An immunohistochemical study of seasonal changes in luteinizing hormone-releasing hormone and vasotocin in the forebrain and the neurohypophysis of the toad, Bufo japonicus

Abstract

Seasonal changes in luteinizing hormone-releasing hormone (LH-RH) and arginine vasotocin (AVT) were examined immunohistochemically in the toad forebrains and neurohypophyses. Strongly immunoreactive (ir-) LH-RH perikarya, from which dense ir-LH-RH fibers project to the median eminence, were localized in the medial septal nucleus and the nucleus of the diagonal band of Broca in the animals captured in the spring and the autumn. While, in the animals collected in the summer, ir-LH-RH perikarya and fibers were sparse, and immunoreactivity in the median eminences was weak. Artificially induced hibernation decreased the density of ir-LH-RH in the median eminence, in contrast with strong immunoreactivity in the control animals kept at room temperature. The amounts of ir-LH-RH in the median eminences of hibernating toads which were captured shortly before the breeding period varied conspicuously among individuals. The median eminences in migrating toads showed relatively weak LH-RH immunoreactivity. After the breeding, the immunoreactivity returned to the strong level that was observed in the spring and the autumn. These seasonal changes in ir-LH-RH seem to correspond to seasonal reproductive activity in this species. However, significant seasonal variations were not found in ir-AVT.