Turnover of serotonin in brain of an anoxia-tolerant vertebrate, the crucian carp.

Serotonin (5-HT) is a neurotransmitter in the vertebrate brain. Estimates of 5-HT turnover times range between 1 h (mammals) and 10 h (goldfish). Synthesis and catabolism of 5-HT are dependent on molecular oxygen, hence, 5-HT turnover comes to a halt during anoxia. Nevertheless, a few vertebrates, such as the crucian carp (Carassius carassius L.), tolerate anoxia for several weeks. To assess the ability of the crucian carp brain to tolerate a stop in 5-HT turnover during anoxia, it is essential to know its normoxic rate of 5-HT turnover. The turnover of 5-HT was estimated in different brain regions of winter-acclimatized crucian carp held in normoxia at 8 degrees C using two different methods, i.e., 1) by measuring the increase seen in 5-HT after monoamine oxidase inhibition (by pargyline), and 2) by measuring the rate of 5-hydroxytryptophan (5-HTP) accumulation, after the conversion of 5-HTP to 5-HT had been inhibited (by N-m-hydroxybenzylhydrazine). Both methods suggested similar turnover rates (approximately 10-20 pmol/g h) and turnover times (1-3 days). This is probably the slowest 5-HT turnover ever measured in a vertebrate (less than 1/100 of that of mammals). The slow 5-HT turnover could be explained by a high degree of recycling of the same 5-HT molecules, in combination with a decreased turnover at low temperatures. Moreover, the slow 5-HT turnover might be a prerequisite for preserving the functional integrity of the 5-HT system during anoxia.