Extrahypothalamic projection of luteinizing hormone-releasing hormone fibers in the brain of the toad, Bufo japonicus.

Extrahypothalamic projection of luteinizing hormone-releasing hormone (LHRH) fibers in the brain of the toad (Bufo japonicus) was examined immunohistochemically by the avidin-biotin-peroxidase complex (ABC) method. Immunoreactive LHRH perikarya are localized in the nuclei medialis septi and of the diagonal band of Broca. A part of the LHRH fibers are sent anteriad to the medial and dorsal pallia. Some fibers reach the olfactory bulb. Dorsocaudally, LHRH neurons in the medial septum project their fibers to the deep layers of the optic tectum and the posterior mesencephalon including the nucleus pretrigeminalis, which is considered to be a generator of mate calling behavior, via the habenular and posterior thalamic regions. In addition, LHRH fibers which run caudad through the dorsal infundibular region and then the mesencephalic reticular formation were widely distributed in both the gray and the white matter of the medulla oblongata. These findings suggest that LHRH acts as a neurotransmitter or a neuromodulator in the various neuronal circuitries for reproductive behavior in the central nervous system, because LHRH has been considered to be related to amphibian seasonal breeding, and many regions where the immunoreactive LHRH fibers were observed are the loci concerned with mating behavior.