Sex differences in the pattern of steroid accumulation by motoneurons of the rat lumbar spinal cord.

We previously reported that male rat lumbar motoneurons accumulate radioactivity after injection of tritiated testosterone (T) or dihydrotestosterone (DHT), but not estradiol (E). We now report autoradiographic evidence that lumbar motoneurons in male rats accumulate T or its metabolites more frequently than do female motoneurons. However, this sex difference in hormone accumulation by motoneurons is not observed following the injection of DHT or E, both of which are normal metabolites of T. Very few motoneurons from any population accumulated E or its metabolites. In fact, some motoneurons had fewer silver grains over their nuclei than would be expected by chance, thus suggesting that these cells not only failed to accumulate E or its metabolites, but actively or passively excluded them from their nuclei. Virtually every motoneuron of either sex accumulated hormone after DHT injection. Following T injection, more motoneurons were labeled in males (72-77%) than in females (28-39%), indicating a sex difference in hormone accumulation. Taken together, these results suggest that either (1) there are separate receptors for T and DHT, and motoneurons have more T receptors (but equal numbers of DHT receptors) in males than in females, or (2) the motoneurons of male rats have greater access to systemically administered T or its metabolites than do those of females.