Insects
An illustrative example of the serotonergic coordination
of feeding-related physiological events is provided by
the South American ‘‘kissing bug’’ Rhodnius prolixus, an
obligatory blood feeder (and vector for the parasite causing
Chagas’ disease). This insect is capable of ingesting
300 mg of blood in 15 min, increasing its body mass by
a factor of 10 (Orchard, 2006). Serotonin, which is widely
distributed throughout insect nervous systems, has been
found to orchestrate remarkably diverse feeding-related
physiological events in Rhodnius, both as a neurotransmitter/
neuromodulator and as a neurohormone.
As Rhodnius feeding begins, serotonergic nerves stimulate
the release of saliva, which contains substances that
block hemostasis of the host’s blood, permitting continuous
ingestion (Orchard, 2006). Simultaneously, serotonergic
nerves of the body wall trigger softening (plasticization)
of the cuticle, allowing the body wall to expand as the crop
(anterior midgut) fills with blood. The crop also receives
serotonergic afferents, and these induce contractions
and increased fluid transport into the circulatory system
(hemolymph). As this occurs, serotonin is released from
an abdominal nerve plexus directly into the hemolymph,
elevating circulating serotonin levels up to 15-fold (Lange
et al., 1989). Serotonin in the hemolymph acts as a neurohormone
at the animal’s Malpighian tubules, inducing
a massive diuresis that is essential for the animal to reduce
its mass and thereby regain its mobility.