DOSE-DEPENDENT EFFECTS OF ACUTE MELATONIN TREATMENTS ON BRAIN CARBOHYDRATE-METABOLISM OF RAINBOW-TROUT

The levels of glycogen, lactate, acetoacetate and beta-hydroxybutyrate in brain as well as glycogen and lactate levels in liver, and glucose levels in plasma were evaluated in rainbow trout, Oncorhynchus mykiss , injected with ethanol/saline (5/95; v/v) alone (controls) or contain ing melatonin at three different doses 0.25, 0.5 and 1 mg kg(-1). The results obtained demonstrate, for the first time in a teleost fish, th e existence of changes in brain carbohydrate and ketone body metabolis m due to melatonin treatment. Thus, a clear dose-dependent decrease wa s observed in brain and liver glycogen levels, whereas a clear dose-re sponse increase was observed in brain for lactate, acetoacetate and be ta-hydroxybutyrate levels, and in plasma for glucose levels. CO2 produ ction from glucose was also tested in brains of pooled fish and these rates were not altered by melatonin treatment. Altogether, these resul ts suggest that melatonin may play an indirect role, possibly through alterations in insulin physiology, in the regulation of carbohydrate a nd ketone body metabolism in brain of rainbow trout.