Skins of Potamotrygon reticulatus are light in color in vitro, exhibiting punctate melanophores. α‐Melanocyte stimulating hormone (EC50 = 4.58 × 10–9 M) and prolactin (EC50 = 1.44 × 10–9 M) darken the skins in a dose‐dependent manner. The endothelins ET‐1, ET‐2 and ET‐3, and the purines, ATP, and uracil triphosphate (UTP) were not able to induce either skin lightening or darkening. Forskolin and the calcium ionophore A23187 promoted a dose‐dependent darkening response, whereas N2, 2′‐O‐dibutyryl guanosine 3′‐5′‐cyclic monophosphate (db cyclic GMP), phorbol‐12‐myristate‐13‐acetate (TPA), and 1‐oleoyl‐2‐acetyl‐sn‐glycerol (OAG) were ineffective. The maximal response obtained with the calcium ionophore A23187 was only 76% of maximal darkening. These results indicate that the cyclic adenosine 3′‐5′‐monophosphate (cAMP) pathway is probably involved in the pigment dispersion of P. reticulatus melanophores. Other experiments should be done to further investigate how cytosolic calcium may be physiologically increased, and the existence of a putative cross‐talk between calcium and cAMP signals. In conclusion, the only hormones effective on P. reticulatus melanophores were prolactin and α‐MSH. No aggregating agent has been shown to antagonize these actions. Prolactin effect on elasmobranch melanophores adds a novel physiological role to this ancient hormone. J. Exp. Zool. 284:485–491, 1999. © 1999 Wiley‐Liss, Inc.

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.

Mammalian seasonalmolting and color change are known to be influenced by photoperiod changes. Calomys laucha, a South American rodent, exhibits seasonal pelage color change; however, unlike Northern hemisphere rodents, which present a gray or brown color during summer and a whitish color during winter, C. laucha pelage changes from an orange color during summer to a dark gray color during winter. Animals maintained for over a year in stationary photoperiod (LD 12:12h, 22 • C) presented orange pelage color during the summer corresponding month (January), and gray color during the winter corresponding month (July). Same age animals were evaluated during summer or winter months, and also showed different colors. Animals exposed for 12 weeks to summer or winter artificial conditions displayed color change, not according to the environmental conditions, as expected, but similar to that of animals maintained in stationary photoperiod. These results suggest that pelage color change in C. laucha is controlled by an endogenous circannual rhythm. The adaptive function of C. laucha color change is discussed.

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