PHI-like immunoreactivity in the nervous system of the cockroach (insect) and Aplysia (mollusc) with special reference to its relationship to VIP-like immunoreactivity.

Studies in mammals have indicated that PHI (peptide histidine isoleucine) and VIP (vasoactive intestinal polypeptide) share a prepropeptide, and that both peptides are contained in the same neurons. The present study proposes that this relationship may not hold true in protostomian invertebrates. In the nervous system of the cockroach (Insecta, Arthropoda) and Aplysia (Gastropoda, Mollusca) distribution of PHI and VIP was examined by immunocytochemistry. In both animals neurons containing PHI-like immunoreactivity were numerously found and only a small part of them in the cockroach contained VIP-like immunoreactivity. These findings suggest that in these invertebrates, PHI may play a more important regulatory role than VIP. Possible reasons for the difference in the distribution of immunoreactivities for PHI and VIP are discussed.