Reward without dopamine

Dopamine (DA) is believed to play a fundamental role in reward processes. Virtually all drugs of abuse activate dopaminergic systems, as do "natural" rewards such as sexual interaction and food. Sweet-tasting solutions, for example, are a well characterized natural reward. In the present experiments, we used mice that cannot make DA (DD mice) to test the hypothesis that DA is necessary for reward. Sucrose preference, assessed with a computerized "lickometer," was used to determine whether DD mice respond preferentially for rewarding stimuli. DD mice preferentially chose sucrose over water, and also preferred the noncaloric sweetener saccharin. Furthermore, the rate of licking, bout size, and length were greater in DD mice drinking sweets than in controls. These data refute the necessity of DA for the reward processes manifested by sucrose preference. However, DD mice initiated licking less frequently than control mice and had fewer total licks. We suggest that DD mice have a deficit of goal-directed behavior that is not specific to reward processes. Lastly, juvenile DD mice demonstrate robust sucrose preference before experience with food in the presence of DA. Thus, DA is not required for mice to learn to consume sweet solutions preferentially. We conclude that DA is not required to find the sweet tastes of sucrose or saccharin rewarding.

The genetics of schizophrenia: glutamate not dopamine?

The major targets of current drugs used in mental health, such as neurotransmitter receptors and transporters, are based on serendipitous findings from several decades ago, and there is currently a severe drought of new drug targets. There is a pressing need for novel drugs, and much hope has been placed on the use of molecular genetics to help define them. However, despite evidence for a genetic basis to schizophrenia stretching back for over a century, and a heritability of about 80%, the identification of susceptibility genes has been an uphill struggle. Candidate gene studies, which have generally focussed on obvious candidates from the dopamine and serotonin systems, as well as genes involved in brain development, have not generally been successful, although meta-analysis indicates that the dopamine D3 receptor gene (DRD3) and the serotonin receptor gene type 2A (HTR2A) may have a very small influence on risk. Linkage analysis has provided robust evidence of genetic loci, for example, on chromosomes 8p, 13q and 22q, and also implies shared genetic aetiology with bipolar disorder. The identification of these loci together with advances in genetic technology, especially the characterisation of polymorphisms, the understanding of haplotypes and the development of statistical methods, has lead to the identification of several plausible susceptibility genes, including neuregulin 1, proline dehydrogenase and dysbindin. Interestingly, these genes point more towards a role for the glutamate pathway rather than the dopamine pathway in schizophrenia. We have attempted to replicate some of these findings in schizophrenic patients from SW China, and we find significant association with a novel neuregulin 1 haplotype, with proline dehydrogenase polymorphisms, but not with catechol-O-methyltransferase (COMT). The replication of neuregulin 1 association on chromosome 8p by several investigators is the most convincing to date, and the presence of a syndrome similar to dementia praecox of 8p linked families, and the lack of linkage of bipolar disorder to this region is a testament to the ideas of Kraepelin more than 100 years ago.

Hyperdopaminergic mutant mice have higher "wanting" but not "liking" for sweet rewards

What is the role of dopamine in natural rewards? A genetic mutant approach was taken to examine the consequences of elevated synaptic dopamine on (1) spontaneous food and water intake, (2) incentive motivation and learning to obtain a palatable sweet reward in a runway task, and (3) affective "liking" reactions elicited by the taste of sucrose. A dopamine transporter (DAT) knockdown mutation that preserves only 10% of normal DAT, and therefore causes mutant mice to have 70% elevated levels of synaptic dopamine, was used to identify dopamine effects on food intake and reward. We found that hyperdopaminergic DAT knockdown mutant mice have higher food and water intake. In a runway task, they demonstrated enhanced acquisition and greater incentive performance for a sweet reward. Hyperdopaminergic mutant mice leave the start box more quickly than wild-type mice, require fewer trials to learn, pause less often in the runway, resist distractions better, and proceed more directly to the goal. Those observations suggest that hyperdopaminergic mutant mice attribute greater incentive salience ("wanting") to a sweet reward in the runway test. But sucrose taste fails to elicit higher orofacial "liking" reactions from mutant mice in an affective taste reactivity test. These results indicate that chronically elevated extracellular dopamine facilitates "wanting" and learning of an incentive motivation task for a sweet reward, but elevated dopamine does not increase "liking" reactions to the hedonic impact of sweet tastes.

Overexpression of the 32-kilodalton dopamine and cyclic adenosine 3?,5?-monophosphate-regulated phosphoprotein in common adenocarcinomas

BACKGROUND

The 32-kilodalton dopamine and cyclic adenosine 3',5'-monophosphate-regulated phosphoprotein (DARPP-32) is the only known protein that can act as a protein phosphatase-1 inhibitor or a protein kinase inhibitor. It is known as a neurogenic protein that plays a major role in dopamine regulation in the central nervous system. Recently, DARPP-32 and a truncated isoform, t-DARPP, have been cloned in gastric carcinoma. The extent of DARPP-32 and t-DARPP expression in human epithelial tissues and carcinomas remained unknown.

METHODS

Using tissue microarrays and an antibody to DARPP-32, the authors evaluated the immunohistochemical findings in 43 different normal epithelium human tissue samples and in 187 samples of common carcinomas of breast, prostate, colon, and stomach.

RESULTS

DARPP-32 proteins were expressed at varying levels in several types of normal epithelial tissues outside of the central nervous system. Using quantitative real-time reverse transcriptase-polymerase chain reaction analysis, the authors demonstrated that both DARPP-32 and t-DARPP mRNAs frequently were overexpressed in carcinomas of the breast, prostate, colon, and stomach compared with normal tissue samples. Immunohistochemical analysis of tissue microarrays that contained 187 carcinoma samples confirmed the strong expression of DARPP-32 proteins in these tumor types.

CONCLUSIONS

The current study provides the first evidence that DARPP-32 expression is not limited to dopamine signaling in normal cells of the central nervous system. The pattern of expression of DARPP-32 proteins in normal epithelial tissues suggests that these proteins play an important role in epithelial signaling that may be tissue specific. Moreover, the observation that DARPP-32 and t-DARPP frequently are overexpressed in common subtypes of human adenocarcinomas suggest that these proteins may be important in tumorigenesis.

Executive subprocesses in working memory: relationship to catechol-O-methyltransferase Val158Met genotype and schizophrenia

BACKGROUND

Cognitive dysfunction in the working memory domain seems to be under genetic control and is a candidate intermediate phenotype in schizophrenia. Genes that affect working memory processing may contribute to risk for schizophrenia.

METHODS

Working memory and attentional processing were assessed in a large and unselected sample of schizophrenic patients, their healthy siblings, and controls (N = 250). We used the n-back task because it allows parametric analysis over increasing loads and delays and parsing of subcomponents of executive cognition and working memory, including temporal indexing and updating. Participants were genotyped for catechol-O-methyltransferase (COMT) at the Val158Met locus, which has been shown to affect executive cognition and frontal lobe function, likely because of genetically determined variation in prefrontal dopamine signaling.

RESULTS

A significant COMT genotype effect was found: Val/Val individuals had the lowest n-back performance, and Met/Met individuals had the highest performance. Effects were similar in the 1- and 2-back conditions and across all groups, whereas no effect on the Continuous Performance Test was seen, suggesting that genotype was not affecting working memory subprocesses related to attention, load, or delay. Siblings also performed significantly worse than controls on the 1- and 2-back conditions.

CONCLUSIONS

A prefrontal cognitive mechanism common to the 1- and 2-back conditions, probably executive processes involved in information updating and temporal indexing, is sensitive to the COMT genotype. Considering that the 3 participant groups were affected more or less linearly by the COMT genotype, an additive genetic model in which the effect of allele load is similar in its effects on prefrontally based working memory irrespective of the genetic or environmental background in which it is expressed is suggested. The findings also provide convergent evidence that an intermediate phenotype related to prefrontal cortical function represents a viable approach to understanding neuropsychiatric disorders with complex genetic etiologies and individual differences in cognition.

Viral restoration of dopamine to the nucleus accumbens is sufficient to induce a locomotor response to amphetamine

Administration of amphetamine to mice evokes hyperlocomotion. Dopamine deficient (DD) mice, in which tyrosine hydroxylase (TH) has been specifically inactivated in dopaminergic neurons, have a blunted response to amphetamine, indicating that the hyperlocomotive response requires dopamine. Dopamine production can be restored to specific brain regions by using adeno-associated viruses expressing TH and GTP cyclohydrolase 1 (GTPCH1). Restoration of dopamine specifically to the nucleus accumbens (NAc) of DD mice completely restores the ability of these mice to respond to amphetamine. This response is specific to the dopamine production in the NAc, as restoration of dopamine production to the caudate putamen (CPu) does not fully restore the hyperlocomotive response to amphetamine. These data support previous studies in which accumbal dopamine is required for producing a normal locomotor response to amphetamine and further show that release of dopamine restricted to the NAc is sufficient for this response

Attention deficit and hyperactivity disorder: review of genetic association studies

Attention Deficit and Hyperactivity Disorder (ADHD) is a common idiopathic childhood neurodevelopmental disorder, exacting a significant clinical and public health toll. It impairs schooling and social adaptation, resulting in high rates of depression, conduct disorder, school dropouts, and substance abuse, and necessitating exposure of many children to prolonged courses of stimulant psychotropic medication. Although the biological basis of ADHD is unknown, it has been shown to possess considerable heritability. Candidate gene association studies proved to be a productive strategy leading to replicated association findings of genetic loci contributing to susceptibility to ADHD. Based on the mechanism of action of stimulant drugs effective in the alleviation of ADHD symptoms, current association studies have focused mainly on dopaminergic genes. Promising exploratory findings have also been reported for genes affecting other neurotransmitter systems. The current article reviews the rationale, methodology, and main findings in the field, and outlines future directions. Locating the actual genes mediating ADHD susceptibility will have far reaching implications for understanding the pathophysiology of ADHD as well as for understanding mechanisms of therapeutic drug action and genetic determinants of response.

Aetiology of eating disorders in the 21(st) century: new answers to old questions

This paper reviews the aetiology of the eating disorders, anorexia nervosa and bulimia nervosa, which is generally thought of as multi-factorial in nature. In recent years with the advent of new bio-technologies interest in the exploration of the contribution of biological, in particular genetic factors to the origins of these disorders has been revived. The challenge for the future is to understand better the relative importance of biological and psychosocial risk-factors and how these factors interact. Moreover, there is a need for a greater appreciation of the developmental perspective in the origins of eating disorders.

Transcriptional control of dopamine neuron development

Recent studies have identified several factors that influence the development of midbrain dopamine (DA) neurons. The identity of early proliferating DA progenitor cells are specified by the secreted factors sonic hedgehog and fibroblast growth factor 8, derived from the floor plate of the ventral midline and the mid/hindbrain border, respectively. While transcription factors specifically expressed in the proliferating DA progenitor cells remain to be identified, several transcription factors important for postmitotic DA cell development have been characterized. These include Nurr1, Lmx1b, Pitx3, and En1/En2. The studies of these transcription factors have not only increased the understanding of how DA neurons are generated in vivo, but also allowed the development of new strategies using stem cells for engineering DA neurons in vitro, results that may have significance in future therapies of patients with Parkinson's disease.

The control of dopamine neuron development, function and survival: insights from transgenic mice and the relevance to human disease

Transgenic technology, especially the use of homologous recombination to disrupt specific genes to produce knockout mice, has added considerably to the understanding of dopamine (DA) neuron develop, survival and function. The current review summarizes results from knockout mice with the target disruption of genes involved in the development of DA neurons (engrailed 1 and 2, lmx1b, and Nurr1), in maintaining DA neurotransmission (tyrosine hydroxylase, vesicular monoamine transporter, DA transporter, DA D2 and D3 receptors) and important for DA neuron survival (alpha-synuclein, glia cell line-derived neurotrophic factor and superoxide dismutase). As alterations in DA neurotransmission have been implicated in a number of human neuropathologies including Parkinson's disease, schizophrenia and attention deficit/hyperactivity disorder, understanding how specific genes are involved in the function of DA neurons and the compensatory changes that result from loss or reduction in gene expression could provide important insight for the treatment of these diseases.

The genetic determinants of smoking

Dependence on tobacco, like many other drug dependencies, is a complex behavior with both genetic and environmental factors contributing to the variance. The heritability estimates for smoking in twin studies have ranged from 46 to 84%, indicating a substantial genetic component to smoking. Candidate gene studies have detected functional polymorphisms in genes coding for the cytochrome P450 enzymes, and variations in these genes that lead to more rapid nicotine metabolism have been implicated in smoking. Similarly, smoking has been associated with polymorphisms in dopaminergic genes that may influence the dopamine receptor number and/or function. Animal experiments have localized specific subunits of the nicotinic receptors that may mediate the reinforcing properties of nicotine and have investigated their role in nicotine dependence. However, environmental factors have also been found to contribute to the risk of initiation and persistence of smoking. We review the scientific evidence that supports a role for genetic influences on smoking, discuss the specific genetic and neurobiological mechanisms that may mediate susceptibility to nicotine dependence, identify possible gene/environmental interactions that may be important in understanding smoking behavior, and suggest directions for future research. Insights into the genetic contributions to smoking can potentially lead to more effective strategies to reduce smoking.

Genetic polymorphisms and personality in healthy adults: a systematic review and meta-analysis

A meta-analysis was conducted on studies reporting data on associations between candidate genes and human personality. Studies reporting data for psychiatric populations (including organic disease and substance abuse) were excluded. A total of 46 studies contributed to the analysis. Pooled data using a fixed-effects model suggested significant associations between the 5HTT LPR, DRD4 c>t, DRD4 length, DRD2 A1/A2, DRD3 A1/A2 polymorphisms and personality traits. A multivariate analysis using a mixed-effects model and including age, sex and predominant ethnicity as covariates was applied to the analyses of 5HTT LPR and DRD4 length polymorphism data. Only the association between the 5HTT LPR polymorphism and avoidance traits remained significant (P=0.038). However, sensitivity analyses excluding data from studies reporting allele frequencies not in Hardy-Weinberg equilibrium and unpublished data resulted in this association no longer being significant. Implications for the design of future association studies of human personality are discussed, including the likely sample sizes that will be required to achieve sufficient power and the potential role of moderating variables such as sex.

Investigating the role of dopaminergic and serotonergic candidate genes in obsessive-compulsive disorder

There is increasing evidence that the aetiology of obsessive-compulsive disorder (OCD) has a marked genetic component, although the precise mechanism of inheritance is unclear. Clinical and pharmacological studies have implicated the serotonergic and dopaminergic systems in disease pathogenesis. This study investigated the role of attractive candidate genes in the serotonergic and dopaminergic pathways in the development of OCD. The distribution of selected polymorphic variants in the serotonin receptor type 2A and 1Dbeta (5-HT(2A), 5-HT(1Dbeta)), dopamine transporter (DAT), dopamine receptor type 4 (DRD4) and monoamine-oxidase A (MAO-A) genes were analysed in 71 OCD cases and 129 control individuals in the genetically homogeneous Afrikaner population, by means of case-control association studies. Although no statistically significant genotypic or allelic associations were detected, the data yielded interesting preliminary results that warrant further discussion and investigation.

Valproate attenuates hyperactive and perseverative behaviors in mutant mice with a dysregulated dopamine system

BACKGROUND

Dopamine transporter (DAT) knockdown (KD) mice, with approximately 90% loss of expression of the DAT, allow for the examination of the behavioral consequences of a chronically dysregulated dopamine system. The DAT KD mice have hyperdopaminergic tone, are hyperactive, and show impaired response inhibition in a number of paradigms. We hypothesized that the DAT KD mice would also display deficits in prepulse inhibition (PPI) and would be perseverative in their locomotor behavior.

METHODS

Basal levels of PPI and patterns of locomotor behavior were measured in two cohorts of DAT KD mice. In addition, measurements of locomotor behavior were recorded after pretreatment with 100 mg/kg valproate in both DAT KD and wildtype mice.

RESULTS

The DAT KD mice were hyperactive and displayed perseverative motor behavior but had normal levels of PPI. The clinically effective antimania drug valproate significantly attenuated the hyperactivity and perseverative locomotor behavior in the DAT KD mice and had no effect in control mice.

CONCLUSIONS

The DAT KD mice appear to provide a model of some aspects of manic behavior. With limited models of bipolar disorder, the DAT KD mice might provide a vehicle to screen for new psychiatric therapies to treat mania and its related symptoms.

Adenosine receptor blockade reverses hypophagia and enhances locomotor activity of dopamine-deficient mice

Adenosine receptors modulate dopaminergic function by regulating dopamine release in presynaptic neurons and intracellular signaling in postsynaptic striatal neurons. To investigate how adenosine impinges on the action of dopamine in feeding and locomotion, genetically altered, dopamine-deficient mice were treated with adenosine receptor antagonists. Acute administration of the nonselective adenosine receptor antagonist, caffeine (5-25 mgkg i.p.), reversed the hypophagia of mutant mice and induced hyperactivity in both control and mutant animals. However, caffeine treatment elicited much less hyperactivity in dopamine-deficient mice than did l-3,4-dihydroxyphenylalanine (l-dopa) administration, which partially restores dopamine content. Caffeine treatment enhanced feeding of l-dopa-treated mutants but, unexpectedly, it reduced their hyperlocomotion. Caffeine administration induced c-Fos expression in the cortex of dopamine-deficient mice but had no effect in the striatum by itself. Caffeine attenuated dopamine agonist-induced striatal c-Fos expression. An antagonist selective for adenosine A(2A) receptors induced feeding and locomotion in mutants much more effectively than an A(1) receptor antagonist. l-dopa-elicited feeding and hyperlocomotion were reduced in mutants treated with an A(1) receptor agonist, whereas an A(2A) receptor agonist decreased l-dopa-induced feeding without affecting locomotion. The observations suggest that the hypophagia and hypoactivity of mutants result not only because of the absence of dopamine but also because of the presence of A(2A) receptor signaling. This study of a genetic model of dopamine depletion provides evidence that A(2A) receptor antagonists could ameliorate the hypokinetic symptoms of advanced Parkinson's disease patients without inducing excessive motor activity.

Candidate genes in eating disorders

Environmental influences, as well as biological and genetic factors influence risk for eating disorders. Family and twin studies have shown that eating disorders are familial and suggest the influence of genetic factors on their etiology. Positive associations have been observed for some candidate genes that have been studied (such as 5HT2A receptor gene); however, the field has been plagued by nonreplications. In this paper we review the extant association studies of eating disorders.

Dopamine genes and attention-deficit hyperactivity disorder: a review

OBJECTIVE

To review the results of genetic studies investigating dopamine-related genes in attention-deficit hyperactivity disorder (ADHD).

DATA SOURCES

Papers (association/linkage, meta-analyses and animal model studies) were identified through searches of the PubMed database and systematically reviewed.

DATA SYNTHESIS

Consistent results from molecular genetic studies are pointing strongly to the possible link between 2 specific genes, the dopamine transporter (SLC3A6) and the dopamine receptor 4 (DRD4), and ADHD.

CONCLUSIONS

The implication of SLC6A3 and DRD4 genes in ADHD appears to be one of the most replicated in psychiatric genetics and strongly suggests the involvement of the brain dopamine systems in the pathogenesis of ADHD. However, more work is required to further these findings by genotype-to-phenotype correlations and identify the functional allelic variants/mutations that are responsible for these associations. The role of other dopamine genes, which may have smaller effects than SLC6A3 and DRD4, needs also to be determined.

Effect of peripheral lipopolysaccharide injection on dopamine content in murine anterior olfactory nucleus

Norepinephrine turnover rate in the murine locus coeruleus (LC) is known to be enhanced by the intraperitoneal (i.p.) injection of lipopolysaccharide (LPS). Approximately 40% of LC neurons are also known to project to the olfactory bulb (OB) and the anterior olfactory nucleus (AON). Therefore, we investigated whether an i.p. injection of 500 microg LPS could modulate the catecholamine biosynthesis in these sites in 8-week-old C3H/HeN male mice. Unexpectedly, the content of norepinephrine was not elevated in both sites during 6-h-observation after LPS injection. The contents of dopamine and its metabolites in the AON were highly increased at 4 h after LPS injection, whereas those in the OB were not elevated during 6-h-observation. Although the AON has been considered not to belong to the dopaminergic neuron system, our report is the first to show an elevated dopamine content in the AON under a stressful condition such as endotoxemia.

Dopamine D2 receptor binding, Drd2 expression and the number of dopamine neurons in the BXD recombinant inbred series: genetic relationships to alcohol and other drug associated phenotypes

BACKGROUND

It has not been established to what extent the natural variation in dopamine systems contribute to the variation in ethanol response. The current study addresses this issue by measuring D2 dopamine (DA) receptor binding, the expression of Drd2, the number of midbrain DA neurons in the BXD recombinant inbred (RI) series and then compares these strain means with those previously reported for a variety of ethanol and other drug-related phenotypes.

METHODS

Data were collected for 21 to 23 of the BXD RI strains and the parental strains. D2 DA receptor autoradiography was performed using 125I-epidepride as the ligand [ Kanes S, Dains K, Cipp L, Gatley J, Hitzemann B, Rasmussen E, Sanderson S, Silverman S, Hitzemann R (1996) Mapping the genes for haloperidol-induced catalepsy. J Pharmacol Exp Ther 277:1016-1025]. Drd2 expression was measured using the Affymetrix oligoarray system. Immunocytochemical techniques were used to determine the number of midbrain DA neurons [Hitzemann B, Dains K, Hitzemann R (1994) Further studies on the relationship between dopamine cell density and haloperidol response. J Pharmacol Exp Ther 271:969-976].

RESULTS AND CONCLUSIONS

The range of difference in receptor binding for the RI strains was approximately 2-fold in all regions examined, the core, the shell of the nucleus accumbens (NAc) and the dorsomedial caudate-putamen (CPu); heritability in all regions was moderate--(h2 approximately 0.35). Drd2 expression in forebrain samples from the RI and parental strains ranged 1.5- to h2-fold and was moderate-0.47. Variation in the number of tyrosine hydroxylase (TH) positive neurons was moderate, 41% and 26% and h2 was low--0.19 and 0.15 for the ventral tegmental area (VTA) and substantia nigra compacta (SNc), respectively. Significant correlations were found between D2 DA receptor binding and the low dose (1.33 g/kg) ethanol stimulant response. (p < 0.002) and between expression and conditioned place preference (CPP) (p < 0.0005). No significant correlations were detected between ethanol preference and either receptor binding or Drd2 expression; however, a significant correlation was found between preference and Ncam expression. Ncam is approximately 0.2 Mb from Drd2. Overall, the data suggest ethanol preference and CPP are associated with the expression of Drd2 or closely linked genetic loci.

Transgenic studies on the regulation of the anterior pituitary gland function by the hypothalamus

The anterior pituitary gland is composed of five different cell types secreting hormones whose functions include the regulation of post-natal growth (growth hormone, GH), lactation (prolactin, PRL), reproduction (luteinising hormone, LH, and follicle stimulating hormone, FSH), metabolism (thyroid stimulating hormone, TSH), and stress (adrenocorticotrophic hormone, ACTH). The synthesis and secretion of the anterior pituitary hormones is under the control of neuropeptides released from the hypothalamus into a capillary portal plexus which flows through the external zone of the median eminence to the anterior lobe. This review describes the ways that gene transfer technologies have been applied to whole animals in order to study the regulation of anterior pituitary function by the hypothalamus. The extensive studies on these neuronal systems, within the context of the physiological integrity of the intact organism, not only exemplify the successful application of transgenic technologies to neuroendocrine systems, but also illustrate the problems that have been encountered, and the challenges that lie ahead.

Endogenous dopamine amplifies ischemic long-term potentiation via D1 receptors

BACKGROUND AND PURPOSE

Several observations indicate that, during energy deprivation, endogenous dopamine may become neurotoxic. Accordingly, the nucleus striatum is a preferential site of silent infarcts in humans, and experimental ischemia caused by homolateral carotid occlusion selectively damages this dopamine-enriched brain area. In an attempt to clarify how dopamine takes part in ischemia-induced neuronal damage, we performed in vitro electrophysiological recordings from neurons of the nucleus striatum.

METHODS

Intracellular recordings with sharp microelectrodes were performed from corticostriatal slices. Slices were obtained from both rats and wild-type and dopamine D1 receptor-lacking mice. In some experiments, the striatum was unilaterally denervated by injecting the dopamine-specific neurotoxin 6-hydroxydopamine in the homolateral substantia nigra. Dopamine agonists and antagonists, as well as drugs targeting the intracellular cascade coupled to dopamine receptor stimulation, were applied at known concentrations.

RESULTS

Manipulation of the dopamine system failed to affect the membrane depolarization of striatal neurons exposed to combined oxygen and glucose deprivation of short duration, but it reduced the amplitude of postischemic long-term potentiation (LTP) expressed at corticostriatal synapses. In particular, pharmacological blockade or genetic inactivation of D1/cAMP/protein kinase A pathway prevented the long-term increase of the excitatory postsynaptic potential (EPSP) amplitude caused by a transient ischemic episode, while it failed to prevent the increase of the EPSP half-decay coupled to ischemic LTP.

CONCLUSIONS

The present data suggest that endogenous dopamine, via D1 receptors, selectively facilitates the expression of ischemic LTP on the AMPA-mediated component of the EPSPs, while it does not alter the expression of this form of synaptic plasticity on the N-methyl-D-aspartate-mediated component of corticostriatal synaptic potentials. Understanding the cellular and molecular mechanisms of ischemia-triggered excitotoxicity offers hope for the development of specific treatments able to interfere with this pathological process.

Clinical and subclinical dopaminergic dysfunction in PARK6-linked parkinsonism: an 18F-dopa PET study

PARK6, a locus for early-onset recessive parkinsonism, has been causally implicated in nine unrelated families from four different countries. The gene is still unidentified and hence the importance of PARK6 as a cause of Parkinson's disease is unknown. To date, no pathology or functional imaging studies are available on PARK6-linked parkinsonism. We have used (18)F-dopa positron emission tomography to study four patients who are homozygous and three asymptomatic relatives who are heterozygous for PARK6. The clinically affected PARK6 subjects had a similar 85% reduction in posterior dorsal putamen (18)F-dopa uptake to a group of idiopathic Parkinson's disease patients matched for clinical disease severity and duration but showed significantly greater involvement of head of caudate and anterior putamen. The group of asymptomatic PARK6 carriers showed a significant mean 20 to 30% reduction in caudate and putamen (18)F-dopa uptake in comparison with controls, individual values falling toward the bottom of the normal range. Our results indicate that PARK6 pathology results in a more uniform loss of striatal dopamine terminal function than Parkinson's disease. The subclinical loss of striatal dopamine storage capacity found in the PARK6 carriers implies that the unidentified gene on the short arm of chromosome 1 exhibits either haploinsufficency or a dominant negative effect.