Abstract:The effect of GABA on melatonin content in vitro was studied in the golden hamster retina. GABA significantly increased melatonin levels in a dose-dependent manner, its effect being reversed by a GABA, receptor antagonist, bicuculline, but not by saclofen, a GABA, antagonist. Moreover, an equimolar concentration of muscimol, a GABA, receptor agonist, significantly increased retinal melatonin content, whereas baclofen, a GABA, receptor agonist, was ineffective. The darknessinduced increase in melatonin content in vitro was inhibited by bicuculline, whereas saclofen was ineffective. Retinal GABA turnover rate was significantly higher at midnight than at midday. GABA significantly decreased cyclic AMP and increased cyclic GMP accumulation in the golden hamster retina. The effect of GABA on both nucleotide levels was reversed by bicuculline, but baclofen had no effect. Cyclic GMP analogues (i.e., 8-bromoguanosine 3',5'-cyclic monophosphate and 2'-0-dibutyrylguanosine 3',5'-cyclic monophosphate) significantly increased retinal melatonin content in vitro. Taken together, these results support the hypothesis that GABA may be important for the "dark message" in the hamster retina. Key Words: GABA-Melatonin-Cyclic AMPCyclic GMP-Hamster retina.

A characterization of nitric oxide synthase (NOS) in the golden hamster retina was performed. Enzymatic activity was partially Ca2+ and calmodulin-dependent, required NADPH, and was inhibited by L-arginine analogs. Retinal NOS activity was higher at midnight than at midday. When the hamster were placed under constant darkness for 24 or 48 h, and killed at equivalent time points, representing subjective day and subjective night respectively, the differences in NOS activity disappeared. One hour of darkness during the day increased, while the same period of light during the night decreased, retinal NOS activity. From these results, it might be presumed that hamster retinal NOS activity is regulated by the photic stimulus.

Ichthyological survey of the Camba Cué lagoon, Apipé Grande Island, Corrientes, Argentina. The Apipé Grande island is a Provincial Reserve located in the Paraná river at 1400 km, from its origin at the Buenos Aires harbor, and two km downstream of the Yacyretá Dam. The outstanding feature of the island is that 70 % of its area comprises wetland environments. At the center of the island there are Hermosa and Cambá Cué «lagunas» with 300 and 1000 hectares respectively. The present paper reports the results of the first ichthyological survey to the referred water bodies. As a result of the sampling fieldwork 55 fish species were identified. The presence of species belonging to lotic and lenitic environments is interpreted as evidence for the close hydrological relationship of these water bodies and the Paraná river system. Key words: Cambá Cué lagoon, protected areas, Apipé Grande Island, Paraná River, Neotropical ichthyology, biogeography, la Plata Basin, Apipé rapids.Resumen: La isla Apipé Grande se encuentra a la altura del kilómetro 1400 del río Paraná, dos kilómetros aguas abajo de la represa Yacyretá y actualmente constituye la Reserva Provincial Isla Apipé Grande. Esta isla se destaca por poseer cerca del 70 % de la superficie compuesta por zonas bajas e inundables integradas por madrejones, esteros y lagunas. En la zona central de la isla se encuentran las lagunas Hermosa y Cambá Cué que cubren un área aproximada de 300 y 1000 hectáreas respectivamente. En el presente trabajo se informan los resultados del primer relevamiento ictiológico de los referidos cuerpos de agua. La toma de muestras realizada en la laguna Cambá Cué permitió identificar 55 especies de peces. La presencia de especies características de ambientes tanto lóticos como leníticos es interpretada como evidencia de la estrecha relación hidrológica entre estos cuerpos de agua y el sistema del río Paraná.Palabras clave: laguna Cambá Cué, áreas protegidas, isla Apipé Grande, río Paraná, ictiología neotropical, biogeografìa, cuenca del Plata, Saltos de Apipé. ____________