Serotonin dynamics and actions in the rat carotid body: preliminary findings

Serotonin or 5-HT is a biogenic amine present in the carotid body (CB) of several species as evidenced in many immunocytochemical studies and in a few biochemical measurements. Early literature on 5-HT actions in the CB in all studied species has lead to the conclusion that it does not participate in the setting of conducted action potentials in the sensory nerve of the CB. However, during the last 10 years very important roles in the cellular physiology of the CB have been proposed for this biogenic amine. These roles include a primary role in setting the excitability of chemoreceptor cells via an autocrine or paracrine action, and thereby, the conducted activity in the carotid sinus nerve, and a critical role in the genesis of long term sensory facilitation observed in CBs of animals exposed to intermittent hypoxia. These facts, along with important discrepancies in the endogenous levels of 5-HT in the CB prompted present study conducted in rat CBs. We measured CB endogenous 5-HT content by HPLC with electrochemical detection and found levels of 5-HT in the range of 15-22 pmole/mg tissue in control and chronically hypoxic animals either sustained or intermittent, with no significant differences among them. 5-HT and the 5-HT2A antagonist ketanserin dose-dependently activated chemoreceptor cells as assessed by their capacity to release catecholamines from freshly isolated CB. In preliminary experiments we have observed that intense hypoxia and high extracellular K(+) promote a small release of 5-HT from CB which is not dependent on the presence of extracellular Ca(2+). Further studies are needed to firmly establish the dynamics of 5-HT in the CB of the rat.