Proctolin is not considered a classical
neurotransmitter as in many systems where it is present it does not change the postsynaptic conductance. It is believed proctolin is a
neurohormone in crustaceans and in some insects. More frequently, proctolin is referred to as a
neuromodulator. This is because all proctolinergic systems change the way impulses are transmitted across a
synapse, using proctolin as a
cotransmitter, often with a more common neurotransmitter such as
glutamate. Proctolin is a potent stimulator in the contraction of a number of visceral and skeletal muscles in insects. Proctolin stimulates contractions of the hindgut in
P. americana, the foregut in
S. gregaria and the midgut of
Diploptera punctata and
L. migratoria. Proctolin also modulates reproductive tissue, stimulating contractions of the oviducts in
P. americana,
Leucophaea maderae,
L. migratoria, and spermathecae in
L. migratoria and
Rhodnius prolixus. Another function of proctolin is that it speeds up heart rate in some insects. A proctolin
receptor in
Drosophila melanogaster has been recently identified as the orphan G-protein coupled receptor CG6986. The
DNA of the
gene sequence was cloned and expressed in
mammalian cells and the expressed receptor was specific for proctolin. In
Drosophila, this receptor is strongly expressed in the
head, the larval
hindgut, the
aorta and on
neuronal endings in adult hearts. == Synthesis and breakdown ==