Showing papers on "Catecholamine published in 2002"

Intricate Regulation of Tyrosine Hydroxylase Activity and Gene Expression

Abstract: Tyrosine hydroxylase catalyzes the rate-limiting step in the biosynthesis of the catecholamines dopamine, norepinephrine, and epinephrine. Therefore, the regulation of tyrosine hydroxylase enzyme number and intrinsic enzyme activity represents the central means for controlling the synthesis of these important biogenic amines. An intricate scheme has evolved whereby tyrosine hydroxylase activity is modulated by nearly every documented form of regulation. Beginning with the genomic DNA, evidence exists for the transcriptional regulation of tyrosine hydroxylase mRNA levels, alternative RNA processing, and the regulation of RNA stability. There is also experimental support for the role of both translational control and enzyme stability in establishing steady-state levels of active tyrosine hydroxylase protein. Finally, mechanisms have been proposed for feedback inhibition of the enzyme by catecholamine products, allosteric modulation of enzyme activity, and phosphorylation-dependent activation of the enzyme by various different kinase systems. Given the growing literature suggesting that different tissues regulate tyrosine hydroxylase mRNA levels and activity in different ways, regulatory mechanisms provide not only redundancy but also diversity in the control of catecholamine biosynthesis.

Effects of methylphenidate on extracellular dopamine, serotonin, and norepinephrine: comparison with amphetamine.

Abstract: Methylphenidate promotes a dose-dependent behavioral profile that is very comparable to that of amphetamine. Amphetamine increases extracellular norepinephrine and serotonin, in addition to its effects on dopamine, and these latter effects may play a role in the behavioral effects of amphetamine-like stimulants. To examine further the relative roles of dopamine, norepinephrine, and serotonin in the behavioral response to amphetamine-like stimulants, we assessed extracellular dopamine and serotonin in caudate putamen and norepinephrine in hippocampus in response to various doses of methylphenidate (10, 20, and 30 mg/kg) that produce stereotyped behaviors, and compared the results with those of a dose of amphetamine (2.5 mg/kg) that produces a level of stereotypies comparable to the intermediate dose of methylphenidate. The methylphenidate-induced changes in dopamine and its metabolites were consistent with changes induced by other uptake blockers, and the magnitude of the dopamine response for a behaviorally comparable dose was considerably less than that with amphetamine. Likewise, the dose-dependent increase in norepinephrine in response to methylphenidate was also significantly less than that with amphetamine. However, in contrast to amphetamine, methylphenidate had no effect on extracellular serotonin. These results do not support the hypothesis that a stimulant-induced increase in serotonin is necessary for the appearance of stereotyped behaviors.

Glucagon-like peptide-1 receptor stimulation increases blood pressure and heart rate and activates autonomic regulatory neurons

Abstract: Glucagon-like peptide-1 (GLP-1) released from the gut functions as an incretin that stimulates insulin secretion. GLP-1 is also a brain neuropeptide that controls feeding and drinking behavior and gastric emptying and elicits neuroendocrine responses including development of conditioned taste aversion. Although GLP-1 receptor (GLP-1R) agonists are under development for the treatment of diabetes, GLP-1 administration may increase blood pressure and heart rate in vivo. We report here that centrally and peripherally administered GLP-1R agonists dose-dependently increased blood pressure and heart rate. GLP-1R activation induced c-fos expression in the adrenal medulla and neurons in autonomic control sites in the rat brain, including medullary catecholamine neurons providing input to sympathetic preganglionic neurons. Furthermore, GLP-1R agonists rapidly activated tyrosine hydroxylase transcription in brainstem catecholamine neurons. These findings suggest that the central GLP-1 system represents a regulator of sympathetic outflow leading to downstream activation of cardiovascular responses in vivo.

Dopamine Neurotoxicity: Inhibition of Mitochondrial Respiration

Abstract: Dopamine, due to metabolism by monoamine oxidase or autoxidation, can generate toxic products such as hydrogen peroxide, oxygen-derived radicals, semiquinones, and quinones and thus exert its neurotoxic effects. Intracerebroventricular injection of dopamine into rats pretreated with the monoamine oxidase nonselective inhibitor pargyline caused mortality in a dose-dependent manner with LD50 = 90 micrograms. Norepinephrine was less effective with LD50 = 141 micrograms. The iron chelator desferrioxamine completely protected against dopamine-induced mortality. In the absence of pargyline more rats survived, indicating that the products of dopamine enzymatic metabolism are not the main contributors to dopamine-induced toxicity. Biochemical analysis of frontal cortex and striatum from rats that received a lethal dose of dopamine did not show any difference from control rats in lipid and protein peroxidation and glutathione reductase and peroxidase activities. Moreover, dopamine significantly reduced the formation of iron-induced malondialdehyde in vitro, thus suggesting that earlier events in cell damage are involved in dopamine toxicity. Indeed, dopamine inhibited mitochondrial NADH dehydrogenase activity with IC50 = 8 microM, and that of norepinephrine was twice as much (IC50 = 15 microM). Dopamine-induced inhibition of NADH dehydrogenase activity was only partially reversed by desferrioxamine, which had no effect on norepinephrine-induced inhibition. These results suggest that catecholamines can cause toxicity not only by inducing an oxidative stress state but also possibly through direct interaction with the mitochondrial electron transport system. The latter was further supported by the ability of ADP to reverse dopamine-induced inhibition of NADH dehydrogenase activity in a dose-dependent manner.

Carrier-mediated release of serotonin by 3,4-methylenedioxymethamphetamine: implications for serotonin-dopamine interactions.

Abstract: In vivo microdialysis was used to determine whether the 3,4-methylenedioxymethamphetamine (MDMA)-induced release of serotonin (5-HT) in vivo involves a carrier-mediated process and to investigate further the state-dependent interaction between 5-HT and dopamine. MDMA produced a dose-dependent increase in the extracellular concentration of 5-HT in the striatum and prefrontal cortex that was attenuated by treatment with fluoxetine but not by tetrodotoxin. Suppression by fluoxetine of the MDMA-induced release of 5-HT was accompanied by a suppression of the MDMA-induced release of dopamine. Administration of MDMA to rats treated with carbidopa and L-5-hydroxytryptophan resulted in a synergistic elevation of the extracellular concentration of 5-HT that was much greater than that produced by either treatment alone. The MDMA-induced release of dopamine by MDMA also was potentiated in 5-hydroxytryptophan-treated rats. These data are consistent with the view that MDMA increases the extracellular concentration of 5-HT by facilitating carrier-mediated 5-HT release, which can be enhanced greatly under conditions in which 5-HT synthesis is stimulated. Moreover, these data are supportive of a state-dependent, stimulatory role of 5-HT in the regulation of dopamine release.

Pituitary adenylate cyclase-activating polypeptide is a sympathoadrenal neurotransmitter involved in catecholamine regulation and glucohomeostasis.

Abstract: The adrenal gland is important for homeostatic responses to metabolic stress: hypoglycemia stimulates the splanchnic nerve, epinephrine is released from adrenomedullary chromaffin cells, and compensatory glucogenesis ensues. Acetylcholine is the primary neurotransmitter mediating catecholamine secretion from the adrenal medulla. Accumulating evidence suggests that a secretin-related neuropeptide also may function as a transmitter at the adrenomedullary synapse. Costaining with highly specific antibodies against the secretin-related neuropeptide pituitary adenylate cyclase-activating peptide (PACAP) and the vesicular acetylcholine transporter (VAChT) revealed that PACAP is found in nerve terminals at all mouse adrenomedullary cholinergic synapses. Mice with a targeted deletion of the PACAP gene had otherwise normal cholinergic innervation and morphology of the adrenal medulla, normal adrenal catecholamine and blood glucose levels, and an intact initial catecholamine secretory response to insulin-induced hypoglycemia. However, insulin-induced hypoglycemia was more profound and longer-lasting in PACAP knock-outs, and was associated with a dose-related lethality absent in wild-type mice. Failure of PACAP-deficient mice to adequately counterregulate plasma glucose levels could be accounted for by impaired long-term secretion of epinephrine, secondary to a lack of induction of tyrosine hydroxylase, normally occurring after insulin hypoglycemia in wild-type mice, and a consequent depletion of adrenomedullary epinephrine stores. Thus, PACAP is needed to couple epinephrine biosynthesis to secretion during metabolic stress. PACAP appears to function as an "emergency response" cotransmitter in the sympathoadrenal axis, where the primary secretory response is controlled by a classical neurotransmitter but sustained under paraphysiological conditions by a neuropeptide.

Regulation of extracellular dopamine by the norepinephrine transporter

Abstract: There is growing evidence of an interaction between dopamine and norepinephrine. To test the hypothesis that norepinephrine terminals are involved in the uptake and removal of dopamine from the extracellular space, the norepinephrine uptake blocker desmethylimipramine (DMI) was infused locally while the extracellular concentrations of dopamine were simultaneously monitored. DMI increased the extracellular concentrations of dopamine in the medial prefrontal cortex and nucleus accumbens shell but had no effect in the striatum. The combined systemic administration of haloperidol and the local infusion of DMI produced an augmented increase in extracellular dopamine in the cortex compared with the increase produced by either drug alone. This synergistic increase in dopamine overflow is likely due to the combination of impulse-mediated dopamine release produced by haloperidol and blockade of the norepinephrine transporter. No such synergistic effects were observed in the nucleus accumbens and striatum. Local perfusion of the alpha2-antagonist idazoxan also increased the extracellular concentrations of dopamine in the cortex. Although the stimulation of extracellular dopamine by idazoxan and DMI could be due to the increased extracellular concentrations of norepinephrine produced by these drugs, an increase in dopamine also was observed in lesioned rats that were depleted of norepinephrine and challenged with haloperidol. This contrasted with the lack of an effect of haloperidol on cortical dopamine in unlesioned controls. These results suggest that norepinephrine terminals regulate extracellular dopamine concentrations in the medial prefrontal cortex and to a lesser extent in the nucleus accumbens shell through the uptake of dopamine by the norepinephrine transporter.

Altered responsiveness of the hypothalamus-pituitary-adrenal axis and the sympathetic adrenomedullary system to stress in patients with atopic dermatitis.

Abstract: A growing number of animal data strongly suggest that a hyporeactive hypothalamus-pituitary adrenal (HPA) axis may be pathologically significant by increasing the susceptibility to chronic inflammation. Following this line of evidence, the specific goal of the present study was to investigate the HPA axis in patients with atopic dermatitis (AD), a chronic allergic inflammatory disease. In addition, the sympathetic adrenomedullary (SAM) system as a second potent immunoregulatory and anti-inflammatory stress-response system has been examined. AD patients (n = 36) and nonatopic control subjects (n = 37) were exposed to a standardized laboratory stressor consisting of a free speech and mental arithmetic task in front of an audience. Cortisol, ACTH, and catecholamine concentrations were assessed before and after the stressor. To investigate feedback sensitivity of the HPA axis, a low dose (0.5 mg) dexamethasone suppression test was also performed. AD patients showed significantly attenuated cortisol and ACTH responses to the stressor, whereas catecholamine levels were significantly elevated in atopic patients. No difference between the experimental groups was found in basal cortisol and ACTH concentrations, whereas basal catecholamine levels were significantly elevated. Analysis of cortisol levels after dexamethasone treatment suggested an intact feedback sensitivity in AD sufferers at the pituitary level. The present findings suggest that patients with AD demonstrate a blunted HPA axis responsiveness with a concurrent overreactivity of the SAM system to psychosocial stress. Considering the important immunoregulatory role of the HPA axis and the SAM system, especially under stressful conditions, an aberrant responsiveness of these neuroendocrine systems may increase the susceptibility to (allergic) inflammation and may be one psychobiological mechanism of stress-related aggravation of the disease.

Growth stimulation of intestinal commensal Escherichia coli by catecholamines: a possible contributory factor in trauma-induced sepsis.

Abstract: Trauma is well recognized to result in the immediate and sustained release of stress-related neurochemicals such as the catecholamine norepinephrine. Past work has shown that in addition to their ability to function as neurotransmitters, catecholamines can also directly stimulate the growth of a number of pathogenic bacteria. The development of trauma-associated sepsis has often been linked to the ability of otherwise normal commensal bacteria to invade and penetrate the gut mucosal barrier. Therefore, the aim of our study was to examine whether catecholamines could also stimulate the growth of commensal Escherichia coli strains of the type present in the intestinal tract at the time of a traumatic event. Herein we report that the growth of a range of non-pathogenic isolates of E. coli of human and environmental origin was significantly increased in the presence of catecholamines. A primary mechanism by which catecholamines increase bacterial growth was shown to be iron removal from lactoferrin and transferrin and subsequent acquisition by bacteria. The 3,4-dihydroxybenzoyl (catechol) structure of the catecholamines was further demonstrated to be critical to iron acquisition. The synthetic catecholamine inotropes dobutamine and isoprenaline, as well as norepinephrine metabolites that retained the catechol structure were also active, whereas norepinephrine metabolites in which the catechol moiety had been modified were not. A role for catecholamine-mediated bacterial iron supply in the pathophysiology of gut-derived sepsis due to trauma is proposed.

Brain catecholamine metabolism in catechol-O-methyltransferase (COMT)-deficient mice.

Abstract: Catechol-O-methyltransferase (COMT) catalyses the O-methylation of compounds having a catechol structure and its main function involves the elimination of biologically active or toxic catechols and their metabolites. By means of homologous recombination in embryonic stem cells, a strain of mice has been produced in which the gene encoding the COMT enzyme is disrupted. We report here the levels of catecholamines and their metabolites in striatal extracellular fluid in these mice as well as in homogenates from different parts of the brain, under normal conditions and after acute levodopa administration. In immunoblotting studies, COMT-knockout mice had no COMT protein in brain or kidney tissues but the amounts of catecholamine synthesizing and other metabolizing enzyme proteins were normal. Under normal conditions, COMT deficiency does not appear to affect significantly brain dopamine and noradrenaline levels in spite of relevant changes in their metabolites. This finding is consistent with previous pharmacological studies with COMT inhibitors and confirms the pivotal role of synaptic reuptake processes and monoamine oxidase-dependent metabolism in terminating the actions of catecholamines at nerve terminals. In contrast, when COMT-deficient mice are challenged with l-dihydroxyphenylalanine, they show an extensive accumulation of 3,4-dihydroxyphenylacetic acid and dihydroxyphenylglycol and even dopamine, revealing an important role for COMT under such situations. Notably, in some cases these changes appear to be Comt gene dosage-dependent, brain-region specific and sexually dimorphic. Our results may have implications for improving the treatment of Parkinson's disease and for understanding the contribution of the natural variation in COMT activity to psychiatric phenotypes.

Real-time measurement of electrically evoked extracellular dopamine in the striatum of freely moving rats.

Abstract: The real-time measurement of electrically evoked dopamine was established in brain extracellular fluid of freely moving rats. Dopamine was monitored by fast-scan cyclic voltammetry at carbon fiber microelectrodes lowered into the striatum by means of a detachable micromanipulator. A stimulating electrode, previously implanted in the substantia nigra, was used to evoke striatal dopamine efflux. Evoked extracellular dopamine was both current and frequency dependent. When low current intensities (+/-125 microA) and frequencies (10-20 Hz) were applied, detectable levels of dopamine were elicited without a perceptible behavioral response. Reproducible concentrations of extracellular dopamine could be evoked in the same rat for at least 2 months. These concentrations, moreover, were significantly higher in freely moving rats compared with rats anesthetized with Equithesin. Analysis of measured curves for dopamine uptake and release rates revealed that anesthesia inhibits release but does not affect uptake. It is concluded that (a) fast-scan cyclic voltammetry at carbon fiber microelectrodes is a viable technique for the measurement of electrically evoked dopamine in brain extracellular fluid of freely moving rats, (b) it is possible to determine in situ rate constants for dopamine release and uptake from these temporally and spatially resolved measurements of levels of dopamine, and (c) transient changes in extracellular dopamine levels elicited by electrical stimulation are affected by anesthesia.

Modification of dopamine transporter function : Effect of reactive oxygen species and dopamine

Abstract: Dopamine can oxidize to form reactive oxygen species and quinones, and we have previously shown that dopamine quinones bind covalently to cysteinyl residues on striatal proteins. The dopamine transporter is one of the proteins at risk for this modification, because it has a high affinity for dopamine and contains several cysteinyl residues. Therefore, we tested whether dopamine transport in rat striatal synaptosomes could be affected by generators of reactive oxygen species, including dopamine. Uptake of [3H]dopamine (250 nM) was inhibited by ascorbate (0.85 mM; -44%), and this inhibition was prevented by the iron chelator diethylenetriaminepentaacetic acid (1 mM), suggesting that ascorbate was acting as a prooxidant in the presence of iron. Preincubation with xanthine (500 microM) and xanthine oxidase (50 mU/ml) also reduced [3H]dopamine uptake (-76%). Preincubation with dopamine (100 microM) caused a 60% inhibition of subsequent [3H]dopamine uptake. This dopamine-induced inhibition was attenuated by diethylenetriaminepentaacetic acid (1 mM), which can prevent iron-catalyzed oxidation of dopamine during the preincubation, but was unaffected by the monoamine oxidase inhibitor pargyline (10 microM). None of these incubations caused a loss of membrane integrity as indicated by lactate dehydrogenase release. These findings suggest that reactive oxygen species and possibly dopamine quinones can modify dopamine transport function.

Feedback inhibition of catecholamine release by two different alpha2-adrenoceptor subtypes prevents progression of heart failure.

Abstract: Background— Elevated plasma norepinephrine levels are associated with increased mortality in patients and in animal models with chronic heart failure. To test which α2-adrenoceptor subtypes operate as presynaptic inhibitory receptors to control norepinephrine release in heart failure, we investigated the response of gene-targeted mice lacking α2-adrenoceptor subtypes (α2-KO) to chronic left ventricular pressure overload. In addition, we determined the functional consequences of genetic variants of α2-adrenoceptors in human patients with chronic heart failure. Methods and Results— Cardiac pressure overload was induced by transverse aortic constriction. Three months after aortic banding, survival was dramatically reduced in α2A-KO (52%) and α2C-KO (47%) mice compared with wild-type and α2B-deficient (86%) animals. Excess mortality in α2A- and α2C-KO strains was attributable to heart failure with enhanced left ventricular hypertrophy and fibrosis and elevated circulating catecholamines. The clinical importan...

Toxicity of dopamine to striatal neurons in vitro and potentiation of cell death by a mitochondrial inhibitor.

Abstract: Intrastriatal injections of the mitochondrial toxins malonate and 3-nitropropionic acid produce selective cell death similar to that seen in transient ischemia and Huntington's disease. The extent of cell death can be attenuated by pharmacological or surgical blockade of cortical glutamatergic input. It is not known, however, if dopamine contributes to toxicity caused by inhibition of mitochondrial function. Exposure of primary striatal cultures to dopamine resulted in dose-dependent death of neurons. Addition of medium supplement containing free radical scavengers and antioxidants decreased neuronal loss. At high concentrations of the amine, cell death was predominantly apoptotic. Methyl malonate was used to inhibit activity of the mitochondrial respiratory chain. Neither methyl malonate (50 microM) nor dopamine (2.5 microM) caused significant toxicity when added individually to cultures, whereas simultaneous addition of both compounds killed 60% of neurons. Addition of antioxidants and free radical scavengers to the incubation medium prevented this cell death. Dopamine (up to 250 microM) did not alter the ATP/ADP ratio after a 6-h incubation. Methyl malonate, at 500 microM, reduced the ATP/ADP ratio by approximately 30% after 6 h; this decrease was not augmented by coincubation with 25 microM dopamine. Our results suggest that dopamine causes primarily apoptotic death of striatal neurons in culture without damaging cells by an early adverse action on oxidative phosphorylation. However, when combined with minimal inhibition of mitochondrial function, dopamine neurotoxicity is markedly enhanced.

Restoration of Norepinephrine and Reversal of Phenotypes in Mice Lacking Dopamine β‐Hydroxylase

Abstract: Mice with a targeted disruption of the dopamine beta-hydroxylase (DBH) gene are unable to synthesize norepinephrine (NE) and epinephrine. These mice have elevated levels of dopamine in most tissues, although the levels are only a fraction of those normally found for NE. It is noteworthy that NE can be restored to normal levels in many tissues after a single injection of the synthetic amino acid precursor of NE, L-threo-3,4-dihydroxyphenylserine (DOPS). In other tissues, NE can be restored to normal levels after multiple injections of DOPS, whereas in the midbrain and cerebellum, restoration of NE is limited to 25-30% of normal. NE levels typically peak approximately 5 h after DOPS administration and are undetectable by 48 h. Epinephrine levels are more difficult to restore. The elevated levels of dopamine fall modestly after injection of DOPS. S(-)-Carbidopa, which does not cross the blood-brain barrier, inhibits aromatic L-amino acid decarboxylase and effectively prevents restoration of NE by DOPS in the periphery, while allowing restoration in the CNS. Ptosis and reductions in male fertility, hind-limb extension, postdecapitation convulsions, and uncoupling protein expression in dopamine beta-hydroxylase-deficient mice are all reversed by DOPS injection.

Circadian rhythms of dopamine in mouse retina: the role of melatonin.

Abstract: Both dopamine and melatonin are important for the regulation of retinal rhythmicity, and substantial evidence suggests that these two substances are mutually inhibitory factors that act as chemical analogs of day and night. A circadian oscillator in the mammalian retina regulates melatonin synthesis. Here we show a circadian rhythm of retinal dopamine content in the mouse retina, and examine the role of melatonin in its control. Using high-performance liquid chromatography (HPLC), we measured levels of dopamine and its two major metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in retinas of C3H+/+ mice (which make melatonin) and C57BL/6J mice that are genetically incapable of melatonin synthesis. In a light/dark cycle both strains of mice exhibited daily rhythms of retinal dopamine, DOPAC, and HVA content. However, after 10 days in constant darkness (DD), a circadian rhythm in dopamine levels was present in C3H, but not in C57 mice. C57 mice given ten daily injections of melatonin in DD exhibited a robust circadian rhythm of retinal dopamine content whereas no such rhythm was present in saline-injected controls. Our results demonstrate that (1) a circadian clock generates rhythms of dopamine content in the C3H mouse retina, (2) mice lacking melatonin also lack circadian rhythms of dopamine content, and (3) dopamine rhythms can be generated in these mice by cyclic administration of exogenous melatonin. Our results also indicate that circadian rhythms of retinal dopamine depend upon the rhythmic presence of melatonin, but that cyclic light can drive dopamine rhythms in the absence of melatonin.

Biochemistry of Somatodendritic Dopamine Release in Substantia Nigra: An In Vivo Comparison with Striatal Dopamine Release

Abstract: The somatodendritic release of dopamine in substantia nigra previously has been suggested to be nonvesicular in nature and thus to differ from the classical, exocytotic release of dopamine described for the dopaminergic nerve terminal in striatum. We have compared the effects of reserpine, a compound that disrupts vesicular sequestration of monoamines, on the storage and release of dopamine in substantia nigra and striatum of rats. Reserpine administration (5 mg/kg, i.p.) significantly decreased the tissue level of dopamine in substantia nigra pars reticulata, substantia nigra pars compacta, and striatum. In these brain areas, reserpine-induced reductions in tissue dopamine level occurred within 2 h and persisted at 24 h postdrug. In vivo measurements using microdialysis revealed that reserpine administration rapidly decreased the extracellular dopamine concentration to nondetectable levels in substantia nigra as well as in striatum. In both structures, it was observed that reserpine treatment significantly attenuated the release of dopamine evoked by a high dose of amphetamine (10 mg/kg, i.p.) given 2 h later. In contrast, dopamine efflux in response to a low dose of amphetamine (2 mg/kg, i.p.) was not altered by reserpine pretreatment either in substantia nigra or in striatum. The present data suggest the existence, both at the somatodendritic and at the nerve terminal level, of a vesicular pool of dopamine that is the primary site of transmitter storage and that can be displaced by high but not low doses of amphetamine. The physiological release of dopamine in substantia nigra and in striatum is dependent on the integrity of this vesicular store.

Prostaglandin H synthetase-mediated metabolism of dopamine: Implication for Parkinson's disease

Abstract: Differences in prostaglandin H synthetase (PHS) activity in the substantia nigra of age- and postmortem interval-matched parkinsonian, Alzheimer's, and normal control brain tissue were assessed. Prostaglandin E 2 (PGE 2 , an index of PHS activity) was higher in substantia nigra of parkinsonian brain tissue than Alzheimer's or control tissue. Incubation of substantia nigra slices with arachidonic acid (AA) increased PGE 2 synthesis. Dopamine stimulated PHS synthesis of PGE 2 . [ 3 H] Dopamine was activated by PHS to electrophilic intermediate(s) that covalently bound to DNA, microtubulin protein, bovine serum albumin, and sulfhydryl reagents. When AA was replaced by hydrogen peroxide, PHS/H 2 O 2 -supported binding proceeded at rates similar to those observed with PHS/AA. Indomethacin and aspirin inhibited AA-mediated cooxidation of dopamine but not H 2 O 2 -mediated metabolism. PHS-mediated metabolism of dopamine was not affected by monoamine oxidase inhibitors. Substrate requirements and effects of specific inhibitors suggest cooxidation of dopamine is mediated by the hydroperoxidase activity of PHS. 32 P-postlabeling was used to detect dopamine-DNA adducts. PHS/AA activation of dopamine in the presence of DNA resulted in the formation of five dopamine-DNA adducts, i.e., 23, 43, 114, 70, and 270 amol/μg DNA. DNA adduct formation was PHS, AA, and dopamine dependent. PHS catalyzed cooxidation of dopamine in dopaminergic neuronal degeneration is discussed

Dopamine antagonists alter response allocation but do not suppress appetite for food in rats: contrast between the effects of SKF 83566, raclopride, and fenfluramine on a concurrent choice task

Abstract: Rationale: Dopamine is important for enabling organisms to overcome work-related response costs. One way of investigating this function has been with concurrent choice procedures using food reinforcement. In the present study, rats were given a choice between pressing a lever for preferred Bioserve pellets, or approaching and consuming a less-preferred laboratory chow that was concurrently available. In previous work with this task, dopamine antagonists and accumbens dopamine depletions decreased lever pressing but increased chow consumption. Objective: The present study assessed three drugs (two dopamine antagonists and one appetite suppressant) using the lever pressing/chow feeding task. Results: Under baseline conditions, rats pressed the lever at high rates (1,300–1,500 responses) to obtain the preferred food, and little of the laboratory chow was eaten (1–2 g). Selective D1 and D2 antagonists (SKF 83566 and raclopride) reduced fixed ratio 5 lever pressing, but substantially increased chow consumption. In contrast, the serotonergic appetite suppressant fenfluramine reduced both lever pressing and chow consumption. With the dopamine antagonists, lever pressing and chow consumption were inversely correlated across treatments, while these two measures were unrelated in the fenfluramine experiment. Conclusions: Dopamine antagonists and accumbens dopamine depletions do not simply reduce appetite. Rats with accumbens dopamine depletions, or rats treated with low doses of selective or non-selective dopamine antagonists, remain directed toward the acquisition and consumption of food. These results demonstrate that fundamental aspects of food reinforcement are left intact after treatment with low doses of dopamine antagonists.

An investigation of monoamine receptors involved in antinociceptive effects of antidepressants.

Abstract: We attempted to determine which monoamine re-ceptor subtypes are predominantly involved in antidepressant-induced antinociception Antinociceptive effects were evaluated by using formalin tests with rats Antidepressants acting as potent inhibitors of norepinephrine reuptake (nisoxetine, nortriptyli

Norepinephrine Release in the Rat Pineal Gland: The Input from the Biological Clock Measured by In Vivo Microdialysis

Abstract: The sympathetic innervation of the rat pineal gland was investigated, measuring the norepinephrine (NE) release by on-line in vivo microdialysis. NE was assayed using an HPLC method with precolumn derivatization and fluorescence detection. Its high sensitivity and reliability made it very suitable to monitor the low levels of NE in the dialysates (12.5 fmol during nighttime, 3 fmol during daytime). To increase NE levels, the monoamine reuptake inhibitor cocaine was added to Ringer's solution at concentrations of 10(-6) and 10(-5) M. This resulted in increases of neurotransmitter output of 167 and 219%, respectively, but did not change the qualitative and/or quantitative outcome of other experiments. Perfusion with 10(-6) M tetrodotoxin for 1 h resulted in a decrease of the NE release by >80%, whereas perfusion with the alpha 2-receptor antagonist yohimbine caused a twofold increase. These results indicate that the NE release in the rat pineal was of neuronal origin and regulated by a negative feedback mechanism involving inhibitory presynaptic alpha 2-receptors. Long-term (i.e., 16 h) measurements are described, showing the circadian properties of NE release. A pronounced rhythm is reported, showing extremely sharp transitions between low daytime and high nighttime values. Increases and decreases are reported to occur within the duration of collecting one sample (20 min). For comparison, the rhythm of melatonin release was also recorded. The on and off switches of the sympathetic input correlated well with the circadian rhythm of melatonin release and can thus be considered as the primary clock signal, inducing the nightly production of melatonin.

l‐3,4‐Dihydroxyphenylalanine Increases the Quantal Size of Exocytotic Dopamine Release In Vitro

Abstract: The catecholamine precursor L-3,4-dihydroxyphenylalanine (L-DOPA) is used to augment striatal dopamine (DA), although its mechanism of altering neurotransmission is not well understood. We observed the effects of L-DOPA on catecholamine release in ventral midbrain neuron and PC12 pheochromocytoma cell line cultures. In ventral midbrain neuron cultures exposed to 40 mM potassium-containing media, L-DOPA (100 microM for 1 h) increased DA release by > 10-fold. The elevated extracellular DA levels were not significantly blocked by the DA/norepinephrine transport inhibitor nomifensine, demonstrating that reverse transport through catecholamine-uptake carriers plays little role in this release. In PC12 cells, where DA release from individual secretory vesicles can be observed, L-DOPA (50 microM for 1 h) elevated DA release in high-potassium media by 370%. Amperometric measurements demonstrated that L-DOPA (50 microM for 40-70 min) did not raise the frequency of vesicular exocytosis but increased the average size of quantal release to at least 250% of control levels. Together, these findings suggest that L-DOPA can increase stimulation-dependent transmitter release from DA cells by augmenting cytosolic neurotransmitter, leading to increased quantal size.

Epinephrine and norepinephrine act as potent agonists at the recombinant human dopamine D4 receptor.

Abstract: The catecholamines dopamine (DA), epinephrine (EP), and norepinephrine (NE) play important roles in learning and memory, emotional states, and control of voluntary movement, as well as cardiovascular and kidney function. They activate distinct but overlapping neuronal pathways through five distinct DA receptors (D1R-D5R) and at least 10 different adrenergic receptors (alpha 1a/b/c, alpha 2a/b/c-1/c-2, and beta 1/beta 2/beta 3). The D4R, which is localized to mesolimbic areas of the brain implicated in affective and emotional behavior, has a deduced amino acid sequence with homology to both adrenergic and dopaminergic receptor subtypes. We report here that DA, EP, and NE all show binding in the nanomolar range to three isoforms of the recombinant human D4R (hD4R): D4.2, D4.4, and D4.7. Submicromolar concentrations of DA, EP, and NE were sufficient to activate hD4R isoforms in two different functional assays: agonist-induced guanosine 5'-O-(3-[35S]thiotriphosphate) binding and modulation of adenylyl cyclase activity. DA was approximately fivefold more potent than EP and NE at the D4R, whereas activation of the human D2R required at least 100-fold higher catecholamine concentrations. Functional activation of the D4R by multiple neurotransmitters may provide a novel mechanism for integration of catecholamine signaling in the brain and periphery.

Estrogen Alters MPTP‐Induced Neurotoxicity in Female Mice: Effects on Striatal Dopamine Concentrations and Release

Abstract: The effects of estrogen on MPTP-induced neurotoxicity of the nigrostriatal dopaminergic system were examined in C57Bl and CD-1 mice. Ovariectomized mice with and without estrogen were treated with MPTP or its vehicle. The effects of these treatments on striatal dopamine concentrations and L-DOPA-stimulated dopamine and L-3,4-dihydroxyphenylacetic acid (DOPAC) release in vitro were determined. Dopamine concentrations of C57Bl mice receiving estrogen before MPTP were significantly greater than those of non-estrogen-treated MPTP mice as well as estrogen-treated mice receiving the MPTP vehicle. Dopamine concentrations of the CD-1 mice did not differ with these treatments. L-DOPA-evoked dopamine release values of estrogen-treated C57Bl mice were significantly increased compared with non-estrogen-treated mice. No such differences were observed in the MPTP-treated C57Bl mice. DOPAC release rates were similar to that of dopamine in these C57Bl mice. In the CD-1 mice estrogen also produced a significant increase in L-DOPA-evoked dopamine release; however, this response was unaltered by MPTP treatment. A significant increase in L-DOPA-evoked DOPAC output was obtained only for estrogen-treated CD-1 mice. Both strains show very similar responses to the estrogen treatment, but differential responses of dopamine release to L-DOPA between the C57Bl and CD-1 mice were obtained with regard to the interactive effects of estrogen and MPTP. Our results suggest that in addition to its role as modulator, estrogen may also function as a neuroprotectant against MPTP neurotoxicity of the nigrostriatal dopaminergic system in the C57Bl mouse.

l‐3,4‐Dihydroxyphenylalanine‐Induced Dopamine Release in the Striatum of Intact and 6‐Hydroxydopamine‐Treated Rats: Differential Effects of Monoamine Oxidase A and B Inhibitors

Abstract: Administration of l-DOPA (50 mg/kg) elicits a significant increase in extracellular dopamine in striata of rats treated with the catecholaminergic neurotoxin 6-hydroxydopamine but not in striata of intact rats. To assess the role of dopaminergic nerve terminals in determining the effects of exogenous l-DOPA on extracellular dopamine levels in striatum, we examined the relative contributions of monoamine oxidase A and monoamine oxidase B to the catabolism of dopamine synthesized from exogenous l-DOPA. Extracellular concentrations of dopamine and its catabolite, 3,4-dihydroxyphenylacetic acid, were monitored with in vivo dialysis in striata of intact rats and of rats with unilateral 6-hydroxydopamine lesions of striatal dopamine. Clorgyline (2 mg/kg), an inhibitor of monoamine oxidase A, significantly increased dopamine and decreased 3,4-dihydroxyphenylacetic acid in intact but not in dopamine-depleted striata. Inhibition of monoamine oxidase B with either l-deprenyl (1 mg/kg) or Ro 19-6327 (1 mg/kg) did not significantly affect dopamine or 3,4-dihydroxyphenylacetic acid in striata of intact or dopamine-depleted rats. In intact rats, administration of clorgyline in conjunction with l-DOPA produced a >20-fold increase in dopamine and prevented the l-DOPA-induced increase in 3,4-dihydroxyphenylacetic acid. Although both l-deprenyl and Ro 19-6327 administered in combination with l-DOPA elicited a small but significant increase in dopamine, levels of 3,4-dihydroxyphenylacetic acid were not affected. In rats pretreated with 6-hydroxydopamine, clorgyline had no significant effect on the increases in dopamine and 3,4-dihydroxyphenylacetic acid elicited by l-DOPA. Furthermore, neither l-deprenyl nor Ro 19-6327 affected l-DOPA-induced increases in dopamine and 3,4-dihydroxyphenylacetic acid in dopamine-depleted striata. The present findings indicate that deamination by monoamine oxidase A is the primary mechanism for catabolism of striatal dopamine, both under basal conditions and after administration of exogenous l-DOPA. Loss of dopaminergic terminals eliminates this action of monoamine oxidase A but does not enhance deamination by monoamine oxidase B. These data support a model in which exogenous l-DOPA elicits enhanced extracellular accumulation of dopamine in the dopamine-depleted striatum because some transmitter synthesis occurs at nondopaminergic sites and the dopamine terminals that normally take up and catabolize this pool of transmitter are absent.