Projections of luteinizing hormone-releasing hormone and vasotocin fibers to the anterior part of the preoptic nucleus in the toad, Bufo japonicus.

The anterior part of the preoptic nucleus (APON) is a crucial locus for triggering male mate calling behavior in anuran amphibia. The projections to this locus of luteinizing hormone-releasing hormone and vasotocin fibers were immunohistochemically demonstrated in the brain of the toad (Bufo japonicus) using the avidin-biotin-peroxidase complex method. Immunoreactive (ir)-LH-RH perikarya are localized in the nucleus medialis septi and the nucleus of the diagonal band of Broca. A part of ir-LH-RH fibers arising from these nuclei project to the lateral part of the APON, where the APON neurons form their dendritic field, and often protrude into its medial neuronal cell mass. Meanwhile, a considerable number of vasotocin fibers arising from the ventral magnocellular part of the preoptic nucleus project anterior to these loci. These observations indicate that the LH-RH and vasotocin fibers may form ordinary and/or en passant synapses with the APON neurons to transmit peptidergic neuronal signals which are concerned with initiation of seasonal reproductive behavior.