Immunoreactive gonadotropin-releasing hormone (GnRH) in the brain and pituitary of adult and juvenile swordtails (Xiphophorus helleri, Teleostei, Poeciliidae)

Different molecular variants of gonadotropin‐releasing hormone (GnRH) were localized in the brain and pituitary of Xiphophorus helleri, from neonates up to mature animals of both sexes. Nine GnRH antisera to salmon (s), mammalian (m), chicken I (c‐I), and chicken II (c‐II) GnRH were utilized. In the first week after birth GnRH immunoreactivity (IR) emerges with pale staining of the nucleus olfactoretinalis (NOR) in the ventral forebrain. The intensity of IR in the NOR increases during the next weeks and an IR tract of nerve fibers appears, protruding from the NOR in dorsocaudal direction. Adult animals exhibit additional GnRH‐positive structures. Some perikarya of the nucleus preopticus periventricularis (NPP) are IR and positive fibers extend from the NPP toward the pituitary. In the pituitary IR fibers are also detectable. A distinctive structure in adult animals is an IR cord of neurons (CN) at the bottom of the forebrain which extends from the NPP to the olfactory nerve. A comparison of antisera against different GnRH species indicates that sGnRH is present in the NOR, whereas a different form of GnRH is present in the NPP, CN, and pituitary. The early onset of GnRH IR in the NOR and the widespread distribution of positive fibers from that nucleus into other brain regions suggest neuromodulatory functions of sGnRH from the NOR. The NPP possibly plays a major role in direct stimulation of pituitary gonadotropes via a different type of GnRH. © 1996 Wiley‐Liss, Inc.