Immunocytochemical studies of luteinizing hormone-releasing hormone in brains of agnathan fishes

An immunocytochemical unlabeled antibody enzyme technique has been applied to localize mammalian-like immunoreactive luteinizing hormone-releasing hormone (ir-LH-RH) in central nervous tissue of Pacific lamprey (Entosphenus tridentata; anadromous spawning migrants) and of Pacific hagfish (Eptatretus stouti, sexually undifferentiated and mature). Specific immunocytochemical staining was located over perikarya in the preoptic nucleus of lampreys; beaded axons containing herring body-like dilatations were stained and appeared to pass ventro-posteriorly into the infundibulum where presumed nerve endings were observed in the neurohypophysis. Cell bodies exhibiting ir-LH-RH were distinct from those, presumably vasotocinergic ones, which stained with aldehyde fuchsin. Specific staining was absent in other regions of the lamprey brain, in control preparations, and in any part of the hagfish central nervous tissues. The possible hypophysiotropic actions of ir-LH-RH in the lamprey preoptico-neurohypophysial system are unclear because no anatomical relationship is evident which might provide for transfer of hypothalamic influences to the pars distalis. However, detection of mammalian-like ir-LH-RH in brain of this primitive agnathan fish indicates that this peptide may be of great evolutionary antiquity.