Androgen stimulates neuronal plasticity in the perineal motoneurons of aged male rats

Ten aged male rats (24 months of age) were castrated and implanted subcutan eously with Silastic capsules containing testosterone (T)(5 males) or nothi ng (5 males). Five sham-castrated males (25 months of age) served as contro ls. Four weeks after castration, cholera toxin-horseradish peroxidase (CT-H RP) was injected into the bulbocavernosus muscles and animals were killed 2 days later. The spinal cords containing the spinal nucleus of the bulbocav ernosus (SNB) were dissected, processed with a modified tetramethylbenzidin e method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 150 CT-HR P-labeled SNB motoneurons were analyzed by measuring the percentage of soma tic membranes covered by synaptic contacts, synaptoid contacts, and neuron- neuron contacts. Most of the neuronal structures in the control and experim ental SNB motoneurons consisted of synaptic contacts. The mean percentage o f somatic membranes covered by synapses in castrated, aged males treated wi th T was significantly greater than that in control or castrated animals. S ize and number of synaptic contacts per unit length of somatic membranes in castrated, aged males treated with T were also significantly greater than those in control or castrated animals. Plasma levels of T in castrated, age d males treated with T were significantly greater than that in controls. Th ese results suggest that the SNB motoneurons of aged male rats retain a con siderable synaptic plasticity in response to androgen, and that androgen ma y be, at least in part, involved in the process of aging of the SNB system in male rats. J. Comp. Neurol. 430: 389-395, 2001. (C) 2001 Wiley-Liss, Inc .