The anionic permeability of the inhibitory postsynaptic membrane of hippocampal pyramidal cells

The anion permeability and current sensitivity of inhibitory postsynaptic potentials (i.p.s.ps) of CA1 hippocampal pyramidal cells have been studied with intracellular recording techniques. Passage of depolarizing current through the microelectrode increased the size of i.p.s.ps while hyperpolarizing current decreased and eventually reversed the i.p.s.ps. The early portion of the i.p.s.p. was most sensitive to current. The results obtained from the injection of anions into the pyramidal cells indicated that a sharp dichotomy existed: either the anion was permeable as shown by a depolarizing shift in the i.p.s.p., or the anion was impermeable, there being as a consequence a hyperpolarizing shift in the i.p.s.p. The permeable anions include Br$^{-}$, NO$_{2}^{-}$, NO$_{3}^{-}$, SCN$^{-}$, OCN$^{-}$, ClO$_{3}^{-}$ and HCO$_{2}^{-}$, while the impermeable anions included citrate, glutamate, sulphate, methylsulphate, bromate, chromate, acetate and fluoride. These results are identical to those obtained by ion injection into motoneurons. Diffusion of anions into pyramidal cells supported the results from ion injection; namely, that there was a sharp dichotomy between permeant and impermeant anions. However, acetate, and to a lesser extent bromate and glutamate, were exceptions, since these anions led to an abolition or reduction of the hyperpolarizing i.p.s.p. and yet from ion injection studies these anions were clearly impermeant. Possible explanations for this paradox are given in the following paper. The present findings suggest that the anion permeability of the inhibitory postsynaptic membrane in hippocampal pyramidal cells is similar to that reported for spinal motoneurons.