Distribution of gonadotropin‐releasinghormone immunoreactivity in the brain of the pacific hagfish, Eptatretus stouti (craniata: Myxinoidea)

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Abstract
The distribution of gonadotropin-releasing hormone (GnRH)-like immunoreactivity in the brain of a myxinoid, the Pacific hagfish (Eptatretus stouti), was investigated via immunohistochemistry, including the use of six different antisera. In the diencephalon, immunoreactive cell bodies were found in two systems: the infundibular hypothalamus, a neuromodulatory nucleus with diffuse projections of varicose fibers to most areas of the brain, and a primarily preoptic system of putatively hypophysiotropic neurons that projects to the neurohypophysis. Some potential neurovascular and CSF contacts were also identified. These findings are consistent with those of similar studies in other craniates and suggest that a preoptic hypophysiotropic system may be present in all craniates. We therefore tentatively accept the homology of this system in hagfish and vertebrates. The homology of the distributed hypothalamic system is more dubious. It may be homologous to a caudal GnRH system of modulatory neurons found in many vertebrates. Antiserum PBL-49 displays a differential affinity for the two systems, indicating that the two systems differ in the amount or identity of the immunoreactive substance. We suggest that the two systems have distinct functions in hagfish. The primitive function of GnRH-like molecules in craniates may have thus been both neuromodulatory and hypophysiotropic. These findings also indicate that the brain-pituitary axis of hagfish is more similar to that of vertebrates than has been previously suggested.