THE SHORT-TERM EFFECTS OF 5,7-DIHYDROXYTRYPTAMINE ON PERIPHERAL SEROTONIN STORES IN RHODNIUS-PROLIXUS AND THEIR LONG-TERM RECOVERY

The serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) appears to affect invertebrate systems differently from vertebrate ones. The basi s for, toxicity in vertebrates appears to involve the intraneuronal ac tions of monoamine oxidase (MAO) upon.the toxin. In insects, MAO is no t present in appreciable amounts. In this study, we demonstrate that i n vitro 5,7-DHT competitively inhibits the uptake of [H-3]serotonin by serotonergic neurohaemal areas. The apparent K(M) increases from 4.9 x 10(-7) to 1.7 x 10(-6) M. This neurotoxin also causes a significant release of previously accumulated [H-3]serotonin in nominally Ca2+-fre e saline. While 5,7-DHT does not affect the uptake of [H-3]tryptophan, it reduces the subsequent synthesis of [H-3]serotonin. In vivo, the t issues appear to have recovered 2 weeks after toxin treatment, as dete rmined by immunohistochemistry. At 24 h, 1 week and 2 weeks after inje ction, the tissues are able to take up and release [H-3]serotonin norm ally. 1 and 2 weeks after injection, insects ingest a normal-sized blo od meal, a behaviour acutely disrupted by 5,7-DHT treatment. The resul ts of this and other invertebrate studies suggest that 5,7-DHT does no t destroy serotonergic neurons, as it does in vertebrates. 5,7-DHT may be a more useful tool to study the functions of serotonin in inverteb rates as one may transiently affect serotonin stores.