Axotomy of developing rat spinal motoneurons: cell survival, soma size, muscle recovery, and the influence of testosterone.

During the period of synapse elimination, motoneurons are impaired in their ability to generate or regenerate axonal branches: following partial denervation of their target muscle, young motoneurons do not sprout to nearby denervated fibers and after axonal injury, they fail to reinnervate the muscle. In the rat levator ani (LA) muscle, which is innervated by motoneurons in the spinal nucleus of the bulbocavernosus (SNB), synapse elimination ends relatively late in development and can be regulated by testosterone. We took advantage of this system to determine if the end of synapse elimination and the development of regenerative capabilities by motoneurons share a common mechanism, or, alternatively, if these two events can be dissociated in time. Axotomy on or before postnatal day 14 (P14) caused the death of SNB motoneurons. By P21, toward the end of synapse elimination in the LA muscle, SNB motoneurons had developed the ability to survive axonal injury. Altering testosterone levels by castration on P7 followed by 4 weeks of either testosterone propionate or control injections did not change the ability of SNB motoneurons to survive axonal injury during development, although these same treatments alter the time course of synapse elimination in the LA muscle. Thus, we dissociated the inability of SNB motoneurons to recover from axonal injury from their developmental elimination of synaptic terminals. We also measured the effect of early axotomy on motoneuronal soma size and on target muscle weight. Axotomy on P14 caused a long-lasting decrease in the soma size of surviving SNB motoneurons, whereas motoneurons axotomized on P28 recovered their normal soma size. Axotomy on or before P7 caused severe atrophy of the target muscles, matching the extensive loss of motoneurons. However, target muscle recovery after axotomy on P14 was as good as recovery after axotomy at later ages, despite greater motoneuronal death after axotomy on P14. This result may reflect an increase in motor unit size, a decrease in polyneuronal innervation by SNB motoneurons that survive axotomy on P14, or a combination of the two.