Functionally distinct chloride-mediated GABA responses in rat cerebellar granule cells cultured in a low-potassium medium

Redox modulation of homomeric rho1 GABA receptors

The activity of many receptors and ion channels in the nervous system can be regulated by redox-dependent mechanisms. Native and recombinant GABA(A) receptors are modulated by endogenous and pharmacological redox agents. However, the sensitivity of GABA(C) receptors to redox modulation has not been demonstrated. We studied the actions of different reducing and oxidizing agents on human homomeric GABArho(1) receptors expressed in Xenopus laevis oocytes. The reducing agents dithiothreitol (2 mM) and N-acetyl-L-cysteine (1 mM) potentiated GABA-evoked Cl(-) currents recorded by two-electrode voltage-clamp, while the oxidants 5-5'-dithiobis-2-nitrobenzoic acid (500 microM) and oxidized dithiothreitol (2 mM) caused inhibition. The endogenous antioxidant glutathione (5 mM) also enhanced GABArho(1) receptor-mediated currents while its oxidized form GSSG (3 mM) had inhibitory effects. All the effects were rapid and easily reversible. Redox modulation of GABArho(1) receptors was strongly dependent on the GABA concentration; dose-response curves for GABA were shifted to the left in the presence of reducing agents, whereas oxidizing agents produced the opposite effect, without changes in the maximal response to GABA and in the Hill coefficient. Our results demonstrate that, similarly to GABA(A) receptors and other members of the cys-loop receptor superfamily, GABA(C) receptors are subjected to redox modulation.

Tonic benzodiazepine-sensitive GABAergic inhibition in cultured rodent cerebellar granule cells

Recent studies have demonstrated that granule cells in rat cerebellar slices exhibit a tonic form of GABAergic inhibition. The presence of a similar constitutive GABAergic conductance was investigated in synaptically coupled cultures of neonatal rat cerebellum. In cells exhibiting spontaneous inhibitory postsynaptic currents (IPSCs), application of the GABA(A) receptor antagonist bicuculline (10 microM) eliminated the IPSCs and also produced a significant decrease in holding current. This latter effect was lacking in cells that did not exhibit IPSCs. Application of TTX (1 microM) and Cd(2+) (100 microM) decreased the IPSC frequency and also produced a change in holding current; these effects were eliminated by the prior application of bicuculline. In the presence of TTX, application of the benzodiazepine (BDZ) Flunitrazepam (1 microM) caused a 85+/-15% increase in the component of holding current that arose from GABA(A) receptor activity. Noise analysis indicated that the GABA(A) receptors underlying this tonic form of GABAergic inhibition exhibited a mean single channel conductance close to 14 pS, a value similar to that seen for somatic GABA(A) receptors in these cells. Thus, like their counterparts in cerebellar slices, cerebellar granule cells in culture exhibit a background GABAergic conductance. The most likely source of this tonic current is GABA spilt over from active inhibitory synapses. As this conductance was sensitive to benzodiazepine receptor agonists it is unlikely to arise entirely from GABA(A) receptors containing the alpha6 subunit.

Permeability and single channel conductance of human homomeric rho1 GABAC receptors

1. Homomeric human rho1 GABAC receptors were expressed in Xenopus oocytes and in human embryonic kidney cells (HEK293) in order to examine their conductance and permeability. 2. Reversal potentials of currents elicited by gamma-aminobutyric acid (GABA) were measured in extracellular solutions of various ionic composition to determine relative permeability of homomeric rho1 receptors. The rank order of anionic permeability was: SCN- > I- > NO3- > Br- > Cl- > formate (For-) > HCO3- > acetate (Ac-) approximately proprionate (Prop-) approximately isethionate (Ise-) approximately F- approximately PO4-. 3. In the oocyte expression system, relative permeabilities to SCN-, I-, NO3-, Br- and HCO3- were higher for rho1 GABAC receptors than alpha1beta2gamma2L GABAA receptors. 4. Expression of rho1 GABAC receptors in Xenopus oocytes and in HEK293 cells gave similar relative permeabilities for selected anions, suggesting that the expression system does not significantly alter permeation properties. 5. The pore diameter of the homomeric rho1 GABAC receptor expressed in oocytes was estimated to be 0.61 nm, which is somewhat larger than the 0.56 nm pore diameter estimated for alpha1beta2gamma2L GABAA receptors. 6. Homomeric rho1 GABA receptors expressed in oocytes had a single channel chord conductance of 0.65 +/- 0.04 pS (mean +/- s.e.m.) when the internal chloride concentration ([Cl-]i) was 20 mM. With a [Cl-]i of 100 mM, the single channel chord conductance was 1.59 +/- 0.24 pS. 7. The mean open time directly measured from 43 GABA-induced channel openings in six patches was 3. 2 +/- 0.8 s. The mean open time in the presence of 100 microM picrotoxin was 0.07 +/- 0.01 s (77 openings from 3 patches). 8. The differences observed in ionic permeabilities, pore size, single channel conductance and mean open time suggest that the rho1 homomeric receptor may not be the native retinal GABAC receptor reported previously.

Mouse cerebellar granule cell differentiation: electrical activity regulates the GABAA receptor alpha 6 subunit gene

GABAA receptor alpha6 subunit gene expression marks cerebellar granule cell maturation. To study this process, we used the Deltaalpha6lacZ mouse line, which has a lacZ reporter inserted into the alpha6 gene. At early stages of postnatal cerebellar development, alpha6-lacZ expression is mosaic; expression starts at postnatal day 5 in lobules 9 and 10, and alpha6-lacZ is switched on inside-out, appearing first in the deepest postmigratory granule cells. We looked for factors regulating this expression in cell culture. Membrane depolarization correlates inversely with alpha6-lacZ expression: granule cells grown in 25 mM [K+]o for 11-15 d do not express the alpha6 gene, whereas cultures grown for the same period in 5 mM [K+]o do. This is influenced by a critical early period: culturing for >/=3 d in 25 mM [K+]o curtails the ability to induce the alpha6 gene on transfer to 5 mM [K+]o. If the cells start in 5 mM [K+]o, however, they still express the alpha6-lacZ gene in 25 mM [K+]o. In contrast to granule cells grown in 5 mM [K+]o, cells cultured in 25 mM [K+]o exhibit no action potentials, mEPSCs, or mIPSCs. In chronic 5 mM [K+]o, factors may therefore be released that induce alpha6. Blockade of ionotropic and metabotropic GABA and glutamate receptors or L-, N-, and P/Q-type Ca2+ channels did not prevent alpha6-lacZ expression, but inhibition of action potentials with tetrodotoxin blocked expression in a subpopulation of cells.