Blood-brain barrier for human growth hormone and insulin-like growth factor-I.

There is a blood-brain barrier (BBB) for GH. A certain, unknown amount of GH passes the BBB, acts on the neuronal GH receptors and directly influences the brain mechanisms serving the feedback and ultradian secretion of GH. The high density of GH receptors in the choroid plexus suggests a possible receptor-mediated transcytosis transport. The effects of GH on brain development, neuronal plasticity and neuroprotection seem to be mediated by IGFs. GH and IGFs are also synthesized in the brain. The relative contributions to brain functions of GHs produced inside and outside the BBB are unknown. The cerebrospinal fluid (CSF) space is the compartment inside the barrier accessible to clinicians. High GH levels in CSF were reported in acromegaly and also a small increase was reported after chronic administration of hGH in GH-deficiency syndromes. For the practitioner it is necessary to determine the normal range of hGH levels in CSF.