Insect neuropetide proctolin and its analogues. An overview of the present literature

Effects of selected neuropeptides, mating status and castration on male reproductive tract movements and immunolocalization of neuropeptides in earwigs

In earwigs, the male reproductive system is complex, comprising accessory glands and long dual intromittent organs for transfer of materials to the female and for removal of rival sperm. We investigated potential factors altering contractions of the male reproductive tracts in vitro. Tracts from 0-day (newly emerged) males displayed relatively little motility in vitro; however, those from 5-day (intermediate stage of sexual maturity) and 8-day (fully mature) males pulsed vigorously. Both 1 and 100 nM proctolin (RYLPT-OH) stimulated the rate of contraction of reproductive tracts from both 5-day and 8-day males. In contrast, 1 nM and 100 nM FGLa AST (cockroach allatostatin) did not affect pulsations. However, 10 microM FGLa AST decreased activity of reproductive tracts. Mating decreased motility of tracts from 5-day old males, but did not alter motility of tracts from 8-day old males. Castration of larvae significantly suppressed reproductive tract motility in subsequent 8-day old adults compared with those of intact or sham-operated adults. Castration also suppressed seminal vesicle size. Lastly, we assessed the presence and distribution of proctolin-like and allatostatin-like immunoreactivity in tissues. Immunoreactivity to FGLa AST and proctolin was widespread, occurring in the brain and ventral ganglia. Surprisingly, we did not detect immunoreactivity to either FGLa AST or proctolin within the reproductive system; however, proctolin immunoreactivity was evident in nerves extending from the terminal ganglion of 8-day, but not 0-day, males. Collectively, these experiments demonstrate that the male earwig reproductive system is an appropriate model for use in addressing sexual maturation and activities in male insects.

Contributions of insect research toward our understanding of neurosecretion

The process of neurosecretion is an important and widespread method of biological communication among animals. Although insects and vertebrates appear to be very different, neurosecretory mechanisms and the neuropeptides themselves are often the same. The gradual acceptance of neurosecretion as a biological phenomenon, largely as a result of research done with insects, is discussed.

Molecular identification of the first insect proctolin receptor

The website of the Drosophila Genome Project (www.flybase.org) contains the sequence of an annotated gene CG6986, which is predicted to code for a G protein-coupled receptor. We cloned the cDNA of this gene and expressed it in Chinese hamster ovary cells. Screening of a neuropeptide library revealed that the expressed receptor was specific for the neuropeptide proctolin (EC(50), 6x10(-10)M). Proctolin (RYLPT) was the first invertebrate neuropeptide to be fully sequenced (already in 1975) and occurs with identical structure in both crustaceans and insects, where it has myo- and neurostimulatory actions. Northern blots showed that the Drosophila proctolin receptor was only weakly expressed in embryos, larvae, pupae, and in the thoraces and abdomina of adult flies, but strongly in the heads of adult animals. The Drosophila receptor reported here is the first invertebrate proctolin receptor to be identified.

Identification and characterization of a G protein-coupled receptor for the neuropeptide proctolin in Drosophilamelanogaster

Proctolin is a bioactive neuropeptide that modulates interneuronal and neuromuscular synaptic transmission in a wide variety of arthropods. We present several lines of evidence to propose that the orphan G protein-coupled receptor CG6986 of Drosophila is a proctolin receptor. When expressed in mammalian cells, CG6986 confers second messenger activation after proctolin application, with an EC(50) of 0.3 nM. In competition-based studies, the CG6986 receptor binds proctolin with high affinity (IC(50) = 4 nM). By microarray analysis, CG6986 transcript is consistently detected in head mRNA of different genotypes, and under different environmental conditions. By blot analysis, anti-CG6986 antibodies detect a band in tissue homogenates similar to the predicted size of the protein. Proctolin receptor immunosignals are found in the hindgut, heart, and in distinct neuronal populations of the CNS; such patterns correlate with previous demonstrations of proctolin biological activity, and in several instances, with areas of proctolin peptide immunosignals. The identification of a bona fide proctolin receptor provides the basis for a mechanistic analysis of this critical synaptic modulator.

Distribution of PACAP-like immunoreactivity in the nervous system of oligochaeta

The marked similarity between the primary structures of human, other vertebrate, and the invertebrate tunicate PACAP suggests that PACAP is one of the most highly conserved peptides during the phylogeny of the metazoans. We investigated the distribution of PACAP-like immunoreactivity in the nervous system of three oligochaete (Annelida) worms with immunocytochemistry. The distribution pattern of immunoreactivity was similar in all three species (Lumbricus terrestris, Eisenia fetida, and Lumbricus polyphemus). The cerebral ganglion contains numerous immunoreactive cells and fibers. A few cells and fibers were found in the medial and lateral parts of the subesophageal and ventral cord ganglia. In the peripheral nervous system, immunoreactivity was found in the enteric nervous system, in epidermal sensory cells, and in the clitellum.

Proctolin, an insect neuropeptide

Synthetic, biological and conformational studies on the insect neuropeptide proctolin (Arg-Tyr-Leu-Pro-Thr) and some of its analogues are reviewed.