Rhodopsin expression in the zebrafish pineal gland from larval to adult stage.

The zebrafish pineal gland plays an important role in different physiological functions including the regulation of the circadian clock. In the fish pineal gland the pinealocytes are made up of different segments: outer segment, inner segment and basal pole. Particularly, in the outer segment the rhodopsin participates in the external environment light reception that represents the first biochemical step in the melatonin production. It is well known that the rhodopsin in the adult zebrafish is well expressed in the pineal gland but both the expression and the cellular localization of this protein during development remain still unclear. In this study using qRT-PCR, sequencing and immunohistochemistry the expression as well as the protein localization of the rhodopsin in the zebrafish from larval (10 dpf) to adult stage (90 dpf) were demonstrated. The rhodopsin mRNA expression presents a peak of expression at 10 dpf, a further reduction to 50 dpf before increasing again in the adult stage. Moreover, the cellular localization of the rhodopsin-like protein was always localized in the pinealocyte at all ages examined. Our results demonstrated the involvement of the rhodopsin in the zebrafish pineal gland physiology particularly in the light capture during the zebrafish lifespan.

The photoreceptive cells of the pineal gland in adult zebrafish (Danio rerio).

The zebrafish pineal gland plays a fundamental role in the regulation of the circadian rhythm through the melatonin secretion. The pinealocytes, also called photoreceptive cells, are considered the morphofunctional unit of pineal gland. In literature, the anatomical features, the cellular characteristics, and the pinealocytes morphology of zebrafish pineal gland have not been previously described in detail. Therefore, this study was undertaken to analyze the structure and ultrastructure, as well as the immunohistochemical profile of the zebrafish pineal gland with particular reference to the pinealocytes. Here, we demonstrated, using RT-PCR, immunohistochemistry and transmission electron microscopy, the expression of the mRNA for rhodopsin in the pineal gland of zebrafish, as well as its cellular localization exclusively in the pinealocytes of adult zebrafish. Moreover, the ultrastructural observations demonstrated that the pinealocytes were constituted by an outer segment with numerous lamellar membranes, an inner segment with many mitochondria, and a basal pole with the synapses. Our results taken together demonstrated a central role of zebrafish pinealocytes in the control of pineal gland functions.

Aquaporin 4 in the sensory organs of adult zebrafish (Danio rerio).

The aquaporins (AQPs) are a family (AQP-AQP10) of transmembrane channel proteins that mediate the transport of water, ions, gases, and small molecules across the cell membrane, thus regulating cell homeostasis. AQP4 has the highest water permeability and it is involved in hearing and vision in mammals. Here, we used immunohistochemistry to map the presence of AQP4 in the sensory organs of adult zebrafish. The antibody used detected by Western blot proteins of 34 kDa (equivalent to that of mammalian AQP4) and maps in the sensory cells of taste buds, the hair sensory cells of the neuromast and of the maculae, and cristae ampullaris of the inner ear. Moreover, the retinal photoreceptors display AQP4 immunoreactivity. The non-sensory cells in these organs were AQP4 negative. These results suggest that the AQP4 could play a role in the regulation of water balance and ion transport in the sensory cells of zebrafish, bringing new data for the utilizing of this experimental model in the biology of sensory system.