Neurotransmitter control of growth hormone secretion in children after cranial radiation therapy

Cranial radiation for childhood cancer can cause growth hormone deficiency (GHD), usually due to hypothalamic rather than pituitary dysfunction. To investigate whether this hypothalamic dysfunction is secondary to altered neurotransmitter input from other brain centers, we used neurotransmitter-excitatory substances to study the GH secretory response in 17 children who had received 12 to 60 Grey (Gy) to the cranium and 40 short children with normal endocrine function. As expected, the irradiated children had decreased mean GH secretion in response to insulin-induced hypoglycemia and arginine infusion, and decreased mean 24 hour GH concentrations, compared to the control group. In contrast, the two groups had similar GH secretory responses to GHRH stimulation and somatostatin suppression. Assessment of neurotransmitter pathways in the irradiated children revealed significantly lower mean peak GH concentrations in response to 5 of the 6 substances tested compared to control children: alpha-adrenergic stimulation (clonidine), beta-adrenergic blockade (propranolol), cholinergic stimulation, dopaminergic stimulation (L-dopa), and GABA-ergic stimulation (valproic acid). Results of serotonergic stimulation (L-tryptophan) were not statistically significant. Eleven patients who had abnormal GH secretion underwent 4 or more tests with neurotransmitter-stimulatory agents; 3 patients had peak GH concentrations of < 2.5 micrograms/l to all tests, whereas 4 patients had a peak GH concentration of > or = 7 micrograms/l to one or more tests but < 5 micrograms/l to one or more other tests. These observations suggest that radiation damage may sometimes spare growth hormone-releasing hormone (GHRH) and somatostatin secretion while affecting neurotransmitter pathways. We postulate that the hierarchy of sensitivity to radiation damage may be hypothalamic and extra-hypothalamic neurotransmitters > hypothalamic GHRH and/or somatostatin secretion > pituitary GH secretion.