Different modes of gonadotropin-releasing hormone (GnRH) release from multiple GnRH systems as revealed by radioimmunoassay using brain slices of a teleost, the dwarf gourami (Colisa lalia).

It has become a general notion that there are multiple GnRH systems in the vertebrate brains. To measure GnRH release activities from different GnRH systems, we conducted a static incubation of brain-pituitary slices under various conditions, and GnRH released into the incubation medium was measured by RIA. The slices were divided into two parts, one containing GnRH neurons in the preoptic area and axon terminals in the pituitary (POA-GnRH slices), and the other containing the cell bodies and fibers of terminal nerve-GnRH neurons and midbrain tegmentum-GnRH neurons (TN-TEG-GnRH slices). We demonstrated that GnRH release was evoked by high [K(+)](o) depolarizing stimuli (in both POA-GnRH and TN-TEG-GnRH slices) via Ca(2+) influx through voltage-gated Ca(2+) channels. The most prominent result was the presence of conspicuous sexual difference in the amount of GnRH release in the POA-GnRH slices. The GnRH release from TN-TEG-GnRH slices also showed a small sexual difference, which was by far more inconspicuous than that of POA-GnRH slices. Immunohistochemical analysis using an antiserum specific to the seabream GnRH (sbGnRH; suggested to be specific to POA-GnRH neurons) revealed the presence of a much larger number of POA-GnRH neurons in males than in females. This clear morphological sexual difference is suggested to underlie that of GnRH release in the POA-GnRH slices.