Co-localization of three gonadotropin-releasing hormone transcripts in larval, parasitic, and adult sea lamprey brains

RNA expression of lamprey gonadotropin-releasing hormone (lGnRH)-I, -II, and -Ill was demonstrated in the brains of larval, parasitic phase and adult sea lampreys, Petromyzon marinus, using a highly sensitive triple-label in situ hybridization technique. In female larval lampreys, IGnRH-I and -II were co-expressed in the same neurons throughout the olfactory bulbs, preoptic area (POA), and rhombencephalon (hind brain); lGnRH-I, -II and -III were triple co-expressed in the hypothalamus and in the paranuclear region of neuronal somas in the rhombencephalon. In female parasitic phase lampreys, IGnRH-I and -II were co-expressed in the POA, thalamus, and preoptico-neurohypophyseal tract (PNT); lGnRH-III was minimally triple co-expressed with IGnRH-I and -II in the hypothalamus. In adult female lampreys, IGnRH-I and -III were co-expressed in the hypothalamus; IGnRH-I was also expressed in the neurohypophysis (NH). In adult male lampreys, lGnRH-I and -IIII were co-expressed in the primordial hippocampus, POA, thalamus, hypothalamus, NH, and PNT; lGnRH-I was also expressed in the epithalamus. In summary, we provide the first study using in situ hybridization of all three IGnRHs lGnRH-I, -II, and -III) at three major life stages (larval, parasitic, and adult) of lampreys, which strongly supports previous immunohistological studies and suggests that IGnRH-I and -II are the predominant IGnRHs in larval and parasitic phase lampreys, and that lGnRH-I and -III are the predominant lGnRHs in adult female and male lampreys. Therefore, our results show that IGnRH-I, -II, and -III have different localization and co-expression in the development and sexual maturation of lampreys, which may suggest unique physiological roles at each life stage and sex in the developing and mature lamprey brain. (C) 2017 Elsevier Inc. All rights reserved.