E. J. Nye

The Insulin Hypoglycemia Test: Hypoglycemic Criteria and Reproducibility

The insulin hypoglycemia test (IHT) is widely regarded as the ‘gold standard’ for dynamic stimulation of the hypothalamic‐pituitary‐adrenal (HPA) axis. This study aimed to investigate the temporal relationship between a rapid decrease in plasma glucose and the corresponding rise in plasma adenocorticotropic hormone (ACTH), and to assess the reproducibility of hormone responses to hypoglycemia in normal humans. Ten normal subjects underwent IHTs, using an insulin dose of 0.15 U/kg. Of these, eight had a second IHT (IHT2) and three went on to a third test (IHT3). Plasma ACTH and cortisol were measured at 15‐min intervals and, additionally, in four IHT2s and the three IHT3s, ACTH was measured at 2.5‐ or 5‐min intervals. Mean glucose nadirs and mean ACTH and cortisol responses were not significantly different between IHT1, IHT2 and IHT3. Combined data from all 21 tests showed the magnitude of the cortisol responses, but not the ACTH responses, correlated significantly with the depth and duration of hypoglycemia. All subjects achieved glucose concentrations of of ≤ 1.6 mmol/l before any detectable rise in ACTH occurred. In the seven tests performed with frequent sampling, an ACTH rise never preceeded the glucose nadir, but occurred at the nadir, or up to 15 min after. On repeat testing, peak ACTH levels varied markedly within individuals, whereas peak cortisol levels were more reproducible (mean coefficient of variation 7%). In conclusion, hypoglycemia of ≤ 1.6 mmol/l was sufficient to cause stimulation of the HPA axis in all 21 IHTs conducted in normal subjects. Nonetheless, our data cannot reveal whether higher glucose nadirs would stimulate increased HPA axis activity in all subjects. Overall, the cortisol response to hypoglycemia is more reproducible than the ACTH response but, in an individual subject, the difference in peak cortisol between two IHTs may exceed 100 nmol/l.

The use of naloxone for investigating disorders of the hypothalamic-pituitary-adrenal axis

The hypothalamic-pituitary-adrenal (HPA) axis forms an integral and vital part of the stress response system. Pituitary ACTH secretion is regulated by two hypothalamic peptides, corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP), with CRH being the primary mediator in humans. Numerous neuronal circuits within the brain influence the activity of the CRH neurons in the paraventricular nuclei of the hypothalamus, including endogenous opioid systems. Opioid receptors are widely distributed in the brain but have a particular association with three major neural networks: the sensory, limbic and neuroendocrine systems. Opioidergic agents affect HPA activity in humans and animals, al though important species differences exist. In vitro and in vivo studies provide considerable evidence for a centrally mediated effect of opioidergic agents on the HPA axis, most likely by direct action on the hypothalamus. Naloxone is a competitive antagonist at multiple opioid receptor subtypes. Clinical investigations utilizing naloxone administration in humans have had three primary aims:1) to elucidate the role of endogenous opioid systems in the normal physiology of the HPA axis; 2) to study the pathophysiology of disorders associated with or caused by dysfunction of the HPA axis; 3) to develop new diagnostic tests. High doses of naloxone stimulate the human HPA ards causing rises in plasma ACTH and cortisol concentrations. Pharmacological stud ies strongly suggest that naloxone blocks a tonic inhibitory effect of endogenous opioids on central alpha-adrenergic pathways, which in turn stimulate ACTH secretion via CRH release from the hypothalamus. Therefore, the naloxone test provides a means of evaluating hypothalamic CRH reserve and assesses the integrity of the entire HPA axis. Several disorders are associated with dysregulation of the HPA axis including major depression, post-traumatic stress disorder, alcoholism, chronic fatigue syndrome, Cushings syndrome, and obesity. Major depression is a condition often associated with clinical and biochemical evidence of hypercortisolism (pseudo-Cushings syndrome) and may be very difficult to distinguish from patients with true Cushings syndrome. Patients with pseudo-Cushings syndrome are postulated to have hypersecretion of hypothalamic CRH producing an upregulation of an otherwise normal HPA axis. Patients with major depression have an ACTH hyperresponse to naloxone compared with healthy subjects and patients with Cushings disease. This increased ACTH response is further exaggerated by combining naloxone administration with the direct corticotrope stimulation provided by exogenous AVP. The naloxone test also has potential diagnostic utility-when central adrenal insufficiency is suspected. Naloxone applies a specific pharmacological stimulus at the hypothalamic level, therefore a normal ACTH and cortisol response implies functional integrity of all three components of the HPA axis. Preliminary studies comparing naloxone administration with the insulin hypoglycemia and metyrapone tests are promising. The naloxone test may provide an alternative in patients with suspected central adrenal insufficiency who are unable to undergo an insulin hypoglycemia test or a metyrapone test because of safety issues.