Methods are described for isolating adult rabbit retinal and maintaining it in a medium designed to resemble CSF. Morphologic, metabolic, nd electrophysiologic measurements obtained on the in vitro retinas showed that they remained in a nearly physiological state for at least 8 h, and even after 2 days in vitro they still exhibited a high level of metabolic activity and electrical responsiveness to light. Physiological activity was modified by photic stimulation, and data are presented to document changes in metabolism in response to the changes in function. The isolated retina appears to offer a number of unusual advantages for studying relationships between function and metabolism in organized mammalian central nervous tissue.

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A number of investigators have related CO2 metabolism to cerebrospinal-fluid (CSF) formation, e.g. Tschirgi, Frost & Taylor (1954); Fisher & Copenhaver (1959); Oppelt, Maren, Owens & Rall (1963); Pollay & Davson (1963). The present studies were designed to investigate certain aspects of this relation, not as it applies to CSF as a whole, but with respect specifically to the fluid formed by the choroid plexus of the lateral ventricle. The effect on the volume and composition of newly formed choroid-plexus fluid was determined when PCo, was altered in the plasma, when Pco2 was altered on the CSF side of the choroid-plexus membrane, and when a carbonic anhydrase inhibitor, acetazolamide, was applied topically to the choroid plexus.In addition, a study was made of the effect on choroid-plexus fluid of the cardiac glycoside, ouabain, applied topically. METHODSThe operative procedure and the methods used to sample and analyse the plasma and choroid-plexus fluid have been described in the preceding paper (Ames, Higashi & Nesbett, 1965).In general pattern, the experimental procedure involved sampling of arterial plasma and choroid-plexus fluid under control conditions; introduction of the experimental variable; sampling again after 30 and (usually) 60 min under the test situation. In some instances, conditions were again returned to the control situation and samples taken 30 min later. Residual choroid-plexus fluid was evacuated and discarded before beginning each collection.The cats were maintained on artificial respiration throughout these studies. An effort was made to adjust the stroke volume and rate of the respiration pump to maintain normal arterial pH under control conditions, though in fact it averaged somewhat above normal at about 7-5. To lower plasma PCO2 the animals were hyperventilated by doubling the stroke volume of the respiration pump and tripling the rate, still using room air. To increase plasma PCO2, a mixture of 10 % CO2 and air was substituted for air without changing the rate or stroke volume of the respiration pump; for this study, most of the animals were curarized.The PCO2 on the CSF side of the choroid plexus was varied by bathing the plexus alternatively in Pantopaque pre-equilibrated with 5 % CO2 in air or in Pantopaque pre-equilibrated simply * Present address: Yamaguchi Medical School, Ube, Yamaguchi Prefecture, Japan.

Hofmann, 1960). The studies to be reported are concerned with the movement of K+ from blood to choroid-plexus fluid and with the regulation of its concentration in the choroid-plexus fluid.The concentrations of K+ in choroid-plexus fluid and in plasma were determined under control conditions and following experimentally induced elevations or reductions in plasma K+. Analyses for Na+ and C1-were also made, and the concentrations of the three ions were measured in cisternal fluid, obtained under control conditions, for comparison with the choroid-plexus fluid. METHODS SamplingAdult cats weighing 1-7-4-4 kg were anaesthetized with intraperitoneal sodium pentobarbitone (Nembutal) 40 mg/kg. A tracheotomy was performed, and the cats were maintained on artificial respiration throughout the rest of the procedure. Volume and rate of respiration were adjusted to maintain normal plasma pH.

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