The short term effects of 5,7-dihydroxytryptamine on peripheral serotonin stores in Rhodnius prolixus and their long-term recovery

Abstract

The serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) appears to affect invertebrate systems differently from vertebrate ones. The basis for toxicity in vertebrates appears to involve the intraneuronal actions of monoamine oxidase (MAO) upon the toxin. In insects, MAO is not present in appreciable amounts. In this study, we demonstrate that in vitro 5.7-DHT competitively inhibits the uptake of [ 3H]serotonin by serotonergic neurohaemal areas. The apparent K M increases from 4.9 × 10 −7 to 1.7 × 10 −6 M. This neurotoxin also causes a significant release of previously accumulated [ 3H]serotonin in nominally Ca 2+-free saline. While 5,7-DHT does not affect the uptake of [ 3H]tryptophan, it reduces the subsequent synthesis of [ 3H]serotonin. In vivo, the tissues appear to have recovered 2 weeks after toxin treatment, as determined by immunohistochemistry. At 24 h, 1 week and 2 weeks after injection, the tissues are able to take up and release [ 3H]serotonin normally. 1 and 2 weeks after injection, insects ingest a normal-sized blood meal, a behaviour acutely disrupted by 5,7-DHT treatment. The results of this and other invertebrate studies suggest that 5,7-DHT does not destroy serotonergic neurons, as it does in vertebrates. 5,7-DHT may be a more useful tool to study the functions of serotonin in invertebrates as one may transiently affect serotonin stores.