Effects of light on plasma somatolactin levels in red drum (Sciaenops ocellatus)

The effects of illumination on circulating somatolactin (SL) levels were studied in red drum sampled at various times during a 24-h light-dark cycle (l2L:l2D) and during a 24-h period of constant light. Plasma SL concentrations were low in red drum sampled during the light phase (0.6 +/- 0.05-1.1 +/- 0.2 ng/ml). The levels were significantly elevated during the early-dark phase (5.8 +/- 1.0 ng/ml) and declined during the late-dark phase (0.39 +/- 0.05 ng/ml). In contrast, plasma SL levels remained low. (0.4 +/- 0.1-1.3 +/- 0.4 ng/ml) in fish sampled through a 24-h constant light period following a 1-week exposure to the light-dark cycle. Circulating SL levels were also significantly elevated during the early-dark phase in fish maintained under a reversed light-dark cycle. The role of the eyes in mediation the SL response to light was evaluated by comparing circulating SL levels in optic-tract sectioned and enucleated fish with those of intact (control) fish. Plasma SL concentrations were significantly higher in optic-tract sectioned and enucleated fish (6.5 +/- 0.9-13.8 +/- 1.5 ng/ml) than in control fish (0.6 +/- 0.05-3.9 +/- 0.7 ng/ml) during both the early-dark phase and the early-light phase of the 24-h light-dark cycle. The absence of significant changes in plasma SL levels between the light and dark phases in optic-tract sectioned and enucleated fish appears to be due to a loss of light perception in these fish. Moreover, intact and sham-operated red drum maintained in constant darkness had dramatically increased plasma SL levels (18.8 +/- 2.0-24.8 +/- 1.8 ng/ml). The present results are consistent with our earlier findings that plasma SL levels are elevated in fish kept in constant darkness and in a dark-background tank (reduction in overall light levels) during the light phase. Taken together, these results suggest that plasma SL levels are elevated in red drum in the absence of light and in response to low illumination. Interestingly, the integument of the fish became light during the dark phase of the light-dark cycle. In our earlier studies, the increase of plasma SL concentrations was associated with aggregation of melanophores, and direct effects of SL on melanophore aggregation were demonstrated. Overall, our studies with red drum suggest a possible role of elevated SL levels on melanophore aggregation during the dark phase of the 24-h day-night cycle.