Inhibition of spinal or hypoglossal motoneurons of the newborn rat by glycine or GABA

Whole-cell recording of intracellular pH with silanized and oiled patch-type single or double-barreled microelectrodes

Conventional ion-sensitive microelectrodes cannot be used in small cells, since they create too large an electrical leak at the site of penetration. Membrane potentials can be measured in such cells with the whole-cell configuration of the patch-clamp technique, after obtaining a high-resistance seal (giga-seal) to the cell membrane. Achieving such seals with patch-type microelectrodes silanized and filled with ion-sensitive cocktails has proved very difficult. Since ion-sensitive microelectrodes offer advantages over fluorescent techniques, we have developed a method which enables whole-cell recordings of membrane potential and intracellular pH to be achieved with silanized microelectrodes. We have been able to obtain high-resistance seals with silanized tips by dipping them in mineral oil. We describe the method for both single-barrel and theta-glass double-barreled microelectrodes. Double-barreled microelectrodes can be used to measure and control membrane potential in the whole-cell patch-clamp configuration while also measuring ionic activities with the adjacent barrel. We present illustrative experiments showing intracellular pH recordings in snail neurones and rat dorsal root ganglion cells, and we suggest the method can also be applied to other liquid-sensor ion-sensitive microelectrodes.

Why glycine transporters have different stoichiometries

In the brain, neurons and glial cells compete for the uptake of the fast neurotransmitters, glutamate, GABA and glycine, through specific transporters. The relative contributions of glia and neurons to the neurotransmitter uptake depend on the kinetic properties, thermodynamic coupling and density of transporters but also on the intracellular metabolization or sequestration of the neurotransmitter. In the case of glycine, which is both an inhibitory transmitter and a neuromodulator of the excitatory glutamatergic transmission as a co-agonist of N-methyl D-aspartate receptors, the glial (GlyT1b) and neuronal (GlyT2a) transporters differ at least in three aspects: (i) stoichiometries, (ii) reverse uptake capabilities and (iii) pre-steady-state kinetics. A 3 Na(+)/1 Cl(-)/gly stoichiometry was established for GlyT2a on the basis of a 2 charges/glycine flux ratio and changes in the reversal potential of the transporter current as a function of the extracellular glycine, Na(+) and Cl(-) concentrations. Therefore, the driving force available for glycine uphill transport in neurons is about two orders of magnitude larger than for glial cells. In addition, GlyT2a shows a severe limitation for reverse uptake, which suggests an essential role of GlyT2a in maintaining a high intracellular glycine pool, thus facilitating the refilling of synaptic vesicles by the low affinity, low specificity vesicular transporter VGAT/VIAAT. In contrast, the 2 Na(+)/1 Cl(-)/gly stoichiometry and bi-directional transport properties of GlyT1b are appropriate for the control of the extracellular glycine concentration in a submicromolar range that can modulate N-methyl D-aspartate receptors effectively. Finally, analysis of the pre-steady-state kinetics of GlyT1b and GlyT2a revealed that at the resting potential neuronal transporters are preferentially oriented outward, ready to bind glycine, which suggests a kinetic advantage in the uptake contest.