Localization of the neuropeptide APGWamide in gastropod molluscs by in situ hybridization and immunocytochemistry.

The amidated tetrapeptide Ala-Pro-Gly-Trp-NH2 (APGWamide) plays a key role in the control of male copulation behavior in the basommatophoran pulmonate freshwater snail Lymnaea stagnalis. The morphological basis for a conserved role of APGWamide in the control of male reproduction in gastropod molluscs is presented. The prosobranch Littorina littorea, the opisthobranch Aplysia californica, the basommatophoran pulmonate Bulinus truncatus, and the stylommatophoran pulmonates Arion ater and Limax maximus have been examined for the presence of APGWamide producing neurons using immunocytochemistry and in situ hybridization. In all species investigated a cluster of APGWamide expressing neurons is present in the anteromedial region of the cerebral ganglia. The asymmetrical distribution which exists in Lymnaea and which coincides with the innervation of the asymmetrically located penial complex is also found in the opisthobranch Aplysia, as well as in the stylommatophoran pulmonate slugs Arion and Limax, in which APGWamide immunoreactive neurons are only found in the mesocerebrum of the right cerebral ganglion. APGWamide immunoreactive varicose fibers innervate muscles of the male accessory sex organs in Bulinus and Aplysia, confirming the hypothesis that APGWamide may be a biochemically and functionally conserved factor in the regulation of gastropod mollusc reproduction.