Antibodies against different forms of GnRH distinguish different populations of cells and axonal pathways in a urodele amphibian, Taricha granulosa

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Abstract
Neurons immunoreactive to the peptide hormone gonadotropin‐releasing hormone (GnRH) have been identified in the posterior diencephalon or anterior midbrain of diverse vertebrates. These cells are located caudal to the more well‐characterized GnRH neurons in the nervus terminalis and septo‐preoptic area, and are believed to express one or more of the nonmammalian forms of the GnRH. The present study utilized immunocytochemical techniques to determine whether the posterior GnRH group is present in a urodele amphibian, the newt Taricha granulosa. Antibodies directed against different molecular forms of GnRH were used to evaluate the immunological properties of GnRH‐containing neurons in amphibians. An antibody selective for mammalian GnRH labeled perikarya in the nervus terminalis (terminal nerve) and septo‐preoptic region, as described previously. Thick fibers that arise from terminal nerve and septo‐preoptic neurons project mainly to the median eminence, medial pallium and habenula. An antibody selective for chicken GnRH II labeled cell bodies in the paraventricular organ and posterior tubercle of the caudal diencephalon, and thin fibers that project widely throughout the central nervous system. Region‐specific staining with different GnRH antibodies supports the interpretation that different molecular forms of GnRH are expressed by neuroanatomically distinguishable systems.