Differences in responsiveness to testosterone of penile reflexes and copulatory behavior of male rats

The display of penile reflexes and copulatory behavior appears to reflect the activity of two different underlying neuronal systems, both of which are modulated by systemic testosterone (T) concentration. To indirectly compare the two systems, the responsiveness to T of penile reflexes and copulatory behavior was examined. In the first experiment castrated spinal male rats were given penile reflex tests while receiving replacement T through Silastic capsule implants filled with T (50 mm T). After capsule removal the number of penile erections and flips declined within 24 hr and gradually decreased for 12 days. Subjects were then reimplanted with new 50-mm T capsules. The number of penile flips and erections increased within 6 and 12 hr, respectively. This is a much more rapid response rate to T than has been established for copulatory behavior. In the second experiment castrated spinal male rats were tested for penile reflexes with a 50-mm T capsule, which was then replaced with a 10-, 5-, or 2-mm T or an empty capsule. The number of penile reflexes declined in a dose-response fashion. In the third experiment, castrated sexually experienced male rats were tested for copulatory behavior with two 25-mm T capsules which were then replaced with a 10 or 2-mm T or an empty capsule. Only males with empty capsules had decrements in copulatory behavior, revealing that a low level of T can maintain virtually normal sexual behavior despite a marked decline in penile reflex activity. The neuronal system underlying penile reflexes (spinal neurons) is apparently much more responsive to changes in T concentrations than the neuronal system underlying motivational and appetitive aspects of copulatory behavior (brain neurons).