Diurnal effects of fluoxetine and naloxone on the human hypothalamic-pituitary-adrenal axis.

1. Central serotonergic pathways are hypothesized to be involved in the stimulation of hypothalamic adrenocorticotropic hormone (ACTH) secretagogue release by both circadian- and stress-induced mechanisms. We aimed to investigate this hypothesis by measuring the effect of the highly specific serotonin re-uptake inhibitor fluoxetine (FX) on ACTH and cortisol release in the morning and in the afternoon in humans, both by itself and in combination with the opioid antagonist naloxone (Nal). Naloxone causes ACTH release in humans by removing an endogenous inhibitory opioid tone on central noradrenergic pathways stimulatory to hypothalamic corticotropin-releasing hormone (CRH) secretion. Serotonergic agents may act directly or indirectly through these central noradrenergic pathways and, if so, would be expected to be additive to or synergistic with Nal in causing ACTH and cortisol release. 2. Oral FX (40 mg) was given at approximately 07.00 or 11.00 h, either alone or with intravenous Nal 3 h later, to normal human volunteers. Plasma ACTH and cortisol levels were measured for 5 h after FX dosing. 3. Fluoxetine produced a small but non-significant increase in Nal-stimulated ACTH and cortisol release in both morning and afternoon studies. Naloxone alone did not cause different ACTH and cortisol responses in the morning and afternoon. 4. These results suggest that serotonergic pathways are not major regulators of the hypothalamic-pituitary-adrenal axis in humans or that FX has counteracting acute inhibitory effects on the axis, such as inhibition of hypothalamic arginine vasopressin secretion, which has been demonstrated in chronic animal studies.