Role of Acetylcholine Receptors and Dopamine Transporter in Regulation of Extracellular Dopamine in Rat Carotid Body Cultures Grown in Chronic Hypoxia or Nicotine
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Using dissociated rat carotid body (CB) cultures, we compared levels of extracellular dopamine (DA) around oxygen-sensitive glomus cells grown for approximately 12 days in normoxia (Nox; 20% O2), chronic hypoxia (CHox; 6% O2), or chronic nicotine (CNic; 10 microM nicotine, 20% O2), with or without acetylcholine (ACh) receptor (AChR) agonists/antagonists and blockers of DA uptake. In Nox cultures, extracellular DA, determined by HPLC and normalized to the number of tyrosine hydroxylase-positive glomus cells present, was augmented by acute (approximately 15-min) exposure to hypoxia (5% O2; approximately 6x basal), high extracellular K+ (30 mM; approximately 10x basal), nomifensine (1 microM; a selective DA uptake inhibitor; approximately 3x basal), and nicotine (100 microM; approximately 5x basal), but not methylcholine (300 microM; a specific muscarinic agonist). In contrast, in CHox cultures where basal DA release is markedly elevated (approximately 9x control), the stimulatory effect of high K+ (3-4x basal) and acute hypoxia (approximately 2x basal) on DA release persisted, but nicotine and nomifensine were no longer effective and methylcholine had a partial inhibitory effect. In CNic cultures, basal DA levels were also elevated (approximately 9x control), similar to that in CHox cultures; however, although acute hypoxia had a stimulatory effect on DA release (approximately 2x basal), nicotine, nomifensine, and high K+ were ineffective. The elevated basal DA in both CHox and CNic cultures was attenuated by acute or chronic treatment with mecamylamine (100 microM), a nicotinic AChR (nAChR) antagonist. In addition, long-term (16-h), but not acute (15-min), treatment with the muscarinic antagonist atropine (1 microM) produced an additional enhancement of basal DA levels in CHox cultures. Thus, after chronic hypoxia or nicotine in vitro, extracellular DA levels around CB chemoreceptor cell clusters appear to be set by a variety of factors including released ACh, positive and negative feedback regulation via nAChRs and muscarinic AChRs, respectively, and modulation of DA transporters. These results provide insight into roles of endogenous transmitters in the adaptation of CB chemoreceptors to chronic hypoxia and suggest pathways by which neuroactive drugs, e.g., nicotine, can interfere with the protective chemoreflex response against hypoxia.
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