Sensory pathways linking social and environmental cues to endocrine control regions of amphibian forebrains.

The secretion of gonadotropin releasing hormone (GnRH), and hence sex steroids, is influenced by social signals produced by conspecifics and by environmental cues such as day length. The sensory systems processing these varied signals must therefore connect with the GnRH control centers in the preoptic area and hypothalamus. Forebrain pathways in frogs provide a model for how vertebrates accomplish this. Auditory information, which can transmit the features of the vocal communication signals used by frogs during reproductive social behavior, reaches both the preoptic area and ventral hypothalamus via thalamic and midbrain nuclei. An examination of these auditory pathways suggests that there are subtle differences in their anatomical, physiological, and histochemical characteristics. The suprachiasmatic nucleus, which is important for controlling circadian rhythms in vertebrates, also projects to both the preoptic area and ventral hypothalamus, providing information about environmental characteristics in parallel with the information about social signals. This pattern of connections suggests that the two major GnRH control centers in amphibian brains receive independent, parallel information about the social and environmental cues influencing reproduction.