GONADOTROPIN-RELEASING-HORMONE (GNRH) PATHWAYS AND REPRODUCTIVE CONTROL IN ELASMOBRANCHS

Immunoreactive (ir) gonadotropin-releasing hormone (GnRH) is localized in many neurons of the terminal nerve (TN) and midbrain tegmentum, wh ile few ir-cells are observed in the preoptic area and ventral hypotha lamus. The paucity of preoptic ir-cells may relate to an unusual featu re of the elasmobranch pituitary, i.e. a lack of portal control of gon adotropin-producing cells. TN and midbrain GnRH-ir neurons may be majo r 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 i n sexual functions. Important regulatory mechanisms, represented by in teractions between GnRH pathways and sex-steroid concentrating neurons , are likely to occur in the preoptic area, habenula and midbrain tegm entum.