Effects of sustained-release testosterone on marking behavior in the Mongolian gerbil.

Utilizing subcutaneous silastic implants containing testosterone the effect of sustained low-level release of testosterone on marking behavior in castrated Mongolian gerbils was determined. Castration resulted in a decrease in marking frequency from 38 +/- 4 to 4 +/- 1. Sustained release of 6 micrograms testosterone per day from implants did not stimulate marking behavior. Daily release of 11 micrograms increased marking 550% after one week. Release of 17 micrograms per day was required to restore marking to intact levels. Similar release rates of cholesterol did not increase marking. Release rates which increased marking in males did not increase marking in females. The minimum effective sustained-release dose of testosterone necessary to stimulate marking in females was five times greater than that in males. This difference is not due to a sex difference in plasma testosterone removal rates, since the circulating half-life of 3H-testosterone measured by testosterone-specific radioassay was not different in castrated males (72 +/- 5 min) and females (66 +/- 4 min). The present data are consistent with the hypothesis that there is a sexual dimorphism in the testosterone responsiveness of the neural substrate regulating territorial marking behavior.