Detection and isolation of proctolin-like immunoreactivity in arachnids: Possible cardioregulatory role for proctolin in the orb-weaving spiders Argiope and Araneus

Abstract

Proctolin-like immunoreactivity was detected in the prosoma of four species of araneid spiders. Proctolin levels as determined by radioimmunoassay ranged from 3.2 ± 0.6 ng proctolin per prosoma in Argiope aurantia to 8.7 ± 0.9 ng proctolin per prosoma in Araneus cavaticus. Immunohistochemical staining of prosomal sections revealed proctolin-positive tracts, fibres and varicosities in both the supraoesophageal and suboesophageal ganglia of Argiope aurantia and Araneus cavaticus. Using whole-mount preparations of spider hearts, proctolin immunoreactive fibres and varicosities were also observed in association with the myocardium. Proctolin-like bioactivity in partially purified prosomal extracts of Argiope aurantia and Araneus cavaticus co-eluted with synthetic proctolin using high-performance liquid chromatography. Application of 10 −7 M synthetic proctolin to the neurogenic spider heart typically caused increases in the strength and rate of heart contractions, and occasionally produced a sustained contracture of the myocardium. Responses were similar in all four species examined. These data suggest that an endogenous proctolin-like peptide in these arachnids may act as a cardioregulatory neuromodulator or as a neurotransmitter in the central nervous system.