify the role of DA D1 and D2 receptors of the rat nucleus accumbens (NAc) shell and core in the motivational effects of morphine as studied by conditioned place preference (CPP).

METHODS

The effect of the intracerebral infusion of DA antagonists specific for DA D1 (SCH 39166) and D2 receptors (L-sulpiride) was studied in a single-trial place conditioning paradigm with fixed assignment of the drug to the unpreferred compartment.

RESULTS

Morphine induced significant CPP at all the doses tested (0.5, 1.0, and 2.0 mg/kg, subcutaneously). A dose of 1.0 mg/kg was selected for further studies. Intra-NAc shell infusion of SCH 39166 and L-sulpiride at doses of 25 and 50 ng/1 microl per side impaired the acquisition of CPP by morphine. No effect was observed at 12.5 ng/1 microl per side. Intra-NAc core infusion of SCH 39166 (12.5, 25, and 50 ng/1 microl per side) did not affect the acquisition of morphine-induced CPP, while L-sulpiride (12.5, 25, and 50 ng/1 microl per side) impaired CPP acquisition only at the dose of 50 ng/1 microl per side. No effect on morphine-induced CPP was observed when the DA antagonists were infused into the NAc shell or core 10 min before the test session.

CONCLUSION

These results indicate that DA D1 and D2 receptors in the NAc shell are involved in the acquisition of morphine-induced CPP.

The neurobiology of positive emotions

Compared to the study of negative emotions such as fear, the neurobiology of positive emotional processes and the associated positive affect (PA) states has only recently received scientific attention. Biological theories conceptualize PA as being related to (i) signals indicating that bodies are returning to equilibrium among those studying homeostasis, (ii) utility estimation among those favoring neuroeconomic views, and (iii) approach and other instinctual behaviors among those cultivating neuroethological perspectives. Indeed, there are probably several distinct forms of positive affect, but all are closely related to ancient sub-neocortical limbic brain regions we share with other mammals. There is now a convergence of evidence to suggest that various regions of the limbic system, including especially ventral striatal dopamine systems are implemented in an anticipatory (appetitive) positive affective state. Dopamine independent mechanisms utilizing opiate and GABA receptors in the ventral striatum, amygdala and orbital frontal cortex are important in elaborating consummatory PA (i.e. sensory pleasure) states, and various neuropeptides mediate homeostatic satisfactions.

Phosphodiesterase inhibitors: a novel mechanism for receptor-independent antipsychotic medications

OVERVIEW

All current antipsychotic medications work by binding to Gi-coupled dopamine (DA) D2 receptors. Such medications are thought to affect cellular function primarily by decreasing DA-mediated regulation of intracellular cyclic adenosine monophosphate (cAMP).However, several studies indicate that cAMP signal transduction abnormalities in schizophrenia may not be limited to D2-containing cells. The current study examines the potential of using non-receptor-based agents that modify intracellular signal transduction as potential antipsychotic medications.

METHODS

The indirect DA agonist amphetamine has been used to model the auditory sensory processing deficits in schizophrenia. Such pharmacologically induced abnormalities are reversed by current antipsychotic treatments. This study examines the ability of the phosphodiesterase-4 inhibitor, rolipram, to reverse amphetamine-induced abnormalities in auditory-evoked potentials that are characteristic of schizophrenia.

RESULTS

Rolipram reverses amphetamine-induced reductions in auditory-evoked potentials.

CONCLUSION

This finding could lead to novel approaches to receptor-independent treatments for schizophrenia.

Endogenous opioids mediate basal hedonic tone independent of dopamine D-1 or D-2 receptor activation

Exogenously administered opiates are recognized as rewarding and the involvement of dopamine systems in mediating their apparent pleasurable effects is contentious. The aversive response to naloxone administration observed in animal studies suggests the presence of an endogenous opioid tone regulating hedonic state. We sought evidence for the requirement for dopamine systems in mediating this action of endogenous opioids by determining whether mice deficient in dopamine D-1 or D-2 receptors were able to display conditioned place aversion to naloxone. Mice received saline in the morning in one chamber and either saline or naloxone (10 mg/kg, s.c.) in the afternoon in another chamber, each day for 3 days. On the test day they were given free access to the testing chambers in the afternoon. Similar to their wild-type littermates, D-1 and D-2 receptor knockout mice receiving naloxone in the afternoon spent significantly less time on the test day in the compartment in which they previously received naloxone, compared with animals receiving saline in the afternoon. The persistence of naloxone-conditioned place aversion in D-1 and D-2 knockout mice suggests that endogenous opioid peptides maintain a basal level of positive affect that is not dependent on downstream activation of dopamine systems involving D-1 or D-2 receptors.

Opiate state controls bi-directional reward signaling via GABAA receptors in the ventral tegmental area

The neural mechanisms that mediate the transition from a drug-naive state to a state of drug dependence and addiction are not yet known. Here we show that a discrete population of GABA(A) receptors in the mammalian ventral tegmental area (VTA) serves as a potential addiction switching mechanism by gating reward transmission through one of two neural motivational systems: either a dopamine-independent (opiate-naive) or a dopaminergic (opiate-dependent or opiate-withdrawn) system. Bi-directional transmission of reward signals through this GABA(A) receptor substrate is dynamically controlled by the opiate state of the organism and involves a molecular alteration of the GABA(A) receptor. After opiate exposure and subsequent withdrawal, the functional conductance properties of the rat VTA GABA(A) receptor switch from an inhibitory to an excitatory signaling mode.

Amphetamine primes motivation to gamble and gambling-related semantic networks in problem gamblers

Previous research suggests that gambling can induce effects that closely resemble a psychostimulant drug effect. Modest doses of addictive drugs can prime motivation for drugs with similar properties. Together, these findings imply that a dose of a psychostimulant drug could prime motivation to gamble in problem gamblers. This study assessed priming effects of oral D-amphetamine (AMPH) (30 mg) in a within-subject, counter-balanced, placebo-controlled design in problem gamblers (n=10), comorbid gamblerdrinkers (n=6), problem drinkers (n=8), and healthy controls (n=12). Modified visual analog scales assessed addictive motivation and subjective effects. A modified rapid reading task assessed pharmacological activation of words from motivationally relevant and irrelevant semantic domains (Gambling, Alcohol, Positive Affect, Negative Affect, Neutral). AMPH increased self-reported motivation for gambling in problem gamblers. Severity of problem gambling predicted positive subjective effects of AMPH and motivation to gamble under the drug. There was little evidence that AMPH directly primed motivation for alcohol in problem drinkers. On the reading task, AMPH produced undifferentiated improvement in reading speed to all word classes in Nongamblers. By contrast, in the two problem gambler groups, AMPH improved reading speed to Gambling words while profoundly slowing reading speed to motivationally irrelevant Neutral words. The latter finding was interpreted as directly congruent with models, which contend that priming of addictive motivation involves a linked suppression of motivationally irrelevant stimuli. This study provides experimental evidence that psychostimulant-like neurochemical activation is an important component of gambling addiction.

Segregation of amphetamine reward and locomotor stimulation between nucleus accumbens medial shell and core

Convergent evidence suggests that amphetamine (AMPH) exerts its rewarding and locomotor stimulating effects via release of dopamine in the nucleus accumbens. However, there is no consensus as to the relative contributions of core and medial shell subregions to these effects. Moreover, the literature is based primarily on intracranial administration, which cannot fully mimic the drug distribution achieved by systemic administration. In the present study, the effects of bilateral 6-hydroxydopamine lesions of the accumbens core or medial shell on rewarding and locomotor stimulating effects of systemically administered amphetamine (0.75 mg/kg, i.p.) were examined in a conditioned place preference (CPP) procedure relying solely on tactile cues (floor texture). Residual dopamine innervation was quantified by [125I]-RTI-55 binding to the dopamine transporter. When lesions were performed before the conditioning phase, AMPH-induced locomotor stimulation and CPP magnitude were positively correlated with residual dopamine transporter binding in core and medial shell, respectively. Medial shell lesions did not affect morphine CPP, arguing that a sensory or mnemonic deficit was not responsible for the lesion-induced reduction in AMPH CPP. Medial shell lesions performed between the conditioning phase and the test day reduced the expression of amphetamine CPP. These results suggest that after systemic amphetamine administration, rewarding and locomotor stimulating effects of the drug are anatomically dissociated within the nucleus accumbens: the medial shell contributes to rewarding effects, whereas the core contributes to behavioral activation.

Suppression of morphine-induced conditioned place preference by l-12-chloroscoulerine, a novel dopamine receptor ligand

The effect of l-12-chloroscoulerine (l-CSL), a novel ligand with dual dopamine D1 receptor agonistic and D2 receptor antagonistic actions, on the development of morphine-induced conditioned place preference (CPP) was investigated in mice. Morphine (10 mg/kg)-induced place preference was dose dependently suppressed by coadministration of l-CSL (5, 10 and 20 mg/kg), which induced neither place preference nor place aversion when administered alone at a dose of 20 mg/kg. The D1 receptor antagonist SCH23390 (0.1 mg/kg) suppressed, whereas the D2 receptor agonist (+/-)-2-(N-phenylethyl-N-propyl)-amino-5-hydroxytetralin (PPHT) (0.5 mg/kg) had no influence on the development of morphine-induced place preference. However, SCH23390 (0.1 mg/kg) did not affect, whereas PPHT (0.5 mg/kg) reversed the suppressive effect of l-CSL on the development of morphine-induced place preference. These results indicate that l-CSL suppresses the development of place preference of morphine by blocking D2 receptors.

Treatment with the atypical antipsychotic, olanzapine, prevents the expression of amphetamine-induced place conditioning in the rat

Place conditioning (PC) experiments were conducted as a means to further elaborate the treatment potential of the atypical antipsychotic, olanzapine (OLZ), for stimulant abuse. The resulting preference/aversion provides an indirect measure of the incentive salience (i.e., euphoria/dysphoria) produced by a drug. Male Sprague-Dawley rats (n=48) were conditioned in two unique environments (i.e., vertical vs. horizontal stripped walls, large vs. small grid flooring) using injections (1.0 mg/kg ip) of either amphetamine (AMPH) or saline (SAL). On average, animals displayed a significant preference for the AMPH-paired location after 2.5 weeks of conditioning (five pairings each of AMPH and SAL). Once the preference was established, animals were pretreated (60 min) with a single dose of OLZ (0.0, 0.56, 1.0 or 1.5 mg/kg sc) given on the test (AMPH-free) day. For the following week's test, animals were injected with SAL (1.0 mg/kg ip) in an attempt to recapture the side preference exhibited before OLZ treatment. OLZ treatment prevented the expression of the AMPH-conditioned preference and reduced locomotor activity. Inhibition of preference resulted from the highest dose of OLZ (1.5 mg/kg), while the inhibition of locomotor activity occurred across all three doses. Additionally, while the effects on preference were no longer apparent by the SAL test the following week (reversible), the activity was still depressed during the SAL tests in animals that had experienced the highest dose of OLZ (1.5 mg/kg). Control experiments, in which OLZ was used as the conditioning drug, suggest that OLZ itself possesses no aversive effects in the PC paradigm, and may even produce a preference for the drug-paired chamber. Because the AMPH preference is dependent on dopamine (DA) release in the nucleus accumbens (NAcc), these experiments suggest that OLZ pretreatment interferes with the rewarding, as well as the subjective effects of AMPH.

Chronic access to a sucrose solution enhances the development of conditioned place preferences for fentanyl and amphetamine in male Long-Evans rats

Consumption of palatable food and fluids alters the behavioral consequences of psychoactive drugs. To further investigate the effects of intake of palatable nutrients on the rewarding properties of these drugs, the effects of chronic intake of a sweet sucrose solution on the development of conditioned place preferences (CPP) to a mu-opioid agonist, fentanyl, and to a stimulant drug, amphetamine, were examined. Male Long-Evans rats consumed laboratory chow and water or chow, water, and a 32% sucrose solution. CPP testing was conducted in a three-chamber apparatus. In Experiment 1 (over four conditioning days), rats received saline, 0.004, or 0.016 mg/kg sc fentanyl citrate before being placed on the nonpreferred side of the apparatus and saline (subcutaneously) before being placed on the preferred side during a separate session on the same day. When given access to all three chambers, rats injected with 0.016 mg/kg fentanyl spent significantly more time on the drug-paired side than rats injected with saline. Furthermore, sucrose-fed rats displayed a significantly greater CPP than chow-fed rats. After conditioning, rats were tested for fentanyl-induced antinociception using the tail-flick test. Using a cumulative dose procedure, fentanyl (0.003, 0.010, 0.030, and 0.100 mg/kg sc) led to dose-dependent increases in tail-flick latencies. Rats fed with sucrose displayed significantly greater responses to fentanyl than those in the chow group. In Experiment 2, rats spent significantly more time on the drug-paired side of the CPP apparatus following injections of 0.33 or 1.0 mg/kg amphetamine than after saline injections. Additionally, following injection of 0.33 mg/kg amphetamine, sucrose-fed rats spent significantly more time on the drug-paired side of the chamber than chow-fed rats.

Blockade of mesolimbic dopamine transmission dramatically increases sensitivity to the rewarding effects of nicotine in the ventral tegmental area

Nicotine produces rewarding and aversive motivational effects in humans and other animal species. Here, we report that the mammalian ventral tegmental area (VTA) represents a critical neural substrate for the mediation of both the rewarding and aversive properties of nicotine. We demonstrate that direct infusions of nicotine into the VTA can produce both rewarding and aversive motivational effects. While the rewarding effects of higher doses of nicotine were not attenuated by dopamine (DA) receptor blockade, blockade of mesolimbic DA signalling with either systemic or intra-nucleus accumbens (NAc) neuroleptic pretreatment potentiated the sensitivity to nicotine's rewarding properties over a three-order-of-magnitude dose range. Furthermore, the behavioural effects of lower doses of intra-VTA nicotine were reversed, switching the motivational valence of nicotine from aversive to rewarding. Our results suggest that blockade of mesolimbic DA signalling induced by neuroleptic medications may block selectively the aversive properties of nicotine, thus increasing the vulnerability to nicotine's rewarding and addictive properties by inducing a unique, drug-vulnerable phenotype.

Nucleus accumbens shell and core dopamine: differential role in behavior and addiction

Drug addiction can be conceptualized as a disturbance of behavior motivated by drug-conditioned incentives. This abnormality has been explained by Incentive-Sensitization and Allostatic-Counteradaptive theories as the result of non-associative mechanisms acting at the stage of the expression of incentive motivation and responding for drug reinforcement. Each one of these theories, however, does not account per se for two basic properties of the motivational disturbance of drug addiction: (1). focussing on drug- at the expenses of non-drug-incentives; (2). virtual irreversibility. To account for the above aspects we have proposed an associative learning hypothesis. According to this hypothesis the basic disturbance of drug addiction takes place at the stage of acquisition of motivation and in particular of Pavlovian incentive learning. Drugs share with non-drug rewards the property of stimulating dopamine (DA) transmission in the nucleus accumbens shell but this effect does not undergo habituation upon repeated drug exposure, as instead is the case of non-drug rewards. Repetitive, non-decremental stimulation of DA transmission by drugs in the nucleus accumbens septi (NAc) shell abnormally strengthens stimulus-drug associations. Thus, stimuli contingent upon drug reward acquire powerful incentive properties after a relatively limited number of predictive associations with the drug and become particularly resistant to extinction. Non-contingent occurrence of drug-conditioned incentive cues or contexts strongly facilitates and eventually reinstates drug self-administration. Repeated drug exposure also induces a process of sensitization of drug-induced stimulation of DA transmission in the NAc core. The precise significance of this adaptive change for the mechanism of drug addiction is unclear given the complexity and uncertainties surrounding the role of NAc core DA in responding but might be more directly related to instrumental performance.

The effects of acute haloperidol or risperidone on subjective responses to methamphetamine in healthy volunteers

Despite extensive evidence that selective dopamine antagonists attenuate the reinforcing effects of stimulants in laboratory animals, there is little evidence that dopamine antagonists block the positive subjective effects of stimulants in humans. However, recent evidence suggests that the subjective effects of stimulants in humans may depend in part on serotonin. The goal of this study was to examine the effects of haloperidol, a drug that primarily blocks dopamine receptors, and risperidone, a drug that blocks both dopamine and serotonin receptors, on the physiological and subjective effects of methamphetamine in healthy volunteers. Two groups of subjects participated in a placebo-controlled, within-subject, 2 x 2 repeated measures design. One group was tested with haloperidol (3 mg; N = 18), the other with risperidone (0.75 mg; N = 18). Each subject participated in four sessions receiving all combinations of antagonist or placebo and methamphetamine (20 mg) or placebo. Dependent measures included vital signs and standardized questionnaires of subjective effects. At these doses, both haloperidol and risperidone produced mild sedative-like effects compared to placebo. However, neither drug consistently reduced the stimulant-like effects of methamphetamine. These results add to the growing body of literature suggesting that D(2) dopamine receptor antagonists do not block the euphorigenic subjective effects of stimulant drugs in humans, and also do not support the idea that serotonin contributes significantly to these effects.

Effects of dopamine antagonists with different receptor blockade profiles on morphine-induced place preference in male mice

The effects of dopamine (DA) antagonists with different selectivity for the DA receptors (SCH 23390, 0.5, 0.25, 0.125 mg/kg; haloperidol, 0.2, 0.1 mg/kg; raclopride, 1.2, 0.6, 0.3 mg/kg; risperidone, 0.4, 0.2, 0.1 mg/kg; U-99194A maleate, 40, 20 mg/kg; clozapine, 2.5, 1.25, 0.625 mg/kg) on the acquisition of place conditioning and morphine-induced conditioned place preference (CPP) were explored in male mice. Morphine (40 mg/kg) produced CPP while SCH 23390, haloperidol and clozapine (highest dose) and risperidone (lowest dose) produced conditioned place aversion (CPA). Raclopride and U-99194A maleate did not produce CPP or CPA. Morphine-induced CPP was reversed by the administration of SCH 23390 and risperidone (all doses), haloperidol (highest dose) and raclopride and clozapine (intermediate and lowest doses). U-99194A maleate did not reverse morphine-induced CPP. These results suggest that the conditioned rewarding effects of morphine are mediated by the different subtypes of DA receptors.

Triazolam attenuates amphetamine but not morphine conditioned place preferences

In a series of four experiments the benzodiazepine triazolam was tested for reinforcing effects and for effects on reinforcement induced by amphetamine and morphine. Reinforcement was assessed in a conditioned place preference paradigm. Triazolam did not produce reinforcing or aversive effects when administered in doses ranging from 0.0625 to 0.5 mg/kg. Triazolam did attenuate reinforcing effects produced by 0.75 and 1.25 mg/kg amphetamine. No effect of triazolam was observed on morphine-induced reinforcement. These results indicate that the administration of triazolam can affect the brain mechanisms that mediate the reinforcing effects of amphetamine but not morphine.

Conditioned locomotor activity but not conditioned place preference following intra-accumbens infusions of cocaine

In the first experiment, the conditioned place preference (CPP) paradigm was used to examine the rewarding properties of bilateral microinfusions of cocaine HCl into the nucleus accumbens (0, 12.5, 25, 50, or 100 micrograms). No dose of intra-accumbens cocaine induced a significant CPP. However, bilateral intra-accumbens infusions of d-amphetamine sulfate (10 micrograms) or intraperitoneal administration of cocaine HCl (5 or 10 mg/kg) both produced a significant preference for the drug-paired compartment. In the second experiment, the ability of bilateral intra-accumbens infusions of cocaine HCl (50 micrograms) to elicit conditioned locomotor activity (CLA) was examined. During the conditioning trials, intra-accumbens cocaine significantly increased locomotor activity. On the test day, when no drug was administered, the group that had previously received cocaine in the activity chamber showed significantly greater locomotor activity than the vehicle control group. This demonstration of CLA indicates that rats are able to associate the effects of intra-accumbens infusions of cocaine with environmental stimuli; however, these infusions are not rewarding as measured by the CPP paradigm. In addition, these results may indicate important differences between the neural substrates for cocaine and amphetamine reward and reveal a dissociation between CPP and CLA.

Cocaine enhances retention of avoidance conditioning in rats

The effects of post-training cocaine administration were tested on retention of a one-way active avoidance task in rats. A 5.0 mg/kg IP dose of cocaine enhanced retention of the avoidance task, in three separate experiments, as indicated by an increase in the number of avoidances made when animals were tested 24 h after training, while both a lower (2.5 mg/kg) and a higher (7.5 mg/kg) cocaine dose had no effect. Lidocaine (4-8 mg/kg) administered post-training did not reliably affect retention in the same task. Cocaine's ability to enhance retention depended on the interval between training and drug injection such that only cocaine administered directly after training enhanced retention the following day. The results show that post-training cocaine administration enhances retention of an active avoidance task in rats, and that this effect is probably independent of the anesthetic properties of the drug.

GABA(A) receptors in the ventral tegmental area control bidirectional reward signalling between dopaminergic and non-dopaminergic neural motivational systems

In the midbrain ventral tegmental area (VTA), both dopaminergic and nondopaminergic neural substrates mediate various behavioural reward phenomena. VTA GABAergic neurons are anatomically positioned to influence the activity of both the mesolimbic dopamine system and nondopamine efferents from the VTA. In order to examine the possible functional role of VTA GABA(A) receptors in neural reward processes, we performed discrete, bilateral microinjections of the GABA(A) receptor agonist, muscimol, or the GABA(A) receptor antagonist, bicuculline, into the VTA. Using a fully counterbalanced, unbiased conditioned place-preference paradigm, we demonstrate that activation of VTA GABA(A) receptors, with the GABA(A) receptor agonist muscimol (5--50 ng/microL), or inhibition of VTA GABA(A) receptors, with the GABA(A) receptor antagonist bicuculline (5--50 ng/microL), both produce robust rewarding effects. Furthermore, these rewarding effects can be pharmacologically dissociated: blockade of dopamine receptors with a dopamine receptor antagonist, alpha-flupenthixol (0.8 mg/kg; i.p.), or concurrent activation of VTA GABA(B) receptors with a GABA(B) receptor agonist, baclofen (70 ng/microL), blocked the rewarding properties of the GABA(A) receptor agonist, but had no effect on the rewarding properties of the GABA(A) receptor antagonist. These results suggest that, within the VTA, a single GABA(A) receptor substrate controls bidirectional reward signalling between dopaminergic and nondopaminergic brain reward systems.

Diazepam modifies the effect of pedunculopontine lesions on morphine but not on amphetamine conditioned place preference

We have previously shown that T-maze learning impairments caused by lesions to the pedunculopontine tegmental nucleus (PPTg) can be reversed by the anxiolytic diazepam. We now report that diazepam also reverses the effect of PPTg lesions on conditioned place preference (CPP) to morphine but not to amphetamine. Rats with bilateral sham or N-methyl-D-aspartate lesions (0.1 or 0.05 M) to the PPTg were trained in a unbiased CPP paradigm with 2 mg/kg morphine or 2 mg/kg D-amphetamine associated with one compartment of the apparatus and vehicle injections in the alternative compartment. After three drug/saline-compartment pairings, the preference of the animals was assessed by allowing them to explore the entire apparatus for 20 min. In contrast to sham-lesioned subjects, the rats with PPTg lesions did not show a preference for the compartment paired with morphine or amphetamine. In two experiments the expression of a morphine CPP was restored by injecting the lesioned animals with 1 mg/kg of diazepam 30 min before the test session. Diazepam pre-treatment did not restore the expression of amphetamine CPP.

The cyclo-oxygenase inhibitor nimesulide induces conditioned place preference in rats

Two cyclo-oxygenase inhibitors, indomethacin and nimesulide, have been shown to potentiate morphine-induced stimulation of meso-accumbens dopamine neurons. In this study, an unbiased conditioned place preference procedure was used to evaluate whether nimesulide produces motivational effect after systemic administration in rats. These results show that nimesulide, at doses 0.1, 0.5 and 1 mg/kg, even lower than those usually applied for inflammatory conditions, induces conditioned place preference in rats, suggesting that it might possess rewarding properties in humans.

Circadian rhythms, aging and memory

In human beings and animal models, cognitive performance is often impaired in natural and experimental situations where circadian rhythms are disrupted. This includes a general decline in cognitive ability and fragmentation of behavioural rhythms in the aging population of numerous species. There is some evidence that rhythm disruption may lead directly to cognitive impairment; however, this causal link has not been made for effects due to aging. We have tested this link by examining rhythms and performance on contextual conditioning with the conditioned place preference task, in elderly, age-matched hamsters. Young healthy hamsters developed a preference for a context that is paired with the opportunity to engage in wheel-running (experiment 1). Aged animals with consolidated locomotor rhythms developed similar degrees of preference, whereas the age-matched hamsters with fragmented rhythms did not (experiment 2). The degree of preference was also correlated with activity amplitude. These results support the notion that age-related rhythm fragmentation contributes to the age-related memory decline.

Circadian rhythms, aging and memory

In human beings and animal models, cognitive performance is often impaired in natural and experimental situations where circadian rhythms are disrupted. This includes a general decline in cognitive ability and fragmentation of behavioural rhythms in the aging population of numerous species. There is some evidence that rhythm disruption may lead directly to cognitive impairment; however, this causal link has not been made for effects due to aging. We have tested this link by examining rhythms and performance on contextual conditioning with the conditioned place preference task, in elderly, age-matched hamsters. Young healthy hamsters developed a preference for a context that is paired with the opportunity to engage in wheel-running (experiment 1). Aged animals with consolidated locomotor rhythms developed similar degrees of preference, whereas the age-matched hamsters with fragmented rhythms did not (experiment 2). The degree of preference was also correlated with activity amplitude. These results support the notion that age-related rhythm fragmentation contributes to the age-related memory decline.

Increased rewarding properties of morphine in dopamine-transporter knockout mice

The activation of dopamine (DA) neurotransmission plays a crucial role in the behavioural responses to drugs of abuse. In particular, increased extracellular levels of DA within the mesolimbic pathway have been implicated in the rewarding and locomotor stimulatory properties of morphine. We investigated the behavioural responses to morphine in mice with a genetic disruption of the DA transporter (DAT), resulting in a constitutively high level of extrasynaptic DA. In the conditioned place preference test, DAT-/- mice exhibited a stronger rewarding response to morphine (5 mg/kg, s.c.) compared with control littermates. However, the same dose of morphine failed to increase locomotor activity in DAT-/- mice, whilst enhancing locomotion in DAT+/- and DAT+/+ animals. Morphine-induced analgesia was unaffected in mutant mice, but the behavioural expression of naloxone-induced withdrawal signs was blunted. In vivo voltammetry in the shell of the nucleus accumbens revealed that morphine was able to stimulate DA neurons in DAT-/- mice, resulting in the accumulation of higher extracellular DA levels compared with control animals. Morphine also induced a higher rate of c-fos transcription in the shell of the nucleus accumbens in mutant mice. We conclude that morphine-induced rewarding responses are firmly established in DAT mutant mice despite a DA transmission that is already tonically activated, and independently of any effect on locomotion. These particular behavioural responses to morphine may be associated with the action of the drug on DA release and c-fos expression in the shell of the nucleus accumbens of DAT-/- mice.

Role of dopamine receptor subtypes in the acquisition of a testosterone conditioned place preference in rats

The present experiments investigated the neurochemical bases of the rewarding properties of testosterone, focusing on the role of dopaminergic function in the acquisition of a testosterone conditioned place preference (CPP). In two experiments for 8 alternating days adult male Long-Evans rats received peripheral injections of testosterone in a water-soluble hydroxypropyl-beta-cyclodextrin inclusion complex (0.8 mg/kg) or saline immediately prior to being confined for 30 min to one of two compartments of a place preference apparatus. On day 10 the rats were given a 20-min test session and allowed access to all compartments of the apparatus. No hormone was injected prior to the test, and the amount of time spent in each compartment of the apparatus was recorded. In each experiment administration of testosterone was found to induce a CPP. Injections of the mixed D1/D2 receptor antagonist alpha-flupenthixol (0.3 mg/kg), the selective D1 antagonist SCH 23390 (0.1 mg/kg), or the selective D2 antagonist sulpiride (20 mg/kg), each blocked acquisition of the testosterone CPP. The findings suggest a role for both dopamine D1 and D2 receptor subtypes in the acquisition of testosterone CPP.

Inhibition by ginsenosides Rb1 and Rg1 of cocaine-induced hyperactivity, conditioned place preference, and postsynaptic dopamine receptor supersensitivity in mice

A single or repeated administration of cocaine (15 mg/kg) in mice produced hyperactivity and conditioned place preference (CPP). Ginsenoside Rb1 (Rb1) and ginsenoside Rg1 (Rg1), prior to and during the cocaine treatment in mice, inhibited cocaine-induced hyperactivity and CPP. The development of enhanced postsynaptic dopamine (DA) receptor sensitivity in mice displaying a cocaine-induced CPP was evidenced by the enhanced response in ambulatory activity to the DA agonist, apomorphine (2 mg/kg). Rb1 and Rg1 inhibited the development of postsynaptic DA receptor supersensitivity. However, Rb1 and Rg1 did not show any antidopaminergic activity at the postsynaptic DA receptors, because the apomorphine-induced climbing behavior was not inhibited by Rb1 and Rg1. Therefore, it is presumed that Rb1 and Rg1 modulate DA activity induced by cocaine at the presynaptic DA receptors, and this modulation results in the inhibition of postsynaptic dopaminergic activation. These results suggest that the cocaine-induced CPP may be associated with enhanced DA receptor sensitivity. The inhibition by Rb1 and Rg1 of cocaine-induced hyperactivity and CPP may be closely related with the inhibition of dopaminergic activation induced by cocaine at the presynaptic DA receptors.

Opioid modulation of ventral pallidal inputs

While the ventral pallidum (VP) is known to be important in relaying information between the nucleus accumbens and target structures, it has become clear that substantial information processing occurs within the VP. We evaluated the possibility that opioid modulation of other transmitters contained in VP afferents is involved in this process. Initially, we demonstrated that opioids hyperpolarized VP neurons in vitro and suppressed spontaneous firing in vivo. The ability of opioids to modulate other transmitters was determined using microiontophoretically applied ligands and extracellular recordings of VP neurons from chloral hydrate-anesthetized rats. With neurons that responded to iontophoresed opioid agonists, the ejection current was reduced to a level that was below that necessary to alter spontaneous firing. This "subthreshold" current was used to determine the ability of mu opioid receptor (microR) agonists to alter VP responses to endogenous (released by electrical activation of afferents) and exogenous (iontophoretically applied) transmitters. microR agonists decreased the variability and enhanced the acuity (e.g., "signal-to-noise" relationship) of VP responses to activation of glutamatergic inputs from the prefrontal cortex and amygdala. By contrast, microR agonists attenuated both the slow excitatory responses to substance P and GABA-induced inhibitions that resulted from activating the nucleus accumbens. Subthreshold opioids also attenuated inhibitory responses to stimulating midbrain dopaminergic cells. These results suggest that a consequence of opioid transmission in the VP is to negate the influence of some afferents (e.g., midbrain dopamine and accumbal GABA and substance P) while selectively potentiating the efficacy of others (e.g., cortical and amygdaloid glutamate). Interpreted in the context of opiate abuse, microR opioids in the VP may serve to diminish the influence of reinforcement (ventral tegmental area and nucleus accumbens) in the transduction of cognition (prefrontal cortex) and affect (amygdala) into behavior. This may contribute to drug craving that occurs even in the absence of reward.

The effects of quinine and 4-aminopyridine on conditioned place preference and changes in motor activity induced by morphine in rats

1. The effects of two unselective potassium (K(+)-) channel blockers, quinine (12.5, 25 and 50 mg/kg) and 4-aminopyridine (1 and 2 mg/kg), on conditioned place preference and biphasic changes in motor activity induced by morphine (10 mg/kg) were tested in Wistar rats. Quinine is known to block voltage-, calcium- and ATP-sensitive K(+)-channels while 4-aminopyridine is known to block voltage-sensitive K(+)-channels. 2. In the counterbalanced method, quinine attenuated morphine-induced place preference, whereas 4-aminopyridine was ineffective. In the motor activity test measured with an Animex-activity meter neither of the K(+)-channel blockers affected morphine-induced hypoactivity, but both K(+)-channel blockers prevented morphine-induced secondary hyperactivity. 3. These results suggest the involvement of quinine-sensitive but not 4-aminopyridine-sensitive K(+)-channels in morphine reward. It is also suggested that the blockade of K(+)-channels sensitive to these blockers is not sufficient to prevent morphine-induced hypoactivity whereas morphine-induced hyperactivity seems to be connected to both quinine- and 4-aminopyridine-sensitive K(+)-channels.