tailed receptors in primates both a tail less and a tailed receptor have been identified these multiple receptor systems not only suggest a complex integration for proper gnrh signaling but offer more importance to the presence or absence of the c terminal tail in this integration applicants have previously characterized two high affinity gnrh binding sites in the lamprey pituitary suggesting the presence of at least two functional gnrh receptors in the lamprey pituitary lampreys along with hagfish represent the only surviving members of the class agnathans the oldest extant vertebrates this ancient lineage diverged from the main vertebrate line over 550 million years ago lampreys are important to the understanding of the reproductive success of the first vertebrates and are likely to have retained the key characteristics of the ancestral gnrh and gnrh receptor from which modern gnrh isoforms and gnrh receptors arose two gnrh isoforms have been identified in the brain of the lamprey these two gnrh isoforms are the only vertebrate forms that vary in the sixth position and lamprey gnrh iii is the only isoform known to vary in the third position containing a tyr instead of the characteristic trp applicants 39 previous work has included physiological.