The carotid body (CB), the main peripheral arterial chemoreceptor, registers the levels of pO(2), pCO(2) and pH in the blood and responds to their changes by regulating the breathing. Previous research suggests that in the wide variety of neuroactive substances with an essential role in chemosensory function, dopamine is the most promising transmitter candidate of the chemoreception process. In this study we demonstrated the expression of tyrosine hydroxylase (TH), the rate-limiting enzyme for catecholamine biosynthesis, in the CB glomus cells, and also in certain petrosal ganglion (PG) and superior cervical ganglion (SCG) neurons that provide their innervation, as revealed by retrograde tracing with Fast Blue. In particular, we found that a large number of glomus cells were TH-immunopositive. Nerve fibres around blood vessels and glomus cells were immunostained as well. TH immunoreactivity was also detected in a subset of small- to medium-sized PG neurons. Conversely, virtually all sympathetic neurons in the SCG contained TH. These results show that a population of chemoreceptor cells in the adult rat CB is dopaminergic and, moreover, receives a dual dopaminergic innervation by both sensory and autonomic fibres from the PG and SCG, respectively. The presence of functional dopamine receptors and the physiological, excitatory or inhibitory, significance of dopamine in presynaptic and postsynaptic chemosensory signalling in the CB of rats still remain to be elucidated.
