Gonadotropin-releasing hormone (GnRH) pathways and reproductive control in elasmobranchs

Synopsis Immunoreactive (ir) gonadotropin-releasing hormone (GnRH) is localized in many neurons of the terminal nerve (TN) and midbrain tegmentum, while few ir-cells are observed in the preoptic area and ventral hypothalamus. The paucity of preoptic ir-cells may relate to an unusual feature of the elasmobranch pituitary, i.e. a lack of portal control of gonadotropin-producing cells. TN and midbrain GnRH-ir neurons may be major sources of GnRH used to modulate or otherwise control both pituitary and brain cells via delivery through the systemic circulation. These ir-nuclei also appear to directly innervate CNS regions (the preoptic area, habenula and clasper control area of the spinal cord) involved in sexual functions. Important regulatory mechanisms, represented by interactions between GnRH pathways and sex-steroid concentrating neurons, are likely to occur in the preoptic area, habenula and midbrain tegmentum.