Evidence for regulatory mechanisms maintaining testosterone-dependent AVP concentrations in terminal regions

The dose and time relationships of testosterone treatment on arginine-vasopressin (AVP) concentrations of steroid-dependent vasopressinergic extrahypothalamic neurons were studied in the castrated male rat. The rate of AVP increase and the extent and limits of accumulation of AVP in the terminal regions were explored. Testosterone propionate (TP) treatment (0.25, 0.50, and 1.00 mg/kg SC given three times a week) started 15 weeks after castration. The periods of treatment lasted for 20, 40, and 80 days. AVP concentrations in the lateral septum (LS) and the lateral habenula (LH), determined by radioimmunoassay, increased dose and time dependently. A significant effect was achieved by 0.25 mg/kg and a maximum level obtained by 0.50 mg/kg after a 40-day period of treatment. The maximum concentration reached corresponded to the level of the intact animal. Increasing the dose or period of treatment did not raise the AVP concentration above that particular level. The data suggest regulatory mechanisms that maintain the AVP concentration in the terminal regions at a precastrational level.