Conditioned saccharin avoidance induced by infusion of amphetamine in the nucleus accumbens shell and morphine in the ventral tegmental area: behavioral and biochemical study.

Drugs of abuse possess the seemingly paradoxical property of conditioning rats to avoid from drinking a saccharin solution that had been predictively paired with their systemic administration (conditioned saccharin avoidance, CSA). CSA is dependent upon an intact dopamine (DA) transmission but the locus, central or peripheral, and eventually the brain area from which this effect originates and its relationship with the rewarding properties of the drug is debated. In order to clarify this issue we tested the ability of amphetamine and morphine to induce CSA after infusion at the same dose-range and in the same areas from which these drugs induce conditioned place preference (CPP). Drugs were infused intracerebrally immediately after saccharin drinking in two acquisition trials and CSA was tested on a two bottle saccharin/water choice. Amphetamine (10 and 20 µg/0.5 µl) induced CSA after infusion in the NAc shell but was ineffective in the NAc core. Morphine (0.5 and 1 µg/0.5 µl) induced CSA from the VTA at both doses tested. Amphetamine (20 µg/0.5 µl) and morphine (1 µg/0.5 µl) failed to induce CSA after infusion 1.2mm dorsal the NAc shell and the VTA respectively. Finally, morphine (1 µg/0.5 µl), infused in the VTA, elicited a selective increase in dialysate DA in the NAc shell. These results indicate that drugs of abuse induce CSA from the same intracerebral sites and at the same doses at which they induce CPP. These observations are consistent with the existence of a strong relationship between CSA and drug reward related to their ability to stimulate DA transmission in the NAc shell.

Behavioural sensitization after repeated exposure to Delta 9-tetrahydrocannabinol and cross-sensitization with morphine.

RATIONALE: Repeated exposure to several drugs of abuse has been reported to induce behavioural sensitization. So far no evidence has been provided that such a phenomenon also applies to cannabinoids. OBJECTIVES: In this study we investigated if repeated exposure to Delta(9)-tetrahydrocannabinol (Delta(9)-THC) induces behavioural sensitization. In addition we tested the possibility of cross-sensitization between Delta(9)-THC and morphine. METHODS: Male Sprague-Dawley rats were administered for 3 days, twice daily, with increasing doses of Delta(9)-tetrahydrocannabinol (2, 4 and 8 mg/kg i.p.) or increasing doses of morphine (10, 20 and 40 mg/kg s.c.) or vehicle. After a washout of 14 days the animals were challenged with Delta(9)-THC (75 and 150 microg/kg i.v.), with a synthetic cannabinoid agonist WIN55212-2 (75 and 150 microg/kg i.v.) or with morphine (0.5 mg/kg i.v.), through a catheter inserted into the left femoral vein 24 h before, and the behaviour recorded. RESULTS: Rats previously administered with Delta(9)-THC showed a greater behavioural activation compared to controls in response to challenge with Delta(9)-THC (150 microg/kg i.v.) and to challenge with morphine (0.5 mg/kg i.v.). Similar to that observed after repeated opiates, this behavioural sensitization was characterized by stereotyped activity. Animals administered with a schedule of morphine that induces behavioural sensitization to morphine also showed a behavioural sensitization to challenge with cannabinoids (Delta(9)-HC and WIN55212-2, 75 and 150 microg/kg i.v.). The effect of the challenge with Delta(9)-THC was prevented by the administration of the CB1 antagonist SR141716A (1 mg/kg i.p.), 40 min beforehand. CONCLUSIONS: The results of the present study demonstrate that repeated exposure to Delta(9)-THC induces behavioural sensitization not only to cannabinoids but also to opiates. This cross-sensitization was symmetrical since rats behaviourally sensitized to morphine were also sensitized to cannabinoids. These observations further support the evidence of an interaction between the opioid and the cannabinoid system and might provide a neurobiological basis for a relationship between cannabis use and opiate abuse.

Intravenous administration of ecstasy (3,4-methylendioxymethamphetamine) enhances cortical and striatal acetylcholine release in vivo.

The effect of intravenous administration of 3,4-methylendioxymethamphetamine (MDMA), in a range of doses (0.32-3.2 mg/kg) that have been shown to maintain self-administration behaviour in rats, on in vivo acetylcholine release from rat prefrontal cortex and dorsal striatum was studied by means of microdialysis with vertical concentric probes. Intravenous administration of MDMA dose-dependently increased basal acetylcholine release from the prefrontal cortex to 57+/-21%, 98+/-20%, 102+/-7% and 141+/-14% above baseline, at doses of 0.32, 0.64, 1.0 and 3.2 mg/kg, respectively. MDMA also stimulated striatal acetylcholine release at the dose of 3.2 mg/kg i.v. (the maximal increase being 32+/-3% above baseline) while at the dose of 1 mg/kg i.v., MDMA failed to affect basal acetylcholine output. Administration of MDMA also dose-dependently stimulated behaviour. The results of the present study show that MDMA affects measures of central cholinergic neurotransmission in vivo and suggest that at least some of the psychomotor stimulant actions of MDMA might be positively coupled with an increase in prefrontal cortical and striatal acetylcholine release.

Psychostimulant sensitization: differential changes in accumbal shell and core dopamine.

The nucleus accumbens has been subdivided into a shell and a core compartment on the basis of histochemical and connectional differences. Recently, we reported that behavioral sensitization to morphine is associated with an increased dopamine transmission in the caudate-putamen and in the nucleus accumbens core as well as a decreased response in the nucleus accumbens shell following acute morphine challenge. We have now performed a similar study in rats sensitized to amphetamine and to cocaine. Behavioral sensitization was induced by daily administration of a single dose of 1 mg/kg s.c. of amphetamine for 10 days or of 10 mg/kg i.p. of cocaine twice a day for 14 days. Microdialysis was performed 10-14 days after the last injection of amphetamine and 7-10 days after the last injection of cocaine. Both schedules resulted in robust behavioral sensitization in response to challenge with 0.25 and 0.5 mg/kg of amphetamine and to 5 and 10 mg/kg of cocaine, respectively. Subjects pre-exposed to amphetamine showed a sensitization of dopamine transmission in the nucleus accumbens core but not in the nucleus accumbens shell. Subjects pre-exposed to cocaine showed sensitization of dopamine transmission in the core only to the lower dose of cocaine. In the shell no change was observed after the lower dose of cocaine while a significant reduction of the dopamine response was observed after the higher dose. These results suggest that behavioral sensitization might result from reciprocal changes in the response of nucleus accumbens dopamine in the shell and in the core to drug challenge.

Differential changes in accumbens shell and core dopamine in behavioral sensitization to nicotine.

Repeated treatment with nicotine has been shown to sensitize rats to its locomotor stimulant effects and to its properties to stimulate mesolimbic dopamine transmission. We investigated the relationship between sensitization of nicotine induced locomotor stimulation and activation of dopamine transmission in the nucleus accumbens shell and core. Rats were administered daily for 5 days with 0.4 mg/kg s.c. of nicotine or with saline and 24 h later, dopamine was monitored by microdialysis in the shell and in the core of nucleus accumbens and behavioral activity was scored after challenge with nicotine (0. 4 mg/kg s.c.). Behavioral sensitization to nicotine was associated with a reduced response of dopamine transmission in the shell and with an increased one in the core of nucleus accumbens.

Drug addiction as a disorder of associative learning. Role of nucleus accumbens shell/extended amygdala dopamine.

Conventional reinforcers phasically stimulate dopamine transmission in the nucleus accumbens shell. This property undergoes one-trial habituation consistent with a role of nucleus accumbens shell dopamine in associative learning. Experimental studies with place- and taste-conditioning paradigms confirm this role. Addictive drugs share with conventional reinforcers the property of stimulating dopamine transmission in the nucleus accumbens shell. This response, however, undergoes one-trial habituation in the case of conventional reinforcers but not of drugs. Resistance to habituation allows drugs to repetitively activate dopamine transmission in the shell upon repeated self-administration. This process abnormally facilitates associative learning, leading to the attribution of excessive motivational value to discrete stimuli or contexts predictive of drug availability. Addiction is therefore the expression of the excessive control over behavior acquired by drug-related stimuli as a result of abnormal strenghtening of stimulus-drug contingencies by nondecremental drug-induced stimulation of dopamine transmission in the nucleus accumbens shell.

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

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

Dopamine-dependent behavioural stimulation by non-peptide delta opioids BW373U86 and SNC 80: 2. Place-preference and brain microdialysis studies in rats.

The motivational properties of the non-peptide delta-opioid receptor agonists BW373U86 and SNC 80 were investigated using the place-conditioning paradigm. BW373U86 (0.5-1.0 mg/kg s.c.) and SNC 80 (1.25-5.0 mg/kg s.c.) elicited significant preference for the drug-paired compartment, in a dose-related fashion. Naltrindole (5.0 mg/kg s.c.) pretreatment, while failing to modify preference when given alone, completely prevented place-preference induced by BW373U86 (1.0 mg/kg s.c.) and SNC 80 (1.25 mg/kg s.c.). The dopamine D1 receptor antagonist SCH23390, given at doses that do not affect place-preference (0.012 mg/kg s.c.), completely prevented the place-preference induced by BW373U86 and SNC 80. At the doses effective in eliciting place-preference, BW373U86 and SNC 80 failed to modify extracellular dopamine in the medial nucleus accumbens, while in the dorso-lateral caudate-putamen BW373U86 (1.0 and 2.5 mg/kg s.c.) reduced extracellular dopamine, and this effect was prevented by naltrindole (5.0 mg/kg s.c.). SNC 80, only at the dose of 5 mg/kg s.c., significantly reduced extracellular DA in the dorso-lateral caudate-putamen. The results indicate that stimulation of delta-opioid receptors has incentive properties that might be related to an indirect amplification of post-synaptic dopamine transmission.

Role of vesicular dopamine in the in vivo stimulation of striatal dopamine transmission by amphetamine: evidence from microdialysis and Fos immunohistochemistry.

The role of vesicular and newly synthesized dopamine in the action of amphetamine was investigated by studying the effect of reserpine and alpha-methyl-p-tyrosine pretreatment on amphetamine-induced changes in extracellular dopamine and acetylcholine, estimated by brain microdialysis, and on c-fos expression, estimated by quantitative immunohistochemistry of the Fos antigene, in the dorsal caudate-putamen of rats. Blockade of dopamine synthesis by alpha-methyl-p-tyrosine pretreatment (1 or 2 h) only partially prevented the increase in extracellular dopamine concentrations elicited by 0.5 and 2 mg/kg s.c. of amphetamine. Inactivation of vesicular amine uptake by reserpine pretreatment (3 h) reduced the increase in extracellular dopamine by 2 mg/kg but not by 0.5 mg/kg of amphetamine. Combined pretreatment with reserpine (3 h) and alpha-methyl-p-tyrosine (1 h) drastically reduced the increase in extracellular dopamine by both doses of amphetamine (0.5 and 2 mg/kg s.c.). alpha-Methyl-p-tyrosine pretreatment reduced c-fos expression stimulated by amphetamine (2 mg/kg) in the dorsomedial and dorsolateral caudate-putamen while reserpine pretreatment reduced it only in the dorsolateral caudate-putamen. Amphetamine (2 mg/kg s.c.) stimulated acetylcholine release but this effect was not modified by reserpine or alpha-methyl-p-tyrosine pretreatment. The results indicate that blockade of dopamine synthesis, by itself, is insufficient to prevent the stimulation of dopamine transmission by amphetamine and, conversely, that inactivation of vesicular dopamine significantly reduces amphetamine effects at pre- and postsynaptic levels. Therefore, vesicular dopamine appears to contribute to the stimulation of dopamine transmission elicited by amphetamine in the dorsal caudate-putamen.

Neuroleptics increase striatal acetylcholine release by a sequential D-1 and D-2 receptor mechanism.

In normal striata, pre-treatment with the D-1 antagonist SCH 23390 (250 and 25 micrograms kg-1, s.c.) completely and lastingly prevented the D-2 antagonist remoxipride (REM) from increasing acetylcholine (ACh) release; post-treatment did not affect REM action. In alpha-methyl-p-tyrosine (alpha-MpT) dopamine (DA)-depleted striata, however, pretreatment with SCH 23390 resulted in a transient impairment of REM induced stimulation of ACh release but post-treatment still had no effect. Two different mechanisms therefore seem to be involved in D-2 antagonist-induced stimulation of ACh release; the antagonists indirectly activate a D-1 receptor-mediated facilitatory mechanism regulating cholinergic function, and directly block a D-2 receptor-mediated inhibitory one. In normal rats, in an early phase after D-2 antagonist administration, only the first mechanism is operative; in a later phase, both mechanisms cooperate in the stimulation of ACh release. When DA release is impaired by alpha-MpT, the D-2 inhibitory receptor mechanism becomes more important than the D-1 facilitatory one in controlling ACh release.

Drugs of abuse: biochemical surrogates of specific aspects of natural reward?

In this paper it is argued that drugs of abuse act on specific neurotransmitter pathways and by this mechanism elicit neurochemical changes that mimic some aspects of the overall pattern of the neurochemical effects of natural rewarding stimuli. Thus, drugs of abuse are biochemically homologous to specific aspects of natural rewarding stimuli. The behavioral similarity between drugs of abuse and natural stimuli, including that of being rewarding, results from their common property of activating neurochemically specific pathways. Natural stimuli accomplish this result indirectly through their sensory properties and incentive learning while drugs stimulate by a direct central action the critical reward pathways. Many drugs of abuse mimic the incentive properties of natural stimuli and their ability to stimulate mesolimbic dopamine pathways (Fig. 1). Both natural rewards and drugs of abuse, including amphetamine, cocaine and other psychostimulants, preferentially stimulate dopamine transmission in the mesolimbic nucleus accumbens compared with the dorsal caudate, an area related to the extrapyramidal motor system. Although many drugs of abuse mimic the incentive aspect of natural reward, this is probably not an absolute prerequisite for conferring to a drug some abuse liability. It might be predicted that certain drugs might be abused as a result of their action at sites located beyond dopamine or by mimicking other aspects of naturally rewarding stimuli such as the 'functional' (or trophotropic). This might be the case with opiates (which also mimic the 'incentive' aspect) and of benzodiazepines, as a result of activation of the central opioid reward system and of the central gamma-aminobutyric acid (GABA)-benzodiazepine system respectively. The hypothesis appears to have heuristic value as it predicts that biochemical mechanisms important for the rewarding properties of drugs of abuse are expected to play a role also in natural reward. One test of this hypothesis is offered by the observation that the 5-hydroxytryptamine (5-HT) system, through 5-HT3 receptors, and the central opioid system, through delta-opioid receptors, can contribute to the mechanism of the dopamine-activating properties of certain drugs of abuse. On this basis it would be predicted that drugs acting on 5-HT3 and on delta-opioid receptors would interfere with or mimic certain aspects of natural rewarding stimuli.

Complex interactions between the steroid derivative RU 5135 and the GABAA-receptor complex.

The modulation of [35S]t-butylbicyclophosporothionate ([35S]TBPS) binding was used to evaluate the actions of the steroid derivative RU 5135 at the gamma-aminobutyric acidA (GABAA) receptor complex. The inhibition of [35S]TBPS binding by GABA in the presence of various concentrations of RU 5135 was consistent with the hypothesis that RU 5135 is a competitive antagonist at the GABAA receptor. Despite common structural features (i.e., 3 alpha-hydroxylated, 5 beta-reduced A ring) with GABAA receptor-active neurosteroids, RU 5135 did not appear to be competitive at the putative steroid site on the GABAA receptor-active, as demonstrated by Schild analysis of 5 alpha-pregnane-3 alpha-ol-20-one (3 alpha,5 alpha-P) modulation of [35S]TBPS binding in the presence of different concentrations of RU 5135. On the other hand, the reduced potency of 3 alpha,5 alpha-P as an inhibitor of [35S]TBPS binding in the presence of RU 5135, as well as blockade of 5 alpha-pregnane-3 alpha-20 alpha-diol (5 alpha-pregnanediol) inhibition of [35S]TBPS binding by RU 5135 provide further support for the GABAA receptor antagonist properties of RU 5135. Moreover, this amidine steroid was able to partially inhibit [35S]TBPS binding independent of GABA with nanomolar potency; yet the mechanism by which this occurs remains to be determined.

Calcium-dependent, tetrodotoxin-sensitive stimulation of cortical serotonin release after a tryptophan load.

The effect of intraperitoneal administration of tryptophan (50, 100, or 200 mg/kg) on extracellular concentrations of tryptophan, serotonin (5-hydroxytryptamine, 5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) was studied in the cortex of freely moving rats by transcerebral dialysis. Rats were implanted with dialysis probes in the frontal cortex, and experiments were performed 24 h later. Tryptophan, 5-HT, and 5-HIAA were quantified in 20-min samples of dialysate by HPLC with electrochemical detection after separation on reverse-phase columns. Tryptophan administration resulted in a significant increase of tryptophan, 5-HT, and 5-HIAA levels in dialysates. The maximal increase of 5-HT and 5-HIAA output was approximately 150% over basal values. Perfusion with Ringer's solution containing tetrodotoxin (1 microM) reduced 5-HT output by 90% and prevented the increase of 5-HT and 5-HIAA content after 100 mg/kg of tryptophan. Similar results were obtained after perfusion with Ringer's solution without Ca2+. The results indicate that a tryptophan load stimulates the physiological release of 5-HT.