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Dexras1 is a novel GTPase that acts at a confluence of signaling mechanisms associated with psychiatric and neurological disease including NMDA receptors, NOS1AP and nNOS. Recent work has shown that Dexras1 mediates iron trafficking and NMDA-dependent neurodegeneration but a role for Dexras1 in normal brain function or psychiatric disease has not been studied. To test for such a role, mice with germline knockout (KO) of Dexras1 were assayed for behavioral abnormalities as well as changes in NMDA receptor subunit protein expression. Because Dexras1 is up-regulated during stress or by dexamethasone treatment, we included measures associated with emotion including anxiety and depression. Baseline anxiety-like measures (open field and zero maze) were not altered, nor were depression-like behavior (tail suspension). Measures of memory function yielded mixed results, with no changes in episodic memory (novel object recognition) but a significant decrement on working memory (T-maze). Alternatively, there was an increase in pre-pulse inhibition (PPI), without concomitant changes in either startle amplitude or locomotor activity. PPI data are consistent with the direction of change seen following exposure to dopamine D2 antagonists. An examination of NMDA subunit expression levels revealed an increased expression of the NR2A subunit, contrary to previous studies demonstrating down-regulation of the receptor following antipsychotic exposure (Schmitt et al., 2003) and up-regulation after exposure to isolation rearing (Turnock-Jones et al., 2009). These findings suggest a potential role for Dexras1 in modulating a selective subset of psychiatric symptoms, possibly via its interaction with NMDARs and/or other disease-related binding-partners. Furthermore, data suggest that modulating Dexras1 activity has contrasting effects on emotional, sensory and cognitive domains.

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Vesicular monoamine transporters (VMAT) are involved in presynaptic storage and release of neurotransmitters. While it was thought initially that only VMAT2 is brain expressed and VMAT1 is present only in the periphery, recent data have challenged the exclusive expression of VMAT2 in the brain. To further elucidate the role of VMAT1 brain expression and its potential role in neuropsychiatric disorders, we have investigated mice lacking VMAT1. Comparison of wildtype and knock-out (KO) mice using qPCR and immunohistochemistry documents the expression of VMAT1 in the brain. Deletion of VMAT1 leads to increased hippocampal apoptosis and reduced neurogenesis as assessed by caspase-3-labeling and 5-bromo-deoxy-uridine-labeling. Behavioral data show that mice lacking VMAT1 have neurocognitive deficits. VMAT2 expression is not altered in VMAT1 KO mice, suggesting a distinct role of VMAT1. Our data support VMAT1 brain expression and suggest that VMAT1 plays a key role in survival of hippocampal neurons and thus might contribute to neurocognitive deficits observed in neuropsychiatric disorders.

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Stress hormones are thought to be involved in the etiology of depression, in part, because animal models show they cause morphological damage to the brain, an effect that can be reversed by chronic antidepressant treatment. The current study examined two mouse strains selected for naturalistic variation of tissue regeneration after injury for resistance to the effects of chronic corticosterone (CORT) exposure on cell proliferation and neurotrophin mobilization. The wound healer MRL/MpJ and control C57BL/6J mice were implanted subcutaneously with pellets that released CORT for 7 days. MRL/MpJ mice were resistant to reductions of hippocampal cell proliferation by chronic exposure to CORT when compared to vulnerable C57BL/6J mice. Chronic CORT exposure also reduced protein levels of brain-derived neurotrophic factor (BDNF) in the hippocampus of C57BL/6J but not MRL/MpJ mice. CORT pellet exposure increased circulating levels of CORT in the plasma of both strains in a dose-dependent manner although MRL/MpJ mice may have larger changes from baseline. The strains did not differ in circulating levels of corticosterone binding globulin (CBG). There were also no strain differences in CORT levels in the hippocampus, nor did CORT exposure alter glucocorticoid receptor or mineralocorticoid receptor expression in a strain-dependent manner. Strain differences were found in the N-methyl-D-aspartate (NMDA) receptor, and BDNF I and IV promoters. Strain and CORT exposure interacted to alter tropomyosine-receptor-kinase B (TrkB) expression and this may be a potential mechanism protecting MRL/MpJ mice. In addition, differences in the inflammatory response of matrix metalloproteinases (MMPs) may also contribute to these strain differences in resistance to the deleterious effects of CORT to the brain.

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Adverse prenatal environment, such as intrauterine growth retardation (IUGR), increases the risk for negative neurobehavioral outcomes. IUGR, affecting approximately 10% of all US infants, is a known risk factor for attention deficit hyperactivity disorder (ADHD), schizophrenia spectrum disorders and addiction. Mouse dams were fed a protein deficient (8.5% protein) or isocaloric control (18% protein) diet through pregnancy and lactation (a well validated rodent model of IUGR). Dopamine-related gene expression, dopamine content and behavior were examined in adult offspring. IUGR offspring have six to eightfold over-expression of dopamine (DA)-related genes (tyrosine hydroxylase (TH) and dopamine transporter) in brain regions related to reward processing (ventral tegmental area (VTA), nucleus accumbens, prefrontal cortex (PFC)) and homeostatic control (hypothalamus), as well as increased number of TH-ir neurons in the VTA and increased dopamine in the PFC. Cyclin-dependent kinase inhibitor 1C (Cdkn1c) is critical for dopaminergic neuron development. Methylation of the promoter region of Cdkn1c was decreased by half and there was a resultant two to sevenfold increase in Cdkn1c mRNA expression across brain regions. IUGR animals demonstrated alterations in dopamine-dependent behaviors, including altered reward-processing, hyperactivity and exaggerated locomotor response to cocaine. These data describe significant dopamine-related molecular and behavioral abnormalities in a mouse model of IUGR. This animal model, with both face validity (behavior) and construct validity (link to IUGR and dopamine dysfunction) may prove useful in identifying underlying mechanisms linking IUGR and adverse neurobehavioral outcomes such as ADHD.

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The cyclic AMP (cAMP)-response element binding protein (CREB) is an activity-dependent transcription factor that plays a role in synaptic plasticity and memory storage in Aplysia, Drosophila, and rodents. Mice with targeted deletions of two CREB isoforms (alpha and delta; CREB alphadelta mice) have been characterized on a mixed genetic background of C57BL/6 (B6) and 129/SvEv (129), as well as on a defined F1 hybrid of B6 and FVB/N, and these results suggest that the phenotype of CREB alphadelta mice depends critically on genetic background. In an examination of the hypothesis that the role of CREB in learning and memory can be influenced by strain differences, we analyzed mice with the CREB alphadelta mutation on an F1 hybrid background of B6 and 129 strains. CREB alphadelta mice on this background had impaired short-term and long-term cued and contextual fear conditioning and normal spatial learning in the Morris water maze. Our results suggest that at least some aspects of hippocampal function are normal in CREB alphadelta mice, and that CREB alphadelta mice on the B6/129 F1 background have alterations in amygdala function. These studies underscore the importance of controlling for genetic background in the behavioral analysis of knockout and transgenic mice.

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The authors evaluated the cognitive and psychomotor effects of serotonin reuptake inhibitors in healthy elderly volunteers. Paroxetine, sertraline, and placebo were compared for 3 weeks of testing in a double-blind study with behavioral testing at baseline and at the end of each week. MANOVA models demonstrated no between-group differences; however, mixed-model random regression analyses revealed that Day 14 plasma paroxetine levels correlated negatively with delayed verbal recall and paired-associate learning scores. In contrast, plasma sertraline levels correlated positively with Day 7 immediate verbal recall, Day 14 tapping, and Day 21 delayed verbal recall scores, and negatively with divided-attention task scores on Day 21. Plasma paroxetine levels were associated with mild behavioral impairment at Day 14, with no other significant adverse effects. Plasma sertraline levels were associated with mild and transient behavioral changes, as well as early termination in several subjects.

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BACKGROUND: Little is known about the hypothalamic-pituitary-adrenal axis response to acute stressful behavioral challenges in patients with social phobia. METHODS: Eighteen patients with social phobia and 17 normal volunteers participated in two behavioral stressors: a speech task and physical exercise. RESULTS: Normal volunteers (n = 14) demonstrated a significant 50% increase in salivary cortisol levels to the speech task. Three nonresponding normal volunteers demonstrated a 17% decrease. In contrast, patients with social phobia demonstrated dichotomous changes. Seven social phobia patients demonstrated a significantly higher 90% increase in salivary cortisol to the speech task, whereas the remaining patients (n = 11) were nonresponders demonstrating a 32% decrease in cortisol. Both patient groups were significantly more anxious than the normal volunteers. In contrast to the response to a speech task, social phobics showed a cortisol response to physical exercise of similar magnitude as normal volunteers. CONCLUSIONS: The results indicated dichotomies in magnitude and in distribution of the cortisol response to a speech task between social phobia patients and normal volunteers. Social phobia patients responded differently than normal volunteers to a stressor associated with social evaluation but not to physical exercise. These results suggest adaptation of distinct biological processes specific to different stressful conditions in social phobia.

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The regulation of extracellular levels of 5-hydroxytryptamine (serotonin) (5-HT) in the striatum and ventral hippocampus was studied using in vivo microdialysis in awake, unrestrained wild-type 5-HT(1A) and 5-HT(1B) receptor knockout mice. Systemic administration of the selective serotonin reuptake inhibitor fluoxetine evoked a significant dose-dependent increase in extracellular 5-HT in both the striatum and hippocampus at both 2.5 mg/kg (i.p.) and 20 mg/kg (i.p.) in wild-type mice. In 5-HT(1A) receptor knockout mice, the response to 2.5 mg/kg fluoxetine was significantly augmented in the striatum but not the hippocampus, whereas the response to 20 mg/kg fluoxetine was significantly greater in both brain regions. In 5-HT(1B) receptor knockout mice, the increase of extracellular 5-HT was augmented in the hippocampus but not the striatum at both doses of fluoxetine. The response pattern to fluoxetine alone in 5-HT receptor mutant mice corresponded with the effects of fluoxetine given with either the 5-HT(1A) receptor antagonist WAY 100635 (0.1 mg/kg i.p.) or the 5-HT(1B/1D) receptor antagonist GR 127935 (0.056 mg/kg) in wild-type mice. These results indicate common topographical regulation of 5-HT release in different brain regions by genetic mutation and pharmacological challenges. The 5-HT(1A) autoreceptor plays a larger role in regulating 5-HT release in the striatum and possibly other brain regions innervated by the dorsal raphe nucleus, whereas the role of the 5-HT(1B) receptor is relatively greater in the hippocampus and possibly other brain regions innervated by the median raphe nucleus.

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Plasticity in serotonergic transmission in serotonin or 5-hydroxytryptamine (5-HT) receptor mutants was examined by measuring the regulation of extracellular 5-HT levels in the striatum and ventral hippocampus of 5-HT(1A) and 5-HT(1B) receptor knockout mice using in vivo microdialysis. The efficacy of genetic deletion was verified by showing blunted regulation of extracellular 5-HT with selective 5-HT receptor agonists. 5-HT(1A) receptor knockout mice failed to demonstrate reduction of extracellular 5-HT in response to systemic administration of the 5-HT(1A) receptor agonist R-8-hydroxydipropylaminotetralin (R-8-OH-DPAT) and 5-HT(1B) receptor knockout mice failed to demonstrate reduction of extracellular 5-HT in response to systemic administration of the 5-HT(1B) receptor agonist CP 94,253. Plasticity also developed to deletion of the complementary autoreceptor. 5-HT(1A) receptor knockout mice demonstrated a significantly greater response to CP 94,253 in the striatum, but not the ventral hippocampus, suggesting the development of enhanced sensitivity of striatal 5-HT(1B) receptors. In 5-HT(1B) receptor knockout mice, R-8-OH-DPAT evoked a significantly diminished response in the ventral hippocampus, but not the striatum, suggesting the potential desensitization of 5-HT(1A) receptors in the median raphe nucleus. The pattern of regional compensations between somatodendritic and terminal autoreceptors was confirmed by pharmacological challenges using the selective serotonin reuptake inhibitor fluoxetine combined with either a 5-HT(1A) (WAY 100635) or a 5-HT(1B/1D) (GR 127935) receptor antagonist. The regional pattern of compensation may be determined by the preferential role of 5-HT(1A) or 5-HT(1B) receptors in regulating 5-HT release. Taken together, these results demonstrate the development of regional plasticity between complementary somatodendritic and terminal autoreceptors after the genetic deletion of 5-HT(1A) or 5-HT(1B) receptors.

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The development of serotonin receptor knockout mice has provided an opportunity to study antidepressant drug effects in animals with targeted genetic deletion of receptors involved in antidepressant responses. In the current study, the effects of two types of antidepressant drugs, the selective serotonin reuptake inhibitors fluoxetine and paroxetine and the selective norepinephrine reuptake inhibitor desipramine, were examined in 5-hydroxytryptamine (5-HT)(1A) and 5-HT(1B) receptor mutant mice using the tail suspension test (TST). Under baseline conditions, the immobility of 5-HT(1A) receptor mutant mice, but not 5-HT(1B) receptor mutant mice, was significantly lower than that of wild-type mice. The decreased baseline immobility in 5-HT(1A) receptor mutant mice was reversed by pretreatment with alpha-methyl-para-tyrosine, but not by para-chlorophenylalanine, suggesting mediation by enhanced catecholamine function. In wild-type mice, fluoxetine (10.0--20.0 mg/kg i.p.) and desipramine (5.0--20.0 mg/kg i.p.) both significantly decreased immobility in the TST. In 5-HT(1A) receptor mutant mice, desipramine (20.0 mg/kg i.p.) significantly decreased immobility, whereas fluoxetine (20.0 mg/kg i.p.) and paroxetine (20.0 mg/kg i.p.) had no effect. The immobility of 5-HT(1B) receptor mutant mice was decreased similarly by desipramine (5.0--20.0 mg/kg i.p.). However, the effect of low doses of fluoxetine were significantly augmented in the 5-HT(1B) receptor mutant mice (2.5--20.0 mg/kg i.p.) compared with wild-type mice. Administration of selective 5-HT receptor antagonists in wild-type mice partially reproduced the phenotypes of the mutant mice. These results suggest that 5-HT(1A) and 5-HT(1B) receptors have different roles in the modulation of the response to antidepressant drugs in the TST.

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RATIONALE: Although the rewarding effects of cocaine are generally attributed to its ability to increase dopamine (DA) transmission, cocaine demonstrates approximately equal affinity for dopamine and serotonin (5-HT) transporters in vitro. However, there have been few direct systematic comparisons of the effects of cocaine on DA and 5-HT transmission in vivo. OBJECTIVES: The present experiments compared the effects of systemic cocaine administration, local cocaine infusion and the systemic administration of cocaine during infusion on extracellular levels of DA and 5-HT in the nucleus accumbens (NAc). METHODS: In vivo microdialysis in awake unrestrained rats was used to measure the effects of systemic administration and local infusion of cocaine on extracellular DA and 5-HT levels simultaneously in the NAc. RESULTS: Systemic cocaine (10-25 mg/kg, IP) dose-dependently increased DA and 5-HT levels, but the increase in DA was larger than for 5-HT at 18 mg/kg. Infusion of cocaine (0.1-10.0 mM) into the NAc increased both DA and 5-HT levels, but the effect on DA was larger than 5-HT at 0.1 and 3 mM cocaine. The influence of cocaine on DA and 5-HT somatodendritic autoreceptors was examined when cocaine (25 mg/kg) was administered systemically during cocaine infusion. The increase in DA and 5-HT levels during cocaine infusion was attenuated by the systemic injection of cocaine during cocaine infusion, but the decrease of 5-HT was greater than that for DA. CONCLUSIONS: Cocaine produced a larger impact on DA than 5-HT neurotransmission under specific conditions. A series of physiological mechanisms, i.e. terminal density, neurotransmitter interactions and somatodendritic regulation, are discussed as factors responsible for facilitating cocaine's effects on DA relative to 5-HT.

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RATIONALE: Recent advances in neurobehavioral genetics have increased the importance of research on the behavioral patterns of different mouse strains. A comprehensive comparison of inbred and outbred mouse strains was conducted to provide information on the range of performance and pharmacological effects in the forced swimming test, a behavioral test commonly used to measure the effects of antidepressant drugs. OBJECTIVES: Baseline performance and pharmacological responses to desipramine, a selective norepinephrine reuptake inhibitor, and fluoxetine, a selective serotonin reuptake inhibitor, were compared in seven inbred and four outbred mouse strains in the forced swimming test. METHODS: Swim sessions were conducted by placing mice in individual glass cylinders filled with water for 6 min. The duration of behavioral immobility during the last 4 min of the test was scored from videotapes. RESULTS: A 10-fold range of immobility values and coefficient of variation supported the existence of substantial behavioral differences between mouse strains in baseline performance in the FST. In general, inbred strains demonstrated lower variability than outbred strains. Desipramine dose-dependently reduced immobility in seven of the 11 strains tested, with DBA/2J and the C57BL/6J mice showing greater sensitivity than the other strains. In contrast, fluoxetine reduced immobility in only three out of the 11 strains tested, DBA/2J, BALB/cJ and NIH Swiss mice. CONCLUSIONS: Background strain is a critical variable in determining baseline performance and the sensitivity to different types of antidepressant drugs in the mouse FST. The use of such mouse strains may provide information on the genetic basis for strain differences in depressive behavior and differential sensitivity to diverse classes of antidepressants.

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Norepinephrine (NE) is thought to play an important role in the pathophysiology of depression, and in the mechanism of action of antidepressant compounds. Previously, we created mice that are unable to synthesize NE and epinephrine due to targeted disruption of the dopamine-beta-hydroxylase gene (Dbh). To specifically test the role of NE in mediating behavioral changes elicited by antidepressants, these mice were examined in the forced swim test. There was no difference in baseline immobility scores in the forced swim test between Dbh(+/-) mice, which have normal levels of NE, and Dbh(-/-) mice. However, the Dbh(-/-) mice failed to demonstrate antidepressant-like behavioral effects following the administration of several classes of antidepressants. These included the NE reuptake inhibitors desipramine and reboxetine, the monoamine oxidase inhibitor pargyline, and the atypical antidepressant bupropion. In addition, desipramine significantly reduced immobility in the Dbh(-/-) mice following pretreatment with the synthetic NE precursor L-threo-3,4-dihydroxyphenylserine, but not saline. Biochemical studies showed that there was no significant difference in the regional brain levels of NE transporter immunoreactivity or monoamine oxidase activity, the primary targets for most of the compounds examined. Taken together, these data show that the use of mice that lack endogenous NE may be an important strategy for unraveling the role of NE in tests sensitive to the effects of various psychotherapeutic agents.

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RATIONALE: The C57BL/6 is one of the most widely used mouse strains in behavioral, pharmacological, and genetic research but little is known about their response on tests for antidepressant drugs. OBJECTIVES: The behavior of C57BL/6 mice, and mice from other strains, was examined in the tail suspension test (TST), a common behavioral test used for the screening of antidepressant compounds. METHODS: C57BL/6J mice from the Jackson Laboratory, C57BL/6N mice from Harlan, A/J, 129-SV-ter and DBA/2 mice were tested under baseline conditions in the TST. RESULTS: The majority of the C57BL/6 mice from the Jackson Laboratory tested in this paradigm (70%) climbed up their tails during the 6-min test session. C57BL/6 mice obtained from Harlan (35%) also demonstrated this climbing behavior, suggesting that it is not specific to mice from a particular supplier. Other strains (A/J 18%), 129-SV-ter (0%) and DBA/2 (0%) mice) showed less propensity for tail climbing. CONCLUSIONS: The occurrence of this behavior is an important consideration when testing antidepressant drugs or the effects of stress using the TST with inbred mouse strains, especially those from the C57BL/6 strain.

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The serotonergic dorsal raphé nucleus (DRN) is innervated by corticotropin-releasing factor (CRF)-immunoreactive fibers and contains CRF receptor-binding sites, suggesting that endogenous CRF regulates this system. The present study examined the possibility that CRF in the DRN regulates the release of serotonin (5-HT) in forebrain terminal regions. Intracerebroventricular administration of CRF produced a bimodal effect on extracellular levels of 5-HT in the lateral septum. Doses of 0.3 and 1.0 microg decreased extracellular 5-HT levels, whereas both a higher (3.0 microg) and a lower (0.1 microg) dose had no effect. The reduction of extracellular 5-HT in the lateral septum by CRF (0.3 microg, i.c.v.) was blocked by pretreatment with the CRF receptor antagonist d-PheCRF(12-41) (3.0 microg, i.c.v.). Direct administration of CRF (30 ng) into the DRN reduced extracellular 5-HT levels in the lateral septum and the striatum. Furthermore, injection of d-PheCRF(12-41) (10 ng) into the DRN before ventricular administration of CRF (0.3 microg, i.c.v.) blocked the decrease in extracellular 5-HT in both the lateral septum and striatum. Taken together, these data support the hypothesis that CRF may modulate 5-HT release in terminal regions via its effects at the level of the DRN. This modulation supports a potential interaction between CRF and 5-HT in stress-related psychiatric disorders in which both systems have been implicated.

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The receptors responsible for mediating the antidepressant effects of selective serotonin reuptake inhibitors are largely unknown. The role of the 5-HT4 receptor in mediating the antidepressant-like effects of fluoxetine in a modified rat forced swim test was examined. Fluoxetine (20 mg/kg) decreased immobility and increased swimming, a pattern shown to represent its actions on the serotonergic system. The selective 5-HT4 receptor antagonist, SB 204070A [8-amino-7-chloro-(N-butyl-4-piperidyl)methylbenzo-1, 4-dioxan-5-carboxylate hydrochloride] (0.1-3 mg/kg), failed to change any of the active behaviors in the test compared with saline-treated animals. Upon combination, SB 204070A (3 mg/kg) failed to alter the effects of fluoxetine effects in the test. These data therefore suggest that activation of postsynaptic 5-HT4 receptors, subsequent to reuptake inhibition by fluoxetine, is not necessary for its antidepressant-like behavioral effects in this test.

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The role of the 5-HT(2C) receptor in mediating active behaviors in the modified rat forced swim test was examined. Three novel selective 5-HT(2C) receptor agonists, WAY 161503 (0.1-3.0 mg/kg), RO 60-0175 (2-20 mg/kg), and RO 60-0332 (20 mg/kg), all decreased immobility and increased swimming, a pattern of behavior similar to that which occurs with the selective serotonin reuptake inhibitor fluoxetine (5-20 mg/kg). However, the prototypical but nonselective 5-HT(2C) receptor agonist m-chlorophenylpiperazine (1-10 mg/kg) increased immobility scores in the forced swim test. The selective 5-HT(2C) receptor antagonist SB 206533 was inactive when given alone (1-20 mg/kg). However, SB 206533 (20 mg/kg) blocked the antidepressant-like effects of both WAY 161503 (1 mg/kg) and fluoxetine (20 mg/kg). The atypical antidepressant (noradrenergic alpha(2) and 5-HT(2C) receptor antagonist) mianserin reduced immobility and increased climbing at 30 mg/kg. At a behaviorally subactive dose (10 mg/kg), mianserin abolished the effects of WAY 161503 (1 mg/kg) on both swimming and immobility scores. Mianserin blocked the effects of fluoxetine (20 mg/kg) on swimming only; mianserin plus fluoxetine reduced immobility and induced a switch to climbing behavior, suggesting activation of noradrenergic transmission. These data exemplify the benefits of using the modified rat forced swim test, which was sensitive to serotonergic compounds and distinguished behavioral changes associated with serotonergic and noradrenergic effects. Taken together, the results strongly implicate a role for 5-HT(2C) receptors in the behavioral effects of antidepressant drugs.

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The regional distribution and cellular localization of dopamine D3 receptors in the rat brain was examined using quantitative autoradiography. [(125)I]7-OH-PIPAT bound in a saturable and reversible manner and exhibited subnanomolar affinity for a single population of GTP-insensitive sites. The pharmacological profile was characteristic of cloned D3 receptors and nonspecific binding was uniformly low. The highest levels of D3 receptors were measured in the islands of Calleja, nucleus accumbens, ventral pallidum, substantia nigra, and lobules 9 and 10 of the cerebellum. The high specific activity of this ligand also allowed detection of D3 receptors in other regions, including the serotonergic dorsal and median raphe nuclei, indicating that the distribution of this receptor is more widespread than previously appreciated. The cellular localization of D3 receptors in regions containing dopaminergic cells and terminals was examined by discrete injection of neurotoxins. Lesion of dopaminergic neurons with 6-hydroxydopamine produced 50% decreases in [(125)I]7-OH-PIPAT binding in the nucleus accumbens and substantia nigra. Quinolinic acid lesion of neurons originating in the nucleus accumbens also produced approximately 50% decreases in D3 receptors in the nucleus accumbens, substantia nigra, and ventral pallidum. 5, 7-Dihydroxytryptamine lesion of serotonergic cells and processes produced no changes in [(125)I]7-OH-PIPAT binding. These results demonstrate the presence of D3 receptors in several brain regions not previously identified and suggest that D3 receptors are expressed at somatodendritic and terminal levels of both dopaminergic and nondo-paminergic cells within the mesolimbic dopamine system.

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Regulation of the expression of dopamine D2 and D3 receptors in the rat brain was examined using quantitative autoradiography after chronic (14 day) drug treatments designed to increase or decrease dopamine receptor stimulation. Reserpine treatment depleted endogenous dopamine by more than 90% and significantly increased the binding of [(125)I]NCQ 298 to D2 receptors in the nucleus accumbens, ventral pallidum, and substantia nigra. In contrast, this treatment significantly decreased the binding of [(125)I]7-OH-PIPAT to D3 receptors in each of these regions. Chronic stimulation of D2-like receptors with quinpirole (1 mg/kg/day) or 7-OH-DPAT (1 mg/kg/day) produced decreases in [(125)I]NCQ 298 binding in the nucleus accumbens, ventral pallidum, and substantia nigra as expected. As with depletion, chronic stimulation elicited an opposite response from D3 receptors with significant increases observed in the ventral pallidum and substantia nigra. D3 receptor expression in the nucleus accumbens was unchanged. Baclofen (30 mg/kg/day) or continuous administration of the psychomotor stimulant cocaine (20 mg/kg/day) produced no significant changes in D2 or D3 receptor binding in any region examined. Acute administration of the irreversible antagonist EEDQ (10 mg/kg) nearly eliminated D2 receptor binding in all regions, but inactivated D3 receptors only in the VP and SN, suggesting subtype-specific and region-specific differences in receptor occupancy. The existence of regional and subtype-specific heterogeneities in the regulation of these receptors supports the contention that despite their similar pharmacological profiles, D2 and D3 receptors may mediate different functional responses.

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