Neuronal systems immunoreactive with antiserum to lamprey gonadotropin-releasing hormone in the brain of Petromyzon marinus

The role of gonadotropin-releasing hormone (GnRH) in mammalian reproduction has been studied extensively; however, the role of a structurally different, but related, decapeptide is not well characterized in the most primitive class of vertebrates, Agnatha. Utilizing an antiserum directed to the recently characterized lamprey GnRH, we examined immunoreactive neuronal perikarya and nerve fibers in sections from the brain of the sea lamprey, Petromyzon marinus, using the unlabeled peroxidase-antiperoxidase method. Neuronal perikarya and fibers were immunopositive with antisera generated to lamprey GnRH and also to certain antisera generated to mammalian GnRH. Immunopositive neuronal perikarya were detected in an arc-shaped population extending from ventral to dorsal preoptic areas. Fibers from these cells projected to the neurohypophysis via the preoptico-hypophyseal tract, but in addition also protruded into the third ventricle. Additionally, some fibers coursed along the external surface of the brain, and may also release GnRH into meningeal compartments. The presence of fully processed, mature decapeptide is indicated within neuronal perikarya, as well as in projecting nerve fibers and terminals. No reaction product was detected in sections incubated with an antiserum to the interior amino acid sequences of mammalian LHRH. This finding supports the structure reported for lamprey GnRH by Sherwood et al. (1986).