SUMMARY1. The ion-selective and ion transport properties of glycine receptor (GlyR) and y-aminobutyric acid receptor (GABAR) channels in the soma membrane of mouse spinal cord neurones were investigated using the whole-cell, cell-attached and outside-out patch versions of the patch-clamp technique.2. Current-voltage (I-V) relations oftransmitter-activated currents obtained from whole-cell measurements with 145 mM-Cl-intracellularly and extracellularly, showed outward rectification. In voltage-jump experiments, the instantaneous I-V relations were linear, and the steady-state I-V relations were rectifying outwardly indicating that the gating of GlyR and GABAR channels is voltage sensitive.3. The reversal potential of whole-cell currents shifted 56 mV per tenfold change in internal Cl-activity indicating activation of Cl--selective channels. The permeability ratio of K+ to Cl-(PK/PC1) was smaller than 0 05 for both channels.4. The permeability sequence for large polyatomic anions was formate > bicarbonate > acetate > phosphate > propionate for GABAR channels; phosphate and propionate were not measurably permeant in GlyR channels. This indicates that open GlyR and GABAR channels have effective pore diameters of 5-2 and 5-6 A, respectively. The sequence of relative permeabilities for small anions was SCN-> 1-> Br-> Cl-> F-for both channels.5. GlyR and GABAR channels are multi-conductance-state channels. In cellattached patches the single-channel slope conductances close to 0 mV membrane potential were 29, 18 and 10 pS for glycine, and 28, 17 and 10 pS for GABA-activated channels. The most frequently observed (main) J. BORMANN, 0. P. HAMILL AND B. SAKMANN I-> SCN-> F-for both channels. This is nearly the inverse sequence of that found for the permeability of these ions indicating the presence of binding sites for ions in the channel.8. The conductance-activity relation of GlyR and GABAR channels for Cl-was described by hyperbolic curves. The maximal conductances (Ymax) were 92 and 72 pS; the half-saturation activities (Ki) were 108 and 155 mM-Cl-for GlyR and GABAR channels, respectively. 9. The conductance-activity relation for SCN-was characterized by maximal conductances of 35 pS (GlyR) and 32 pS (GABAR). The half-saturation activities were 46 and 57 mm, respectively. In equal mixtures of SCN-and Cl-on both membrane sides the channel conductance showed anomalous mole-fraction dependence. This indicates that the channels contain at least two binding sites for anions.10. The difference between GlyR and GABAR channels in their main-state Clconductance could be caused by a small energy difference (-1 kJ mol-1) in the interaction of Cl-ions with the channel wall. The observation that both GlyR and GABAR channels display comparable conductance states, show similar permeability and conductance sequences for small anions, a similar conductance saturation curve for Cl-and SCN-and similar mole-fraction dependence of their conductance in Cl--SCN-mixtures could indicate that both receptor channels have a similar molecular st...

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gamma-Aminobutyric acid (GABA), a major inhibitory neurotransmitter in the mammalian nervous system, is known to operate bicuculline-sensitive Cl- channels through GABAA receptors and bicuculline-insensitive cation channels through GABAB receptors. Recent observations indicate that the retina may contain GABA receptors with unusual pharmacological properties. Here we report that GABA gates bicuculline-insensitive Cl- channels in rod bipolar cells of the rat retina, which were not modulated by flunitrazepam, pentobarbital and alphaxalone and were only slightly blocked by picrotoxinin. Moreover, the GABAB receptor agonist baclofen, and the antagonist 2-hydroxysaclofen had no effect. The underlying single-channel conductance was 7 pS and the open time 150 ms. These values are clearly different from those obtained for GABAA receptor channels recorded in other neurons of the same preparation, and in other parts of the brain. The bicuculline- and baclofen-insensitive GABA receptors were activated selectively by the GABA analogue cis-4-aminocrotonic acid (CACA). Hence they may be similar to those receptors termed GABAC receptors.

Purified preparations of the inhibitory glycine receptor (GlyR) contain alpha and beta subunits, which share homologous primary structures and a common transmembrane topology with other members of the ligand‐gated ion channel superfamily. Here, a beta subunit‐specific antiserum was shown to precipitate the [3H]strychnine binding sites localized on alpha subunits from membrane extracts of both rat spinal cord and mammalian cells co‐transfected with alpha and beta cDNAs. Further, inhibition of alpha homo‐oligomeric GlyRs by picrotoxinin, a non‐competitive blocker of ion flow, was reduced 50‐ to 200‐fold for alpha/beta hetero‐oligomeric receptors generated by cotransfection. Site‐directed mutagenesis identified residues within the second predicted transmembrane segment (M2) of the beta subunit as major determinants of picrotoxinin resistance. These data implicate the M2 segment in blocker binding to and lining of the GlyR chloride channel.

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