The Effect of Ouabain on the Efflux of Sodium Ions in the Nerve Cords of Two Insect Species (Periplaneta Americana and Carausius Morosus)

Nerve Structure-Function: Unusual Structural Details and Unmasking of Sulfhydryl Groups by Electrical Stimulation or Asphyxia in Axon Membranes and Gap Junctions

This review describes and discusses unusual axonal structural details and evidence for unmasking sulfhydryl groups (-SH) in axoplasmic membranes resulting from electrical stimulation or asphyxia. Crayfish axons contain fenestrated septa (FS) that, in phase contrast, micrographs appear as repeated striations. In the electron microscope, each septum is made of two cross-sectioned membranes containing ~55 nm pores, each occupied by a microtubule. Thin filaments, which we believe are made of kinesin, bridge the microtubule to the edge of the pore. FS are believed to play a role in axoplasmic flow. The axons also display areas in which axon and sheath glial cell plasma membranes are sharply curved and project into the axoplasm. In freeze-fractures, the protoplasmic leaflet (P-face) of the projections appears as elongated indentations containing parallel chains of particles. The sheath glial cell plasma membrane also contains particles, but they are irregularly aggregated. The axons also display areas where axonal and glial plasma membranes fuse, creating intercellular pores. In axons fixed during electrical stimulation, the plasma membrane, the outer membrane of mitochondria, membranes of other cytoplasmic organelles, and gap junctions increase in electron opacity and thickness, resulting from unmasking of sulfhydryl groups (-SH). Similar changes occur in asphyxiated nerve cords.

Barrier mechanisms in the Drosophila blood-brain barrier

The invertebrate blood-brain barrier (BBB) field is growing at a rapid pace and, in recent years, studies have shown a physiologic and molecular complexity that has begun to rival its vertebrate counterpart. Novel mechanisms of paracellular barrier maintenance through G-protein coupled receptor signaling were the first demonstrations of the complex adaptive mechanisms of barrier physiology. Building upon this work, the integrity of the invertebrate BBB has recently been shown to require coordinated function of all layers of the compound barrier structure, analogous to signaling between the layers of the vertebrate neurovascular unit. These findings strengthen the notion that many BBB mechanisms are conserved between vertebrates and invertebrates, and suggest that novel findings in invertebrate model organisms will have a significant impact on the understanding of vertebrate BBB functions. In this vein, important roles in coordinating localized and systemic signaling to dictate organism development and growth are beginning to show how the BBB can govern whole animal physiologies. This includes novel functions of BBB gap junctions in orchestrating synchronized neuroblast proliferation, and of BBB secreted antagonists of insulin receptor signaling. These advancements and others are pushing the field forward in exciting new directions. In this review, we provide a synopsis of invertebrate BBB anatomy and physiology, with a focus on insights from the past 5 years, and highlight important areas for future study.

Potassium fluxes across the blood brain barrier of the cockroach, Periplaneta americana

Potassium fluxes across the blood-brain barrier of the cockroach Periplaneta americana were measured using the scanning ion-selective microelectrode technique. In salines containing 15 mM or 25 mMK(+), an efflux of K(+) from the ganglia of isolated nerve cords was counterbalanced by an influx across the connectives. Metabolic inhibition with CN(-) resulted in an increase in K(+) efflux across both the ganglia and the connectives. Depletion of K(+) by chilling the nerve cords in K(+)-free saline was associated with subsequent K(+) influx across the connectives in K(+)-replete saline at room temperature. There were dramatic increases in K(+) efflux across both ganglia and connectives when the nerve cords were exposed to the pore-forming antibiotic amphotericin B. K(+) fluxes across the ventral nerve cord were also altered when paracellular leakage was augmented by transient exposure to 3M urea. K(+) efflux was reduced by the K(+) channel blockers Ba(2+) and tetraethylammonium or by exposure to Ca(2+)-free saline and K(+) efflux from the ganglia was increased by addition of ouabain to the bathing saline. The results provide direct support for a model proposing that K(+) is cycled through a current loop between the ganglia and the connectives and that both the Na(+)/K(+)-ATPase and K(+) channels are implicated in extracellular K(+) homeostasis within the central nervous system.