The localization of FMRFamide in the nervous and somatic tissues of Nereis virens and its effects upon the isolated esophagus.

1. The tetrapeptide FMRFamide which is present in extracts of Nereis virens was localized in various nereid tissues immunohistochemically. 2. Immunoreactive FMRFamidergic cells and fibers were found in the supraesophageal (brain) and subesophageal ganglia, as well as in the intersegmental ganglia of the ventral nerve cord. Immunoreactive fibers were present in the neuropile of, and the connectives between, the supraesophageal, subesophageal, and intersegmental ganglia. 3. In the periphery immunoreactive FMRFamidergic fibers and a few cell bodies were observed in the gut. Sparse fibers were also seen in the body wall, parapodia, and cephalic palps. When the antiserum was preabsorbed with FMRFamide, no specific immunoreactivity was detected. 4. The esophagus of Nereis, isolated and suspended in a tissue bath, responded to FMRFamide with a dose-dependent relaxation; threshold was between 30 and 300 nM, and the EC50 was 1.55 +/- 0.60 microM. Benzoquinonium did not modify this response. 5. Thus, FMRFamide seems to be a neurotransmitter in both the central and peripheral nervous system of Nereis virens, and may be involved in the control of esophageal motility.