Testosterone raises LHRH levels exclusively in the median eminence of castrated rats.

Although LHRH is widely distributed in the diencephalon, previous studies show that testosterone (T) treatment for 72-96 h of castrated male rats raised LHRH levels only in the medial basal hypothalamus. In the present study, LHRH concentrations were analyzed in microdissected brain regions shown to contain LHRH perikarya and their projections to identify the discrete regions which may display this T-dependent accumulation of luteinizing hormone-releasing hormone (LHRH) and to study the temporal sequence of LHRH changes in those regions. Rats were killed at 12-hour intervals from 60 to 96 h after subcutaneous implantation of Silastic capsules containing T. Results showed that the physiological range of serum T levels attained by these implants suppressed LH release at all times, however, there was no immediate effect on LHRH concentrations in any region. In fact, out of the 9 regions in the preoptic-tuberal pathway examined, 8 regions displayed no change in LHRH concentrations at any time. On the other hand, for up to 84 h, LHRH concentrations in the median eminence region remained unchanged; an additional 12 h of T exposure significantly raised LHRH levels. These results suggest that this T-dependent accumulation of LHRH may arise either by de novo synthesis or by increase in the rate of processing of the precursor LHRH protein into the immunoreactive form within nerve terminals in the median eminence.