Effects of gonadal steroids on the input-output relationship of the corticospinal pathway in humans

Most of our knowledge about gonadal steroid effects on the nervous system come from studies of limbic structures, while virtually nothing is known about the action of these hormones on the motor system. We carried out experiments on six healthy volunteer males to determine the threshold and gain of the input-output relationship (i.e. stimulus intensity vs. response size) of the corticospinal motoneurones in relation to the transient increase of gonadal steroids induced by a single intramuscular injection of human chorionic gonadotropin (hCG). The motor cortex was excited by focal transcranial magnetic stimuli and the evoked responses from the first dorsal interosseous (FDI) were recorded with surface electromyographic electrodes. In some experiments, the threshold and recruitment curve of the soleus H reflex were determined to assess the gonadal steroid effects on spinal motoneurones. All the subjects showed a significant decrease of cortical motor threshold concomitant with a significant increase of testosterone and oestradiol plasma concentrations. By contrast, there was no significant change of the slope of the curve expressing the relationship between the intensity of magnetic stimulation and the size of the muscle response. Both the threshold and slope of the H reflex were unaffected by the gonadal steroid levels. Latencies of the maximal FDI response to cortical and cervical magnetic stimulation were also unmodified. We conclude that gonadal steroids are involved in regulating the threshold of corticospinal motoneurones in humans. Our observations confirm that the threshold and gain of corticospinal motoneurones are determined by different neural mechanisms.