Injections of 5-hydroxytryptamine (5-HT, serotonin) are found to cause plasticization of the abdominal cuticle of Rhodnius larvae. This plasticization is a direct action of 5-HT on some element in the body wall; the central nervous system is not required. It is probable that 5-HT acts directly at a receptor on the epidermal cells. The relationship between structure and plasticizing activity for a number of 5-HT analogues has been investigated. The receptor resembles other ‘classical’ 5-HT receptors in its requirements, but is unlike the 5-HT/diuretic hormone receptor of Rhodnius Malpighian tubules.

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