Histochemistry 69, 61 68 (1980) Histochemistry 9 by Springer-Verlag 1980 Gastrin/CCK-Like Immunoreactivity in the Nervous System of Coelenterates C.J.P. Grimmelikhuijzen 1, 2, ,, F. Sundler 1 and J.F. Rehfeld 3 Department of Histology, University of Lund, Lund, Sweden, 2 Max-Planck-Institutffir medizinischeForschung, D-6900 Heidelberg, Federal Republic of Germany, and 3 Institute of Medical Biochemistry,Universityof Aarhus, Aarhus, Denmark Summary. Using immunocytochemistry, gastrin/CCK-like immunoreactivity is found in sensory nerve cells in the ectoderm of the mouth region of hydra and in nerve cells in the endoderm of all body regions of the sea anemone tealia. These results are corroborated by radioimmunoassay: One hydra contains at least 5 fmole and one tealia at least 2 nmole gastrin/CCK- like immunoreactivity. Reactivities towards gastrin and CCK antisera with different specificities suggest that the coelenterate gastrin/CCK-like peptide contains the C-terminal amino-acid sequence common to mammalian gastrin and CCK. In addition the radioimmunochemical data indicate that the coe- lenterate peptide also contains an amino-acid sequence that resembles the sequence 20-30 of porcine CCK-33, but that no other sequences of gastrin are present. Thus, it is probably more CCK-like than gastrin-like. Introduction Hydra is one of the simplest multicellular organisms. Its body wall consists of only two cell layers, ectoderm and endoderm which are separated by an acellular layer called mesoglea. Hydra has the most primitive nervous system in the animal kingdom. The nerve cells form a continuous network, which is densest in the head and in the foot region (Hadii 1909). The nerve cells seem to be "multifunctional" because they all possess a cilium, form synaptic junctions with nerve cells, nematocytes and epitheliomuscular cells, and have neurosecretory granules (Westfall 1973 a, b; Westfall and Kinnamon 1978). The neurosecretory granules are similar in size range and morphology to those present in peptide containing nerves of higher animals (cf. Baumgarten et al. 1970). Reports have appeared on the possible presence of substance P in coelente- rates (Taban and Cathieni 1979; Creagh etal. 1980). The presence of neuro- peptides in coelenterates is interesting, not only from an evolutionary point * Permanent address: Max-Planck-Institut ffir medizinische Forschung, Abteilung Biophysik, Jahnstrasse 29, D-6900 Heidelberg, Federal Republic of Germany 0301-5564/80/0069/0061/$01.60