Mild hypoxia exposure impacts peripheral serotonin uptake and degradation in Gulf toadfish (Opsanus beta)

Plasma serotonin (5-hydroxytryptamine, 5-HT) homeostasis is maintained through the combined processes of uptake (via the 5-HT transporter SERT, and others), degradation (via monoamine oxidase, MAO) and excretion. Previous studies have shown that inhibiting SERT, which would inhibit 5-HT uptake and degradation, attenuates parts of the cardiovascular hypoxia reflex in gulf toadfish (Opsanus beta), suggesting that these 5-HT clearance processes may be important during hypoxia exposure. Therefore, the goal of this experiment was to determine the effects of mild hypoxia on 5-HT uptake and degradation in the peripheral tissues of toadfish. We hypothesized that 5-HT uptake and degradation would be upregulated during hypoxia, resulting in lower plasma 5-HT, with uptake occurring in the gill, heart, liver and kidney. Fish were exposed to normoxia (97.6% O2 saturation, 155.6 Torr) or 2 min, 40 min or 24 h mild hypoxia (50% O2 saturation, ∼80 Torr), then injected with radiolabeled [3H]5-HT before blood, urine, bile and tissues were sampled. Plasma 5-HT levels were reduced by 40% after 40 min of hypoxia exposure and persisted through 24 h. 5-HT uptake by the gill was upregulated following 2 min of hypoxia exposure, and degradation in the gill was upregulated at 40 min and 24 h. Interestingly, there was no change in 5-HT uptake by the heart and degradation in the heart decreased by 58% within 2 min of hypoxia exposure and by 85% at 24 h. These results suggest that 5-HT clearance is upregulated during hypoxia and is likely driven, in part, by mechanisms within the gill and not the heart.