Effect of melatonin on body coloration and spontaneous swimming activity in rainbow trout, Salmo gairdneri

• 1.

  1. Melatonin causes body blanching in rainbow trout (Salmo gairdneri) and the response is dose-dependent. A significant change in coloration was evoked by doses as low as 0·007 μg/g (intraperitoneal) and 2 μg/ml (in ambient water). Evidence is presented that melatonin given in ambient water affects coloration both through uptake in the blood as well as via possible diffusion through the skin. In spite of the apparently pharmacological nature of dose levels, the feasibility of a biological role of melatonin is evaluated.

• 2.

  2. Melatonin induced a dose-dependent impairment of body movements and equilibrium indicating that it has neural effects. The relative time course of impairment and blanching responses supports the view that the regulation of coloration may be mediated by action of melatonin primarily on peripheral sites.

• 3.

  3. Observations on blinded trout (with or without pineal) and the results of background reversal experiments indicate possible involvement of a melanophore-stimulating hormone. The possibility of an interaction of melatonin with this hormone and/or direct effect at the sympathetic terminal-melanophore level is considered.

• 4.

  4. Pinealectomy in trout with eyes intact had no effect either on responses to melatonin or on swimming activity. However, melatonin led to a significant inhibition of the latter. It was not possible to differentiate between the toxic effect and possibly specific effect of melatonin on swimming activity.

• 5.

  5. The significance of the present observations in relation to pineal function is also discussed.