School of Physiology & Pharmacology, The University of New South Wales, Sydney, 2052 Australia
Both GABA- and glycine-activated currents were observed in tissue-cultured hippocampal neurons of one day old rat pups and single channel currents were measured through these inhibitory neurotransmitter-activated chloride channels to determine their anion selectivity and concentration dependence of permeation. Current-voltage relations for both agonists displayed rectification with single channel conductance increasing at positive potentials. Permeabilities determined from reversal potentials, were maximal for anions with diameters between 4 and 5 Å. Larger diameter anions had lower permeabilities consistent with a minimum pore diameter of 5.6 Å for both agonist-activated channels. The permeability for anions of similar size was largest for those ions with a more symmetrical charge distribution (e.g., NO3- > HCO3-). The permeability sequence was SCN- > NO3- > I- > Br- > Cl- > Formate- > Acetate- > HCO3- > Gluconate- > F- > Phosphate-, whereas the conductance sequence was Cl- > Br- > NO3- > I- > SCN- > Formate- > Acetate- > Bicarbonate- > Gluconate- > F- > Phosphate-. These results suggest that the ions interact with sites within the channel, with hydration forces contributing an important component to the barrier for ion entry into the channel. The spherically symmetrical halides displayed an exponential relationship between relative permeability and hydration energy. The less permeant anions also displayed partial channel block. Cl- and SCN- exhibited mole-fraction effects. These results suggest that both agonist-activated channels act as multi-ion pathways with fairly similar permeation characteristics.