Proctolin in the antennal circulatory system of lower Neoptera: a comparative pharmacological and immunohistochemical study

Neuropeptides are important with respect to almost all physiological processes and behavioural patterns in an organism. In the present study, muscle bioassays, immunohistochemistry and matrix‐assisted laser desorption ionization time‐of‐flight (MALDI‐TOF) mass spectrometry are used to investigate the distribution and efficacy of proctolin in the antennal heart of 36 species of lower Neoptera. In total, 20 species of Dictyoptera (cockroaches, termites, praying mantids), eight species of Saltatoria (crickets and grasshoppers) and eight species of Phasmatodea (stick insects) are investigated. The antennal heart of all tested Blattoidea, including the termite Mastotermes darwiniensis Froggatt, exhibit a strong proctolin‐like immunoreactivity, as well as a high sensitivity to proctolin, whereas members of the second clade of cockroaches (Blaberoidea) are largely insensitive to proctolin and show only a weak proctolin‐like immunoreactivity. The antennal heart of praying mantids (Mantodea) also contains only few proctolin‐like immunoreactive fibres but is highly sensitive to proctolin. Such a high sensitivity is found also in Phasmatodea, although the antennal heart of these insects does not have proctolin‐like immunoreactive fibres. These findings are supported by MALDI‐TOF mass spectrometry; no trace of proctolin is detected in antennal heart preparations of Phasmatodea. In Saltatoria, only weak (or no) effects of proctolin are observed and, in most subtaxa, no proctolin‐like immunoreactivity is visible in the antennal heart preparations. Only in Grylloidea is strong proctolin‐like immunostaining found in processes of the antennal heart. In these species, weak or moderate effects of proctolin are observed in the antennal heart bioassay. In general, the differences in distribution of proctolin and effects on the antennal heart within the basal Neoptera cannot be deduced from their phylogenetic position, although they show conformity within each (sub)taxon.