Biomedical subjects

Richard D Palmiter

Publications and source records attributed to Richard D Palmiter.

At least 19 recordsLinked to original sources

PubMed2006

NPY/AgRP neurons are not essential for feeding responses to glucoprivation.

Animals respond to hypoglycemia by eating and by stimulating gluconeogenesis. These responses to glucose deprivation are initiated by glucose-sensing neurons in the brain, but the neural circuits that control feeding behavior are not well established. Neurons in the arcuate region of the hypothalamus that express neuropeptide Y (NPY) and agouti-related protein (AgRP) have been implicated in mediating the feeding response to glucoprivation. We devised a method to selectively ablate these neurons in neonatal mice and then tested adult mice for their feeding responses to fasting, mild hypoglycemia, 2-deoxy-d-glucose and a ghrelin receptor agonist. Whereas the feeding response to the ghrelin receptor agonist was completely abrogated, the feeding response to glucoprivation was normal. The feeding response after a fast was attenuated when standard chow was available but normal with more palatable solid or liquid diet. We conclude that NPY/AgRP neurons are not necessary for generating or mediating the orexigenic response to glucose deficiency, but they are essential for the feeding response to ghrelin and refeeding on standard chow after a fast.

Agouti-Related Protein↗

PubMed2006

Lack of neuropeptide Y attenuates the somatic signs of opiate withdrawal.

Recent evidence suggests that neuropeptide Y (NPY) may be involved in the neurobiological responses to drugs of abuse. This study was designed to assess the possible contribution of NPY to opiate withdrawal behaviors. Here we report that mice lacking the NPY gene show normal conditioned place aversion to opiate withdrawal, but show attenuated opiate withdrawal somatic signs.

Animals↗

PubMed2006

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.

Adenoviruses, Canine↗

PubMed2006

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.

Adenoviridae↗

PubMed2006

Role of noradrenergic signaling by the nucleus tractus solitarius in mediating opiate reward.

Norepinephrine (NE) is widely implicated in opiate withdrawal, but much less is known about its role in opiate-induced locomotion and reward. In mice lacking dopamine beta-hydroxylase (DBH), an enzyme critical for NE synthesis, we found that NE was necessary for morphine-induced conditioned place preference (CPP; a measure of reward) and locomotion. These deficits were rescued by systemic NE restoration. Viral restoration of DBH expression in the nucleus tractus solitarius, but not in the locus coeruleus, restored CPP for morphine. Morphine-induced locomotion was partially restored by DBH expression in either brain region. These data suggest that NE signaling by the nucleus tractus solitarius is necessary for morphine reward.

Animals↗

PubMed2006

Local dopamine production in the dorsal striatum restores goal-directed behavior in dopamine-deficient mice.

To determine whether dopamine signaling in the dorsal striatum is sufficient for performance of goal-directed behaviors, local dopamine production was restored in the dorsal striatum of dopamine-deficient (DD) mice through viral-mediated gene therapy. Virally rescued DD (vrDD) mice were tested for learning of an appetitive T-maze task designed to measure goal-directed behavior. The results indicate that in contrast with the performance of DD mice that have dysregulated dopamine signaling, vrDD mice were able to perform the T-maze task and reverse their behavior as well as sham-operated control mice. The authors conclude that finely tuned dopaminergic signaling within the dorsal striatum is sufficient for performance of goal-directed behavior.

Animals↗

PubMed2006

Clioquinol and docosahexaenoic acid act synergistically to kill tumor cells.

Clioquinol, an 8-hydroxyquinoline derivative (5-chloro-7-iodo-8-hydroxyquinoline) with antimicrobial properties, has recently been found to have cytotoxic activity towards human cancer cell lines at concentrations achieved by oral administration. This study was initiated to determine whether clioquinol could potentiate the antitumor effects of two drugs, doxorubicin and docosahexaenoic acid (DHA), believed to act in part via the generation of reactant oxidant species. At low micromolar concentrations, clioquinol had little effect upon cell viability and did not potentiate doxorubicin's cytotoxicity. Clioquinol significantly enhanced DHA's cytotoxic effects, an interaction that was shown to be synergistic by isobolographic analysis. Clioquinol exhibited a synergistic interaction with DHA in reducing nuclear factor-kappaB activity and inducing apoptosis, and the combination reduced the level of several molecules that promote cell survival, including Akt, p65, and Bcl-2. Interestingly, clioquinol neither induced lipid peroxidation itself nor increased peroxidation brought about by the addition of DHA. However, when cells were pretreated with antioxidant vitamin E, the synergism of clioquinol and DHA was blocked, indicating the essential role of lipid peroxidation for their action. These findings reveal a novel antitumor drug combination that synergistically targets major cell survival signaling pathways.

Antineoplastic Combined Chemotherapy Protocols↗

PubMed2005

Parkin-deficient mice are not more sensitive to 6-hydroxydopamine or methamphetamine neurotoxicity.

BACKGROUND: Autosomal recessive juvenile parkinsonism (AR-JP) is caused by mutations in the parkin gene which encodes an E3 ubiquitin-protein ligase. Parkin is thought to be critical for protecting dopaminergic neurons from toxic insults by targeting misfolded or oxidatively damaged proteins for proteasomal degradation. Surprisingly, mice with targeted deletions of parkin do not recapitulate robust behavioral or pathological signs of parkinsonism. Since Parkin is thought to protect against neurotoxic insults, we hypothesized that the reason Parkin-deficient mice do not develop parkinsonism is because they are not exposed to appropriate environmental triggers. To test this possibility, we challenged Parkin-deficient mice with neurotoxic regimens of either methamphetamine (METH) or 6-hydroxydopamine (6-OHDA). Because Parkin function has been linked to many of the pathways involved in METH and 6-OHDA toxicity, we predicted that Parkin-deficient mice would be more sensitive to the neurotoxic effects of these agents. RESULTS: We found no signs consistent with oxidative stress, ubiquitin dysfunction, or degeneration of striatal dopamine neuron terminals in aged Parkin-deficient mice. Moreover, results from behavioral, neurochemical, and immunoblot analyses indicate that Parkin-deficient mice are not more sensitive to dopaminergic neurotoxicity following treatment with METH or 6-OHDA. CONCLUSION: Our results suggest that the absence of a robust parkinsonian phenotype in Parkin-deficient mice is not due to the lack of exposure to environmental triggers with mechanisms of action similar to METH or 6-OHDA. Nevertheless, Parkin-deficient mice could be more sensitive to other neurotoxins, such as rotenone or MPTP, which have different mechanisms of action; therefore, identifying conditions that precipitate parkinsonism specifically in Parkin-deficient mice would increase the utility of this model and could provide insight into the mechanism of AR-JP. Alternatively, it remains possible that the absence of parkinsonism in Parkin-deficient mice could reflect fundamental differences between the function of human and mouse Parkin, or the existence of a redundant E3 ubiquitin-protein ligase in mouse that is not found in humans. Therefore, additional studies are necessary to understand why Parkin-deficient mice do not display robust signs of parkinsonism.

Aging↗

PubMed2005

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.

Analgesics, Opioid↗

PubMed2005

Modulation of neuropeptide Y expression in adult mice does not affect feeding.

Despite numerous experiments showing that administration of neuropeptide Y (NPY) to rodents stimulates feeding and obesity, whereas acute interference with NPY signaling disrupts feeding and promotes weight loss, NPY-null mice have essentially normal body weight regulation. These conflicting observations suggest that chronic lack of NPY during development may lead to compensatory changes that normalize regulation of food intake and energy expenditure in the absence of NPY. To test this idea, we used gene targeting to introduce a doxycycline (Dox)-regulated cassette into the Npy locus, such that NPY would be expressed until the mice were given Dox, which blocks transcription. Compared with wild-type mice, adult mice bearing this construct expressed approximately 4-fold more Npy mRNA, which fell to approximately 20% of control values within 3 days after treatment with Dox. NPY protein also fell approximately 20-fold, but the half-life of approximately 5 days was surprisingly long. The biological effectiveness of these manipulations was demonstrated by showing that overexpression of NPY protected against kainate-induced seizures. Mice chronically overexpressing NPY had normal body weight, and administration of Dox to these mice did not suppress feeding. Furthermore, the refeeding response of these mice after a fast was normal. We conclude that, if there is compensation for changes in NPY levels, then it occurs within the time it takes for Dox treatment to deplete NPY levels. These observations suggest that pharmacological inhibition of NPY signaling is unlikely to have long-lasting effects on body weight.

Aging↗

PubMed2005

NPY/AgRP neurons are essential for feeding in adult mice but can be ablated in neonates.

Hypothalamic neurons that express neuropeptide Y (NPY) and agouti-related protein (AgRP) are thought to be critical regulators of feeding behavior and body weight. To determine whether NPY/AgRP neurons are essential in mice, we targeted the human diphtheria toxin receptor to the Agrp locus, which allows temporally controlled ablation of NPY/AgRP neurons to occur after an injection of diphtheria toxin. Neonatal ablation of NPY/AgRP neurons had minimal effects on feeding, whereas their ablation in adults caused rapid starvation. These results suggest that network-based compensatory mechanisms can develop after the ablation of NPY/AgRP neurons in neonates but do not readily occur when these neurons become essential in adults.

Aging↗

PubMed2005

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.

Amphetamine↗

PubMed2005

Dopamine depletion does not protect against acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity in vivo.

Dopamine (DA) has been postulated to play a role in the loss of dopaminergic substantia nigra (SN) neurons in Parkinson's disease because of its propensity to oxidize and form quinones and other reactive oxygen species that can alter cellular function. Moreover, DA depletion can attenuate dopaminergic cell loss in vitro. To test the contribution of DA to SN impairment in vivo, we used DA-deficient mice, which lack the enzyme tyrosine hydroxylase in dopaminergic cells, and mice pharmacologically depleted of DA by alpha-methyl-p-tyrosine pretreatment. Mice were treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a toxin that produces parkinsonian pathology in humans, nonhuman primates, and rodents. In contrast to in vitro results, genetic or pharmacologic DA depletion did not attenuate loss of dopaminergic neurons in the SN or dopaminergic neuron terminals in the striatum. These results suggest that DA does not contribute to acute MPTP toxicity in vivo.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗

PubMed2005

Expression of mutant NMDA receptors in dopamine D1 receptor-containing cells prevents cocaine sensitization and decreases cocaine preference.

The interaction of dopamine and glutamate in limbic brain regions mediates behaviors associated with psychostimulants, which act in part to increase dopamine signaling at both D1 receptors (D1Rs) and D2 receptors. Many addictive behaviors are a result of learned associations, and NMDA receptor activation has been shown to be important for these behaviors. We hypothesized that if NMDA receptor activation in dopamine receptor-containing cells is required for the addictive properties of psychostimulants, then mice with reduced NMDA receptor activity in D1R-containing cells would have attenuated long-term behavioral changes to these drugs. We generated a mouse line in which D1R-containing cells express an NR1 NMDA receptor subunit containing a mutation in the pore that reduces calcium flux. Mice expressing the mutant NMDA receptors in D1R-containing cells have normal basal activity and display similar increases in locomotor activity when treated with acute amphetamine or cocaine. However, the mutant mice fail to display locomotor sensitization to repeated cocaine administration. In addition, these mice also have a decreased ability to form a conditioned place preference to cocaine. These data suggest that intact NMDA receptor signaling in D1R-containing cells is required for the manifestation of behaviors associated with repeated drug exposure.

Amino Acid Substitution↗

PubMed2005

Neuropeptide Y protects against methamphetamine-induced neuronal apoptosis in the mouse striatum.

Methamphetamine (METH) is an illicit drug that causes neuronal apoptosis in the mouse striatum, in a manner similar to the neuronal loss observed in neurodegenerative diseases. In the present study, injections of METH to mice were found to cause the death of enkephalin-positive projection neurons but not the death of neuropeptide Y (NPY)/nitric oxide synthase-positive striatal interneurons. In addition, these METH injections were associated with increased expression of neuropeptide Y mRNA and changes in the expression of the NPY receptors Y1 and Y2. Administration of NPY in the cerebral ventricles blocked METH-induced apoptosis, an effect that was mediated mainly by stimulation of NPY Y2 receptors and, to a lesser extent, of NPY Y1 receptors. Finally, we also found that neuropeptide Y knock-out mice were more sensitive than wild-type mice to METH-induced neuronal apoptosis of both enkephalin- and nitric oxide synthase-containing neurons, suggesting that NPY plays a general neuroprotective role within the striatum. Together, our results demonstrate that neuropeptide Y belongs to the class of factors that maintain neuronal integrity during cellular stresses. Given the similarity between the cell death patterns induced by METH and by disorders such as Huntington's disease, our results suggest that NPY analogs might be useful therapeutic agents against some neurodegenerative processes.

Animals↗

PubMed2005

Attenuated feeding responses to circadian and palatability cues in mice lacking neuropeptide Y.

Neuropeptide Y (NPY) is a potent orexigenic peptide that is implicated in the feeding response to a variety of stimuli. The current studies employed mice lacking NPY (Npy-/-) and their wild-type (Npy+/+) littermates to investigate the role of this peptide in the feeding response to circadian and palatability cues. To investigate the response to a circadian stimulus, we assessed food intake during the 4-h period following dark onset, a time of day characterized by maximal rates of food consumption. Compared to Npy+/+ controls, intake of Npy-/- mice was reduced by 33% during this period (0.6+/-0.1g versus 0.9+/-0.1g; p<or=0.05). In contrast, intake did not differ between genotypes when measured over a 24-h period (3.7+/-0.2g versus 3.5+/-0.3g; p=ns). Furthermore, reduced dark cycle 4h food intake in Npy-/- mice was not evident after a 24-h fast (1.4+/-0.1g for both genotypes; p=ns), despite a pronounced delay in the initiation of feeding (636+/-133 s versus 162+/-29 s; p<or=0.05). To investigate the role of NPY in the feeding response to palatability cues, mice were presented with a highly palatable diet (HP) for 1h each day (in addition to having ad libitum access to chow) for 18 days. Npy+/+ mice rapidly increased daily HP intake such that by the end of the first week, they derived a substantial fraction of daily energy from this source (41+/-3%). By comparison, HP intake was markedly reduced in Npy-/- mice during the first week (24+/-7% of daily energy intake, p<or=0.05 versus Npy+/+), although it eventually increased (by Day 9) to values comparable to those of Npy+/+ controls. These experiments suggest that NPY contributes to the mechanism whereby food intake increases in response to circadian and palatability cues and that mechanisms driving food intake in response to these stimuli differ from those activated by energy restriction.

Animals↗

PubMed2005

Dopamine supersensitivity correlates with D2High states, implying many paths to psychosis.

Dopamine supersensitivity occurs in schizophrenia and other psychoses, and after hippocampal lesions, antipsychotics, ethanol, amphetamine, phencyclidine, gene knockouts of Dbh (dopamine beta-hydroxylase), Drd4 receptors, Gprk6 (G protein-coupled receptor kinase 6), Comt (catechol-O-methyltransferase), or Th-/-, DbhTh/+ (tyrosine hydroxylase), and in rats born by Cesarean-section. The functional state of D2, or the high-affinity state for dopamine (D2High), was measured in these supersensitive animal brain striata. Increased levels and higher proportions (40-900%) for D2High were found in all these tissues. If many types of brain impairment cause dopamine behavioral supersensitivity and a common increase in D2High states, it suggests that there are many pathways to psychosis, any one of which can be disrupted.

Animals↗

PubMed2005

Parkin-deficient mice are not a robust model of parkinsonism.

Mutations in the human parkin gene cause autosomal recessive juvenile parkinsonism, a heritable form of Parkinson's disease (PD). To determine whether mutations in the mouse parkin gene (Park2) also result in a parkinsonian phenotype, we generated mice with a targeted deletion of parkin exon 2. Using an extensive behavioral screen, we evaluated neurological function, motor ability, emotionality, learning, and memory in aged Parkin-deficient mice. The behavioral profile of Parkin-deficient mice on a B6;129S4 genetic background was strikingly similar to that of control mice, and most differences were not reproducible by using coisogenic mice on a 129S4 genetic background. Moreover, catecholamine levels in the striatum, olfactory bulb, and spinal cord of Parkin-deficient mice were normal. In contrast to previous studies using independently generated Parkin-deficient mice, we found no evidence for nigrostriatal, cognitive, or noradrenergic dysfunction. Understanding why Parkin-deficient mice do not exhibit robust signs of parkinsonism could advance knowledge and treatment of PD.

Amphetamines↗