A Quantitative Study of the Ionic Basis of Extraneuronal Potential Changes in the Central Nervous System of the Cockroach (Periplaneta Americana L.)

An electrophysiological method for measuring the potassium permeability of the nerve perineurium

An electrophysiological method is described for measuring the potassium permeability (PK) of the perineurium of the sciatic nerve of the frog. The method is based on the principle of grease-gap recording, in which an insulating compartment separates two surface recording electrodes. The sciatic nerves of frogs Rana temporaria and R. pipiens were isolated and mounted across a five compartment chamber, with Vaseline grease seals on the partitions between compartments. Compartments #1, #2 and #5 contained frog Ringer solution, #4 was filled with Vaseline and formed the grease gap, and #3 was the test compartment in which solutions could be changed. The nerve was stimulated via platinum electrodes in compartments #1 and #2, and DC potentials and compound action potentials (CAP) were recorded between Ag/AgCl electrodes connected through Ringer-agar bridges to compartments #3 and #5. In nerves with undamaged perineurium, changing from normal Ringer to high [K+] Ringer (100 mM, KCl replacing NaCl) for 2 min caused negligible change in DC potential or CAP, indicating that raised [K+] was not reaching the axon surface, and hence that the perineurium was exerting a diffusional restriction on K+ entry. In nerves damaged by stretching or drying, K+ pulses caused a depolarising change in DC potential (delta DC), and corresponding decline in CAP amplitude, consistent with a leaky perineurium allowing K+ entry and axonal depolarisation. Ringer made hypertonic by the addition of 2.5 M sucrose or 5 M NaCl caused increased perineurial permeability to K+. The method was calibrated by measuring the delta DC in response to raised [K+] in the range 5-100 mM [K+] in desheathed nerves; from this calibration curve relating delta DC to endoneurial [K+] it was possible to calculate the change in endoneurial [K+] occurring in intact preparations. The calculations showed that the undamaged perineurium had a PK of < 6.3 x 10(-7) cm.s-1, similar to the value calculated for in situ nerves using radioisotopic techniques, but less than the value reported for isolated perineurial cylinders. The method gives real-time information on the K+ permeability of the nerve perineurium and its modulation by experimental treatments.

Modulation of a glial blood-brain barrier

CNS interstitial fluid homeostasis by the glial perineurial blood-brain barrier in the crayfish and cockroach is dependent on glial uptake mechanisms, low paracellular permeability, and the cation-binding properties of the extracellular matrix. Potassium selective permeability of the crayfish perineurium is modulated by a Ca(2+)-dependent mechanism at the basolateral membranes of the glial barrier and is ion and voltage dependent. In addition, extracellular charged sites are significant in perineurial K+ and Ca2+ homeostasis and may be modified by changes in pH. In the cockroach, and probably the crayfish, perineurial K+ transport may also be modulated by receptor-mediated changes in glial membrane permeability. The factors acting at the crayfish and cockroach blood-brain barrier are summarized in FIGURE 8 and would be well suited for providing efficient K+ spatial buffering of the CNS. Analogous processes have been described in vertebrate glial cells and in the endothelial blood-brain barrier, which implies a common primary function. The CNS is protected from large fluctuations in the body fluids by the blood-brain barrier, whereas glial uptake mechanisms control the composition of the brain interstitial fluid, and modulation of both barrier permeability and glial transport by the altered chemical environment following neuronal activity allows precise adjustment of the brain extracellular fluids to the changing needs of the CNS. The insect and crustacean ventral nerve cord and perineurial blood-brain barrier provide an excellent preparation in which the interactions between these factors can be investigated in intact CNS tissue.

Octopamine reduces potassium permeability of the glia that form the insect blood-brain barrier

Octopamine caused only a slight reduction in the potential across the perineurial glia of the cockroach, had no effect upon sodium-induced changes in potential, but did reduce potassium-induced changes (at 10(-7) M and above). The effect of 10(-7) M octopamine was accompanied by a rise in resistance, was mimicked by 10(-7) M synephrine and blocked by 10(-6) M phentolamine. Transperineurial potassium permeability was reduced by 10(-6) M octopamine. It is concluded that octopamine receptors mediate a reduction in potassium conductance of the basolateral membrane of these glia, and a reduction in the net potassium permeability of the barrier.

Intercellular junctions and the development of the blood-brain barrier in Manduca sexta

In early embryonic development of the tobacco horn moth no blood-brain barrier is present, as shown by the unimpeded entry of exogenous tracers into the nervous system. However, later on, just before hatching, lanthanum and horseradish peroxidase (HRP) are unable to move inwardly beyond the level of the perineurium, which is the morphological site of the blood--brain barrier in the adult moth, as well as in other insects. Freeze-fracture studies indicate that in the early embryo, 10 nm particles are scattered about in the perineurial membrane PF, either as separate entities or as short linear arrays. By hatching or just before, however, the 10 nm particles have become aligned into lengthy linear aggregates as PF ridges with EF grooves. These would appear to be the simple, arthropod-form of tight junction, and are presumed to be the basis of the perineurial blood-brain barrier. At about the same time, gap junctional elements appear both between adjacent perineurial cells and between glial cells. In both cell types, the gap junctions form from free 13 nm EF particles which gradually become aligned or clumped into strands and aggregates which ultimately coalesce to form first irregular masses and then the macular plaques typical of mature gap junctions. Many of the latter stages are coincident with the hatching of a motile larvae, so that the perineurial and glial cells are by this stage coupled via the channels of the gap junctional particles. They are therefore able to undergo both ionic and metabolic exchange and cooperation during larval life, in addition to being able to respond to hormonal substances in an integrated way. During the 5 larval instars more gap junctions form as the perineurial layer grows thicker. These junctions become more regular in outline and their particles more tightly packed; these larval structures are compared with junctions found in the adult which tend to be more extensive but otherwise similar. Since no septate junctions are apparent during Manduca embryonic or larval life when the blood-brain barrier forms, nor in adults, the results of this report support the contention that it is the tight junctions rather than septate ones which form the basis of permeability barriers in this, and probably other, arthropod systems.