The pineal organ of teleost fish is a directly photosensory organ that contains photoreceptor cells similar to those of the retina. It conveys photoperiod information to the brain via neural pathways and by release of indoleamines, primarily melatonin, into the circulation. The photoreceptor cells respond to changes in ambient illumination with a gradual modulation of neurotransmission to second-order neurons that innervate various brain centres, and by modulation of indoleamine synthesis. Melatonin is produced rhythmically, and melatonin synthesis may be regulated either directly by ambient photoperiod, or by an endogenous circadian oscillator that is entrained by the photoperiod. During natural conditions, melatonin is produced at highest levels during the night. Although the pineal organ undoubtedly influences a variety of physiological parameters, as assessed by experimental removal of the pineal organ and/or administration of exogenous indoleamines, its role in any physiological situation is not clear cut. The effects of any interference with pineal functions appear to vary with the time of year and experimental photothermal regimes. There are strong indications that the pineal organ is one component in a central neural system that constitutes the photoperiod-responding system of the animal, i.e. the system that is responsible for correct timing of daily and seasonal physiological rhythms. It is important to envisage the pineal organ as a part of this system; it interacts with other photosensory structures (the retina, possibly extraretinal non-pineal photoreceptors) and circadian rhythm generators

The pineal organ of teleost fishes

The stimulatory effects of long wavelengths of light on the ovarian development in the tropical damselfish, Chrysiptera cyanea

Although the exact etiology of Chagas disease is not completely elucidated, thymic atrophy and oxidative stress are believed to be important contributors to the pathogenesis during acute Trypanosoma cruzi (T. cruzi) infection. We hypothesized that exogenous melatonin, administered by gavage (5 mg/kg, p.o., gavage) to young (5 weeks old) and middle-aged (18 months old) male Wistar rats, would modulate thymic oxidative damage and reverse the age-related thymus regression during T. cruzi acute infection. Increased levels of superoxide anion (O2- ) were detected in the thymus of infected animals, and treatment with melatonin reverted this response. We found reduced TBARS levels as well as a significant increase in superoxide dismutase (SOD) activity in the thymus of all middle-aged melatonin-treated animals, infected or not with T. cruzi. Furthermore, melatonin increased the thymic expression of SOD1 and SOD2 in middle-aged control animals. Nox2 expression was not affected by melatonin treatment in young or middle-aged animals. Melatonin reverted the age-related thymic regression as revealed by the increase in thymus weight, total number of thymocytes, and reduction in age-related accumulation of double-negative thymocytes. This is the first report to directly examine the effects of melatonin treatment on the thymic antioxidant/oxidant status and thymic changes during T. cruzi infection. Our results revealed new antioxidant features that turn melatonin a potentially useful compound for the treatment of Chagas disease, a condition in which an excessive oxidative damage occurs.

Effects of melatonin on thymic and oxidative stress dysfunctions during Trypanosoma cruzi infection

The present study attempted for the first time to explore the importance of photoperiod in the regulation of seasonal ovarian functions in any subtropical major carp. Adult Indian major carp Catla catla were transferred to a long photoperiod (LP; LD 16:8) or a short photoperiod (SP; LD 8:16) for 30 days on 4 dates corresponding to the beginnings of 4 reproductive phases in an annual cycle, and responsiveness of the ovary was evaluated by comparison with the gonadal weight (IG), relative number of developing oocytes, serum levels of vitellogenin, and the activity of 2 important steroidogenic enzymes, that is, Δ53β-hydroxysteroid dehydrogenase and 17.β-hydroxysteroid dehydrogenase, in the ovary of fish in a natural photoperiod. Exposure of fish to LP during the preparatory phase (February-March) resulted in a significant (p < 0.001) increase in the values of vitellogenin and in the activity of both the steroidogenic enzymes but not in the ovarian weight and in the relative number of different stages of oocy...

Importance of photoperiods in the regulation of ovarian activities in indian major carp Catla catla in an annual cycle.

Cadmium effects on the retina of adult Danio rerio.

To investigate the relative importance of pineal and eyes in ovarian activity, catfish subjected to pinealectomy, blinding, or both were exposed to continuous light (LL) or continuous darkness (DD) during the different phases of the annual reproductive cycle. Pineal or eyes have no influence on ovarian activity during the preparatory, prespawning, and spawning periods of the annual reproductive cycle under these photoregimes. However, the pineal accelerated ovarian activity under LL and DD during the postspawning period. Blinding alone has no effect on ovarian recrudescence under these regimes. However, combined surgery (blinded-pinealectomy) inhibited ovarian development under both these conditions, indicating that the pineal organ is more important than the eyes. Ovarian recrudescence occurred even in the absence of both pineal and eyes, indicating the involvement of extrapineal and extraocular photoreception in the regulation of reproductive activity in the catfish.

Role of pineal and eyes in the regulation of ovarian activity and vitellogenin levels in the catfish exposed to continuous light or continuous darkness.

S. K. Garg

Papers

Role of pineal and eyes in the regulation of ovarian activity and vitellogenin levels in the catfish exposed to continuous light or continuous darkness.