Immunohistochemical localization of a GnRH-like peptide in the brain of the cephalopod spear-squid, Loligo bleekeri

We examined whether a gonadotropin-releasing hormone (GnRH)-like peptide exists in the brain of the cephalopod spear-squid, Loligo bleekeri, by performing a time-resolved fluoroimmunoassay and immunohistochemistry. The displacement curve obtained for serially diluted extracts of the spear-squid brain paralleled the chicken GnRH-II (cGnRH-II) standard curve, indicating the existence of a cGnRH-II-like peptide in the brain. For immunohistochemistry, a mouse monoclonal antibody raised against the common amino acid sequence of GnRH (LRH13) and a rabbit polyclonal antibody raised against cGnRH-II were used. GnRH-like-immunoreactive (ir) cell bodies (that reacted with LRH13) were mainly detected in the central part of the ventral magnocellular lobe (vmL), and a few cell bodies were also detected in the olfactory lobe and palliovisceral lobe (pvL). Bundles of GnRH-like-ir axons were observed running from the vmL to the internal brain regions. GnRH-like-ir fibers were widely distributed in almost all the brain regions. cGnRH-II-ir cell bodies were localized in the optic gland, outer region of the vmL, and pvL. Further, cGnRH-II-ir fibers were distributed in the wide areas of the brain. These results suggest that at least two forms of GnRH-like peptidergic neuronal systems exist in the spear-squid brain.