Dopamine D4 receptors: beyond schizophrenia

Dopamine D4 receptors mediate a wide range of neuronal signal transduction cascades. Malfunctions of these mechanisms may contribute to the pathophysiology of neuropsychiatric disorders, and their modification underlies the actions of many psychotropic drugs. Postmortem neuropathological and genetic studies provide inconclusive associations between D4 receptors and schizophrenia. Clinical trials of partially selective lead D4 antagonists have proved them to be ineffective against psychotic symptoms in patients diagnosed with schizophrenia. However, associations are emerging between D4 receptors and other neuropsychiatric disorders, including attention-deficit hyperactivity disorder as well as specific personality traits such as novelty seeking. Preclinical studies indicate that D4 receptors play a pivotal role in the cellular mechanisms of hyperactivity, impulsivity, and working memory. Accordingly, D4 receptors have broader implications for human illnesses than has been suggested by early focus on psychotic illness as a clinical target, and selective D4 agents may yield clinically useful drugs for several neuropsychiatric disorders that require improved treatments.

Repeated antipsychotic drug exposurein developing rats: Dopamine receptor effects

Antipsychotic drugs are often prescribed to juvenile psychiatric patients, though their cerebral effects during development are incompletely described. Accordingly, we studied the effects of repeated treatment with dissimilar antipsychotic drugs on dopamine (DA) receptors in juvenile vs. adult rats. Tissue levels of DA receptor types (D1, D2, D3, and D4) in forebrain regions of juvenile rats were quantified after 3 weeks of daily treatment with representative first- (fluphenazine) and second-generation (clozapine and olanzapine) antipsychotics, and compared with similarly treated adult rats examined in previous studies. Fluphenazine, clozapine, and olanzapine all decreased D1 receptors in dorsolateral frontal and medial prefrontal cortex (MPC) of juvenile, but not adult rats. Conversely, all three test agents increased D2 labeling in MPC of adult, but not young animals. Fluphenazine and olanzapine, but not clozapine, also increased D2 receptor levels in hippocampus, and D4 levels in nucleus accumbens (NAc) and caudate-putamen (CPu) in both juvenile and adult brain. D3 receptors were not altered by any treatment in any brain region at either age. Only some DA receptor adaptations to antipsychotic treatment are shared by developing and mature animals. Developmental differences in DA receptor responses may account for differences in clinical effects of antipsychotic drugs between young and adult psychiatric patients.

Regulation of working memory by dopamine D4 receptor in rats

Working memory is regulated by neurotransmitters in prefrontal cortex (PFC), including dopamine and norepinephrine. Previous studies of dopamine function in working memory have focused on the D1 and D2 receptors, with most evidence suggesting a dominant role for the D1 receptor. Since the dopamine D4 receptor is highly expressed in PFC, we hypothesize that it may also contribute to working memory. To test this hypothesis, we examined behavioral effects of L-745,870, a highly selective, centrally active, D4 antagonist, using a delayed alternation task in rats. Task performance was dose-dependently affected by the D4 antagonist, depending on individual baseline functional status of working memory. In rats with good baseline performance, the D4 antagonist had no effects at low doses, whereas high doses disrupted working memory. In rats with poor baseline working memory, the D4 antagonist significantly improved working memory at low doses, and higher doses were not distinguishable from vehicle controls. Effects of the D4 antagonist among poor performers were most robust when task demand for working memory was high, with lesser effects at lower demand level, suggesting that such effects were selective for working memory. The present findings indicate a significant role of the D4 receptor in working memory, and suggest innovative, D4-based, treatment of cognitive deficits associated with neuropsychiatric disorders.

Early adoption modifies the effects of prenatal stress on dopamine and glutamate receptors in adult rat brain

Stressful stimuli during pregnancy induce complex effects that influence the development of offspring. These effects can be prevented by environmental manipulations during the early postnatal period. Repeated restraint during the last week of pregnancy was used as a model of prenatal stress, and adoption at birth was used to change the postnatal environment. No differences were found in various physical landmarks, except for testis descent, for which all prenatally stressed pups showed a 1-day delay in comparison with control rats, regardless of the postnatal adoption procedure. Levels of dopamine (DA) D(2) and glutamate (Glu) N-methyl-D-aspartate (NMDA) receptors were differentially regulated in different forebrain regions of cross-fostered adult offspring. Increased concentrations of cortical D(2) receptors detected in stressed pups, raised by a gestationally stressed biological mother, were not detected when the pups were raised by a control mother. Control pups raised by a foster mother whether gestationally stressed or not had higher levels of NMDA receptors in cortical areas. These findings suggest that the normal expression of DA and Glu receptors is influenced by in utero experience and by lactation. The complex pattern of receptor changes reflects the high vulnerability of DA and Glu systems to variations both in prenatal and in postnatal environment, particularly for cortical D(2) receptors and NMDA receptors in cerebral cortex and nucleus accumbens. In contrast, testis descent appears to be more susceptible to prenatal than to postnatal environmental events.

Enhanced Expression of Dopamine D1 and Glutamate NMDA Receptors in Dopamine D4 Receptor Knockout Mice

Expression of dopamine ([DA] D1 and D2) and glutamate ([Glu], (N-methyl-d-aspartic acid [NMDA], alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA], and kanaic acid [KA]) receptor types were analyzed autoradiographically in forebrain regions of D4 receptor knockout mice and their wild-type controls. Selective radioligand binding to D4 receptors was virtually absent in D4 receptor knockout mouse brain in contrast to significant specific D4 binding in forebrain tissue of wild-type controls. Labeling of D1 receptors was significantly increased in nucleus accumbens (NAc; 39%) and caudate putamen (CPu; 42%) of D4-knockout mice vs wild-type controls. In addition, NMDA receptor labeling was significantly increased in NAc (31%), CPu (40%), and hippocampal CA1 (21%) and CA3 (25%) regions of D4 knockouts vs wild-type controls. No changes in D2, AMPA or KA receptors were found. The findings suggest that D1, D4, and NMDA receptors might interact functionally and that developmental absence of D4 receptors might trigger compensatory mechanisms that enhance expression of D1 receptors in NAc and CPu, and NMDA receptors in NAc, CPu, and hippocampus. The findings also encourage cautious interpretation of results in knockout mice with targeted absence of specific genes, as complex adaptive changes not directly related to the missing gene might contribute to physiological and behavioral responses.

Dopamine depletion abolishes apomorphine- and amphetamine-induced increases in extracellular serotonin levels in the striatum of conscious rats: a microdialysis study

We investigated how serotonergic neurotransmission was affected by 6-hydroxydopamine (6-OHDA) lesioning of the adult rat brain dopamine (DA) system. In this animal model for Parkinson's disease (PD), the effect of destroying ascending DA pathways on extracellular levels of serotonin (5-HT) and 5-HT innervation in rat striatum were examined. Profound unilateral lesions of the nigrostriatal DA pathways were made by infusing 6-OHDA unilaterally into either the right medial forebrain bundle or the right substantia nigra. At 5 weeks after lesioning extracellular levels of DA and 5-HT were determined with microdialysis and high-pressure liquid chromatography under basal conditions and after systemic injections of apomorphine or amphetamine. DA nerve-terminal destruction and 5-HT innervation were determined with quantitative autoradiography. 6-OHDA lesioning reduced extracellular levels of DA below detection limits and led to statistically significant increases in extracellular 5-HT. Apomorphine, and amphetamine, respectively increased extracellular 5-HT to 8.2- and 2.2-fold above baseline levels in intact animals; these effects were absent in 6-OHDA-lesioned animals. Basal levels of [(3)H]paroxetine binding to 5-HT transporters in caudate-putamen increased by 41% in 6-OHDA-lesioned animals. These results suggest that 6-OHDA lesioning led to hyperinnervation of 5-HT nerve terminals and increases in basal extracellular 5-HT levels, but also to an unexplained loss of apomorphine and amphetamine-induced release of 5-HT. Addressing whether this impairment has significance in the onset of PD might lead to development of new strategies to manage parkinsonian symptoms.

Long-term effects of olanzapine, risperidone, and quetiapine on ionotropic glutamate receptor types: implications for antipsychotic drug treatment

Levels of ionotropic glutamate (Glu) N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and kainic acid (KA) receptors in rat forebrain regions were compared by quantitative in vitro receptor autoradiography after continuous treatment for 28 days with the atypical antipsychotics olanzapine, risperidone, and quetiapine, or vehicle controls. All three treatments significantly decreased NMDA binding in caudate-putamen (CPu; by 30, 34, and 26%, respectively) but increased AMPA receptor levels in same region (by 22, 30, and 28%). Olanzapine and risperidone, but not quetiapine, also reduced NMDA receptor labeling in hippocampal CA1 (21 and 19%) and CA3 (23 and 22%) regions. KA receptors were unaltered by any treatment in the brain regions examined. These findings suggest that the antipsychotic effects of olanzapine and risperidone may be mediated in part by NMDA receptors in hippocampus, and perhaps AMPA receptors in CPu. The findings also support the hypothesis that down-regulation of NMDA receptors by atypical antipsychotic agents in CPu contributes to their low risk of extra-pyramidal side effects. Inability of olanzapine, risperidone, and quetiapine to alter KA receptors suggests their minimal role in mediating the central nervous system actions of these drugs.

Animal models of attention-deficit hyperactivity disorder

Attention-deficit hyperactivity disorder (ADHD) involves clinically heterogeneous dysfunctions of sustained attention, with behavioral overactivity and impulsivity, of juvenile onset. Experimental models, in addition to mimicking syndromal features, should resemble the clinical condition in pathophysiology, and predict potential new treatments. One of the most extensively evaluated animal models of ADHD is the spontaneously hypertensive rat. Other models include additional genetic variants (dopamine transporter gene knock-out mouse, coloboma mouse, Naples hyperexcitable rat, acallosal mouse, hyposexual rat, and population-extreme rodents), neonatal lesioning of dopamine neurons with 6-hydroxydopamine, and exposure to other neurotoxins or hippocampal irradiation. None is fully comparable to clinical ADHD. The pathophysiology involved varies, including both deficient and excessive dopaminergic functioning, and probable involvement of other monoamine neurotransmitters. Improved models as well as further testing of their ability to predict treatment responses are required.

Long-term effects of newer antipsychotic drugs on neuronal nitric oxide synthase in rat brain

Neuronal nitric oxide synthase (nNOS) catalyzes the synthesis of neuronal nitric oxide from L-arginine. Behavioral and neurochemical studies implicate neuronal nitric oxide in the pathophysiology of schizophrenia and in the actions of standard antipsychotic drugs. However, involvement of nNOS in the actions of newer antipsychotic drugs requires further investigation. Accordingly, density levels of nNOS, a marker for neuronal nitric oxide production, were examined in rat forebrain regions by computed autoradiography after repeated treatment (28 days) with three newer antipsychotic agents, olanzapine, risperidone, and quetiapine. No significant differences in nNOS levels were detected in representative cortical, limbic, and extrapyramidal brain regions of drug-treated vs vehicle-treated animals. The findings suggest that nNOS plays a minimal role in mediating the long-term actions of newer antipsychotic drugs.

Long-term effects of prenatal stress on dopamine and glutamate receptors in adult rat brain

Prenatal stress greatly influences the ability of an individual to manage stressful events in adulthood. Such vulnerability may result from abnormalities in the development and integration of forebrain dopaminergic and glutamatergic projections during the prenatal period. In this study, we assessed the effects of prenatal stress on the expression of selective dopamine and glutamate receptor subtypes in the adult offsprings of rats subjected to repeated restraint stress during the last week of pregnancy. Dopamine D2-like receptors increased in dorsal frontal cortex (DFC), medial prefrontal cortex (MPC), hippocampal CA1 region and core region of nucleus accumbens (NAc) of prenatally stressed rats compared to control subjects. Glutamate NMDA receptors increased in MPC, DFC, hippocampal CA1, medial caudate-putamen, as well as in shell and core regions of NAc. Group III metabotropic glutamate receptors increased in MPC and DFC of prenatally stressed rats, but remained unchanged in all other regions examined. These results indicate that stress suffered during the gestational period has long lasting effects that extend into the adulthood of prenatally stressed offsprings. Changes in dopamine and glutamate receptor subtype levels in different forebrain regions of adult rats suggest that the development and formation of the corticostriatal and corticolimbic pathways may be permanently altered as a result of stress suffered prenatally. Maldevelopment of these pathways may provide a neurobiological substrate for the development of schizophrenia and other idiopathic psychotic disorders.

Serotonin transporter binding increases in caudate-putamen and nucleus accumbens after neonatal 6-hydroxydopamine lesions in rats: implications for motor hyperactivity

We examined serotonin (5-HT) transporters in rat forebrain using quantitative autoradiography at three distinct developmental stages after neonatal 6-hydroxydopamine lesions. The lesions substantially increased 5-HT transporter binding in both caudate-putamen and nucleus accumbens, but not cerebral cortex. The effects reached maximal levels as early as postnatal day (PD) 24, and were sustained until early adulthood. Behavioral analyses indicated that neonatal lesions resulted in motor hyperactivity on PD 24, but not on PD 36 or 59. These findings suggest that excess 5-HT transporters reflect serotonin hyperinnervation reported to occur in lesioned rats, and may modulate motor hyperactivity.

Effects of norepinephrine and serotonin transporter inhibitors on hyperactivity induced by neonatal 6-hydroxydopamine lesioning in rats

Consistent with their clinical effects in attention deficit-hyperactivity disorder (ADHD), the stimulants methylphenidate and amphetamine reduce motor hyperactivity in juvenile male rats with neonatal 6-hydroxydopamine (6-OHDA) lesions of the forebrain dopamine (DA) system. Since stimulants act on several aminergic neurotransmission systems, we investigated underlying mechanisms involved by comparing behavioral actions of d-methylphenidate, selective inhibitors of the neuronal transport of DA [GBR-12909 (1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-[3-phenylpropyl]piperazine dihydrochloride), amfonelic acid], serotonin [5-hydroxytryptamine (5-HT), citalopram, fluvoxamine], and norepinephrine (NE; desipramine, nisoxetine) in 6-OHDA lesioned rats. Selective dopamine lesions were made using 6-OHDA (100 microg, intracisternal) on postnatal day (PD) 5 after desipramine pretreatment (25 mg/kg, s.c.) to protect noradrenergic neurons. Rats were given test agents or vehicle, intraperitoneally, before recording motor activity for 90 min at PD 25 in a novel environment. d-Methylphenidate stimulated motor activity in sham controls and antagonized hyperactivity in lesioned rats. Selective DA transport inhibitors GBR-12909 and amfonelic acid greatly stimulated motor activity in sham control subjects, too, but did not antagonize hyperactivity in lesioned rats. In contrast, all selective 5-HT and NE transporter antagonists tested greatly reduced motor hyperactivity in 6-OHDA lesioned rats but did not alter motor activity in sham controls. The findings indicate that behavioral effects of stimulants in young rats with neonatal 6-OHDA lesions may be mediated by release of NE or 5-HT and support interest in using drugs that increase activity of norepinephrine or serotonin to treat ADHD.

Organotypic rat cerebellar slice culture as a model to analyze the molecular pharmacology of GABAA receptors

The preservation of the neuronal circuitry in rat cerebellar slice cultures provides an advantage in monitoring the development and characterizing the pharmacology of GABA(A) receptor subtypes. Sprague-Dawley rats, 8-11 days of age, were decapitated, their cerebella were cut into 400-microm slices and transferred into culture dishes. Cell viability and organotypic cerebellar organization of the culture remained well preserved up to 3 weeks. Autoradiographic procedures were introduced in these advanced culture technique and employed [(3)H]Ro 15-4513 in the absence and presence of 10 microM diazepam to visualize all benzodiazepine (BZD) and diazepam-insensitive (DIS) binding sites, respectively. Since expression of the alpha6 subunit variant of the GABA(A)/BZD receptor is restricted to the cerebellar granule cells and the BZD receptor agonist diazepam has very low affinity for this subunit, changes in DIS [(3)H]Ro 15-4513 binding sites during cultivation time can be attributed to changes in alpha6 subunit expression. A time-dependent development of total and DIS [(3)H]Ro 15-4513 binding sites were observed in the culture with a trend towards an increase in GABA(A) receptor alpha6 subunit levels during the first week. These findings suggest that explant preparations can be used to examine morphological changes in rat cerebellar slices. In addition, these preparations can be utilized to study the pharmacological effects of GABA(A)/BZD selective drugs on postnatal development of GABA(A) receptors in rat cerebellum.

Long-term effects of olanzapine, risperidone, and quetiapine on serotonin 1A, 2A and 2C receptors in rat forebrain regions

RATIONALE

Serotonin (5-HT) and its receptors have been implicated in various neuropsychiatric disorders. Altered serotonergic neurotransmission and interactions between 5-HT and dopamine (DA) systems may contribute to the pathophysiology of idiopathic psychotic or manic disorders. Interactions with 5-HT receptors may also contribute to special properties of modern antipsychotic drugs not yet evaluated for long-term effects on 5-HT receptors.

OBJECTIVE AND METHODS

We surveyed effects of newer atypical antipsychotics on 5-HT receptor types 1A, 2A, and 2C in rat forebrain regions by quantitative receptor autoradiography with selective radioligands following 28 days of continuous infusion of drugs or control vehicle.

RESULTS

Infusion of olanzapine, risperidone, and quetiapine increased 1A, but decreased 2A receptor labeling in frontal cerebral cortex. Olanzapine decreased binding at 2C receptors in hippocampal CA(1) and CA(3) regions and perhaps entorhinal cortex; olanzapine, but neither risperidone nor quetiapine, also decreased 2C labeling in caudate-putamen.

CONCLUSIONS

The findings suggest that altered 5-HT(1A) and 5-HT(2A)receptor levels in frontal cortex, and 5-HT(2C) receptors in other forebrain regions, may contribute to psychopharmacological properties of these novel atypical antipsychotic agents, perhaps including their antipsychotic or antimanic actions, and low risk of adverse extrapyramidal effects.

Plasticity of dopamine D4 receptors in rat forebrain: temporal association with motor hyperactivity following neonatal 6-hydroxydopamine lesioning

Genetic studies suggest that dopamine D(4) receptor polymorphism is associated with attention deficit hyperactivity disorder (ADHD). We recently reported that motor hyperactivity in juvenile male rats with neonatal 6-hydroxydopamine lesions of the central dopamine system can be reversed by dopamine D(4) receptor-selective antagonists. In this study, effects of such lesions on D(4) as well as other dopamine receptors (D(1) and D(2)) were autoradiographically quantified at selected developmental stages. Neonatal lesions resulted in motor hyperactivity at postnatal day (PD) 25, but not at PD 37 or 60. Correspondingly, D(4) receptor levels in lesioned rats were substantially increased in caudate-putamen and decreased in nucleus accumbens at PD 25, but not at PD 37 or 60. Neonatal lesions also led to relatively minor changes in D(1) and D(2) receptor binding in various forebrain regions. However, the time-course of lesion-induced motor hyperactivity correlated only with changes in D(4), but not D(1) and D(2) receptors. These results further support the hypothesis that D(4) receptors may play a pivotal role in lesion-induced hyperactivity, and possibly in clinical ADHD.

Effects of dopamine D4 receptor-selective antagonists on motor hyperactivity in rats with neonatal 6-hydroxydopamine lesions

RATIONALE

Dopamine D4 receptor gene polymorphism has been repeatedly associated with attention deficit hyperactivity disorder (ADHD) and related personality traits. We recently reported that motor hyperactivity in an animal model of ADHD was dose-dependently reversed by CP-293,019, a D4 receptor-selective antagonist. However, behavioral effects of this agent may not be attributed exclusively to D4 receptor blockade, since it interacts with other sites including serotonin receptors.

OBJECTIVES

To test further the hypothesis that D4 receptor blockade can reduce motor hyperactivity, behavioral effects of three chemically and pharmacologically dissimilar D4 antagonists were compared to that of ketanserin, a serotonin 5-HT(2A/2C) antagonist.

METHODS

Selective dopamine lesions were made in male rats at postnatal day (PD) 5 with intracisternal 6-hydroxydopamine (100 microg) after desipramine pretreatment (25 mg/kg, SC) to protect noradrenergic neurons. Effects of D4 receptor-selective antagonists and ketanserin on lesion-induced motor hyperactivity were examined during the periadolescent period (postnatal days 23-26) with an infrared photobeam activity system.

RESULTS

The D4 antagonists L-745,870 and U-101,958 dose-dependently inhibited motor hyperactivity in rats with neonatal lesions, whereas S-18126 lacked this effect at doses up to 30 mg/kg. None of these drugs affected motor behavior in sham control rats. In contrast, ketanserin produced apparent sedative effects in both lesioned and intact control rats without normalizing hyperactivity.

CONCLUSIONS

Motor hyperactivity in this ADHD model was selectively antagonized by three of four dopamine D4 receptor antagonists evaluated, encouraging clinical assessment of D4 antagonists in patients with ADHD.

Effects of newer antipsychotics on extrapyramidal function

Following acceptance of clozapine as a superior antipsychotic agent with low risk of adverse extrapyramidal syndromes (EPS), such as dystonia, parkinsonism, akathisia or tardive dyskinesia, several novel antipsychotic drugs have been developed with properties modelled on those of clozapine. Though generally considered 'atypical' in their relatively low risk of inducing EPS, these agents vary considerably in their pharmacology and impact on neurological functioning. Although few comparative data are available, the atypical antipsychotics can be tentatively ranked by EPS risk (excluding akathisia and neuroleptic malignant syndrome) in the following order: clozapine < quetiapine < olanzapine = ziprasidone. At higher doses, risperidone is ranked with a higher EPS risk than olanzapine and ziprasidone, but its risk of EPS is lower with lower doses. In general, this ranking is inversely related to antidopaminergic (D2 receptor) potency. The high antiserotonergic (5-HT2A receptor) potency of risperidone, clozapine, ziprasidone and olanzapine, but not quetiapine, as well as the antimuscarinic activity of olanzapine and clozapine may also limit EPS. For the treatment of psychotic reactions to dopamine agonist therapy in Parkinson's disease, clozapine is both effective and relatively well tolerated; quetiapine may be tolerated, olanzapine is not well tolerated, risperidone is poorly tolerated, and amisulpride and ziprasidone have not been well evaluated. Clozapine, perhaps because of its anticholinergic activity, can reduce parkinsonian tremor. It is useful for ongoing psychosis with tardive dyskinesia, especially for dystonic features. No atypical antipsychotic is clearly effective for motor abnormalities in Huntington's disease or Tourette's syndrome, and the effect of these drugs on other neurological disorders have been well evaluated in only small numbers of patients. In summary, with the exception of clozapine, and perhaps quetiapine, atypical antipsychotics have brought only relative avoidance of EPS, strongly encouraging continued searches for novel antipsychotic agents.

Stereoselective effects of methylphenidate on motor hyperactivity in juvenile rats induced by neonatal 6-hydroxydopamine lesioning

RATIONALE

The psychostimulant dl-threo-methylphenidate is commonly used to treat attention deficit-hyperactivity disorder (ADHD). Consistent with its effects in ADHD patients, racemic methylphenidate antagonizes behavioral hyperactivity in several animal models of ADHD, including juvenile rats with neonatal 6-hydroxydopamine (6-OHDA) lesions of forebrain dopamine projections. The enantiomers of methylphenidate differ markedly in stimulant potency but have not been compared in the 6-OHDA lesion model.

OBJECTIVE

Locomotor-inhibiting effects of methylphenidate enantiomers were compared in 6-OHDA-lesioned rats to test the hypothesis that d-methylphenidate is more potent than dl- and l-methylphenidate.

METHODS

Selective dopamine lesions were made using 6-OHDA (100 microg, intracisternal, IC) on postnatal day (PD) 5 after desipramine (25 mg/kg, SC) pretreatment to protect noradrenergic neurons. Effects of d-, l- and dl-threo-methylphenidate on locomotor activity of lesioned and sham control rats were quantified at PD 23-27.

RESULTS

Lesioning yielded robust motor hyperactivity at PD 23-27. Both d- and dl-methylphenidate stimulated locomotor activity in intact rats, and inhibited activity in lesioned rats. l-Methylphenidate did not affect locomotor activity in either lesioned rats or controls. d-Methylphenidate (ED(50)=1.66 mg/kg) was 3.3 times more potent than dl-methylphenidate (ED(50)=5.45 mg/kg) in reducing locomotor hyperactivity in lesioned rats. In addition, pretreatment of lesioned rats with l-methylphenidate significantly reduced the motor inhibiting effects of d-methylphenidate.

CONCLUSIONS

The more active enantiomer, as predicted, was d-methylphenidate, but the l-enantiomer interfered with its effects, suggesting that clinical potency of d-methylphenidate may be more than twice that of the racemate.

Role of dopamine D(4) receptors in motor hyperactivity induced by neonatal 6-hydroxydopamine lesions in rats

The role of dopamine D(4) receptors in behavioral hyperactivity was investigated by assessing D(4) receptor expression in brain regions and behavioral effects of D(4) receptor-selective ligands in juvenile rats with neonatal 6-hydroxydopamine lesions, a laboratory model for attention deficit-hyperactivity disorder (ADHD). Autoradiographic analysis indicated that motor hyperactivity in lesioned rats was closely correlated with increases in D(4) but not D(2) receptor levels in caudate-putamen. D(4)-selective antagonist CP-293,019 dose-dependently reversed lesion-induced hyperactivity, and D(4)-agonist CP-226,269 increased it. These results indicate a physiological role of dopamine D(4) receptors in motor behavior, and may suggest much-needed innovative treatments for ADHD.

Neuropharmacology of dopamine receptors:: Implications in neuropsychiatric diseases

There has been an extraordinary recent accumulation of information concerning the neurobiology and neuropharmacology of dopamine (DA) receptors in the mammalian central nervous system. Many new DA molecular entities have been cloned, their gene, peptide sequences and structures have been identified, their anatomical distributions in the mammalian brain described, and their pharmacology characterized. Progress has been made toward developing selective ligands and drug-candidates for different DA receptors. The new discoveries have greatly stimulated preclinical and clinical studies to explore the neuropharmacology of DA receptors and their implications in the neuropathophysiology of different neuropsychiatric diseases including schizophrenia, Parkinson's disease and attention-deficit hyperactivity disorder. Accordingly, it seems timely to review the salient aspects of this specialized area of preclinical neuropharmacology and its relevance to clinical neuropsychiatry.

Long-term effects of olanzapine, risperidone, and quetiapine on dopamine receptor types in regions of rat brain: implications for antipsychotic drug treatment

Changes in members of the dopamine (DA) D(1)-like (D(1), D(5)) and D(2)-like (D(2), D(3), D(4)) receptor families in rat forebrain regions were compared by quantitative in vitro receptor autoradiography after prolonged treatment (28 days) with the atypical antipsychotics olanzapine, risperidone, and quetiapine. Olanzapine and risperidone, but not quetiapine, significantly increased D(2) binding in medial prefrontal cortex (MPC; 67% and 34%), caudate-putamen (CPu; average 42%, 25%), nucleus accumbens (NAc; 37%, 28%), and hippocampus (HIP; 53%, 30%). Olanzapine and risperidone, but not quetiapine, produced even greater up-regulation of D(4) receptors in CPu (61%, 37%), NAc (65%, 32%), and HIP (61%, 37%). D(1)-like and D(3) receptors in all regions were unaltered by any treatment, suggesting their minimal role in mediating actions of these antipsychotics. The findings support the hypothesis that antipsychotic effects of olanzapine and risperidone are partly mediated by D(2) receptors in MPC, NAc, or HIP, and perhaps D(4) receptors in CPu, NAc, or HIP, but not in cerebral cortex. Selective up-regulation of D(2) receptors by olanzapine and risperidone in CPu may reflect their ability to induce some extrapyramidal effects. Inability of quetiapine to alter DA receptors suggests that nondopaminergic mechanisms contribute to its antipsychotic effects.

Nigrostriatal dopaminergic denervation enhances dopamine D(4) receptor binding in rat caudate-putamen

Radioligand binding to dopamine (DA) D(4) receptors was examined in adult rat forebrain 5 weeks after unilateral 6-hydroxydopamine (6-OHDA) lesioning of substantia nigra to remove ascending nigrostriatal dopaminergic projections. D(4) receptor binding was increased by up to 47% in denervated caudate-putamen (CPu) in rats that rotated away from the lesioned side with apomorphine challenge, with lesser changes in rats that failed to rotate with apomorphine. Functional significance of D(4) receptor upregulation induced by the lesions was investigated by examining behavioral effects of the highly selective D(4) agonist CP-226,269 and antagonist CP-293,019. Neither agent induced rotation at doses as high as 30 mg/kg ip. Pretreatment with the D(4) antagonist CP-293,019 did not affect rotation induced by either a D(1)-like (SKF-38393) or D(2)-like receptor (quinpirole) agonist. These findings provide the first evidence that D(4) receptors can be upregulated by nigrostriatal dopaminergic denervation. They also suggest that, unlike D(1) and D(2) receptors, D(4) receptors do not play a pivotal role in rotational behavior in rats with unilateral dopaminergic lesions.

GABA(B) receptors: altered coupling to G-proteins in rats sensitized to amphetamine

Modified dopamine and glutamate neurotransmission in discrete brain regions is implicated in stimulant-induced behavioral sensitization. Release of both neurotransmitters is influenced by GABA(B) metabotropic receptors for the principal inhibitory neurotransmitter GABA. Accordingly, GABA(B) receptors were examined in rats sensitized to amphetamine by measuring receptor density and coupling to G-proteins indicated as [(3)H]baclofen binding and baclofen-mediated [(35)S]GTP gamma S binding. Repeated treatment with (+)-amphetamine (5mg/kg per day, i.p., for five days) sensitized the rats to amphetamine challenge (1mg/kg) at 14 days, but not one day, later. GABA(B) receptor density was not altered at either time. Baclofen-mediated [(35)S]GTP gamma S binding, however, was selectively augmented in the prefrontal cortex and attenuated in the nucleus accumbens at 14 days, but not one day, after amphetamine treatment. Changes in GABA(B) receptor coupling to G-proteins in rats sensitized to amphetamine, but not in similarly treated but unsensitized rats, lead us to suggest that altered GABA(B) receptor functioning may contribute to the expression of amphetamine-induced behavioral sensitization.

Effects of nigrostriatal dopamine denervation on ionotropic glutamate receptors in rat caudate-putamen

Changes in ionotropic glutamate NMDA, AMPA and KA receptor binding in rat caudate-putamen were examined by quantitative in vitro receptor autoradiography 5 weeks after lesioning nigrostriatal dopaminergic projections. In this animal model of Parkinson's disease, density of binding in caudate-putamen increased at KA, but not NMDA or AMPA receptors. The findings indicate that nigrostriatal dopamine denervation can selectively enhance KA receptor levels in rat basal ganglia, suggest that KA receptors contribute to the pathophysiology of Parkinson's disease, and may suggest innovative treatments.

Alkylation of rat dopamine transporters and blockade of dopamine uptake by EEDQ

Effects of the alkylating agent EEDQ (N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline) on levels of dopamine transporter (DA(T)) and function were examined in caudate-putamen (CPu) tissue from rat brain. EEDQ produced profound, dose-dependent decreases in DA(T) binding in homogenates (IC(50)=78 microM) and frozen sections (IC(75)=200 microM) that were not reversed by washing. EEDQ also blocked uptake of [(3)H]DA in CPu synaptosomes (IC(50)=17 microM). However, single (10 mg/kg) or repeated administration of EEDQ in vivo (15 mg/kg/day x 3) did not alter DA(T) levels or DA uptake in CPu. Pretreatment of rats with alpha-methyl-p-tyrosine and reserpine to deplete endogenous dopamine also failed to lower DA(T) levels in CPu after injections of EEDQ. EEDQ is an effective alkylating agent for DA(T) in vitro, but not to evaluate metabolic turnover or function of DA(T) in vivo. The results encourage development of selective and in vivo-active DA(T)-alkylating agents.

Postnatal development of dopamine D1-like receptors in rat cortical and striatolimbic brain regions: An autoradiographic study

Postnatal development of dopamine D1-like (D1/D5) receptors in rat caudate-putamen (CPu), nucleus accumbens (NAc), hippocampus, frontal and entorhinal cerebral cortex was assessed between postnatal days (PD) 7-60 by in vitro receptor autoradiography. Density of [3H]SCH-23390 binding to D1-like receptors increased from PD-7 to a peak at PD-28 in CPu (11-fold) and NAc (23-fold), then declined by 20-40% in both regions over PD-35-60, to adult levels. In hippocampus, frontal and entorhinal cortex, D1-like receptors increased by lesser amounts (3- to 4-fold) from PD-7 to stable, maximal adult levels at PD-60. Evidently, excess D1-like receptors were eliminated during maturation of CPu and NAc, but not in the other forebrain regions. Postnatal D1-like receptor development in rat forebrain paralleled that of D2- and D4-like receptors in the same regions.

Dopamine D(4) receptors in rat forebrain: unchanged with amphetamine-induced behavioral sensitization

Dopamine D(2) receptors are implicated in stimulant-induced behavioral sensitization.(7,10) Studies using selective receptor antagonists also implicate the D(4) receptor, a member of the dopamine D(2)-like receptor family.(3) Accordingly, dopamine D(4) and D(2)-like receptor levels in rat forebrain were examined by computed autoradiography after repeated (+)-amphetamine treatment that induced behavioral sensitization. Receptor binding was quantified in critical brain regions including caudate-putamen, nucleus accumbens septi, medial prefrontal cortex and hippocampus. No significant differences in D(4) or D(2)-like receptor levels were detected among rats sensitized to amphetamine, those exposed to amphetamine but killed before behavioral sensitization emerged or vehicle-treated controls. The findings indicate that expression of amphetamine-induced behavioral sensitization is not associated with altered D(4) (or D(2)) receptor density in rat forebrain.

Olanzapine, quetiapine, and risperidone: long-term effects on monoamine transporters in rat forebrain

Long-term effects of novel atypical antipsychotic drugs on monoamine transporters are unknown. We compared labeling of dopamine (DAT) and serotonin (SERT) transporter proteins in subregions of rat corpus striatum by quantitative autoradiography with [(3)H]2-beta-carbomethoxy-3-beta-[4'-iodophenyl]tropane ([(3)H]beta-CIT) and [(3)H]paroxetine after 28 days of continuous subcutaneous infusion of olanzapine, quetiapine, risperidone, or vehicle controls. Drug treatment did not significantly alter the abundance of either transporter type in caudate-putamen or nucleus accumbens, indicating that transporter proteins required to inactivate synaptically released dopamine and serotonin resist adaptations to long-term treatment with novel antipsychotics that affect neurotransmission by these amines.

Comparative postnatal development of dopamine D(1), D(2) and D(4) receptors in rat forebrain

Postnatal development of dopamine D(1), D(2) and D(4) receptors in the caudate-putamen, nucleus accumbens, frontal cortex and hippocampus was assessed in rat brain between postnatal days 7 and 60. In the caudate-putamen and nucleus accumbens, density of all three receptor subtypes increased to a peak at postnatal day 28, then declined significantly in both regions (postnatal days 35-60) to adult levels. In the frontal cortex and hippocampus, these receptors rose steadily and continuously to stable, maximal adult levels by postnatal day 60. Evidently, D(1), D(2) and D(4) receptors follow a similar course of development in several cortical, limbic and extrapyramidal regions of rat forebrain, with selective elimination of excess dopamine receptors at the time of puberty in the caudate-putamen and accumbens but not other brain regions.

[3H]beta-CIT: a radioligand for dopamine transporters in rat brain tissue

[3H]2-beta-carbomethoxy-3-beta-[4'-iodophenyl]tropane (beta-CIT) was prepared and evaluated. With rat forebrain tissue, [3H]beta-CIT showed high affinity for dopamine transporters (DAT), with selectivity for DAT over norepinephrine transporters, but not serotonin transporters, as well as DAT-stereoselectivity with beta-CIT, amphetamine and methylphenidate. Affinity and selectivity for 53 compounds assayed with [3H]beta-CIT and standard DAT radioligand [3H]GBR-12935 were highly correlated (r0.95). [3H]beta-CIT is proposed as a useful, high-affinity DAT radioprobe.

Effects of alkylating agents on dopamine D(3) receptors in rat brain: selective protection by dopamine

Dopamine D(3) receptors are structurally highly homologous to other D(2)-like dopamine receptors, but differ from them pharmacologically. D(3) receptors are notably resistant to alkylation by 1-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), which readily alkylates D(2) receptors. We compared EEDQ with N-(p-isothiocyanatophenethyl)spiperone (NIPS), a selective D(2)-like receptor alkylating agent, for effects on D(3) and D(2) receptors in rat brain using autoradiographic analysis. Neither agent occluded D(3) receptors in vivo at doses that produced substantial blockade of D(2) receptors, even after catecholamine-depleting pretreatments. In vitro, however, D(3) receptors were readily alkylated by both NIPS (IC(50)=40 nM) and EEDQ (IC(50)=12 microM). These effects on D(3) sites were blocked by nM concentrations of dopamine, whereas microM concentrations were required to protect D(2) receptors from the alkylating agents. The findings are consistent with the view that alkylation of D(3) receptors in vivo is prevented by its high affinity for even minor concentrations of endogenous dopamine.

Dopamine D4 receptors: significance for molecular psychiatry at the millennium

Extraordinary progress has been made in the molecular, genetic, anatomical, and pharmacological characterization of dopamine D4 receptors in animal and human brain. Clarification of the neurochemical and physiological roles of these cerebral receptors is emerging. Postmortem neuropathological studies have inconsistently linked D4 receptors to psychotic disorders, and genetic studies have failed to sustain conclusive associations between D4 receptors and schizophrenia. However, associations are emerging between D4 receptors and other neuropsychiatric disorders, including attention deficit hyperactivity disorder, mood disorders, and Parkinson's disease, as well as specific personality traits such as novelty-seeking. Selective D4 agonists and antagonists have been developed as useful experimental probes. D4antagonists, so far, have proved ineffective in treatment of schizophrenia, but testing in a broader range of disorders may yield clinically useful drugs. D4 receptors appear to have broad implications for the pathophysiology of neuropsychiatric illnesses and their improved treatment.

Selective alkylatation of dopamine D2 and D4 receptors in rat brain by N-(p-isothiocyanatophenethyl)spiperone

Effects of the D2-like receptor alkylating agent NIPS (N-[p-isothiocyanatophenethyl]spiperone) on dopamine receptors in rat brain were characterized by radioreceptor assays and quantitative autoradiography. NIPS alkylated D2 and D4 receptors concentration-dependently in brain sections and transfected cells. NIPS also alkylated both receptors dose-dependently in vivo, with no effect on dopamine D1-like or serotonin 5-HT2 receptors at a dose that occluded 75% of D2 and D4 receptors. Pretreatment with D2-like receptor selective antagonist haloperidol completely blocked the effects of NIPS. The findings demonstrate that NIPS selectively alkylates D2 and D4 receptors, indicating its potential utility for studies of these receptors.

Effects of chronic treatment with typical and atypical antipsychotic drugs on the rat striatum

Human MRI studies have demonstrated that treatment with typical antipsychotics may increase the volume of the caudate nucleus while clozapine treatment is associated with either no change or a reversal of the previous volume increase. In this study four groups of seven rats were treated for 8 months with either the typical antipsychotic haloperidol, the atypical antipsychotic clozapine, the D2/D3 receptor antagonist raclopride, or vehicle (plain drinking water). Striatal sections were prepared using D1-like and D2-like receptor ligand autoradiography. Images (4-6 sections per rat, per ligand) were digitized and the area of the striatum was measured on each section. Rats treated with haloperidol did not have a larger mean striatum area than the control group on either D1- or D2-like ligand autoradiograms. Using the D2-like ligand autoradiograms, the clozapine treated animals had a smaller mean striatum area than the control group. Mean left striatum area was larger than mean right striatum area in each treatment group and in the control group. In contrast to the MRI findings reported in schizophrenia, the area of the striatum was not increased in rats treated with typical antipsychotic agents, but the clozapine-associated area reduction may parallel the clinical studies.

Brain dopamine D(4) receptors: basic and clinical status

Since their discovery in 1991, an extraordinary amount of information has accumulated about the neurobiology and pharmacology of D(4) receptors in the mammalian central nervous system, making it timely to review salient aspects of this rapidly evolving research story and its relevance to clinical neuroscience. Recent progress in the molecular, genetic, anatomical, and functional characterization of D(4) receptors in the animal and human brain is yielding insights into their neurochemical and physiological roles in brain function. The temporal patterns of postnatal D(4) receptor development appear to differ in specific regions of mammalian forebrain. Postmortem neuropathological and clinical genetic studies have generally been disappointing regarding possible relationships of D(4) receptors to the pathophysiology or treatment of schizophrenia, however, they suggest relationships to other neuropsychiatric conditions, including attention deficit hyperactivity disorder, mood disorders, and Parkinson's disease. Several selective agonists and antagonists for D(4) receptors have been developed. Some are employed as experimental D(4) probes. The potential of D(4)-selective ligands as innovative treatments for neuropsychiatric disorders requires further investigation.

Regional localization of dopamine and ionotropic glutamate receptor subtypes in striatolimbic brain regions

Localization of dopamine (D(1)-, D(2)-like, and D(4)) and ionotropic glutamate (NMDA, AMPA, and KA) receptor subtypes within the striatolimbic forebrain remains incomplete, but basic to understanding the functional organization of this important brain region. We found that frontal cortical ablation supported colocalization of D(4) and NMDA receptors on corticostriatal afferents to caudate-putamen and nucleus accumbens in rat forebrain. Local injection of kainic acid into caudate-putamen, nucleus accumbens, or hippocampus produced massive local postsynaptic losses of D(1)- and D(2)-like, as well as NMDA, AMPA, and KA receptors, and kainic acid ablation of hippocampal-striatal projections indicated the selective expression of presynaptic NMDA and KA autoreceptors. Degeneration of nigrostriatal dopamine projections with 6-hydroxydopamine showed that all three glutamatergic subtypes exist as heteroceptors on nigrostriatal dopaminergic terminals. Our findings suggest common interactions between excitatory glutamatergic and inhibitory dopaminergic receptors in rat forebrain. Further localization of these receptor subtypes in striatolimbic forebrain should help to clarify their contributions to the pathophysiology of neuropsychiatric disorders and their treatment.

Neuropharmacological assessment of potential dopamine D4 receptor-selective radioligands

Radiolabeled dopamine D4 receptor-selective agents ([3H]1-benzyl-4-[ N-(3-isopropoxy-2-pyridinyl)-N-methyl]-aminopiperidine maleate; [3 H]PNU-101958. and [125I]1-[4-iodobenzyl]-4-[ N-(3-isopropoxy-2-pyridinyl)-N-methyl]-aminopiperidine; [125I]RBI-257) were prepared and characterized. With D4.2- and D2L receptor-transfected cell membranes, [3H]PNU-101958 showed high dopamine D4 receptor affinity and selectivity, and potent inhibition by dopamine D4 receptor-selective compounds. However, its binding with rat brain homogenates showed little regional selectivity, and pharmacology inconsistent with selective dopamine D4 receptor labeling. Autoradiography indicated partial displacement of [3H]PNU-101958 by unlabeled dopamine D4 receptor ligands without regional selectivity, and lack of selective labeling with [125I]RBI-257. The results encourage further efforts to develop better dopamine D4 receptor-selective radioligands.

Effects of hippocampal kainic acid lesions on striatolimbic dopamine D1-, D2-, and D4-like receptors

Changes in D1-, D2- and D4-like dopamine receptor binding in rat caudate-putamen, nucleus accumbens and hippocampus were examined by quantitative in vitro receptor autoradiography after unilateral injections of kainic acid into hippocampus to degenerate local intrinsic neurons and remove hippocampal-striatal projections. Radioligands were: [3H]SCH-23390 for D1-like (D1/D5) receptors, [3H]nemonapride alone for D2-like (D2/D3/D4) receptors and with 300 nM S[-]-raclopride and other masking agents for D4-like receptors. Kainic acid injections produced massive local hippocampal losses of D1-like (75%), D2-like (77%) and D4-like (79%) receptor binding but did not significantly alter D1-, D2- or D4-like receptor density in caudate-putamen or nucleus accumbens. Most D1-, D2- and D4-like receptors in rat hippocampus evidently arise on intrinsic postsynaptic neurons whereas none was detected on presynaptic terminals of hippocampal-striatal afferents innervating the striatal complex.

Localization of ionotropic glutamate receptors in caudate-putamen and nucleus accumbens septi of rat brain: comparison of NMDA, AMPA, and kainate receptors

Changes in binding of selective radioligands at NMDA ([3H]MK-801), AMPA ([3H]CNQX), and kainate ([3H]kainic acid) glutamate (GLU) ionotropic receptors in rat caudate-putamen (CPu) and nucleus accumbens (NAc) were examined by quantitative autoradiography following: 1) unilateral surgical ablation of frontal cerebral cortex to remove descending corticostriatal GLU projections, 2) unilateral injection of kainic acid (KA) into CPu or NAc to degenerate local intrinsic neurons, or 3) unilateral injections of 6-hydroxydopamine (6-OH-DA) into substantia nigra to degenerate ascending nigrostriatal dopamine (DA) projections. Cortical ablation significantly decreased NMDA receptor binding in ipsilateral medial CPu (20%), and NAc (16%), similar to previously reported losses of DA D4 receptors. KA lesions produced large losses of NMDA receptor labeling in CPu and NAc (both by 52%), AMPA (41% and 45%, respectively), and kainate receptors (40% and 45%, respectively) that were similar to the loss of D2 receptors in CPu and NAc after KA injections. Nigral 6-OH-DA lesions yielded smaller but significant losses in NMDA (17%), AMPA (12%), and kainate (11%) receptor binding in CPu. The results indicate that most NMDA, AMPA, and kainate receptors in rat CPu and NAc occur on intrinsic postsynaptic neurons. Also, some NMDA, but not AMPA or kainate, receptors are also found on corticostriatal projections in association with D4 receptors; these may, respectively, represent excitatory presynaptic NMDA autoreceptors and inhibitory D4 heteroceptors that regulate GLU release from corticostriatal axons in medial CPu and NAc. Conversely, the loss of all three GLU receptor subtypes after lesioning DA neurons supports their role as excitatory heteroceptors promoting DA release from nigrostriatal neurons.

Postnatal development of dopamine D4-like receptors in rat forebrain regions: comparison with D2-like receptors

Development of dopamine D4-like receptors in rat caudate-putamen (CPu), nucleus accumbens (NAc), frontal cortex, hippocampus, and entorhinal cortex was assessed at seven points between postnatal days 7 and 60 by computed in vitro receptor autoradiography, and compared with dopamine (DA) D2-like receptors. Density of radioligand binding to both receptor types increased from day 7 to a peak at day 28 in caudate-putamen (D4, 3.3-fold; D2, 4.3-fold) and nucleus accumbens (2.9- and 3.6-fold), then declined by 28%-33% over days 35-60 to adult levels in both brain regions. In hippocampus, frontal and entorhinal cortex, both receptor types increased by 3.8- to 5.8-fold from day 7 to maximal levels at day 35 that remained unchanged to day 60. These findings suggest: (1) D4- and D2-like receptors follow a similar course of development in several cortical, extrapyramidal, and limbic regions of rat forebrain; (2) elimination of excessive receptors of both types occurred in caudate-putamen and nucleus accumbens but not in the other brain regions.

Postnatal development of dopamine and serotonin transporters in rat caudate-putamen and nucleus accumbens septi

Density of dopamine transporter (DAT) and serotonin transporter (5-HTT) membrane proteins in the caudate-putamen (CPu) and nucleus accumbens (NAc) of rat brain was assessed at seven ages at postnatal days (PD) 7-60, by in vitro quantitative autoradiography. Binding of [3H]GBR-12935 (to DAT) and [3H]paroxetine (to 5-HTT) increased steadily and very similarly, from low levels at PD-7 to maximal levels, to 6-7-fold higher density at PD-60 in both regions. These findings indicate that DAT and 5-HTT follow a synchronized course of development in rat CPu and NAc. In contrast to reported elimination of excessive receptors in CPu and NAc during maturation, there was no evidence of pruning of DAT or 5-HTT in these regions of rat forebrain.

Effects of hippocampal lesions on striatolimbic ionotropic glutamatergic receptors

Changes in ionotropic N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate glutamatergic (Glu) receptor binding in rat caudate putamen (CPu) and nucleus accumbens (NAc) and hippocampal CA1 and CA3 regions were examined after unilateral injections of kainic acid (KA) into hippocampus to degenerate local intrinsic neurons and remove the glutamatergic afferents projecting to CPu and NAc. KA injections produced large losses of NMDA, AMPA and kainate receptors in hippocampal CA1 and CA3 regions. In addition, NMDA and kainate, but not AMPA, receptors were significantly reduced in NAc and CPu. These results suggest that a large proportion of NMDA, AMPA and kainate receptors in rat hippocampus arise on intrinsic postsynaptic neurons, and that some NMDA and kainate, but not AMPA, receptors also occur on presynaptic terminals of hippocampostriatal afferents innervating the CPu and NAc.

Medial prefrontal cortical D2 and striatolimbic D4 dopamine receptors: common targets for typical and atypical antipsychotic drugs

1. In vitro receptor autoradiography was used to examine the long-term effects of a typical (fluphenazine), atypical (clozapine), or potential atypical antipsychotic (S[+]-N-n-propylnorapomorphine; [+]-NPA) on different dopamine (DA) receptor subtypes. 2. D1-Like and D3 receptor levels were not changed with any treatment in any brain region examined. 3. D2 Receptors in caudate-putamen (CPu), nucleus accumbens (NAc) and olfactory tubercle (OT) were significantly increased by long-term treatment with fluphenazine, but not with clozapine or S[+]-NPA. 4. D2 Receptor levels in medial prefrontal cortex (MPC), but not dorsolateral frontal cortex (DFC), were elevated after repeated daily administration of fluphenazine, clozapine, and S[+]-NPA. 5. D4-Like receptors, assayed under D4-selective conditions, were increased by fluphenazine, clozapine and S(+)-NPA in both NAc and CPu, but by none of these treatments in OT, DFC or MPC. 6. These results support a common role for medial prefrontal cortical D2 and striatolimbic D4 receptors in mediating the clinical actions of typical and atypical antipsychotic drugs.

Localization of dopamine receptor subtypes in corpus striatum and nucleus accumbens septi of rat brain: comparison of D1-, D2- and D4-like receptors

Changes in D1-, D2- and D4-like dopamine receptor binding in rat brain were examined by quantitative autoradiography following: (i) unilateral surgical ablation of frontal cerebral cortex to remove descending projections to corpus striatum and nucleus accumbens, (ii) unilateral injections of kainic acid into corpus striatum or nucleus accumbens to degenerate local intrinsic neurons, (iii) unilateral injections of 6-hydroxydopamine into substantia nigra to degenerate ascending dopamine projections. Rats were killed one week after lesioning, with contralateral tissue controls. Radioligands were: [3H]SCH-23390 for D1-like (D1/D5) receptors, [3H]nemonapride alone for D2-like (D2/D3/D4) receptors, and [3H]nemonapride with 300 nM S[-]-raclopride and other masking agents for D4-like receptors (identified by blockade with D4 selective L-745,870). Frontal cerebral cortex ablation did not alter D1- or D2-like receptor density, but D4-like binding decreased significantly in both corpus striatum (18%) and nucleus accumbens (23%). Kainic acid markedly reduced D1-like (75% and 84%) and D2-like binding (44% and 52%), with smaller D4-like losses (28% and 27%) in corpus striatum and nucleus accumbens, respectively. Nigral 6-hydroxydopamine lesions (verified by autoradiographic loss of dopamine transporters labelled with [3H]GBR-12935) did not significantly change D1-, D2-, or D4-like binding in the corpus striatum. These results suggest that the majority of D1-, and D2-like, and a smaller portion of D4-like receptors in corpus striatum and nucleus accumbens arise on intrinsic postsynaptic neurons, and that some D4-like, but neither D1- nor D2-like, receptors are found on presynaptic corticostriatal afferents.

Regional distribution of dopamine D4 receptors in rat forebrain

Binding of the D2-like (D2/D3/D4) radioligand [3H]nemonapride under selective conditions (with 300 nM S[-]-raclopride and other masking agents to occlude D2/D3 receptors and non-specific binding sites) revealed a subset of raclopride-insensitive binding sites considered D4-like receptors. These sites were stereoselective to R(-)-N-n-propylnorapomorphine (NPA) over its S(+)-NPA in a similar fashion to cloned D4 receptors expressed in cell lines. In addition, the highly D4-selective agent L-745,870 displaced 74-83% of these sites in rat brain regions, suggesting that most were D4 receptors. These apparent D4 receptors represented a relatively high proportion of D2-like receptors in hippocampus, dorsolateral frontal, medial prefrontal and entorhinal cortex, but fewer in caudate-putamen and nucleus accumbens.

Long-term effects of S(+)N-n-propylnorapomorphine compared with typical and atypical antipsychotics: differential increases of cerebrocortical D2-like and striatolimbic D4-like dopamine receptors

Changes in D2-like dopamine (DA) receptor binding in rat brain regions were compared by quantitative in vitro receptor autoradiography after 21-d treatment with a typical (fluphenazine), atypical (clozapine), or candidate atypical antipsychotic (S[+]-N-n-propylnorapomorphine, [+]-NPA). Fluphenazine treatment significantly increased binding of the D2,3,4 radioligands [3H]nemonapride and [3H]spiperone in caudate-putamen (CPu: 22%, 32%), nucleus accumbens (ACC: 67%, 52%), olfactory tubercle (OT: 53%, 43%), and medial prefrontal cerebral cortex (MPC: 46%, 47%) but not dorsolateral frontal cortex (DFC). D2-like binding in MPC was also increased by (+)-NPA (49%, 39%) and clozapine (60%, 40%), but not in DFC, CPu, ACC, or OT. Binding of D2,3-selective [3H]raclopride increased less after fluphenazine in ACC (27%) and CPu (16%) than with the nonselective radioligands, and not after clozapine or (+)-NPA. D3-selective binding of [3H]R (+)-7-OH-DPAT was not changed with any treatment or region including islands of Calleja. Binding of [3H]nemonapride or [3H]spiperone under D4-selective conditions (with 300 nM S[-]-raclopride and other masking agents, at sites occluded by D4 ligand L-745,870), was increased by fluphenazine, (+)-NPA, clozapine in ACC (120%, 76%, 70%, respectively), and CPu (54%, 37%, 35%), but not in OT, DFC or MPC. These results support the hypothesis that cerebrocortical D2-like and striatolimbic D4-like receptors contribute to antipsychotic actions of both typical and atypical drugs and encourage further consideration of S(+)aporphines as potential atypical antipsychotics.

Differential regulation of dopamine receptors after chronic typical and atypical antipsychotic drug treatment

Changes in dopamine receptor subtype binding in different brain regions were examined after 28 days treatment of rats with haloperidol, raclopride, clozapine or SCH23390 using in vitro receptor autoradiography. [3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin binding to dopamine D3 receptors was not changed in any brain region by any of the drug treatments. [3H]SCH23390 was only increased by chronic SCH23390 treatment. Haloperidol significantly increased [3H]nemonapride and [3H]spiperone binding to dopamine D2-like receptors in the caudate putamen. In contrast, haloperidol caused a small, significant increase in [3H]raclopride binding in the lateral caudate putamen only. Raclopride also elevated, but to a lesser extent [3H]nemonapride and [3H]spiperone binding in caudate putamen, whereas it did not affect [3H]raclopride binding. Clozapine did not significantly change D2-like striatal binding of [3H]nemonaipride, [3H]spiperone or [3H]raclopride. The differences in radioligand binding suggest that [3H]nemonapride and [3H]spiperone may be binding to additional subsets of dopamine D2-like receptors (including D4-like receptors) that are not recognized by [3H]raclopride, which has high affinity for D2 and D3 receptors only. Quantification of [3H]nemonapride or [3H]spiperone binding in the presence of 300 nM raclopride (to block D2 and D3 receptors) revealed that haloperidol, raclopride and clozapine up-regulated D4-like receptors in the caudate putamen using either radioligand. These results suggest that D4-like receptors may be a common site of action of both typical and atypical antipsychotics.

Dopamine receptor subtypes: differential regulation after 8 months treatment with antipsychotic drugs

Regulation of dopamine receptor subtypes was determined after long-term (8 mo) administration of typical and atypical antipsychotic drugs using 3H-nemonapride, 3H-raclopride, 3H-spiperone, 3H-7-hydroxy-N,N-di-n-propyl-2-aminotetralin, 3H-SCH23390 and 125I-sulpiride in vitro receptor autoradiography. Drug-induced receptor upregulation was remarkably different across the various D2-like receptor radioligands. Chronic haloperidol treatment resulted in a strong increase in 3H-nemonapride, 3H-spiperone and 125I-sulpiride binding to striatal areas, whereas 3H-raclopride binding was marginally affected. Raclopride treatment elevated striatal binding of 3H-nemonapride and 3H-spiperone to a lesser extent, and did not alter 3H-raclopride binding. Clozapine treatment did not affect the binding of the tritiated radioligands. These differences suggest that 3H-nemonapride and 3H-spiperone are binding to an additional subset of D2-like receptors, not recognized by 3H-raclopride. 3H-Nemonapride binding in the presence of 300 nM raclopride uncovered a striatal binding site (designated as D4-like receptor), that was up-regulated after chronic haloperidol, raclopride and clozapine treatment. The 125I-sulpiride binding sites in the prefrontal cortex were also up-regulated by the three antipsychotics. In contrast, 3H-spiperone binding sites were down-regulated in the prefrontal and dorsolateral cortical area. Chronic antipsychotic treatment did not affect Dl-like or D3 dopamine receptor subtype binding.

Regulation of ionotropic glutamate receptors following subchronic and chronic treatment with typical and atypical antipsychotics

Quantitative in vitro receptor autoradiography was used to examine changes in three ionotropic glutamate receptor subtypes using 3H-MK801 (NMDA-R antagonist), 3H-CNQX (AMPA-R antagonist) and 3H-kainic acid (kainate-R agonist) following subchronic (28 days) and chronic (8 months) treatment of rats with a typical antipsychotic, haloperidol (1.5 mg/kg per day), atypical antipsychotic, clozapine (25 mg/kg per day), the dopamine D2/D3 receptor antagonist, raclopride (10 mg/kg per day), and the dopamine D1 (D1/D5) receptor antagonist SCH23390 (0.5 mg/kg per day). Subchronic and chronic drug treatments did not significantly alter 3H-CNQX or 3H-kainate binding in any of brain regions examined. Subchronic SCH23390 treatment elevated 3H-MK801 binding in the hippocampal formation with significant increases in the CA1 and dentate gyrus, suggesting a specific role for dopamine D1 receptors in the regulation of hippocampal NMDA receptor function. Subchronic, but not chronic, haloperidol and clozapine treatment significantly reduced 3H-MK801 binding in the medial prefrontal cortex. This suggests that typical and atypical antipsychotics may exert some of their clinical effects by affecting NMDA receptors in the medial prefrontal cortex. Both subchronic and chronic clozapine treatment decreased 3H-MK801 binding in the caudate putamen. The minimal extrapyramidal side effects produced by clozapine may result, in part, from the reduction in NMDA receptor binding in the caudate putamen.