Relaxin inhibits central angiotensin II expression in killifish: a central osmoregulatory role for relaxin and angiotensin II in the killifish Fundulus heteroclitus.

We investigated whether relaxin and brain angiotensin II (AII) are osmoregulatory hormones in killifish. Four groups of killifish acclimatized to different saline environments (freshwater, 25% sea water, 50% sea water [brackish], and sea water) were euthanized and their brains were embedded in paraffin. A separate group of brackish water killifish were injected with relaxin 1 h before dissection and embedding. Serial sagittal sections were immunohistochemically stained for AII. Angiotensin II immunostaining did not differ between saline environments. Contrary to expectation, relaxin decreased the number of AII-immunoreactive cells in hypothalami of brackish water fish.