Behavioral sensitization, alternative splicing, and d3 dopamine receptor-mediated inhibitory function

Behavioral sensitization, the progressive and enduring augmentation of certain behaviors following repetitive drug use, alters rodent locomotion in a long-standing manner. The same dopamine pathways playing an important role in drug dependence and psychosis also play a critical role in sensitization. Individual dopamine receptor subtypes have markedly different functional responses to stimulation, with D3 dopamine receptor stimulation inhibiting rodent locomotion. The D3 receptor has highest affinity of the dopamine receptor subtypes for dopamine, and is occupied to a greater degree following stimulant drug administration. D3 receptor activity may be regulated through the expression of an alternatively spliced, truncated receptor isoform (termed 'D3nf') altering receptor localization and function via dimerization with the full-length subunit. The expected physiological response to repetitive drug administration is tolerance. Tolerance of D3 receptor inhibition of locomotion would contribute to sensitization to stimulant drugs. We hypothesize that repetitive D3 receptor stimulation contributes to the development of behavioral sensitization through decreased responsivity of D3-receptor-mediated locomotor inhibition. Increased D3nf expression may direct altered receptor localization and subsequent release of D3-receptor-mediated inhibition, contributing to the expression of sensitization. These hypotheses follow directly from the affinities of the receptor subtypes for dopamine; dopamine concentrations following stimulant administration; the effects of individual dopamine receptor subtype stimulation on locomotion; and the expected homeostatic response of the system to perturbation by drug. Clarifying these mechanisms underlying sensitization may suggest new interventions for neuropsychiatric conditions in which dopamine plays an important role, including psychosis, drug dependence, and Parkinson's disease. This information may also elucidate a previously unrecognized mechanism regulating receptor trafficking and desensitization.

Dose-response analysis of locomotor activity and stereotypy in dopamine D3 receptor mutant mice following acute amphetamine

Accumulating evidence suggests that dopamine D3 receptor (D3R) stimulation is inhibitory to spontaneous and psychostimulant-induced locomotion through opposition of concurrent D1R and D2R-mediated signaling. To evaluate this model, we used homozygous D3R mutant mice and wild-type controls to investigate the role of the D3R in locomotor activity and stereotypy stimulated by acute amphetamine (AMPH) (0.2, 2.5, 5.0, 10.0 mg/kg). At the lowest dose tested (0.2 mg/kg), neither D3R mutant mice nor wild-type mice exhibited measurable change in locomotor activity or stereotypy relative to their respective saline-treated controls. D3R mutant mice exhibited a significantly greater increase in locomotor activity, but not stereotypy, relative to wild-type mice in response to treatment with AMPH 2.5 mg/kg. AMPH-induced locomotor activity and stereotypy were similar in both wild-type and D3R mutant mice at both the 5.0 and 10 mg/kg AMPH doses. These findings provide further support for an inhibitory role for the D3R in AMPH-induced locomotor activity, and demonstrate a more limited role for the D3R in modulating AMPH-induced stereotypy.

Modulation of phosphoinositide-protein kinase C signal transduction by omega-3 fatty acids: implications for the pathophysiology and treatment of recurrent neuropsychiatric illness

The phosphoinositide (PI)-protein kinase C (PKC) signal transduction pathway is initiated by pre- and postsynaptic Galphaq-coupled receptors, and regulates several clinically relevant neurochemical events, including neurotransmitter release efficacy, monoamine receptor function and trafficking, monoamine transporter function and trafficking, axonal myelination, and gene expression. Mounting evidence for PI-PKC signaling hyperactivity in the peripheral (platelets) and central (premortem and postmortem brain) tissues of patients with schizophrenia, bipolar disorder, and major depressive disorder, coupled with evidence that PI-PKC signal transduction is down-regulated in rat brain following chronic, but not acute, treatment with antipsychotic, mood-stabilizer, and antidepressant medications, suggest that PI-PKC hyperactivity is central to an underlying pathophysiology. Evidence that membrane omega-3 fatty acids act as endogenous antagonists of the PI-PKC signal transduction pathway, coupled with evidence that omega-3 fatty acid deficiency is observed in peripheral and central tissues of patients with schizophrenia, bipolar disorder, and major depressive disorder, support the hypothesis that omega-3 fatty acid deficiency may contribute to elevated PI-PKC activity in these illnesses. The data reviewed in this paper outline a potential molecular mechanism by which omega-3 fatty acids could contribute to the pathophysiology and treatment of recurrent neuropsychiatric illness.

C57BL/6J mice exhibit reduced dopamine D3 receptor-mediated locomotor-inhibitory function relative to DBA/2J mice

Previous reports have identified greater sensitivity to the locomotor-stimulating, sensitizing, and reinforcing effects of amphetamine in inbred C57BL/6J mice relative to inbred DBA/2J mice. The dopamine D3 receptor (D3R) plays an inhibitory role in the regulation of rodent locomotor activity, and exerts inhibitory opposition to D1 receptor (D1R)-mediated signaling. Based on these observations, we investigated D3R expression and D3R-mediated locomotor-inhibitory function, as well as D1R binding and D1R-mediated locomotor-stimulating function, in C57BL/6J and DBA/2J mice. C57BL/6J mice exhibited lower D3R binding density (-32%) in the ventral striatum (nucleus accumbens/islands of Calleja), lower D3R mRNA expression (-26%) in the substantia nigra/ventral tegmentum, and greater D3R mRNA expression (+40%) in the hippocampus, relative to DBA/2J mice. There were no strain differences in DR3 mRNA expression in the ventral striatum or prefrontal cortex, nor were there differences in D1R binding in the ventral striatum. Behaviorally, C57BL/6J mice were less sensitive to the locomotor-inhibitory effect of the D3R agonist PD128907 (10 microg/kg), and more sensitive to the locomotor-stimulating effects of novelty, amphetamine (1 mg/kg), and the D1R-like agonist +/- -1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8,-diol hydrochloride (SKF38393) (5-20 mg/kg) than DBA/2J mice. While the selective D3R antagonist N-(4-[4-{2,3-dichlorphenyl}-1 piperazinyl]butyl)-2-fluorenylcarboxamide (NGB 2904) (0.01-1.0 mg/kg) augmented novelty-, amphetamine-, and SKF38393-induced locomotor activity in DBA/2J mice, it reduced novelty-induced locomotor activity in C57BL/6J mice. Collectively, these results demonstrate that C57BL/6J mice exhibit less D3R-mediated inhibitory function relative to DBA/2J mice, and suggest that reduced D3R-mediated inhibitory function may contribute to heightened sensitivity to the locomotor-stimulating effects of amphetamine in the C57BL/6J mouse strain. Furthermore, these data demonstrate that comparisons between C57BL/6J and DBA/2J mouse strains provide a model for elucidating the molecular determinants of genetic influence on D3R function.

Relative expression of D3 dopamine receptor and alternative splice variant D3nf mRNA in high and low responders to novelty

Studies in rodents suggest an important role for the D3 dopamine receptor in regulating locomotor responses to spatial novelty and psychostimulants. The D3 receptor alternatively spliced variant D3nf produces a non-dopamine binding protein that may alter D3 receptor localization by dimerizing with the full-length receptor. In the high responder/low responder (HR/LR) model, the locomotor response to an inescapable, novel spatial environment predicts individual differences in the locomotor and rewarding effects of psychostimulants. We hypothesized that individual differences in D3 receptor expression could contribute to individual differences in the locomotor response to novelty in the HR/LR model. To test this hypothesis, we screened rats for response to a novel spatial environment and analyzed brain tissue for mRNA levels of the D3 receptor and D3nf by real-time RT-PCR. The ratios of D3/D3nf mRNA in prefrontal cortex and substantia nigra/ventral tegmentum were significantly lower in HRs than in LRs. There were no differences in relative expression of D3/D3nf between HRs and LRs in nucleus accumbens. These data further support a role for the D3 dopamine receptor in behavioral responses to novelty and, given the established relationship between novelty and psychostimulant responses, suggest that the D3 receptor may be an important target for assessment of drug abuse vulnerability. Additionally, these findings are consistent with the hypothesis that alternative splicing may contribute to regulation of D3 dopamine receptor function.

Effects of cytochrome P450 3A modulators ketoconazole and carbamazepine on quetiapine pharmacokinetics

AIMS

To explore the potential for drug interactions on quetiapine pharmacokinetics using in vitro and in vivo assessments.

METHODS

The CYP enzymes responsible for quetiapine metabolite formation were assessed using recombinant expressed CYPs and CYP-selective inhibitors. P-glycoprotein (Pgp) transport was tested in MDCK cells expressing the human MDR1 gene. The effects of CYP3A4 inhibition were evaluated clinically in 12 healthy volunteers that received 25 mg quetiapine before and after 4 days of treatment with ketoconazole 200 mg daily. To assess CYP3A4 induction in vivo, 18 patients with psychiatric disorders were titrated to steady-state quetiapine levels (300 mg twice daily), then titrated to 600 mg daily carbamazepine for 2 weeks.

RESULTS

CYP3A4 was found to be responsible for formation of quetiapine sulfoxide and N- and O-desalkylquetiapine and not a Pgp substrate. In the clinical studies, ketoconazole increased mean quetiapine plasma C(max) by 3.35-fold, from 45 to 150 ng ml(-1) (mean C(max) ratio 90% CI 2.51, 4.47) and decreased its clearance (Cl/F) by 84%, from 138 to 22 l h(-1) (mean ratio 90% CI 0.13, 0.20). Carbamazepine decreased quetiapine plasma C(max) by 80%, from 1042 to 205 ng ml(-1) (mean C(max) ratio 90% CI 0.14, 0.28) and increased its clearance 7.5-fold, from 65 to 483 l h(-1) (mean ratio 90% CI 6.04, 9.28).

CONCLUSIONS

Cytochrome P450 3A4 is a primary enzyme responsible for the metabolic clearance of quetiapine. Quetiapine pharmacokinetics were affected by concomitant administration of ketoconazole and carbamazepine, and therefore other drugs and ingested natural products that strongly modulate the activity or expression of CYP3A4 would be predicted to change exposure to quetiapine.

Hypoactivity of the hypothalamo-pituitary-adrenocortical axis during recovery from chronic variable stress

Chronic stress induces both functional and structural adaptations within the hypothalamo-pituitary-adrenocortical (HPA) axis, suggestive of long-term alterations in neuroendocrine reactivity to subsequent stressors. We hypothesized that prior chronic stress would produce persistent enhancement of HPA axis reactivity to novel stressors. Adult male rats were exposed to chronic variable stress (CVS) for 1 wk and allowed to recover. Plasma ACTH and corticosterone levels were measured in control or CVS rats exposed to novel psychogenic (novel environment or restraint) or systemic (hypoxia) stressors at 16 h, 4 d, 7 d, or 30 d after CVS cessation. Plasma ACTH and corticosterone responses to psychogenic stressors were attenuated at 4 d (novel environment and restraint) and 7 d (novel environment only) recovery from CVS, whereas hormonal responses to the systemic stressor were largely unaffected by CVS. CRH mRNA expression was up-regulated in the paraventricular nucleus of the hypothalamus (PVN) at 16 h after cessation of CVS, but no other alterations in PVN CRH or arginine vasopressin mRNA expression were observed. Thus, in contrast to our hypothesis, reductions of HPA axis sensitivity to psychogenic stressors manifested at delayed recovery time points after CVS. The capacity of the HPA axis to respond to a systemic stressor appeared largely intact during recovery from CVS. These data suggest that chronic stress selectively targets brain circuits responsible for integration of psychogenic stimuli, resulting in decreased HPA axis responsiveness, possibly mediated in part by transitory alterations in PVN CRH expression.

Risperidone pretreatment prevents elevated locomotor activity following neonatal hippocampal lesions

Long-standing behavioral abnormalities emerge after puberty in rats following neonatal hippocampal lesion, providing a developmental model of abnormal rat behavior that may have predictive validity in identifying compounds effective in treating symptoms of schizophrenia. We sought to test the predictive validity of the neonatal hippocampal lesion model in identifying preventive treatment for first-episode psychosis. We determined the effect of risperidone, recently studied for prevention of first-episode psychosis, on the development of elevated locomotor activity following neonatal hippocampal lesions. Rat pups received hippocampal or sham lesions on postnatal day 7, followed by treatment with risperidone or vehicle from postnatal days 35 to 56. Locomotor activity in response to novelty, amphetamine, and nocturnal locomotion were determined on postnatal day 57. Low-dose risperidone (45 microg/kg) pretreatment prevented elevated locomotor activity in some, but not all, of the behavioral tasks following neonatal hippocampal lesions. In contrast, higher risperidone pretreatment was less effective in preventing elevated locomotor activity following neonatal hippocampal lesions. Because low risperidone dosages were also found to be effective in preventing first-episode psychosis in human studies, these data support the predictive validity of the hippocampal lesion model in identifying medications for prevention of first-episode psychosis. Additionally, these data support the use of low-dose risperidone in psychosis prevention, and suggest the possibility that higher risperidone doses could be less effective in this application.

Expression of the glucocorticoid-induced receptor mRNA in rat brain

The glucocorticoid-induced receptor (GIR) is an orphan G-protein-coupled receptor awaiting pharmacological characterization. GIR was originally identified in murine thymoma cells, and shows a widespread, yet not completely complementary distribution in mouse and human brain. Expression of the mouse GIR gene is modulated by dexamethasone in the brain and periphery, suggesting that GIR function is directly responsive to glucocorticoid signals. The rat GIR was cloned from rat prefrontal cortex by our group and was shown to be up-regulated following chronic amphetamine. The physiological role of GIR in the rat is not known at present. In order to gain a clearer understanding of the potential functions of GIR in the rat, we performed a detailed mapping of GIR mRNA expression in the rat brain. GIR mRNA showed widespread distribution in forebrain limbic and thalamic structures, and a more restricted distribution in hindbrain areas such as the spinal trigeminal nucleus and the median raphe nucleus. Areas with moderate to high levels of GIR include olfactory regions such as the nucleus of olfactory tract, hippocampus, various thalamic nuclei, cortical layers, and some hypothalamic nuclei. In comparison with previous studies, significant regional differences exist in GIR distribution in mouse and rat brain, particularly in the thalamus, striatum and in hippocampus at a cellular level. Overall, the expression of GIR in rat brain more closely approaches that seen previously in human than mouse, suggesting that rat models may be more informative for understanding the role of GIR in glucocorticoid physiology and glucocorticoid-related disease states. GIR mRNA distribution in the rat indicates a potential role of this receptor in the control of feeding and ingestive behavior, regulation of stress and emotional behavior, learning and memory, and, drug reinforcement and reward.

Stress and amphetamine induce Fos expression in medial prefrontal cortex neurons containing glucocorticoid receptors

Exposure to stress or amphetamine potently activates the immediate early gene, c-fos, within medial prefrontal cortex neurons, but the phenotype of these neurons is not known. Fluorescence immunohistochemistry was used to determine that a large subpopulation of medial prefrontal cortex cells expressing Fos protein after restraint and amphetamine also co-express nuclear glucocorticoid receptors (GRs). These findings suggest exposure to amphetamine activates the same medial prefrontal cortex regions responsible for integration of responses to stress, and suggest the potential for AP1-glucocorticoid cross-talk in these cell populations.

Altered behavioral response to dopamine D3 receptor agonists 7-OH-DPAT and PD 128907 following repetitive amphetamine administration

Behavioral sensitization, the progressive and enduring enhancement of certain behaviors following repetitive drug use, is mediated in part by dopaminergic pathways. Increased locomotor response to drug treatment, a sensitizable behavior, is modulated by an opposing balance of dopamine receptor subtypes, with D1/D2 dopamine receptor stimulation increasing and D3 dopamine receptor activation inhibiting amphetamine-induced locomotion. We hypothesize that tolerance of D3 receptor locomotor inhibition contributes to behavioral sensitization. In order to test the hypothesis that expression of behavioral sensitization results in part from release of D3 receptor-mediated inhibition, thereby resulting in decreased response to D3 receptor agonists, we examined the effect of repetitive amphetamine administration on the behavioral response to the D3 receptor preferring agonists 7-OH-DPAT and PD 128907. D3-selective effects have recently been described for both drugs at a low dose. At 1 week following completion of a repetitive treatment regimen, amphetamine-pretreated rats displayed a decreased response to D3-selective doses of both 7-OH-DPAT and PD 128907, when compared to animals receiving saline pretreatment. Moreover, in addition to the quantitative alteration in response, there was a change in the inter-relation between response to amphetamine and D3 agonist. A highly significant inverse relation between locomotor inhibitory response to PD 128907 and the locomotor-stimulant response to amphetamine was observed prior to amphetamine treatment. In contrast, 10 days following repetitive amphetamine treatment, the relation between response to PD 128907 and amphetamine was not detected. The observed behavioral alteration could not be accounted for by changes in D3 receptor binding in ventral striatum. These findings suggest a persistent release of D3 receptor-mediated inhibitory influence contributes to the expression of behavioral sensitization to amphetamine.

7-OH-DPAT and PD 128907 selectively activate the D3 dopamine receptor in a novel environment

The D3 dopamine receptor is expressed primarily in limbic brain areas, and appears to play an inhibitory role in rodent locomotor behavior. Evidence suggests a potential role for the D3 receptor in the pathology of neuropsychiatric disease. Progress in elucidating D3 receptor function has been hampered, however, by a lack of well-characterized, selective ligands and by conflicting information regarding the behavioral phenotype of D3 receptor knockout mice. Here, we describe studies evaluating the behavioral effects of (+/-)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) and PD 128907, two D3 receptor agonists whose in vivo selectivity has been a topic of considerable controversy. We demonstrate that both compounds inhibit locomotion under novel environmental conditions in wild-type (WT) mice, but are without measurable behavioral effect under identical conditions in D3 receptor knockout mice. Additionally, we demonstrate that at low, D3 selective doses, these compounds are without behavioral effect in both WT and D3 receptor knockout mice that have acclimated to the testing environment. These findings suggest that D3 receptor stimulation inhibits novelty-stimulated locomotion, and establish conditions for the use of 7-OH-DPAT and PD 128907 as D3 receptor agonists in vivo. Potential implications of these observations are discussed.

The D3 dopamine receptor and substance dependence

Behavioral sensitization, the progressive and enduring enhancement of certain stimulant-induced behaviors following repetitive drug use, is mediated in part by dopaminergic pathways known to play a role in drug dependence. It has been theorized that sensitization underlies the development of drug craving and initiates addictive behaviors of drug dependence. We propose that down-regulation of D3 dopamine receptor function contributes to sensitization. Rodent locomotion is regulated by the opposing influence of dopamine receptor subtypes, with D3 stimulation inhibiting and concurrent D1/D2 receptor activation stimulating locomotion. The D3 receptor has greater occupancy than D1 or D2 receptors following stimulant drug administration. Sensitization may therefore result in part from greater accommodation of the inhibitory D3 receptor "brake" on locomotion, leading to progressive locomotion increase following repeated stimulant exposure. Further study is needed to test this proposed model, and to clarify the role of individual dopamine receptor subtypes in sensitization and drug dependence.

Regression modeling of rodent locomotion data

We present the case for a model-based approach to the statistical analysis of rodent locomotion data for which previous research has established that the time course of behavioral change follows a specified parametric form. Inadequacies of statistical methods commonly applied to such data are described. Regression models can reveal time-dependent aspects of response that might escape more elementary analyses, as we illustrate with experimental data regarding the locomotion response to amphetamine. Inter-individual variation in response profiles is incorporated through the use of random coefficients (i.e. mixed effects models). These models provide an effective means of quantifying the extent of individual differences in behavioral response to stimulant drugs not attributable to treatment. The need to adjust for multiple comparisons when searching for regions of significant difference is discussed. We present specific models that we have found useful for characterizing responses to low doses of amphetamine or cocaine (the log-logistic random effects model) and higher or repetitive doses of amphetamine or cocaine (the polynomial random effects model). Code for performing these analyses with standard statistical software is provided.

Cloning, expression, and regulation of a glucocorticoid-induced receptor in rat brain: effect of repetitive amphetamine

Behavioral sensitization to psychostimulants involves neuroadaptation of stress-responsive systems. We have identified and sequenced a glucocorticoid-induced receptor (GIR) cDNA from rat prefrontal cortex. The full-length GIR cDNA encodes a 422 amino acid protein belonging to G-protein-coupled receptor superfamily. Although the ligand for GIR is still unknown, the dendrogram construction indicates that GIR may belong to peptide receptor subfamily (e.g., substance P receptor), with more distant relationship to subfamilies of glycoprotein hormone receptors (e.g., thyrotropin receptor) and biogenic amine receptors (e.g., dopamine receptor). GIR shares 31-34% amino acid identity to the tachykinin receptors (substance P receptor, neurokinin A receptor, and neurokinin B receptor). GIR mRNA is expressed preferentially in brain, and its neuronal expression is relegated to limbic brain regions, particularly in forebrain. GIR transcript levels are increased significantly and persistently in prefrontal cortex for 7 d after discontinuation of chronic amphetamine exposure. The induction of GIR expression by amphetamine is associated with augmented behavioral activation. These findings suggest that modulation of GIR expression may be involved in behavioral sensitization, and GIR may play a role at the interface between stress and neuroadaptation to psychostimulants.

D3 dopamine receptor, behavioral sensitization, and psychosis

Behavioral sensitization is a progressive, enduring enhancement of behaviors that develops following repeated stimulant administration. It is mediated in part by dopaminergic pathways that also modulate a number of psychiatric conditions including the development of psychosis. We propose that down-regulation of D3 dopamine receptor function in critical brain regions contributes to sensitization. Rodent locomotion, a sensitizable behavior, is regulated by the opposing influence of dopamine receptor subtypes, with D3 stimulation opposing concurrent D1 and D2 receptor activation. The D3 dopamine receptor has a 70-fold greater affinity for dopamine than D1 or D2 dopamine receptors. This imbalance in ligand affinity dictates greater occupancy for D3 than D1 or D2 receptors at typical dopamine concentrations following stimulant drug administration, resulting in differences in the relative tolerance at D3 vs D1 and D2 receptors. Sensitization may therefore result in part from accommodation of the inhibitory D3 receptor 'brake' on D1/D2 mediated behaviors, leading to a progressive locomotion increase following repeated stimulant exposure. The requirement for differential tolerance at D3 vs D1 and D2 receptors may explain the observed development of sensitization following application of cocaine, but not amphetamine, directly into nucleus accumbens. If correct, the 'D3 Dopamine Receptor Hypothesis' suggests D3 antagonists could prevent sensitization, and may interrupt the development of psychosis when administered during the prodromal phase of psychotic illness. Additional study is needed to clarify the role of the D3 dopamine receptor in sensitization and psychosis.

Uncoupling of serotonergic and noradrenergic systems in depression: preliminary evidence from continuous cerebrospinal fluid sampling

We used the technique of continuous cerebrospinal fluid (CSF) sampling to test the following hypotheses regarding CNS monoaminergic systems in depression: (1) absolute concentrations of the informational substances tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) are altered in the CNS of depressed patients (2) abnormal rhythms of tryptophan and/or 5-HIAA, or defective conversion of tryptophan to serotonin (5HT), exist in the CNS of depressed patients, and (3) the relationship between the CNS 5HT and norepinephrine (NE) systems is disrupted in depressed patients. We obtained 6-h concentration time series of tryptophan, 5-HIAA, NE, and 3-methoxy-4-hydroxyphenylglycol (MHPG) in the CSF of 10 patients with major depression and in 10 normal volunteers. No significant differences in CSF tryptophan, 5-HIAA, NE, or MHPG concentrations or rhythms were observed between normal volunteers and depressed patients. Neither were there differences in the mean tryptophan-to-serotonin ratio. However, a negative linear relationship was observed between mean concentrations of 5-HIAA and NE in the CSF of the normal volunteers (r = 0.916 [r2 = 0.839], df = 9, P < 0.001) while, in contrast, depressed patients showed no such relationship (r = +0.094 [r2 = 0.00877], df = 9, n.s.). Furthermore, the correlation coefficients expressing the relationship between CSF MHPG and CSF 5-HIAA within the normal and depressed groups were significantly different. These data support the hypothesis that a disturbance in the interaction between the serotonergic and noradrenergic systems can exist in depressive illness in the absence of any simple 5HT or NE deficit or surplus.

The dopamine D3 receptor antagonist nafadotride inhibits development of locomotor sensitization to amphetamine

Behavioral sensitization is a well-studied model of behavioral plasticity mediated at least in part by dopaminergic systems believed to play an important role in several psychiatric conditions. In the rodent, locomotion is regulated by the opposing balance of D3 and D2 receptors, with D2 activation increasing and D3 stimulation inhibiting locomotion. However, receptor occupancy of D3 dopamine receptors is far greater than D2 or D1 occupancy at typical post-stimulant dopamine concentrations. We therefore hypothesized that tolerance of D3 receptor inhibition of locomotion contributes to the development of sensitization. To test this hypothesis, we examined the effect of the D3 receptor antagonist nafadotride on sensitization. As predicted, nafadotride inhibits augmentation of the locomotion response to repetitive amphetamine. This finding is consistent with the proposed model of adaptive down-regulation of D3 dopamine receptor function contributing to the development of behavioral sensitization.

Olfactory receptor neurons express D2 dopamine receptors

The role of the dopamine (DA) in the olfactory bulb (OB) was explored by determining which of the potential target cells express dopamine receptors (DARs). Previously, it was reported that D2-like DAR (D2, D3, and D4 subtypes) radioligand binding is restricted to the outer layers of the OB. The neuronal elements present only in these layers are the axons of the olfactory receptor neurons (ORNs) and the juxtaglomerular (JG) neurons of the glomerular layer. Based on this pattern of D2-like ligand binding, it was suggested that D2-like receptors might be located presynaptically on ORN terminals. The present study was undertaken to investigate this hypothesis. In the outer bulb layers of rats in which the ORNs were destroyed by nasal lavage with ZnSO(4), D2-like radioligand binding was reduced severely. The receptor subtype D2 mRNA, but not D3 mRNA, was detected in adult rat olfactory epithelial tissue. By using in situ hybridization, this D2 mRNA was located preferentially in epithelial layers that contain ORN perikarya. D2 mRNA was eliminated after bulbectomy, a manipulation known to cause retrograde degeneration of the mature ORNs. Taken together, the surgical manipulations indicate that mature ORNs express D2 DARs and are consistent with the hypothesis that functional receptors are translocated to their axons and terminals in the bulb. This suggests that dopamine released from JG interneurons could be capable of presynaptically influencing neurotransmission from the olfactory nerve terminals to OB target cells through the D2 receptor.

Quantification of dopamine D3 receptor mRNA level associated with the development of amphetamine-induced behavioral sensitization in the rat brain

We hypothesized that changes in expression of dopamine (DA) D3 receptor gene in the rat brain would correlate with the behavioral sensitization induced by amphetamine (AMPH). In order to test this hypothesis, we measured D3 receptor mRNA levels in the striatum, nucleus accumbens and prefrontal cortex, in individual rats following AMPH treatment (2.5 mg/kg s.c., for 5 consecutive days) using a ribonuclease protection assay method. We observed similar levels of D3 receptor mRNA in saline and AMPH treated animals in each brain region examined. These results suggest behavioral sensitization to AMPH is not mediated through postsynaptic transcriptional regulation of D3 receptor.

A case of treatment-refractory psychosis responsive to sertindole

Sertindole is an antipsychotic with atypical in vitro and ex vivo binding profiles and little propensity to cause extrapyramidal symptoms. However, its potential usefulness in the treatment of psychosis resistant to the 'classical' neuroleptics has not been determined. In the present study we used a double blind, placebo-controlled trial of sertindole and observed dramatic, sustained resolution of formerly-chronic positive and negative psychotic symptoms in a schizophrenic patient. This patient had averaged 2.5 inpatient admissions per year for the 8 years preceding initiation of sertindole therapy, but has had no hospitalizations or psychosis in the 3.5 years since. Improved cognitive function has also been documented. This preliminary, but controlled, experience suggests that sertindole may possess a spectrum of clinical activity distinct from that of the typical antipsychotic agent.

Quantification of dopamine D1 and D2 receptor mRNA levels associated with the development of behavioral sensitization in amphetamine treated rats

We hypothesized that changes in expression of dopamine (DA) D1 and D2 receptor genes in caudate/putamen (CP) would correlate with the development of behavioral changes in amphetamine treated rats. In order to test this hypothesis, we measured DA D1 and D2 receptor mRNA in CP, as well as locomotor behavior, in individual rats following amphetamine treatment. D1 and D2 mRNA levels were similar in caudate/putamen of rats treated with acute amphetamine, chronic amphetamine or saline injection. We found no correlation between D1 or D2 mRNA levels in caudate/putamen and the behavioral response to either acute or chronic amphetamine. These results suggest that behavioral sensitization to amphetamine is not mediated through transcriptional regulation of D1 or D2 mRNA levels in caudate/putamen.

Regional quantification of dopamine transporter mRNA in rat brain using a ribonuclease protection assay

We describe the regional distribution of dopamine transporter (DAT) mRNA in selected regions of rat brain using a highly sensitive and specific nuclease protection assay. This method determines the absolute quantity of mRNA expressed in the brain regions surveyed. DAT mRNA level varied widely between brain regions, and was detected only in cell body regions of the major dopaminergic pathways. Highest expression was seen in substantia nigra/ventral tegmentum (SN/VTA). Lower but detectable expression of a protected mRNA of the expected size was observed within hypothalamus. Expression could not be detected by this method in other brain regions studied. Our results indicate that this method is sufficiently sensitive to allow study of mRNA expression in individual animals.