Effects produced on inhibitory postsynaptic potentials by the coupled injections of cations and anions into motoneurons

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Abstract
Ions have been injected into cat motoneurons impaled by a double microelectrode. Current has been passed down one barrel and up the other, so injecting cations out of the former and anions out of the latter. The injection of (K$^{+}$+Cl$^{-}$) ions gave a depolarizing shift of the E$_{IPSP}$ almost as large as for a Cl$^{-}$ ion injection, and with a time course comparable with that for injections of Cl$^{-}$ or K$^{+}$ ions alone. The injection of (Na$^{+}$+Cl$^{-}$) ions displaced the E$_{IPSP}$ by much the same amount and again the recovery time was almost as fast as after a Cl$^{-}$ ion injection, and much faster than after the injection of Na$^{+}$ ions alone. On the other hand injection of (2Na$^{+}$+SO$_{4}^{2-}$) ions caused a slight displacement of the E$_{IPSP}$ in the hyperpolarizing direction. It was postulated that, when (K$^{+}$)$_{i}$ is depleted, there is an accelerated operation of an inward pump for (K$^{+}$+Cl$^{-}$) ions with the consequence that the decline of high (Cl$^{-}$)$_{i}$ is slowed by a factor of three or more. The ion injection procedures did not provide evidence for or against the participation of K$^{+}$ ion movements in the generation of the IPSP, but this participation at a level comparable with that of Cl$^{-}$ ions had to be postulated in order to account for the normal hyperpolarizing character of the IPSP.