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.

Apelin, the natural ligand of the orphan seven-transmembrane receptor APJ, inhibits human immunodeficiency virus type 1 entry

In addition to the CCR5 and CXCR4 chemokine receptors, a subset of primary human immunodeficiency virus type 1 (HIV-1) isolates can also use the seven-transmembrane-domain receptor APJ as a coreceptor. A previously identified ligand of APJ, apelin, specifically inhibited the entry of primary T-tropic and dualtropic HIV-1 isolates from different clades into cells expressing CD4 and APJ. Analysis of apelin analogues demonstrated that potent and specific antiviral activity was retained by a 13-residue, arginine-rich peptide. Antiviral potency was influenced by the integrity of methionine 75, which contributes to APJ-binding affinity, and by the retention of apelin residues 63 to 65. These studies demonstrate the ability of a small peptide ligand to block the function of APJ as an HIV-1 coreceptor, identify apelin sequences important for the inhibition, and provide new reagents for the investigation of the significance of APJ to HIV-1 infection and pathogenesis.

Personality and risk-taking: common biosocial factors

The first part of this article describes a study of the relationships between personality and risk-taking in six areas: smoking, drinking, drugs, sex, driving, and gambling. The participants, 260 college students, were given self-report measures of risky behaviors in each of the six areas and the Zuckerman-Kuhlman five-factor personality questionnaire. Generalized risk-taking (across all six areas) was related to scales for impulsive sensation seeking, aggression, and sociability, but not to scales for neuroticism or activity. Gender differences on risk-taking were mediated by differences on impulsive sensation seeking. The second part discusses biological traits associated with both risk-taking and personality, particularly sensation seeking, such as the D4 dopamine receptor gene. the enzyme monoamine oxidase, and augmenting or reducing of the cortical evoked potential. Comparative studies show relationships between biological markers shared with other species and correlated behaviors similar to sensation seeking in humans. A biosocial model of the traits underlying risk-taking is presented.

Distinct functions of the two isoforms of dopamine D2 receptors

Signalling through dopamine D2 receptors governs physiological functions related to locomotion, hormone production and drug abuse. D2 receptors are also known targets of antipsychotic drugs that are used to treat neuropsychiatric disorders such as schizophrenia. By a mechanism of alternative splicing, the D2 receptor gene encodes two molecularly distinct isoforms, D2S and D2L, previously thought to have the same function. Here we show that these receptors have distinct functions in vivo; D2L acts mainly at postsynaptic sites and D2S serves presynaptic autoreceptor functions. The cataleptic effects of the widely used antipsychotic haloperidol are absent in D2L-deficient mice. This suggests that D2L is targeted by haloperidol, with implications for treatment of neuropsychiatric disorders. The absence of D2L reveals that D2S inhibits D1 receptor-mediated functions, uncovering a circuit of signalling interference between dopamine receptors.

Role of D(1) and D(2) dopamine receptors in the acquisition and expression of flavor-preference conditioning in sham-feeding rats

The present study examined the effects of D(1) and D(2) antagonists on flavor-preference conditioning by the sweet taste of sucrose. All sessions were conducted under sham-feeding conditions to minimize post-ingestive influences. The rats were trained in alternating, one-bottle sessions to sham-feed a 16% sucrose solution containing one novel flavor (CS+) and a less-preferred 0.2% saccharin solution containing a different flavor (CS-). Three groups of food-restricted rats were treated with either vehicle (control group), the D(1) antagonist, SCH23390 (200 nmol/kg), or the D(2) antagonist, raclopride (200 nmol/kg) during one-bottle training. A fourth group (yoked group) was vehicle-treated and its training intakes were matched to that of the D(1) and D(2) drug groups. Preferences were assessed in two-bottle tests with the CS+ and CS- flavors presented in mixed 8% sucrose+0.1% saccharin solutions following systemic doses of 0, 200, or 800 nmol/kg of either the D(1) or D(2) antagonists. All groups significantly preferred the CS+ flavor in vehicle tests, although the preferences were weaker in the D(1), D(2), and yoked groups compared to the control group. All groups selectively reduced their CS+ intakes when treated with either D(1) or D(2) antagonists during two-bottle testing, and the CS+ preference was blocked at the higher doses. These data show that D(1) and D(2) receptor antagonists block the expression of a sucrose-conditioned preference, but produces substantially lesser effects upon the acquisition of this form of flavor conditioning.

Imaging the glutamatergic system in vivo--relevance to schizophrenia

Schizophrenia is a devastating psychiatric illness. Its pathophysiology is not fully clarified. Animal data, in vitro and indirect in vivo imaging support glutamatergic N-methyl-D-aspartate (NMDA) receptor hypofunction in the disorder. A lack of suitable ligands has obstructed direct evaluation of the NMDA receptor hypofunction hypothesis of schizophrenia. Many research groups are working towards developing appropriate single-photon emission tomography and positron emission tomography ligands for the NMDA receptor. This paper briefly presents evidence for links between glutamatergic system dysfunction and schizophrenia. It reviews the radioligands to evaluate glutamatergic receptors in vivo and discusses issues in developing novel ligands for the glutamatergic system.

Biological markers and schizophrenia

OBJECTIVE

The delivery of biological markers for schizophrenia would greatly assist preventative strategies by identifying at-risk individuals who could then be monitored and treated in a manner with a view to minimising subsequent morbidity. This paper aims to present a selection of biological measures that may indicate risk of schizophrenia.

METHOD

A selective and brief review is provided of intensively studied putative markers, including enlarged cerebral ventricles, dopamine D2 receptor density, amphetamine-stimulated central nervous system dopamine release, plasma homovanillic acid and smooth pursuit eye tracking dysfunction.

RESULTS

A number of biological measures have been reported to be correlated with schizophrenia.

CONCLUSIONS

Presently, none of these measures has satisfactory performance characteristics in terms of predictive validity, noninvasiveness, ease of testing and low cost that would enable their widespread use. However, a few have potential for further investigation and development.

Electrophysiological and behavioural evidence for an antagonistic modulatory role of adenosine A2A receptors in dopamine D2 receptor regulation in the rat dopamine-denervated striatum

It has been shown that striatal adenosine A2A receptors can antagonistically interact with dopamine D2 receptors at the membrane level leading to a decrease in the affinity and efficacy of D2 receptors. Extracellular recordings and rotational behaviour were employed to obtain a correlate to these findings in an animal model of Parkinson's disease (PD). The recordings were performed in rats with unilateral 6-hydroxydopamine (6-OHDA)-induced catecholamine depletion. While recording in the dopamine-depleted striatum, local applications of the dopamine D2 agonist quinpirole reduced neuronal activity. However, when the adenosine A2A antagonist MSX-3 was applied simultaneously with quinpirole, the inhibition of neuronal firing seen after quinpirole alone was significantly potentiated (P< 0.001, n = 11). In contrast, local application of CGS 21680 attenuated the effect of quinpirole. The doses of MSX-3 and CGS 21680 used to achieve the modulation of quinpirole action had no effect per se on striatal neuronal firing. Furthermore, rotational behaviour revealed that MSX-3 dose-dependently increased the number of turns when administrated together with a threshold dose of quinpirole while no enhancement was achieved when MSX-3 was combined with SKF 38393. MSX-3 alone did not induce rotational behaviour. In conclusion, this study shows that low ineffective doses of MSX-3 enhance the effect of quinpirole on striatal firing rate, while the A2A agonist exerts the opposite action. This mechanism gives a therapeutic potential to A2A antagonists in the treatment of PD by enhancing D2 receptor function.

Intra-accumbens infusion of D(3) receptor agonists reduces spontaneous and dopamine-induced locomotion

The present study investigated whether PD 128907 and 7-OH-DPAT, described as preferential dopamine (DA) D(3) receptor agonists, produce hypolocomotion by acting at postsynaptic dopaminergic receptors within the nucleus accumbens. Bilateral infusion of PD 128907 (1.5 and 3 microg/0.5 microl) induced a dose-dependent hypolocomotion, whereas its enantiomer, PD 128908, was inactive. Local infusion of 7-OH-DPAT and the preferential DA autoreceptor agonist, B-HT 920, at the same dose range also decreased spontaneous locomotion. In addition, both drugs induced yawning with B-HT 920 producing the greatest effect. In the second experiment, the ability of these agonists to reduce the locomotor activity induced by intra-accumbens injection of DA (10 microg/0.5 microl) was studied. Pretreatment with either PD 128907 or 7-OH-DPAT (3 microg) reduced DA-induced hyperactivity. Local infusion of B-HT 920 (3 microg) failed to antagonise the locomotor effects of DA. Altogether these findings suggest that PD 128907 and 7-OH-DPAT induce hypolocomotion by acting in part at postsynaptic DA receptors. The possible role of D(2) and/or D(3) receptors in the mediation of these effects is discussed.

Reward deficiency syndrome: a biogenetic model for the diagnosis and treatment of impulsive, addictive, and compulsive behaviors

The dopaminergic system, and in particular the dopamine D2 receptor, has been implicated in reward mechanisms. The net effect of neurotransmitter interaction at the mesolimbic brain region induces "reward" when dopamine (DA) is released from the neuron at the nucleus accumbens and interacts with a dopamine D2 receptor. "The reward cascade" involves the release of serotonin, which in turn at the hypothalmus stimulates enkephalin, which in turn inhibits GABA at the substania nigra, which in turn fine tunes the amount of DA released at the nucleus accumbens or "reward site." It is well known that under normal conditions in the reward site DA works to maintain our normal drives. In fact, DA has become to be known as the "pleasure molecule" and/or the "antistress molecule." When DA is released into the synapse, it stimulates a number a DA receptors (D1-D5) which results in increased feelings of well-being and stress reduction. A consensus of the literature suggests that when there is a dysfunction in the brain reward cascade, which could be caused by certain genetic variants (polygenic), especially in the DA system causing a hypodopaminergic trait, the brain of that person requires a DA fix to feel good. This trait leads to multiple drug-seeking behavior. This is so because alcohol, cocaine, heroin, marijuana, nicotine, and glucose all cause activation and neuronal release of brain DA, which could heal the abnormal cravings. Certainly after ten years of study we could say with confidence that carriers of the DAD2 receptor A1 allele have compromised D2 receptors. Therefore lack of D2 receptors causes individuals to have a high risk for multiple addictive, impulsive and compulsive behavioral propensities, such as severe alcoholism, cocaine, heroin, marijuana and nicotine use, glucose bingeing, pathological gambling, sex addiction, ADHD, Tourette's Syndrome, autism, chronic violence, posttraumatic stress disorder, schizoid/avoidant cluster, conduct disorder and antisocial behavior. In order to explain the breakdown of the reward cascade due to both multiple genes and environmental stimuli (pleiotropism) and resultant aberrant behaviors, Blum united this hypodopaminergic trait under the rubric of a reward deficiency syndrome.

Evidence for antagonist activity of the dopamine D3 receptor partial agonist, BP 897, at human dopamine D3 receptor

The dopaminergic system has long been implicated in the mechanisms of reward and addiction. 1-(4-(2-Naphthoylamino)butyl)-4-(2-methoxyphenyl)-1A-piperazine HCl (BP 897) has been claimed to be a selective dopamine D3 receptor partial agonist and has recently been shown to inhibit cocaine-seeking behaviour, suggesting a role for dopamine D3 receptor agonists in the treatment of addiction. We have previously characterised the pharmacological profile of the human dopamine D3 and D2(long) receptors using microphysiometry and radioligand binding and we have now studied the interaction of BP 897 with the dopamine D2 and D3 receptors using these methods. At both human dopamine D3 and D2 receptors, BP 897 lacked agonist activity but was a potent and selective antagonist with pK(b) values of 8.05+/-0.16 (4) and 9.43+/-0.22 (4) at human dopamine D2 and D3 receptors, respectively. These results, therefore, suggest that it may be the dopamine D3 receptor antagonist properties of BP 897 which have potential in the treatment of addiction and withdrawal.

Expression and coreceptor function of APJ for primate immunodeficiency viruses

APJ is a seven transmembrane domain G-protein-coupled receptor that functions as a coreceptor for some primate immunodeficiency virus strains. The in vivo significance of APJ coreceptor function remains to be elucidated, however, due to the lack of an antibody that can be used to assess APJ expression, and because of the absence of an antibody or ligand that can block APJ coreceptor activity. Therefore, we produced a specific monoclonal antibody (MAb 856) to APJ and found that it detected this receptor in FACS, immunofluorescence, and immunohistochemistry studies. MAb 856 also recognized APJ by Western blot, enabling us to determine that APJ is N-glycosylated. Using this antibody, we correlated APJ expression with coreceptor activity and found that APJ had coreceptor function even at low levels of expression. However, we found that APJ could not be detected by FACS analysis on cell lines commonly used to propagate primate lentiviruses, nor was it expressed on human PBMC cultured under a variety of conditions. We also found that some viral envelope proteins could mediate fusion with APJ-positive, CD4-negative cells, provided that CD4 was added in trans. These findings indicate that in some situations APJ use could render primary cell types susceptible to virus infection, although we have not found any evidence that this occurs. Finally, the peptide ligand for APJ, apelin-13, efficiently blocked APJ coreceptor activity.

Binding of calmodulin to the D2-dopamine receptor reduces receptor signaling by arresting the G protein activation switch

Signaling by D(2)-dopamine receptors in neurons likely proceeds in the presence of Ca(2+) oscillations. We describe here the biochemical basis for a cross-talk between intracellular Ca(2+) and the D(2) receptor. By activation of calmodulin (CaM), Ca(2+) directly inhibits the D(2) receptor; this conclusion is based on the following observations: (i) The receptor contains a CaM-binding motif in the NH(2)-terminal end of the third loop, a domain involved in activating G(i/o). A peptide fragment encompassing this domain (D2N) bound dansylated CaM in a Ca(2+)-dependent manner (K(D) approximately 0.1 micrometer). (ii) Activation of purified Galpha(i1) by D2N, and D(2) receptor-promoted GTPgammaS (guanosine 5'-(3-O-thio)triphosphate) binding in membranes was suppressed by Ca(2+)/CaM (IC(50) approximately 0.1 micrometer). (iii) If Ca(2+) influx was elicited in D(2) receptor-expressing HEK293 cells, agonist-dependent inhibition of cAMP formation decreased. This effect was not seen with other G(i)-coupled receptors (A(1)-adenosine and Mel(1A)-melatonin receptor). (iv) The D(2) receptor was retained by immobilized CaM and radiolabeled CaM was co-immunoprecipitated with the receptor. Specifically, inhibition by CaM does not result from uncoupling the D(2) receptor from its cognate G protein(s); rather, CaM directly targets the D(2) receptor to block the receptor-operated G protein activation switch.

Genomic analysis and functional expression of canine dopamine D2 receptor

Dopamine D2 receptor (DRD2) is one of the five dopamine receptors with seven transmembrane domains that are coupled to the G protein. We have cloned and characterized the genomic and cDNA sequences of the canine DRD2 gene, which are 12.7 and 2.7 kb in size, respectively. The genomic DNA is composed of seven exons and six introns, encoding a 443 amino acid protein with 95% amino acid identity to other mammalian D2 receptors. A length polymorphism was detected in intron 3 of the receptor gene. We also characterized alternatively spliced forms of DRD2 cDNAs, DRD2L and DRD2S. They showed a higher level of expression in midbrain and thalamus. The ratio between the long and short form is similar in RT-PCR reaction. In human and rodent, the same two spliced forms are known to be coupled to G(i)-type heterotrimeric GTP binding protein, thereby opening an inwardly rectifying potassium channel, GIRK1. When the canine DRD2L and DRD2S were heterologously expressed in Xenopus oocytes, both forms activated GIRK1 potassium channels through coupling with G(i) protein. This activation was dose-dependent, demonstrating its ligand specificity.

Enhancement of ethanol reward by dopamine D3 receptor blockade

This experiment examined the influence of U-99194A, a dopamine D3 receptor antagonist, on ethanol's rewarding effect in a place conditioning paradigm. Swiss-Webster mice received six pairings of a tactile stimulus with ethanol (2 g/kg, i.p.), U-99194A (20 mg/kg, i. p.) with ethanol, or U-99194A alone. A different stimulus was paired with saline. During conditioning, ethanol or ethanol/U-99194A produced similar increases in locomotor activity. U-99194A alone produced modest increases in activity on some trials. As expected, the 2 g/kg ethanol dose produced a nonsignificant trend towards conditioned place preference. However, U-99194A enhanced the acquisition of ethanol preference, whereas U-99194A alone did not produce either place preference or aversion. The results are consistent with the notion that dopamine D3 systems are important in the response to ethanol and further suggest that D3 receptor blockade increases ethanol reward.

Chronic hypoxia induces changes in the central nervous system processing of arterial chemoreceptor input

Chronic hypoxia increases the hypoxic ventilatory response (HVR) in awake rats and the phrenic nerve response to carotid sinus nerve stimulation in anesthetized rats. An increased O2 sensitivity of the arterial chemoreceptors contributes to the increase in the HVR, but changes in the CNS processing of afferent information from arterial chemoreceptors are also involved. Adult male Sprague-Dawley rats were exposed to 0-7 days of hypobaric hypoxia (PIO2 = 80 Torr). Ventilation was measured in rats exposed to 0, 2 and 7 days of hypoxia using whole-body plethysmography. Ventilation increased after 2 days and remained elevated after 7 days of hypoxia. Following dopamine D2 receptor (D2-R) blockade in the CNS, frequency significantly decreased after 0 and 7 days of hypoxia, but did not change significantly after 2 days of hypoxia. In anesthetized rats, the phrenic nerve response to carotid sinus nerve stimulation was reduced following systemic D2-R blockade in control rats and those exposed to 7 days of hypoxia. After 2 days of hypoxia, there was no effect of blocking systemic D2-R. To determine whether changes in D2-R mRNA precede physiological changes, competitive RT-PCR was used to quantify D2-R mRNA in micropunches from the nucleus tractus solitarius (NTS) in normoxic and chronically hypoxic rats. In hypoxia, D2-R mRNA in the caudal NTS initially increased (6-12 hours) and then decreased below control levels (24 hours-7 days). These results show that chronic hypoxia causes time-dependent changes in D2-R that could result in changes in the ventilatory response to hypoxia.

Lack of operant ethanol self-administration in dopamine D2 receptor knockout mice

RATIONALE

Dopamine D2 receptors are postulated to play an important role in modulating the reinforcing effects of abused drugs including ethanol.

OBJECTIVES

This experiment examined operant ethanol self-administration in dopamine D2 receptor knockout (KO) mice and wild-type (WT) mice using a continuous access procedure.

METHODS

Adult male KO and WT mice were trained in 30-min sessions to perform a lever press response for access to 10% v/v ethanol. After training, the mice were placed in test chambers on a continuous (23 h/day) basis with access to food (one lever press, i.e., FR1), 10% v/v ethanol (four lever presses, i.e., FR4), and water from a sipper tube (phase 1). After 30 consecutive sessions, response patterns were determined for 0, 5, 10, 20 and 30% v/v ethanol (phase 2). Saccharin (0.2% w/v) was subsequently added to the ethanol mixture and responding was examined for 0, 5, 10 and 20% ethanol (phase 3).

RESULTS

During phase 1, WT mice displayed higher ethanol-lever responding compared to KO mice. Food lever responding and water intake was the same in both genotypes. During phase 2, WT mice displayed concentration-dependent ethanol lever responding, whereas KO mice responded at low rates regardless of ethanol concentration. WT mice also responded more for food compared to KO mice. Each genotype showed similar water intakes except at the 20% ethanol concentration, where WT mice had lower intakes. During phase 3, WT mice continued to show higher responding for all concentrations including saccharin alone. WT mice also continued to respond more for food compared to KO mice, but drank less water. In each phase, WT mice displayed episodic (bout) responding on the ethanol lever. KO mice did not respond for ethanol in bouts.

CONCLUSIONS

Reduced responding in the KO mice for several reinforcers including ethanol indicates a more general role for dopamine D2 receptors in motivated responding rather than a specific role in ethanol reinforcement.

The serotonin 5-HT(2A) receptor subtype does not mediate apomorphine-induced aggressive behaviour in male Wistar rats

We have studied the effect of the 5-HT(2A) receptor antagonists on apomorphine-induced aggressive behaviour in male Wistar rats. In acute behavioural experiments with apomorphine-pretreated (1.0 mg/kg, s.c., once daily, 2 weeks) animals, risperidone (0.5 and 1.0 mg/kg) inhibited aggressive behaviour, but ketanserin and ritanserin (0.5-5. 0 mg/kg) had no effect on the latency and intensity of aggressive behaviour. Concomitant risperidone (0.5 mg/kg) and haloperidol (0.03 and 0.3 mg/kg) administration blocked aggressive behaviour completely. In conclusion, our experiments confirm that inhibition of the apomorphine-induced aggressive behaviour is elicited by drugs with dopamine (DA) but not with 5-HT(2A) antagonistic activity. Moreover, it may be concluded that the serotonin 5-HT(2A) receptor subtype does not alter the DA-mediated behaviour.

The selective dopamine D4 receptor antagonist, PNU-101387G, prevents stress-induced cognitive deficits in monkeys

Stress exposure impairs the cognitive functioning of the prefrontal cortex (PFC). Previous research has examined the dopamine (DA) D1 receptor mechanisms underlying this response. The current study performed a preliminary examination of the role of D4 receptor mechanisms by determining whether the selective D4 receptor antagonist, PNU-101387G, could prevent stress-induced working memory deficits in monkeys. Animals were tested on the delayed response task following treatment with PNU-101387G (0 or 0.1-0.8 mg/kg, 60-min pretreatment), and the pharmacological stressor, FG7142 (0 or 0.2 mg/kg, 30-min pretreatment). FG7142 significantly impaired delayed response performance relative to vehicle; PNU-101387G pretreatment produced a dose-related reversal of the FG7142 response. PNU-101387G had no significant effects on its own, but there were trends toward improvement at low doses and impairment at higher doses. Further studies in a larger number of animals appear warranted. These preliminary findings suggest that D4 receptor mechanisms contribute to stress-induced cognitive dysfunction.

Cellular colocalization of dopamine D1 mRNA and D2 receptor in rat brain using a D2 dopamine receptor specific polyclonal antibody

1. The main objective of this work was to investigate the extent of cellular colocalization of dopamine D1 and D2 receptors in the rat brain. A double labeling technique, that combined immunocytochemical labeling of the D2 receptor using polyclonal antibodies raised against the third intracellular loop of the short isoform of the human D2 receptor in combination with in situ hybridization detecting D1 mRNA expression, was designed to accomplish this goal. 2. The specificity of the antisera obtained was confirmed by immunoprecipitation assay, Western blot analysis, and immunocytochemistry on D2R transfected cells and murine brain tissue. Western blot using the D2 receptor antibody revealed a specific broad band centered at 67 kDa in transfected cells and a major protein of 88 kDa corresponding to D2R expressed in the caudate-putamen, to a lesser extent in the cortex, and not at all detected in the hypothalamic region. 3. The content of neurons double-labeled for D1/D2 receptors was observed at in differing intensities in the dorsal endopiroform nucleus, the intercalated nucleus of amygdala, the anterior part of the cortical nucleus amygdala, the nucleus of the lateral olfactory tract, the piriform cortex, the parabrachial nucleus, the supraoptic nucleus and the parabigeminal nucleus. All other regions of the brain revealed neurons expressing either D1 or D2 dopamine receptors but not both at that same time. 4. These results clearly demonstrated that specific neurons expressed both receptors D1 and D2, and that this colocalization was restricted to particular regions of the rat brain.

Endogenous dopamine potentiates the effects of glutamate on extracellular GABA in the prefrontal cortex of the freely moving rat

Using microdialysis, the effects of endogenous dopamine on basal extracellular concentrations of gamma-aminobutyric acid (GABA) and on the increases of GABA produced by glutamate were investigated in the medial prefrontal cortex of the awake rat. The dopamine uptake inhibitor nomifensine (1, 100 and 1000 microM), used to increase extracellular dopamine, produced a dose-related increase of dialysate dopamine (0.1-1 nM) but did not change dialysate concentrations of GABA or glutamate at any dose used. The glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxilic acid (PDC; 0.5 and 2 mM), used to increase extracellular glutamate, produced a dose-related increase of dialysate glutamate (1.5-5.5 microM) and increased dialysate GABA by 125%. When a simultaneous increase of endogenous dopamine and glutamate was produced, the increases of dialysate GABA were significantly higher (185% of baseline) than those produced by glutamate alone. These effects on dialysate GABA were attenuated by the D2 receptor antagonist (-) sulpiride, but not by the D1 receptor antagonist SCH-23390, all of which suggests that extracellular dopamine plays an important role in modulating endogenous glutamate-GABA interactions in the prefrontal cortex of the rat.

Striatal dopamine release in reading and writing measured with [123I]iodobenzamide and single photon emission computed tomography in right handed human subjects

Competition between endogenous dopamine and a radioligand for postsynaptic dopamine D(2) receptor binding was examined in two groups of eight subjects each who had to read or write off a text, respectively, and in a control group. Single photon emission computed tomography (SPECT) and the ligand [(123)I]iodobenzamide (IBZM) were used for in vivo imaging. Subjects commenced reading or writing immediately before IBZM injection and continued for 30min thereafter. SPECT images were acquired 60min later. Striatum-to-parietal-cortex IBZM uptake ratios were lower in subjects who wrote off the text than in controls indicating competition of IBZM and dopamine. There was no difference between subjects who read the text and controls. Thus, dopamine release occurs as a consequence of the motoric activity involved in writing rather than of cognitive functions necessary for reading the text.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

Suppression of transient 40-Hz auditory response by haloperidol suggests modulation of human selective attention by dopamine D2 receptors

Cognitive processes including selective attention may depend on synchronous activity of neurons at the gamma-band (around 40Hz). To determine the effect of neuroleptic challenge on transient auditory evoked 40-Hz response, simultaneous measurement of 122-channel magnetoencephalogram (MEG) and 64-channel electroencephalogram (EEG) was used. Either 2mg of dopamine D(2)-receptor antagonist haloperidol or a placebo was administered orally to 11healthy subjects in a double-blind randomized crossover design in two separate sessions. The subjects attended to tones presented to one ear and ignored those presented to the other ear. Haloperidol significantly suppressed the transient 40-Hz electric response to the attended stimuli, while no significant effect was observed in the electric responses to the unattended tones or in the magnetic responses. The present result suggests that dopamine D(2) receptors modulate selective attention.