The dopamine D3 receptor and autoreceptor preferring antagonists (+)-AJ76 and (+)-UH232; a microdialysis study

Preclinical Pharmacology of [2-(3-Fluoro-5-Methanesulfonyl-phenoxy)Ethyl](Propyl)amine (IRL790), a Novel Dopamine Transmission Modulator for the Treatment of Motor and Psychiatric Complications in Parkinson Disease

IRL790 ([2-(3-fluoro-5-methanesulfonylphenoxy)ethyl](propyl)amine, mesdopetam) is a novel compound in development for the clinical management of motor and psychiatric disabilities in Parkinson disease. The discovery of IRL790 was made applying a systems pharmacology approach based on in vivo response profiling. The chemical design idea was to develop a new type of DA D3/D2 receptor type antagonist built on agonist rather than antagonist structural motifs. We hypothesized that such a dopamine antagonist with physicochemical properties similar to agonists would exert antidyskinetic and antipsychotic effects in states of dysregulated dopaminergic signaling while having little negative impact on physiologic dopamine transmission and, hence, minimal liability for side effects related to dopamine-dependent functions. At the level of in vivo pharmacology, IRL790 displays balancing effects on aberrant motor phenotypes, reducing l-DOPA-induced dyskinesias in the rodent 6-hydroxydopamine lesion model and reducing psychostimulant-induced locomotor hyperactivity elicited by pretreatment with either d-amphetamine or dizocilpine, without negatively impacting normal motor performance. Thus, IRL790 has the ability to normalize the behavioral phenotype in hyperdopaminergic as well as hypoglutamatergic states. Neurochemical and immediate early gene (IEG) response profiles suggest modulation of DA neurotransmission, with some features, such as increased DA metabolites and extracellular DA, shared by atypical antipsychotics and others, such as increased frontal cortex IEGs, unique to IRL790. IRL790 also increases extracellular levels of acetylcholine in the prefrontal cortex and ventral hippocampus. At the receptor level, IRL790 appears to act as a preferential DA D3 receptor antagonist. Computational docking studies support preferential affinity at D3 receptors with an agonist-like binding mode. SIGNIFICANCE STATEMENT: This paper reports preclinical pharmacology along with molecular modeling results on IRL790, a novel compound in clinical development for the treatment of motor and psychiatric complications in advanced Parkinson disease. IRL790 is active in models of perturbed dopaminergic and glutamatergic signaling, including rodent 6-hydroxydopamine l-DOPA-induced dyskinesias and psychostimulant-induced hyperactivity, in a dose range that does not impair normal behavior. This effect profile is attributed to interactions at dopamine D2/D3 receptors, with a 6- to 8-fold preference for the D3 subtype.

Ghrelin increases intake of rewarding food in rodents

We investigated whether ghrelin action at the level of the ventral tegmental area (VTA), a key node in the mesolimbic reward system, is important for the rewarding and motivational aspects of the consumption of rewarding/palatable food. Mice with a disrupted gene encoding the ghrelin receptor (GHS-R1A) and rats treated peripherally with a GHS-R1A antagonist both show suppressed intake of rewarding food in a free choice (chow/rewarding food) paradigm. Moreover, accumbal dopamine release induced by rewarding food was absent in GHS-R1A knockout mice. Acute bilateral intra-VTA administration of ghrelin increased 1-hour consumption of rewarding food but not standard chow. In comparison with sham rats, VTA-lesioned rats had normal intracerebroventricular ghrelin-induced chow intake, although both intake of and time spent exploring rewarding food was decreased. Finally, the ability of rewarding food to condition a place preference was suppressed by the GHS-R1A antagonist in rats. Our data support the hypothesis that central ghrelin signaling at the level of the VTA is important for the incentive value of rewarding food.

Glycine receptors involved in acamprosate's modulation of accumbal dopamine levels: an in vivo microdialysis study

BACKGROUND

Glycine receptors (GlyRs) in the nucleus accumbens (nAc) and nicotinic acetylcholine receptors (nAChRs) in the ventral tegmental area (VTA) have been suggested to be involved in the positive reinforcing and dopamine elevating effects of ethanol. Recent studies have also shown that ethanol high-preferring rats substantially decrease their ethanol intake when treated with a glycine transporter 1 inhibitor (ORG 25935). Acamprosate, a drug used for relapse prevention in treatment of alcohol dependence, has also been demonstrated to elevate extracellular dopamine levels in the nAc. However, the underlying mechanism of action of acamprosate is not fully understood. Here we investigated whether acamprosate interferes with a neuronal circuitry that previously has been demonstrated to be involved in the dopamine elevating effects of ethanol and taurine.

METHODS

In vivo microdialysis in freely moving rats was used to assess accumbal dopamine levels before and during local (nAc) or systemic administration of acamprosate.

RESULTS

Perfusion of 0.5 mM acamprosate in the nAc significantly increased dopamine levels. Pretreatment either with 10 microM strychnine in the nAc or 100 microM mecamylamine in the VTA, completely antagonized the acamprosate-induced elevation of accumbal dopamine levels. Also, systemic acamprosate administration elevated accumbal dopamine output, an effect that was abolished by local (nAc) pretreatment with 10 microM strychnine.

CONCLUSIONS

These results suggest that both systemic and local application of acamprosate elevate extracellular dopamine levels in the nAc by activating accumbal GlyRs, and, secondarily, tegmental nAChRs.

Alpha-conotoxin MII-sensitive nicotinic acetylcholine receptors are involved in mediating the ghrelin-induced locomotor stimulation and dopamine overflow in nucleus accumbens

Previously, we have reported that the orexigenic peptide ghrelin activates the cholinergic-dopaminergic reward link, involving nicotinic acetylcholine receptors (nAChR). The alpha(3)-alpha(7) and beta(2)-beta(4) subunits of the nAChR can be combined into pentameric nAChRs, with different functional roles. The present experiments show that the locomotor stimulatory effects of ghrelin, either into laterodorsal tegmental area (LDTg) or ventral tegmental area (VTA), are mediated via ventral tegmental nAChR, but neither the alpha(4)beta(2) (using dihydro-beta-erythroidine) nor the alpha(7) (using methyllycaconitine) subtypes appears to be involved. On the other hand, the alpha(3)beta(2), beta(3) and/or alpha(6) (using alpha-conotoxin MII) subtypes in the VTA mediate the stimulatory and DA-enhancing effects of ghrelin, a pattern that ghrelin shares with ethanol (n=5-8). Radioligand-binding experiments shown that ghrelin does not interfere directly with nAChRs (n=26). We therefore suggest that the alpha(3)beta(2), beta(3) and/or alpha(6) subtypes might be pharmacological targets for treatment of addictive behaviours including compulsive overeating and alcoholism.

Nicotinic acetylcholine receptors in the ventral tegmental area mediate the dopamine activating and reinforcing properties of ethanol cues

RATIONALE

Cues associated with alcohol can elicit craving, support drug-seeking and precipitate relapse.

OBJECTIVES

We investigated the possible involvement of nicotinic acetylcholine receptors (nAChRs) in the ventral tegmental area (VTA) in the conditioned reinforcing properties of ethanol-associated stimuli in the rat.

MATERIALS AND METHODS

First, using in vivo microdialysis, we analyzed the effect of VTA perfusion of the nonselective nAChR antagonist mecamylamine (MEC) or the selective alpha4beta2* nAChR antagonist dihydro-beta-erythroidine (DHbetaE) on the nucleus accumbens (nAc) dopaminergic response to the presentation of an ethanol-associated conditioned stimulus (CS). Second, rats were trained to associate a tone+light CS with the presentation of 10% ethanol and were subsequently tested on the acquisition of a new instrumental response with conditioned reinforcement (CR) after local VTA infusion of MEC, DHbetaE, or alpha-Conotoxin MII (alpha-CtxMII, a selective alpha3beta2* and alpha6* nAChR antagonist).

RESULTS

The ethanol-associated CS elevated nAc dopamine, an effect that was blocked by VTA perfusion of MEC but not DHbetaE. Systemic administration of MEC or local VTA infusion of MEC or alpha-CtxMII selectively blocked ethanol-associated CR, whereas systemic DHbetaE had no effect.

CONCLUSIONS

We hypothesize a novel mechanism by which alcohol-associated cues promote drug-seeking behavior via activation of dopamine-stimulating alpha-CtxMII-sensitive nAChRs in the VTA. Pharmacological manipulations of selective nAChRs may thus be possible treatment strategies to prevent cue-induced relapse.

Ghrelin administration into tegmental areas stimulates locomotor activity and increases extracellular concentration of dopamine in the nucleus accumbens

Ghrelin stimulates appetite, increases food intake and causes adiposity by mechanisms that include direct actions on the brain. Previously, we showed that intracerebroventricular administration of ghrelin has stimulatory and dopamine-enhancing properties. These effects of ghrelin are mediated via central nicotine receptors, suggesting that ghrelin can activate the acetylcholine-dopamine reward link. This reward link consists of cholinergic input from the laterodorsal tegmental area (LDTg) to the mesolimbic dopamine system that originates in the ventral tegmental area (VTA) and projects to the nucleus accumbens. Given that growth hormone secretagogue receptors (GHSR-1A) are expressed in the VTA and LDTg, brain areas involved in reward, the present series of experiments were undertaken to examine the hypothesis that these regions may mediate the stimulatory and dopamine-enhancing effects of ghrelin, by means of locomotor activity and in vivo microdialysis in freely moving mice. We found that local administration of ghrelin into the VTA (1 microg in 1 microl) induced an increase in locomotor activity and in the extracellular concentration of accumbal dopamine. In addition, local administration of ghrelin into the LDTg (1 microg in 1 microl) caused a locomotor stimulation and an increase in the extracellular levels of accumbal dopamine. Taken together, this indicates that ghrelin might, via activation of GHSR-1A in the VTA and LDTg, stimulate the acetylcholine-dopamine reward link, implicating that ghrelin is a part of the neurochemical overlap between the reward systems and those that regulate energy balance.

Ethanol-induced dopamine elevation in the rat — Modulatory effects by subchronic treatment with nicotinic drugs

Chronic nicotine administration is associated with increased ethanol consumption in laboratory animals and in humans. Some smokers report less sedation during acute ethanol intoxication after nicotine administration and the sedative effects from ethanol are mediated by inhibitory GABA(A)-receptors. In a series of in vivo microdialysis experiments we investigated whether subchronic pre-treatment with nicotinic drugs known to enhance ethanol consumption in the rat (nicotine or the peripheral nicotinic antagonist hexamethonium) could modulate the alterations in extracellular dopamine observed in response to administration of ethanol or the sedative GABA(A)-agonist diazepam. In the nucleus accumbens and the dorsal striatum, systemic and/or local ethanol administration resulted in transient increases in extracellular dopamine levels that returned to baseline before the local levels of ethanol started to decline. In hexamethonium pre-treated rats, however, the nucleus accumbens dopamine levels were time-locked to the ethanol levels in the same area after systemic or local ethanol administration. Perfusion of diazepam into the nucleus accumbens produced a significant reduction in nucleus accumbens dopamine in controls. Prior subchronic treatment with nicotine or hexamethonium abolished this effect. The present results suggest that subchronic treatment with the nicotinic acetylcholine receptor antagonist hexamethonium reduces a GABA(A)-R mediated counteraction of the nucleus accumbens dopamine response to ethanol. Additionally, we demonstrate that modulation of nicotinic receptors may reduce the sensitivity of GABA(A) receptors to benzodiazepines. These phenomena may offer a novel explanation to why nicotine and alcohol are often co-abused.

Characterization of ethanol-induced dopamine elevation in the rat nucleus accumbens

Ethanol-induced accumbal dopamine elevations have been linked to ethanol consumption. It is unclear, however, where along the mesolimbic dopamine system this effect is initiated and why the ethanol-induced dopamine elevations are transient, returning to pre-drug baseline before brain and blood ethanol levels decline. Using in vivo microdialysis, Experiment 1 investigated the effect of local ethanol application in the nucleus accumbens, the ventral tegmental area and the nucleus accumbens+the ventral tegmental area, on accumbal dopamine. Experiment 2 examined whether the rapid withdrawal of dopamine response to ethanol involves activation of GABA(A)-receptors, by analyzing the effect of accumbal co-perfusion of picrotoxin and ethanol. In Experiment 1, ethanol perfusion into the ventral tegmental area alone did not affect accumbal dopamine. Ethanol co-perfusion of one of the tested doses into the ventral tegmental+the nucleus accumbens produced higher dopamine levels than ethanol perfusion into the nucleus accumbens alone during 120-160 min following perfusion onset. In Experiment 2, accumbal ethanol perfusion caused a transient increase in nucleus accumbens dopamine. Co-perfusion of ethanol and picrotoxin produced a sustained dopamine elevation. These data support the hypothesis that the primary effect of ethanol on accumbal dopamine is in the nucleus accumbens, but that a secondary effect of nucleus accumbens ethanol perfusion, such as release of acetylcholine in the ventral tegmental area, enables ethanol to act as a nicotinic acetylcholine receptor co-agonist in this area. Moreover, recruitment of GABA(A)-receptor activity appears responsible for the second, declining phase with respect to dopamine levels following ethanol administration.

Role of the subunit composition of central nicotinic acetylcholine receptors for the stimulatory and dopamine-enhancing effects of ethanol

AIMS

The stimulatory, rewarding, and dopamine (DA)-enhancing effects of ethanol may involve central nicotinic acetylcholine receptors (nAChR), especially those located in the ventral tegmental area (VTA). Identifying the subunit composition that mediates these effects of ethanol would increase the understanding of the neurochemical basis underlying the addictive properties of ethanol. In the present series of experiments, the role of the alpha(3)beta(2)(*) and/or beta(3)(*) and/or alpha(6)(*) subunits of the nAChR for the stimulatory and DA-enhancing effects of ethanol was investigated by using alpha-conotoxin MII (alphaCtxMII), selective to the alpha(3)beta(2)(*) and/or beta(3)(*) and/or the alpha(6)(*) subunits of the nAChR, and the alpha-conotoxin PIA-analogue (alphaCtxPIA-analogue), suggested to be selective to the alpha(6)(*) subunits.

METHODS

alphaCtxMII and the alphaCtxPIA-analogue were synthesized using a modified literature procedure. The purity and identity of the peptides were confirmed with HPLC and FAB-MS analyses, respectively. Locomotor activity and in vivo microdialysis in freely moving mice were used.

RESULTS

alphaCtxMII and the alphaCtxPIA-analogue were synthesized in good yields (>95%; >90%). In addition, we found that synthesized alphaCtxMII antagonized ethanol-induced locomotor stimulation, which confirms our previous results with the commercially available alphaCtxMII. Furthermore, the synthesized alphaCtxPIA-analogue, assumably also selective for alpha(6)(*) subunits of the nAChR, did neither antagonize the stimulatory nor the accumbal DA-enhancing effects of ethanol.

CONCLUSION

These results indicate that alphaCtxMII- but not alphaCtxPIA-analogue-sensitive receptors, i.e. the alpha(3)beta(2)(*) and/or beta(3)(*) rather than the alpha(6)(*) subunits of the nAChR, appear to be of greater importance for these effects of ethanol and that these subunits could constitute neurochemical targets for developing new drugs for the treatment of alcohol dependence.

Taurine elevates dopamine levels in the rat nucleus accumbens; antagonism by strychnine

The mesolimbic dopamine (DA) system, projecting from the ventral tegmental area (VTA) to the nucleus accumbens (nAcc), is involved in reward-related behaviours and addictive processes, such as alcoholism and drug addiction. It was recently suggested that strychnine-sensitive glycine receptors (GlyR) in the nAcc regulate both basal and ethanol-induced mesolimbic DA activity via a neuronal loop involving endogenous activation of nicotinic acetylcholine receptors (nAChR) in the VTA. However, as the nAcc appears to contain few glycine-immunoreactive cell bodies or fibres, the question as to what may be the endogenous ligand for GlyRs in this brain region remains open. Here we have investigated whether the amino acid taurine could serve this purpose using in vivo microdialysis in awake, freely moving male Wistar rats. Local perfusion of taurine (1, 10 or 100 mm in the perfusate) increased DA levels in the nAcc. The taurine (10 mm)-induced DA increase was, similarly to that previously observed after ethanol, completely blocked by (i) perfusion of the competitive GlyR antagonist strychnine in the nAcc, (ii) perfusion of the nAChR antagonist mecamylamine (100 microm) in the VTA, and (iii) systemic administration of the acetylcholine-depleting drug vesamicol (0.4 mg/kg, i.p). The present results suggest that taurine may be an endogenous ligand for GlyRs in the nAcc and that the taurine-induced elevation of DA levels in this area, similarly to that observed after local ethanol, is mediated via a neuronal loop involving endogenous activation of nAChRs in the VTA.

Ghrelin stimulates locomotor activity and accumbal dopamine-overflow via central cholinergic systems in mice: implications for its involvement in brain reward

It is becoming increasingly apparent that there is a degree of neurochemical overlap between the reward systems and those regulating energy balance. We therefore investigated whether ghrelin, a stomach-derived and centrally derived orexigenic peptide, might act on the reward systems. Central ghrelin administration (1 microg/microL, to the third ventricle) induced an acute increase in locomotor activity as well as dopamine-overflow in the nucleus accumbens, suggesting that ghrelin can activate the mesoaccumbal dopamine system originating in the ventral tegmental area, a system associated with reward and motivated behaviour. The cholinergic afferents to the ventral tegmental area have been implicated in natural reward and in regulating mesoaccumbal dopamine neurons. The possibility that nicotinic receptors are involved in mediating the stimulatory and dopamine-enhancing effects of ghrelin is supported by the findings that peripheral injection of the unselective nicotinic antagonist mecamylamine (2.0 mg/kg) blocked these ghrelin-induced effects. Tentatively, ghrelin may, via activation of the acetylcholine-dopamine reward link, increase the incentive values of signals associated with motivated behaviours of importance for survival such as feeding behaviour. It will be important to discover whether this has therapeutic implications for compulsive addictive behaviours, such as eating behaviour disorders and drug dependence.

Glycine receptors regulate dopamine release in the rat nucleus accumbens

BACKGROUND

The mesolimbic dopamine (DA) system seems to be centrally involved in regulating reward-related behavior and consequently has been implicated in addictive processes, such as alcoholism and drug addiction. This DA system has also been implicated in psychosis and in regulating hedonia/anhedonia, important components of mania and depression. Given the potentially great importance of the mesolimbic DA system for several psychiatric disorders, it is of major interest to delineate the mechanisms and dynamics underlying DA regulation and release. Recently strychnine-sensitive glycine receptors (GlyR) have attracted some interest in this matter.

METHODS

Western blot and in vivo microdialysis (couplied to high-pressure liquid chromatography with electrochemical detection), as well as reversed microdialysis, in awake, freely moving, adult male Wistar rats.

RESULTS

Here we demonstrate by means of Western blot that alpha GlyR subunit proteins are expressed in the rat nucleus accumbens (nAc), a major target of the mesolimbic DA system. We further show that reversed microdialysis of the competitive GlyR antagonist strychnine into the nAc concentration-dependently (2-200 microM) and in a reversible manner decreases accumbal extracellular DA levels. Conversely, reversed microdialysis of the agonist glycine increases accumbal DA levels in some rats but not others. The strychnine-induced depression of the accumbal DA levels is antagonized by simultaneous local perfusion of glycine.

CONCLUSIONS

The present results indicate that GlyRs in the nAc are tonically activated and of importance for regulating extracellular DA levels. The possibility of pharmacologically interfering with GlyRs to combat psychiatric disorders, in which the mesolimbic DA system is implicated, such as alcoholism, drug addiction, and psychosis, should be explored.

Involvement of accumbal glycine receptors in the regulation of voluntary ethanol intake in the rat

BACKGROUND

Extracellular dopamine (DA) levels in the nucleus accumbens (nAc) increase after ethanol (EtOH) administration in the rat, a response that may be involved in the positive reinforcing effects of EtOH. The mechanisms underlying this DA activation and how they relate to EtOH reinforcement remain to be elucidated, but recent data indicate that glycine receptors (GlyRs) in the nAc may be involved. Here this hypothesis was further challenged by examining the influence of bilateral accumbal application of glycine (a GlyR agonist), strychnine (a GlyR competitive antagonist), or Ringer on EtOH intake and preference, as well as on the concomitant DA output in the nAc, in EtOH high-preferring male Wistar rats.

METHODS

EtOH high-preferring male Wistar rats [EtOH preference >60% during continuous access to a bottle of EtOH (6% v/v) and a bottle of water] were limited to drink 1 hr/day (limited access drinking). Thereafter, the animals were equipped bilaterally with microdialysis probes aimed at the mAc, and were subjected to in vivo microdialysis (coupled to high-pressure liquid chromatography with electrochemical detection) and reversed microdialysis (for drug application) during two experimental days (balanced study), during which the animals were allowed a choice between EtOH and water.

RESULTS

The EtOH consumption in rats that were perfused with Ringer in the nAc was approximately 0.9 g/kg/hr and associated with a significant increase in extracellular accumbal DA levels. In a subpopulation of rats, bilateral accumbal glycine (100 microM) perfusion produced a significant increase in accumbal DA output and a decrease in EtOH preference and intake. In these glycine responders, the EtOH consumed (approximately 0.7 g/kg/hr) did not produce a further increase of DA levels. In other rats, bilateral glycine perfusion did not change the accumbal DA output, and voluntary EtOH intake was not altered. In these glycine nonresponders, EtOH tended to increase accumbal DA levels. Bilateral accumbal strychnine (20 microM) perfusion significantly decreased DA output in the nAc, and the DA levels remained decreased despite a statistically significant increase of EtOH intake. Finally, the increase in accumbal DA levels observed after EtOH consumption in Ringer-treated rats was significantly larger in glycine responders than in glycine nonresponders.

CONCLUSIONS

The present findings suggest that glycine and strychnine alter extracellular DA levels in the nAc, probably via GlyR stimulation and blockade, respectively, and concomitantly glycine and strychnine reciprocally alter also EtOH consumption in EtOH high-preferring male Wistar rats. The possibility of developing selective GlyR agonists and/or antagonists should be explored. Such agents could prove of value in the treatment of alcoholism.

D3 dopamine receptor activates phospholipase D through a pertussis toxin-insensitive pathway

Within the dopamine receptor family, the D(3) dopamine receptor's function remains inadequately described. The D(3) receptor has been shown to couple to inhibition of adenylyl cyclase, stimulation of mitogenesis, and regulation of K(+) and Ca(2+) currents, all in a pertussis toxin (PTX)-sensitive manner. Here we report D(3) receptor activation of the phospholipase D (PLD) enzyme in HEK 293 cells heterologously expressing the human D(3) receptor. Activation by agonist is dose dependent and displays the pharmacology expected of the D(3) receptor. The D(3) receptor specific antagonists AJ-76 and U99194A ablated the increase in activity by the preferring D(3) agonist (+) 7-OH DPAT. In addition, the D(3) receptor-mediated activation of PLD is not mediated by G-proteins of the G(i)/G(o) family, as pretreatment with PTX had no effect. PLD activation is a novel finding for the D(3) receptor, and is the first example of an effector system where D(3) signals without G(i)/G(o) protein intermediates.

Accumbal Strychnine-Sensitive Glycine Receptors: An Access Point for Ethanol to the Brain Reward System

BACKGROUND

Ethanol (EtOH), like other drugs of abuse, increases extracellular dopamine (DA) levels in the nucleus accumbens (nAc) of the brain reward system, an effect that may be of importance for alcohol addiction. How this DA increase is produced is not fully understood, although previous studies from the present laboratories indicate that nicotinic acetylcholine receptors in the ventral tegmental area play an important role in mediating this effect. Furthermore, activation of these receptors may be secondary to some priming effect produced by EtOH in the nAc. We recently demonstrated that strychnine-sensitive glycine receptors (GlyRs) are present in the nAc and that they are involved in regulating extracellular DA levels. Here we examine the tentative role of these accumbal GlyRs in the above-mentioned priming mechanism of EtOH.

METHOD

In vivo microdialysis (coupled to high pressure liquid chromatography with electrochemical detection) and reversed microdialysis, in awake, freely moving adult male Wistar rats.

RESULTS

Local perfusion of strychnine decreased accumbal DA levels per se and completely prevented the increase of accumbal DA levels after both local and systemic EtOH administration. Accumbal perfusion of the GlyR agonist glycine instead increased DA levels in a subpopulation of rats and prevented the EtOH-induced increase after local but not systemic EtOH in all animals.

CONCLUSION

The present results suggest that GlyRs in the nAc might constitute targets for EtOH in its mesolimbic DA-activating effect. Gene polymorphism and drug developmental studies that focus on this receptor population and its relation to alcohol dependence are warranted.

Is an alpha-conotoxin MII-sensitive mechanism involved in the neurochemical, stimulatory, and rewarding effects of ethanol?

Ethanol and nicotine are the most commonly abused drugs among human beings, and a large body of evidence, from both epidemiologic and preclinical studies, indicates that there is a positive correlation between intake of both drugs. Findings of studies from our research group have demonstrated that nicotinic acetylcholine receptors, especially those located in the ventral tegmental area, are important for the stimulatory, rewarding, and dopamine-enhancing effects of ethanol. Furthermore, results of recent work indicate that the alpha4beta2* and the alpha7* receptor subunits of the nicotinic acetylcholine receptors do not seem to be involved in the neurochemical and behavioral effects of ethanol in rodents. The aim of the current study was to investigate further the role of different nicotinic acetylcholine receptor subunits in the stimulatory, dopamine-enhancing, and rewarding effects of ethanol in rodents by using the peptide alpha-conotoxin MII (5 nmol; an antagonist of the alpha3beta2*, beta3*, and alpha6* subunits of the nicotinic acetylcholine receptor) administered locally into the ventral tegmental area. A significant reduction of ethanol-induced accumbal dopamine overflow, measured by means of in vivo microdialysis, and of locomotor stimulation was observed in mice. Furthermore, alpha-conotoxin MII was demonstrated to reduce voluntary ethanol intake significantly in both rats and mice. These results indicate that alpha-conotoxin MU-sensitive receptors may be important in mediating the stimulatory, dopamine-enhancing, and rewarding effects of ethanol, and that alpha-conotoxin MII-sensitive receptors may constitute targets for development of new adjuvant for treatment of ethanol dependence.

Topographical Assessment of Ethological and Dopamine Receptor Agonist-Induced Behavioral Phenotype in Mutants with Congenic DARPP-32 'Knockout'

Congenic (10 backcrosses into C57BL/6J) mutants with targeted gene deletion of DARPP-32, a neuronal phosphoprotein regarded as an essential mediator of the biological effects of dopamine (DA), were assessed phenotypically using an ethologically based approach that resolves all topographies of behavior in the mouse repertoire. Over initial exploration, female, but not male, DARPP-32 mutants evidenced increased locomotion and decreased grooming, while a decrease in rearing seated was evident in mutants of both genders; continuing assessment over several hours did not reveal additional phenotypic effects. Following challenge with the nonselective DA receptor agonist apomorphine, low doses were associated with reduced levels of sniffing, grooming, total rearing, and rearing seated in DARPP-32 mutants relative to wildtypes; this would suggest some role for DARPP-32 in mediating the biological effects of presynaptic D(2)-like autoreceptor or inhibitory postsynaptic D(2)-like receptor activation. Following challenge with higher doses, while stereotyped sniffing and locomotion with chewing was largely unaltered, the additional murine response of Straub tail was essentially abolished in DARPP-32 mutants, indicating some specific involvement of DARPP-32 in mediating this topography of behavior; additionally, there were overall reductions in levels of sniffing, total rearing, rearing seated, and grooming in DARPP-32 mutants that were unrelated to the dose of apomorphine administered, indicating broader topographical effects following the stress of the injection procedure relative to more naturalistic conditions. The developmental absence of DARPP-32 following targeted gene deletion appears to be associated with compensatory processes that maintain certain topographies of spontaneous and agonist-induced DAergic function, while other topographies remain impaired.

Acute and subacute effects of risperidone and cocaine on accumbens dopamine and serotonin release using in vivo microvoltammetry on line with open-field behavior

In vivo microvoltammetry was used to detect dopamine (DA) and serotonin (5-HT) release from nucleus accumbens (NAcc) of freely moving, male, Sprague-Dawley laboratory rats, while animals' locomotor (forward ambulations) and stereotypic behavior (fine movements of sniffing and grooming) were monitored at the same time with infrared photobeams. Monoamine release mechanisms were determined by using a depolarization blocker (gamma-butyrolactone, gamma BL). Miniature carbon sensors (BRODERICK PROBES microelectrodes) smaller than a human hair were used in conjunction with a semidifferential electrochemical circuit to detect release of each monoamine in separate signals and within seconds. The purpose was to evaluate the neuropharmacology of the 5-HT(2)/DA(2) antagonist risperidone in its current therapeutic role as an atypical antipsychotic medication as well as in its potential role as pharmacotherapy for cocaine psychosis and withdrawal symptoms. Acute (single drug dose) and subacute (24-h follow-up studies in the same animal, no drug administration) studies were performed for each treatment group. The hypothesis for the present studies is derived from a growing body of evidence that cocaine-induced psychosis and schizophrenic psychosis share similar neurochemical and behavioral manifestations. Results showed that (1) Acute administration of risperidone (2 mg/kg sc) significantly increased DA and 5-HT release in NAcc above baseline (habituation) values (P<.001) while locomotion and stereotypy were virtually unaffected. In subacute studies, DA release did not differ from baseline (P>.05), whereas 5-HT release was significantly increased above baseline (P<.001). Locomotion increased over baseline but not to a significant degree, while stereotypy was significantly increased above baseline (P<.05). (2) Acute administration of cocaine (10 mg/kg ip) significantly increased both DA and 5-HT release above baseline (P<.001), while locomotion and stereotypy were significantly increased over baseline (P<.001). In subacute studies, DA decreased significantly below baseline (P<.001) and significant decreases in 5-HT release occurred at 15, 20, 50 and 55 min (P<.05). Behavior increased above baseline but did not reach a statistically significant degree. (3) Acute administration of risperidone/cocaine (2 mg/kg sc and 10 mg/kg ip, respectively) showed a significant block of the cocaine-induced increase in DA release in the first hour (P<.001) and 5-HT release in both hours of study (P<.001). Cocaine-induced locomotion and stereotypy were blocked simultaneously with the monoamines (P<.001). In subacute studies, DA and 5-HT release returned to baseline while locomotion and stereotypy increased insignificantly above baseline. Thus, (a) these studies were able to tease out pharmacologically the critical differences between presynaptic and postsynaptic responses to drug treatment(s) and these differences may lead to more effective therapies for schizophrenic and/or cocaine psychosis. (b) Taken together with other data, these acute studies suggest that risperidone may possibly act via inhibition of presynaptic autoreceptors to produce the observed increases in accumbens DA and 5-HT release, whereas cocaine may be acting at least in part via serotoninergic modulation of DA postsynaptically. The subacute data suggest that pharmacokinetics may play a role in risperidone's action and that neuroadaptation may play a role in the mechanism of action of cocaine. Finally, the ability of risperidone to block cocaine-induced psychostimulant neurochemistry and behavior during acute studies while diminishing the withdrawal symptoms of cocaine during subacute studies suggests that risperidone may be a viable pharmacotherapy for cocaine psychosis and withdrawal.

Ethanol elevates accumbal dopamine levels via indirect activation of ventral tegmental nicotinic acetylcholine receptors

It was previously demonstrated that the central nicotinic acetylcholine receptor antagonist mecamylamine perfused in the ventral tegmental area (VTA) counteracts the elevation of extracellular dopamine levels in the nucleus accumbens after systemic ethanol, as measured by in vivo microdialysis. In the present study we investigated the effect of different concentrations of ethanol perfused locally in the VTA or in the nucleus accumbens on extracellular accumbal dopamine levels. Ethanol (10-1000 mM) perfused in the VTA did not influence dopamine output in the nucleus accumbens. However, ethanol (300 mM) perfused in the nucleus accumbens increased accumbal dopamine levels to approximately the same extent (30%) as observed after systemic ethanol, whereas ethanol (1000 mM) decreased the dopamine output by approximately 50%. Next, the hypothesis that endogenous acetylcholine is required for the increased accumbal dopamine levels after ethanol was challenged. It was shown that in animals pre-treated with vesamicol, a potent inhibitor of vesicular acetylcholine storage, ethanol (300 mM) in the nucleus accumbens failed to elevate extracellular accumbal dopamine levels. Similarly, in animals perfused with mecamylamine in the VTA, but not in the nucleus accumbens, ethanol in the nucleus accumbens (300 mM) failed to increase accumbal dopamine levels. However, whereas dihydro-beta-erythroidine (antagonist for the nicotinic receptor subtype alpha4beta2) perfused in the VTA prevented the increase in accumbal dopamine after systemic nicotine, the antagonist was unable to prevent the dopamine elevating effects of ethanol. Finally, to investigate whether mecamylamine exerts its antagonizing effect of ethanol induced accumbal dopamine levels through an interaction with the NMDA receptor MK-801, the effects of the prototypic NMDA receptor antagonist were examined and compared to those of mecamylamine. After perfusion in the VTA, MK-801 enhanced accumbal dopamine levels by itself but did not antagonize the enhancing effect of ethanol. The present set of experiments indicate that the mesolimbic dopamine activating effects of ethanol may be due to an indirect rather than direct activation of ventral tegmental nicotinic acetylcholine receptors of a subtype composition different from the alpha4beta2. Furthermore, it is argued that the primary site of action of ethanol in its accumbal dopamine elevating effect may be located to the nucleus accumbens or nearby regions.

Role of different nicotinic acetylcholine receptors in mediating behavioral and neurochemical effects of ethanol in mice

Ethanol and nicotine are the most abused drugs, and it is well known that co-abuse of ethanol and nicotine is frequent in human beings. We have previously obtained results indicating that the ethanol-induced stimulation of both the mesolimbic dopamine system and locomotor activity may involve activation of central nicotinic acetylcholine receptors (nAChRs), especially those located in the ventral tegmental area. Different subpopulations of nAChRs have been identified, and, in the present series of experiments, we have studied the effects of various nAChR antagonists on the stimulation of dopamine overflow in the nucleus accumbens and on locomotor activity induced by ethanol in male mice. Ethanol (2.0 g/kg, i.p.) enhanced dopamine overflow in the nucleus accumbens by approximately 40%, measured by means of in vivo microdialysis in awake, freely moving mice. Mecamylamine (negative allosteric modulator of nAChR; 2.0 mg/kg, i.p.) blocked the ethanol-induced stimulation of both locomotor activity and accumbal dopamine overflow. Methyllycaconitine citrate (alpha(7) antagonist; 2.0 mg/kg, i.p.) and dihydro-beta-erythroidine (competitive and selective alpha(4)beta(2) antagonist; 0.5 mg/kg, s.c.), in doses that had no marked effects per se, did not significantly reduce the behavioral and neurochemical stimulation caused by ethanol. The present results support the suggestion that the stimulatory effects of ethanol on locomotor activity and dopamine release do not involve the alpha(4)beta(2) or alpha(7) subunit compositions of the nAChR and that the effects of mecamylamine are mediated through a site not directly associated with the alpha(4)beta(2) or alpha(7) nAChR subunits.

Differential effect of quinpirole and 7-OH-DPAT on the spontaneous [(3)H]-dopamine efflux from rat striatal synaptosomes

The effect of quinpirole and 7-OH-DAPT, two D(2)-like agonists, were examined using superfused rat striatal synaptosomes to study the autoregulation of spontaneous [(3)H]-dopamine ([(3)H]-DA) release. Basal [(3)H]-DA efflux was Ca(2+)-dependent by approximately 45% and was inhibited by cadmium 10 microM by 24%. Quinpirole (1 nM to 3 microM) inhibited spontaneous [(3)H]-DA efflux in a concentration-dependent manner (pEC(50) = 7.56 +/- 0.07 and E(max) = 26 +/- 0.09%) and this effect was competitively antagonized by haloperidol (0.3-1 nM) (apparent pA(2) = 9.61 +/- 0.08). In addition, activation of the D(2) DA autoreceptor by quinpirole only modulates the calcium-dependent component of [(3)H]-DA efflux. Low concentrations of a putative-selective D(3) DA agonist, (+/-)-7-OH-DPAT (0.03-0.1 microM), inhibited spontaneous [(3)H]-DA release by 13% (P < 0.05), but higher drug concentrations (> or =1 microM) increased basal [(3)H]-DA efflux in a concentration-dependent, nonsaturable, but reversible manner. Haloperidol (1-10 nM) reversed the (+/-)-7-OH-DPAT-induced inhibition, but not the increase in [(3)H]-DA outflow. The effect of (+/-)-7-OH-DPAT was mimicked by (+)-7-OH-DPAT. However, another putative D(3) DA agonist, PD 128,907 (1 nM to 3 microM), decreased spontaneous tritium efflux (maximal inhibition of 19 +/- 3.06% at 3 microM, P < 0.01). The effect of 7-OH-DPAT 10 microM was independent of the presence of extracellular Ca(2+), since its effect on basal [(3)H]-DA outflow was not significantly modified in a 200 nM free-Ca(2+) medium. In addition, the 7-OH-DPAT-induced enhancement of basal [(3)H]-DA efflux does not involve depolarization of nerve terminals or the reversal of the DA uptake system, as tetrodotoxin (1 microM) and nomifensine (1microM) did not modify the effect of 7-OH-DPAT 10 microM. The present data indicate that activation of D(2) DA autoreceptor subtype by quinpirole inhibits Ca(2+)-dependent spontaneous [(3)H]-DA efflux. 7-OH-DPAT activates the D(2) DA autoreceptor at low concentrations, whereas its action in releasing [(3)H]-DA effect is not receptor-mediated and could involve other mechanisms other than either conventional vesicular exocytosis or the DA uptake system.

Effects of intrastriatal infusion of D2 and D3 dopamine receptor preferring antagonists on dopamine release in rat dorsal striatum (in vivo microdialysis study)

Dopamine D2-like receptor antagonists haloperidol, spiperone, clozapine, cis -( +)- (1S,2R)-5-methoxy-1-methyl-2-(n -propylamino)tetralin, ( +)-AJ76, cis -( +)- (1S,2R)-5-methoxy-1-methyl-2-(n -di-propylamino)tetralin, ( +)-UH232, and putative D3 dopamine receptor agonist ( +/-)- 7-hydroxy-N,N-di- n -propyl-2-aminotetralin, 7-OH-DPAT, were infused via a transcerebral microdialysis probe into the dorsal striatum of freely moving rats. Local infusion of all the dopamine antagonists studied resulted in concentration-dependent increase of striatal dopamine release in vivo. Subsequent i.p. administration of the drugs did not produce a further rise of dopamine release as compared to the maximal increase elicited by local administration of the same substances. The difference between effects of D2 and D3 dopamine receptor preferring antagonists applied locally was observed only in the degree of dopamine release elevation [the maximal responses were about 160% for haloperidol and spiperone, 190% for clozapine and ( +)-UH232 and 400% for ( +)-AJ76, of basal]. Striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels were elevated only slightly following local infusion of haloperidol, spiperone and clozapine, while systemic administration of the drugs resulted in a marked increase of metabolite extracellular levels. Both ( +)-UH232 and ( +)-AJ76 were found to increase significantly DOPAC and HVA levels during infusion, but the effect was less pronounced in comparison to that produced by systemic drug administration. Infusion of 7-OH-DPAT in the concentration range 5 x 10(-9)to 10(-6) M significantly decreased dopamine release but not metabolite levels down to the values observed following systemic drug administration. The present results give further evidence for the hypothesized leading role of nerve terminal dopamine autoreceptors, presumably of D3 type, in neuroleptic-induced augmentation of dopamine release in rat dorsal striatum.

The dopamine D2/D3 antagonist DS121 potentiates the effect of cocaine on locomotion and reduces tolerance in cocaine tolerant rats

To explore the significance of dopamine (DA) autoreceptors in cocaine tolerance and cocaine induced locomotor activity rats were treated with saline and cocaine (40 mg/kg per day via osmotic minipump; normal and cocaine tolerant rats, respectively). Injections of DS121 (0-7 mg/kg, i.p.; S(-)-3-(3-(cyanophenyl)-N-n-propylpiperidine), a DA D2/3 and autoreceptor preferring antagonist, either alone (i.e. DS121 + saline injection) or in combination with cocaine (7.5 mg/kg, i.p.) were also given. DS121 (+ saline) increased locomotor activity in both saline and cocaine pump (CP) treated animals. DS121 also potentiated the effect of cocaine on locomotor activity; this effect was greatest in CP (tolerant) animals. It is concluded that DS121 can increase locomotor activity and that this effect is greatest when the DA tone is high, that is when cocaine is present, suggestive of a presynaptic mechanism. Furthermore, because DS121 potentiation of cocaine induced locomotor activity is greatest in tolerant animals it is concluded that supersensitive DA autoreceptors underlie this effect. These data further support our previous data, which show that DA autoreceptors are sensitized after continuous cocaine (minipump) treatment.

Positron emission tomography of radioligand binding in porcine striatum in vivo: haloperidol inhibition linked to endogenous ligand release

The ligands N-methylspiperone and haloperidol both bind to D(2)-like dopamine receptors. The competitive nature of the binding over a wide range of haloperidol concentrations and the effect on dopamine release have never been tested in vivo. We determined the competitive interaction between 3-N-[(11)C]methylspiperone ([(11)C]NMSP) and haloperidol binding to striatal dopamine D(2)-like receptors with positron emission tomography (PET) of pig brain. [(11)C]NMSP tomography was performed with haloperidol at five different plasma concentrations maintained constant by programmed infusion. Kinetic parameters of ligand competition for binding in the striatum were determined by deconvolving time-activity curves of the striatum and cerebellum from metabolite-corrected arterial plasma [(11)C]NMSP and haloperidol concentrations. Two types of [(11)C]NMSP-binding sites were evident in the striatum, both saturable by haloperidol administration. The preponderant or primary sites bound [(11)C]NMSP irreversibly, as dopamine D2-like receptors, while the secondary sites bound [(11)C]NMSP reversibly, as do serotonin S2 receptors. Woolf-Hanes plots revealed the predicted approximately linear relationships between the binding indices and the haloperidol plasma concentration. For the irreversible binding sites, this relationship indicated a 50% inhibitory concentration of haloperidol of 2 nM in plasma and a maximum binding capacity of 64 pmol cm(-3) in striatum. For the reversible binding sites, the relationship indicated a 50% inhibitory plasma concentration of haloperidol of 1 nM and a maximum binding capacity of 4.5 pmol cm(-3). Second-order polynomial Eadie-Hofstee-Scatchard plots were consistent with increased competition from an endogenous ligand of the irreversibly binding sites only with increasing doses of haloperidol. At the highest haloperidol dose, this hypothetical endogenous ligand had risen 6-7-fold. We contend that this reveals the release of dopamine by high concentrations of haloperidol.

Monoamine neurotransmitters in resected hippocampal subparcellations from neocortical and mesial temporal lobe epilepsy patients: in situ microvoltammetric studies

It is known that epilepsy patients diagnosed with neocortical temporal lobe epilepsy (NTLE), differ from those diagnosed with mesial temporal lobe epilepsy (MTLE), e.g., in hippocampal (HPC) pathology. In the present studies, we tested the hypothesis that NTLE and MTLE subtypes of human epilepsy might differ in regards to their HPC monoamine neurochemistry. Monoamine neurotransmitters were studied in separate signals and within s with semiderivative microvoltammetry, used in combination with stearate indicator, Ag-AgCl reference and stainless steel auxiliary microelectrodes. Anterior HPC specimens from the patients' epileptogenic zone, defined by electrocorticography, were resected neurosurgically from 13 consecutive patients with intractable temporal lobe epilepsy. Four patients were diagnosed with NTLE and nine with MTLE. The criteria for the diagnosis of NTLE versus MTLE was absence versus presence of HPC sclerosis, respectively, based on MRI examination of resected tissue. In addition, NTLE patients demonstrated seizure onset in anterolateral temporal neocortex on electroencephalography (EEG). HPC subparcellations studied were: (a) Granular Cells of the Dentate Gyrus (DG), (b) Polymorphic Layer of DG and (c) Pyramidal Layer: subfields, CA1 and CA2. Dopamine (DA), serotonin (5-HT), norepinephrine (NE) and ascorbic acid (AA) (co-factor in DA to NE synthesis), exhibited separate and characteristic half-wave potentials in millivolts. Each half-wave potential, i.e., the potential at which maximum current was generated, was experimentally established in vitro. Concentrations of neurotransmitters found in HPC subparcellations were interpolated from calibration curves derived in vitro from electrochemical detection of monoamines and AA in saline phosphate buffer. Significant differences between subtypes in concentration of monoamines were analyzed by the Mann Whitney rank sum test and those differences in probability distribution of monoamines were analyzed by the Fisher Exact test; in each case, P<0.01 was the criteria selected for determining statistical significance. DA concentrations were higher in NTLE compared with MTLE in each HPC subparcellation [P=0.037, 0.024 and 0.007, respectively (P<0.01)] and DA occurred more frequently in NTLE in the Pyramidal Layer [P=0.077 (P<0.01)]. AA was present in one NTLE patient. NE concentrations were higher in MTLE vs. NTLE in each subparcellation [P=0.012, 0.067 and 0.07, respectively (P<0.01)] and NE occurred more frequently in MTLE in Granular Cells of DG and Pyramidal Layer [P=0.052 and 0.014, respectively (P<0.01)]. In MTLE, NE concentrations in the CA1 subfield of the Pyramidal Layer were decreased vs. the CA2 subfield [P=0.063 (P<0.01)]. Serotonin was found in every HPC subparcellation of each subtype but 5-HT concentrations were higher in NTLE vs. MTLE in the Granular Cells of DG and the Pyramidal Layer (CA1 subfield) [P=0.076 and 0.095, respectively (P<0.01)]. Thus, this preliminary study showed that marked differences in HPC monoamine neurochemistry occurred in NTLE patients as compared with MTLE patients.

Thiophene analogs of naphthoxazines and 2-aminotetralins: bioisosteres with improved relative oral bioavailability, as compared to 5-OH-DPAT

In the present study, a series of thiophene analogs of 2-aminotetralins and hexahydronaphthoxazines were studied in vivo for their ability to decrease striatal dopamine release, their effects on locomotor activity, and their behavioral characteristics in reserpinized rats, in order to investigate whether a thiophene moiety can act as a bioisostere for the phenol moiety. In general, the new compounds showed lower in vivo activities than 5-hydroxy-2-(N,N,-di-n-propylamino)tetralin (5-OH-DPAT). However, the introduction of the thiophene moiety gave a significant improvement of the relative oral bioavailability, compared to 5-OH-DPAT. Our results suggest that the thiophene moiety can act as a bioisostere for a phenol group in hydroxylated 2-aminotetralins. For the thianaphthoxazines it was not possible to discriminate between bioisosterism for a phenyl or a phenol moiety. The tetrahydrobenzo[b]thiophenes could be used as lead compounds for the development of novel dopamine receptor ligands with improved relative oral bioavailability.

Dopamine D3 receptor mutant and wild-type mice exhibit identical responses to putative D3 receptor-selective agonists and antagonists

Previous studies using a variety of drugs with different affinities for the dopamine (DA) D3 receptor suggested that this receptor is involved in regulating motor activity and hypothermia. However, the in vivo selectivity of many of these compounds has been repeatedly questioned. To examine the precise roles of the DA D3 receptor in motor activity and hypothermic responses, we used mutant mice lacking the DA D3 receptor to evaluate the in vivo effects of several putative D3 receptor-selective agonists and antagonists. Using automated photocell activity chambers, we observed that the decreases in locomotor activity produced by putative D3 receptor-selective agonists as well as increases in locomotor activity produced by putative D3 receptor antagonists are identical in D3 receptor mutant and wild-type mice. In addition, the hypothermia produced by the putative D3 receptor-selective agonist PD 128907 is identical in both groups of mice. Based on these findings, we propose that D3 receptors are unlikely to be involved in these effects and we caution that the putative D3 ligands that have been used to reach conclusions regarding the functional roles of D3 receptors lack the necessary in vivo selectivity to support such conclusions.

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.

Voluntary ethanol intake in the rat and the associated accumbal dopamine overflow are blocked by ventral tegmental mecamylamine

The mesocorticolimbic dopamine system is believed to be involved in mediating the positive reinforcing effects of drugs of abuse, including ethanol. The nicotinic acetylcholine receptor antagonist mecamylamine perfused via reversed microdialysis in the ventral tegmental area antagonizes the increase of accumbal extracellular dopamine levels after systemic ethanol, and, after systemic injection, lowers ethanol intake in the rat. In the present study the effect of ventral tegmental mecamylamine on ethanol intake and preference, as well as on extracellular accumbal dopamine levels, was investigated in the same animal. To this end, in vivo microdialysis using a double probe approach (one in the nucleus accumbens and one in the ventral tegmental area) was combined with an ethanol preference model invoking a free choice between a bottle of water and a bottle of ethanol 6% (v/v) solution. Wistar rats drinking more than 60% of their total daily fluid intake from the ethanol solution (ethanol high-preferring animals) were selected during a screening period and used for the experiments. The animals received vehicle or mecamylamine (100 microM) in the ventral tegmental area and were then presented with a choice between water and ethanol in a limited access paradigm to which they previously had been adapted. On the next day the rats that received vehicle day 1 now received mecamylamine, and vice versa. When treated with vehicle, ethanol intake and preference were unaltered, as compared to baseline behavior, and accumbal dopamine levels increased significantly to approximately 130% of the pre-drug baseline level. When receiving mecamylamine, ethanol intake and preference were reduced markedly and dopamine levels were unaltered, as compared to pre-drug baseline levels. The present results further indicate that nicotinic acetylcholine receptors in the ventral tegmental area are involved in the positive reinforcing effects of ethanol. Thus, mecamylamine or other antagonists specifically aimed at ventral tegmental nicotinic acetylcholine receptors could represent a new pharmacological treatment principle against alcohol abuse, the efficacy of which should be explored in high-scale alcohol consumers or alcoholics.

Nefazodone attenuates the behavioral and neurochemical effects of ethanol

This study evaluated the influence of nefazodone, a combined 5-HT2A receptor antagonist and 5-HT reuptake inhibitor, on the behavioral and neurochemical effects of ethanol in nonselected male Wistar rats. In microdialysis experiments, ethanol (2.5 g/kg, i.p.) increased extracellular accumbal dopamine levels by 36% (p = 0.0073) compared to baseline levels, and elevated the maximal DOPAC and HVA levels by 26% (p = 0.0093) and 52% (p = 0.0010), respectively, Nefazodone (50 mg/kg, s.c.) per se increased accumbal dopamine levels by 28% (p = 0.0199) but, when injected 40 min before ethanol, reduced the ethanol-induced elevation of accumbal dopamine overflow (p = 0.0132) and decreased the ethanol-induced HVA levels (p = 0.0159). In an ethanol(6% v/v)/water free-choice paradigm, nefazodone (50 mg/kg, s.c.) decreased ethanol intake by 51% (p = 0.0251) and preference by 22% (p = 0.0251) in high- but not low-preferring rats from a nonselected Wistar strain. These results show that nefazodone modulates the mesolimbic dopamine system in a dopamine activity-dependent manner, and influences the neurochemical and behavioral effects of ethanol in the rat.

Investigations into the nature of a 7-OH-DPAT discriminative cue: comparison with D-amphetamine

In the present study, separate squads of rats were trained to discriminate either the dopamine D3 receptor preferring ligand 7-hydroxy-2-(di-N-propylamino)-tetralin (7-OH-DPAT) (0.03 mg/kg) from saline, or D-amphetamine (0.3 mg/kg) from saline using a standard operant schedule (FR10 schedule reinforcement). Following stable acquisition of responding, tests of generalisation and antagonism were conducted. A number of dopamine agonists having high dopamine D2-like receptor (D2, D3 or D4) affinity generalised to the 7-OH-DPAT, but not amphetamine, cue. The dopamine D2/3 receptor agonist SKF38393 showed no generalisation to either drug cue. Subsequent correlational analysis suggested that this effect was most likely mediated through the dopamine D3 receptor. The dopamine D2/3 receptor antagonist raclopride significantly attenuated both cues. The failure of these drugs to generalise to amphetamine, suggest that there is little involvement of the dopamine D3 receptor subtype in mediating its discriminative stimulus properties.

Accumbal dopamine overflow after ethanol: localization of the antagonizing effect of mecamylamine

It has been suggested that ethanol exerts its mesolimbic dopamine activating effects and its reinforcing effects via interaction with central nicotinic acetylcholine receptors, thus providing a basis for the often observed covariation between ethanol and nicotine consumption. We have previously demonstrated that the central nicotinic acetylcholine receptor antagonist mecamylamine totally counteracts the ethanol-induced elevation of extracellular dopamine in the nucleus accumbens, as measured by in vivo microdialysis. A contribution of peripheral nicotinic receptor blockade could, however, not be excluded. In the present study, mecamylamine (1.0 mg/kg, i.p.) again totally counteracted the ethanol-induced dopamine overflow, as measured by in vivo microdialysis, while the quarternary nicotinic receptor antagonist hexamethonium (10 mg/kg, i.p.) did not. Furthermore, the increase in accumbal dopamine overflow after systemic ethanol (2.5 g/kg, i.p.) was counteracted by local perfusion of mecamylamine (50 microM) in the ipsilateral ventral tegmental area, but not by mecamylamine perfusion in the nucleus accumbens. Ethanol-induced accumbal dopamine overflow was also counteracted by perfusion of hexamethonium (250 microM) in the ventral tegmental area. These results provide further evidence that ethanol-induced activation of the mesolimbic dopamine system is mediated via stimulation of central nicotinic acetylcholine receptors, and that the receptor population within the ventral tegmental area may be the most important in this regard. It is suggested that antagonists of central nicotinic acetylcholine receptors may be useful in the treatment of alcoholism.

Interactions between cocaine and (-)-DS 121: studies with 2-deoxyglucose autoradiography and microdialysis in the rat brain

(-)-DS 121 [S-(-)-3-(3-cyanophenyl)-N-n-propyl piperidine], a dopamine autoreceptor preferring antagonist, has been shown to stimulate locomotor activity and induce conditioned place preference. However, the drug fails to facilitate intracranial self-stimulation or substitute for cocaine in cueing experiments, and it blocks cocaine self-administration. In the present study using 2-deoxyglucose autoradiography, (-)-DS 121 (at 50 but not 15 mg/kg i.p.) significantly and selectively increased local cerebral glucose utilization in the olfactory cortex, medial and lateral septum, hippocampal areas, substantia nigra pars reticulata, caudate, and mammillary body. Local cerebral glucose utilization was depressed in caudal areas of the cortex. Interestingly, however, both doses of (-)-DS 121 blocked the increases in local cerebral glucose utilization produced by 5 mg/kg i.v. cocaine. The present study also evaluated the effects of (-)-DS 121 of extracellular striatal dopamine levels using microdialysis in freely moving rats. By itself, 15 mg/kg of (-)-DS 121 increased extracellular striatal dopamine levels to approximately 300% of controls. Cocaine (5 mg/kg i.v.) produced a 370% increase in striatal dopamine levels. When rats were pretreated with (-)-DS 121, a subsequent dose of cocaine augmented the increase in extracellular striatal dopamine to 870% of controls. The results support the contention that (-)-DS 121 possesses weak cocaine-mimetic effects and that its antagonism of cocaine's subjective effects are due to interactions with dopamine at postsynaptic sites. It is hypothesized that, like other preferential autoreceptor antagonists, (-)-DS 121 may be useful as a pharmacotherapy in drug addiction.

Lineweaver-Burk analysis for the blocking effects of mammalian dopamine receptor antagonists on dopamine-induced currents in Achatina giant neurones

1. We had demonstrated (Emaduddin et al., 1995) the blocking effects of the three mammalian dopamine receptor antagonists, (+/-)-SKF83566 (mammalian dopamine D1-like receptor antagonist), (+)-UH232 (D2 and D3-like receptor antagonist) and (+/-)-sulpiride (D2-like receptor antagonist) on the dose (pressure duration)-response curves of dopamine in the three giant neurone types, LVMN (left visceral multiple spike neurone), d-RPeAN (dorsal-right pedal anterior neurone) and v-LCDN (ventral-left cerebral distinct neurone), of Achatina fulica Férussac under voltage clamp. In the present study, we analyzed these data by Lineweaver-Burk plot. 2. Dopamine-induced inward currents (Iin) of the two neurone types, LVMN and d-RPeAN, were blocked by (+/-)-SKF83566 and (+)-UH232 in partly noncompetitive and partly uncompetitive manners. (+/-)-Sulpiride had no effect on these currents. 3. In contrast, dopamine-induced outward current (Iout) of v-LCDN was inhibited competitively by (+/-)-sulpiride and noncompetitively by (+)-UH232. (+/-)-SKF83566 had no effect on this current. 4. Therefore, we consider that the pharmacological features of the dopamine receptors of Achatina neurones are not identical in detail to those of the mammalian dopamine receptors.

Aminotetralin drugs and D3 receptor functions. What may partially selective D3 receptor ligands tell us about dopamine D3 receptor functions?

The dopamine D3 receptor gene was identified by Sokoloff and colleagues in 1990. This finding rapidly gained the interest of the scientific community because this unexpected dopamine receptor subtype may play an important role in the antipsychotic activity of neuroleptic drugs. It recognizes most neuroleptics with a high affinity, and its brain distribution is restricted mainly to the ventral part of the striatal complex. However, the characterization and the subsequent identification of functions of the D3 receptor were hampered initially by at least four important factors that are still partially unresolved: (1) the absence of selective drugs that can discriminate between the D2 and D3 receptor subtype functions in vivo, (2) the lack of apparent coupling with GTP-dependent proteins, (3) the absence of effects on second messenger systems, and (4) the low level of expression of this receptor in brain tissue; these factors have contributed to tempering the interest of scientists. However, this situation has begun to change with the identification of [3H]7-hydroxy-N,N-(di-n-propyl)-2-aminotetralin ([3H]7-OH-DPAT), the first selective ligand for the dopamine D3 receptor. Although its binding selectivity for the D3 versus the D2 receptor is somewhat artificial, the potentially important impact of identification of a function for the D3 receptor encouraged scientists to use this aminotetralin compound for in vivo studies with, however, limited success. This commentary is focused on the impact and controversies generated by the use of 7-OH-DPAT and its congeners, on new conceptual views that may arise from this research, and on what partially selective D3 receptor ligands may tell us about dopamine D3 receptor functions.

In vivo evidence for preferential role of dopamine D3 receptor in the presynaptic regulation of dopamine release but not synthesis

Brain microdialysis was used to investigate the effects of the putative dopamine D3 receptor agonist (+/-)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) on dopamine release, metabolism and synthesis in the dorsal striatum and nucleus accumbens of awake rats. The drug administered i.p. dose dependently decreased the release, metabolism and synthesis of dopamine in both brain areas. The potency of 7-OH-DPAT to decrease dopamine release was found to be higher in the nucleus accumbens than in the dorsal striatum (ED50 for nucleus accumbens 0.0096 mg/kg, i.p.; for dorsal striatum 0.068 mg/kg, i.p.). Dopamine metabolism, assessed by measuring 3,4-dihydroxyphenylacetic acid extracellular levels, and dopamine synthesis, determined as 3,4-dihydroxyphenylalanine output following perfusion with the L-aromatic acid decarboxylase inhibitor 3-hydroxybenzylhydrazine (10(-5) M), were decreased at higher dose ranges of 7-OH-DPAT (ED50 for decrease of 3,4-dihydroxyphenylalanine output in nucleus accumbens 0.124 mg/kg, i.p.; in dorsal striatum 0.101 mg/kg, i.p.). The hypomotility of rats induced by 7-OH-DPAT in doses of 0.002-0.25 mg/kg, i.p., was shown to correlate with the decreased dopamine release in the nucleus accumbens. Pretreatment of animals with 7-OH-DPAT at the putative dopamine D3 receptor 'selective' dose of 0.05 mg/kg, i.p., was found to prevent the increase of dopamine release but not the increase in metabolism in the dorsal striatum of freely moving rats induced by (+)-AJ76, cis (+)-(1S,2R)-5-methoxy-1-methyl-1-2-(n-propylamino)tetralin HCI (7 mg/kg, i.p.) and haloperidol (0.1 mg/kg, i.p.). Local application of 7-OH-DPAT by addition into the perfusing medium also resulted in a preferential decrease of dopamine release in the nucleus accumbens as compared with the dorsal striatum (EC50 for nucleus accumbens 1.9 nM; for dorsal striatum 11.3 nM). The present results give further support to the hypothesis that the dopamine D3 autoreceptor is preferentially involved in the presynaptic regulation of dopamine release, while the D2 autoreceptor controls dopamine synthesis.

Effects on locomotor activity after local application of (+)-UH232 in discrete areas of the rat brain

The preferential dopamine autoreceptor, and slightly D3 preferring, antagonist (+)-UH232 (cis-(+)-(1S,2R)-5-methoxy-1-methyl-2-(n-dipropylamino) tetralin) increases locomotor activity and synaptic dopamine release in the nucleus accumbens and striatum after systemic administration to the rat. As shown in this study, (+)-UH232, was unable to produce an increase in locomotor activity measured for 60 minutes after local administration into the terminal or somato-dendritic regions of the mesolimbic dopamine pathways or into the lateral ventricle. Instead, a dose dependent decrease of spontaneous locomotor activity after local application (0.05-50.0 nmol/side) of (+)-UH232 into the nucleus accumbens, was seen. A similar reduction in locomotor activity was produced by the classical dopamine antagonist raclopride. Analysis of the dose*time interactions on locomotor activity did, however, indicate that there is a significant dose*time interaction after local application of (+)-UH232 into the lateral ventricle and VTA. Raclopride, on the other hand, produced only a weak time dependent effect in the VTA. The potential problem of Leao's spreading depression in micro-injection experiments were considered, however, spreading depression does not seem to influence the effects of (+)-UH232 locally applied into the nucleus accumbens. In conclusion, both (+)-UH232 and raclopride produced a dose dependent decrease in spontaneous locomotor activity when examined as the total activity count over 60 minutes after local application into the N Acc.

Simultaneous monitoring of dopamine, its metabolites and trans-isomer of atypical neuroleptic drug carbidine concentrations in striatal dialysates of conscious rats

1. Transcerebral microdialysis was used to monitor dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and trans-isomer of atypical neuroleptic drug carbidine concentrations in the dialysates from dorsal striatum of freely moving rats following i.p. administration of the drug at doses 0.5, 1,5 and 25 mg/kg. The changes in locomotor activity as well as catalepsy in rats following transcarbidine administration were also evaluated. 2. The microdialysis "point of no net flux" method was used to measure interstitial free concentration (IFC) of trans-carbidine in the dorsal striatum of freely moving rats following i.p. administration of the drug at dose 5 mg/kg. The maximal IFC of trans-carbidine was found to be approximately 1 mu M 20-40 min after injection. 3. The drug at doses up to 1 mg/kg produces elevation of dopamine release not affecting sufficiently its metabolite dialysate levels. IFC of the drug calculated for these doses will not exceed 0.24 pM. At the dose 5 mg/kg, i.p., elevation of both dopamine release and metabolism was observed and dopamine release increased slightly more than DOPAC dialysate levels. 4. Stimulatory action of trans-carbidine on locomotor activity of non-operated rats has been observed at doses 0.2 and 0.5 mg/kg, i.p.. 5. Only the dose 25 mg/kg of trans-carbidine (maximal calculated IFC 4.53 mu M) was found to be cataleptogenic. The drug at this dose failed to increase DA release but induced a marked increase of DOPAC and HVA output. 6. It is concluded that trans-carbidine in in vivo neurochemical and behavioural studies demonstrates the preferential antagonistic action on dopamine release-regulating autoreceptors.

Identifiable Achatina giant neurones: their localizations in ganglia, axonal pathways and pharmacological features

1. An African giant snail (Achatina fulica Férussac), originally from East Africa, is now found abundantly in tropical and subtropical regions of Asia, including Okinawa in Japan. This is one of the largest land snail species in the world. The Achatina central nervous system is composed of the buccal, cerebral and suboesophageal ganglia. The 37 giant neurones were identified in these ganglia by the series of studies conducted over about 20 years. The identifications were made by the localization of these neurones in the ganglia, their axonal pathways and their pharmacological features. 2. In the left buccal ganglion, the four giant neurones, d-LBAN, d-LBMB, d-LBCN and d-LBPN, were identified. In the left and right cerebral ganglia, d-LCDN, d-RCDN, v-LCDN and v-RCDN were identified. The suboesophageal ganglia are further composed of the left and right parietal, the visceral, the left and right pleural, and the left and right pedal ganglia. In the right parietal ganglion, PON, TAN, TAN-2, TAN-3, RAPN, d-RPLN, BAPN, LPPN, LBPN, LAPN and v-RPLN were identified. In the visceral ganglion, VIN, FAN, INN, d-VLN, v-VLN, v-VAN, LVMN, RVMN and v-VNAN were identified. In the left parietal ganglion, v-LPSN was identified. In the left and right pedal ganglia, LPeNLN, RPeNLN, d-LPeLN, d-LPeCN, d-RPeAN, d-LPeDN, d-LPeMN and d-LPeEN were identified. 3. Of the small molecule compounds tested, dopamine, 5-hydroxytryptamine, GABA, L-glutamic acid, threo- or erythro-beta-hydroxy-L-glutamic acid were effective on the Achatina giant neurones. We suppose that these compounds act as the neurotransmitters for these neurones. 4. Of the neuroactive peptides, achatin-I(Gly-D-Phe-Ala-Asp). APGW-amide(Ala-Pro-Gly-Trp-NH2) and Achatina cardioexcitatory peptide (ACEP-1)(Ser-Gly-Gln-Ser-Trp-Arg-Pro-Gln-Gly-Arg-Phe-NH2) were proposed as neurotransmitters, because these were effective on the Achatina giant neurones and their presence was demonstrated in the Achatina ganglia. Further, myomodulin (Pro-Met-Ser-Met-Leu-Arg-Leu-NH2), buccalin (Gly-Met-Asp-Ser-Leu-Ala-Phe-Ser-Gly-Gly-Leu-NH2), FMRFamide (Phe-Met-Arg-Phe-NH2). [Ser2]-Mytilus inhibitory peptide ([Ser2]-MIP) (Gly-Ser-Pro-Met-Phe-Val-NH2), catch-relaxing peptide (CARP) (Ala-Met-Pro-Met-Leu-Arg-Leu-NH2), oxytocin (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2) and small cardioactive peptideB (SCPB) (Met-Asn-Tyr-Leu-Ala-Phe-Pro-Arg-Met-NH2) could also be neurotransmitters because these peptides were also effective on the Achatina giant neurones, though their presence in the ganglia of this animal has not yet been demonstrated. 5. Calcium current (ICa) was recorded from Achatina giant neurones in the Na(+)-free solution containing K(+)-channel blockers under voltage clamp. The Ca2+ antagonistic effects of brovincamine, verapamil, eperisone, diltiazem, monatepil, etc., were compared using the ICa of the Achatina neurones. 6. Almost all of the mammalian small molecule neurotransmitters were effective on the Achatina giant neurones, suggesting that these compounds are acting on the neurones of a wide variety of animal species. However, the pharmacological features of the Achatina neurone receptors to these compounds were not fully comparable to those of the mammalian receptors. For example, we proposed that beta-hydroxy-L-glutamic acid (either threo- or erythro-) could be an inhibitory neurotransmitter for an Achatina neurone. 7. In contrast, the Achatina giant neurones appear to have no receptor for the mammalian neuroactive peptides, except for oxytocin and Arg-vasotocin. On the other hand, many neuroactive peptides were isolated from invertebrate nervous tissues, including achatin-I, a neuroexcitatory tetrapeptide having a D-phenylalanine residue.

Concurrent locomotor stimulation and decrease in dopamine release in rats and mice after treatment with the competitive NMDA receptor antagonists D-CPPene and CGS 19755

The present study was aimed at investigating the effects of the competitive N-methyl-D-aspartate (NMDA) receptor antagonists D-CPPene (3-(2-carboxypiperazine-4-yl)-propenyl-1-phosphonic acid) and CGS 19755 (cis-4-(phosphonomethyl)piperidine-2-carboxylic acid) on dopamine (DA) transmission and motor activity in mice and rats. As measures of DA release we used mouse brain 3-methoxytyramine (3-MT) levels, and indirect estimate of DA release, and striatal dialysate measures of DA in conscious and freely moving rats by means of microdialysis. To obtain additional information about monoaminergic neurotransmission, brain tissue levels of DA, DOPAC, HVA, 5-HT and 5-HIAA were measured in both mice and rats. The animals were sacrificed at the time when NMDA antagonist-induced locomotor stimulation was maximal. In mice, D-CPPene and CGS 19755 decreased striatal 3-MT levels, whereas, in general, 3-MT levels in the limbic forebrain were not significantly altered. Treatment with CGS 19755 decreased rat striatal dialysate levels of DA but increased 5-HIAA at time points when locomotor activity was increased. D-CPPene and CGS 19755 have been observed to produce psychotic symptoms in man. The present study suggests that these symptoms are not a result of an increase in central dopamine release.

7-OH-DPAT injected into the accumbens reduces locomotion and sucrose ingestion: D3 autoreceptor-mediated effects?

7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) injected bilaterally in the nucleus accumbens (NAC) resulted in profound, noncatatonic, dose-dependent (0.3-3 mg total dose) hypolocomotion but without inducing yawning. It also decreased intake of a highly preferred 3% sucrose solution (1 microgram total dose). Systemic injection of 7-OH-DPAT (0.1-3.0 mg/kg, i.p.) similarly induced hypolocomotion while failing to induce yawning. In none of these studies did rats show any signs of hyperlocomotion or any stereotyped responses normally associated with D2 or mixed D1/D2 receptor stimulation. These data suggest that hypolocomotion elicited by 7-OH-DPAT in the NAC may be mediated at the D3 receptor as distinct from the D2 dopamine receptor. We discuss the possibility that the behavioural effects we observed are mediated at D3 autoreceptors.

Effects of dopamine on snail neurones

The pharmacological features of dopamine receptors in identifiable giant neurone types of a snail (Achatina fulica Férussac) were studied. Under voltage clamp, two neurone types, LVMN (left ventral multiple spike neurone) and d-RPeAN (dorsal-right pedal anterior neurone), produced an inward current (Iin) in response to dopamine, (-)-noradrenaline and epinine, whereas v-LCDN (ventral-left cerebral distinct neurone) produced an outward current (Iout) in response to dopamine and epinine. Mammalian dopamine receptor agonists, fenoldopam (dopamine D1-like receptor agonist), (+/-)-SKF 38393 (1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8- diol) (D1-like), apomorphine (D2-like), (-)-quinpirole (D3 and D4) and methylergometrine showed slight or no effect. (+/-)-SKF 83566 ((+/-)-7-bromo-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benza zepine) (dopamine D1-like receptor antagonist) and (+)-UH 232 (cis-(+)-5-methoxy-1-methyl-2-(di-n-propylamino)tetralin) (D3 and D2) non-competitively inhibited the Iin of LVMN and d-RPeAN, but (+/-)-sulpiride (D2-like) was without effect. In contrast, (+/-)-sulpiride competitively inhibited Iout of v-LCDN, (+)-UH 232 non-competitively inhibited Iout of v-LCDN but (+/-)-SKF 83566 was without effect. H-7 (1-(5-isoquinolinesulfonyl)-2-methylpiperazine) (non-selective protein kinase inhibitor) inhibited Iin of LVMN and d-RPeAN, but did not affect Iout of v-LCDN. Dopamine-induced Iin was Na(+)-dependent; Iout was K(+)-dependent. Ouabain did not affect these currents. We propose that the pharmacological features of Achatina neuronal dopamine receptors are not fully comparable to those of mammals, although intracellular signal transduction systems linked with dopamine receptors may similarly exist in different animal species.

Effect of (-)-DS 121 and (+)-UH 232 on cocaine self-administration in rats

The novel dopamine autoreceptor antagonists (-)-DS 121 and (+)-UH 232 were tested for their ability to alter cocaine self-administration behavior in rats reinforced on a progressive ratio (PR) schedule. (-)-DS 121 (15 mg/kg) and (+)-UH 232 (30 mg/kg) produced significant decreases in breaking point. (-)-DS 121 produced variable results on rate of cocaine intake on an FR1 schedule, indicating that rate may on occasion be insensitive to changes in cocaine reinforcement. In animals previously trained to self-administer cocaine, (-)-DS 121 failed to maintain responding when substituted for cocaine. This profile suggests that (-)-DS 121 is a promising new candidate for the treatment of cocaine abuse.

The putative dopamine D3 agonist, 7-OH-DPAT, reduces dopamine release in the nucleus accumbens and electrical self-stimulation to the ventral tegmentum

The present experiments were designed to test further the idea that 7-OH-DPAT (7-hydroxy-N,N-di-n-propyl-2-aminotetralin), a putative dopamine (DA) D3 agonist, has effects at DA autoreceptors to reduce intracranial DA levels and to reduce behaviours that are DA-dependent. Rats were trained to respond on a self-stimulation protocol for electrical stimulation to the ventral tegmental area (VTA). Each press of a lever delivered a 0.5 s train of square wave, 1.5 ms duration, 100 Hz, 90-120 mA stimulation. Systemic administration of 7-OH-DPAT at 0.01-0.3 mg/kg i.p., quickly dose-dependently reduced responding. Electrical stimulation using similar parameters to those that supported self-stimulation were then applied to the VTA of anaesthetized rats. Fast cyclic voltammetry (FCV) revealed that this stimulation released DA in the nucleus accumbens (NAC). 7-OH-DPAT i.p. (0.1-3.0 mg/kg) quickly and potently reduced the size of the DA-generated voltammetric signal. This effect of 0.3 mg/kg 7-OH-DPAT was not blocked by sulpiride (60 mg/kg, i.p.) a D2-specific antagonist that may preferentially block D2 autoreceptors. These data are discussed with reference to the possibility that 7-OH-DPAT reduces the release of dopamine in the NAC, at D3, but not at D2, autoreceptors and that this in turn may reduce the rewarding effect of VTA stimulation.