Electrical properties and structure of the frog arachnoid membrane

We have used an in vitro preparation of the frog arachnoid membrane to study the role of this membrane in the maintenance of the "blood-cerebrospinal fluid (csf) barrier". Electron microscopy showed that the membrane was made up of 10-15 layers of flat epithelial cells joined together by numerous cell junctions. The electrical resistance of the preparation was about 2000 ohms cm2. The steady-state transmural potential difference (pd) ranged up to 45 mV, csf positive, and this eliminated by either the addition of ouabain to the csf, or by replacing the NaC1 with TEA C1. The pd across the membrane increased when bicarbonate was added to the external bathing solutions. We conclude that this pd is due to the active transport of sodium from the subural fluid to the csf. In some preparations the transmural pd was reversed, i.e., csf negative, and this was also abolished by the addition of ouabain to the csf, or by replacing chloride with isethionate. We conclude that this pd is related to active chloride transport. These, and other experiments, lead us to the conclusion that the arachnois membrane is involved in the production of the cerebrospinal fluid and the maintenance of the blood-cerebrospinal fluid barrier.