Hormonal Control of the Anatomical Specificity of Motoneuron-to-Muscle Innervation in Rats

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Abstract
Motoneurons of the spinal nucleus of the bulbocavernosus innervate bulbocavernosus muscles in male rats. Adult female rats normally lack both the spinal nucleus and its target muscles. Prenatal treatment of females with testosterone propionate resulted in adults having, like males, both the spinal nucleus and its target muscles. However, prenatal treatment with dihydrotestosterone propionate preserves the muscles but not the motoneurons. This paradoxical condition might result from (i) bulbocavernosus muscles without innervation; (ii) muscles innervated by morphologically unrecognizable motoneurons; (iii) muscles innervated by a very few spinal nucleus cells, each innervating many bulbocavernosus fibers; or (iv) muscles innervated by motoneurons outside their normal anatomical locus in the spinal nucleus. The results of retrograde marker injections into the bulbocavernosus muscles of females treated with androgen refute the first three possibilities and confirm the last: the different androgen treatments result in anatomically distinct spinal motor nuclei innervating bulbocavernosus muscles.