Hormonal control of neuron number in sexually dimorphic spinal nuclei of the rat: III. Differential effects of the androgen dihydrotestosterone.

The spinal cord of the rat contains two sexually dimorphic nuclei: the spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN). These nuclei and the perineal muscles they innervate are present in males but reduced or absent in females. The sex difference in motoneuron number in these nuclei is due to an androgen-regulated motoneuron death. Developing females treated with the androgen testosterone propionate (TP) have a fully masculine number of SNB and DLN motoneurons and retain the perineal muscles they would normally have lost. Paradoxically, females treated prenatally with the androgen dihydrotestosterone propionate (DHTP) also retain the perineal musculature but as adults lack the SNB motoneurons which would normally innervate them. The SNB target muscles retained by DHTP females are anomalously innervated by motoneurons in the DLN. Counts of motoneurons and degenerating cells in the developing SNB of DHTP-treated females showed that their feminine number is the result of a failure of DHTP to prevent the death of SNB motoneurons. Furthermore, the peak number of SNB motoneurons was below that of normal females, suggesting that DHTP treatment may also have inhibited motoneuronal migration. However, DHTP treatment fully masculinized both motoneuron number and degenerating cell counts in the DLN of these females, and it is this masculinized DLN that gives rise to the anomalous projection. Taken together, these results suggest that the effects of different androgens during development are specific and complex, involving the regulation of motoneuron death, migration, and specification of peripheral projections.