Bimodal distribution of dopamine receptor densities in brains of schizophrenics.

The dopamine hypothesis of schizophrenia was examined by measuring the density of dopamine receptors in the postmortem brains of 81 control subjects and 59 schizophrenics from four different countries. The densities of dopamine receptors in the tissues from the schizophrenic patients had a bimodal distribution in the caudate nucleus, putamen, and nucleus accumbens. One mode occurred 25 percent above the control density, and a second mode occurred at a density 2.3 times that of the control density for all three regions. Although almost all the patients had been medicated with neuroleptics, the two modes had the same dissociation constant for the labeled ligand used, suggesting that the neuroleptic doses were similar for the two populations of schizophrenics. The results thus provide direct evidence for two distinct categories of schizophrenia.

Schizophrenia: dopamine D1 receptor sequence is normal, but has DNA polymorphisms.

Genes that regulate dopamine (DA) receptors may underlie the overactive DA system in schizophrenia. Since it is known that there is an abnormally reduced or absent regulation of the DA D2 receptor by the DA D1 receptor in the postmortem schizophrenia brain, the human DA D1 receptor gene was sequenced from genomic deoxyribonucleic acid (DNA) of seven schizophrenic individuals. The tissues from two schizophrenics had previously been found to have a reduced link between DA D1 and D2 receptors. The D1 receptor genes were amplified by the polymerase chain reaction, subcloned, and sequenced. Although three DNA polymorphisms were found, the deduced amino acid sequence of the DA D1 receptor was normal in these tissues.

Schizophrenia: normal sequence in the dopamine D2 receptor region that couples to G-proteins. DNA polymorphisms in D2.

Because dopamine (DA) D2 receptors are a target in neuroleptic therapy and have been found to be elevated in schizophrenia, the human DA D2 receptor gene was examined for possible abnormalities in schizophrenia. Moreover, since D2 receptors in psychosis have a reduced coupling to D1 receptors, the cytoplasmic third loop of D2 was chosen for deoxyribonucleic acid (DNA) sequencing, since this region is essential for coupling to G-proteins. This region also contains exon 5, which is expressed in the long form of D2, but not in the short form of D2. In eight schizophrenia cases, this region had normal exon sequences (exons 4, 5 and 6), and normal sequences at its intron-exon junctions. However, exon 6 contained three DNA polymorphic base changes, and introns 4 and 5 revealed three missing bases and two polymorphic base changes, none of which would be expected to alter the D2 receptor protein in schizophrenia.

Isolation and characterization of myosin from the adrenal medulla.

A protein with adenosine triphosphatase activity was isolated from bovine adrenal medulla and subsequently purified by ammonium sulfate precipitation and agarose gel filtration using a discontinuous two-buffer system. Characterization of this protein by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate, by assay of the activity of Ca2+, K(+)-EDTA and Mg2+ dependent adenosine triphosphatases by amino acid analysis and by electron microscopy has shown that the adrenal medullary myosin closely resembles those myosins isolated from muscle and other non-muscle cells. The possible roles of myosin in the adrenal medulla are discussed.

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.

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.

New dopamine receptor, D2(Longer), with unique TG splice site, in human brain.

Brain dopamine receptor agonists alleviate the signs of Parkinson's disease, while dopamine receptor antagonists alleviate hallucinations and delusions in psychosis. The dopamine type 2 receptor (or D2) is blocked by antipsychotic drugs, including even the "atypical" drugs such as clozapine or remoxipride, in direct relation to their clinical potencies. Compared to the long form of the D2 receptor (D2(Long)), the short form (D2(Short)) may be three times more sensitive to benzamide antipsychotic drugs. Hence, it is essential to identify additional variants of dopamine receptors for which more selective antipsychotic drugs can be found. Although no family linkage has been found between the D2 receptor and schizophrenia, there can be brain region abnormalities in the RNA transcript expression of dopamine receptors. Therefore, in order to identify variant dopamine D2 receptors, we searched for mutations in the RNA transcripts for the dopamine D2 receptor in the striatum of post-mortem brains from individuals who died with psychosis, including schizophrenia. A new splice variant of the D2 receptor, D2(Longer), with a unique TG splice site, was found in one control brain and in two psychotic brains.

A serotonin-4 receptor-like pseudogene in humans.

During a search for new G-protein-linked receptors for dopamine and serotonin, we found a serotonin-4 receptor-like pseudogene. This receptor-like pseudogene is intronless, contains an in-frame stop codon following transmembrane-3, and has two one-nucleotide insertions between transmembrane-5 and -6 regions which alter the reading frame. The predicted amino acid sequence of the human pseudogene is about 35% identical with that of the rat serotonin-4 receptor.

Schizophrenia: elevated mRNA for dopamine D2(Longer) receptors in frontal cortex.

Because dopamine D2 receptors are the primary targets for antipsychotic drugs, including clozapine and quetiapine, and because some studies have found D2 receptors to be elevated in schizophrenia, we examined the mRNA of three forms of the D2 receptor, particularly the new form of the dopamine D2 receptor, D2(Longer), in post-mortem brains from patients who died with schizophrenia. Using quantitative competitive RT-PCR (reverse transcriptase-polymerase chain reaction), the D2(Longer) mRNA was higher in the frontal cortex, compared to control tissues. The mRNA concentration of D2(Long) and D2(Short) was also higher in the frontal cortex, compared to control tissues. Although most of the schizophrenia patients had received different antipsychotic drugs for varying periods of time, the mRNA of D2(Longer), as well as that for D2(Long) and D2(Short), in such medicated tissues was similar to that in a frontal cortex tissue from a patient who had reliably never received antipsychotic drugs. It is possible, therefore, that the elevation of the mRNAs for D2(Longer), D2(Long) and D2(Short) in the frontal cortex may be related to the disease of schizophrenia itself.

A human serotonin-7 receptor pseudogene.

Although the serotonin-7 receptor was cloned several years ago, its localization in brain tissues remains confusing because of the existence of a related expressed pseudogene, the sequence of which has not hitherto been reported. During the course of searching for related receptor genes, we also searched for this pseudogene to determine its sequence. Human genomic DNA was screened for dopamine and serotonin receptor-like genes, using the polymerase chain reaction method and degenerate oligonucleotide primers based on the similar sequences in the transmembrane-6 and -7 regions of the serotonin-5A, the serotonin-7, and the dopamine D2, D3 and D4 receptors. This resulted in one of the clones having a 115 bp fragment, of which 89% of the bases were identical to the transmembrane-6 and -7 regions of the serotonin-7 receptor sequence. The fragment was radiolabelled and used to screen a human fetal brain cDNA library. A novel cDNA clone of 1326 bp was isolated. Based on the nucleotide sequence, 88% of the bases in this sequence of the pseudogene are identical to the human serotonin-7 receptor coding sequence. However, compared to the serotonin-7 receptor DNA sequence, the pseudogene sequence has nucleotide deletions and insertions, resulting in frame-shifts and stop codons. It was concluded that this sequence represented a pseudogene related to the serotonin-7 receptor gene.

Neuroleptics have identical potencies in human brain limbic and putamen regions.

In order to examine whether some neuroleptic drugs were specifically more potent on human limbic dopamine receptors than on striatal dopamine receptors, we tested the potency of eight neuroleptics on their ability to inhibit the binding of [3H]spiperone to D2 dopamine receptors in human putamen and nucleus accumbens. Each of the neuroleptics had an identical potency in both tissues, the IC50 values being 0.2 nM for spiperone, 2.5 nM for haloperidol, 2.6 nM for trifluperidol, 5 nM for fluphenazine, 20 nM for thioridazine, 25 nM for chlorpromazine, 100 nM for metoclopramide and 300 nM for clozapine. There is no evidence, therefore, for the concept of a limbic-specific dopamine receptor antagonist.

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.

Dopamine D2 receptor promoter polymorphism: no association with schizophrenia.

A functional polymorphism in the promoter region of the dopamine D2 receptor gene, the -141C Del allele, which may be associated with schizophrenia susceptibility, has previously been described in a Japanese sample. The present study was done in order to examine whether such an association would also be found in a North American schizophrenia patient population. However, analysis of the -141C Del allele frequency in the present group of schizophrenia patients (n = 50) and control subjects (n = 51) did not identify any significant differences. These data support the recent reports on German and British subjects that this genetic variation in the 5'-flanking region of the dopamine D2 receptor gene does not play a major role in the genetic predisposition to schizophrenia.

Dopamine D1 and D2 receptor selectivities of agonists and antagonists.

The selectivities of various dopamine agonists and antagonists for dopamine D1 and D2 receptors were obtained by comparing their relative dissociation constants for inhibiting the binding of [3H]SCH 23390 at D1 receptors (calf caudate nucleus) and at D2 receptors (pig anterior pituitary tissue). The most selective agonists were SK&F 38393 (for D1) and (+)-PHNO (for D2), while the most selective antagonists were SCH 23390 (for D1) and raclopride or eticlopride (for D2).

Anti-myosin stains chromaffin cells.

The presence in fixed chromaffin cells of antigenic sites for a myosin antibody was demonstrated using immunofluorescence techniques. Tests on viable cells showed that at least some of the antigenic sites seem to be localized on or close to the cell surface and explained the cell agglutination that occurred with the addition of the myosin antibody to cells isolated by a method described in this paper.

Link between D1 and D2 dopamine receptors is reduced in schizophrenia and Huntington diseased brain.

Dopamine receptor types D1 and D2 can oppose or enhance each other's actions for electrical, biochemical, and psychomotor effects. We report a D1-D2 interaction in homogenized tissue as revealed by ligand binding. D2 agonists lowered the binding of [3H]raclopride to D2 receptors in striatal and anterior pituitary tissues. Pretreating the tissue with the D1-selective antagonist SCH 23390 prevented the agonist-induced decrease in [3H]raclopride binding to D2 sites in the striatum but not in the anterior pituitary, which has no D1 receptors. Conversely, a dopamine-induced reduction in the binding of [3H]SCH 23390 to D1 receptors could be prevented by the D2-selective antagonist eticlopride. Receptor photolabeling experiments confirmed both these D1-D2 interactions. The blocking effect by SCH 23390 was similar to that produced by a nonhydrolyzable guanine nucleotide analogue, and SCH 23390 reduced the number of agonist-labeled D2 receptors in the high-affinity state. Thus, the D1-D2 link may be mediated by guanine nucleotide-binding protein components. The link may underlie D1-D2 interactions influencing behavior, since the link was missing in over half the postmortem striata from patients with schizophrenia and Huntington disease (both diseases that show some hyperdopamine signs) but was present in human control, Alzheimer, and Parkinson striata.

Dopamine receptors labelled by PHNO.

Since the high-affinity state of dopamine D2 receptors may be abnormal in psychomotor diseases, it is desirable to develop a radioactive agonist to label this high-affinity site for possible clinical diagnostic use. (+)PHNO is a selective D2 agonist used to treat Parkinson's disease. We prepared [3H](+)PHNO from allyl-des-propyl(+)PHNO. In binding to dopamine receptors in homogenates of canine brain striata, [3H](+)PHNO had a dissociation constant of 0.35 nM in the absence of NaCl, and 0.56 nM in the presence of NaCl. Dopamine agonists and antagonists inhibited the binding of [3H](+)PHNO at drug concentrations similar to those inhibiting other [3H]ligands at D2 receptors, but not similar to those acting at D4 receptors. Approximately 90% of the total [3H](+)PHNO binding was specific. Guanilylimidodiphosphate markedly inhibited [3H](+)PHNO binding, suggesting that [3H](+)PHNO was binding primarily to the high-affinity state of dopamine D2 receptors rather than to D3 receptors. The density of the [3H](+)PHNO binding sites was equal to that of [3H]emonapride (or [3H]YM-09151-2), both densities of which were 1.5- to 2-fold higher than that of [3H]spiperone, compatible with the idea that [3H](+)PHNO binds to monomers of D2, while [3H]spiperone binds to dimers of D2. Although [3H](+)PHNO has good selectivity and affinity for the high-affinity state of D2, the [3H]ligand was sensitive to endogenous dopamine, since washing the tissue lowered the dissociation constant. For future in vivo labelling of D2 by an agonist, therefore, it will be essential to search for a related [3H]ligand with an even lower dissociation constant.

Dopamine receptor parameters detected by [3H]spiperone depend on tissue concentration: analysis and examples.

The binding of lipophilic radioligands to homogenized tissue was investigated with the help of a simple, two-component model: a specific component reflects binding to a single and uniform population of sites; a nonspecific component reflects partitioning into the membrane and the entrapment of some drug present in the aqueous phase prior to separation of the particulate fraction. The results indicate that the capacity and the affinity of the receptor may be underestimated when the data are analyzed in terms of total rather than free radioligand. Errors in capacity arise when for a significant fraction of the radioligand access to the receptor is blocked by an unlabelled drug and this appears as nonspecific binding. This is most likely to occur when the partition coefficient is such that the free radioligand is located predominantly in the particulate phase. Errors in affinity reflect the tendency of the membrane to reduce the free concentration of a lipophilic drug in the aqueous phase. A further complication arises when a significant fraction of the total radioligand binds to the receptor. [3H]Spiperone binds to dopamine D2 receptors with a dissociation constant of about 50 pM and partitions into the particulate phase of brain homogenates with a membrane/buffer partition coefficient of 410. As expected, both capacity and affinity can appear to depend on the concentration of tissue used in the assay. If the partition coefficient is known, corrected estimates of both parameters can be obtained knowing only the total concentration of radioligand; if the partition coefficient is not known, the free concentration of radioligand in the aqueous phase must be measured independently. The former procedure requires that the aqueous and particulate components of the system be separated by centrifugation; with filtration, the removal of an indeterminate amount of radioligand from the membrane during washing precludes any correction based on the partition coefficient. For the specific example of [3H]spiperone in human brain, the artifacts become negligible at concentrations of protein below 0.1 mg/ml of incubate. The capacity per unit of original tissue is best determined using unwashed preparations, since about 30% of the total protein and a comparable percentage of the receptors are lost on washing.