Li+ stimulates ATP-regulated dopamine uptake in PC12 cells

The relationship between oxidative stress and post-translational modification of the dopamine transporter in bipolar disorder

Bipolar disorder (BD) has been consistently associated with altered levels of oxidative stress markers, although the cause and consequences of these alterations remain to be elucidated. One of the main hypotheses regarding the pathogenesis of mania involves increased dopaminergic transmission. In this review, the authors aim to discuss a potential mechanism by which increased oxidative stress inhibits the uptake of dopamine through the post-translational modification of the dopamine transporter and its implications for BD. Within the next 5 years, the authors believe that the mechanisms of dopamine transporter oxidation and its impact on the pathophysiology of BD will be elucidated, which may open avenues for the development of more specific interventions for the treatment of this debilitating illness.

Transtorno de estresse pós-traumático e transtorno de humor bipolar

O Transtorno Bipolar (THB) não é somente uma condição endógena. Severos eventos negativos durante a vida influenciam o desenvolvimento do primeiro episódio e alteram o curso do THB durante a vida. O Transtorno de Estresse Pós-Traumático (TEPT) é uma severa e incapacitante doença mental que afeta uma significativa parcela da população, em algum momento de suas vidas. A presença concomitante de TEPT e THB parece mais freqüente que anteriormente sugerido, e pacientes psicóticos com história de trauma tem sintomas mais severos e maior tendência a abusar de substância psicoativas ilícitas. Pensamentos intrusivos e pesadelos ocorrem com freqüência nos pacientes com TEPT e têm sido associados aos transtornos de humor. O tratamento farmacológico dessa comorbidade ainda está relacionado a estudo empíricos ou não-controlados. Neste artigo, são revisados aspectos atuais relacionados a essa comorbidade e enfatizados aspectos referentes à epidemiologia, etiologia, curso e tratamento farmacológico da comorbidade entre TEPT e THB. Especialmente, este estudo enfatiza a importância de avaliar sistematicamente a história de trauma em pacientes com THB.

The comorbidity of bipolar and anxiety disorders: prevalence, psychobiology, and treatment issues

BACKGROUND

Although symptoms of anxiety as well as anxiety disorders commonly occur in patients with bipolar disorder, the pathophysiologic, theoretical, and clinical significance of their co-occurrence has not been well studied.

METHODS

The epidemiological and clinical studies that have assessed the overlap of bipolar and anxiety disorders are reviewed, with focus on panic disorder and obsessive-compulsive disorder (OCD), and to a lesser extent, social phobia and post-traumatic stress disorder. Potential neural mechanism and treatment response data are also reviewed.

RESULTS

A growing number of epidemiological studies have found that bipolar disorder significantly co-occurs with anxiety disorders at rates that are higher than those in the general population. Clinical studies have also demonstrated high comorbidity between bipolar disorder and panic disorder, OCD, social phobia, and post-traumatic stress disorder. Psychobiological mechanisms that may account for these high comorbidity rates likely involve a complicated interplay among various neurotransmitter systems, particularly norepinephrine, dopamine, gamma-aminobutyric acid (GABA), and serotonin. The second-messenger system constituent, inositol, may also be involved. Little controlled data are available regarding the treatment of bipolar disorder complicated by an anxiety disorder. However, adequate mood stabilization should be achieved before antidepressants are used to treat residual anxiety symptoms so as to minimize antidepressant-induced mania or cycling. Moreover, preliminary data suggesting that certain antimanic agents may have anxiolytic properties (e.g. valproate and possibly antipsychotics), and that some anxiolytics may not induce mania (e.g. gabapentin and benzodiazepines other than alprazolam) indicate that these agents may be particularly useful for anxious bipolar patients.

CONCLUSIONS

Comorbid anxiety symptoms and disorders must be considered when diagnosing and treating patients with bipolar disorder. Conversely, patients presenting with anxiety disorders must be assessed for comorbid mood disorders, including bipolar disorder. Pathophysiological, theoretical, and clinical implications of the overlap of bipolar and anxiety disorders are discussed.

The role of neuronal and extraneuronal plasma membrane transporters in the inactivation of peripheral catecholamines

Catecholamines are translocated across plasma membranes by transporters that belong to two large families with mainly neuronal or extraneuronal locations. In mammals, neuronal uptake of catecholamines involves the dopamine transporter (DAT) at dopaminergic neurons and the norepinephrine transporter (NET) at noradrenergic neurons. Extraneuronal uptake of catecholamines is mediated by organic cation transporters (OCTs), including the classic corticosterone-sensitive extraneuronal monoamine transporter. Catecholamine transporters function as part of uptake and metabolizing systems primarily responsible for inactivation of transmitter released by neurons. Additionally, the neuronal catecholamine transporters, recycle catecholamines for rerelease, thereby reducing requirements for transmitter synthesis. In a broader sense, catecholamine transporters function as part of integrated systems where catecholamine synthesis, release, uptake, and metabolism are regulated in a coordinated fashion in response to the demands placed on the system. Location is also important to function. Neuronal transporters are essential for rapid termination of the signal in neuronal-effector organ transmission, whereas non-neuronal transporters are more important for limiting the spread of the signal and for clearance of catecholamines from the bloodstream. Besides their presynaptic locations, NET and DAT are also present at several extraneuronal locations, including syncytiotrophoblasts of the placenta and endothelial cells of the lung (NET), stomach and pancreas (DAT). The extraneuronal monoamine transporter shows a broad tissue distribution, whereas the other two non-neuronal catecholamine transporters (OCT1 and OCT2) are mainly localized to the liver, kidney, and intestine. Altered function of peripheral catecholamine transporters may be involved in disturbances of the autonomic nervous system, such as occurs in congestive heart failure and hypernoradrenergic hypertension. Peripheral catecholamine transporters provide important targets for clinical imaging of sympathetic nerves and diagnostic localization and treatment of neuroendocrine tumors, such as neuroblastomas and pheochromocytomas.

Chronic verapamil modifies striatal and frontal cortex dopamine levels

In an attempt to elucidate if a change in dopamine (DA) levels was involved in the antimanic action of verapamil reported in various clinical studies, monoamine concentrations in three brain regions (striatum, frontal cortex and hippocampus) obtained from verapamil-treated rats (10 mg/kg i.p. per day for 21 days) were quantified by HPLC coupled to electrochemical detection, and compared with monoamine concentrations in haloperidol-treated animals (5 mg/kg i.p. per day for 21 days). We have found that verapamil and haloperidol, when injected for 3 weeks to rats sacrificed 2 h after the last injection, decreased the striatal DA concentration to a similar extent. This decrease was not observed in short-term (one injection 2 h before sacrifice) verapamil- or haloperidol-treated rats. Moreover, after such a single injection of verapamil the striatal DA concentration was even increased. The striatal concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) was increased about two-fold by haloperidol, but not by verapamil. This haloperidol-induced increase in striatal DOPAC was similar after one injection and after 21 days of haloperidol administration. Neither verapamil nor haloperidol modified the concentrations of homovanillic acid (HVA) or 3-methoxytyramine (3-MT) in the striatum. In the frontal cortex, chronic verapamil increased the concentrations of DA two-fold, and chronic haloperidol increased the concentration of DOPAC two-fold. The other DA metabolites, namely HVA and 3-MT were not significantly changed. The concentration of serotonin (5-HT) and its main metabolite, 5-hydroxyindoleacetic acid (5-HIAA), in control, verapamil- and haloperidol-treated rats were similar in the three brain regions studied. We conclude that DA autoreceptors are implicated in verapamil's effects on frontal cortex and striatum DA levels; and that the presumed antimanic action exerted by verapamil is due to its long-term effect on these receptors.

Effects of a chronic lithium treatment on central dopamine neurotransporters

The objectives of this study were to characterize the effects of a chronic lithium (Li+) treatment on dopamine (DA) uptake sites, as well as on the levels of mRNA encoding for these transporters, and to determine the eventual reversibility of the treatment. Quantitative autoradiography was carried out on sections from rat brain using 3beta-(4-[125I]iodophenyl)tropane-2beta-carboxylic acid isopropyl ester ([125I]RT1-121) to label DA transporters, and mRNA levels were measured by in situ hybridization. Following chronic Li+ treatment (28 days), the labelling to DA transporters increased (60-90%) in all sections of the rostral and caudal neostriatum, whereas no alteration was observed in the other regions studied, namely the substantia nigra, the ventral tegmental area, and the dorsal raphe nucleus. These effects were reversed completely following a withdrawal period of 2 days without Li+. Also, there were no modifications in the labelling of DA transporters after only 2 days of Li+ treatment. In addition, we measured the levels of mRNA encoding for DA transporters in the substantia nigra and the ventral tegmental area; however, no alterations were observed following a chronic Li+ treatment, and thus the hypothesis of an increased synthesis is not supported. This could mean that the Li+ treatment modified the affinity of DA transporters for the radioligand, possibly a consequence of conformational changes induced by the disruption of the nerve terminal membrane environment; however, a modification in the number of transporters could not be ruled out. The results of this study further support the hypothesis of the implication of central dopaminergic transmission in the pathology and treatment of affective disorders.

Identification of the major transport pathway for the parkinsonism-inducing neurotoxin 1-methyl-4-phenylpyridinium

1-Methyl-4-phenylpyridinium is a potent parkinsonism-inducing neurotoxin which has become a valuable tool for the examination of the mechanisms and therapeutic treatment strategies for Parkinson's syndrome. Recently, it has been found that physiological levels of extracellular ATP (0.1-1 mM) stimulate dopamine uptake into both rat and bovine brain synaptosomes and rat pheochromocytoma cells in a dose-dependent manner. In this study we report that physiological levels of extracellular ATP (0.1-2 mM) stimulate the transport of 1-methyl-4-phenylpyridinium into the pheochromocytoma cell line by 270% over basal levels. Kinetically, the presence of ATP increases both the K(m) and Vmax of 1-methyl-4-phenylpyridinium transport. In addition, 1-methyl-4-phenylpyridinium is far more effective at inhibiting ATP-stimulated dopamine transport (IC50 = 11 microM) than basal dopamine transport (IC50 100 microM) into pheochromocytoma cells. These data show that the ATP-regulated 1-methyl-4-phenylpyridinium transport pathway is the major component (approximately 95%) of total 1-methyl-4-phenylpyridinium transport, and provide the first evidence for the involvement of extracellular ATP in the bulk transport of 1-methyl-4-phenylpyridinium.

Cation requirements of basal and ATP-regulated dopamine transport in rat pheochromocytoma cells

The transport of dopamine into presynaptic nerve terminals is the primary mechanism for the termination of dopaminergic neurotransmission. This transport process has recently been found to be composed of two components, a basal dopamine transport pathway which exists in the absence of extracellular ATP and an ATP-regulated moiety which comprises approximately 66% of the total transport system [Cao C. J. et al. (1990) Biochem. Pharmac. 39, R9-R14; Cao C. J. et al. (1989) Biochemistry 8, 207-220; Dunigan C. D. and Shamoo A. E. (1995) Neuroscience 65, 1-4; Eshleman A. et al. (1995) Life Sci. 56, 1613-1621]. Using a rat pheochromocytoma cell line and a Krebs bicarbonate buffering system, the present study examined the effect of several cations on both basal and ATP-regulated dopamine transport. In the absence of extracellular ATP, dopamine transport had an absolute dependence on the presence of Na+, but exhibited no requirement for Mg2+. Kinetically, the addition of 120 mM NaCl increased the Vmax of basal dopamine transport by approximately 150%. In contrast, the ATP-regulated dopamine transport pathway displayed a different sensitivity to Na+ and was completely dependent upon the presence of Mg2+. The addition of 1.2 mM MgSO4 increased the Vmax of transport in the presence of 0.7 mM extracellular ATP by 222%. Both basal and ATP-regulated transport were unaffected by the removal of either Ca2+ or K+ from the assay buffer. When the effects of ouabain, a potent inhibitor of Na+, K(+)-ATPase, were tested in the rat pheochromocytoma cell model, it was found that concentrations of ouabain as high as 1 mM were ineffective at inhibiting either the basal or ATP-regulated dopamine transport components. These results imply that the Na+ gradient supplied by Na+, K(+)-ATPase is not the sole provider of energy needed to drive either transport process. The ionic requirements of the basal and ATP-regulated dopamine transport pathways demonstrate the distinction between the two transport processes. In addition, the ionic dependency profile of the ATP-regulated moiety has provided some mechanistic insights into ATP-regulated catecholamine uptake, as the absolute Mg2+ requirement and the ineffectiveness of Ca2+ argues against the involvement of either purinergic receptors or a Ca(2+)-dependent, Mg(2+)-independent ectokinase in the ATP-regulated transport system.