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.