Light sensitive rhabdoms in the octopus retina increase in cross-sectional area in the dark and shrink in the light. Growth in the dark is due to the formation of microvilli in an avillar region of the photoreceptor cell membrane and lengthening of rhabdomere microvilli already present. Diminution in the light is the result of the disassembly and shortening of the same microvilli. Each microvillus contains an actin filament core that must be assembled or disassembled in the dark or light, respectively. To understand the regulation of the construction and breakdown of rhabdomere microvilli in the light and dark, we used centrifugation to separate the rhabdom membranes followed by Western blotting and Rho pull-down assays to investigate the role of Rho GTPases in this process. Western blotting showed a difference in the distribution of Rho in rhabdom membrane and supernatant fractions. In the light, Rho was mostly present in the supernatant but in the dark it was found in the fraction enriched with rhabdom membranes. Complementing these results, pull-down assays showed that Rho is activated in the dark but in the light, Rho is mostly inactive. We believe that in the dark, activated Rho binds to the rhabdom membrane and initiates signaling pathways, leading to growth of rhabdomere microvilli. In the light, Rho is present in the soluble fraction, is inactivated, and is likely bound to a Rho GDI. Receptors involved in the activation of Rho in the dark are undetermined and may involve rhodopsin or another membrane protein.

In the cephalopod retina, light/dark adaptation is accompanied by a decrease/increase in rhabdom size and redistribution of rhodopsin and retinochrome. Rearrangements in the actin cytoskeleton probably govern changes in rhabdom size by regulating the degradation/formation of rhabdomere microvilli. Photopigment movements may be directed by microtubules present in the outer segment core cytoplasm. We believe that rhodopsin activation by light stimulates Rho and Rac signaling pathways, affecting these cytoskeletal systems and their possible functions in controlling rhabdom morphology and protein movements. In this study, we localized cytoskeletal and signaling proteins in octopus photoreceptors to determine their concurrence between the lighting conditions. We used toxin B from Clostridium difficile to inhibit the activity of Rho/Rac and observed its effect on the location of signaling proteins and actin and tubulin. In both lighting conditions, we found Rho in specific sets of juxtaposed rhabdomeres in embryonic and adult retinas. In the light, Rho and actin were localized along the length of the rhabdomere, but, in the dark, both proteins were absent from a space beneath the inner limiting membrane. Rac colocalized with tubulin in the outer segment core cytoplasm and, like Rho, the two proteins were also absent beneath the inner limiting membrane in the dark. The distribution of actin and Rho was affected by toxin B and, in dark-adapted retinas, actin and Rho distribution was similar to that observed in the light. Our results suggest that the Rho/Rac GTPases are candidates for the regulation of rhabdomere size and protein movements in light-dark-adapted octopus photoreceptors.

The following paper serves, first, as an attempt to posit some initial parameters that may better orient an understanding of the relationship between psychoanalytic praxis and the social realm. These initial remarks are used as a starting point to consider a case where the issue of the social is brought into a psychoanalytically informed therapeutic practice. The second part of this essay focuses on a case presentation that has been published by the relational psychoanalyst, Ken Corbett, who explores family romance, the primal scene, and the difficulties of a young boy who has (the now proverbial) two mothers.

No abstract

The amount of S-crystallin present in the octopus retina is significantly greater in dark-adapted retinas and it binds to F-actin. In the light, the level of S-crystallin is greatly reduced and there is no apparent F-actin binding. No other studies, to our knowledge, show that S-crystallin binds to the actin cytoskeleton or that its expression is regulated by light. Arginine kinase may provide energy for cytoskeletal remodeling as it may in other neural tissues.