Comparative Aspects of Luteinizing Hormone-Releasing Hormone Structure and Function in Vertebrate Phylogeny*

Abstract

Immunoreactive LHRH was detected in hypothalamic and extrahypothalamic brain extracts of the rat, bird (pigeon and chicken), reptile (tortoise and lizard), amphibian (frog and toad), teleost (cichlid), and elasmobranch (dogfish) and in the whole brain of the cyclostome (hagfish). The concentration of hypothalamic immunoreactive LHRH was more than 5-fold greater than that of the extrahypothalamic brain. Mammalian and amphibian hypothalamic immunoreactive LHRH yielded displacement curves parallel to those of synthetic LHRH in assays employing four antisera which recognize different regions of the decapeptide, thus suggesting a similarity in the structure of their LHRH. Hypothalamic immunoreactive LHRH from the bird, reptile, teleost, and elasmobranch differed from the mammalian and amphibian peptide in yielding displacement curves nonparallel to those of synthetic LHRH with three different antisera (1076, 743, and 744) which bind between Trp³ and Pro⁹ of LHRH. The difference in structure appears to be near Leu⁷. With antiserum 422 which binds the NH₂- and CO₂Htermini of LHRH, bird, reptile, and teleost hypothalamic extracts yielded displacement curves parallel to that of synthetic LHRH. Bird, reptile, and teleost hypothalamic extracts showed displacement curves parallel to each other in all assays. In studies on the relative quantitation of LHRH, all four antisera gave similar values of immunoreactive LHRH concentration in mammalian hypothalamic extracts and in amphibian hypothalamic extracts. By contrast, assay of bird, reptile, teleost, and elasmobranch hypothalamic extracts with antiserum 422 gave much higher values than did the other antisera, suggesting that the LHRH peptide is structurally different from mammalian and amphibian LHRH in the region of Leu⁷ but similar at the NH₂- and CO₂H-termini. These conclusions are supported by studies on the biological activity of hypothalamic LHRH from the different species using dispersed ovine anterior pituitary cells in culture. The LH release responses to equivalent amounts of immunoreactive LHRH (as measured by antiserum 422 which binds a region of LHRH essential for biological activity) from the various species were similar, indicating that the biologically active region of the molecule has been conserved in evolution. Structural differences in vertebrate hypothalamic immunoreactive LHRH were confirmed by cation exchange and high pressure liquid chromatography. Our findings of differences and similarities in vertebrate LHRH support a contemporary phylogenetic scheme. (Endocrinology106: 707, 1980)