Dopamine receptors in human and calf brains, using [3H]apomorphine and an antipsychotic drug

Pimozide Increases a Delayed Rectifier K+ Conductance in Chicken Embryo Vestibular Hair Cells

Pimozide is a conventional antipsychotic drug largely used in the therapy for schizophrenia and Tourette's syndrome. Pimozide is assumed to inhibit synaptic transmission at the CNS by acting as a dopaminergic D₂ receptor antagonist. Moreover, pimozide has been shown to block voltage-gated Ca²⁺ and K⁺ channels in different cells. Despite its widespread clinical use, pimozide can cause several adverse effects, including extrapyramidal symptoms and cardiac arrhythmias. Dizziness and loss of balance are among the most common side effects of pimozide. By using the patch-clamp whole-cell technique, we investigated the effect of pimozide [3 μM] on K⁺ channels expressed by chicken embryo vestibular type-II hair cells. We found that pimozide slightly blocks a transient outward rectifying A-type K⁺ current but substantially increases a delayed outward rectifying K⁺ current. The net result was a significant hyperpolarization of type-II hair cells at rest and a strong reduction of their response to depolarizing stimuli. Our findings are consistent with an inhibitory effect of pimozide on the afferent synaptic transmission by type-II hair cells. Moreover, they provide an additional key to understanding the beneficial/collateral pharmacological effects of pimozide. The finding that pimozide can act as a K⁺ channel opener provides a new perspective for the use of this drug.

The High Affinity Dopamine D2 Receptor Agonist MCL-536: A New Tool for Studying Dopaminergic Contribution to Neurological Disorders

The dopamine D₂ receptor exists in two different states, D₂^high and D_2low; the former is the functional form of the D₂ receptor and associates with intracellular G-proteins. The D₂ agonist [³H]MCL-536 has high affinity for the D₂ receptor (K_d 0.8 nM) and potently displaces the binding of (R-(-)-N-n-propylnorapomorphine (NPA; K_i 0.16 nM) and raclopride (K_i 0.9 nM) in competition binding assays. Here, we further characterize [³H]MCL-536. [³H]MCL-536 was metabolically stable, with about 75% of the compound remaining intact after 1 h incubation with human liver microsomes. Blood-brain barrier penetration in rats was good, attaining at 15 min a % injected dose per gram of wet tissue (%ID/g) of 0.28 in males versus 0.42 in females in the striatum. Specific uptake ratios ([%ID/g striatum]/[%ID/g cerebellum]) were stable in males during the first 60 min and in females up to 15-30 min. The D₂-rich striatum exhibited the highest uptake and slowest washout compared to D₂-poor cortex or cerebellum. In peripheral organs, uptake peaked at 15 min but declined to baseline at 60 min, indicating good clearance from the body. In vitro autoradiography on transaxial and coronal brain sections showed specific binding of [³H]MCL-536, which was abolished by preincubation with D₂/D₃ ligands sulpiride, NPA, and raclopride and in the presence of the stable GTP analogue guanylylimidodiphosphate. In amphetamine-sensitized animals, striatal binding was higher than in controls, indicating specificity for the D₂^high receptor state. [³H]MCL-536's unique properties make it a valuable tool for research on neurological disorders involving the dopaminergic system like Parkinson's disease or schizophrenia.

Repeated administration of 5-hydroxytryptamine 2C agonist MK212 produces a sensitization effect of antipsychotic activity

5-Hydroxytryptamine 2C (5-HT_2C ) receptor agonists have been suggested to possess an antipsychotic activity in several acute preclinical tests of antipsychotic drugs with low extra-pyramidal side effect liability. However, little is known about the long-term effect associated with chronic use of 5-HT_2C receptor agonists. The present study examined whether repeated activation of 5-HT_2C receptor with a highly selective 5-HT_2C receptor agonist MK212 would induce a long-term change in its antipsychotic-like activity (either a sensitization or tolerance) in the conditioned avoidance response and MK801-induced hyperlocomotion tests. Sprague-Dawley rats were first tested under the intraperitoneal (i.p.) treatment of MK212 (0.25, 0.5, 1.0 mg/kg) for 5 consecutive days. Three days later, when all rats were injected with a low dose of MK 212 (0.25 mg/kg) and tested for avoidance responding, rats that had been pretreated with 1.0 and 0.5 mg/kg MK212 made significantly fewer avoidance responses than those that had been treated with vehicle (0.9% saline). However, this past drug exposure-induced group difference was not significant in the MK801-induced hyperlocomotion test. Overall, results from this study suggest that repeated treatment of MK212 is capable of inducing a dose-dependent sensitization of antipsychotic activity in conditioned avoidance response. The discrepancy in sensitization of MK212 in CAR and MK801-induce hyperlocomotion may be related to the different mechanism underlying the effect of MK212 in these two tests. © 2016 IUBMB Life, 68(12):985-993, 2016.

Agonist high- and low-affinity states of dopamine D₂ receptors: methods of detection and clinical implications

Dopamine D(2) receptors, similar to other G-protein-coupled receptors, exist in a high- and low-affinity state for agonists. Based upon a review of the methods for detecting D(2) receptor agonist high-affinity states, we discuss alterations of such states in animal models of disease and the implications of such alterations for their labelling with positron emission tomography (PET) and single-photon emission computed tomography (SPECT) tracers. The classic approach of detecting agonist high-affinity states compares agonist competition for antagonist radioligands, in most cases using [(3)H]-spiperone as the radioligand; alternative approaches and radioligands have been proposed, but their claimed advantages have not been substantiated by other investigators. In view of the advantages and disadvantages of various techniques, we critically have reviewed reported findings on the detection of D(2) receptor agonist high-affinity states in a variety of animal models. These data are compared to the less numerous findings from human in vivo studies based on PET and SPECT tracers; they are interpreted in light of the finding that D(2) receptor agonist high-affinity states under control conditions may differ between rodent and human brain. The potential advantages of agonist ligands in studies of pathophysiology and as diagnostics are being discussed.

Abnormal negative feedback processing in first episode schizophrenia: evidence from an oculomotor rule switching task

BACKGROUND

Previous studies have shown that patients with schizophrenia are impaired on executive tasks, where positive and negative feedbacks are used to update task rules or switch attention. However, research to date using saccadic tasks has not revealed clear deficits in task switching in these patients. The present study used an oculomotor 'rule switching' task to investigate the use of negative feedback when switching between task rules in people with schizophrenia.

METHOD

A total of 50 patients with first episode schizophrenia and 25 healthy controls performed a task in which the association between a centrally presented visual cue and the direction of a saccade could change from trial to trial. Rule changes were heralded by an unexpected negative feedback, indicating that the cue-response mapping had reversed.

RESULTS

Schizophrenia patients were found to make increased errors following a rule switch, but these were almost entirely the result of executing saccades away from the location at which the negative feedback had been presented on the preceding trial. This impairment in negative feedback processing was independent of IQ.

CONCLUSIONS

The results not only confirm the existence of a basic deficit in stimulus-response rule switching in schizophrenia, but also suggest that this arises from aberrant processing of response outcomes, resulting in a failure to appropriately update rules. The findings are discussed in the context of neurological and pharmacological abnormalities in the conditions that may disrupt prediction error signalling in schizophrenia.

The pipeline and future of drug development in schizophrenia

While the current antipsychotic medications have profoundly impacted the treatment of schizophrenia over the past 50 years, the newer atypical antipsychotics have not fulfilled initial expectations, and enormous challenges remain in long-term treatment of this debilitating disease. In particular, improved treatment of the negative symptoms and cognitive dysfunction in schizophrenia which greatly impact overall morbidity is needed. In this review we will briefly discuss the current pipeline of drugs for schizophrenia, outlining many of the strategies and targets currently under investigation for the development of new schizophrenia drugs. Many of these compounds have great potential as augmenting agents in the treatment of negative symptoms and cognition. In addition, we will highlight the importance of developing new paradigms for drug discovery in schizophrenia and call for an increased role of academic scientists in discovering and validating novel drug targets. Indeed, recent breakthroughs in genetic studies of schizophrenia are allowing for the development of hypothesis-driven approaches for discovering possible disease-modifying drugs for schizophrenia. Thus, this is an exciting and pivotal time for the development of truly novel approaches to drug development and treatment of complex disorders like schizophrenia.

Disrupted prediction-error signal in psychosis: evidence for an associative account of delusions

Delusions are maladaptive beliefs about the world. Based upon experimental evidence that prediction error-a mismatch between expectancy and outcome--drives belief formation, this study examined the possibility that delusions form because of disrupted prediction--error processing. We used fMRI to determine prediction-error-related brain responses in 12 healthy subjects and 12 individuals (7 males) with delusional beliefs. Frontal cortex responses in the patient group were suggestive of disrupted prediction-error processing. Furthermore, across subjects, the extent of disruption was significantly related to an individual's propensity to delusion formation. Our results support a neurobiological theory of delusion formation that implicates aberrant prediction-error signalling, disrupted attentional allocation and associative learning in the formation of delusional beliefs.

From prediction error to psychosis: ketamine as a pharmacological model of delusions

Recent cognitive neuropsychiatric models of psychosis emphasize the role of attentional disturbances and inappropriate incentive learning in the development of delusions. These models highlight a pre-psychotic period in which the patient experiences perceptual and attentional disruptions. Irrelevant details and numerous associations between stimuli, thoughts and percepts are imbued with inappropriate significance and the attempt to rationalize and account for these bizarre experiences results in the formation of delusions. The present paper discusses delusion formation in terms of basic associative learning processes. Such processes are driven by prediction error signals. Prediction error refers to mismatches between an organism's expectation in a given environment and what actually happens and it is signalled by both dopaminergic and glutamatergic mechanisms. Disruption of these neurobiological systems may underlie delusion formation. We review similarities between acute psychosis and the psychotic state induced by the NMDA receptor antagonist drug ketamine, which impacts upon both dopaminergic and glutamatergic function. We conclude by suggesting that ketamine may provide an appropriate model to investigate the formative stages of symptom evolution in schizophrenia, and thereby provide a window into the earliest and otherwise inaccessible aspects of the disease process.

Monoaminergic synapses and schizophrenia: 45 years of neuroleptics

In 1952 Delay and Deniker introduced the first antipsychotic, chlorpromazine, into the treatment of mental patients. They subsequently defined the word 'neuroleptic' to describe drugs as different as reserpine and chlorpromazine which seemed to have similar effects on the mental life of patients. In the 1960s the hypothesis was developed, mainly due to Carlsson, that the principal mode of action of neuroleptics was to interfere with synaptic transmission mediated by dopamine (DA) in the brain. This concept was given substantial credence with the discovery by Seeman and Snyder in the 1970s that many of the neuroleptics acted as DA receptor blockers. Subsequently two different classes of DA receptor were defined on the basis of their coupling to adenylate cyclase by Kebabian. In the 1980s molecular biology led to the cloning of five different DA receptors, and at the end of this period vanTol and his colleagues cloned the D4 DA receptor, which has been of considerable interest in the 1990s as it is greatly elevated in the brains of schizophrenics. This historical review ends with a consideration of the possibility that in addition to DA receptors, serotonin and perhaps other transmitter receptors are involved in the aetiology of schizophrenia.

Concentration and distribution of thioridazine and metabolites in schizophrenic post-mortem brain tissue

Thioridazine (THD) and its major metabolites, mesoridazine (MES), sulforidazine (SULF), and northioridazine (norTHD) accumulate at a predictable rate in human brain tissue after chronic medication. Although the concentration of THD is normally lower than or the same as its major metabolite, MES, in the plasma, it was found to be up to five times as concentrated in the brain tissue of treated patients. THD and its metabolites were evenly distributed throughout all regions of the brain in chronically medicated patients. Brain concentrations of THD were also compared with those of chlorpromazine (CPZ) when both drugs had been given at the same dose before death, and were shown to be up to 10 times more concentrated in brain at doses greater than 300 mg/day. Because some of the metabolites of THD are pharmacologically active, it is important to know how they accumulate in the brain in relation to the parent compound to understand how this drug mediates its clinical effect.

Calcium-activated potassium conductance. An alternative to the dopamine hypothesis of neuroleptic action?

Neuroleptics are structurally a heterogenous group of compounds which possess antipsychotic activity. They increase dopamine metabolites by blocking dopamine receptors and enhancing presynaptic turnover. This forms the cornerstone of the dopamine hypothesis of neuroleptic action, which is supported by wide-ranging behavioural, physiological and biochemical studies. It is, however, clear that neuroleptics are far less specific for the dopamine receptor than was previously considered. They influence a range of neuronal activities, including calcium-activated potassium conductance, which governs the rate of action potential generation by many neurones. Recent physiological studies indicate that all commonly used neuroleptics alter calcium-activated potassium conductance in central neurones, in concentrations similar to those achieved clinically. An adaptive increase in calcium-activated potassium conductance mechanisms in key sensory processing neurones would render the psychotic patient less susceptible to bombardment by environmental stimuli. This action may explain in part the therapeutic effect of neuroleptics.

Autoradiographic localisation of 3H-apomorphine binding sites in rat brain

The best experimental conditions for a selective binding of 3H-apomorphine to dopamine receptors on cryostat sections were first selected by liquid scintillation quantification of the bound radioactivity. In the corpus striatum, a specific binding occurred with a half-maximal saturation concentration of about 1 nM and a maximal capacity of 180 fmol/mg of slice protein, both values in agreement with previous binding data on either membranes or slices incubated in a physiological medium. Inhibition with domperidone was clearly biphasic, indicating two classes of sites corresponding to the D-2 and D-3 sites as previously defined on membranes. When 3H-apomorphine was used at low concentrations (0.8-1.5 nM), a condition ensuring a preferential labelling of D-2 sites, rather well contrasted autoradiographic pictures were generated. The major dopaminergic projection fields in telencephalon (caudate-putamen, nucleus accumbens, olfactory tubercles) were visualised as well as other catecholaminergic regions such as the superficial gray layer of superior colliculi. Within the striatum, differences in density of these sites were observed in three perpendicular planes and confirmed by a computer densitometric image analysis. Labelling of areas of origin of the cerebral dopaminergic neurons in substantia nigra or ventral tegmental area were also observed. When a higher concentration of 3H-apomorphine (3.5 nM) was used in the presence of domperidone, another, but autoradiographically less distinct subclass of sites (D-3 sites) was demonstrated.

Dopamine agonist activity of EMD 23,448

EMD 23,448 was examined in tests of dopaminergic function and was found to be an atypical dopamine (DA) agonist. EMD 23,448 was a weak or inactive DA agonist when examined in tests of normal postsynaptic DA receptor function: production of stereotypy in the rat (ED50 greater than 5.0 mg/kg i.p.); production of emesis in beagles (minimum effective dose = 81 micrograms/kg i.v.); and, enhanced locomotor activity of the mouse (no excitation in doses less than or equal to 50 mg/i.p.). Moreover, EMD 23,448 was relatively weak in competing for [3H]-apomorphine binding to rat striatal membranes (Ki, 205 nM). On the other hand, this indolyl-3-butylamine did activate supersensitive postsynaptic DA receptors. Specifically, it elicited contralateral turning in rats with a unilateral 6-hydroxydopamine lesion of the substantia nigra (ED50 value = 0.9 mg/kg) and did elicit stereotypy in rats given chronic daily haloperidol treatments. EMD 23,448 also exerted pharmacological effects in tests designed to measure activation of dopamine autoreceptors. It inhibited the gamma-butyrolactone-induced increase in striatal dopa levels (ED50 = 1 mg/kg i.p.) and produced a dose-related fall in the locomotor activity of the mouse. The results are discussed and contrasted with data derived for apomorphine and the putatively selective autoreceptor agonist (+/-)-3-PPP.

Halopemide, a new psychotropic agent. Cerebral distribution and receptor interactions

Halopemide is a new psychotropic agent, a structural analogue of the neuroleptics of the butyrophenone type but with different pharmacological and clinical properties. Preliminary clinical findings indicate that halopemide lacks the ability to induce parkinsonism and may be an effective drug in the treatment of psychosis characterized by autism, emotional withdrawal or apathy. Its pharmacological effects at a molecular level in comparison to structurally related neuroleptics and putative metabolites are reviewed.

Effects of age on dopamine and serotonin receptors measured by positron tomography in the living human brain

D2 dopamine and S2 serotonin receptors were imaged and measured in healthy human subjects by positron emission tomography after intravenous injection of 11C-labeled 3-N-methylspiperone. Levels of receptor in the caudate nucleus, putamen, and frontal cerebral cortex declined over the age span studied (19 to 73 years). The decline in D2 receptor in males was different from that in females.

Solubilization of dopamine-D2 receptors from synaptosomal membranes of the bovine caudate nucleus

Dopamine D2-receptors were solubilized from synaptosomal membranes of the bovine caudate nucleus using different detergents. They were labelled with [3H]-spiperone and assayed by polyethylene glycol precipitation. CHAPS was found to be the best solubilizing agent among all detergents used. Optimal conditions for solubilization were: 0.25% CHAPS, 3.5 mg ml-1 protein, 25 min, 4 degrees C and the yield of D2-receptors was 18.6%. Addition of some sulphobetain detergents increased the extent of solubilization, 125 mM NaCl and 0.25 M sucrose decreased it, while SH-group protecting agents (2 mM dithiothreitol and 6 mM beta-mercaptoethanol), as well as MEGA-9 and MEGA-12 were almost ineffective. -log IC50 values for solubilized dopamine D2-receptors are in linear correlation with the corresponding values for membrane-bound receptors (r = 0.962, slope factor 0.96) and Kd value of solubilized receptors was 3.61 +/- 0.94 nM, while that of membrane-bound receptors was 1.25 +/- 0.10 nM. Specific binding of [3H]-spiperone to the solubilized receptors resolved by linear sucrose density gradient centrifugation shows two maxima, one in the first several fractions from the bottom and the other with an apparent S value of 7.3.

Is there a "dopaminergic glial cell"?

Intracellular cAMP increased 9-fold in cerebral hemisphere primary cultures after incubation with dopamine (10(-4) M). The effect was dose- and time-dependent (10(-6) M-10(-4) M; 2-10 minutes). It was mimicked, to some extent, by the partial agonist apomorphine (10(-5) M-10(-4) M) and antagonized by fluphenazine (10(-5) M-10(-4) M). The elevation of cAMP caused by dopamine was incompletely antagonized by propanolol (10(-5) M-10(-4) M), obviating an interaction with beta-adrenergic receptors. A beta-adrenergic effect was antagonized by propranolol but only slightly by fluphenazine. The effect of dopamine on cAMP-level was more pronounced in a subpopulation of the hemisphere culture, i.e. in astroglial cultures from the striatum, 12-fold compared with controls at 10(-4) M. No dopamine stimulated formation of cAMP was found in primary cultures from brain-stem. The results demonstrated some heterogeneity among astroglial cells. The cultures used contained mainly astroglial-like cells, as judged from immunohistochemical localization of the glial specific proteins S 100 and GFA (alpha-albumin). No mature neurons or oligodendroglial cells have so far been demonstrated in the cultures.

Dopaminergic binding sites in rat striatal slices and the action of guanyl nucleotides

Dopaminergic binding sites were studied in slices from rat striatum incubated in a physiological medium and using the two highly selective ligands 3H-apomorphine and 3H-domperidone. The clearly biphasic or stretched inhibition of the specific binding of these two ligands by domperidone or apomorphine, respectively allowed to define three distinct classes of binding site. It was demonstrated, by comparing the binding of the 3H-ligand added at the beginning of slice incubation or just before homogenisation of tissue and filtration, that the "specific" bindings only occurred during the incubation of slices. The inhibition constants (Ki values) of dopaminergic agents for the three classes of binding site as also the dissociation constants (Kd values) of 3H-ligands and the maximal capacity (Bmax) of the three classes of binding site were closely similar to those of binding sites previously demonstrated on rat striatal membranes, namely D-2, D-3 and D-4 sites (Sokoloff et al. 1980 a, b). Their identification on a preparation in which the cellular organisation is largely preserved rules out the possibility that these sites represent an artifact due to membrane preparation. Unexpectedly the addition of guanyl nucleotides like GTP or GppNHp to the slice preparation decreased the binding of 3H-apomorphine to the high affinity sites (particularly to the D-2 sites) while D-4 site binding was correspondingly increased. The guanyl nucleotide effect apparently took place before cell disruption and occurred at concentrations similar to those required in striatal membrane preparations. These observations, together with those indicating the presence of high affinity binding sites for dopaminergic agonists in intact striatal cells, suggest that a putative nucleotide regulatory unit of dopamine receptors, is not fully occupied by intracellular GTP but could be interacted with from the external face of the cell membrane.

Assessment of dopamine receptor densities in the human brain with carbon-11-labeled N-methylspiperone

We describe the use of carbon-11-labeled 3-N-methylspiperone, a ligand that preferentially binds to dopamine receptors in vivo, to image the receptors by positron emission tomography scanning in baboons and, for the first time, in a human. The method has now been used in 58 humans for noninvasive assessment of the state of brain dopamine receptors under normal and pathological conditions.

Interaction of the component enantiomers of the putative dopamine autoreceptor agonist, TL-99 (6,7-dihydroxy-2-dimethylamino tetralin) with dopaminergic systems in mammalian brain and teleost retina

The enantiomers of the putative dopamine autoreceptor agonist, TL-99 (6,7-dihydroxy-2-dimethylaminotetralin) were examined in a number of in vivo and in vitro test paradigms to further examine the reported autoreceptor selectivity of this compound. The (+)-isomer of the aminotetralin was more active as a dopamine agonist than either the racemate or the (-)-enantiomer. In addition to this dopaminergic activity, TL-99 was found to be a potent alpha 2-adrenoceptor agonist, this activity being more prominent in the (+)-isomer. The (-)-isomer, however, was a weak alpha 2/DA receptor agonist and unlike the (+)-enantiomer was devoid of activity in the D-1-selective carp retina adenylate cyclase assay. Pharmacological examination of the effects of TL-99 on mouse locomotor activity showed that the effects of the aminotetralin in this dopamine autoreceptor test system were antagonized by either the alpha 2-antagonist, yohimbine or by the dopamine antagonist, sulpiride. TL-99 also produced contralateral turning in 6-OHDA lesioned rats. It is concluded that the apparent dopamine autoreceptor selectivity of TL-99 as assessed by in vivo animal test systems may be due partially to its alpha 2-agonist activity. The sedation and consequent reduction in mouse locomotor activity and in turning in the rat as the dose level is increased undoubtedly occurs via alpha 2-agonist and dopamine autoreceptor activity and cannot be interpreted as selectivity for the dopamine autoreceptor.

Imaging dopamine receptors in the human brain by positron tomography

Neurotransmitter receptors may be involved in a number of neuropsychiatric disease states. The ligand 3-N-[11C]methylspiperone, which preferentially binds to dopamine receptors in vivo, was used to image the receptors by positron emission tomography scanning in baboons and in humans. This technique holds promise for noninvasive clinical studies of dopamine receptors in humans.

Effect of acute and chronic cholinesterase inhibition with diisopropylfluorophosphate on muscarinic, dopamine, and GABA receptors of the rat striatum

The effects of acute and chronic administration of diisopropylfluorophosphate (DFP) to rats on acetylcholinesterase (AChE) activity (in striatum, medulla, diencephalon, cortex, and medulla) and muscarinic, dopamine (DA), and gamma-aminobutyric acid (GABA) receptor characteristics (in striatum) were investigated. After a single injection of (acute exposure to) DFP, striatal region was found to have the highest degree of AChE inhibition. After daily DFP injections (chronic treatment), all brain regions had the same degree of AChE inhibition, which remained at a steady level despite the regression of the DFP-induced cholinergic overactivity. Acute administration of DFP increased the number of DA and GABA receptors without affecting the muscarinic receptor characteristics. Whereas chronic administration of DFP for either 4 or 14 days reduced the number of muscarinic sites without affecting their affinity, the DFP treatment caused increase in the number of DA and GABA receptors only after 14 days of treatment; however, the increase was considerably lower than that observed after the acute treatment. The in vitro addition of DFP to striatal membranes did not affect DA, GABA, or muscarinic receptors. The results indicate an involvement of GABAergic and dopaminergic systems in the actions of DFP. It is suggested that the GABAergic and dopaminergic involvement may be a part of a compensatory inhibitory process to counteract the excessive cholinergic activity produced by DFP.

[3H]dopamine labeling of D3 dopaminergic sites in human, rat, and calf brain

The binding of [3H]dopamine to brain regions of calf, rat, and human was investigated. The calf caudate contained the highest density of [3H]dopamine binding sites, with a Bmax value of 185 fmol/mg protein, whereas rat and human striatum contained one-third this number of sites. The KD values for [3H]dopamine in all tissues were 2-3 nM. Dopaminergic catecholamines (dopamine, apomorphine, 6,7-dihydroxy-2-aminotetralin, and N-propylnorapomorphine) inhibited the binding of [3H]dopamine in all three species, at low concentrations, with IC50 values of 1.5 to 6 nM. Neuroleptics, in contrast, inhibited the binding at high concentrations (with IC50 values of 200 to 40,000 nM). The [3H]dopamine binding sites were saturable, heat-labile, and detectable only in dopamine-rich brain regions; these sites differed from D2 dopamine sites (labeled by [3H]butyrophenone neuroleptics), and from D1 dopamine sites (labeled by [3H]thioxanthene neuroleptics) associated with the dopamine-stimulated adenylate cyclase. We have, therefore, called these high-affinity [3H]dopamine binding sites D3 sites. [3H]Apomorphine and [3H]ADTN also appeared to label D3 sites. These ligands however, were less selective than [3H]dopamine, and labeled sites other than D3 as well. Assay conditions were important in determining the parameters of [3H]dopamine binding. The optimum conditions for selective labeling of the D3 dopaminergic sites, using [3H]dopamine, required the presence of EDTA and ascorbate.

Soluble spiroperidol binding factors from bovine caudate nucleus

Several properties of soluble spiroperidol binding factors separated from bovine caudate nucleus have been investigated by a previously unreported procedure. Data consistent with high particle weight and rapid binding equilibration are reported for high-affinity (+)butaclamol-sensitive components of a digitonin extract. A slower sedimenting component is found that also exhibits high affinity for spiroperidol but is not sensitive to (+)butaclamol. Centrifugation of a caudate nucleus homogenate yields a supernatant that appears to contain a component that exhibits spiroperidol binding that is more sensitive to displacement by (-) than by (+)butaclamol. The procedure used effects rapid separation of bound from unbound tritiated ligand on short columns of Sephadex G-15 followed by extrusion and sectioning of the Sephadex. The radioactivity remaining with each section is determined. The procedure is very rapid; the addition of active phases or the changing of the ionic environment, which may disturb the equilibrium, is avoided; and recovery of the protein free of bound ligand is easily affected.

Dopamine receptors in canine caudate nucleus

Physiological, pharmacological, histochemical and biochemical studies indicate that dopamine receptors are heterogenous in the central nervous system with each individual functions. This review describes pharmacological and biochemical characteristics of dopamine receptors, particularly in canine caudate nucleus, which have been studied in our laboratory with a brief comparison to the current studies by other workers in similar research fields. Two distinct dopamine receptors have been characterized by means of [3H]dopamine binding to the synaptic membranes from canine caudate nucleus. One of the receptors with a Kd of about 3 muM for dopamine may be associated with adenylate cyclase and referred to as D2 receptor. The other receptor with a Kd of about 10 nM for dopamine is independent of adenylate cyclase and referred to as D2. A photochemical irreversible association of [3H]dopamine with the membraneous receptors makes it possible to separate D1 and D2 receptors from one another by gel filtration on a Sephadex G-200 column after solubilization with Lubrol PX. On the basis of selective inhibition of [3H]dopamine binding to D1 and D2 receptors, dopamine antagonists can be classified into three classes: D1-selective (YM-09151-2), D2-selective (sulpiride) and nonselective (haloperidol, chlorpromazine). Effects of these typical antagonists on the metabolism of rat brain dopamine suggest that D1 receptor is more closely associated with the neuroleptic-induced increase in dopamine turnover. Studies with 28 benzamide derivatives and some classical neuroleptics reveal that apomorphine-induced stereotypy displays a greater association with D1 than with D2 receptors. Dopamine-sensitive adenylate cyclase in canine caudate nucleus can be solubilized with Lubrol PX in a sensitive form to either dopamine. Gpp(NH)p or fluoride Sephadex G-200 gel filtration separates adenylate cyclase from D1 receptors with a concomitant loss of dopamine sensitivity. Addition of the D1 receptor fraction to the adenylate cyclase restores the responsiveness to dopamine. The solubilized dopamine-unresponsive adenylate cyclase can be further separated into two distinct fractions by a batch-wise treatment with GTP-sepharose: a catalytic unit which does not respond to fluoride, and a guanine nucleotide regulatory protein. The regulatory protein confers distinct responsiveness to Gpp(NH)p and fluoride upon adenylate cyclase. These results indicate that dopamine-sensitive adenylate cyclase is composed of at least three distinct units; D1 receptor, guanine nucleotide regulatory protein and adenylate cyclase.

Multiple postsynaptic dopamine receptors and behavioral manifestation

Previously we demonstrated the existence of non- and positive-cooperative dopamine receptors in rat striatum in [3H] apomorphine binding experiments. Non-cooperative sites were sensitive to the inhibition of sulpiride while cooperative sites were not. In the present study results dealing with the involvement of those two types of postsynaptic dopamine receptors in different behavior manifestations is shown employing lisuride hydrogen maleate (LHM). LHM elicited contralateral turnings in 6-hydroxydopamine lesioned rats unilaterally in the striatum whereas it caused ipsilateral turnings in kainic acid lesioned rats as was observed following the administration of apomorphine. Furthermore, the effect of LHM on rotating behavior was abolished by the pretreatment with sulpiride. On the other hand, LHM inhibited apomorphine induced stereotyped behavior whereas the sulpiride failed to block it. These results suggested the dual action of LHM on multiple postsynaptic dopamine receptors. The results also indicated that non-cooperative postsynaptic dopamine receptors are involved in rotating behavior while cooperative receptors participate in the elicitation of stereotypy.

Genetic regulation of neurotransmitter enzymes and receptors: relationship to the inheritance of psychiatric disorders

This report begins with a summary of the evidence for genetic involvement in certain major psychiatric syndromes. The relation of these disorders to deficits in central nervous system neurotransmitters is also summarized. These reviews serve as an introduction to our studies on the genetic regulation of neurotransmitters and their enzymes and receptors in inbred mice. The steady-state levels of the adrenal catecholamine biosynthetic enzymes are controlled genetically; not only is each enzyme regulated by a single locus, but also there is statistical evidence that the phenotypic expression of the entire pathway is regulated by a single gene. Studies on the biochemical mechanism of gene action suggest that genetic regulation is exerted on proteolysis of the enzymes, rather than their synthesis. In addition, we have examined the genetic control of dopamine receptors in inbred mice. Dopaminergic receptors in the nigrostriatal and mesolimbic pathways are under genetic control. Preliminary evidence suggests that the pathways are regulated by different genetic systems. If this early speculation proves true, it would have important clinical implications.

Comparison of the binding of [3H]spiperone and [3H]domperidone in homogenates of mammalian retina and caudate nucleus

The specific binding of [3H]spiperone and [3H]domperidone, as defined by 1 microM-(+)butaclamol, was compared in homogenates of bovine retina and caudate nucleus. Scatchard analyses of saturation data for [3H]spiperone binding yielded dissociation constants (Kd) of 0.35 nM in the retina and 0.64 nM in the caudate nucleus. Comparison of the maximum number of binding sites (Bmax) present in each tissue indicated that the density of sites in bovine caudate nucleus (270 fmol/mg protein) was approximately three times higher than in bovine retina (92 fmol/mg protein). This difference was even more marked in guinea pig tissues, with a ratio of 7:1 between corpus striatum and retina. The pharmacological analysis of [3H]spiperone binding in both the bovine retina and caudate nucleus indicated an interaction with dopaminergic rather than serotonergic sites. However, inhibition curves obtained to dopaminergic agonists in the bovine retina were significantly steeper than those observed in the bovine caudate nucleus, as reflected in the greater Hill coefficients obtained for these agents in the retina. Furthermore, only a small amount of specific [3H]domperidone binding was observed in either the bovine caudate nucleus or the guinea pig striatum, whilst no specific [3H]domperidone binding was detectable in homogenates of either bovine or guinea pig retina. These data suggest that the retina possesses only a small population of dopaminergic D2 sites and that these binding sites may differ from those present in the caudate nucleus.

Dopamine receptor mediated inhibition by pergolide of electrically-evoked 3H-dopamine release from striatal slices of cat and rat: slight effect of ascorbate

The dopamine receptor agonist pergolide inhibited the calcium-dependent, electrically evoked overflow of tritium from slices of the striatum of cat or rat prelabelled with 3H-dopamine. This inhibition of tritium overflow by nanomolar concentrations of pergolide was antagonized by the benzamide neuroleptic S-sulpiride (0.1 microM). In millimolar concentrations, L- ascorbate had slight or no effects on this dopamine receptor mediated inhibition, in striatal slices of either the cat or the rat. Since these same concentrations of ascorbate have been reported to completely block the specific binding of 3H-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (ADTN) and of 3H-apomorphine to presumed dopamine receptors, the present results suggest a dissociation between the characteristics of 3H-ADTN and 3H-apomorphine binding and the dopamine autoreceptor. Previous contradictory results concerning the existence of inhibitory dopamine receptors which modulate depolarization-evoked overflow of dopamine from the striatum of the rat are thus apparently not due to a species difference nor to the use of ascorbate, but rather to differences in experimental conditions.

Biochemical genetics of neurotransmitter enzymes and receptors: relationships to schizophrenia and other major psychiatric disorders

Genetic control of catecholamine biosynthetic enzymes and dopamine receptors is described. The steady-state levels of each of the catecholamine biosynthetic enzymes in the adrenal is regulated by a single genetic locus. The entire biosynthetic pathway gives the appearance of concerted inheritance under the control of a single locus. Mesolimbic and nigrostriatal dopamine receptors are also genetically regulated. Preliminary evidence suggests that agonist binding sites differ from antagonist sites in both brain regions, and that the genetic controls, which are expressed on receptor site number, are independent in the two brain regions.

Resolution of dopamine and serotonin receptor components of [3H]spiperone binding to rat brain regions

A procedure was developed to identify receptors for dopamine and serotonin separately and selectively by means of [3H]spiperone and to measure the density of each receptor in different regions of the rat brain. In the striatum, the binding of [3H]spiperone to dopamine receptors was inhibited by sulpiride but not by quinazolinedone R43448 (R43448); in the frontal cortex, however, the binding of [3H]spiperone to serotonin receptors was inhibited by R43448 but not by sulpiride. Thus, the density of dopamine receptors (D2 sites) was measured by [3H]spiperone binding in the presence of 0.1 microM R43448 (to preclude the attachment of the 3H-labeled ligand to serotonin sites), while the density of serotonin receptors (S2 sites) was measured by [3H]spiperone binding in the presence of 10 microM sulpiride (to preclude the attachment of the 3H-labeled ligand to dopamine sites). The density of D2 sites was highest in the striatum, followed by the olfactory tubercle, hypothalamus, substantia nigra, and the lower pons--medulla region. All five regions had similar dissociation constants (Kd values) of 0.05--0.15 nM. The density of S2 sites was highest in the frontal cortex, followed by the posterior cortex, olfactory tubercle, striatum, hypothalamus, and thalamus, and all regions had Kd values in the range 0.6--2.3 nM. Thus, because the Kd values were similar for all regions, and because Scatchard analyses revealed a single set of sites for either D2 or S2 (where detected), the main criteria for resolving the dopamine and serotonin components of [3H]spiperone binding were considered fulfilled.

Optimal conditions for [3H]apomorphine binding and anomalous equilibrium binding of [3H]apomorphine and [3H]spiperone to rat striatal membranes: involvement of surface phenomena versus multiple binding sites

I. Binding of [3H] apomorphine to dopaminergic receptors in rat striatum was most reproducible and clearly detectable when incubations were run at 25 degrees C in Tris-HCl buffer, pH 7.5, containing 1 mM-EDTA and 0.01% ascorbic acid, using a washed total-membrane fraction. The receptor binding was stereospecifically inhibited by (+)-butaclamol, and dopamine agonists and antagonists showed high binding affinity for these sites. Unlabelled apomorphine inhibited an additional nonstereospecific binding site, which was unrelated to dopamine receptors. EDTA in the incubation mixture considerably lowered nonstereospecific [3H]apomorphine binding, apparently by preventing the complexation of the catechol moiety with metal ions which were demonstrated in membrane preparations. Stereospecific [3H]apomorphine binding was not detectable in the frontal cortex, whereas in the absence of EDTA much saturable nonstereospecific binding occurred. II. Kinetic patterns of stereospecific [3H]spiperone and [3H]apomorphine binding to rat striatal membranes and the inhibition patterns of a dopamine antagonist and an agonist were evaluated at different temperatures in high-ionic-strength Tris buffer with salts added and low-ionic-strength Tris buffer with EDTA. Apparent KD values of spiperone decreased with decreasing tissue concentrations. KD values of both spiperone and apomorphine were little influenced by temperature changes. Scatchard plots of the stereospecific binding changed from linear to curved; the amount of nonstereospecific binding of the 3H ligands varied considerably, but in opposite directions for spiperone and apomorphine in the different buffers. In various assay conditions, interactions between agonists, and between antagonists, appeared fully competitive, but agonist-antagonist interactions were of mixed type. The anomalous binding patterns are interpreted in terms of surface phenomena occurring upon reactions of a ligand with complex physicochemical properties and nonsolubilized sites on membranes suspended in a buffered aqueous solution. It is concluded that anomalous binding patterns are not necessarily an indication of binding to multiple sites or involvement of distinct receptors for high-affinity agonist and antagonist binding.

Effects of propranolol on the locomotor stimulation induced by activation of postsynaptic catecholamine receptors

The present study was undertaken in order to clarify the possible involvement of a central beta-adrenoceptor mediated action on the stimulation of locomotor activity by the dopamine agonist apomorphine and the noradrenergic agonist clonidine. The effect of pretreatment with various doses of d-and dl-propranolol on apomorphine- and apomorphine plus clonidine-induced locomotor stimulation in reserpinized mice was measured in photocell activity chambers. Pretreatment with dl-propranolol prolonged the duration of apomorphine-induced locomotor stimulation without affecting the maximal level of activity. A similar tendency was seen after pretreatment with the d-form of propranolol, which has a much lower beta-receptor blocking activity. The potentiation by clonidine of the apomorphine-induced locomotor stimulation in reserpinized mice was dose-dependently reduced by pretreatment with dl-propranolol whereas d-propranolol was found to be ineffective. The results indicate that central beta-receptor mechanisms might be involved in the apomorphine plus clonidine-induced locomotor stimulation of reserpinized mice.

Presynaptic dopamine receptors: insensitivity to kainic acid and the development of supersensitivity following chronic haloperidol

The effects of kainic acid lesions and chronic haloperidol treatment on rat striatal dopaminergic presynaptic receptors were studied. Following the gamma-butyrolactone-induced inhibition of dopaminergic impulse flow, and after dopa decarboxylase inhibition, dopa accumulation and its reversal by dopamine agonists was measured in vivo. 3H-apomorphine (a dopamine receptor ligand with purported presynaptic specificity) was used for in vitro binding experiments. Presynaptic dopamine receptors, as assessed by both methods, were unaffected by intrastriatal kainic acid injection 5-6 days before sacrifice. Seven days after termination of chronic haloperidol treatment (28 days, 0.5 mg/kg/day s.c.) both an increased apomorphine response using the dopa accumulation method and an increase in 3H-apomorphine binding were observed, indicating the development of presynaptic dopamine receptor supersensitivity.