Potassium channels in crustacean glial cells

Glial potassium channels activated by neuronal firing or intracellular cyclic AMP in Helix

1. Cell-attached and whole cell patch clamp experiments were performed on satellite glial cells adhering to the cell body of neurones in situ within the nervous system of the snail Helix pomatia. The underlying neurone was under current or voltage-clamp control. 2. Neuronal firing induced a delayed (20-30 s) persistent (3-4 min) increase in the opening probability of glial K+ channels. The channels were also activated by perfusing the ganglion with a depolarizing high-K+ saline, except when the underlying neurone was prevented from depolarizing under voltage-clamp conditions. 3. Two K(+)-selective channels were detected in the glial membrane. The channel responding to neuronal firing was present in 95% of the patches (n = 393). It had a unitary conductance of 56 pS, a Na+ :K+ permeability ratio < 0.02 and displayed slight inward rectification in symmetrical [K+] conditions. It was sensitive to TEA, Ba2+ and Cs+. The following results refer to this channel as studied in the cell-attached configuration. 4. The glial K+ channel was activated by bath application of the membrane-permeant cyclic AMP derivatives 8-bromo-cAMP and dibutyryl-cAMP, the adenylyl cyclase activator forskolin and the diesterase inhibitors IBMX, theophylline and caffeine. It was insensitive to cyclic GMP activators and to conditions that might alter the intracellular [Ca2+] (ionomycin, low-Ca2+ saline and Ca2+ channel blockers). 5. The forskolin-induced changes in channel behaviour (open and closed time distributions, burst duration, short and long gaps within bursts) could be accounted for by a four-state model (3 closed states, 1 open state) by simply changing one of the six rate parameters. 6. The present results suggest that the signal sent by an active neurone to satellite glial cells is confined to the glial cells round that neurone. The effect of this signal on the class of glial K+ channels studied can be mimicked by an increase in glial cAMP concentration. The subsequent delayed opening of the glial K+ channels does not appear to play a role in siphoning the excess K+ released by active neurones. It is hypothesized that the cAMP-gated glial K+ channels may be involved in the control of glial cell proliferation.

Cockroach glial cell cultures: morphological development and voltage-gated potassium channels

Insect glial cells derived from the embryonic brain of Periplaneta americana cockroaches have been kept in primary culture conditions for up to 3 weeks. Under the culture conditions used, the glial cells differentiated and formed a complex cellular network on the floor of the culture vessels from which glial-glial and glial-neuronal contacts could be seen. Single-channel currents from cell-attached glial membrane patches were recorded using the gigaseal technique. Depolarisation of membrane patches activated outward currents, which were abolished in the presence of extracellular 50 mM tetraethylammonium or 5 mM 4-aminopyridine, and were insensitive to 1 microM tetrodotoxin, 10 microM picrotoxin and 2 mM Cd2+. The amplitude of the outward currents increased linearly with depolarisation, and amplitude histograms obtained at several pipette potentials could be reasonably fitted with a single Gaussian corresponding to a single channel type with a slope conductance of 37 +/- 11 pS. The extrapolated equilibrium potential of the outward current was about 5 +/- 10 mV positive to the resting potential and both the channel open time constant and relative open time probability were sensitive to membrane potential, increasing markedly with depolarisation. The results presented in this paper show the presence of a cadmium-insensitive, voltage-dependent outward potassium channel in insect glial cells in vitro.

Single ion channel currents in neuropile glial cells of the leech central nervous system

The patch-clamp technique was used to investigate the activity of single ion channels in neuropile glial (NG) cells in the central nervous system (CNS) of the medicinal leech, Hirudo medicinalis. We found evidence for two distinct Cl- channels that could be distinguished by their basic electrical properties and their responses to different inhibitors on single ion channel currents. In the inside-out configuration in symmetrical Cl- solutions, these channels showed current-voltage relationships with slight outward rectification, mean conductances of 70 and 80 pS, and reversal potentials near 0 mV. Significant permeability to Na+, K+, or SO4(2-) could not be detected. The open-state probability of the 70 pS Cl- channel increased with membrane depolarization, whereas the open-state probability of the 80 pS Cl- channel was voltage-independent. The application of the stilbene derivative DIDS (100 microM) to the cytoplasmic side of the glial cell membrane blocked both Cl- channels. The activity of the 70 pS channel was blocked irreversibly by DIDS, whereas the activity of the 80 pS channel reappeared after wash-out of DIDS. Both channels were blocked reversibly by 1 mM Zn2+. K+ channels could only be observed occasionally in the soma membrane of the NG cells. We have characterized a 60 pS K+ channel with a high selectivity for K+ over Na+. The low density of K+ channels in the soma membrane may indicate a non-uniform distribution of this channel type in NG cells.

Single-channel and whole-cell recordings from on-neurone glial cells in Helix pomatia ganglia

A procedure is described for performing patch-clamp recordings on satellite glial cells kept in place within the nervous ganglia in the mollusc Helix. Glial cell properties were deduced from whole-cell and cell-attached recordings. The glial membrane was found to contain densely packed inwardly rectifying K+ channels. Activation of the neurones, under either current-clamp or voltage-clamp conditions, depolarized the glial cell layer wrapped around the neurones and induced a delayed persistent increase in the K+ channel opening probability. These results suggest that the glial channels opened in response to a signal emanating from the active neurones. This preparation provides a useful means of detecting and analysing neurone-glial interactions at the cell and unitary channel levels.