Physiological color change in the bullfrog, Rana catesbeiana

Adults of Rana catesbeiana maintained for 4 days in 12:12 light/dark regimen exhibited a rhythmic color change of 24 hr. Under constant light, however, the rhythm disappeared, and the reflectance values gradually became greater, that is the animals became lighter. Under constant darkness, the rhythm was also abolished, but the animals tended to a darker color. On black background the skin darkening proceeded at a faster rate as compared to the skin lightening of animals adapting to a white background. The difference in color change rate suggests that the darkening responses are probably mediated by an increase in a circulating hormone, whereas skin lightening probably results from the serum level decrease of the same hormone. Most certainly, this hormone is alpha-MSH, as the in vitro assays demonstrated its high potency as a full darkening agonist (EC50 = 9 x 10(-10) M). Prolactin (EC50 = 7.7 x 10(-8) M) and endothelins 2 (EC50 = 1.3 x 10(-6) M) and 3 (EC50 = 4.8 x 10(-7) M) were also full agonists, but 100- to 1000-fold less potent than alpha-MSH. Isoproterenol, in the absence or presence of dibenamine, and endothelin-1 also elicited darkening responses in a dose-related manner, but reaching only 23% and 35% of the maximal darkening, respectively. Isoproterenol darkening effect was completely blocked by propranolol, confirming its action through beta-adrenoceptors. These results, taken together with the lack of lightening activity of norepinephrine on alpha-MSH-darkened skins, suggest that R. catesbeiana melanophores do not possess very active beta-adrenoceptors and lack alpha-adrenoceptors. On the other hand, the lightening agonist melatonin elicited only half-maximal dose-dependent reversal of MSH-induced darkening. Our results suggest that the chromatic rhythm is not endogenous, and most likely is determined by the light/dark cycle effect on alpha-MSH secretion.