Mediation of light-induced changes in pineal receptor and supporting cell nuclei and nucleoli in steelhead trout (Salmo gairdneri)

The morpho-anatomy and histology of the pineal complex in a major Indian carp, Catla catla: identification of the pineal photoreceptor cells and their responsiveness to constant light and constant darkness during different phases of the annual reproductive cycle

In contrast to mammals in which the pineal gland is a discrete structure situated dorsally in the brain, the "pineal gland" in teleost fishes is composed of a number of separate but connected constituent parts, collectively described as the "pineal complex." In this paper, we have described the pineal complex in a common Indian carp, Catla catla, which exhibits an annual reproductive cycle. Attempts have been made to (a) provide an in-depth description of the structure of the pineal complex; and (b) identify the photoreceptor cells of the pineal, by exposing the animals to constant light (LL) and constant darkness (DD). Furthermore, we examined any possible influence of the reproductive status of the fish on the responsiveness of the pineal photoreceptor cells in C. catla following exposure to LL and DD. To this end, a total of four experiments were carried out during the four different phases of the annual reproductive cycle that is characteristic of this species. Each of these four experiments was carried out for a period of 30 days after which the fishes were sacrificed, different parts of the pineal complex were dissected out, and processed for histological and karyometric studies. Our results showed that the pineal complex in this species is composed of three separate but connected parts, (a) an end vesicle (EV); (b) a dorsal sac (DS); and (c) a long and thin pineal stalk (PS) that attaches the EV to the DS. Detailed karyometric and histo-morphologic studies following exposure of the animals to DD and LL showed that constant darkness led to a stimulatory effect on the pineal photoreceptor cells of the EV as evident from a significant increase in the nuclear diameter. In contrast, the nuclear diameter of the photoreceptor cells in animals subjected to constant light showed a significant reduction. Furthermore, the observed cellular changes in the EV of fish exposed either to LL or DD were independent of the stage of the gonadal cycle. The apparent lack of any cellular responses either in the PS, or in the DS, following exposure to LL and DD, suggests that in C. catla the photoreceptor cells are located only within the epithelial lining of the EV and that these cells respond in a manner similar to mammalian pinealocytes when subjected to comparable photoperiod-induced experimental conditions.

Influence of continuous light and darkness on the secretory pinealocytes of Heteropneustes fossilis

In an earlier study on Heteropneustes fossilis, evidence of secretory activity in the pinealocytes had been demonstrated at the electron microscopic (EM) level and it was found to exist in two phases: a secretory phase (light cells) and a storage phase (dark cells). In the present investigation, H. fossilis was subjected to artificial photoperiods of continuous illumination and continuous darkness for a period of ten days and the effect on the secretory pinealocytes was studied at the EM level. Marked results were observed within the short period of ten days emphasizing the role of environmental photoperiod on the secretory activity of the pinealocytes. During continuous illuminated phase, both light and dark cells were observed: the light cells showed intense secretory activity and dark cells a storage one. During the dark phase both types of cells were present but in different metabolic states and neither of the cells demonstrated synthetic nor storage activity. Light cells were metabolically active but not secretory active and dark cells showed a necrotic condition. Phagocytotic activity of the dark cells was also seen. Intense neural activity was also observed during exposure to both the artificial photoperiods. The results highlight the role of light on the secretory activities of the pinealocytes of the catfish pineal organ.

The structure of the pineal complex in a common Indian teleost, Catla catla: evidence for pineal-induced inhibition of testicular function within an annual reproductive cycle

The structure of the pineal complex and the annual reproductive cycle in a major Indian carp, Catla catla, were investigated in the present study. Additionally, given the well-known inhibitory effects of the pineal on reproductive function in mammals, attempts were made to investigate whether or not the pineal exerts an inhibitory influence on reproductive function in this piscine species as well. Sexually adult animals were utilized in all experiments. The cytomorphology of the pineal complex and a number of parameters for testicular function--such as testicular cytology, serum testosterone levels, and testicular activities of two steroidogenic enzymes, 17beta-hydroxysteroid dehydrogenase (17beta-HSD) and delta5-3beta-hydroxysteroid dehydrogenase (delta5-3beta-HSD) were examined over a period of two years. Our studies showed that the pineal complex in this species consists of three separate but distinctly connected components: (a) an end vesicle (EV); (b) a long pineal stalk (PS); and (c) a dorsal sac (DS). Of these, the epithelial lining of the EV consists of cells that have rounded vesicular nuclei and long apical cytoplasmic processes that reach the lumen, features suggestive of photoreceptor cells. The cells of the PS have some similarity with those of the EV, while DS cells appear columnar and ciliated. With regard to gonadal activity, germ cell profiles revealed that this species has four distinct phases during the annual reproductive cycle: (a) preparatory (January-April); (b) pre-spawning (May-June); (c) spawning (July); and (d) post-spawning (August-December). During the spawning phase (July), seminiferous tubular diameter, percentage of late spermatids within seminiferous tubules, and serum testosterone levels showed the highest values compared to those obtained in most of the other phases of the reproductive cycle. Also in July, along with peak serum testosterone levels, the activities of 17beta-HSD and delta5-3beta-HSD were at their highest levels. In a correlation between the pineal cytology and testicular functional status, it was noted that both the nuclear diameter and the apical cytoplasmic projections of the EV photoreceptor cells showed a significant reduction, thus suggesting a reduced synthetic activity, during the month of July, the spawning phase of the reproductive cycle. In contrast, the same features of the EV cells during the other phases of the reproductive cycle showed an increased cellular and metabolic activity--a time when the gonads were less active and in a quiescent stage. These data suggest an inhibitory role of the pineal on gonadal function and thus provide additional credence to the concept that, as in higher mammals, there exists an inverse relationship between the pineal activity and gonadal function in teleost fishes as well.

Ultrastructural study of the cellular types in the pineal organ of Gambusia affinis (teleost)

The pineal organ of Gambusia affinis was studied via light and electron microscopy. The cell types studied included photoreceptor cells, supporting cells, and a third cell type. The photoreceptor cells, which appear to form clusters, are divided into four regions: outer segment, inner segment, cell soma, and synaptic pedicle. Synaptic ribbons are commonly observed in the synaptic pedicle. The supporting cells separate the photoreceptor cells from the thick basal lamina that surrounds the entire pineal organ. The supporting cells show highly organized membrane formations, some lipid-like inclusions, and a diplosome. One of the centrioles gives rise to an invaginated cilium. The third cell type is observed infrequently and appears to be located mainly in the vicinity of the outer segments. The morphological characteristics of this cell type are similar to those of phagocytic cells. The ultrastructural features of the pineal organ of G. affinis are compared with those of other teleosts.

Effect of pinealectomy, eye enucleation, and melatonin treatment on ovarian activity and vitellogenin levels in the catfish exposed to short photoperiod or long photoperiod

To investigate the relative importance of pineal, and eyes, and melatonin treatment on ovarian activity, catfish. Heteropneustes fossilis subjected to pinealectomy, eye enucleation, or both were exposed to short (LD 9:15) or long (LD 14:10) photoperiod during the different stages of the annual reproductive cycle. During the preparatory period, pinealectomy accelerated ovarian recrudescence under long photoperiod, but no effects of pinealectomy were observed under short photoperiod. Pineal has no influence on ovarian activity during the prespawning and spawning periods of the reproductive cycle. In the late postspawning period, ovarian recrudescence is accelerated after pinealectomy in catfish maintained under short photoperiod (LD 9:15 at 25 degrees C). But under long photoperiod (LD 14:10 at 25 degrees C), pinealectomy delayed ovarian recrudescence. An investigation into the role of the eyes revealed that eye enucleation nullifies the inhibitory effects of LD 9:15 and represses the stimulatory effects of LD 14:10. Combined surgery inhibited ovarian development under both the regimes. It is significant that pinealectomy and/or blinding neither counteracted nor delayed the postspawning ovarian regression. The findings suggest that the role of the pineal in catfish reproduction is variable and depends upon the photoperiod to which they are exposed as well as on the time of the year and the stage of the reproductive cycle. It is concluded that extraretinal and extrapineal photoreceptors also are involved in influencing seasonal reproduction. Further, no effects of melatonin treatment on ovarian activity or on vitellogenin levels during the preparatory or prespawning periods were observed, indicating that the pineal effects on gonadal activity in catfish may not be mediated through the secretion of melatonin.

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

To investigate the relative importance of pineal and eyes in ovarian activity, catfish subjected to pinealectomy, binding, 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.

Immunocytochemical and electron-microscopic investigations of the pineal organ in adult agamid lizards, Uromastix hardwicki

Lacertilian species display a remarkable diversity in the organization of the neural apparatus of their pineal organ (epiphysis cerebri). The occurrence of immunoreactive S-antigen and opsin was investigated in the retina and pineal organ of adult lizards, Uromastix hardwicki. In this species, numerous retinal photoreceptors displayed S-antigen-like immunoreactivity, whereas only very few pinealocytes were labeled. Immunoreactive opsin was found neither in retinal photoreceptors nor in pinealocytes. Electron microscopy showed that all pinealocytes of Uromastix hardwicki resemble modified pineal photoreceptors. A peculiar observation is the existence of a previously undescribed membrane system in the inner segments of these cells. It is evidently derived from the rough endoplasmic reticulum but consists of smooth membranes. The modified pineal photoreceptor cells of Uromastix hardwicki were never seen to establish synaptic contacts with somata or dendrites of intrapineal neurons, which are extremely rare. Vesicle-crowned ribbons are prominent in the basal processes of the receptor cells, facing the basal lamina or establishing receptor-receptor and receptor-interstitial type synaptoid contacts. Dense-core granules (60-250 nm in diameter) speak in favor of a secretory activity of the pinealocytes. Attention is drawn to the existence of receptor-receptor and receptor-interstitial cell contacts indicating intramural cellular relationships that deserve further study.

The effects of constant light and constant darkness on daily changes in the morphology of the pineal organ in the goldfish, Carassius auratus

The fine structure of photoreceptor cells in the pineal organ of the goldfish was found to vary quantitatively over a 24-hour period. Stereological analysis revealed significant daily changes in the volume of the cell and inner segment, nuclear volume and nucleolar diameter, volume of endoplasmic reticulum and Golgi bodies, area of both rough and smooth endoplasmic reticulum, and number of vesicles associated with each Golgi body. Peak values of these variables occurred either during the dark phase or latter part of the light phase. These findings agree closely with those reported in higher vertebrates, and suggest that metabolic activities, and possible secretory functions, of the pineal organ of fishes are synchronized to the light:dark cycle. Daily changes in these variables generally persisted in fish exposed to constant darkness for seven days, with the peak in these rhythms coinciding closely with those observed in fish exposed to a light:dark cycle. In contrast, the rhythms in all variables were abolished in fish kept in continual light for seven days. Photoreceptor cells from fish exposed to continuous light had larger nucleoli and greater amounts of rough endoplasmic reticulum indicating a further effect of light on pineal metabolism in lower vertebrates.

The effects of constant light and constant darkness on the pineal organ of the goldfish, Carassius auratus

Goldfish (Carassius auratus) were subjected to both long- and short-term (1, 3, 6, and 170 days) constant light or constant darkness to determine the effects of environmental lighting on the morphology of photoreceptor cells in the pineal organ. Morphometric analysis of electron micrographs from each experimental group suggested that the effects of constant darkness are morphologically expressed only after long-term (6 months) exposure. These included an increase in the volume of photoreceptor and phagocytic cells, area of rough endoplasmic reticulum, and volume of mitochondria per photoreceptor cell, and a decrease in the size of individual Golgi bodies. On the other hand, the response to constant light occurred generally within the first 6 days. Under these conditions, there was a reduction in outer-segment volume, but an increase in both the volume of Golgi per photoreceptor cell and size of individual Golgi bodies. Synaptic ribbons in these specimens were longer at every time sampled. The results of this study provide evidence that photoreceptor cell organelles involved in synthesis and secretion, as well as photosensory functions, are influenced by environmental lighting.