Increased expression of the dopamine transporter leads to loss of dopamine neurons, oxidative stress and l-DOPA reversible motor deficits

The dopamine transporter is a key protein responsible for regulating dopamine homeostasis. Its function is to transport dopamine from the extracellular space into the presynaptic neuron. Studies have suggested that accumulation of dopamine in the cytosol can trigger oxidative stress and neurotoxicity. Previously, ectopic expression of the dopamine transporter was shown to cause damage in non-dopaminergic neurons due to their inability to handle cytosolic dopamine. However, it is unknown whether increasing dopamine transporter activity will be detrimental to dopamine neurons that are inherently capable of storing and degrading dopamine. To address this issue, we characterized transgenic mice that over-express the dopamine transporter selectively in dopamine neurons. We report that dopamine transporter over-expressing (DAT-tg) mice display spontaneous loss of midbrain dopamine neurons that is accompanied by increases in oxidative stress markers, 5-S-cysteinyl-dopamine and 5-S-cysteinyl-DOPAC. In addition, metabolite-to-dopamine ratios are increased and VMAT2 protein expression is decreased in the striatum of these animals. Furthermore, DAT-tg mice also show fine motor deficits on challenging beam traversal that are reversed with l-DOPA treatment. Collectively, our findings demonstrate that even in neurons that routinely handle dopamine, increased uptake of this neurotransmitter through the dopamine transporter results in oxidative damage, neuronal loss and l-DOPA reversible motor deficits. In addition, DAT over-expressing animals are highly sensitive to MPTP-induced neurotoxicity. The effects of increased dopamine uptake in these transgenic mice could shed light on the unique vulnerability of dopamine neurons in Parkinson's disease.

Novel roles for β-arrestins in the regulation of pharmacological sequestration to predict agonist-induced desensitization of dopamine D3 receptors

BACKGROUND AND PURPOSE

In addition to typical GPCR kinase (GRK)-/β-arrestin-dependent internalization, dopamine D3 receptor employed an additional GRK-independent sequestration pathway. In this study, we investigated the molecular mechanism of this novel sequestration pathway.

EXPERIMENTAL APPROACH

Radioligand binding, flow cytometry and cell surface biotinylation assay were used to characterize trafficking properties of D2 and D3 receptors. Serine/threonine and N-linked glycosylation mutants of the D3 receptor were utilized to locate receptor regions involved in pharmacological sequestration and desensitization. Various point mutants of the D2 and D3 receptors, whose sequestration and desensitization properties were altered, were combined with knockdown cells of GRKs or β-arrestins to functionally correlate pharmacological sequestration and desensitization.

KEY RESULTS

The D3 receptor, but not the D2 receptor, showed characteristic trafficking behaviour in which receptors were shifted towards the more hydrophobic domains within the plasma membrane without translocation into other intracellular compartments. Among various amino acid residues tested, S145/S146, C147 and N12/19 were involved in pharmacological sequestration and receptor desensitization. Both pharmacological sequestration and desensitization of D3 receptor required β-arrestins, and functional relationship was observed between two processes when it was tested for D3 receptor variants and agonists.

CONCLUSIONS AND IMPLICATIONS

Pharmacological sequestration of D3 receptor accompanies movement of cell surface receptors into a more hydrophobic fraction within the plasma membrane and renders D3 receptor inaccessible to hydrophilic ligands. Pharmacological sequestration is correlated with desensitization of the D3 receptor in a Gβγ- and β-arrestin-dependent manner. This study provides new insights into molecular mechanism governing GPCR trafficking and desensitization.

The physiological relevance of functional selectivity in dopamine signalling

We sought to determine the role of functionally selective dopamine (DA) signalling pathways (G protein or β-arrestin) in DA-dependent behaviours. Mice that were globally deficient for β-arrestins or mice deficient in GSK3β in D2 receptor (D2R)-expressing neurons were used to investigate the role of functional selectivity in DA-dependent behaviours such as locomotor activity and conditioned place preference (CPP). Wild-type or knockout mice were injected with drugs such as morphine and amphetamine, which are known to increase DA levels in the brain and to induce a hyper-locomotor response and CPP. Unlike β-arrestin1 (βarr1)-deficient mice, mice globally deficient for β-arrestin2 (βarr2) mount a reduced hyperlocomotor response to either morphine or amphetamine. However, mice deficient in GSK3β in D2R-expressing neurons show a significantly reduced locomotor response to only amphetamine but not morphine. Interestingly, all mice tested show a normal CPP response to either morphine or amphetamine. β-arrestin-mediated DA receptor signalling has an important role in the locomotor response, but not CPP, to drugs such as morphine and amphetamine, demonstrating a functional selectivity of DA-dependent behaviours in mice. It is likely that G-protein-dependent signalling through DA receptors mediates the CPP response.

N-aryl piperazine metabotropic glutamate receptor 5 positive allosteric modulators possess efficacy in preclinical models of NMDA hypofunction and cognitive enhancement

Impaired transmission through glutamatergic circuits has been postulated to play a role in the underlying pathophysiology of schizophrenia. Furthermore, inhibition of the N-methyl-d-aspartate (NMDA) subtype of ionotropic glutamate receptors (NMDAR) induces a syndrome that recapitulates many of the symptoms observed in patients with schizophrenia. Selective activation of metabotropic glutamate receptor subtype 5 (mGlu5) may provide a novel therapeutic approach for treatment of symptoms associated with schizophrenia through facilitation of transmission through central glutamatergic circuits. Here, we describe the characterization of two novel N-aryl piperazine mGlu5 positive allosteric modulators (PAMs): 2-(4-(2-(benzyloxy)acetyl)piperazin-1-yl)benzonitrile (VU0364289) and 1-(4-(2,4-difluorophenyl)piperazin-1-yl)-2-((4-fluorobenzyl)oxy)ethanone (DPFE). VU0364289 and DPFE induced robust leftward shifts in the glutamate concentration-response curves for Ca(2+) mobilization and extracellular signal-regulated kinases 1 and 2 phosphorylation. Both PAMs displayed micromolar affinity for the common mGlu5 allosteric binding site and high selectivity for mGlu5. VU0364289 and DPFE possessed suitable pharmacokinetic properties for dosing in vivo and produced robust dose-related effects in reversing amphetamine-induced hyperlocomotion, a preclinical model predictive of antipsychotic-like activity. In addition, DPFE enhanced acquisition of contextual fear conditioning in rats and reversed behavioral deficits in a mouse model of NMDAR hypofunction. In contrast, DPFE had no effect on reversing apomorphine-induced disruptions of prepulse inhibition of the acoustic startle reflex. These mGlu5 PAMs also increased monoamine levels in the prefrontal cortex, enhanced performance in a hippocampal-mediated memory task, and elicited changes in electroencephalogram dynamics commensurate with procognitive effects. Collectively, these data support and extend the role for the development of novel mGlu5 PAMs for the treatment of psychosis and cognitive deficits observed in individuals with schizophrenia.

Deficient serotonin neurotransmission and depression-like serotonin biomarker alterations in tryptophan hydroxylase 2 (Tph2) loss-of-function mice

Probably the foremost hypothesis of depression is the 5-hydroxytryptamine (5-HT, serotonin) deficiency hypothesis. Accordingly, anomalies in putative 5-HT biomarkers have repeatedly been reported in depression patients. However, whether such anomalies in fact reflect deficient central 5-HT neurotransmission remains unresolved. We employed a naturalistic model of 5-HT deficiency, the tryptophan hydroxylase 2 (Tph2) R439H knockin mouse, to address this question. We report that Tph2 knockin mice have reduced basal and stimulated levels of extracellular 5-HT (5-HT(Ext)). Interestingly, cerebrospinal fluid (CSF) 5-hydroxyindoleacetic acid (5-HIAA) and fenfluramine-induced plasma prolactin levels are markedly diminished in the Tph2 knockin mice. These data seemingly confirm that low CSF 5-HIAA and fenfluramine-induced plasma prolactin reflects chronic, endogenous central nervous system (CNS) 5-HT deficiency. Moreover, 5-HT(1A) receptor agonist-induced hypothermia is blunted and frontal cortex 5-HT(2A) receptors are increased in the Tph2 knockin mice. These data likewise parallel core findings in depression, but are usually attributed to anomalies in the respective receptors rather than resulting from CNS 5-HT deficiency. Further, 5-HT(2A) receptor function is enhanced in the Tph2 knockin mice. In contrast, 5-HT(1A) receptor levels and G-protein coupling is normal in Tph2 knockin mice, indicating that the blunted hypothermic response relates directly to the low 5-HT(Ext). Thus, we show that not only low CSF 5-HIAA and a blunted fenfluramine-induced prolactin response, but also blunted 5-HT(1A) agonist-induced hypothermia and increased 5-HT(2A) receptor levels are bona fide biomarkers of chronic, endogenous 5-HT deficiency. Potentially, some of these biomarkers could identify patients likely to have 5-HT deficiency. This could have clinical research utility or even guide pharmacotherapy.

A functional alternative splicing mutation in human tryptophan hydroxylase-2

The brain serotonergic system has an essential role in the physiological functions of the central nervous system and dysregulation of serotonin (5-HT) homeostasis has been implicated in many neuropsychiatric disorders. The tryptophan hydroxylase-2 (TPH2) gene is the rate-limiting enzyme in brain 5-HT synthesis, and thus is an ideal candidate gene for understanding the role of dysregulation of brain serotonergic homeostasis. Here, we characterized a common, but functional single-nucleotide polymorphism (SNP rs1386493) in the TPH2 gene, which decreases efficiency of normal RNA splicing, resulting in a truncated TPH2 protein (TPH2-TR) by alternative splicing. TPH2-TR, which lacks TPH2 enzyme activity, dominant-negatively affects full-length TPH2 function, causing reduced 5-HT production. The predicted mRNA for TPH2-TR is present in postmortem brain of rs1386493 carriers. The rs13864923 variant does not appear to be overrepresented in either global or multiplex depression cohorts. However, in combination with other gene variants linked to 5-HT homeostasis, this variant may exhibit important epistatic influences.

Multiscale imaging characterization of dopamine transporter knockout mice reveals regional alterations in spine density of medium spiny neurons

The dopamine transporter knockout (DAT KO) mouse is a model of chronic hyperdopaminergia used to study a wide range of neuropsychiatric disorders such as schizophrenia, attention deficit hyperactivity disorder (ADHD), drug abuse, depression, and Parkinson's disease (PD). Early studies characterizing this mouse model revealed a subtle, but significant, decrease in the anterior striatal volume of DAT KO mice accompanied by a decrease in neuronal cell body numbers (Cyr et al., 2005). The present studies were conducted to examine medium spiny neuron (MSN) morphology by extending these earlier reports to include multiscale imaging studies using correlated light microscopy (LM) and electron microscopy (EM) techniques. Specifically, we set out to determine if chronic hyperdopaminergia results in quantifiable or qualitative changes in DAT KO mouse MSNs relative to wild-type (WT) littermates. Using Neurolucida Explorer's morphometric analysis, we measured spine density, dendritic length and synapse number at ages that correspond with the previously reported changes in striatal volume and progressive cell loss. Light microscopic analysis using Neurolucida tracings of photoconverted striatal MSNs revealed a highly localized loss of dendritic spines on the proximal portion of the dendrite (30 μm from the soma) in the DAT KO group. Next, thick sections containing MSN dendritic segments located at a distance of 20-60 μm from the cell soma, a region of the dendrite where spine density is reported to be the highest, were analyzed using electron microscope tomography (EMT). Because of the resolution limits of LM, the EM analysis was an extra measure taken to assure that our analysis included nearly all spines. Spine density measurements collected from the EMT data revealed only a modest decrease in the DAT KO group (n=3 mice) compared to age-matched WT controls (n=3 mice), a trend that supports the LM findings. Finally, a synaptic quantification using unbiased stereology did not detect a difference between DAT KO mice (n=6 mice) and WT controls (n=7 mice) at the EM level, supporting the focal nature of the early synaptic loss. These findings suggest that DAT KO mice have MSNs with highly localized spine loss and not an overall morphologically distinct cell shape. The characterization of morphological changes in DAT KO mice may provide information about the neural substrates underlying altered behaviors in these mice, with relevance for human neurological disorders thought to involve altered dopaminergic homeostasis. Results from this study also indicate the difficulty in correlating structural changes across scales, as the results on fine structure revealed thus far are subtle and non-uniform across striatal MSNs. The complexities associated with multiscale studies are driving the development of shared online informatics resources by gaining access to data where it is being analyzed.

A greater role for the norepinephrine transporter than the serotonin transporter in murine nociception

Norepinephrine and serotonin involvement in nociceptive functions is supported by observations of analgesic effects of norepinephrine transporter (NET) and serotonin transporter (SERT) inhibitors such as amitriptyline. However, the relative contribution of NET and SERT to baseline nociception, as well as amitriptyline analgesia, is unclear. Amitriptyline and morphine analgesia in wild-type (WT) mice and littermates with gene knockout (KO) of SERT, NET or both transporters was conducted using the hotplate and tail-flick tests. Hypoalgesia was observed in NET KO mice, and to a lesser extent in SERT KO mice. The magnitude of this hypoalgesia in NET KO mice was so profound that it limited the assessment of drug-induced analgesia. Nonetheless, the necessary exclusion of these subjects because of profound baseline hypoalgesia strongly supports the role of norepinephrine and NET in basal nociceptive behavior while indicating a much smaller role for serotonin and SERT. To further clarify the role of NET and SERT in basal nociceptive sensitivity further experiments were conducted in SERT KO and NET KO mice across a range of temperatures. NET KO mice were again found to have pronounced thermal hypoalgesia compared to WT mice in both the hotplate and tail-flick tests, while only limited effects were observed in SERT KO mice. Furthermore, in the acetic acid writhing test of visceral nociception pronounced hypoalgesia was again found in NET KO mice, but no change in SERT KO mice. As some of these effects may have resulted from developmental consequences of NET KO, the effects of the selective NET blocker nisoxetine and the selective SERT blocker fluoxetine were also examined in WT mice: only nisoxetine produced analgesia in these mice. Collectively these data suggest that NET has a far greater role in determining baseline analgesia, and perhaps other analgesic effects, than SERT in mice.

Implanted reuptake-deficient or wild-type dopaminergic neurons improve ON L-dopa dyskinesias without OFF-dyskinesias in a rat model of Parkinson's disease

OFF-L-dopa dyskinesias have been a surprising side-effect of intrastriatal foetal ventral mesencephalic transplantation in patients with Parkinson's disease. It has been proposed that excessive and unregulated dopaminergic stimulation of host post-synaptic striatal neurons by the grafts could be responsible for these dyskinesias. To address this issue we transplanted foetal dopaminergic neurons from mice lacking the dopamine transporter (DATKO) or from wild-type mice, into a rat model of Parkinson's disease and L-dopa-induced dyskinesias. Both wild-type and DATKO grafts reinnervated the host striatum to a similar extent, but DATKO grafts produced a greater and more diffuse increase in extra-cellular striatal dopamine levels. Interestingly, grafts containing wild-type dopaminergic neurons improved parkinsonian signs to a similar extent as DATKO grafts, but provided a more complete reduction of L-dopa induced dyskinesias. Neither DATKO nor wild-type grafts induced OFF-L-dopa dyskinesias. Behavioural and receptor autoradiography analyses demonstrated that DATKO grafts induced a greater normalization of striatal dopaminergic receptor supersensitivity than wild-type grafts. Both graft types induced a similar downregulation and normalization of PEnk and fosb/Deltafosb in striatal neurons. In summary, DATKO grafts causing high and diffuse extra-cellular dompamine levels do not per se alter graft-induced recovery or produce OFF-L-dopa dyskinesias. Wild-type dopaminergic neurons appear to be the most effective neuronal type to restore function and reduce L-dopa-induced dyskinesias.

Reduced expression of the vesicular acetylcholine transporter causes learning deficits in mice

Storage of acetylcholine in synaptic vesicles plays a key role in maintaining cholinergic function. Here we used mice with a targeted mutation in the vesicular acetylcholine transporter (VAChT) gene that reduces transporter expression by 40% to investigate cognitive processing under conditions of VAChT deficiency. Motor skill learning in the rotarod revealed that VAChT mutant mice were slower to learn this task, but once they reached maximum performance they were indistinguishable from wild-type mice. Interestingly, motor skill performance maintenance after 10 days was unaffected in these mutant mice. We also tested whether reduced VAChT levels affected learning in an object recognition memory task. We found that VAChT mutant mice presented a deficit in memory encoding necessary for the temporal order version of the object recognition memory, but showed no alteration in spatial working memory, or spatial memory in general when tested in the Morris water maze test. The memory deficit in object recognition memory observed in VAChT mutant mice could be reversed by cholinesterase inhibitors, suggesting that learning deficits caused by reduced VAChT expression can be ameliorated by restoring ACh levels in the synapse. These data indicate an important role for cholinergic tone in motor learning and object recognition memory.

Dopamine levels modulate the updating of tastant values

To survive, animals must constantly update the internal value of stimuli they encounter; a process referred to as incentive learning. Although there have been many studies investigating whether dopamine is necessary for reward, or for the association between stimuli and actions with rewards, less is known about the role of dopamine in the updating of the internal value of stimuli per se. We used a single-bottle forced-choice task to investigate the role of dopamine in learning the value of tastants. We show that dopamine transporter knock-out mice (DAT-KO), which have constitutively elevated dopamine levels, develop a more positive bias towards a hedonically positive tastant (sucrose 400 mM) than their wild-type littermates. Furthermore, when compared to wild-type littermates, DAT-KO mice develop a less negative bias towards a hedonically negative tastant (quinine HCl 10 mM). Importantly, these effects develop with training, because at the onset of training DAT-KO and wild-type mice display similar biases towards sucrose and quinine. These data suggest that dopamine levels can modulate the updating of tastant values, a finding with implications for understanding sensory-specific motivation and reward seeking.

Functional polymorphisms of the brain serotonin synthesizing enzyme tryptophan hydroxylase-2

Many neuropsychiatric disorders are considered to be related to the dysregulation of brain serotonergic neurotransmission. Tryptophan hydroxylase-2 (TPH2) is the neuronal-specific enzyme that controls brain serotonin synthesis. There is growing genetic evidence for the possible involvement of TPH2 in serotonin-related neuropsychiatric disorders; however, the degree of genetic variation in TPH2 and, in particular, its possible functional consequences remain unknown. In this short review, we will summarize some recent findings with respect to the functional analysis of TPH2.

Norepinephrine transporter knockout-induced up-regulation of brain alpha2A/C-adrenergic receptors

The norepinephrine transporter (NET) is responsible for the rapid removal of norepinephrine released from sympathetic neurons; this release is controlled by inhibitory alpha(2)-adrenergic receptors (alpha(2)ARs). Long-term inhibition of the NET by antidepressants has been reported to change the density and function of pre- and postsynaptic ARs, which may contribute to the antidepressant effects of NET inhibitors such as desipramine. NET-deficient (NET-KO) mice have been described to behave like antidepressant-treated mice. By means of quantitative real-time PCR we show that mRNAs encoding the alpha(2A)-adrenergic receptor (alpha(2A)AR) and the alpha(2C)-adrenergic receptor (alpha(2C)AR) are up-regulated in the brainstem, and that alpha(2C)AR mRNA is also elevated in the hippocampus and striatum of NET-KO mice. These results were confirmed at the protein level by quantitative autoradiography. The NET-KO mice showed enhanced binding of the selective alpha(2)AR antagonist [(3)H]RX821002 in several brain regions. Most robust increases (20-25%) in alpha(2)AR expression were observed in the hippocampus and in the striatum. Significant increases (16%) were also seen in the extended amygdala and thalamic structures. In an 'in vivo' test, the alpha(2)AR agonist clonidine (0.1 mg/kg) caused a significantly greater reduction of locomotor activity in NET-KO mice than in wild-type mice, showing the relevance of our findings at the functional level.

Norepinephrine transporter-deficient mice respond to anxiety producing and fearful environments with bradycardia and hypotension

The study of anxiety and fear involves complex interrelationships between psychiatry and the autonomic nervous system. Altered noradrenergic signaling is linked to certain types of depression and anxiety disorders, and treatment often includes specific transporter blockade. The norepinephrine transporter is crucial in limiting catecholaminergic signaling. Norepinephrine transporter-deficient mice have increased circulating catecholamines and elevated heart rate and blood pressure. We hypothesized, therefore, that reduced norepinephrine clearance would heighten the autonomic cardiovascular response to anxiety and fear. In separate experiments, norepinephrine transporter-deficient (norepinephrine transporter-/-) mice underwent tactile startle and trace fear conditioning to measure hemodynamic responses. A dramatic tachycardia was observed in norepinephrine transporter-/- mice compared with controls following both airpuff or footshock stimuli, and pressure changes were also greater. Interestingly, in contrast to normally elevated home cage levels in norepinephrine transporter-deficient mice, prestimulus heart rate and blood pressure were actually higher in norepinephrine transporter+/+ animals throughout behavioral testing. Upon placement in the behavioral chamber, norepinephrine transporter-deficient mice demonstrated a notable bradycardia and depressor effect that was more pronounced in females. Power spectral analysis indicated an increase in low frequency oscillations of heart rate variability; in mice, suggesting increased parasympathetic tone. Finally, norepinephrine transporter-/- mice exhibited sexual dimorphism in freeze behavior, which was greatest in females. Therefore, while reduced catecholamine clearance amplifies immediate cardiovascular responses to anxiety- or fear-inducing stimuli in norepinephrine transporter-/- mice, norepinephrine transporter deficiency apparently prevents protracted hemodynamic escalation in a fearful environment. Conceivably, chronic norepinephrine transporter blockade with transporter-specific drugs might attenuate recognition of autonomic and somatic distress signals in individuals with anxiety disorders, possibly lessening their behavioral reactivity, and reducing the cardiovascular risk factors associated with persistent emotional arousal.

Relative opioid efficacy is determined by the complements of the G protein-coupled receptor desensitization machinery

G protein-coupled receptor regulation by G protein-coupled receptor kinases and beta-arrestins can lead to desensitization and subsequent internalization of the receptor. In in vitro and cellular systems, beta-arrestins do not seem to play a major role in regulating micro opioid receptor (microOR) responsiveness. Removal of the betaarrestin2 (betaarr2) gene in mice leads paradoxically to enhanced and prolonged microOR-mediated antinociception. The betaarr2 knockout (betaarr2-KO) mice also fail to develop morphine antinociceptive tolerance in the hot-plate test, further indicating that the betaarr2 protein plays an essential role in microOR regulation in vivo. In this study, the contribution of betaarr2 to the regulation of the microOR was examined in both human embryonic kidney 293 cells and in betaarr2-KO mice after treatment with several opiate agonists. A green fluorescent protein tagged betaarr2 was used to assess receptor-betaarr2 interactions in living cells. Opiate agonists that induced robust betaarr2-green fluorescent protein translocation produced similar analgesia profiles in wild-type and betaarr2-KO mice, whereas those that do not promote robust betaarr2 recruitment, such as morphine and heroin, produce enhanced analgesia in vivo. In this report, we present a rationale to explain the seemingly paradoxical relationship between beta-arrestins and microOR regulation wherein morphine-like agonists fail to promote efficient internalization and resensitization of the receptor.

G protein-coupled receptor kinase 5 regulates airway responses induced by muscarinic receptor activation

G protein-coupled receptors (GPCRs) transduce extracellular signals into intracellular events. The waning responsiveness of GPCRs in the face of persistent agonist stimulation, or desensitization, is a necessary event that ensures physiological homeostasis. GPCR kinases (GRKs) are important regulators of GPCR desensitization. GRK5, one member of the GRK family, desensitizes central M(2) muscarinic receptors in mice. We questioned whether GRK5 might also be an important regulator of peripheral muscarinic receptor responsiveness in the cardiopulmonary system. Specifically, we wanted to determine the role of GRK5 in regulating muscarinic receptor-mediated control of airway smooth muscle tone or regulation of cholinergic-induced bradycardia. Tracheal pressure, heart rate, and tracheal smooth muscle tension were measured in mice having a targeted deletion of the GRK5 gene (GRK5(-/-)) and littermate wild-type (WT) control mice. Both in vivo and in vitro results showed that the airway contractile response to a muscarinic receptor agonist was not different between GRK5(-/-) and WT mice. However, the relaxation component of bilateral vagal stimulation and the airway smooth muscle relaxation resulting from beta(2)-adrenergic receptor activation were diminished in GRK5(-/-) mice. These data suggest that M(2) muscarinic receptor-mediated opposition of airway smooth muscle relaxation is regulated by GRK5 and is, therefore, excessive in GRK5(-/-) mice. In addition, this study shows that GRK5 regulates pulmonary responses in a tissue- and receptor-specific manner but does not regulate peripheral cardiac muscarinic receptors. GRK5 regulation of airway responses may have implications in obstructive airway diseases such as asthma or chronic obstructive pulmonary disease.

Differential psychostimulant-induced activation of neural circuits in dopamine transporter knockout and wild type mice

Dopamine (DA) is a neurotransmitter that has been implicated in a wide variety of psychiatric disorders that include attention deficit-hyperactivity disorder (ADHD), schizophrenia, and drug abuse. Recently, we have been working with a mouse in which the gene for the DA transporter (DAT) has been disrupted. This mouse is hyperactive in the open field, displays an inability to inhibit ongoing behaviors, and is deficient on learning and memory tasks. Psychostimulants such as amphetamine and methylphenidate attenuate the hyperlocomotion of the mutants, but stimulate activity of the wild type (WT) controls. The objective of the present study is to examine the neural basis for the differential responses to psychostimulants in these mice. WT and DAT knockout (KO) animals were given vehicle or methylphenidate, amphetamine, or cocaine and brain sections were immunostained for Fos. In WT mice, methylphenidate induced Fos-like immunoreactivity (Fos-LI) in the mesostriatal and mesolimbocortical DA pathways that included the anterior olfactory nucleus, frontal association cortex, orbitofrontal cortex, cingulate cortex, caudate-putamen, globus pallidus, claustrum, lateral septum, nucleus accumbens, basolateral and central nuclei of the amygdala, bed nucleus of stria terminalis, subthalamic nucleus, substantia nigra, ventral tegmental area, and dorsal raphe. Additional areas of activation included the granular dentate gyrus, Edinger-Westphal nucleus, and periaqueductal gray. While the mutants showed little response in most of these same areas, the anterior olfactory nucleus, caudal caudate-putamen, lateral septum, basolateral and central nuclei of the amygdala, and bed nucleus of stria terminalis were activated. Amphetamine and cocaine produced similar changes to that for methylphenidate, except these psychostimulants also induced Fos-LI in the nucleus accumbens of the KO animals. Since the DAT gene is disrupted in the KO mouse, these findings suggest that dopaminergic mechanisms may mediate the WT responses, whereas non-dopaminergic systems predominate in the mutant. In the mutants, it appears that limbic areas and non-dopaminergic transmitter systems within these brain regions may mediate responses to psychostimulants. Inasmuch as the KO mouse may represent a useful animal model for ADHD and because psychostimulants such as cocaine are reinforcing to these animals, our results may provide some useful insights into the neural mechanisms-other than DA-that may contribute to the symptoms of ADHD and/or drug abuse in human patients.

Dopamine autoreceptor regulation of release and uptake in mouse brain slices in the absence of D(3) receptors

The effects of the dopamine D(3) receptor, a putative autoreceptor, have been investigated by comparing behavioral and neurochemical properties of wild-type mice and mice with a genetic deletion of the D(3) receptor. The D(3) knock-out mice were modestly hyper-responsive to a novel environment relative to wild-type mice, and, consistent with this, quantitative in vivo microdialysis revealed elevated striatal dopamine extracellular levels. The dynamic actions of autoreceptors on electrically evoked dopamine release were examined in striatal brain slices from these animals and monitored with fast scan cyclic voltammetry at carbon-fiber microelectrodes. Quinpirole, a dopamine receptor agonist with potency at both D(2) and D(3) receptors, inhibited evoked dopamine in a dose-dependent manner with a slightly higher dose required in the knock-out animals (EC(50) of 60+/-10 nM in wild-type animals and 130+/-40 in D(3) knock-out animals; both curves had a Hill slope near 2). Dopamine synthesis inhibition with alpha-methyl-p-tyrosine caused released dopamine levels to decrease in each genotype. However, regulation of secretion by autoreceptors was still operant. Dose-response curves to quinpirole were unchanged in D(3) knock-out tissue, but secretion-regulated release exhibited a Hill slope decreased to 1 in the wild-type animals. In both genotypes, similar quinpirole-evoked increases in uptake rate were evident following synthesis inhibition. These data are consistent with the D(3) receptor having a small but significant role as a dopamine autoreceptor that partially regulates secretion, but not synthesis, in the caudate-putamen.

Desensitization of the isolated .beta.2-adrenergic receptor by .beta.-adrenergic receptor kinase, cAMP-dependent protein kinase, and protein kinase C occurs via distinct molecular mechanisms

Exposure of beta 2-adrenergic receptors (beta 2ARs) to agonists causes a rapid desensitization of the receptor-stimulated adenylyl cyclase response. Phosphorylation of the beta 2AR by several distinct kinases plays an important role in this desensitization phenomenon. In this study, we have utilized purified hamster lung beta 2AR and stimulatory guanine nucleotide binding regulatory protein (Gs), reconstituted in phospholipid vesicles, to investigate the molecular properties of this desensitization response. Purified hamster beta 2AR was phosphorylated by cAMP-dependent protein kinase (PKA), protein kinase C (PKC), or beta AR kinase (beta ARK), and receptor function was determined by measuring the beta 2AR-agonist-promoted Gs-associated GTPase activity. At physiological concentrations of Mg2+ (less than 1 mM), receptor phosphorylation inhibited coupling to Gs by 60% (PKA), 40% (PKC), and 30% (beta ARK). The desensitizing effect of phosphorylation was, however, greatly diminished when assays were performed at concentrations of Mg2+ sufficient to promote receptor-independent activation of Gs (greater than 5 mM). Addition of retinal arrestin, the light transduction component involved in the attenuation of rhodopsin function, did not enhance the uncoupling effect of beta ARK phosphorylation of beta 2AR when assayed in the presence of 0.3 mM free Mg2+. At concentrations of Mg2+ ranging between 0.5 and 5.0 mM, however, significant potentiation of beta ARK-mediated desensitization was observed upon arrestin addition. At a free Mg2+ concentration of 5 mM, arrestin did not potentiate the inhibition of receptor function observed on PKA or PKC phosphorylation. These results suggest that distinct pathways of desensitization exist for the receptor phosphorylated either by PKA or PKC or alternatively by beta ARK.

Hyperactivity, elevated dopaminergic transmission, and response to amphetamine in M1 muscarinic acetylcholine receptor-deficient mice

Acetylcholine serves an important modulatory role in the central nervous system. Pharmacological evidence has suggested that cholinergic activity can modulate central dopaminergic transmission; however, the nature of this interaction and the receptors involved remain undefined. In this study we have generated mice lacking the M1 muscarinic acetylcholine receptor and examined the effects of M1 deletion on dopaminergic transmission and locomotor behavior. We report that M1 deficiency leads to elevated dopaminergic transmission in the striatum and significantly increased locomotor activity. M1-deficient mice also have an increased response to the stimulatory effects of amphetamine. Our results provide direct evidence for regulation of dopaminergic transmission by the M1 receptor and are consistent with the idea that M1 dysfunction could be a contributing factor in psychiatric disorders in which altered dopaminergic transmission has been implicated.

The NH(2)-terminus of norepinephrine transporter contains a basolateral localization signal for epithelial cells

When expressed in epithelial cells, dopamine transporter (DAT) was detected predominantly in the apical plasma membrane, whereas norepinephrine transporter (NET) was found in the basolateral membrane, despite 67% overall amino acid sequence identity. To identify possible localization signals responsible for this difference, DAT-NET chimeras were expressed in MDCK cells and localized by immunocytochemistry and transport assays. The results suggested that localization of these transporters in MDCK cells depends on their highly divergent NH(2)-terminal regions. Deletion of the first 58 amino acids of DAT (preceding TM1) did not change its apical localization. However, the replacement of that region with corresponding sequence from NET resulted in localization of the chimeric protein to the basolateral membrane, suggesting that the NH(2)-terminus of NET, which contains two dileucine motifs, contains a basolateral localization signal. Mutation of these leucines to alanines in the context of a basolaterally localized NET/DAT chimera restored transporter localization to the apical membrane, indicating that the dileucine motifs are critical to the basolateral localization signal embodied within the NET NH(2)-terminal region. However, the same mutation in the context of wild-type NET did not disrupt basolateral localization, indicating the presence of additional signals in NET directing its basolateral localization within the plasma membrane.

A novel radioiodinated high affinity ligand for the D2 -dopamine receptor

A novel high affinity dopaminergic ligand, N-(p-aminophenethyl)spiroperidol, has been synthesized and radioiodinated to a specific radioactivity of 2175 Ci/mmol. Binding of this ligand to bovine anterior pituitary membranes is: rapid (40-60 min to equilibrium at 25 degrees C) and reversible (t1/2 = 1 h at 25 degrees C); saturable and of high affinity (KD approximately 20 pM) and displays a typical D2-dopaminergic specificity. The ligand, which identifies the same number of receptor sites as other tritiated antagonist ligands, can be used in different tissues and preparations to delineate the characteristics of the D2 receptor. Thus, this high affinity, high specific radioactivity ligand (N-(p-amino-m-[125I]iodophenethyl)spiroperidol) represents a tool which until now had not been available for the characterization of the D2-dopamine receptor.

Differential regulation of the dopamine D2 and D3 receptors by G protein-coupled receptor kinases and beta-arrestins

The D(2) and D(3) receptors (D(2)R and D(3)R), which are potential targets for antipsychotic drugs, have a similar structural architecture and signaling pathway. Furthermore, in some brain regions they are expressed in the same cells, suggesting that differences between the two receptors might lie in other properties such as their regulation. In this study we investigated, using COS-7 and HEK-293 cells, the mechanism underlying the intracellular trafficking of the D(2)R and D(3)R. Activation of D(2)R caused G protein-coupled receptor kinase-dependent receptor phosphorylation, a robust translocation of beta-arrestin to the cell membrane, and profound receptor internalization. The internalization of the D(2)R was dynamin-dependent, suggesting that a clathrin-coated endocytic pathway is involved. In addition, the D(2)R, upon agonist-mediated internalization, localized to intracellular compartments distinct from those utilized by the beta(2)-adrenergic receptor. However, in the case of the D(3)R, only subtle agonist-mediated receptor phosphorylation, beta-arrestin translocation to the plasma membrane, and receptor internalization were observed. Interchange of the second and third intracellular loops of the D(2)R and D(3)R reversed their phenotypes, implicating these regions in the regulatory properties of the two receptors. Our studies thus indicate that functional distinctions between the D(2)R and D(3)R may be found in their desensitization and cellular trafficking properties. The differences in their regulatory properties suggest that they have distinct physiological roles in the brain.

Catecholamine release and uptake in the mouse prefrontal cortex

Monitoring the release and uptake of catecholamines from terminals in weakly innervated brain regions is an important step in understanding their importance in normal brain function. To that end, we have labeled brain slices from transgenic mice that synthesize placental alkaline phosphatase (PLAP) on neurons containing tyrosine hydroxylase with antibody-fluorochrome conjugate, PLAP-Cy5. Excitation of the fluorochrome enables catecholamine neurons to be visualized in living tissue. Immunohistochemical fluorescence with antibodies to tyrosine hydroxylase and dopamine beta-hydroxylase revealed that the PLAP labeling was specific to catecholamine neurons. In the prefrontal cortex (PFC), immunohistochemical fluorescence of the PLAP along with staining for dopamine transporter (DAT) and norepinephrine transporter (NET) revealed that all three exhibit remarkable spatial overlap. Fluorescence from the PLAP antibody was used to position carbon-fiber microelectrodes adjacent to catecholamine neurons in the PFC. Following incubation with L-DOPA, catecholamine release and subsequent uptake was measured and the effect of uptake inhibitors examined. Release and uptake in NET and DAT knockout mice were also monitored. Uptake rates in the cingulate and prelimbic cortex are so slow that catecholamines can exist in the extracellular fluid for sufficient time to travel approximately 100 microm. The results support heterologous uptake of catecholamines and volume transmission in the PFC of mice.

Glutamatergic modulation of hyperactivity in mice lacking the dopamine transporter

In the brain, dopamine exerts an important modulatory influence over behaviors such as emotion, cognition, and affect as well as mechanisms of reward and the control of locomotion. The dopamine transporter (DAT), which reuptakes the released neurotransmitter into presynaptic terminals, is a major determinant of the intensity and duration of the dopaminergic signal. Knockout mice lacking the dopamine transporter (DAT-KO mice) display marked changes in dopamine homeostasis that result in elevated dopaminergic tone and pronounced locomotor hyperactivity. A feature of DAT-KO mice is that their hyperactivity can be inhibited by psychostimulants and serotonergic drugs. The pharmacological effect of these drugs occurs without any observable changes in dopaminergic parameters, suggesting that other neurotransmitter systems in addition to dopamine might contribute to the control of locomotion in these mice. We report here that the hyperactivity of DAT-KO mice can be markedly further enhanced when N-methyl-d-aspartate receptor-mediated glutamatergic transmission is blocked. Conversely, drugs that enhance glutamatergic transmission, such as positive modulators of l-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate glutamate receptors, suppress the hyperactivity of DAT-KO mice. Interestingly, blockade of N- methyl-d-aspartate receptors prevented the inhibitory effects of both psychostimulant and serotonergic drugs on hyperactivity. These findings support the concept of a reciprocal functional interaction between dopamine and glutamate in the basal ganglia and suggest that agents modulating glutamatergic transmission may represent an approach to manage conditions associated with dopaminergic dysfunction.

Genetic animal models: focus on schizophrenia

The neurobiology of schizophrenia remains poorly understood. Symptoms of schizophrenia are classically thought to be associated with an imbalance of the dopaminergic system. However, the contribution of other neurotransmitters, in particular glutamate, has been increasingly appreciated. The role of individual components of neurotransmitter systems in aberrant behaviors can be experimentally tested in transgenic animals. Dopamine transporter knockout mice display persistently elevated dopaminergic tone and therefore might be appropriate substrates to evaluate the dopamine hypothesis. Similarly, NMDA receptor-deficient mice can be used to evaluate the glutamate hypothesis of schizophrenia. In this review we discuss how such animal models might be relevant for understanding the neurochemical underpinnings of certain manifestations of schizophrenia.

Agonist-dependent recruitment of phosphoinositide 3-kinase to the membrane by beta-adrenergic receptor kinase 1. A role in receptor sequestration

Agonist-dependent desensitization of the beta-adrenergic receptor requires translocation and activation of the beta-adrenergic receptor kinase1 by liberated Gbetagamma subunits. Subsequent internalization of agonist-occupied receptors occurs as a result of the binding of beta-arrestin to the phosphorylated receptor followed by interaction with the AP2 adaptor and clathrin proteins. Receptor internalization is known to require D-3 phosphoinositides that are generated by the action of phosphoinositide 3-kinase. Phosphoinositide 3-kinases form a family of lipid kinases that couple signals via receptor tyrosine kinases and G-protein-coupled receptors. The molecular mechanism by which phosphoinositide 3-kinase acts to promote beta-adrenergic receptor internalization is not well understood. In the present investigation we demonstrate a novel finding that beta-adrenergic receptor kinase 1 and phosphoinositide 3-kinase form a cytosolic complex, which leads to beta-adrenergic receptor kinase 1-mediated translocation of phosphoinositide 3-kinase to the membrane in an agonist-dependent manner. Furthermore, agonist-induced translocation of phosphoinositide 3-kinase results in rapid interaction with the receptor, which is of functional importance, since inhibition of phosphoinositide 3-kinase activity attenuates beta-adrenergic receptor sequestration. Therefore, agonist-dependent recruitment of phosphoinositide 3-kinase to the membrane is an important step in the process of receptor sequestration and links phosphoinositide 3-kinase to G-protein-coupled receptor activation and sequestration.

Molecular determinants underlying the formation of stable intracellular G protein-coupled receptor-beta-arrestin complexes after receptor endocytosis*

beta-Arrestins bind agonist-activated G protein-coupled receptors (GPCRs) and mediate their desensitization and internalization. Although beta-arrestins dissociate from some receptors at the plasma membrane, such as the beta2 adrenergic receptor, they remain associated with other GPCRs and internalize with them into endocytic vesicles. Formation of stable receptor-beta-arrestin complexes that persist inside the cell impedes receptor resensitization, and the aberrant formation of these complexes may play a role in GPCR-based diseases (Barak, L. S., Oakley, R. H., Laporte, S. A., and Caron, M. G. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 93-98). Here, we investigate the molecular determinants responsible for sustained receptor/beta-arrestin interactions. We show in real time and in live human embryonic kidney (HEK-293) cells that a beta-arrestin-2-green fluorescent protein conjugate internalizes into endocytic vesicles with agonist-activated neurotensin-1 receptor, oxytocin receptor, angiotensin II type 1A receptor, and substance P receptor. Using receptor mutagenesis, we demonstrate that the ability of beta-arrestin to remain associated with these receptors is mediated by specific clusters of serine and threonine residues located in the receptor carboxyl-terminal tail. These clusters are remarkably conserved in their position within the carboxyl-terminal domain and serve as primary sites of agonist-dependent receptor phosphorylation. In addition, we identify a beta-arrestin mutant with enhanced affinity for the agonist-activated beta2-adrenergic receptor that traffics into endocytic vesicles with receptors that lack serine/threonine clusters and normally dissociate from wild-type beta-arrestin at the plasma membrane. By identifying receptor and beta-arrestin residues critical for the formation of stable receptor-beta-arrestin complexes, these studies provide novel targets for regulating GPCR responsiveness and treating diseases resulting from abnormal GPCR/beta-arrestin interactions.

Functional interaction between monoamine plasma membrane transporters and the synaptic PDZ domain-containing protein PICK1

PDZ domain-containing proteins play an important role in the targeting and localization of synaptic membrane proteins. Here, we report an interaction between the PDZ domain-containing protein PICK1 and monoamine neurotransmitter transporters in vitro and in vivo. In dopaminergic neurons, PICK1 colocalizes with the dopamine transporter (DAT) and forms a stable protein complex. Coexpression of PICK1 with DAT in mammalian cells and neurons in culture results in colocalization of the two proteins in a cluster pattern and an enhancement of DAT uptake activity through an increase in the number of plasma membrane DAT. Deletion of the PDZ binding site at the carboxyl terminus of DAT abolishes its association with PICK1 and impairs the localization of the transporter in neurons. These findings indicate a role for PDZ-mediated protein interactions in the localization, expression, and function of monoamine transporters.

Role of the sphingosine-1-phosphate receptor EDG-1 in PDGF-induced cell motility

EDG-1 is a heterotrimeric guanine nucleotide binding protein-coupled receptor (GPCR) for sphingosine-1-phosphate (SPP). Cell migration toward platelet-derived growth factor (PDGF), which stimulates sphingosine kinase and increases intracellular SPP, was dependent on expression of EDG-1. Deletion of edg-1 or inhibition of sphingosine kinase suppressed chemotaxis toward PDGF and also activation of the small guanosine triphosphatase Rac, which is essential for protrusion of lamellipodia and forward movement. Moreover, PDGF activated EDG-1, as measured by translocation of beta-arrestin and phosphorylation of EDG-1. Our results reveal a role for receptor cross-communication in which activation of a GPCR by a receptor tyrosine kinase is critical for cell motility.

Hyperactivity and impaired response habituation in hyperdopaminergic mice

Abnormal dopaminergic transmission is implicated in schizophrenia, attention deficit hyperactivity disorder, and drug addiction. In an attempt to model aspects of these disorders, we have generated hyperdopaminergic mutant mice by reducing expression of the dopamine transporter (DAT) to 10% of wild-type levels (DAT knockdown). Fast-scan cyclic voltammetry and in vivo microdialysis revealed that released dopamine was cleared at a slow rate in knockdown mice, which resulted in a higher extracellular dopamine concentration. Unlike the DAT knockout mice, the DAT knockdown mice do not display a growth retardation phenotype. They have normal home cage activity but display hyperactivity and impaired response habituation in novel environments. In addition, we show that both the indirect dopamine receptor agonist amphetamine and the direct agonists apomorphine and quinpirole inhibit locomotor activity in the DAT knockdown mice, leading to the hypothesis that a shift in the balance between dopamine auto and heteroreceptor function may contribute to the therapeutic effect of psychostimulants in attention deficit hyperactivity disorder.

Constitutive arrestin-mediated desensitization of a human vasopressin receptor mutant associated with nephrogenic diabetes insipidus

Agonist-dependent desensitization and internalization of G protein-coupled receptors (GPCR) are mediated by the binding of arrestins to phosphorylated receptors. The affinity of arrestins for the phosphorylated GPCR regulates the ability of the internalized receptor to be dephosphorylated and recycled back to the plasma membrane. In this study, we show that the naturally occurring loss of function vasopressin receptor mutation R137H, which is associated with familial nephrogenic diabetes insipidus, induces constitutive arrestin-mediated desensitization. In contrast to the wild-type vasopressin receptor, the nonsignaling R137H receptor is phosphorylated and sequestered in arrestin-associated intracellular vesicles even in the absence of agonist. Eliminating molecular determinants on the receptor that promote high affinity arrestin-receptor interaction reestablishes plasma membrane localization and the ability of the mutated receptors to signal. These findings suggest that unregulated desensitization can contribute to the etiology of a GPCR-based disease, implying that pharmacological targeting of GPCR desensitization may be therapeutically beneficial.

Dopamine transporter and vesicular monoamine transporter knockout mice : implications for Parkinson's disease

One of the most valuable methods for understanding the function of a particular protein is the generation of animals that have had the gene encoding for the protein of interest disrupted, commonly known as a "quo;knockout"quo; or null mutant. By incorporating a sequence of DNA (typically encoding antibiotic resistance to aid in the selection of the mutant gene) into embryonic stem cells by homologous recombination, the normal transcription of the gene is effectively blocked (Fig. 1). Since a particular protein is encoded by two copies of a gene, it is necessary to have the gene on both alleles "quo;knocked out."quo; This is performed by cross-breeding animals with one affected allele (heterozygote) to generate offspring that have inherited two mutant alleles (homozygote). This procedure has been used to generate animals lacking either the plasma membrane dopamine transporter (DAT; Fig. 2) or the vesicular monoamine transporter (VMAT2; Fig. 3). Both DAT and VMAT2 are essential for dopamine homeostasis and are thought to participate in the pathogenesis of Parkinson's disease (1-5). Fig. 1. Maps of the targeting vector and the mock construct. The mouse genomic fragment (clone 11) was isolated from a Stratagene 129 SvJ library by standard colony hybridization using a PCR probe from the 5' end of rat cDNA. The restriction site abbreviations are as follows: H, HindIII; N, NotI; Sc, SacI; Sn, SnaI; X, XbaI; and Xh, XhoI. The region between HindIII and SnaI on clone 11 containing the coding sequence from transmembrane domains 3 and 4 of VMAT2 was deleted and replaced with PGK-neo. The 3' fragment of clone 11 was reserved as an external probe for Southern analysis. To facilitate PCR screening of embryonic stem cell clones, a mock construct containing the SnaI/XbaI fragment and part of the Neo cassette was generated as a positive control. pPNT and pGEM4Z were used to construct knockout and mock vectors, respectively. (Reproduced with permission from ref. 1). Fig. 2. DAT and VMAT2 expression in wild-type and DAT knockout midbrain. DAT immunoreactivity in wild-type (A) and DAT knockout midbrain (B). VMAT2 immunoreactivity in wild-type (C) and DAT knockout midbrain (D). Robust immunoreactivity was observed in the ventral tegmental area and substantia nigra pars compacta and reticulata in the wild-type brain. Note absence of DAT immunoreactivity and modest reduction of VMAT2 immunoreactivity in the DAT knockout. Fig. 3. Characterization of VMAT2 gene disruption. (A) Southern blot analysis of mouse genomic DNA. The Southern blot was prepared with 15 μg of genomic DNA per lane and probed with a 1.4-kb 3' external genomic fragment. +/+, wild type littermates; +/-, heterozygote; -/-, homozygote. (B) RT-PCR analysis of mouse brain poly(A)+ RNA. For each reverse transcription assay, 0.5 μg of poly(A)+ RNA was used. Equal volumes of cDNA templates were used for each PCR assay. The PCR primers used flank the neomycin cassette for the purpose of detecting potential readthrough of the neomycin DNA. The heterozygote has a reduced amount of transcripts compared with the wild-type littermate; the homozygote is devoid of VMAT2 transcripts. G3PDH was used as internal control. (C) Western blot analysis of wholebrain synaptic vesicles. Samples (25 μg) of vesicles were solubilized and separated by SDS-PAGE, transferred to nitrocellulose, subjected to Western blot analysis with anti-VMAT2-Ct (top) or anti-a-tubulin (bottom) antibodies, and developed with chemiluminescence. Molecular mass markers (kDa) are shown to the left. To confirm equal loading and transfer of proteins, the blots were stripped and reprobed with an antibody to α-tubulin. (Reproduced with permission from ref. 1). The importance of DAT in neuronal function is highlighted in animals in which DAT has been genetically deleted (DAT KO) (3). In the homozygote DAT KO mice, released dopamine remains in the extracellular space up to 300 times longer than normal. As expected, these animals display behaviors consistent with persistent activation of dopamine receptors, such as hyperlocomotion. Genetic deletion of VMAT2 reveals the essential role of vesicular storage and release of monoamines. Homozygote VMAT2 knockout mice survive for only a few days, whereas heterozygotes appear normal. Studies performed in homozygote pups and heterozygote adults clearly show that the level of VMAT2 expression calibrates the level of vesicular filling (1,2,bi4). With only 50% of normal VMAT2, heterozygote animals have reduced vesicular filling and release. These alterations in presynaptic monoamine function in the heterozygotes are thought to be responsible for the observed sensitization to the psychostimulants cocaine and amphetamine and to ethanol (1). Knockout animals also appear to parallel the changes that occur in reserpinized animals, suggesting that the adverse actions of this drug are mediated by VMAT2.

Prepulse inhibition deficits and perseverative motor patterns in dopamine transporter knock-out mice: differential effects of D1 and D2 receptor antagonists

Dopamine is known to regulate several behavioral phenomena, including sensorimotor gating and aspects of motor activity. The roles of dopamine D1 and D2 receptors in these behaviors have been documented in the rat literature, but few reports exist on their role in mice. We used dopamine transporter (DAT) (-/-) mice to examine the behavioral consequences of a chronically hyperdopaminergic state, challenging them with the preferential dopamine D2 receptor antagonist raclopride and D1 receptor antagonist SCH23390. At baseline, DAT (-/-) mice exhibited deficient sensorimotor gating as measured by prepulse inhibition (PPI) of the startle response, exhibited nonfocal preservative patterns of locomotion, and were hyperactive in a novel environment. Pretreatment with raclopride significantly increased PPI in the DAT (-/-) mice, whereas SCH23390 had no significant effect. Blockade of D2 receptors did not affect the predominantly straight patterns of motor behavior produced by the DAT (-/-) mice, but antagonism of D1 receptors significantly attenuated the preservative patterns, producing more of a meandering behavior seen in the DAT (+/+) control mice. Both D1 and D2 receptor antagonists decreased the hyperactivity seen in the DAT (-/-) mice. These findings support the role of the D2, but not the D1, receptor in the modulation of PPI in mice. Furthermore, D1 receptor activation appears to be the critical substrate for the expression of preservative patterns of motor behavior, whereas both D1 and D2 receptors appear to regulate the amount of motor activity.

Potentiated opioid analgesia in norepinephrine transporter knock-out mice

Several studies have shown that activation of alpha(2)-adrenergic receptors (alpha(2)ARs) leads to mild analgesic effects. Tricyclic antidepressants (TCAs), such as desipramine (DMI), which block norepinephrine transporters (NETs), also produce mild antinociception. The coadministration of either alpha(2)AR agonists or TCAs with opiates produces synergistically potentiated antinociception. It has been postulated that the analgesic effects of TCAs are determined by their ability to inhibit norepinephrine reuptake via interactions with the NET. To test this idea, we studied mice lacking a functional NET in spontaneous and morphine-induced antinociceptive paradigms. Morphine (10 mg/kg, s.c. ) treatment produced greater analgesia, as assayed in the warm water tail-flick assay, in NET-knock-out (-KO) mice than in wild-type (WT) mice. As anticipated, yohimbine, an inhibitor of alpha(2)ARs, blocked this potentiation. Moreover, a warm water swim-stress paradigm, which is known to induce the release of endogenous opioids, produced greater antinociception in NET-KO than in the WT mice. Naloxone, an inhibitor of opioid receptors, blocked the development of the swim-evoked analgesia in both WT and NET-KO mice, confirming the involvement of the endogenous opioid system. In the NET-KO mice, DMI did not further enhance analgesia but was still able to produce inhibitory effects on the locomotor activity of these mutants, suggesting that the effects of this TCA are not exclusively via interactions with the NET. In summary, these results demonstrate in a genetic model that both endogenous and exogenous opiate-mediated analgesia can be enhanced by elimination of the NET, indicating that the interaction of TCAs with NET mediates these effects.

Mu-opioid receptor desensitization by beta-arrestin-2 determines morphine tolerance but not dependence

Morphine is a powerful pain reliever, but also a potent inducer of tolerance and dependence. The development of opiate tolerance occurs on continued use of the drug such that the amount of drug required to elicit pain relief must be increased to compensate for diminished responsiveness. In many systems, decreased responsiveness to agonists has been correlated with the desensitization of G-protein-coupled receptors. In vitro evidence indicates that this process involves phosphorylation of G-protein-coupled receptors and subsequent binding of regulatory proteins called beta-arrestins. Using a knockout mouse lacking beta-arrestin-2 (beta arr2-/-), we have assessed the contribution of desensitization of the mu-opioid receptor to the development of morphine antinociceptive tolerance and the subsequent onset of physical dependence. Here we show that in mice lacking beta-arrestin-2, desensitization of the mu-opioid receptor does not occur after chronic morphine treatment, and that these animals fail to develop antinociceptive tolerance. However, the deletion of beta-arrestin-2 does not prevent the chronic morphine-induced up-regulation of adenylyl cyclase activity, a cellular marker of dependence, and the mutant mice still become physically dependent on the drug.

Dopamine D(5) receptor immunolocalization in rat and monkey brain

Dopamine D(5) receptor localization has been difficult because even the most specific ligands cannot distinguish between molecular subtypes of the D(1)-like receptor subfamily. Antifusion protein rabbit polyclonal antibodies directed against the C-terminus of human D(5) receptor were therefore developed for immunolocalization of the D(5) receptor protein in brain. The antibodies were characterized by immunoblot analysis and immunoprecipitation and used for light microscopic immunocytochemistry in rat and monkey brain. Affinity purified D(5) antibodies were specific for D(5) fusion protein as well as cloned and native D(5) receptor on Western blots, and D(5) antisera specifically immunoprecipitated solubilized, cloned D(5) receptor. Regional distribution of D(5) receptor immunoreactivity was consistent across species and correlated well with D(5) mRNA distribution previously reported in monkey brain. Immunoreactivity was widespread and tended to label perikarya and proximal dendrites of neurons in cerebral cortex, basal ganglia, basal forebrain, hippocampus, diencephalon, brainstem, and cerebellum. Neuropil was immunoreactive in olfactory bulb, islands of Calleja, cerebral cortex, superior colliculus, and molecular layer of cerebellum. The distribution of D(5) in brain was clearly different from that of other dopamine receptor subtypes, including D(1), the other member of the D(1)-like receptor subfamily. This unique distribution corroborates the idea that the D(5) receptor subtype has a distinct role in dopamine neurotransmission.

Mice lacking the dopamine transporter display altered regulation of distal colonic motility

The mechanisms by which dopamine (DA) influences gastrointestinal (GI) tract motility are incompletely understood and complicated by tissue- and species-specific differences in dopaminergic function. To improve the understanding of DA action on GI motility, we used an organ tissue bath system to characterize motor function of distal colonic smooth muscle segments from wild-type and DA transporter knockout (DAT -/-) mice. In wild-type mice, combined blockade of D(1) and D(2) receptors resulted in significant increases in tone (62 +/- 9%), amplitude of spontaneous phasic contractions (167 +/- 24%), and electric field stimulation (EFS)-induced (40 +/- 8%) contractions, suggesting that endogenous DA is inhibitory to mouse distal colonic motility. The amplitudes of spontaneous phasic and EFS-induced contractions were lower in DAT -/- mice relative to wild-type mice. These differences were eliminated by combined D(1) and D(2) receptor blockade, indicating that the inhibitory effects of DA on distal colonic motility are potentiated in DAT -/- mice. Motility index was decreased but spontaneous phasic contraction frequency was enhanced in DAT -/- mice relative to wild-type mice. The fact that spontaneous phasic and EFS-induced contractile activity were altered by the lack of the DA transporter suggests an important role for endogenous DA in modulating motility of mouse distal colon.

The interaction of beta-arrestin with the AP-2 adaptor is required for the clustering of beta 2-adrenergic receptor into clathrin-coated pits

Beta-arrestins are cytosolic proteins that regulate the signaling and the internalization of G protein-coupled receptors (GPCRs). Although termination of receptor coupling requires beta-arrestin binding to agonist-activated receptors, GPCR endocytosis involves the coordinate interactions between receptor-beta-arrestin complexes and other endocytic proteins such as adaptor protein 2 (AP-2) and clathrin. Clathrin interacts with a conserved motif in the beta-arrestin C-terminal tail; however, the specific molecular determinants in beta-arrestin that bind AP-2 have not been identified. Moreover, the respective contributions of the interactions of beta-arrestin with AP-2 and clathrin toward the targeting of GPCRs to clathrin-coated vesicles have not been established. Here, we identify specific arginine residues (Arg(394) and Arg(396)) in the beta-arrestin 2 C terminus that mediate beta-arrestin binding to AP-2 and show, in vitro, that these domains in beta-arrestin 1 and 2 interact equally well with AP-2 independently of clathrin binding. We demonstrate in HEK 293 cells by fluorescence microscopy that beta(2)-adrenergic receptor-beta-arrestin complexes lacking the beta-arrestin-clathrin binding motif are still targeted to clathrin-coated pits. In marked contrast, receptor-beta-arrestin complexes lacking the beta-arrestin/AP-2 interactions are not effectively compartmentalized in punctated areas of the plasma membrane. These results reveal that the binding of a receptor-beta-arrestin complex to AP-2, not to clathrin, is necessary for the initial targeting of beta(2)-adrenergic receptor to clathrin-coated pits.

Differential affinities of visual arrestin, beta arrestin1, and beta arrestin2 for G protein-coupled receptors delineate two major classes of receptors

Visual arrestin, betaarrestin1, and betaarrestin2 comprise a family of intracellular proteins that desensitize G protein-coupled receptors (GPCRs). In addition, betaarrestin1 and betaarrestin2 target desensitized receptors to clathrin-coated pits for endocytosis. Whether arrestins differ in their ability to interact with GPCRs in cells is not known. In this study, we visualize the interaction of arrestin family members with GPCRs in real time and in live cells using green fluorescent protein-tagged arrestins. In the absence of agonist, visual arrestin and betaarrestin1 were found in both the cytoplasm and nucleus of HEK-293 cells, whereas betaarrestin2 was found only in the cytoplasm. Analysis of agonist-mediated arrestin translocation to multiple GPCRs identified two major classes of receptors. Class A receptors (beta2 adrenergic receptor, mu opioid receptor, endothelin type A receptor, dopamine D1A receptor, and alpha1b adrenergic receptor) bound betaarrestin2 with higher affinity than betaarrestin1 and did not interact with visual arrestin. In contrast, class B receptors (angiotensin II type 1A receptor, neurotensin receptor 1, vasopressin V2 receptor, thyrotropin-releasing hormone receptor, and substance P receptor) bound both betaarrestin isoforms with similar high affinities and also interacted with visual arrestin. Switching the carboxyl-terminal tails of class A and class B receptors completely reversed the affinity of each receptor for the visual and non-visual arrestins. In addition, exchanging the betaarrestin1 and betaarrestin2 carboxyl termini reversed their extent of binding to class A receptors as well as their subcellular distribution. These results reveal for the first time marked differences in the ability of arrestin family members to bind GPCRs at the plasma membrane. Moreover, they show that visual arrestin can interact in cells with GPCRs other than rhodopsin. These findings suggest that GPCR signaling may be differentially regulated depending on the cellular complement of arrestin isoforms and the ability of arrestins to interact with other cellular proteins.

Mice lacking the norepinephrine transporter are supersensitive to psychostimulants

The action of norepinephrine (NE) is terminated, in part, by its uptake into presynaptic noradrenergic neurons by the plasma-membrane NE transporter (NET), which is a target for antidepressants and psychostimulants. Disruption of the NET gene in mice prolonged the clearance of NE and elevated extracellular levels of this catecholamine. In a classical test for antidepressant drugs, the NET-deficient (NET-/-) animals behaved like antidepressant-treated wild-type mice. Mutants were hyper-responsive to locomotor stimulation by cocaine or amphetamine. These responses were accompanied by dopamine D2/D3 receptor supersensitivity. Thus altering NET expression significantly modulates midbrain dopaminergic function, an effect that may be an important component of the actions of antidepressants and psychostimulants.