5-Hydroxytryptamine: a second diuretic hormone in Rhodnius prolixus

Area of Science:

• Insect physiology
• Endocrinology
• Comparative physiology

Background:

• The regulation of fluid secretion in insects is crucial for homeostasis.
• Insect Malpighian tubules are analogous to vertebrate kidneys, playing a key role in excretion and ion balance.
• Hormonal control of diuresis in Rhodnius prolixus involves both peptide hormones and potentially other signaling molecules.

Purpose of the Study:

• To investigate the presence and role of 5-hydroxytryptamine (5-HT) in regulating Malpighian tubule fluid secretion in Rhodnius prolixus.
• To determine if 5-HT acts as a diuretic hormone in this insect species.
• To identify the presence of specific receptors for 5-HT on Rhodnius Malpighian tubules.

Main Methods:

• Bioassays were performed on fifth-instar Rhodnius prolixus haemolymph using Calliphora salivary glands.
• The effects of 5-HT on Malpighian tubule fluid secretion were measured in unfed and fed insects.
• Specific 5-HT receptor antagonists, ketanserin and spiperone, were used to assess receptor involvement and selectivity.

Main Results:

• Biologically active 5-hydroxytryptamine (5-HT) was detected in Rhodnius prolixus haemolymph.
• In unfed insects, 5-HT levels were insufficient to stimulate Malpighian tubule fluid secretion.
• Soon after feeding initiation, 5-HT levels rose sufficiently to stimulate rapid Malpighian tubule secretion.
• Ketanserin and spiperone selectively and reversibly inhibited 5-HT-induced fluid secretion, confirming specific 5-HT receptors.

Conclusions:

• 5-hydroxytryptamine (5-HT) is a naturally occurring hormone in Rhodnius prolixus.
• 5-HT plays a significant role in regulating diuresis in this species, acting in concert with a known peptide hormone.
• Specific 5-HT receptors are present on the Malpighian tubules of Rhodnius prolixus, mediating the fluid secretion response.