Structural Investigation of the Dopamine-2 Receptor Agonist Bromocriptine Binding to Dimeric D2HighR and D2LowR States.

The active (D2HighR) and inactive (D2LowR) states of dimeric dopamine D2 receptor (D2R) models were investigated to clarify the binding mechanisms of the dopamine agonist bromocriptine, using Molecular Dynamics (MD) simulation. The aim of this comprehensive study was to investigate the critical effects of bromocriptine binding on each distinct receptor conformation. The different binding modes of the bromocriptine ligand in the active and inactive states have a significant effect on the conformational changes of the receptor. Based on the MM/GBSA approach, the calculated binding enthalpies of bromocriptine demonstrated selectivity toward the D2HighR active state. There is good agreement between the calculated and experimentally measured D2HighR selectivity. In the ligand-binding site, the key amino acids identified for D2HighR were Asp114(3.32) and Glu95(2.65), and for D2LowR, it was Ser193(5.42). Moreover, analysis of replicate MD trajectories demonstrated that the bromocriptine structure was more rigid at the D2HighR state and more flexible at the D2LowR state. However, the side chains of the ligand-receptor complex of D2HighR showed larger variations relative to the corresponding regions of D2LowR. The present study is part of an ongoing research program to study D2R conformational changes during ligand activation and to evaluate the conformational state selectivity for ligand binding.

Parkinson's disease treatment may cause impulse-control disorder via dopamine D3 receptors.

In treating Parkinson's disease with dopaminergic agonists, such as pramipexole, ropinirole, pergolide, rotigotine, apomorphine, or bromocriptine, it has been observed that a significant number of patients develop impulse-control disorders, such as compulsive shopping, pathological gambling, or hypersexuality. Because the dopamine agonists have high affinities for the dopamine D2 and D3 receptors, the drug dissociation constants of these drugs at the functional high-affinity states of these receptors, namely D2High and D3High, were compared. The data show that, compared to the other dopamine agonist drugs, pramipexole has a relatively high selectivity for the dopamine D3 receptor, as compared to D2, suggesting that the D3 receptor may be a primary target for pramipexole. There is a trend showing that the proportion of impulse-control disorders is related to the selectivity for D3 receptors over D2 receptors, with pramipexole having the highest association with, or frequency of, impulse-control disorders. While the number of studies are limited, the proportion of patients with impulse-control disorder in Parkinson patients treated with an add-on agonist were 32% for pramipexole, 25% for ropinirole, 16% for pergolide, 22% for rotigotine, 10% for apomorphine, and 6.8% for bromocriptine. Clinically, temporary replacement of pramipexole by bromocriptine may provide relief or reversal of the impulsive behavior associated with selective D3 stimulation by either pramipexole or ropinirole, while maintaining D2 stimulation needed for the anti-Parkinson action.

Dopamine agonist radioligand binds to both D2High and D2Low receptors, explaining why alterations in D2High are not detected in human brain scans.

The features of schizophrenia are consistent with increased sensitivity to endogenous dopamine. Animal models of schizophrenia reveal an increase in the in vitro proportion of striatal dopamine D2 receptors in the high-affinity state for dopamine (i.e., D2High), as measured by dopamine/[(3) H]domperidone competition. However, in vivo studies did not reveal the dopamine agonist [(11) C](+)PHNO to be elevated in amphetamine-sensitized rats. Also, no increase was found in the in vivo binding of [(11) C](+)PHNO in schizophrenia patients. This work was done to resolve the contradictory findings. It was found that the in vitro density of rat striatal D2 receptors was 18 pmol/g for [(3) H]raclopride and 12 pmol/g for [(3) H](+)PHNO; most of the latter sites disappeared in the presence of guanine nucleotide. Using 2 nM [(3) H](+)PHNO (K(d) of 0.72 nM at D2) to label D2 receptors in the striata and the human D2 clone, 10 nM to 100 nM dopamine inhibited 10-20% of the [(3) H](+)PHNO bound, representing high-affinity binding of [(3) H](+)PHNO, with the remainder inhibited above 100 nM dopamine, representing low-affinity binding of [(3) H](+)PHNO. It was found that (+)PHNO and (-)NPA dissociated from the D2 clone with half-times of 96 and 600 s, respectively. These rates are slower than the reported sub-second dissociation of the G protein from a receptor, suggesting that these two ligands still occupy the D2Low receptor after the G protein has separated. Thus, the radio-agonist label for (+)PHNO is not selective for dopamine D2High receptors, but also binds to the D2Low state of the dopamine receptor.

Cell membrane lytic action of metoclopramide and its relation to tardive dyskinesia.

The long-term use of many antipsychotic medications carries a risk of tardive dyskinesia in a small proportion of patients. Although metoclopramide is an antipsychotic at high doses, this drug is more commonly used at low daily doses to accelerate stomach movement of food. Because prolonged use of metoclopramide has also been associated with tardive dyskinesia, this drug is convenient to study to examine the possible basis of tardive dyskinesia. Previous work proposed that antipsychotics accumulated in the melanin granules of the human substantia nigra, ultimately building up to high concentrations that could disrupt cell membranes of nigral neurons. While previous work demonstrated the accumulation of metoclopramide in postmortem human nigral tissue, it remained to be tested whether high concentrations of metoclopramide would actually disrupt cell membranes. Therefore, the present work examined whether metoclopramide could disrupt cell membranes, using human erythrocytes directly exposed to various concentrations of metoclopramide in vitro. It was found that metoclopramide caused disruption of the red cells starting at a threshold of 1 mM, which would result in ~280 µmoles of metoclopramide per kilogram of dry red cell membranes. However, the nonspecific adsorption of metoclopramide to human substantia nigra is ~23 µmol/kg of dry solids (measured at the clinical spinal fluid concentration of metoclopramide). Therefore, the membrane-lytic concentration of metoclopramide is only about 12 times higher than that after a single exposure of the drug to the nigral tissue. Hence, metoclopramide accumulation in the substantia nigra over a matter of months may lead to nigral neuron damage.

Quantitative proteomic analysis of human substantia nigra in Alzheimer's disease, Huntington's disease and Multiple sclerosis.

The substantia nigra plays important roles in the brain function and is critical in the development of many diseases, particularly Parkinson's disease. Pathological changes of the substantia nigra have also been reported in other neurodegenerative diseases. Using a quantitative proteomic approach, we investigated protein expressions in the substantia nigra of Alzheimer's disease, Huntington's disease, and Multiple sclerosis. The expression level of one hundred and four proteins that were identified in at least three samples of each group were compared with the control group, with nineteen, twenty-two and thirteen proteins differentially expressed in Alzheimer's diseases, Huntington's disease and Multiple sclerosis respectively. The result indicates that the substantia nigra also undergoes functional adaption or damage in these diseases.

The dopamine D1-D2 receptor heteromer localizes in dynorphin/enkephalin neurons: increased high affinity state following amphetamine and in schizophrenia.

The distribution and function of neurons coexpressing the dopamine D1 and D2 receptors in the basal ganglia and mesolimbic system are unknown. We found a subset of medium spiny neurons coexpressing D1 and D2 receptors in varying densities throughout the basal ganglia, with the highest incidence in nucleus accumbens and globus pallidus and the lowest incidence in caudate putamen. These receptors formed D1-D2 receptor heteromers that were localized to cell bodies and presynaptic terminals. In rats, selective activation of D1-D2 heteromers increased grooming behavior and attenuated AMPA receptor GluR1 phosphorylation by calcium/calmodulin kinase IIα in nucleus accumbens, implying a role in reward pathways. D1-D2 heteromer sensitivity and functional activity was up-regulated in rat striatum by chronic amphetamine treatment and in globus pallidus from schizophrenia patients, indicating that the dopamine D1-D2 heteromer may contribute to psychopathologies of drug abuse, schizophrenia, or other disorders involving elevated dopamine transmission.

Alzheimer's disease: ß-amyloid plaque formation in human brain.

Although the precise cause of Alzheimer's disease is not known, the ß-amyloid peptide chains of 40-42 amino acids are suspected to contribute to the disease. The ß-amyloid precursor protein is found on many types of cell membranes, and the action of secretases (ß and γ) on this precursor protein normally releases the ß-amyloids at a high rate into the plasma and the cerebrospinal fluid. However, the concentrations of the ß-amyloids in the plasma and the spinal fluid vary considerably between laboratories. The ß-amyloids adsorb in the nanomolar concentration range to receptors on neuronal and glial cells. The ß-amyloids are internalized, become folded in the ß-folded or ß-pleated shape, and then stack on each other to form long fibrils and aggregates known as plaques. The ß-amyloids likely act as monomers, dimers, or multimers on cell membranes to interfere with neurotransmission and memory before the plaques build up. Treatment strategies include inhibitors of ß- and γ-secretase, as well as drugs and physiological compounds to prevent aggregation of the amyloids. Several immune approaches and a cholesterol-lowering strategy are also being tested to remove the ß-amyloids.

Antipsychotic drug binding in the substantia nigra: an examination of high metoclopramide binding in the brains of normal, Alzheimer's disease, Huntington's disease, and Multiple Sclerosis patients, and its relation to tardive dyskinesia.

This project was done in order to determine why the annual incidence of metoclopramide-associated tardive dyskinesia is much higher than that for the commonly used antipsychotics. To test the hypothesis that metoclopramide tardive dyskinesia may be associated with high concentrations of metoclopramide in the substantia nigra under clinical conditions, the nonspecific binding of tritiated antipsychotics to the dissected melaninized regions of postmortem human substantia nigra was measured. The nonspecific binding at 1 nM [³H]ligand was 7.3, 4.2, 2.6, 0.91 and 0.66 fmoles/mg for [³H]haloperidol, [³H]clozapine, [³H]raclopride, [³H]metoclopramide, and [³H]olanzapine, respectively. After adjusting these values for the known free concentrations of these drugs in plasma or spinal fluid, the amounts that would be bound under clinical conditions would be 231, 113, 15, 11, and 3.4 fmoles/mg for metoclopramide, clozapine, raclopride, haloperidol, and olanzapine, respectively. Using rat striatum as baseline to define antipsychotic binding to nonnigral tissue, the excess amount of binding to the Alzheimer nigral tissue under clinical conditions would be 209, 19, 0, 3.4 and 0.8 fmole/mg for metoclopramide, clozapine, raclopride, haloperidol, and olanzapine, respectively, with a similar pattern for nigral tissues from Huntington and Multiple Sclerosis patients. The high accumulation of metoclopramide is sufficiently high to cause nigral nerve cell membrane damage by metoclopramide's detergent-like action, possibly explaining metoclopramide's toxic ability to elicit early tardive dyskinesia. In addition, the nonspecific binding of metoclopramide was much higher in Alzheimer-diseased substantia nigra, consistent with the fact that older individuals are relatively more vulnerable to metoclopramide tardive dyskinesia.

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, D2high and D2low; 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 (Kd 0.8 nM) and potently displaces the binding of (R-(-)-N-n-propylnorapomorphine (NPA; Ki 0.16 nM) and raclopride (Ki 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 D2high 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.

Hyperactive mice show elevated D2(High) receptors, a model for schizophrenia: Calcium/calmodulin-dependent kinase II alpha knockouts.

The cerebral frontal cortex of patients who had schizophrenia shows elevated levels of RNA for calcium/calmodulin-dependent protein kinase II beta (CaMKIIbeta). In addition, recent research shows that animal models for schizophrenia, such as amphetamine-sensitized rats, consistently show elevated levels of D2 receptors in their high-affinity state (D2(High)), the major target for antipsychotic medication. The present study was done, therefore, to examine whether an alteration in the levels of CaMKIIbeta could lead to altered levels of D2(High) receptors. We found that the CaMKII inhibitor, KN-93, markedly reduced D2(High) states in rat striatum. In addition, we studied heterozygous CaMKIIalpha knock-out mice that show features analogous to schizophrenia. The striata of these mice revealed a 2.8-fold increase in D2(High) receptors. In frontal cortex of the heterozygous CaMKIIalpha knock-out mice, CaMKIIalpha mRNA levels were reduced by 50%, while CaMKIIbeta mRNA levels were unaltered. In striatum, CaMKIIbeta mRNA levels were increased by 29%, suggesting the presence of a new CaMKIIbeta regulatory pathway not previously described. The elevated levels of CaMKIIbeta mRNA in the striatum suggest that this enzyme may increase D2(High) in animals and possibly in schizophrenia itself.

Cocaine self-administration markedly increases dopamine D2 receptor negative cooperativity for dopamine binding: A receptor dimer-based analysis.

The proportion of dopamine D(2)(High) receptors has been reported to increase after cocaine self-administration without a significant change in the total number of D(2) receptors. In the present article, the data of competition by dopamine of [(3)H]domperidone binding to striatal samples from naïve and cocaine-addicted animals (Briand et al., [2008] Eur Neuropsychopharmacol 18:551-556) are analyzed assuming that D(2) receptors are constitutive dimers. The results show another way of interpreting those previous results and lead to the finding that cocaine affects agonist affinity and may strongly affect the receptor-receptor cooperation between dimers, a result that can contribute to dopamine supersensitivity.

Exposure to nicotine produces an increase in dopamine D2(High) receptors: a possible mechanism for dopamine hypersensitivity.

Dopamine D2 receptors exist in both low- and high-affinity states (D2(High)), the latter being the functionally relevant state. Cocaine self-administration produces an increase in D2(High), a phenomenon that could explain why cocaine administration results in hypersensitivity to dopamine, even though drug addicts were found to have a decreased number of striatal dopamine D2 receptors. As nicotine acts through the same mesocortical dopaminergic signaling pathways as other stimulant drugs, which are known to increase the levels of D2(High), we hypothesized that nicotine exposure could produce an increase in D2(High) levels. We determined D2(High) levels in rats after nicotine administration (1.5 mg/kg/day; 14 days), in rats voluntarily self-administering nicotine using an intravenous self-administration (IVSA) protocol (mean dose 0.5 mg/kg/day; 14 days), as well as after a prolonged withdrawal. An increase in the levels of D2(High) was found in rats who had nicotine administered at a uniform dose, as well as in rats who self-administered nicotine via IVSA, but these changes appear to normalize over time, as indicated by lower D2(High) levels in rats after a prolonged withdrawal period. We suggest that nicotine-induced elevation in D2(High) levels could be participating in hypersensitivity to dopamine following nicotine exposure.

Dopamine D2High receptors measured ex vivo are elevated in amphetamine-sensitized animals.

Although dopamine supersensitivity is a fundamental aspect of diseases such as schizophrenia and Parkinson's disease, the molecular basis of dopamine supersensitivity is not known. Because behavioral dopamine supersensitivity is associated with a marked elevation of striatal dopamine D2(High) receptors in vitro, it is important to develop methods to measure D2(High) receptors in vivo. The present ex vivo study found that the dopamine agonist NPA ([-]-N-propyl-norapomorphine) inhibited the binding of the agonist [(3)H](+)PHNO to rat striatal D2 receptors significantly more than the D2 antagonist [(3)H]raclopride, when NPA was coinjected i.v. with each radioligand. These results suggest that the greater sensitivity of [(3)H](+)PHNO to inhibition by the coinjected NPA reflects in vivo competition at D2(High) receptors. Using rats that had been sensitized to amphetamine, this ex vivo method found that the specific binding of [(3)H](+)PHNO that was displaced by 10 microg/kg of NPA was 2.4-fold higher than that for control rats. These data agree with in vitro data showing a marked increase in D2(High) sites after amphetamine sensitization. Therefore, it is recommended that this method of co-injecting the D2 radioligand and the dopamine agonist displacer be used in human positron tomography to detect D2(High) receptors in health and disease.

Glutamate agonists for treating schizophrenia have affinity for dopamine D2High and D3 receptors.

Although the glutamate agonist LY 404,039 has been used to treat schizophrenia, its closest congener LY 379,268 has an affinity for both glutamate and dopamine (DA) D2(High) receptors. Considering that all antipsychotics act on dopamine receptors, and considering that another laboratory reported that LY 379,268 did not have any affinity for the D2(High) receptor, it was necessary to examine whether such glutamate agonists have an affinity for D2 and D3 dopamine receptors in vitro. The present data show that 50-200 nM LY 379,268 inhibited the binding of [(3)H]domperidone and [(3)H](+)PHNO to cloned dopamine D2 receptors consistently and reproducibly by 16% with dissociation constants of 2.1 and 2.5 nM at D2(High), respectively. In addition, LY 379,268 inhibited the binding of [(3)H]domperidone and [(3)H](+)PHNO to cloned dopamine D3 receptors with dissociation constants of 130 and 10 nM, respectively. LY 379,268 also inhibited the binding of [(3)H]domperidone to rat striata with a dissociation constant of 22 nM, predicting a clinical antipsychotic dose of 80-100 mg/day. LY 379,268 appears to act as an agonist at D2(High) and as an antagonist at D3, because guanine nucleotide eliminated the competition at D2(High) but had no effect on the competition at D3. The findings indicate that this type of glutamate agonist, LY 379,268, has a significant affinity for D2(High) and D3 receptors.

Glutamate agonist LY404,039 for treating schizophrenia has affinity for the dopamine D2(High) receptor.

The glutamate agonist LY404,039 has been used to treat schizophrenia. Because all currently used antipsychotics act on dopamine receptors, it was decided to examine whether this glutamate agonist also had an affinity for dopamine D2 receptors in vitro. The present data show that LY404,039 inhibited the binding of [3H]domperidone and [3H]+PHNO by 15.5 +/- 1.5% to the high-affinity state, D2(High), of cloned dopamine D2(Long) receptors and rat striatal tissue with dissociation constants of between 8.2 and 12.6 nM. This high-affinity component of LY404,039 on the binding of [3H]domperidone was inhibited by the presence of guanine nucleotide, indicating an agonist action of the drug at D2(High). LY404,039 also stimulated the incorporation of [35S]GTP-gamma-S into D2(Long) receptors (EC50% = 80 +/- 15 nM) over the same range of concentrations as occurred for the inhibition of [3H]domperidone by LY404,039 at D2(High) (IC50%(High) = 50 +/- 10 nM). A possible clinical antipsychotic action of LY404,039 may depend on the combined stimulation of glutamate receptors and a partial dopamine agonist action that would interfere with neurotransmission at D2(High) receptors.

Dopamine D2High receptors stimulated by phencyclidines, lysergic acid diethylamide, salvinorin A, and modafinil.

Although it is commonly stated that phencyclidine is an antagonist at ionotropic glutamate receptors, there has been little measure of its potency on other receptors in brain tissue. Although we previously reported that phencyclidine stimulated cloned-dopamine D2Long and D2Short receptors, others reported that phencyclidine did not stimulate D2 receptors in homogenates of rat brain striatum. This study, therefore, examined whether phencyclidine and other hallucinogens and psychostimulants could stimulate the incorporation of [(35)S]GTP-gamma-S into D2 receptors in homogenates of rat brain striatum, using the same conditions as previously used to study the cloned D2 receptors. Using 10 microM dopamine to define 100% stimulation, phencyclidine elicited a maximum incorporation of 46% in rat striata, with a half-maximum concentration of 70 nM for phencyclidine, when compared with 80 nM for dopamine, 89 nM for salvinorin A (48 nM for D2Long), 105 nM for lysergic acid diethylamide (LSD), 120 nM for R-modafinil, 710 nM for dizocilpine, 1030 nM for ketamine, and >10,000 nM for S-modafinil. These compounds also inhibited the binding of the D2-selective ligand [(3)H]domperidone. The incorporation was inhibited by the presence of 200 microM guanylylimidodiphosphate and also by D2 blockade, using 10 microM S-sulpiride, but not by D1 blockade with 10 microM SCH23390. Hypertonic buffer containing 150 mM NaCl inhibited the stimulation by phencyclidine, which may explain negative results by others. It is concluded that phencyclidine and other psychostimulants and hallucinogens can stimulate dopamine D2 receptors at concentrations related to their behavioral actions.

Glutamate receptor mGlu2 and mGlu3 knockout striata are dopamine supersensitive, with elevated D2(High) receptors and marked supersensitivity to the dopamine agonist (+)PHNO.

The finding that the mGlu2/3 metabotropic glutamate receptor agonist, LY404039, improves clinical symptoms in schizophrenia warrants a search for a possible interaction between mGlu2/3 receptors and dopamine D2 receptors. Here, this topic is examined in striatal tissue of mice lacking either mGlu2 or mGlu3 receptor. Such mice are known to be behaviorally supersensitive to dopamine receptor agonists. Therefore, to determine the basis of this dopamine supersensitivity, the proportion of dopamine D2(High) receptors was measured in the striata of mGlu2 and mGlu3 receptor knockout mice. The proportion of D2(High) receptors was found to be elevated by 220% in the striata of both knockouts. To measure the functional dopamine supersensitivity, the D2 agonist (+)PHNO was used to stimulate the incorporation of GTP-gamma-S in the striatal homogenates in the presence of drugs that blocked the dopamine D1, D3, and D5 receptors. Compared with control striata, the mGlu2 receptor knockout tissues were 67-fold more sensitive to (+)PHNO, while the mGlu3 receptor knockout tissues were 17-fold more sensitive. These data suggest that group II mGlu receptors-mGlu2 receptors in particular-may normally regulate D2 receptors by reducing the proportion of high-affinity D2 receptors in membranes. Such regulation may contribute to the antipsychotic action of mGlu2/3 receptor agonists.

Rats with persistently high exploratory activity have both higher extracellular dopamine levels and higher proportion of D(2) (High) receptors in the striatum.

Increases in both striatal dopamine release and the proportion of the D(2) receptors in the high affinity state (D(2) (High)) accompany the behavioral sensitization to psychostimulants, but it is not known whether the physiological substrate of the interindividual differences locomotor and exploratory behavior is similar. Thus, we examined whether persistently high spontaneous exploratory activity is associated with extracellular dopamine as well as the proportion of D(2) (High) in the striatum. Extracellular dopamine levels were found to be significantly higher in rats with high exploratory activity (high explorers, HE) as compared with low explorers (LE) in baseline conditions as well as after administration of amphetamine (0.5 mg/kg, i.p.). Also, the HE animals had significantly higher proportion of striatal D(2) (High) receptors than the LE-rats (43.8 +/- 4.4% and 22.5 +/- 1.5%, respectively). Thus, the present findings support the notion that concomitant higher extracellular dopamine levels and the proportion of D(2) (High) receptors in the striatum, whether naturally occurring and persistent or pharmacologically induced, are causally related to high behavioral activity.

Increased dopamine D2High receptors in rats reared in social isolation.

Postweaning social isolation in the rat induces lasting alterations that parallel several of the core symptoms seen in human schizophrenics, including hyperreactivity to novel environments, cognitive impairment, and deficits in sensorimotor gating. The current study determined whether these changes are accompanied by any elevation in the proportion of striatal dopamine receptors in the functional high affinity state (D(2) (High)), as observed in other preclinical models of psychosis. Male Lister hooded rats (20-24 days) were housed in groups of three or alone. On Day 36 postweaning locomotor activity was monitored for 60 min in a novel arena, and on Day 37 novel object discrimination was assessed using a 2 h intertrial interval. Three days later striata were collected, homogenized, washed three times to remove endogenous dopamine, and the proportion of D(2) (High) determined by competition between dopamine and 2.27 nM [(3)H]domperidone. Isolates were significantly more active than group housed controls for both ambulation and rears. Although both groups exhibited comparable levels of familiarization trial object exploration, group housed animals were able to discriminate between novel and familiar objects during the choice trial while isolates were not. Social isolation was associated with a highly significant elevation in the proportion of striatal D(2) (High), equivalent to a 3.3-fold increase (group 15.2% +/- 1.4%, isolate 49.8% +/- 4.8%; P < 0.0001, Student's unpaired t-test). These findings support both the hypothesis that elevated D(2) (High) is a common feature of multiple animal models of psychosis, and the validity of isolation rearing as a neurodevelopmental model of a "schizophrenia-like" state.