SEASONAL EFFECTS OF PROLACTIN ON FRESHWATER OSMOREGULATION OF THE MARINE FORM (TRACHURUS) OF THE STICKLEBACK GASTEROSTEUS ACULEATUS

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Abstract
Effects of prolactin on plasma and urine osmolalities of winter and late-spring sticklebacks (Gasterosteus aculeatus L.) were studied at intervals following transfer from seawater to fresh water. Winter fish injected with the solvent of prolactin (control), when transferred into fresh water, showed a significant drop in plasma osmolality and excreted only slightly hypoosmotic urine. Prolactin injections reduced significantly the drop in plasma osmolality and enabled the fish to excrete highly hypoosmotic urine. Late-spring fish injected with the solvent of prolactin (control) showed, when transferred into fresh water, patterns of changes of plasma and urine osmolalities not significantly different from those of prolactin-injected winter fish. Prolactin injections in this case did not produce significant effects. The data suggest a seasonal variation in prolactin secretion associated with migration from salt to fresh water.