Feeding behavior in dopamine-deficient mice

Neuronal control of energy homeostasis

Neuronal control of body energy homeostasis is the key mechanism by which animals and humans regulate their long-term energy balance. Various hypothalamic neuronal circuits (which include the hypothalamic melanocortin, midbrain dopamine reward and caudal brainstem autonomic feeding systems) control energy intake and expenditure to maintain body weight within a narrow range for long periods of a life span. Numerous peripheral metabolic hormones and nutrients target these structures providing feedback signals that modify the default "settings" of neuronal activity to accomplish this balance. A number of molecular genetic tools for manipulating individual components of brain energy homeostatic machineries, in combination with anatomical, electrophysiological, pharmacological and behavioral techniques, have been developed, which provide a means for elucidating the complex molecular and cellular mechanisms of feeding behavior and metabolism. This review will highlight some of these advancements and focus on the neuronal circuitries of energy homeostasis.

TUB is a candidate gene for late-onset obesity in women

AIMS/HYPOTHESES

We recently reported significant associations between BMI and three TUB single nucleotide polymorphisms (SNPs) in two Dutch cohorts enriched for type 2 diabetes. Here, we attempted a replication of these associations in a large population-based cohort of female twins comprehensively phenotyped for measures of general and central obesity.

METHODS

Two TUB SNPs (rs2272382, rs2272383) and a third (rs1528133), 22 kb distal to RIC3, were genotyped in 2694 Europid women from the St Thomas' UK Adult Twin Registry (Twins UK) (mean age +/- SD: 47.6 +/- 12.7 years; 42.8% postmenopausal). We explored the hypothesis that TUB is a candidate gene for late-onset obesity in humans through testing the interaction of the SNPs by menopausal status.

RESULTS

In the whole cohort, none of the three SNPs showed a significant main effect on measures of general or central obesity. However, for central obesity the rs2272382 SNP showed a significant interaction with menopausal status (p = 0.036). Postmenopausal women homozygous for the minor allele of rs2272382 showed significantly more general obesity (p = 0.022) and central obesity (p = 0.009) than carriers of the major allele. Differences (beta [95% CI]) between the two genotype groups were 0.92 kg/m2 (0.03-1.81) for BMI (p = 0.036), 2.73 cm (0.62-4.84) for waist circumference (p = 0.013) and 2.43% (0.27-4.60) for per cent central fat (p = 0.027). These associations were confirmed by a sibling transmission disequilibrium test for central obesity, waist circumference and per cent central fat.

CONCLUSIONS/INTERPRETATION

We have replicated associations of TUB SNP rs2272382 with measures of general and central obesity in normal postmenopausal women. These findings confirm TUB as a candidate gene for late-onset obesity in humans.

Cocaine-conditioned place preference by dopamine-deficient mice is mediated by serotonin

Rodents learn to associate the rewarding effects of drugs with the environment in which they are encountered and, subsequently, will display a conditioned place preference (CPP) for that environment. Cocaine-induced CPP is generally thought to be mediated through inhibition of the dopamine transporter and the consequent increase in extracellular dopamine. However, here we report that dopamine-deficient (DD) mice formed a CPP for cocaine that was not blocked by a dopamine D1-receptor antagonist. Fluoxetine, a serotonin transporter (SERT) inhibitor, produced CPP in DD, but not control mice, suggesting that serotonin mediates cocaine CPP in DD mice. Inhibition of dopamine neuron firing by pretreatment with quinpirole, a dopamine D2-receptor agonist, blocked both cocaine- and fluoxetine-induced CPP in DD mice. These findings are consistent with the hypothesis that, in the absence of dopamine, cocaine-mediated SERT blockade activates dopamine neurons, which then release some other neurotransmitter that contributes to cocaine reward in DD mice.

Dopamine reward circuitry: two projection systems from the ventral midbrain to the nucleus accumbens-olfactory tubercle complex

Anatomical and functional refinements of the meso-limbic dopamine system of the rat are discussed. Present experiments suggest that dopaminergic neurons localized in the posteromedial ventral tegmental area (VTA) and central linear nucleus raphe selectively project to the ventromedial striatum (medial olfactory tubercle and medial nucleus accumbens shell), whereas the anteromedial VTA has few if any projections to the ventral striatum, and the lateral VTA largely projects to the ventrolateral striatum (accumbens core, lateral shell and lateral tubercle). These findings complement the recent behavioral findings that cocaine and amphetamine are more rewarding when administered into the ventromedial striatum than into the ventrolateral striatum. Drugs such as nicotine and opiates are more rewarding when administered into the posterior VTA or the central linear nucleus than into the anterior VTA. A review of the literature suggests that (1) the midbrain has corresponding zones for the accumbens core and medial shell; (2) the striatal portion of the olfactory tubercle is a ventral extension of the nucleus accumbens shell; and (3) a model of two dopamine projection systems from the ventral midbrain to the ventral striatum is useful for understanding reward function. The medial projection system is important in the regulation of arousal characterized by affect and drive and plays a different role in goal-directed learning than the lateral projection system, as described in the variation-selection hypothesis of striatal functional organization.

Neurobiology of feeding and energy expenditure

Significant advancements have been made in the past century regarding the neuronal control of feeding behavior and energy expenditure. The effects and mechanisms of action of various peripheral metabolic signals on the brain have become clearer. Molecular and genetic tools for visualizing and manipulating individual components of brain homeostatic systems in combination with neuroanatomical, electrophysiological, behavioral, and pharmacological techniques have begun to elucidate the molecular and neuronal mechanisms of complex feeding behavior and energy expenditure. This review highlights some of these advancements that have led to the current understanding of the brain's involvement in the acute and chronic regulation of energy homeostasis.

Is dopamine a physiologically relevant mediator of feeding behavior?

The hypothalamus integrates various hormonal and neuronal signals to regulate appetite and metabolism and thereby serves a homeostatic purpose in the regulation of body weight. Additional neural circuits that are superimposed on this system have the potential to override the homeostatic signals, resulting in either gluttony or anorexia at the extremes. Midbrain dopamine neurons have long been implicated in mediating reward behavior and the motivational aspects of feeding behavior. Recent results reveal that hormones implicated in regulating the homeostatic system also impinge directly on dopamine neurons; for example, leptin and insulin directly inhibit dopamine neurons, whereas ghrelin activates them. Here, I discuss the predictions and implications of these new findings as they relate to dopamine signaling and the physiology of appetite control.

Viral restoration of dopamine signaling to the dorsal striatum restores instrumental conditioning to dopamine-deficient mice

INTRODUCTION

Instrumental responding was evaluated to determine whether mice lacking dopamine [dopamine-deficient mice (DD mice)] could learn to preferentially press a visually cued, active lever for food reward over an inactive lever.

RESULTS

When DD mice were treated with 3,4-L: -dihydroxyphenalanine (L-dopa) to restore dopamine signaling systemically, they were able to learn to press the active lever as well as control mice, whereas mice lacking dopamine would not perform the task. Importantly, DD mice treated with caffeine (to stimulate locomotor and feeding behaviors) also failed to show preference for the active lever and were slower to retrieve rewards after making a reinforced operant response. Selective restoration of dopamine signaling to the nigrostriatal pathway of DD mice via viral-mediated gene transfer completely restored learning and performance of this simple instrumental task. Furthermore, the virally treated DD mice were willing to lever press as much as control mice for reward in progressive-ratio and high fixed-ratio schedules of reinforcement.

CONCLUSION

These results suggest that the deficit in goal-directed behavior observed in mice without dopamine signaling is the result of decreased motivation to obtain reward, and that dopamine signaling in the dorsal striatum is sufficient to restore normal goal-directed behavior on a variety of operant responding tasks.

Local knockdown of genes in the brain using small interfering RNA: a phenotypic comparison with knockout animals

BACKGROUND

Recent reports have suggested effectiveness of RNA interference (RNAi) for the analysis of gene functions in the brain. This study sought to determine the efficiency of local small interfering RNA (siRNA) injections, comparing this approach with animals generated through classical gene targeting.

METHODS

Small interfering RNA against dopamine transporter (DAT) (35 microg/14 days) or tyrosine hydroxylase (TH) (15 microg/3 days) was injected into the ventral tegmental/substantia nigra areas of the brain of adult wildtype or DAT-knockout mice, respectively.

RESULTS

Local injections of siRNA resulted in a 35% to 40% reduction of DAT and TH protein levels in the striatum, respectively. Despite negligible effect of DAT knockdown on novelty-induced locomotion, the locomotor response of DAT siRNA treated animals to amphetamine was blunted similar to what is observed in the DAT heterozygote animals. Since incomplete reduction of TH levels in normal mice does not produce behavioral effects, TH siRNA experiments were carried out in DAT-knockout animals that show increased dependence on newly synthesized dopamine. Knockdown of TH in these animals resulted in reduced basal locomotion.

CONCLUSIONS

Local injection of siRNA in the brain reduced gene expression by 40% to 50%, suggesting that siRNA-mediated knockdown of genes in the brain can be a complementary tool to classical transgenesis for the analysis of gene functions.

Essential function of HIPK2 in TGFbeta-dependent survival of midbrain dopamine neurons

Transforming growth factor beta (TGFbeta) is a potent trophic factor for midbrain dopamine (DA) neurons, but its in vivo function and signaling mechanisms are not entirely understood. We show that the transcriptional cofactor homeodomain interacting protein kinase 2 (HIPK2) is required for the TGFbeta-mediated survival of mouse DA neurons. The targeted deletion of Hipk2 has no deleterious effect on the neurogenesis of DA neurons, but leads to a selective loss of these neurons that is due to increased apoptosis during programmed cell death. As a consequence, Hipk2(-/-) mutants show an array of psychomotor abnormalities. The function of HIPK2 depends on its interaction with receptor-regulated Smads to activate TGFbeta target genes. In support of this notion, DA neurons from Hipk2(-/-) mutants fail to survive in the presence of TGFbeta3 and Tgfbeta3(-/-) mutants show DA neuron abnormalities similar to those seen in Hipk2(-/-) mutants. These data underscore the importance of the TGFbeta-Smad-HIPK2 pathway in the survival of DA neurons and its potential as a therapeutic target for promoting DA neuron survival during neurodegeneration.

Leptin in energy balance and reward: two faces of the same coin?

Leptin receptors are expressed on mesolimbic dopamine neurons, yet little is known about the functional significance of this anatomical relationship. In this issue of Neuron, Hommel et al. reveal a novel site for leptin's regulation of feeding. In turn, Fulton et al. propose a novel role for leptin in regulating non-feeding-related motivated behaviors.

Thoughts for food: brain mechanisms and peripheral energy balance

The past decade has witnessed dramatic advancements regarding the neuroendocrine control of food intake and energy homeostasis and the effects of peripheral metabolic signals on the brain. The development of molecular and genetic tools to visualize and selectively manipulate components of homeostatic systems, in combination with well-established neuroanatomical, electrophysiological, behavioral, and pharmacological techniques, are beginning to provide a clearer picture of the intricate circuits and mechanisms of these complex processes. In this review, we attempt to provide some highlights of these advancements and pinpoint some of the shortcomings of the current understanding of the brain's involvement in the regulation of daily energy homeostasis.

Ghrelin modulates the activity and synaptic input organization of midbrain dopamine neurons while promoting appetite

The gut hormone ghrelin targets the brain to promote food intake and adiposity. The ghrelin receptor growth hormone secretagogue 1 receptor (GHSR) is present in hypothalamic centers controlling energy metabolism as well as in the ventral tegmental area (VTA), a region important for motivational aspects of multiple behaviors, including feeding. Here we show that in mice and rats, ghrelin bound to neurons of the VTA, where it triggered increased dopamine neuronal activity, synapse formation, and dopamine turnover in the nucleus accumbens in a GHSR-dependent manner. Direct VTA administration of ghrelin also triggered feeding, while intra-VTA delivery of a selective GHSR antagonist blocked the orexigenic effect of circulating ghrelin and blunted rebound feeding following fasting. In addition, ghrelin- and GHSR-deficient mice showed attenuated feeding responses to restricted feeding schedules. Taken together, these data suggest that the mesolimbic reward circuitry is targeted by peripheral ghrelin to influence physiological mechanisms related to feeding.

Murine models of inherited monoaminergic and GABAergic neurotransmitter disorders

Monoamine and amino acid neurotransmitters perform diverse biological functions in mammals, including the regulation of inhibitory/excitatory neurotransmission in the brain and spinal cord, movement and sleep, autonomic function, mood and reward, and numerous other processes. The primary transmitters involved include dopamine, serotonin, epinephrine, norepinephrine and γ-aminobutyric acid (GABA). With the exception of the amino acid transmitter GABA, the cofactor integrating these systems is tetrahydrobiopterin, an oxidizable intermediate found in high concentrations in dopaminergic neurons. With growing awareness of the clinical phenotypes, expanding numbers of patients with monoaminergic and GABAergic neurotransmitter disorders are being identified. For some people, therapeutic intervention demonstrates remarkably positive benefits; conversely, for most other disorders therapy offers limited efficacy. Decoding of the complete mouse genome, coupled with methodology capable of ablating specific genes, has revolutionized how geneticists understand and treat human genetic disease. This is well-exemplified in the disorders covered in this review, which focuses predominantly on monoaminergic (tetrahydrobiopterin-dependent) and GABAergic signaling neurotransmitter disorders.

Neuroanatomical phenotyping in the mouse: the dopaminergic system

Voluntary movement in animals is modulated by a number of subcortical systems. One of these resides in the basal nuclei and their associated projections and utilizes dopamine as a neurotransmitter. Apart from regulating movement, the dopaminergic axis is also involved in the control of goal-oriented behavior, cognition, and mood. Disorders of this system result in common human neurologic disorders such as Parkinson's and Huntington's diseases, as well contributing to a host of behavioral conditions, such as schizophrenia, attention deficit hyperactivity disorder, and addiction. Many individual mouse models of human dopaminergic dysfunction have been described in varying degrees of detail. However, when evaluating this region of the brain, the veterinary pathologist is confronted by a paucity of information summarizing the comparative aspects of the anatomy, physiology, and pathology of the central dopaminergic system. In this review, a systematic approach to anatomic phenotyping of the central dopaminergic system in the mouse is described and illustrated using tyrosine hydroxylase immunohistochemistry. Differences between murine neuroanatomy and comparable regions of the nonhuman primate brain are highlighted. Although the mouse is the focus of this review, conditions in domestic animals characterized by lesions within the basal nuclei and its projections are also briefly described. Murine behavioral and motor tests that accompany abnormalities of specific anatomic regions of the dopaminergic axis are summarized. Finally, we review mouse models of Parkinson's and Huntington's diseases, as well as those genetically altered mice that elucidate aspects of dopamine metabolism and receptor function.

Role of brain dopamine in food reward and reinforcement

The ability of food to establish and maintain response habits and conditioned preferences depends largely on the function of brain dopamine systems. While dopaminergic transmission in the nucleus accumbens appears sufficient for some forms of reward, the role of dopamine in food reward does not appear to be restricted to this region. Dopamine plays an important role in both the ability to energize feeding and to reinforce food-seeking behaviour; the role in energizing feeding is secondary to the prerequisite role in reinforcement. Dopaminergic activation is triggered by the auditory and visual as well as the tactile, olfactory, and gustatory stimuli of foods. While dopamine plays a central role in the feeding and food-seeking of normal animals, some food rewarded learning can be seen in genetically engineered dopamine-deficient mice.

Subjective feeling of appetite modulates brain activity: an fMRI study

Obesity and overweight are important risk factors for the development of diabetes mellitus type 2 and associated chronic diseases, and therefore, they have become serious global problems in the western and developed countries. But little is known about the neuroanatomical correlates of eating behavior and its influences on the central nervous processing in humans. We used functional magnetic resonance imaging (fMRI) to measure the cortical activation in 12 lean healthy humans during visual stimulation with food-related and nonfood pictures after a fasting period of at least 5 h. Compared to the nonfood pictures, the food stimuli elicited a significantly greater activity in the left orbitofrontal cortex and the insular/opercular cortex bilaterally with a stronger focus on the left side. Subjective ratings of appetite during the presentation of food-related stimuli modulated the activity in the insula bilaterally, the left operculum and the right putamen. These results provide further insights in the central nervous processing of food relevant stimuli in humans, specifically with respect to the subjective experience of appetite.

Cre recombinase-mediated restoration of nigrostriatal dopamine in dopamine-deficient mice reverses hypophagia and bradykinesia

A line of dopamine-deficient (DD) mice was generated to allow selective restoration of normal dopamine signaling to specific brain regions. These DD floxed stop (DDfs) mice have a nonfunctional Tyrosine hydroxylase (Th) gene because of insertion of a NeoR gene flanked by lox P sites targeted to the first intron of the Th gene. DDfs mice have trace brain dopamine content, severe hypoactivity, and aphagia, and they die without intervention. However, they can be maintained by daily treatment with l-3,4-dihydroxyphenylalanine (L-dopa). Injection of a canine adenovirus (CAV-2) engineered to express Cre recombinase into the central caudate putamen restores normal Th gene expression to the midbrain dopamine neurons that project there because CAV-2 efficiently transduces axon terminals and is retrogradely transported to neuronal cell bodies. Bilateral injection of Cre recombinase into the central caudate putamen restores feeding and normalizes locomotion in DDfs mice. Analysis of feeding behavior by using lickometer cages revealed that virally rescued DDfs mice are hyperphagic and have modified meal structures compared with control mice. The virally rescued DDfs mice are also hyperactive at night, have reduced motor coordination, and are thigmotactic compared with controls. These results highlight the critical role for dopamine signaling in the dorsal striatum for most dopamine-dependent behaviors but suggest that dopamine signaling in other brain regions is important to fine-tune these behaviors. This approach offers numerous advantages compared with previous models aimed at examining dopamine signaling in discrete dopaminergic circuits.

Reversal of supersensitive striatal dopamine D1 receptor signaling and extracellular signal-regulated kinase activity in dopamine-deficient mice

Lesions of dopaminergic nigrostriatal neurons cause supersensitivity to dopamine in the striatum. Previous work has shown that such supersensitivity, an important aspect of rodent models of Parkinson's disease, is associated with anatomically abnormal patterns in the activation of extracellular signal-regulated kinase. After lesions of dopaminergic neurons, dopamine D1-receptor agonists activate extracellular signal-regulated kinase in the dorsal striatum, something not observed in intact animals. Here we used a more selective method of dopamine depletion. Dopamine-deficient mice, in which the tyrosine hydroxylase gene is specifically inactivated in dopaminergic neurons, were used to investigate dopamine D1-receptor-mediated activation of extracellular signal-regulated kinase. In wild-type mice, acute treatment with a dopamine D1-receptor agonist results in activation of extracellular signal-regulated kinase in the nucleus accumbens without activation in the dorsal striatum. In contrast, in dopamine-deficient mice, dopamine D1-receptor-agonist treatment results in activation of extracellular signal-regulated kinase not only in the nucleus accumbens, but also throughout most of the dorsal striatum. Chronic replacement of dopamine by repeated injection of L-DOPA for 36 h reverses this supersensitive extracellular signal-regulated kinase activation. This reversal displays a dorsal to ventral progression such that, by 36 h, extracellular signal-regulated kinase activation is virtually restricted to the nucleus accumbens, as in wild-type mice. The reversal of dopamine D1-receptor activation of extracellular signal-regulated kinase in dopamine-deficient mice following chronic L-DOPA treatment shows that the lack of dopamine, rather than absence of other factors secreted from dopaminergic neurons, is responsible for dopamine supersensitivity.

Cannabinoids, opioids and eating behavior: the molecular face of hedonism?

Obesity represents nowadays one of the most devastating health threats. Published reports even project a decline in life expectancy of US citizens due to the rapidly increasing prevalence of obesity. This alarming increase is intimately linked with recent changes of environment and lifestyle in western countries. In this context, the rewarding or even addictive properties of popular food may represent one of the most serious obstacles to overcome for an effective anti-obesity therapy. Therefore, in addition to molecular networks controlling energy homeostasis, now researchers are starting to define central nervous mechanisms governing hedonic and addictive components of food intake. A recently emerging body of data suggests that the endogenous cannabinoid and opioid systems both represent key circuits responding to the rewarding value of food. This review focuses on the role of these two systems for the homeostatic and hedonic aspects of eating behavior and includes their anatomical and functional interactions. Independent from the degree to which eating can be considered an addiction, cannabinoid and opioid receptor antagonists are promising anti-obesity drugs, since they are targeting both hedonic and homeostatic components of energy balance control.

Morphine reward in dopamine-deficient mice

Dopamine has been widely implicated as a mediator of many of the behavioural responses to drugs of abuse. To test the hypothesis that dopamine is an essential mediator of various opiate-induced responses, we administered morphine to mice unable to synthesize dopamine. We found that dopamine-deficient mice are unable to mount a normal locomotor response to morphine, but a small dopamine-independent increase in locomotion remains. Dopamine-deficient mice have a rightward shift in the dose-response curve to morphine on the tail-flick test (a pain sensitivity assay), suggesting either a decreased sensitivity to the analgesic effects of morphine and/or basal hyperalgesia. In contrast, dopamine-deficient mice display a robust conditioned place preference for morphine when given either caffeine or l-dihydroxyphenylalanine (a dopamine precursor that restores dopamine throughout the brain) during the testing phases. Together, these data demonstrate that dopamine is a crucial component of morphine-induced locomotion, dopamine may contribute to morphine analgesia, but that dopamine is not required for morphine-induced reward as measured by conditioned place preference.

Overeating after restraint stress in cholecystokinin-a receptor-deficient mice

In mammals, including humans, a brain-gut hormone, cholecystokinin (CCK) mediates the satiety effect via CCK-A receptor (R). We generated CCK-AR gene-deficient (-/-) mice and found that the daily food intake, energy expenditure, and gastric emptying of a liquid meal did not change compared with those of wild-type mice. Because CCK-AR(-/-) mice show anxiolytic status, we examined the effects of restraint stress. Seven hours of restraint stress was found to significantly decrease both body weight and food intake during the subsequent 3 days in all tested animals. On the fourth day after restraint stress, the CCK-AR(-/-) mice showed a significantly higher level of daily food intake than prior to stress, and food intake recovered to prestress levels in the wild-type mice. Since peripheral CCK-AR has been known to mediate gastric emptying, both gastric emptying and gastric acid secretion were determined to examine the mechanism of overeating in CCK-AR(-/-) mice. Neither gastric emptying nor gastric acid secretion differed between CCK-AR(-/-) and wild-type mice on the fourth day after stress. In contrast, however, the contents of dopamine and its metabolites in the cerebral cortex of CCK-AR(-/-) mice were increased by stress, but were rather decreased in wild-type mice. Changes in 5-hydroxytryptamine (5-HT) and its metabolite 5HIAA did not differ between the genotypes. In conclusion, CCK-AR(-/-) mice showed overeating after restraint stress, and dopaminergic hyperfunction in the brain of these mice was observed. The present evidence suggests that the CCK-AR function, possibly via altering the dopaminergic function, might be involved in overeating after stress.

Dysregulation of dopamine signaling in the dorsal striatum inhibits feeding

Dopamine signaling is an important component of many goal-directed behaviors, such as feeding. Acute disruption of dopamine signaling using pharmacological agents tends to inhibit normal feeding behaviors in rodents. Likewise, genetically engineered dopamine-deficient (DD) mice are unable to initiate sufficient feeding and will starve by approximately 3 weeks of age if untreated. Adequate feeding by DD mice can be achieved by daily administration of L-3,4-dihydroxyphenylalanine (L-dopa), a precursor of dopamine, which can be taken up by dopaminergic neurons, converted to dopamine, and released in a regulated manner. In contrast, adequate feeding cannot be restored with apomorphine (APO), a mixed agonist that activates D1 and D2 receptors. Viral restoration of dopamine production in neurons that project to the dorsal striatum also restores feeding in DD mice. Administration of amphetamine (AMPH) or nomifensine (NOM), drugs which increase synaptic dopamine concentration, inhibits food intake in virally rescued DD mice (vrDD) as in control animals. These results indicate that the dysregulation of dopamine signaling in the dorsal striatum is sufficient to induce hypophagia and suggest that regulated release of dopamine in that brain region is essential for normal feeding and, probably, many other goal-directed behaviors.

Association of multiple DRD2 polymorphisms with anorexia nervosa

To investigate whether the dopaminergic system plays a role in the etiology of anorexia nervosa (AN) via the dopamine D2 receptor, we investigated association and transmission disequilibrium at seven single-nucleotide polymorphisms (SNPs) spanning about 75 kbp of the gene DRD2. We studied 191 probands with a DSM-IV diagnosis of AN, 457 parents and affected relatives with a DSM-IV eating disorder diagnosis, and 98 unrelated, female, normal weight controls. The -141 C/- insertion/deletion (-141 Indel), previously shown to affect DRD2 transcription efficiency, and multiple exon seven polymorphisms, one of which has previously been shown to affect DRD2 transcript stability, exhibited statistically significant association with diagnosis in haplotype transmission disequilibrium and in haplotype case : control analyses. Significant linkage disequilibrium between the -141 Indel and two exon seven SNPs (939Y and 957Y) was observed over a distance of >50 kbp in the AN probands but not in the controls. Genetically transmitted variation in D2 dopamine receptor expression mediated by functional polymorphisms affecting transcription and translation efficiency may play a role in vulnerability to AN.

Dopamine-independent locomotor actions of amphetamines in a novel acute mouse model of Parkinson disease

Brain dopamine is critically involved in movement control, and its deficiency is the primary cause of motor symptoms in Parkinson disease. Here we report development of an animal model of acute severe dopamine deficiency by using mice lacking the dopamine transporter. In the absence of transporter-mediated recycling mechanisms, dopamine levels become entirely dependent on de novo synthesis. Acute pharmacological inhibition of dopamine synthesis in these mice induces transient elimination of striatal dopamine accompanied by the development of a striking behavioral phenotype manifested as severe akinesia, rigidity, tremor, and ptosis. This phenotype can be reversed by administration of the dopamine precursor, L-DOPA, or by nonselective dopamine agonists. Surprisingly, several amphetamine derivatives were also effective in reversing these behavioral abnormalities in a dopamine-independent manner. Identification of dopamine transporter- and dopamine-independent locomotor actions of amphetamines suggests a novel paradigm in the search for prospective anti-Parkinsonian drugs.

Identification of dopamine transporter- and dopamine- independent locomotor actions of amphetamines suggests a novel paradigm in the search for prospective anti-Parkinsonian drugs.

Aromatic l-amino acid decarboxylase turnover in vivo in rhesus macaque striatum: A microPET study

The aromatic L-amino acid decarboxylase (AAAD) is involved in the de novo synthesis of dopamine, a neurotransmitter crucial in cognitive, neurobehavioral and motor functions. The goal of this study was to assess the in vivo turnover rate of AAAD enzyme protein in the rhesus macaque striatum by monitoring, using microPET imaging with the tracer [(18)F]fluoro-m-tyrosine (FMT), the recovery of enzyme activity after suicide inhibition. Results showed the AAAD turnover half-life to be about 86 h while total recovery was estimated to be 16 days after complete inhibition. Despite this relatively slow AAAD recovery, the animals displayed normal movement and behavior within 24 h. Based on the PET results, at 24 h, the animals have recovered about 20% of normal AAAD function. These findings show that normal movement and behavior do not depend on complete recovery of AAAD function but likely on pre-synaptic and post-synaptic compensatory mechanisms.

Dopamine is not required for the hyperlocomotor response to NMDA receptor antagonists

N-methyl-D-aspartate (NMDA) receptor antagonists can elicit symptoms in humans that resemble those seen in schizophrenic patients. Rodents manifest locomotor and stereotypic behaviors when treated with NMDA receptor antagonists such as phencyclidine (PCP) or dizocilpine maleate (MK-801); these behaviors are usually associated with an activated dopamine system. However, recent evidence suggests that increased glutamatergic transmission mediates the effects of these NMDA receptor antagonists. The role of dopamine in PCP- and MK-801-induced behavior (eg hyperlocomotion) remains unclear. We used dopamine-deficient (DD) mice in which tyrosine hydroxylase is selectively inactivated in dopaminergic neurons to determine whether dopamine is required for the locomotor and molecular effects of PCP and MK-801. DD mice showed a similar increase in locomotor activity and c-fos mRNA induction in the striatum in response to these NMDA receptor antagonists as control mice. Restoration of dopamine signaling in DD mice enhanced their locomotor response to PCP and MK-801. Administration of LY379268, a group II metabotropic glutamate receptor agonist that inhibits glutamate release, blocked PCP- and MK-801-induced hyperlocomotion in both DD and control mice. These results suggest that glutamate, rather than dopamine, is required for the locomotor and molecular effects of NMDA receptor antagonists, but that glutamate and dopamine can act cooperatively.

Dopamine modulates release from corticostriatal terminals

Normal striatal function is dependent on the availability of synaptic dopamine to modulate neurotransmission. Within the striatum, excitatory inputs from cortical glutamatergic neurons and modulatory inputs from midbrain dopamine neurons converge onto dendritic spines of medium spiny neurons. In addition to dopamine receptors on medium spiny neurons, D2 receptors are also present on corticostriatal terminals, where they act to dampen striatal excitation. To determine the effect of dopamine depletion on corticostriatal activity, we used the styryl dye FM1-43 in combination with multiphoton confocal microscopy in slice preparations from dopamine-deficient (DD) and reserpine-treated mice. The activity-dependent release of FM1-43 out of corticostriatal terminals allows a measure of kinetics quantified by the halftime decay of fluorescence intensity. In DD, reserpine-treated, and control mice, exposure to the D2-like receptor agonist quinpirole revealed modulation of corticostriatal kinetics with depression of FM1-43 destaining. In DD and reserpine-treated mice, quinpirole decreased destaining to a greater extent, and at a lower dose, consistent with hypersensitive corticostriatal D2 receptors. Compared with controls, slices from DD mice did not react to amphetamine or to cocaine with dopamine-releasing striatal stimulation unless the animals were pretreated with l-3,4-dihydroxyphenylalanine (l-dopa). Electron microscopy and immunogold labeling for glutamate terminals within the striatum demonstrated that the observed differences in kinetics of corticostriatal terminals in DD mice were not attributable to aberrant cytoarchitecture or glutamate density. Microdialysis revealed that basal extracellular striatal glutamate was normal in DD mice. These data indicate that dopamine deficiency results in morphologically normal corticostriatal terminals with hypersensitive D2 receptors.

Prenatal ethanol preferentially enhances reactivity of the dopamine D1 but not D2 or D3 receptors in offspring

Reports of prenatal ethanol (ETOH) effects on the dopamine system are inconsistent. In an attempt to clarify this issue, dams were given 35% ethanol-derived calories as the sole nutrient source in a liquid diet from the 10th through the 20th day of gestation (ETOH). Controls were pair-fed (PF) an isocaloric liquid diet or given ad libitum access to laboratory chow (LC). Prenatal exposure to both liquid diets reduced body weight of offspring relative to LC controls, more so for ETOH than for PF exposure. Prenatal ETOH also decreased litter size and viability, relative to both LC and PF control groups. On postnatal days 21-23, male and female offspring were given an injection of saline vehicle or one of eight specific dopamine receptor agonists or antagonists. Immediately after injection subjects were placed in individual observation cages, and over the following 30 min, eight behaviors (square entries, grooming, rearing, circling, sniffing, yawning, head and oral movements) were observed and quantified. No prenatal treatment effects on drug-induced behaviors were observed for dopamine D2 (Apomorphine, DPAT or Quinpirole) or D3 (PD 152255, Nafadotride, Apo or Quin effects on yawning) receptor agonists or antagonists, or for the vehicle control. In contrast, prenatal treatment effects were seen with drugs affecting the dopamine D1 receptor. Both D1 agonists (SKF 38393) and antagonists (SCH 23390 and high doses of spiperone) altered behaviors, especially oral and sniffing behaviors, in a manner which suggested enhanced dopamine D1 drug sensitivity in both ETOH and PF offspring relative to LC controls. These results suggest that at this age, both sexes experience a prenatal undernutrition-linked increase in the behavioral response to dopamine D1 agonists and antagonists, which can be intensified by gestational exposure to alcohol.

Distinguishing whether dopamine regulates liking, wanting, and/or learning about rewards

To determine whether dopamine regulates liking, wanting, and/or learning about rewards during goal-directed behavior, the authors tested genetically engineered dopamine-deficient (DD) mice for acquisition of an appetitive T-maze task with and without endogenous dopamine signaling. Experiment 1 established that DD mice treated with L-dihydroxyphenylalanine (L-dopa [LD]) perform similarly to controls on a T-maze task designed to measure liking, wanting, and learning about rewards. Experiment 2, which tested saline-, caffeine-, and LD-treated DD mice on the T maze, separated performance factors from cognitive processes and revealed that dopamine is not necessary for mice to like or learn about rewards but is necessary for mice to seek (want) rewards during goal-directed behavior.

Dysregulation of striatal dopamine signaling by amphetamine inhibits feeding by hungry mice

Amphetamine (AMPH) releases monoamines, transiently stimulates locomotion, and inhibits feeding. Using a genetic approach, we show that mice lacking dopamine (DA-deficient, or DD, mice) are resistant to the hypophagic effects of a moderate dose of AMPH (2 microg/g) but manifest normal AMPH-induced hypophagia after restoration of DA signaling in the caudate putamen by viral gene therapy. By contrast, AMPH-induced hypophagia in response to the same dose of AMPH is not blunted in mice lacking the ability to make norepinephrine and epinephrine (Dbh(-/-)), dopamine D(2) receptors (D2r(-/-)), dopamine D(1) receptors (D1r(-/-)), serotonin 2C receptors (Htr2c(-/Y)), neuropeptide Y (Npy(-/-)), and in mice with compromised melanocortin signaling (A(y)). We suggest that, at this moderate dose of AMPH, dysregulation of striatal DA is the primary cause of AMPH-induced hypophagia and that regulated striatal dopaminergic signaling may be necessary for normal feeding behaviors.

Huntingtin-associated protein 1 (Hap1) mutant mice bypassing the early postnatal lethality are neuroanatomically normal and fertile but display growth retardation

Huntingtin-associated protein 1 (Hap1) is the first huntingtin interacting protein identified in a yeast two-hybrid screen. Although Hap1 expression has been demonstrated in neuronal and non-neuronal tissues, its molecular role is poorly understood. Recently, it has been shown that targeted disruption of Hap1 in mice results in early postnatal death as a result of depressed feeding behavior. Although this result clearly demonstrates an essential role of Hap1 in postnatal feeding, the mechanisms leading to this deficiency, as well as the role of Hap1 in adults, remain unclear. Here we show that Hap1 null mutants display suckling defects and die within the first days after birth due to starvation. Upon reduction of the litter size, some mutants survive into adulthood and display growth retardation with no apparent brain or behavioral abnormalities, suggesting that Hap1 function is essential only for early postnatal feeding behavior. Using a conditional gene repair strategy, we also show that the early lethality can be rescued if Hap1 expression is restored in neuronal cells before birth. Furthermore, no synergism was observed between Hap1 and huntingtin mutation during mouse development. Our results demonstrate that Hap1 has a fundamental role in regulating postnatal feeding in the first 2 weeks after birth and a non-essential role in the adult mouse.

Endogenous neurotensin attenuates dopamine-dependent locomotion and stereotypy

The neuropeptide neurotensin (NT) is highly sensitive to changes in dopaminergic signaling in the striatum, and is thought to modulate dopamine-mediated behaviors. To explore the interaction of NT with the dopamine system, we utilized mice with a targeted deletion of dopamine synthesis specifically in dopaminergic neurons. Dopamine levels in dopamine-deficient (DD) mice are less than 1% of control mice, and they require daily administration of the dopamine precursor L-dihydroxyphenylalanine (L-DOPA) for survival. DD mice are supersensitive to the effects of dopamine, becoming hyperactive relative to control mice in the presence of L-DOPA. We show that 24 h after L-DOPA treatment, when DD mice are in a "dopamine-depleted" state, Nt mRNA levels in the striatum of DD mice are similar to those in control mice. Administration of L-DOPA or L-DOPA plus the L-amino acid decarboxylase inhibitor, carbidopa, (C/L-DOPA) induced Nt expression in the striatum of DD mice. The dopamine D1 receptor antagonist, SCH23390, blocked C/L-DOPA-induced Nt. To test the hypothesis that this striatal Nt expression modulated dopamine-mediated behavior in DD mice, we administered SR 48692, an antagonist of the high affinity NT receptor, together with L-DOPA or C/L-DOPA. L-DOPA-induced hyperlocomotion and C/L-DOPA-induced stereotypy were potentiated by peripheral administration of SR 48692. Furthermore, intrastriatal microinjections of SR 48692 augmented L-DOPA-induced hyperlocomotion. These results demonstrate a dynamic regulation of striatal Nt expression by dopamine via D1 receptors in DD mice, and point to a physiological role for endogenous striatal NT in counteracting motor behaviors induced by an overactive dopamine system.

A role for dopamine in feeding responses produced by orexigenic agents

Dopamine-deficient (DD) mice become hypophagic and die of starvation by 3 to 4 weeks of age unless dopamine is restored by daily treatment with l-3-4-dihydroxyphenylalanine (l-dopa). We demonstrate here that DD mice mount qualitatively normal counter-regulatory blood glucose responses to insulin and 2-deoxy-d-glucose (2-DG). However, unlike control mice, DD mice fail to eat in response to acute glucoprivation induced by insulin or 2-DG. They also have a severely blunted response to central administration of peptide YY (PYY). Viral-mediated restoration of dopamine synthesis to the central caudate putamen (CPu) of DD mice rescues feeding and survival. However, this treatment fails to restore insulin- and 2-DG-induced feeding despite normalizing feeding in response to food deprivation and PYY. Since dopamine signaling in the CPu is not sufficient for glucoprivation-induced feeding, we propose that this feeding behavior may be mediated by dopamine in an anatomically distinct brain region.

Firing properties of dopamine neurons in freely moving dopamine-deficient mice: effects of dopamine receptor activation and anesthesia

To examine the regulation of midbrain dopamine neurons, recordings were obtained from single neurons of freely moving, genetically engineered dopamine-deficient (DD) mice. DD mice were tested without dopamine signaling (basal state) and with endogenous dopamine signaling (after L-dopa administration). In the basal state, when dopamine concentration in DD mice is <1% of that in control animals, the firing properties of midbrain dopamine neurons were remarkably similar among genotypes. However, L-dopa treatment, which restores dopamine and feeding and locomotor behavior in DD mice, profoundly inhibited the firing rate and bursting of dopamine neurons in DD mice. In addition, dopamine neurons in DD mice were hypersensitive to the dopamine receptor agonists quinpirole and SKF 81297. Anesthesia markedly reduced the firing rate of dopamine neurons in DD mice but did not significantly decrease the firing rate in control dopamine neurons. These data suggest that restoration of endogenous dopamine signaling activates hypersensitive long-loop feedback pathways that serve to limit dopamine release and underscore the importance of recording from awake animals.

Simultaneous absence of dopamine D1 and D2 receptor-mediated signaling is lethal in mice

Dopamine (DA) controls a wide variety of physiological functions in the central nervous system as well as in the neuroendocrine and gastrointestinal systems. DA signaling is mediated by five cloned receptors named D1-D5. Knockout mouse models for the five receptors have been generated, and, albeit impaired for some important DA-mediated functions, they are viable and can reproduce. D1 and D2 receptors are the most abundant and widely expressed DA receptors. Cooperative/synergistic effects mediated by these receptors have been suggested, in particular, in the control of motor behaviors. To analyze the extent of such interrelationship, we have generated double D1/D2 receptor mutants. Interestingly, in contrast to single knockouts, we found that concurrent ablation of the D1 and D2 receptors is lethal during the second or third week after birth. This dramatic phenotype is likely to be related to altered feeding behavior and dysfunction of the gastrointestinal system, especially because major anatomical changes were not identified in the brain. Similarly, in the absence of functional D1, heterozygous D2 mutants (D1r(-/-);D2r(+/-)) showed severe growth retardation and did not survive their postweaning period. The analysis of motor behavior in D1r/D2r compound mutants showed that loss of D2-mediated functions reduces motor abilities, whereas the effect of D1r ablation on locomotion strongly depends on the experimental paradigms used. These studies highlight the interrelationship between D1 and D2 receptor-mediated control of motor activity, food intake, and gastrointestinal functions, which has been elusive in the single-gene ablation studies.

Tubby proteins: the plot thickens

The tubby mouse, which shows late-onset obesity and neurosensory deficits, arises from a mutation in the Tub gene. Tub shares homology with the genes for tubby-like proteins Tulp1, Tulp2 and Tulp3. Ablation of Tub, Tulp1 or Tulp3 causes disease phenotypes that are indicative of their importance in nervous-system function and development. Despite this importance, the biochemical functions of tubby-like proteins are only now beginning to be understood. At present, data indicate that tubby-like proteins might function as heterotrimeric-G-protein-responsive intracellular signalling factors, although an array of data also implicates them in other processes.

The role of dopamine in learning, memory, and performance of a water escape task

Dopamine-deficient (DD) mice have selective inactivation of the tyrosine hydroxylase gene in dopaminergic neurons, and they die of starvation and dehydration at 3-4 weeks of age. Daily injections of L-DOPA (50 mg/kg, i.p.) starting approximately 2 weeks after birth allow these animals to eat and drink enough for survival and growth. They are hyperactive for 6-9 h after receiving L-DOPA and become hypoactive thereafter. Because these animals can be tested in the presence or absence of DA, they were used to determine whether DA is necessary for learning to occur. DD mice were tested for learning to swim to an escape platform in a straight alley in the presence (30 min after an L-DOPA injection) or absence (22-24 h after an L-DOPA injection) of dopamine. The groups were split 24 h later and retested 30 min or 22-24 h after their last L-DOPA injection. In the initial test, DD mice without dopamine showed no evidence of learning, whereas those with dopamine had a learning curve similar in slope to controls but significantly slower. A retest after 24 h showed that DD mice can learn and remember in the absence of dopamine, leading to the inference that the lack of dopamine results in a performance/motivational decrement that masks their learning competence for this relatively simple task.

Transgenic mouse models of dopamine deficiency

The dopamine system is implicated in several neurological and psychiatric disorders. Genetic mutations or variations that affect dopamine system functions either directly cause or contribute to these disorders, even though other genetic and environmental factors may contribute significantly to some of these disorders as well. Transgenic mice increasingly become important tools in revealing functions of genes that are essential components of the dopamine system as well as in modeling human genetic disorders. We have reviewed a comprehensive list of those genes and compared genetic mutations/variations in humans and transgenic mouse models. The significance and limitations of these animal models as well as future directions are discussed.

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.

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.

Refinement of behavioural traits in animals for the genetic dissection of eating disorders

Both twin and family studies have revealed the involvement of genetic factors in disorders that affect the regulation of body weight, such as obesity and anorexia nervosa. However, pinpointing the genes that contribute to these human disorders has not yet been very successful. In contrast, genetic studies in animals have been basic for the identification of many genes involved in the regulation of various physiological processes of energy metabolism. We thus plan to review here ways in which findings from animal studies and what is known about behavioural diversity in the human population with eating disorders can be combined. This would probably optimise phenotype-based candidate gene analysis in humans.

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

Characterization and mapping of bovine dopamine receptors 1 and 5

A cDNA encoding the bovine dopamine receptor 1 (DRD1) was isolated from a bovine cDNA library, cloned and completely sequenced. The coding region showed 93 and 91% sequence identity on DNA level and 96 and 94% on protein level with its respective porcine and human orthologs. The bovine DRD1 and dopamine receptor 5 (DRD5) were mapped, respectively, to BTA10 and 6 by radiation hybrid mapping. One SNP was found in DRD1 and four in DRD5. Using polymerase chain reaction-restriction fragment length polymorphism, 11 different European cattle breeds were screened for the presence of the DRD1 and DRD5 substitutions. Allele frequencies for DRD1 and DRD5 alleles were very similar across all the breeds examined. Allele frequency discrepancies were found between Belgian Blue beef breed and the other breeds.

Dwarfism and low insulin-like growth factor-1 due to dopamine depletion in Pts-/- mice rescued by feeding neurotransmitter precursors and H4-biopterin

The tetrahydrobiopterin (BH4) cofactor is essential for the biosynthesis of catecholamines and serotonin and for nitric-oxide synthase (NOS). Alterations in BH4 metabolism are observed in various neurological and psychiatric diseases, and mutations in one of the human metabolic genes causes hyperphenylalaninemia and/or monoamine neurotransmitter deficiency. We report on a knockout mouse for the Pts gene, which codes for a BH4-biosynthetic enzyme. Homozygous Pts-/- mice developed with normal morphology but died after birth. Upon daily oral administration of BH4 and neurotransmitter precursors the Pts-/- mice eventually survived. However, at sexual maturity (6 weeks) the mice had only one-third of the normal body weight and were sexually immature. Biochemical analysis revealed no hyperphenylalaninemia, normal brain NOS activity, and almost normal serotonin levels, but brain dopamine was 3% of normal. Low dopamine leads to impaired food consumption as reflected by the severe growth deficiency and a 7-fold reduced serum insulin-like growth factor-1 (IGF-1). This is the first link shown between 6-pyruvoyltetrahydropterin synthase- or BH4-biosynthetic activity and IGF-1.

Animal models in the investigation of anorexia

Anorexia nervosa (AN) is an eating disorder of unknown origin that most commonly occurs in women and usually has its onset in adolescence. Patients with AN invariably have a disturbed body image and an intense fear of weight gain. There is currently no definitive treatment for this disease, which carries a 20% mortality over 20 years. Development of an appropriate animal model of AN has been difficult, as the etiology of this eating disorder likely involves a complex interaction between genetic, environmental, social, and cultural factors. In this review, we focus on several possible rodent models of AN. In our laboratory, we have developed and studied three different mouse models of AN based on clinical profiles of the disease; separation stress, activity, and diet restriction (DR). In addition, we discuss the spontaneous mouse mutation anx/anx and several mouse gene knockout models, which have resulted in an anorexic phenotype. We highlight what has been learned from each of these models and possibilities for future models. It is hoped that a combination of the study of such models, together with genetic and clinical studies in patients, will lead to more rational and successful prevention/treatment of this tragic, and often fatal, disease.

Neonatal 6-hydroxydopamine administration to mice is fatal

Depletion of dopamine in adult rats by treatment with the neurotoxin 6-hydroxydopamine (6-OHDA) causes severe deficits in feeding, drinking, and movement that often lead to death. However, when neonatal rats are treated similarly, they survive normally, suggesting that compensatory adaptation to dopamine depletion occurs. In contrast, dopamine-deficient mice that have a selective genetic deficiency in dopamine production die 2-4 weeks after birth. Thus, we tested the hypothesis that killing dopaminergic neurons with 6-OHDA might promote survival of dopamine-deficient mice. Body weights, motor coordination, catecholamine levels, and survival were monitored for several weeks after bilateral administrations of 6-OHDA to 3-day-old mice. Some treated mice were raised in a heated chamber to help them conserve energy. The results demonstrate that regardless of genotype or environmental temperature, bilateral neonatal 6-OHDA lesions are lethal to mice.

Dopamine controls the firing pattern of dopamine neurons via a network feedback mechanism

Changes in the firing pattern of midbrain dopamine neurons are thought to encode information for certain types of reward-related learning. In particular, the burst pattern of firing is predicted to result in more efficient dopamine release at target loci, which could underlie changes in synaptic plasticity. In this study, the effects of dopamine on the firing patterns of dopaminergic neurons in vivo and their electrophysiological characteristics in vitro were examined by using a genetic dopamine-deficient (DD) mouse model. Extracellular recordings in vivo showed that, although the firing pattern of dopamine neurons in normal mice included bursting activity, DD mice recordings showed only a single-spike pattern of activity with no bursts. Bursting was restored in DD mice after systemic administration of the dopamine precursor, L-3,4-dihydroxyphenylalanine (L-dopa). Whole-cell recordings in vitro demonstrated that the basic electrophysiology and pharmacology of dopamine neurons were identical between DD and control mice, except that amphetamine did not elicit a hyperpolarizing current in slices from DD mice. These data suggest that endogenously released dopamine plays a critical role in the afferent control of dopamine neuron bursting activity and that this control is exerted via a network feedback mechanism.

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.