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The functions of the fish pineal organ Fenwick, James Clarke


The role of the fish pineal organ has been studied using the goldfish Carassius auratus and the Pacific salmon Oncorhynchus tshawytscha. To this end, the effects of pinealectomy in goldfish on various behavioural responses, endocrine systems, and the reproductive system were studied. The pineal organs and the retinal tissue from mature and immature salmon were examined by thin-layer chromatography and fluorometry to determine if melatonin, a mammalian hormone, is present in the fishes. Goldfish were injected with melatonin to see if the effect of exogenous melatonin was opposite to that of pinealectomy. Pinealectomized goldfish lost the photo-negative response seen in normal goldfish. Blinding had the same effect on phototaxis as pinealectomy and a combination of the two had the same effect as blinding or pinealectomy alone. It was concluded that the normal phototactic response depended upon both the pineal organ and the eyes. Pinealectomy, blinding, or both was followed by a marked increase in swimming activity. Although this increase was correlated with a decrease in the whole brain serotonin level, a causal relationship was not established between the two. Further, pinealectomy alone produced no significant changes in whole brain serotonin level. Melatonin was localized within the pineal organ of salmon and its concentration in this tissue was analyzed. The pineal melatonin store varied during the reproductive cycle and was found in lower concentrations in the pineal organs of mature salmon. Stored melatonin could not be found in the retinal tissue despite evidence for an active tryptophane metabolism in this tissue. Injection of melatonin into goldfish inhibited the increase in gonad size under long photoperiod; this was accompanied by larger gonadotrophs in the melatonin injected fish. Removal of the pineal organ from goldfish held under short photoperiod caused an increase in gonad size similar to that seen in untreated goldfish exposed to long photoperiod. The effect of pinealectomy on the gonads was limited to that season during which the gonads could be stimulated by increasing day length. At other times of the year, neither photoperiod nor pinealectomy produced any significant effect on the gonad size. From this it was concluded that the pineal gland of the goldfish is related to the reproductive cycle and that its function depends upon photoperiod and the production of melatonin. Pinealectomy had no effect on the interrenal tissue, thyroid tissue, plasma osmotic concentration, or plasma levels of Na⁺, Cl⁻, or Ca⁺⁺, indicating that the effects of this operation are specific for the reproductive system. The data obtained from these studies support the hypothesis that the pineal organ of fishes serves a secretory as well as a sensory function. Further, the functional aspects of the mammalian and fish pineal organs are discussed and it is concluded that the role of the pineal organ is similar in the two groups; that is, the pineal organ of mammals and fish is involved in the timing of reproductive events.

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