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Biomedical subjects

P Seeman

Publications and source records attributed to P Seeman.

At least 181 records · Page 10Linked to original sources
PubMed1984

High affinity of quinidine for a stereoselective microsomal binding site as determined by a radioreceptor assay.

The techniques of the radioreceptor binding assay were applied to detect stereoselective binding of quinidine and quinine to a site on human liver microsomes. Binding of 3H-dihydroquinidine was 50% inhibited by 20-100 nM quinidine, while its enantiomer quinine did not displace the 3H-ligand at concentrations up to 500 nM. This stereoselectivity agreed with the affinity values measured by functional enzyme assays of cytochrome P450 activity using sparteine or debrisoquine as substrates.

Binding, Competitive↗
PubMed1984

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.

Antipsychotic Agents↗
PubMed1984

Total and free plasma neuroleptic levels in schizophrenic patients.

Nineteen male patients, under 35 years of age, newly admitted with a diagnosis of schizophrenia, were treated with either chlorpromazine or haloperidol at a fixed dosage for 25 days. Both total and free plasma neuroleptic levels were measured using a radioreceptor assay. Clinical response was measured by the Brief Psychiatric Rating Scale (BPRS). On day 25, nonresponders (those with a decrease of less than 8 points on the BPRS) had both total and free plasma neuroleptic levels within the range observed in responders. It is therefore concluded that lack of therapeutic response is generally not due to inadequate plasma drug levels.

Adult↗
PubMed1984

The dopamine/neuroleptic receptor.

The neuroleptic/dopamine receptor, with its picomolar affinity for potent neuroleptics, is the functional dopamine receptor of the brain. This receptor has been termed the D2 dopamine receptor, and it inhibits or interferes with dopamine-stimulated adenylate cyclase. This D2 receptor has two states, each having different affinity for dopamine. The high-affinity state, termed D2 high, has a 10 nM affinity for dopamine and is the functional correlate for dopamine autoreceptors and for the dopamine receptor in the pituitary gland. The low-affinity state, termed D2 low, has a 2000 nM affinity for dopamine, and may possibly represent the desensitized state of the dopamine receptor or the functional post-synaptic receptor.

Animals↗
PubMed1984

Aporphines. 58. N-(2-chloroethyl) [8,9-2H]norapomorphine, an irreversible ligand for dopamine receptors: synthesis and application.

The synthesis of the title compounds (1c and its 2H isomer 1b) from N-(2-hydroxyethyl)norapomorphine was carried out by ring bromination, followed by chlorination to the 2-chloroethyl compound 6. Further reduction with 2H2 or 3H2 and Pd/C gave 1b or 1c. Radiochemically pure (97%) 1c was obtained with a specific activity of 16.3 Ci/mmol. The purity of 1c was determined by LC, HPLC, UV, and NMR. [3H]NCA was shown to label the D2 receptor; however, the D2 signal appears to be only a small portion of the total signal, which may include binding to other dopamine receptor subtypes (D1 and D3).

Animals↗
PubMed1984

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.

Brain↗
PubMed1984

Agonist high- and low-affinity states of the D2-dopamine receptor in calf brain. Partial conversion by guanine nucleotide.

In order to determine whether D2-dopaminergic receptors in brain exist in different affinity states for agonists and whether these receptors could be completely converted from their agonist high-affinity state to their agonist low-affinity state, we examined the effect of a guanine nucleotide on the competition between [3H]spiperone and dopamine agonists for binding to homogenates of calf caudate nucleus. [3H]spiperone labeled sites having different affinities for agonists as well as antagonists. Agonists recognized three components of [3H]spiperone binding. Two of these components were related to the D2-dopaminergic receptor. These two sites appeared to represent interconvertible states, each having different affinities for agonists. This was supported by the observation of an apparent guanine nucleotide-induced "conversion" of sites with high affinity to those having low affinity for the agonist. This effect of the guanine nucleotide was incomplete, such that a significant proportion of the high-affinity sites (21%) remained in the presence of an excess of the nucleotide. These high-affinity, guanine nucleotide-insensitive sites may represent a distinct class of binding sites having high affinity for both agonists and antagonists or may be the result at equilibrium of an agonist-independent interaction of the receptor and the guanine nucleotide.

Animals↗
PubMed1983

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.

Antipsychotic Agents↗
PubMed1983

Development of receptors for dopamine and noradrenaline in rat brain.

In order to examine the receptor basis for the development of spontaneous locomotion and for the effects of clonidine in the infant rat, we measured the densities of alpha 1-adrenoceptors, alpha 2-adrenoceptors and D2-dopamine receptors in various brain regions of the developing infant Wistar rat. The mesolimbic D2-dopamine receptors paralleled the rise in spontaneous locomotion of the infant rat. No alpha 1-adrenoceptors were detectable at birth in any of the four regions examined (mesolimbic, hippocampus, frontal cortex, and hypothalamus), thus providing no basis for the locomotor-stimulating action of clonidine in the first week of life. Mesolimbic alpha 2-adrenoceptors matured between 21 and 28 days (at which time clonidine yields its usual sedating action).

Animals↗
PubMed1983

Molecular size of the canine and human brain D2 dopamine receptor as determined by radiation inactivation.

Target-size analysis (radiation inactivation) has been utilized for determination of the molecular size of the striatal D2 dopamine receptor of both canine and human membranes. The dog and human receptors were found to have a molecular size of 123,000 daltons. The identity of molecular size values is consistent with available pharmacological and biochemical evidence supporting D2 dopamine receptor identity in canine and human tissues. These data suggest that the canine receptor may be a valid model for molecular and structural investigation of the human D2 dopamine receptor.

Animals↗
PubMed1983

Dopamine receptor sites and states in human brain.

Of the several dopamine-sensitive sites in the brain, the one that most correlates with psychomotor behaviour is the D2 receptor. This receptor has two states. The D2Hi state is characterized by its nM affinity for dopamine. The D2Lo state is typified by its microM affinity for dopamine. Neuroleptics have the same affinity (50 pM for spiperone) for both states. Guanine nucleotides convert most, if not all, of the D2Hi sites into D2Lo sites. Any D2Hi sites which are insensitive to GTP, if such exist, may be separate D4 sites. In addition to the binding of 3H-spiperone being higher in post-mortem schizophrenic brains, the proportions of the D2 receptor in the D2Hi state appear to be less in preliminary data on the schizophrenic brain.

Brain↗
PubMed1982

Ascorbate injury and EDTA (or manganese) protection of D2-dopamine receptors.

Ascorbic acid, EDTA, and Mn2+ modulated neuroleptic binding to membrane receptors but not to solubilized receptors. Preincubation with ascorbic acid at 22 degrees C produced a profound time-dependent decline in [3H]spiperone binding to membrane receptors (prepared in Tris-ascorbate buffer). Ascorbate had minimal effect if introduced simultaneously with [3H]spiperone in the incubation medium, and receptor binding was performed and assayed at 0 degrees C. Binding to the solubilized receptor was not ascorbate sensitive. Both EDTA and Mn2+ blocked the ascorbate effect on membrane receptors but were without effect on solubilized receptors. The preparation and incubation buffers used for [3H]spiperone binding studies should include EDTA if ascorbate is present in the buffer.

Animals↗
PubMed1982

Solubilized receptors for [3H]dopamine (D3 binding sites) from canine brain.

The objective of the present study was to solubilize the D3 site which binds [3H]dopamine, using the same digitonin method that had been successful in solubilizing the D2 dopamine receptor. Canine brain striatal membranes were solubilized by a final concentration of 1% digitonin. The specific binding of [3H]dopamine to the soluble D3 binding sites was measured using Sephadex G-50 gel filtration. The density of D3 sites was identical in the membrane and soluble preparations (82-90 fmoles/mg protein), although the dissociation constant (KD value) went from 1.2 nM in the membranes to the value of 3.4 nM in the soluble material. The concentrations of various drugs which inhibited the binding of [3H]dopamine were similar in the two preparations. The agonists [dopamine, apomorphine and (+/-)-6,7-dihydroxy-2-aminotetralin (ADTN)] all inhibited the binding of [3H]dopamine by 50% at concentrations between 2 and 20 nM in both the intact and soluble preparations. The neuroleptics were all equally weak in inhibiting the binding of [3H]dopamine, with IC50 values in the micromolar concentration range, values typical for the D3 site. Approximately 36% of the D3 sites were recovered from the original tissue. Since the densities and recoveries of the D2 and D3 sites differed upon digitonin solubilization, this provided further indirect evidence that these two sites are distinct and separate entities which might ultimately be separated.

Animals↗