Seasonal changes in luteinizing hormone-releasing hormone concentrations in microdissected brain regions of male rough-skinned newts ( Taricha granulosa)

Abstract

Luteinizing hormone-releasing hormone (LHRH) concentrations were measured in specific brain areas of male rough-skinned newts collected from a single population throughout the reproductive cycle. Plasma androgen and corticosterone (B) concentrations were also measured. Androgen concentrations were highest during the breeding season (winter) and lowest during the summer. In contrast, plasma B was lowest during the breeding season and highest in the summer. Concentrations of LHRH in the infundibulum (I) and rostral hypothalamus (RH) were positively correlated throughout the reproductive cycle; LHRH was always higher in the I than in the RH. Concentrations of LHRH in the ventral preoptic area (POA) fluctuated independently of concentrations of LHRH in the I and RH. However, LHRH concentrations decreased to undetectable levels in all three brain areas in March and April, which was before the end of the breeding season and before plasma androgen concentrations had decreased. An injection of LHRH into postbreeding males resulted in a significant increase in plasma androgen concentrations, indicating that the pituitary-gonad axis was still functional at the end of the breeding season. These results support the hypothesis that an abrupt decline in LHRH secretion is the initial endocrine event that signals the end of the breeding season in this amphibian.