Dopamine Abstracts 3

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The role of dopamine in alcohol self-administration in humans: individual differences.
            (Barrett et al., 2008) Download
OBJECTIVE:  To clarify dopamine's role in alcohol self-administration in a heterogeneous sample of drinkers using acute phenylalanine/tyrosine depletion (APTD). METHODS:  Sixteen men with variable drinking histories were characterized on their ethanol-induced cardiac response, a marker previously proposed to index dopamine system reactivity and vulnerability to alcohol abuse. During separate sessions participants were administered (i) a nutritionally balanced (BAL) amino acid (AA) mixture, (ii) a mixture lacking the dopamine precursors, phenylalanine and tyrosine, and (iii) APTD followed by the dopamine precursor, L-DOPA. Five hours after AA administration, participants could earn units of alcohol using a progressive ratio breakpoint task. RESULTS:  Alcohol self-administration was reduced in the APTD and APTD+L-DOPA conditions relative to the BAL condition. In both cases the changes were predicted by ethanol-induced cardiac change. CONCLUSIONS:  The motivation to drink is likely regulated by more than one neurobiological mechanism. Individual differences in cardiac responsivity to ethanol might provide a peripheral marker of responsiveness to pharmacological manipulations of dopamine.

A dose-finding study on the effects of branch chain amino acids on surrogate markers of brain dopamine function.
            (Gijsman et al., 2002) Download
RATIONALE:  We have previously shown in healthy volunteers that an amino acid mixture lacking tyrosine and phenylalanine reduces tyrosine availability to the brain and produces cognitive and neuroendocrine effects consistent with reduced dopamine function. This could provide a potential nutritional approach to disorders such as mania and schizophrenia, which are characterised by overactivity of dopamine pathways. The amino acid mixture we tested previously is unpalatable, whereas mixtures containing only branch chain amino acids can be made more palatable. However, the effects of such mixtures on dopamine function in humans have not been studied. OBJECTIVE:  To assess the tolerability of different doses of branch chain amino acids and to measure their effects on neuroendocrine and cognitive measures sensitive to changes in dopamine function. METHODS:  We used a randomised, double-blind, cross-over design in 12 healthy volunteers to assess the effect of single oral doses of 10 g, 30 g and 60 g branch chain amino acids on plasma prolactin and a test of spatial recognition memory RESULTS:  The branch chain amino acids were well tolerated. The availability of tyrosine for brain catecholamine synthesis decreased in a dose-related manner. As hypothesised, the drink increased both the plasma prolactin and the latency to respond on the spatial recognition memory task. CONCLUSIONS:  A drink containing branch chain amino acids is well tolerated in healthy volunteers and produces effects consistent with lowered dopamine function.

Ketamine reduces muscle pain, temporal summation, and referred pain in fibromyalgia patients.
(Graven-Nielsen et al., 2000) Download
Central mechanisms related to referred muscle pain and temporal summation of muscular nociceptive activity are facilitated in fibromyalgia syndrome (FMS) patients. The present study assessed the effects of an NMDA-antagonist (ketamine) on these central mechanisms. FMS patients received either i.v. placebo or ketamine (0.3 mg/kg, Ketalar((R))50% decrease in pain intensity at rest by active drug on two consecutive VAS assessments). Fifteen out of 17 ketamine-responders were included in the second part of the study. Before and after ketamine or placebo, experimental local and referred pain was induced by intramuscular (i.m.) infusion of hypertonic saline (0.7 ml, 5%) into the tibialis anterior (TA) muscle. The saline-induced pain intensity was assessed on an electronic VAS, and the distribution of pain drawn by the subject. In addition, the pain threshold (PT) to i.m. electrical stimulation was determined for single stimulus and five repeated (2 Hz, temporal summation) stimuli. The pressure PT of the TA muscle was determined, and the pressure PT and pressure pain tolerance threshold were determined at three bilaterally located tenderpoints (knee, epicondyle, and mid upper trapezius). VAS scores of pain at rest were progressively reduced during ketamine infusion compared with placebo infusion. Pain intensity (area under the VAS curve) to the post-drug infusion of hypertonic saline was reduced by ketamine (-18. 4+/-0.3% of pre-drug VAS area) compared with placebo (29.9+/-18.8%, P<0.02). Local and referred pain areas were reduced by ketamine (-12. 0+/-14.6% of pre-drug pain areas) compared with placebo (126.3+/-83. 2%, P<0.03). Ketamine had no significant effect on the PT to single i.m. electrical stimulation. However, the span between the PT to single and repeated i.m. stimuli was significantly decreased by the ketamine (-42.3+/-15.0% of pre-drug PT) compared with placebo (50. 5+/-49.2%, P<0.03) indicating a predominant effect on temporal summation. Mean pressure pain tolerance from the three paired tenderpoints was increased by ketamine (16.6+/-6.2% of pre-drug thresholds) compared with placebo (-2.3+/-4.9%, P<0.009). The pressure PT at the TA muscle was increased after ketamine (42.4+/-9. 2% of pre-drug PT) compared with placebo (7.0+/-6.6%, P<0.011). The present study showed that mechanisms involved in referred pain, temporal summation, muscular hyperalgesia, and muscle pain at rest were attenuated by the NMDA-antagonist in FMS patients. It suggested a link between central hyperexcitability and the mechanisms for facilitated referred pain and temporal summation in a sub-group of the fibromyalgia syndrome patients. Whether this is specific for FMS patients or a general phenomena in painful musculoskeletal disorders is not known.

Tyrosine depletion attenuates dopamine function in healthy volunteers.
            (Harmer et al., 2001) Download
RATIONALE:  Tyrosine depletion has been shown to reduce dopamine over activity in animal and human investigations. However, the effects on basal dopamine function have not been explored. Such information could establish tyrosine depletion as an effective probe of dopamine function in healthy volunteers and would also have relevance for future therapeutic applications of this manipulation. OBJECTIVE:  The present study investigated the effect of acute tyrosine depletion on dopamine function in healthy volunteers using a combination of neuroendocrine, neuropsychological and subjective measures. METHODS:  On one occasion, volunteers received an amino acid drink selectively lacking tyrosine and phenylalanine (TYR-free), whilst on the other they received a balanced (BAL) amino acid drink. Plasma prolactin, amino acid levels and subjective state were monitored over 6 h following the two drinks, and volunteers also completed a battery of tests from the CANTAB, including measures of spatial memory previously found to be sensitive to changes in dopamine function. RESULTS:  Plasma prolactin levels rose following the TYR-free drink relative to the balanced mixture, indicative of decreased dopamine neurotransmission within the hypothalamus. Following the TYR-free drink, volunteers were impaired at spatial recognition memory and spatial working memory. Volunteers also tended to report that they felt less good following the TYR-free than the BAL mixture. CONCLUSION:  Tyrosine depletion in healthy volunteers affected baseline dopamine function on the different measures employed in this study. Tyrosine depletion would thereby seem valuable as a probe of dopamine function in human volunteers. Ratings of depression and other aspects of cognitive function were unaffected, suggesting that this manipulation may be free of significant side effects when used as a treatment for conditions characterised by dopamine over activity, such as acute mania and schizophrenia.

A randomized, double-blind, placebo-controlled trial of pramipexole, a dopamine agonist, in patients with fibromyalgia receiving concomitant medications.
            (Holman and Myers, 2005) Download
OBJECTIVE:  To assess the efficacy and safety of pramipexole, a dopamine 3 receptor agonist, in patients with fibromyalgia. METHODS:  In this 14-week, single-center, double-blind, placebo-controlled, parallel-group, escalating-dose trial, 60 patients with fibromyalgia were randomized 2:1 (pramipexole:placebo) to receive 4.5 mg of pramipexole or placebo orally every evening. The primary outcome was improvement in the pain score (10-cm visual analog scale [VAS]) at 14 weeks. Secondary outcome measures were the Fibromyalgia Impact Questionnaire (FIQ), the Multidimensional Health Assessment Questionnaire (MDHAQ), the pain improvement scale, the tender point score, the 17-question Hamilton Depression Inventory (HAM-d), and the Beck Anxiety Index (BAI). Patients with comorbidities and disability were not excluded. Stable dosages of concomitant medications, including analgesics, were allowed. RESULTS:  Compared with the placebo group, patients receiving pramipexole experienced gradual and more significant improvement in measures of pain, fatigue, function, and global status. At 14 weeks, the VAS pain score decreased 36% in the pramipexole arm and 9% in the placebo arm (treatment difference -1.77 cm). Forty-two percent of patients receiving pramipexole and 14% of those receiving placebo achieved > or =50% decrease in pain. Secondary outcomes favoring pramipexole over placebo included the total FIQ score (treatment difference -9.57) and the percentages of improvement in function (22% versus 0%), fatigue (29% versus 7%), and global (38% versus 3%) scores on the MDHAQ. Compared with baseline, some outcomes showed a better trend for pramipexole treatment than for placebo, but failed to reach statistical significance, including improvement in the tender point score (51% versus 36%) and decreases in the MDHAQ psychiatric score (37% versus 28%), the BAI score (39% versus 27%), and the HAM-d score (29% versus 9%). No end points showed a better trend for the placebo arm. The most common adverse events associated with pramipexole were transient anxiety and weight loss. No patient withdrew from the study because of inefficacy or an adverse event related to pramipexole. CONCLUSION:  In a subset of patients with fibromyalgia, approximately 50% of whom required narcotic analgesia and/or were disabled, treatment with pramipexole improved scores on assessments of pain, fatigue, function, and global status, and was safe and well-tolerated.

Cerebrospinal fluid biogenic amine metabolites, plasma-rich platelet serotonin and [3H]imipramine reuptake in the primary fibromyalgia syndrome.
            (Legangneux et al., 2001) Download
BACKGROUND:  Primary fibromyalgia syndrome (PFS) is a chronic disorder commonly seen in rheumatological practice. The pathophysiological disturbances of this syndrome, which was defined by the American College of Rheumatology in 1990, are poorly understood. This study evaluated, in 30 patients, the hypothesis that PFS is a pain modulation disorder induced by deregulation of serotonin metabolism. OBJECTIVES:  To compare platelet [(3)H]imipramine binding sites and serotonin (5-HT) levels in plasma-rich platelets (PRP) of PFS patients with those of matched healthy controls and to compare the levels of biogenic amine metabolites in the cerebrospinal fluid (CSF) of PFS patients with those of matched controls. METHODS:  Platelet [(3)H]imipramine binding sites were defined by two criteria, B(max) for their density and K(d) for their affinity. PRP 5-HT and CSF metabolites of 5-HT (5-hydroxyindoleacetic acid, 5-HIAA), norepinephrine (3-methoxy, 4-hydroxy phenylglycol, MHPG) and dopamine (homovanillic acid, HVA) were assayed by reversed-phase high-performance liquid chromatography with coulometric detection. RESULTS:  [(3)H]Imipramine platelet binding was similar (P=0.43 for B(max) and P=0.30 for K(d)) in PFS patients (B(max)=901+/-83 fmol/mg protein, K(d)=0.682+/-0.046) and in matched controls (B(max)=1017+/-119 fmol/mg protein, K(d)=0.606+/-0.056). PRP 5-HT was significantly higher (P=0.0009) in PFS patients (955+/-101 ng/10(9) platelets) than in controls (633+/-50 ng/10(9) platelets). When adjusted for age, the levels of all CSF metabolites were lower in PFS patients. The CSF metabolite of norepinephrine (MHPG) was lower (P:=0.003) in PFS patients (8.33+/-0.33 ng/ml) than in matched controls (9.89+/-0.31 ng/ml) and 5-HIAA was lower (P=0.042) in PFS female patients (22.34+/-1.78 ng/ml) than in matched controls (25.75+/-1.75 ng/ml). For HVA in females, the difference between PFS patients (36.32+/-3.20 ng/ml) and matched controls (38.32+/-2.90 ng/ml) approached statistical significance (P=0.054). CONCLUSION:  Changes in metabolites of CSF biogenic amines appear to be partially correlated to age but remained diagnosis-dependent. High levels of PRP 5-HT in PFS patients were associated with low CSF 5-HIAA levels in female patients but were not accompanied by any change in serotonergic uptake as assessed by platelet [(3)H]imipramine binding sites. These findings do not allow us to confirm that serotonin metabolism is deregulated in PFS patients.

Dopamine depletion impairs frontostriatal functional connectivity during a set-shifting task.
            (Nagano-Saito et al., 2008) Download
We investigated the effect of transient dopamine depletion on functional connectivity during performance of the Wisconsin Card Sorting Task. Functional magnetic resonance imaging data were analyzed as a psychophysiological interaction, a statistical method used to identify functional connectivity during experimental manipulations. Nineteen healthy subjects were scanned, double blind, on 2 separate days: once after drinking an amino acid mixture deficient in the dopamine precursors tyrosine and phenylalanine, and once after drinking a nutritionally balanced mixture. In the balanced drink session, statistically significant connectivity between the frontal lobes and striatum was observed during set shifting, and the greater the prefrontostriatal connectivity, the faster the response time after a shift. Neither of these associations were observed after dopamine depletion. Moreover, dopamine depletion also reduced the degree of deactivation in areas normally suppressed during attention-demanding tasks, including the medial prefrontal cortex, posterior cingulate cortex, and hippocampus. Together, these results suggest that functional connectivity between the frontal lobes and basal ganglia during set shifting contributes to more efficient performance and that dopamine modulates this corticostriatal connectivity.


 

Acute dopamine depletion with branched chain amino acids decreases auditory top-down event-related potentials in healthy subjects.
            (Neuhaus et al., 2009) Download
Cerebral dopamine homeostasis has been implicated in a wide range of cognitive processes and is of great pathophysiological importance in schizophrenia. A novel approach to study cognitive effects of dopamine is to deplete its cerebral levels with branched chain amino acids (BCAAs) that acutely lower dopamine precursor amino acid availability. Here, we studied the effects of acute dopamine depletion on early and late attentive cortical processing. Auditory event-related potential (ERP) components N2 and P3 were investigated using high-density electroencephalography in 22 healthy male subjects after receiving BCAAs or placebo in a randomized, double-blind, placebo-controlled crossover design. Total free serum prolactin was also determined as a surrogate marker of cerebral dopamine depletion. Acute dopamine depletion increased free plasma prolactin and significantly reduced prefrontal ERP components N2 and P3. Subcomponent analysis of N2 revealed a significant attenuation of early attentive N2b over prefrontal scalp sites. As a proof of concept, these results strongly suggest that BCAAs are acting on basic information processing. Dopaminergic neurotransmission seems to be involved in auditory top-down processing as indexed by prefrontal N2 and P3 reductions during dopamine depletion. In healthy subjects, intact early cortical top-down processing can be acutely dysregulated by ingestion of BCAAs. We discuss the potential impact of these findings on schizophrenia research.

The role of stress in the pathophysiology of the dopaminergic system.
            (Pani et al., 2000) Download
In this review, we will examine the most recent preclinical evidence in support of the fact that both acute and chronic stress may have a detrimental impact on the normal function of the dopaminergic system. In recent decades, the term stress has changed its meaning from that of a 'non-specific body response' to a 'monitoring system of internal and external cues'; that is a modality of reaction of the mammalian central nervous system (CNS) which is critical to the adaptation of the organism to its environment. Compelling results have demonstrated that the dopaminergic system is important not only for hedonic impact or reward learning but also, in a broader sense, for reactivity to perturbation in environmental conditions, for selective information processing, and for general emotional responses, which are essential functions in the ability (or failure) to cope with the external world. In this, stress directly influences several basic behaviors which are mediated by the dopaminergic system such as locomotor activity, sexual activity, appetite, and cross sensitization with drugs of abuse. Studies using rat lines which are genetically different in dopamine (DA) physiology, have shown that even small alterations in the birth procedure or early life stress events may contribute to the pathophysiology of psychiatric disorders-in particular those involving central DA dysfunction-and may cause depression or psychotic derangement in the offspring. Finally, the fact that the dopaminergic system after stress responds, preferentially, in the medial prefrontal cortex (MFC), is thought to serve, in humans, as a protection against positive psychotic symptoms, since the increased DA activity in the MFC suppresses limbic DA transmission. However, excessive MFC dopaminergic activity has a negative impact on the cognitive functions of primates, making them unable to select and process significant environmental stimuli. Thus it appears that a critical range of DA turnover is necessary for optimal cognitive functioning after stress, in the response of the CNS to ever-changing environmental demands. Molecular Psychiatry (2000) 5, 14-21.

Cerebrospinal fluid biogenic amine metabolites in fibromyalgia/fibrositis syndrome and rheumatoid arthritis.
            (Russell et al., 1992) Download
OBJECTIVE:  To compare the levels of biogenic amines in the cerebrospinal fluid (CSF) of primary fibromyalgia syndrome (PFS) patients with those in the CSF of controls. METHODS:  Metabolites of serotonin, norepinephrine, and dopamine were identified in CSF, using high performance liquid chromatography with coulometric detection. RESULTS:  CSF levels of metabolites from all 3 neurotransmitters were lower in PFS patients than in controls. CONCLUSION:  A low rate of turnover of several neurotransmitters supports the proposed hypothesis of a metabolic defect in PFS and suggests that the defect occurs at a neuroregulatory level.

Stress and dopamine: implications for the pathophysiology of chronic widespread pain.
            (Wood, 2004) Download
Fibromyalgia has been called a "stress-related disorder" due to the onset and exacerbation of symptoms in the context of stressful events. Evidence suggests that inhibition of tonic pain is mediated by activation of mesolimbic dopamine neurons, arising from the cell bodies of the ventral tegmental area and projecting to the nucleus accumbens. This pain-suppression system is activated by acute stress, via the release of endogenous opioids and substance P within the ventral tegmental area. However, prolonged exposure to unavoidable stress produces both reduction of dopamine output in the nucleus accumbens and development of persistent hyperalgesia. It is proposed that a stress-related reduction of dopaminergic tone within the nucleus accumbens contributes to the development of hyperalgesia in the context of chronic stress and thus plays a role in the pathogenesis of fibromyalgia. A stress-related dysfunction of mesolimbic dopaminergic activity might serve as the basis for other fibromyalgia-associated phenomena as well.


 

Melatonin and dopamine as biomarkers to optimize treatment in phenylketonuria: effects of tryptophan and tyrosine supplementation.
            (Yano et al., 2014) Download
OBJECTIVE:  To determine whether additional supplementation of tryptophan (Trp) and tyrosine (Tyr) improve serotonin and dopamine metabolism in individuals with phenylketonuria treated with large neutral amino acid (LNAA) tablets. STUDY DESIGN:  Ten adult individuals with phenylketonuria participated in a randomized, double-blind, placebo-controlled cross-over study consisting of three 3-week phases: washout, treatment with LNAA tablets plus supplementation with either Trp and Tyr tablets or placebo, and LNAA tablets plus the alternate supplementation. An overnight protocol to measure blood melatonin, a serotonin metabolite in the pinealocytes, and urine 6-sulfatoxymelatonin and dopamine in first-void urine specimens was conducted after each phase. RESULTS:  Serum melatonin and urine 6-sulfatoxymelatonin and dopamine levels were increased in the LNAA phase (LNAA plus placebo) compared with the washout phase. Serum melatonin and urine 6-sulfatoxymelatonin were not increased in the active phase (LNAA plus Trp + Tyr) compared with the LNAA phase, although plasma Trp:LNAA was increased compared with the LNAA phase. Among 7 subjects with a plasma Trp/LNAA >0.03, a negative correlation between urine 6-sulfatoxymelatonin and plasma phenylalanine levels was observed (r = -0.072). Urine dopamine levels and plasma Tyr:LNAA were increased in the active phase compared with the LNAA phase. CONCLUSION:  Melatonin levels were not increased with the higher dose of Trp supplementation, but dopamine levels were increased with the higher dose of Tyr supplementation. Serotonin synthesis appears to be suppressed by high phenylalanine levels at the Trp hydroxylase level.

 


References

Barrett, SP, et al. (2008), ‘The role of dopamine in alcohol self-administration in humans: individual differences.’, Eur Neuropsychopharmacol, 18 (6), 439-47. PubMed: 18367384
Gijsman, HJ, et al. (2002), ‘A dose-finding study on the effects of branch chain amino acids on surrogate markers of brain dopamine function.’, Psychopharmacology (Berl), 160 (2), 192-97. PubMed: 11875637
Graven-Nielsen, T, et al. (2000), ‘Ketamine reduces muscle pain, temporal summation, and referred pain in fibromyalgia patients.’, Pain, 85 (3), 483-91. PubMed: 10781923
Harmer, CJ, et al. (2001), ‘Tyrosine depletion attenuates dopamine function in healthy volunteers.’, Psychopharmacology (Berl), 154 (1), 105-11. PubMed: 11291999
Holman, AJ and RR Myers (2005), ‘A randomized, double-blind, placebo-controlled trial of pramipexole, a dopamine agonist, in patients with fibromyalgia receiving concomitant medications.’, Arthritis Rheum, 52 (8), 2495-505. PubMed: 16052595
Legangneux, E, et al. (2001), ‘Cerebrospinal fluid biogenic amine metabolites, plasma-rich platelet serotonin and [3H]imipramine reuptake in the primary fibromyalgia syndrome.’, Rheumatology (Oxford), 40 (3), 290-96. PubMed: 11285376
Nagano-Saito, A, et al. (2008), ‘Dopamine depletion impairs frontostriatal functional connectivity during a set-shifting task.’, J Neurosci, 28 (14), 3697-706. PubMed: 18385328
Neuhaus, AH, et al. (2009), ‘Acute dopamine depletion with branched chain amino acids decreases auditory top-down event-related potentials in healthy subjects.’, Schizophr Res, 111 (1-3), 167-73. PubMed: 19356906
Pani, L, A Porcella, and GL Gessa (2000), ‘The role of stress in the pathophysiology of the dopaminergic system.’, Mol Psychiatry, 5 (1), 14-21. PubMed: 10673764
Russell, IJ, et al. (1992), ‘Cerebrospinal fluid biogenic amine metabolites in fibromyalgia/fibrositis syndrome and rheumatoid arthritis.’, Arthritis Rheum, 35 (5), 550-56. PubMed: 1374252
Wood, PB (2004), ‘Stress and dopamine: implications for the pathophysiology of chronic widespread pain.’, Med Hypotheses, 62 (3), 420-24. PubMed: 14975515
Yano, S, K Moseley, and C Azen (2014), ‘Melatonin and dopamine as biomarkers to optimize treatment in phenylketonuria: effects of tryptophan and tyrosine supplementation.’, J Pediatr, 165 (1), 184-189.e1. PubMed: 24857519