FMRFamide-like substances in the leech. III. Biochemical characterization and physiological effects.

FMRFamide-like immunoreactivity has been previously localized to identified neurons in the CNS of the leech, Hirudo medicinalis (Kuhlman et al., 1985a). These leech antigens have been characterized biochemically by reverse-phase high-pressure liquid chromatography (HPLC) followed by radioimmunoassay (RIA). The majority of the FMRFamide-like immunoreactivity recovered by HPLC from extracts of leech nerve cords coelutes with authentic FMRFamide. We have tentatively identified this major leech peptide as authentic FMRFamide. Two neurons that control heartbeat in the leech, the HE motor and HA modulatory neurons, and their neural processes on the heart are FMRFamide-like immunoreactive (Kuhlman et al., 1985a). Single individually dissected HE and HA cells were analyzed by HPLC and RIA. Only 1 FMRFamide-like peptide was found in extracts of HA cells; this peptide was chromatographically indistinguishable from authentic FMRFamide. The FMRFamide-like peptide in HE cells could not be isolated by experimental procedures used. Most of the FMRFamide-like immunoreactivity contained within the neural processes on the heart also coeluted with authentic FMRFamide. HE motor neurons, which are believed to be cholinergic (Wallace, 1981a, b; Maranto and Calabrese, 1984a, b), were examined for their FMRFamide-like effects on the heart. The presence of curare in the bathing medium did not block the ability of FMRFamide to induce myogenic activity in heart muscle, suggesting that FMRFamide and ACh act at different receptor sites on the heart. Prolonged firing in HE cells in the presence of curare also induced myogenic activity in heart muscle. This FMRFamide-like action of the HE motor neurons may be normally masked by their cholinergic actions.