ROLE OF ACETYLCHOLINE-RECEPTORS AND DOPAMINE TRANSPORTER IN REGULATION OF EXTRACELLULAR DOPAMINE IN RAT CAROTID-BODY CULTURES GROWN IN CHRONIC HYPOXIA OR NICOTINE

Using dissociated rat carotid body (CB) cultures, we compared levels o f extracellular dopamine (DA) around oxygen-sensitive glomus cells gro wn for similar to 12 days in normoxia (Nor; 20% O-2), chronic hypoxia (CHox; 6% O-2), or chronic nicotine (CNic; 10 mu M nicotine, 20% O-2), with or without acetylcholine (ACh) receptor (AChR) agonists/antagoni sts and blockers of DA uptake. In Nor cultures, extracellular DA, dete rmined by HPLC and normalized to the number of tyrosine hydroxylase-po sitive glomus cells present, was augmented by acute (similar to 15-min ) exposure to hypoxia (5% O-2; similar to 6x basal), high extracellula r K+ (30 mM; similar to 10x basal), nomifensine (1 mu M; a selective D A uptake inhibitor; similar to 3x basal), and nicotine (100 mu M; simi lar to 5x basal), but not methylcholine (300 mu M; a specific muscarin ic agonist). In contrast, in CHox cultures where basal DA release is m arkedly elevated (similar to 9x control), the stimulatory effect of hi gh K+ (3-4x basal) and acute hypoxia (similar to 2x basal) on DA relea se persisted, but nicotine and nomifensine were no longer effective an d methylcholine had a partial inhibitory effect. In CNic cultures, bas al DA levels were also elevated (similar to 9x control), similar to th at in CHox cultures; however, although acute hypoxia had a stimulatory effect on DA release (similar to 2x basal), nicotine, nomifensine, ac id high K+ were ineffective. The elevated basal DA in both CHox and CN ic cultures was attenuated by acute or chronic treatment with mecamyla mine (100 mu M), a nicotinic AChR (nAChR) antagonist. In addition, lon g-term (16-h), but not acute (15-min), treatment with the muscarinic a ntagonist atropine (1 mu M) produced an additional enhancement of basa l DA levels in CHox cultures. Thus, after chronic hypoxia or nicotine in vitro, extracellular DA levels around CB chemoreceptor cell cluster s appear to be set by a variety of factors including released ACh, pos itive and negative feedback regulation via nAChRs and muscarinic AChRs , respectively, and modulation of DA transporters. These results provi de insight into roles of endogenous transmitters in the adaptation of CB chemoreceptors to chronic hypoxia and suggest pathways by which neu roactive drugs, e.g., nicotine, can interfere with the protective chem oreflex response against hypoxia.