The blood-brain barrier of the insect nervous system is thought to be formed by the neuroglia. It is associated with a positive interstitial potential relative to the bathing medium (‘sheath’ potential), and with positive changes in potential that can be induced by raising the potassium level in the bathing medium. In central nervous connectives of the cockroach, Periplaneta americana, these potential differences (p.d.s) were measured just below the perineurium at the same magnitude as comparable p.d.s recorded deeper, indicating that they are produced by the perineurium. The interstitial p.d. apparently results from the presence of a paracellular resistance, and from the generation of less electromotive force (e.m.f.) by the basolateral membrane than by the apical (adglial). Recordings from cells identified by injection of peroxidase showed that the initial change in interstitial p.d. induced by high K was accompanied by: a depolarization of greater magnitude across the basolateral membrane, a small depolarization of the apical membrane, and no depolarization of sub-perineurial glial cells or axons. It therefore appears that the depolarization of the basolateral membrane by high K gives rise, by electrical coupling, to the shift in interstitial p.d. and the depolarization of the apical membrane. There was then a gradual depolarization of all membranes, consistent with a leak of K into the sub-perineurial interstitial system, and where the depolarization of the basolateral membrane would be produced by coupling to the apical. If the perineurium contains an intercellular resistance, then it must restrict diffusion of water-soluble substances.

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