Potassium channel-dependent changes in the volume of developing mouse Schwann cells

Developmental and activity-dependent changes in K+ currents in satellite glial cells in mouse superior cervical ganglion

Voltage-gated K+ currents were recorded from freshly dissociated satellite glial cells wrapping around ganglion cells in mouse superior cervical ganglion (SCG) by whole-cell recordings of patch clamp techniques. Both inward and outward K+ currents during membrane hyperpolarization and depolarization were observed in these glial cells. The current-voltage relation of these K+ currents became almost linear in cells obtained more than 4 weeks after birth. The magnitude of the density of inward K+ currents, which were elicited during membrane hyperpolarization and were eliminated by external barium, progressively increased during the first month after birth. This developmental increase in the magnitude of inward K+ current density was not affected by decentralization of SCG done by transection of cervical sympathetic trunk (CST) 5 days after birth. In adult mice, the magnitude of the inward K+ current density decreased after chronic conduction blockade of CST by local application of tetrodotoxin. On the other hand, the magnitude of the inward K+ current density increased after daily intraperitoneal injection of reserpine and this increase was abolished by pre-treatment of decentralization of SCG. These results suggested that preganglionic innervation was not prerequisite for developmental increase in the inward K+ currents and preganglionic neuronal activity upregulates the inward K+ currents in adult mice. Neuronal regulation of glial K+ channel expression would assist in K+ clearance from periganglionic space to maintain neuronal activity.

Axons regulate the expression of Shaker-like potassium channel genes in Schwann cells in peripheral nerve

We examined potassium channel gene expression of two members of the Shaker subfamily, MK1 and MK2, in sciatic nerves from rats and mice. In Northern blot analysis, MK1 and MK2 probes detected single transcripts of approximately 8 kb and approximately 9.5 kb, respectively, in sciatic nerve and brain from both species. Polymerase chain reaction amplification of a cDNA library of cultured rat Schwann cells using MK1- and MK2- specific primers produced DNA fragments that were highly homologous to MK1 and MK2. To determine whether these channel genes were axonally regulated, we performed Northern blot analysis of developing, permanently transected, and crushed rat sciatic nerves. The mRNA levels for both MK1 and MK2 increased from P1 to P15 and then declined modestly. Permanent nerve transection in adult animals resulted in a dramatic and permanent reduction in the mRNA levels for both MK1 and MK2, whereas normal levels of MK1 and MK2 were restored when regeneration was allowed to occur following crush injury. In all cases, MK1 and MK2 mRNA levels paralleled that of the myelin gene P0. Elevating the cAMP in cultured Schwann cells by forskolin, which mimics axonal contact but not myelination, did not induce detectable levels of MK1 and MK2 mRNA by Northern blot analysis. Further, the level of MK1 mRNA in the vagus nerve, which contains relatively fewer myelinating Schwann cells and relatively more non-myelinating Schwann cells than the sciatic nerve, is reduced relative to the sciatic nerve. In conclusion, we have identified two Shaker-like potassium channel genes in sciatic nerves whose expressions are regulated by axons. We suggest that MK1 and MK2 mRNA are expressed in high levels only in myelinating Schwann cells and that these Shaker-like potassium channel genes have specialized roles in these cells.

Activity-dependent regulation of inwardly rectifying potassium currents in non-myelinating Schwann cells in mice

1. Voltage-gated potassium currents were recorded from freshly dissociated non-myelinating Schwann cells of sural and sympathetic nerves from 1- to 12-week-old mice using the whole-cell or a single channel variation of the patch-clamp technique. 2. All sural cells from 2-week-old mice showed inwardly rectifying potassium (Kir+) currents in whole-cell recordings. Kir+ currents were virtually undetectable in sural cells from mice more than 6 weeks old, which also showed depolarization of the resting membrane potential. On the other hand, the magnitude of Kir+ currents increased in cervical sympathetic trunk (CST) cells in parallel with an increase of cell capacitance 1-6 weeks after birth. The density of Kir+ currents in CST cells increased 1-4 weeks after birth and then stayed constant for up to 12 weeks. 3. The unitary conductance of a single Kir+ channel in CST cells was 30 pS 2-12 weeks after birth; this was recorded in a cell-attached configuration with 154 mM K+ in the pipette. The steady-state open channel probability of single Kir+ channels in CST cells decreased with membrane hyperpolarization, but was not markedly changed 2-12 weeks after birth. 4. Conduction block of CST for 5 days induced by local application of tetrodotoxin (TTX) resulted in a significant decrease in both the magnitude and the density of Kir+ currents in whole-cell recordings in CST cells rostral to the sites of TTX block. Similar changes of Kir+ currents in whole-cell recordings were observed in cells in the inferior postganglionic branch of a superior cervical ganglion after 5 days of TTX block of CST. 5. These results suggest that neuronal activity regulates the expression of functional Kir+ channels in non-myelinating Schwann cells in adult nerves. The activity-dependent regulation of the expression of glial potassium channels could play an important role in the regulation of the potassium microenvironment around active axons to maintain impulse conduction in unmyelinated fibres.

cAMP-mediated expression of inwardly rectifying potassium channels in cultured mouse Schwann cells

Voltage-gated ionic currents were recorded from freshly dissociated or cultured mouse Schwann cells obtained from neonatal sciatic nerves by the whole-cell variation of the patch-clamp technique. Schwann cells virtually lost inwardly rectifying potassium (Kir) currents within 2 days after nerve transection or in culture conditions of neonatal sciatic nerves confirming the previous results that axonal signals were suggested to play an important role in the expression of functional Kir channels. To see the effects of adenosine 3',5'-monophosphate (cAMP) analogues or forskolin on the expression of Kir channels in cultured Schwann cells, these agents were added to the culture medium 4 days after the start of the culture, when Kir currents were almost eliminated from cultured Schwann cells. Cultured Schwann cells restored the expression of Kir currents by co-culture with agents which elevate intracellular cAMP level. The dose-response of 8-(4-chlorophenylthio) (CPT) cAMP for the incidence of the expression of Kir currents showed a steep increase in the percentage of cells with Kir currents between 0.02 and 0.1 mM of external CPT cAMP and approximately two thirds of cells had Kir currents in higher concentrations of more than 0.1 mM of CPT cAMP after 4 days of incubation. After removal of CPT cAMP from the culture media after 4 days of incubation, Kir currents disappeared from cells within 2 days. The simultaneous application of cycloheximide (1 microgram/ml), an inhibitor of protein synthesis, with CPT cAMP suppressed the expression of Kir currents for up to 6 days of incubation.(ABSTRACT TRUNCATED AT 250 WORDS)