Melatonin interactions with the hypothalamus-pituitary-interrrenal axis in goldfish (Carassius auratus): effect on cortisol release

Melatonin (MEL, N-acetyl-5-methoxytryptamine) reduces plasma cortisol levels under basal and stressful situations. The aim of this study was to investigate whether the hypothalamus-pituitary-interrenal (HPI) axis is the target for MEL exerting this effect. First, we investigated if IP-injected MEL counteracts the stimulation of the HPI axis by CRH (ICV injection) or ACTH (IP injection). We considered 4 experimental groups injected with saline (control group), CRH (1 ?g/?l) or ACTH (50 ng/g), MEL (20 ?g/g) and MEL plus CRH or ACTH. Immediately before the injections, and 2 h afterwards, blood was sampled to measure plasma cortisol levels by ELISA. Secondly, we focused on the effect of MEL on cortisol release by the interrenal tissue in vitro. Head kidneys (containing interrenal tissue) were placed in a flow through culture system, and incubated with ACTH (50 nM) or ACTH+MEL (dose response, 1-100 nM). Medium was withdrawn at different time intervals and preserved for posterior cortisol measurement by RIA. Results showed that IP injection of MEL did not counteract the increase in CRH- or ACTH-stimulated circulating cortisol levels. On the other hand, MEL addition (1 and 10 nM) to the culture medium reduced both the total and the maximum peak of cortisol released by the gland, respect to stimulation with ACTH alone. These results suggest that although MEL does not modulate the pharmacological stimulation of the HPI axis by CRH or ACTH injections, at least in our experimental conditions, the reductions of circulating cortisol levels by MEL observed in stressful conditions in vivo could be mediated by a direct action of this neurohormone on the head kidney.