Immunocytochemical localization of gonadotropin-releasing hormones in the brain of a viviparous caecilian amphibian, Typhlonectes natans (Amphibia: Gymnophiona).

The molecular forms and brain distribution of gonadotropin-releasing hormone (GnRH) have been well studied in the amphibian orders Urodela (salamanders and newts) and Anura (frogs and toads). In the order Gymnophiona (caecilians), however, few species have been investigated. Antibodies against different molecular forms of GnRH were used to immunohistochemically localize the GnRH-containing neurons in the brain of the caecilian, Typhlonectes natans which differs from most other amphibians in that it is viviparous. An antibody selective for mammalian GnRH recognized cell bodies predominantly in the septo-preoptic area but only with occasional cell bodies in the lateral hypothalamus and ventral thalamic eminence. Thick, prominent fibers in the septal region and fibers within the terminal nerve were also labeled. An antibody selective for chicken-II GnRH labeled a population of cell bodies in the dorsal hypothalamus, ventral thalamus and midbrain tegmentum. Thin fibers projected laterally from these cells. An antibody specific for salmon GnRH did not label cell bodies but did show intense terminal field immunoreactivity. The brain of this caecilian, therefore, contains three antigenically distinct forms of GnRH. The mammalian and chicken-II GnRH peptides have been shown in other amphibians but the distribution of cells and fibers was unique in this caecilian. Copyright (R) 2000 S. Karger AG, Basel