Dopamine Induces Ca2+ Signaling in Astrocytes through Reactive Oxygen Species Generated by Monoamine Oxidase

Abstract

Dopamine is a neurotransmitter that plays a major role in a variety of brain functions, as well as in disorders such as Parkinson disease and schizophrenia. In cultured astrocytes, we have found that dopamine induces sporadic cytoplasmic calcium ([Ca 2؉ ] c ) signals. Importantly, we show that the dopamine-induced calcium signaling is receptor-independent in midbrain, cortical, and hippocampal astrocytes. We demonstrate that the calcium signal is initiated by the metabolism of dopamine by monoamine oxidase, which produces reactive oxygen species and induces lipid peroxidation. This stimulates the activation of phospholipase C and subsequent release of calcium from the endoplasmic reticulum via the inositol 1,4,5-trisphosphate receptor mechanism. These findings have major implications on the function of astrocytes that are exposed to dopamine and may contribute to understanding the physiological role of dopamine. Dopamine (DA)2 is the predominant catecholamine neurotransmitter in the mammalian brain and controls a variety of functions, including locomotor activity, cognition, emotion, positive reinforcement, food intake, and endocrine regulation. The neurotransmitter DA is a monoamine that is synthesized in dopaminergic neurons in the substantia nigra in the midbrain and transferred to the striatum through very fine C-fibers (1, 2). Dopaminergic terminals constitute ϳ21% of total axon terminals in the striatum and contact mostly dendritic spines and dendritic shafts. The actions of DA are mediated by specific G protein-coupled receptors, which are divided into two major families based on their ability to stimulate (D1-like) or inhibit adenylate cyclase (D2-like). Three human D2-like receptors have been cloned: D2, D3, and D4 (3). Dopamergic receptors are mostly distributed in the striatum and to a lesser degree in other parts of the brain.The signaling pathways of DA in different parts of the brain are of broad clinical and scientific interest. The neurodegenerative disorder, Parkinson disease, is caused by a loss of dopamine-secreting neurons from the midbrain; this leads to rigidity, tremor, and the characteristic slowness of movement. Impairment of DA signaling is thought to play a role not only in Parkinson disease but also Alzheimer disease and psychomotor syndromes such as schizophrenia (4 -6).DA is catabolized by monoamine oxidase (MAO), which breaks down monoamines using FAD, producing aldehydes and hydrogen peroxide. Astrocytes express both forms of MAO: MAO-A and MAO-B (7).Activation of D1 and D2 receptors is believed to modulate intracellular calcium levels by a single mechanism, that is, the stimulation of phosphatidylinositol hydrolysis by phospholipase C, resulting in the production of inositol 1,4,5-trisphosphate (IP 3 ), which mobilizes intracellular calcium stores (2, 3). Other mechanisms of release of Ca 2ϩ from internal stores have also been proposed. DA increases cAMP levels (8 -10). DA appears to affect the activity of calcium channels. In neurons and PC12 cells, DA reduced calcium currents...

Functional role of mitochondrial reactive oxygen species in physiology

“…Considering this, activation of endogenous ROS formation in mitochondria should play an important role in the initiation of signal transduction. Thus, dopamine-induced calcium signal in astrocytes is due to production of ROS in MAO and it can be blocked by molecular or pharmacological inhibition of this enzyme or by application of antioxidants (Vaarmann et al, 2010). Generation of hydrogen peroxide in MAO stimulates lipid peroxidation, activation of phospholipase C and generation IP 3 -dependent calcium signal (Vaarmann et al, 2010) confirming the important role of mitochondrial ROS in signal transduction in CNS.…”

“…Inhibition of respiration and decrease of mitochondrial membrane potential by anoxia or hypoxia can stimulate ROS production in the same way but only for short time (Abramov et al, 2007;Nohl et al, 1993). High mitochondrial membrane potential also increases a rate of ROS production in ETC and it can be elevated by application of the mitochondrial substrates (Abramov et al, 2010;Kovac et al, 2015). Mild uncoupling with low doses of FCCP or physiological uncouplers significantly reduces production of superoxide and for long time is used as a basis for cell protective strategy (Brennan et al, 2006;Skulachev, 1996).…”

Lipid peroxidation is essential for phospholipase C activity and IP3 related calcium signal

“…Under physiological conditions, Ca 2+ mobilization from the ER stores is mediated by a variety of agonists, which are known to trigger InsP 3 synthesis (Rossi et al, 2012). InsP 3 -related Ca 2+ signals are evoked through stimulation of P2Y receptors by ATP or inorganic polyphosphate (Burnstock, 2013;Holmström et al, 2013), or by direct activation of PLC by H 2 O 2 produced during the utilisation of dopamine (Vaarmann et al, 2010). We here focus on purinoreceptor signalling in astrocytes, which is one important PLC-and InsP 3 -mediated signalling pathway in the brain (James and Butt, 2001).…”

“…ROS can activate such release of Ca 2+ from endoplasmic reticulum (ER) stores; a mechanism which has been thought to be mediated by ROS-mediated activation of RyR receptors on the ER (Camello-Almaraz et al, 2006). However, there is also strong evidence that ROS activates PLC, which ultimately contributes to Ca 2+ release (Servitja et al, 2000;Vaarmann et al, 2010). These indirect, PLC-mediated, ROSinduced Ca 2+ signals are likely to play an important role in the brain, where there are high levels of oxidisable polyunsaturated fatty acids and high ROS load due to high oxygen consumption (Halliwell, 2006).…”

Adrenaline and Noradrenaline: Partners and Actors in the Same Play"

“…Of relevance, the oxidative deamination of catecholamines by MAO leads to another relevant product, hydrogen peroxide (H 2 O 2 ) which subsequently may be converted into the highly reactive hydroxyl radical (HO • ) (Valko et al, 2007), that may cause oxidative stressrelated damage (Carvalho et al, 2001, Duarte et al, 2004, Vaarmann et al, 2010, Costa et al, 2011.…”