Absence of detectable melatonin and preservation of cortisol and thyrotropin rhythms in tetraplegia

The human circadian timing system regulates the temporal organization of se veral endocrine functions, including the production of melatonin (via a neu ral pathway that includes the spinal cord), TSH, and cortisol. In traumatic spinal cord injury, afferent and efferent circuits that influence the basa l production of these hormones may be disrupted. We studied five subjects w ith chronic spinal cord injury (three tetraplegic and two paraplegic, all n eurologically complete injuries) under stringent conditions in which the un derlying circadian rhythmicity of these hormones could be examined. Melaton in production was absent in the three tetraplegic subjects with injury to t heir lower cervical spinal cord and was of normal amplitude and timing in t he two paraplegic subjects with injury to their upper thoracic spinal cord. The amplitude and the timing of TSH and cortisol rhythms were robust in th e paraplegics and in the tetraplegics. Our results indicate that neurologic ally complete cervical spinal injury results in the complete loss of pineal melatonin production and that neither the loss of melatonin nor the loss o f spinal afferent information disrupts the rhythmicity of cortisol or TSH s ecretion.