Dopamine D2-like sites in schizophrenia, but not in Alzheimer's, Huntington's, or control brains, for [3H]benzquinoline

Although the basis of schizophrenia is not known, evidence indicates a possible overactivity of dopamine pathways. In order to detect any new dopamine receptor-like sites which may be altered in schizophrenia, the present study used a new radioligand, a [3H]benzo[g]quinoline. The receptors were labelled by this ligand in the presence of other drugs to block the known dopamine D1, D2, D3, or D5 receptors (no D4-selective ligands are available to block D4). Using this method, we found that schizophrenia brain striata had elevated levels of a D2-like site not detected in control human postmortem brains or in Alzheimer's, Huntington's, or Parkinson's disease brains. The ligand acted as an agonist at this D2-like site, because binding was abolished by guanine nucleotide. The binding of the ligand to the D4 receptor, however, was not sensitive to guanine nucleotide. The site differed from D2 itself, because S- and R-sulpiride were equally potent at the D2-like site. The D2-like sites were present in rat and mouse brain but were absent in brain slices from transgenic mice where D2 had been knocked out. The abundance of the receptor was not related to premortem use of antipsychotic drugs. Future research should examine the biochemical differences between the D2 dopamine receptor and these D2-like sites in schizophrenia.

Clozapine occupies high levels of dopamine D2 receptors

An important discrepancy has been noted concerning the number of dopamine D2 receptors which must be occupied in patients by clozapine, in contrast to other antipsychotic drugs, in order to achieve an antipsychotic effect. For example, when D2 receptors are labelled by radioactive raclopride or spiperone congeners in patients, psychosis-controlling doses of all antipsychotic drugs occupy about 70% or more of the D2 receptors in patients. However, equi-effective psychosis-controlling doses of clozapine (approximately 400 mg/day; approximately 70-130 nM in spinal fluid) only occupy between 20% and 50% of the D2 receptors in patients. This discrepancy of a consistently lower occupancy of D2 by psychosis-controlling doses of clozapine may be resolved when one considers that the neuroleptic concentration for half-occupancy of D2 receptors in vitro (i.e. the inhibition constant) depends on the radioligand used to label the receptor. Radioligands with higher tissue/buffer partition coefficients are less displaced by clozapine. This principle applies both in vitro and in vivo. Thus, allowing for this principle, psychosis-controlling doses of clozapine can be shown to occupy over 70% of the brain dopamine D2 receptors in patients, as found with other neuroleptics.

Dopamine D2 receptor dimers and receptor-blocking peptides

Dopamine D2 receptors exist as dimers in whole cell lysate, crude membranes prepared from human caudate, and following solubilization and immunoprecipitation of the receptor from these tissues. Photoaffinity labelling experiments confirmed that D2 receptors exist either as monomers that are selective targets for spiperone or as dimers that are targets for nemonapride. Incubation of D2 dimers with peptides derived from the putative transmembrane (TM) domains of the D2 receptor, or incubation under high temperatures or low pH resulted in the dissociation of the dimer to monomer. D2-TM peptides were unable to dissociate dopamine D1 and serotonin 5-HT1B receptor dimers, suggesting that receptor dimers are formed by specific intermolecular noncovalent interactions involving TM regions. This opens a path to new selective therapeutic receptor-blocking compounds based on this principle of mimicking transmembrane portions of neurotransmitter receptors.

Dopamine and serotonin receptors: amino acid sequences, and clinical role in neuroleptic parkinsonism

This review summarizes the amino acid sequences of the human dopamine and serotonin receptors and their human variants. The review also examines the receptor basis of the atypical antipsychotic drugs that elicit less parkinsonism than the typical antipsychotics. Because the dissociation constant of a drug varies with the radioligand, the dissociation constants of many neuroleptics are here summarized for the dopamine D2-, D4- and serotonin S2A-receptors using different radioligands. Radioligands of low solubility in the membrane (having low tissue/buffer partition) result in lower values for the neuroleptic dissociation constants, compared to radioligands of high membrane solubility. Such studies yield the intrinsic K value for a neuroleptic in the absence of a competing ligand. Clozapine, for example, has an intrinsic K value of 1.6 nM at the D4-receptor, in agreement with the value of 1.6 nM when directly measured with [3H]clozapine at D4. However, because clozapine competes with endogenous dopamine, the in vivo clozapine concentration to occupy 75% of the dopamine D4-receptors is derived to be approximately 13 nM. This agrees with the value of 12 to 20 nM in the plasma water (or spinal fluid) observed in treated patients. Moreover, in L-DOPA psychosis (in Parkinson's disease), the clozapine concentration for 75% blockade of D4 is predicted to be approximately 3 nM. This agrees with the value of approximately 1.2 nM observed by Meltzer et al. in plasma water (Neuropsychopharmacology, 12, 39-45 (1995)). This analysis supports the concept and practical value of the intrinsic K values. Some atypical neuroleptics (remoxipride, clozapine, perlapine, seroquel and melperone) have high intrinsic K values (ranging from 30 to 88 nM) at the D2-receptor, making them displaceable by high levels of endogenous dopamine in the caudate/putamen. In contrast, however, typical neuroleptics (i.e., those that typically cause parkinsonism) have intrinsic K values of 0.3 to 6 nM, making them less displaceable by endogenous dopamine. A relationship exists between the neuroleptic doses for rat catalepsy and the D2/D4 ratio of the intrinsic K values. Thus, the atypical neuroleptics appear to fall into two groups, those that bind loosely to D2 and those that are selective at D4.

A polymorphic dinucleotide repeat in the human dopamine D5 receptor gene promoter

Previously, we have reported the cloning and characterization of the 5'-flanking region of the human dopamine D5 receptor encoding gene (D5) and that the major transactivation domain was 119-182 bp upstream of the transcriptional start site [Beischlag, T.V. et al., Biochemistry, 34 (1995) 5960-5970]. Within this region existed a small dinucleotide repeat termed (TC)13. In this report, we describe the screening of genomic DNAs from 18 unrelated individuals by single-strand conformation polymorphism (SSCP) analysis. SSCP analysis revealed the existence of two additional alleles, termed (TC)12 and (TC)14. Neither form significantly altered D5 promoter-mediated luciferase activity when compared to that of the wild-type control, suggesting that small differences in the number of dinucleotide repeats are not likely of any functional consequence for D5 transactivation.

Radioreceptor binding profile of the atypical antipsychotic olanzapine

The affinities of olanzapine, clozapine, haloperidol, and four potential antipsychotics were compared on binding to the neuronal receptors of a number of neurotransmitters. In both rat tissues and cell lines transfected with human receptors olanzapine had high affinity for dopamine D1, D2, D4, serotonin (5HT)2A, 5HT2C, 5HT3, alpha 1-adrenergic, histamine H1, and five muscarinic receptor subtypes. Olanzapine had lower affinity for alpha 2-adrenergic receptors and relatively low affinity for 5HT1 subtypes, GABAA, beta-adrenergic receptors, and benzodiazepine binding sites. The receptor binding affinities for olanzapine was quite similar in tissues from rat and human brain. The binding profile of olanzapine was comparable to the atypical antipsychotic clozapine, while the binding profiles for haloperidol, resperidone, remoxipride, Org 5222, and seroquel were substantially different from that of clozapine. The receptor binding profile of olanzapine is consistent with the antidopaminergic, antiserotonergic, and antimuscarinic activity observed in animal models and predicts atypical antipsychotic activity in man.

Dopamine D4 receptor variant in Africans, D4valine194glycine, is insensitive to dopamine and clozapine: report of a homozygous individual

The D4Valine194Glycine receptor is a variant of the dopamine D4 receptor and is found in 12.5% of the Afro-Caribbean population. Glycine replaces valine at a position one amino acid away from a serine which is critical for the attachment of dopamine. To determine whether this mutation had an effect on the properties of the dopamine D4 receptor, we constructed this variant and tested the sensitivity of the expressed protein with various drugs. We found that the variant receptor was two orders of magnitude less sensitive to dopamine, clozapine and olanzapine. The variant receptor was insensitive to guanine nucleotide, indicating the absence of a high-affinity state or functional state. The one 15-year-old individual found homozygous for this variant also had sickle cell disease. The patient revealed an overall pattern of low weight and no axillary or pubic hair.

Schizophrenia: elevation of dopamine D4-like sites, using [3H]nemonapride and [125I]epidepride

We here report a three-fold elevation of dopamine D4-like sites in schizophrenia, using [3H]nemonapride to measure dopamine D2 and D3 receptors and D4-like sites, and using [125I]epidepride to measure D2 and D3 sites in ten control and nine schizophrenia post-mortem brain putamen tissues. This result differs from a recent report which did not detect significant D4-like sites in control or schizophrenia putamen (Reynolds and Mason, 1995, Eur. J. Pharmacol. 281, R5). The present finding agrees with other reports wherein an elevation in D4-like sites was found in schizophrenia, using [3H]nemonapride for D2, D3 and D4-like sites, but [3H]raclopride for D2 and D3 sites. The nature of these D4-like sites is not known.

Deriving the therapeutic concentrations for clozapine and haloperidol: the apparent dissociation constant of a neuroleptic at the dopamine D2 or D4 receptor varies with the affinity of the competing radioligand

The apparent dissociation constant, Ki, for a neuroleptic at the dopamine D2 or D4 receptor was consistently higher when competed against [3H]nemonapride than against [3H]spiperone which was in turn higher than that against [3H]raclopride. This finding obtained for all four types of dopamine receptors studied, including the native dopamine D2 receptor in the anterior pituitary tissue, the human D2long receptor, the human D2short receptor and the human D4.4 receptor. Some neuroleptics revealed a difference of over 10-fold between the Ki using [3H]nemonapride and the Ki using [3H]raclopride. The KD values of the three 3H-ligands and the neuroleptic Ki values were lower when using a much lower concentration of tissue, indicating that depletion of ligand presumably accounted for the phenomenon. The Ki values of each neuroleptic were related to the the tissue/buffer partition coefficients of the three 3H-ligands. Extrapolating the neuroleptic Ki value down to a tissue/buffer partition coefficient of unity or zero led to a Ki value for competition versus a water-soluble ligand such as dopamine. Clozapine extrapolated to a Ki value of 1.3 nM. Direct measurement gave a Ki value of 1.6 nM for [3H]clozapine at the dopamine D4 receptor. When competing versus endogenous dopamine, this clozapine value of 1.6 nM would rise to 20 nM for the blockade of 75% of dopamine D4 receptors, matching the observed therapeutic concentration of 18 nM. These data also explain why clozapine occupies 48% of the D2 receptors in patients when measured with [11C]raclopride, but between 0% and 22% when measured with [18F]methylspiperone or [18F]fluoroethylspiperone.

Therapeutic receptor-blocking concentrations of neuroleptics

The therapeutic concentrations of antipsychotic drugs in the patient's plasma water or spinal fluid are identical to their blocking potencies in vitro at the dopamine D2 receptor, with the exception of clozapine which acts at D4. The variation in K values between laboratories stems from the fact that the apparent K value for any antipsychotic drug depends on the affinity of the competing radioligand for the receptor or the membranes. Clozapine at the D2 receptor has a K value of 420 nM using [3H]nemonapride, 180 nM using [3H]spiperone and 82 nM using [3H]raclopride. These K values are related to the tissue/buffer partition coefficients of the ligands. Extrapolating down to either 1 or 0 partition yields the intrinsic K values for the antipsychotic in the absence of any competing ligand. The extrapolated or intrinsic K value for clozapine at D4 is 1.3 nM, in agreement with the value of 1.1 nM measured directly with [3H]clozapine at D4. Clozapine in vivo, however, must compete with endogenous dopamine in the synapse, estimated as 50 nM. Thus, the in vivo concentration of clozapine for 75% occupation of dopamine D4 receptors can be derived as approximately 14 nM, in agreement with the observed value of 12-20 nM in the plasma water or spinal fluid in treated patients. In L-DOPA psychosis in Parkinson's disease, the clozapine concentration (in the plasma water or spinal fluid) for 75% blockade of dopamine D4 receptors may be predicted as approximately 3 nM, in general agreement with the value of approximately 1.2 nM in Parkinson patients who have L-DOPA psychosis. These considerations provide strong support for the conclusion that clozapine primarily targets the D4 receptor in psychosis. Using the same considerations for haloperidol, it can be shown that the haloperidol therapeutic concentration required for 75% blockade of dopamine D2 receptors in vivo will be approximately 2-3 nM, in agreement with the observed value in the spinal fluid or plasma water of 1-3 nM.

The human dopamine D5 receptor gene: cloning and characterization of the 5'-flanking and promoter region

Genomic and overlapping cDNA clones encompassing the entire 5'-untranslated region of the human D5 receptor gene were cloned and sequenced. Comparison of these human D5 receptor genomic and cDNA clones revealed the presence of two exons separated by a small and variably sized intron (of either 179 or 155 bp). We have determined that the major site of transcription initiation of the D5 gene is 2125 bp upstream from the translational initiation start site. The region 5' to the transcription initiation site lacked conventional TATA and CAAT sequences, but contained several putative binding sites for transcription factors, such as Sp1 and Ap1. Luciferase reporter gene constructs containing D5 gene sequence information up to 500 bp 5' of the transcription initiation site were able to stimulate transcription only in SK-N-SH cells but not in COS-7, CHO, P19EC, NB41A3, and SK-N-MC cell lines. Promoter deletion analysis indicated that the D5 gene promoter contained a positive modulator at 119-182 and a negative modulator 251-500 bases upstream from the site of transcription initiation. In addition, in order to detect the expression of functional D5 receptor mRNAs and not those of its expressed pseudogenes, in situ hybridization analysis of monkey and human brain using a 5' D5-specific riboprobe revealed that D5 receptor mRNA was most abundant in discrete cortical areas (layers II, IV, and VI), the dentate gyrus, and hippocampal subfields with very little message detected in the striatum. Unexpectedly, D5 mRNA antisense riboprobes labeled discrete cell bodies in the pars compacta of the substantia nigra. The characterization of the genomic organization of the D5 receptor gene and of those factors involved in its transcriptional regulation may aid in our understanding of the role this gene product plays in the generation and maintenance of dopamine D1-like receptor-mediated events.

Long-term haloperidol elevates dopamine D4 receptors by 2-fold in rats

Chronic treatment of rats with neuroleptics results in elevated numbers of dopamine D2-like receptors. The present study was done to determine whether neuroleptics altered the density of one type of dopamine D2-like receptors, namely the dopamine D4 receptor. We here describe the effect of a one-month treatment with haloperidol on dopamine D4 receptor mRNA and protein levels in rat striatum. Endogenous levels of dopamine D4 receptor mRNA in rat striatum are very low and, therefore, reverse transcription and subsequent amplification were used for quantification. Dopamine D4 receptor density was, because of the absence of a dopamine D4 receptor specific [3H]ligand, determined by the difference between the number of binding sites for [3H]nemonapride and [3H]raclopride. Scatchard analysis of [3H]nemonapride and [3H]raclopride binding show that treatment for one month with halperidol elevates the density of dopamine D4 receptors in rat striatum by approximately 2-fold, whereas dopamine D2 and D3 receptors together show a 19% higher receptor density. Dopamine D4 receptor mRNA was also approximately increased by 2-fold.

Dopamine D4-like receptor elevation in schizophrenia: cloned D2 and D4 receptors cannot be discriminated by raclopride competition against [3H]nemonapride

Three independent studies have found that the density of dopamine D4-like receptors is elevated in postmortem brain striata in schizophrenia. This elevation has been questioned by a fourth study that used a different method and failed to detect a biphasic component when raclopride was used to complete against the binding of 1 nM [3H]nemonapride to schizophrenia tissue. To test whether this competition method could distinguish between dopamine D2 and D4 receptors, the present study used mixtures of only these two cloned receptors, free of all other receptors. Using combinations of cloned dopamine D2 and D4 receptors, this competition method could not resolve these components up to a level of 48% D4 receptors. Thus, the objections raised by the findings of the fourth study, mentioned above, do not appear valid. Furthermore, the present results indicate that the data using such a competition method actually mask a manyfold marked elevation in the density of dopamine D4-like receptors in schizophrenia.

Binding of 5H-dibenzo[a,d]cycloheptene and dibenz[b,f]oxepin analogues of clozapine to dopamine and serotonin receptors

Series of 5,11-dicarbo- and 11-carbo-5-oxy-10-(1-alkyl-1,2,3,6-tetrahydro-4 pyridinyl) analogues and a 11-carbo-5-oxy-10-(1-methyl-4-piperidinyl) analogue of the atypical antipsychotic agent clozapine were prepared and tested for binding to the dopamine D-2L and D-4 and serotonin S-2A and S-2C receptors. Some of these analogues were found to have dopamine D-2L and D-4 and serotonin S-2A and S-2C receptor binding activities as high as or higher than those of clozapine, indicating that neither the diazepine structure nor the piperazine ring present in clozapine is essential for high antidopamine activity and or for high dopamine D-4 selectivity (Ki for the dopamine D-2L receptor/Ki for the dopamine D-4 receptor). Increasing in the effective size of the alkyl substituent at the tertiary amine nitrogen atom in the 1,2,3,6-tetrahydro-4-pyridinyl moiety in the 5H-dibenzo[a,d]cycloheptene series reduces the affinity for the dopamine D-4 receptor, but in the dibenz[b,f]oxepin series, no significant change in binding affinity to the dopamine D-4 receptor was observed. Equal or slightly higher affinity for the serotonin S-2A and S-2C receptors was observed for the 10-(1-ethyl-1,2,3,6-tetrahydro-4- pyridinyl) analogues in both series, but for the 10-[1,2,3,6-tetrahydro-1-(2-propenyl)-4- pyridinyl] analogues, any favourable steric factor is overshadowed by an unfavorable electronic effect as a result of change in the basicity of the tertiary amino group in the pyridinyl moiety. Replacement of three of the four nitrogen atoms in clozapine with three carbon or two carbon atoms and an oxygen atom and removal of the chlorine atoms gives 10-(1,2,3,6-tetrahydro-1- methyl-4-pyridinyl)dibenzo[a,d]cycloheptene and 10-(1-methyl-4-piperidinyl)dibenz[b,f]oxepin, each having twice the binding activity to the dopamine D-4 receptor as does clozapine and a dopamine D-4 selectivity equal to that of clozapine.

Dopamine D4 receptor variant, D4GLYCINE194, in Africans, but not in Caucasians: no association with schizophrenia

Because antipsychotic drugs selectively block dopamine receptors and since dopamine D4 receptors are elevated sixfold in postmortem schizophrenia brain, we searched for possible abnormalities in the coding region of the genomic DNA sequence for the dopamine D4 receptor in control and schizophrenia tissues. The DNA sequence for the first 250 bases of exon 3 of this receptor was examined in the genomic DNA from 296 control individuals and 58 schizophrenics. Twenty-three out of 183 control blacks (12.6%) and 3 out of 24 (12.5%) schizophrenic blacks revealed a replacement of T by G, predicting a substitution of valine by glycine at amino acid position 194. The identical prevalence of 12.5% indicates that the variant is not associated with schizophrenia. The amino acid replacement occurs one amino acid away from a serine amino acid which is critical for the attachment of dopamine. None of the 147 Caucasians (113 controls; 34 schizophrenics) revealed this variant, termed D4GLYCINE194.

Binding of 5H-dibenzo[b,e][1,4]diazepine and chiral 5H-dibenzo[a,d]cycloheptene analogues of clozapine to dopamine and serotonin receptors

5H-Dibenzo[b,e][1,4]diazepine, dibenz[b,f]oxepin, and 5H-dibenzo[a,d]cycloheptene analogues of clozapine [8-chloro-11-(4-methylpiperazino)-5H- dibenzo[b,e][1,4]diazepine] were evaluated for their binding affinity to dopamine D-1, D-2, and D-4 and serotonin S-2A (5-HT2A), S-2C (5-HT2C) and S-3 (5-HT3) receptors. The diazepine analogues display selective binding to the dopamine D-4 and serotonin S-2A receptors similar to that of clozapine, but none has a dopamine D-4 selectivity (Ki for the dopamine D-2A receptor/Ki for the dopamine D-4 receptor) greater than that of clozapine. All of the oxepin analogues also show substantial binding to the dopamine D-4 and serotonin S-2A receptors with 10-(4-methylpiperazino)dibenz[b,f]oxepin having a dopamine D-4 selectivity greater than that of clozapine. Some of the 5H-dibenzo-[a,d]cycloheptene analogues also show strong binding to both the dopamine D-4 and serotonin S-2A receptors, 5-methyl-10-(4-methylpiperazino)-5H-dibenzo[a,d]cycloheptene having a dopamine D-4 selectivity of 7.8 as compared to 10 for clozapine but a serotonin S-2A selectivity (Ki for the dopamine D-2 receptor/Ki for the serotonin S-2A receptor) of 2.0 as compared to 28 for clozapine. The serotonin S-2A selectivity of 2-chloro-10-(4-methylpiperazino)-5H-dibenzo[a,d]-cycloheptene++ + is 200. As an extension of these studies, chiral 5-substitute 10-(1,2,3,6-tetrahydro-1-methyl-4-pyridinyl)-5H-dibenzo[a,d]cyclohept ene analogues show a substantial enantiospecificity toward dopamine and serotonin receptor subtypes, (R)-(-)-5-methyl compound having a 2-fold higher dopamine D-4 selectivity than its (S)-(+) enantiomer as the result of enhanced binding to the dopamine D-4 receptor rather than diminished binding to the dopamine D-2 receptor. (pRa,pSb)-(+)-5-(2-Propylidene)-10-(1,2,3,6-tetrahydro-1-met hyl- 4-pyridinyl)-5H-dibenzo[a,d]cycloheptene is 17 times more active in binding to the dopamine D-4 receptor than is its pSa,pRb enantiomer while being only 1.5 times more active in binding to the dopamine D-2 receptor.

Dopamine D2 receptors mapped in rat brain with [3H](+)PHNO

Previous work with membrane preparations had demonstrated that the agonist (+)-4-propyl-9-hydroxynaphthoxazine (PHNO) labels the high-affinity state of dopamine D2 receptors with 67-fold selectivity over D1 sites. In this study, quantitative autoradiography was used to examine the binding of [3H](+)PHNO to rat brain sections. Highest binding densities were found in caudate-putamen, accumbens, and olfactory tubercles, as expected, and also in specific layers of the olfactory bulb. In addition, a second group of brain regions, including lateral septum, entorhinal cortex, molecular layer of hippocampus, and several brainstem structures showed low but consistent levels of binding. In all brain regions [3H](+)PHNO binding (2 nM) was completely displaced by 10 microM sulpiride (> 99%). Addition of 150 microM guanilylimidodiphosphate, which normally converts D2 receptors from high to low affinity states, abolished [3H](+)PHNO binding in all brain regions (> 99%), except for the islands of Callejas. This is likely to reflect binding to D3 sites in this area. Omission of preincubation in binding assays decreased [3H](+)PHNO binding in a regionally dependent manner, with strongest effects (22%) seen in high-density areas. These preincubation results confirm that (+)PHNO may have limitations for in vivo imaging studies. On the other hand, [3H](+)PHNO's negligible levels of non-specific binding compared to other agonists and overall selectivity would make it an excellent tool for in vitro autoradiographic monitoring of the high affinity state of D2 receptors.

Dopamine receptor pharmacology

Dopamine receptors are the primary targets in the treatment of schizophrenia, Parkinson's disease, and Huntington's chorea, and are discussed in this review by Philip Seeman and Hubert Van Tol. Improved therapy may be obtained by drugs that selectively target a particular subtype of dopamine receptor. Most antipsychotic drugs block D2 receptors in direct correlation to clinical potency, except clozapine, which prefers D4 receptors. D1 and D2 receptors can enhance each other's actions, possibly through subunits of the G proteins. In schizophrenia, the D2 and D3 receptor density is elevated by 10%, while the D4 receptor density is elevated by 600%. Therefore, D4 receptors may be a target for future antipsychotic drugs. While antipsychotics originally helped to discover dopamine receptors, the five cloned dopamine receptors are now facilitating the discovery of selective antipsychotic and antiparkinson drugs.

Receptor-receptor link in membranes revealed by ligand competition: example for dopamine D1 and D2 receptors

An interaction or link between dopamine D1 receptors and dopamine D2 receptors was found by a ligand competition method, using [3H]raclopride to label dopamine D2 receptors and SCH 23390 to block dopamine D1 receptors. In the presence of endogenous or exogenous dopamine, SCH 23390 increased the binding of [3H]raclopride in post-mortem human striata homogenates or in tissue culture cells containing human dopamine D1 and D2 receptors. In order to reveal such intramembrane receptor-receptor interactions in general, therefore, it appears essential to add two agonists, one for each receptor, and then to block one of the receptors when measuring the binding of a ligand to the second receptor.

Dopamine receptors and antipsychotic drug response

Dopamine receptors have been divided into two major types--D1 and D2--based primarily on pharmacological and biochemical criteria. Recent advances in the molecular biology of the dopamine receptor system have allowed the identification and characterisation of at least five distinct neuronal dopamine receptor genes (D1 to D5). These genes encode dopamine receptors belonging to the D1 receptor family, termed D1 and D5, and three D2-like receptors, termed D2, D3 and D4. These receptors are distinguished on the basis of their primary structure, chromosomal location, mRNA size and tissue distribution, and biochemical and pharmacological differences. Although individually these receptor subtypes may not be directly and exclusively involved in the maintenance or expression of schizophrenia, alterations of any of the receptors may contribute to the perturbation or instability of dopaminergic homeostasis in the brain. What was once thought to be a simple two-receptor system seems to have emerged as an intricate and interactive entity. This review summarises what is currently understood about dopamine receptors, their role in antipsychotic drug action, and their association with psychosis.

Dopamine D4 receptors elevated in schizophrenia

Although the biological basis of schizophrenia is not known, possible causes include genetic defects, viruses, amines, brain structure and metabolism, neuroreceptors, and G proteins. The hypothesis of dopamine overactivity in schizophrenia is based on the fact that neuroleptics block dopamine D2 receptors in direct relation to their clinical antipsychotic potencies. Moreover, dopamine D2 or D2-like receptors are elevated in postmortem schizophrenia brain tissue. This elevation, however, is only found in vivo using [11C]methylspiperone but not [11C]raclopride. The dopamine D4 receptor gene has not yet been excluded in schizophrenia because the 21 gene variants of D4 have not yet been tested. Because the link between D1 and D2 receptors is reduced in schizophrenia tissue, we tested whether one component of this link was sensitive to guanine nucleotide. We report here that the binding of [3H]raclopride to D2 receptors in schizophrenia was not sensitive to guanine nucleotide. This finding permitted analysis of data on the binding of [3H]emonapride to the D2, D3 and D4 receptors. We conclude that the combined density of D2 and D3 receptors (labelled by [3H]raclopride) is increased by only 10% in schizophrenia brain, as found by Farde et al., but that it is the density of dopamine D4 receptors which is sixfold elevated in schizophrenia. These findings resolve the apparent discrepancy, mentioned above, wherein the density of [11C]methylspiperone-labelled sites (D2, D3 and D4), but not that of [11C]raclopride-labelled sites (D2 and D3), was found elevated in the schizophrenia striatum.