Ciliary neurotrophic factor receptor ␣ (CNTFR␣) is the ligandbinding component of the CNTF receptor. CNTFR␣ expression is essential for the normal development of spinal motoneurons and is required for the development of a sex difference in motoneuron number in androgen-sensitive perineal motoneurons. We used immunocytochemistry to examine the expression and hormone regulation of CNTFR␣ protein in the spinal nucleus of the bulbocavernosus (SNB), dorsolateral nucleus and retrodorsolateral nucleus

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... of the lower lumbar spinal cord of adult rats. CNTFR␣ immunoreactivity (CNTFR␣-IR) was observed in the somata and dendrites of virtually all motoneurons. In all three motor pools, the intensity of motoneuron soma labeling was greatest among gonadally intact males and was reduced in females and gonadectomized males. The density of CNTFR␣-IR in neuropil also tended to be highest in intact males. Short-term (2 d) testosterone propionate treatment reversed the decline in the density of soma labeling in the SNB of castrated males but did not reverse any other effects of castration. Long-term hormone treatment, achieved by implanting males with testosterone capsules at the time of gonadectomy, prevented the decline in soma labeling in all motor pools and partially prevented the decline in neuropil label caused by castration. We conclude that expression of CNTFR␣ protein is androgen-regulated in spinal motoneurons.