EFFECTS OF INTRACELLULAR POTASSIUM AND SODIUM INJECTIONS ON THE INHIBITORY POSTSYNAPTIC POTENTIAL

John Eccles’ pioneering role in understanding central synaptic transmission

This chapter deals with the central role that Sir John Eccles played in the elucidation of the mechanisms of synaptic transmission within the central nervous system during the three decades between the late 1930s and 1966. His seminal discoveries involved studies of synaptic input to spinal motoneurons using intracellular recording via glass micropipettes after their introduction in the late 1940s. After defending the hypothesis that electrical currents alone explained central synaptic events, his observations of reversal potentials and sensitivity to ion injections instantly converted Eccles to the idea that central synapses generate postsynaptic potentials, designated IPSPs and EPSPs, by liberating chemical transmitters. He and his collaborators used pharmacological manipulations of recurrent inhibition to support the idea that a given neuron liberates the same chemical transmitter substance at all of its synapses, which he called "Dale's Principle". His team worked out the mechanisms and spinal circuits underlying disynaptic and recurrent inhibition, as well as those of presynaptic inhibition. Not content with the view that central synapses were static entities, Eccles also made seminal observations on synaptic plasticity induced by alterations in use and disuse. Although his firmly held belief that the extensive dendritic trees of motoneurons were essentially irrelevant to synaptic events at the soma was later refuted by others in the mid-1960s, Eccles stands as a towering figure in the history of neuroscience. His prodigious energy and commanding intellect gave the field of central synaptic transmission the conceptual bases that have guided it for over 40 years.

The effects of alkaline cations on the responses of cat spinal motoneurons, and their removal from the cells

Potassium, rubidium, caesium and lithium ions were electrophoretically injected into cat spinal motoneurons. The rising slope of the spike potential was slowed by lithium as well as sodium ions, but not by potassium, rubidium or caesium ions. The falling slope of the spike potential and the after-hyperpolarization following the spike were changed by lithium and caesium as well as by sodium ions, but not by potassium or rubidium ions. It is postulated that lithium ions pass through the sodium channels in the active membrane, and rubidium ions through the potassium channels, but caesium ions through neither. Injections of any alkaline cations change the inhibitory postsynaptic potential in the depolarizing direction. This is explained by assuming that chloride ions move into the cell during the passage of cation-injecting currents. The rates of cation extrusion from motoneurons were estimated from the recovery time courses of the motoneuronal potentials. It is suggested that caesium ions are removed from the cell through the diffusion channels in the resting membrane with a rate comparable to that for sodium extrusion; and that lithium ions are extruded by the sodium pump with about half the rate for sodium extrusion.

EFFECTS PRODUCED ON INHIBITORY POSTSYNAPTIC POTENTIALS BY THE COUPLED INJECTIONS OF CATIONS AND ANIONS INTO MOTONEURONS

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 2 4 - 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 .

THE EXTRUSION OF SODIUM FROM CAT SPINAL MOTONEURONS

Sodium ions were injected into cat spinal motoneurons electrophoretically through an intracellular NaCl-filled microelectrode. Following an injection there were characteristic changes in the resting and spike potentials, the after-potential and the inhibitory postsynaptic potential, all of which recovered within about 7 min. The maximum rising slope of the spike recovered exponentially, suggesting the exponential decrease of the intracellular sodium concentration by the operation of the sodium pump in actively extruding excess sodium. The time course of the recovery of the maximum falling slope of the spike paralleled that of the rising slope, indicating a reciprocal change in the intracellular sodium and potassium concentrations. There was a good parallelism in the time courses of the recovery of the amplitude of the after-potential and the maximum falling slope of the spike, as would be expected from their postulated dependence on the same internal potassium concentration. The inhibitory postsynaptic potential recovered from its displacement in the depolarizing direction with the same time course as did the other potentials, which indicates parallel decreases of the intracellular sodium and chloride concentrations. From the exponential recovery curves obtained for these potentials, the rate constant of active sodium extrusion was estimated as 40 h -1 . The fast rate of sodium extrusion in cat motoneurons is related to the dynamic ionic balance in neurons of the central nervous system, and is explained by the geometry and by the membrane properties of motoneurons.

The response of cat spinal motoneurones to the intracellular application of agents with local anaesthetic action

QX-222 (the trimethyl analogue of lignocaine), methylxylocholine, lignocaine and pentobarbitone were iontophoresed intracellularly into cat lumbosacral motoneurones. Iontophoresis and recording was either from a triple-barrelled microelectrode unit or from two separately advanced microelectrodes. QX-222 and methylxylocholine caused a very slow reversible block of the current-evoked and antidromic action potentials (AP) with no significant change of membrane potential (EM). Lignocaine had a minimal blocking effect on the AP. No change, or only a small decrease, of membrane slope conductance (GM) was seen when the APs had been totally abolished. QX-222 and methylxylocholine reduced the massive GM increase evoked by the passage of large depolarizing currents and converted the post-current hyperpolarization (time constant 120-150 ms) into a depolarization of similar time course. It is suggested that the quaternary local anaesthetics can reduce the fast and slow voltage-dependent potassium conductances. Both agents totally blocked AP generation without decreasing the magnitude of the Ia e.p.s.p. It is suggested that intracellularly iontophoresed QX-222 (on account of its low lipid solubility) could be used as a pharmacological tool to block specifically the active Na and channels in only the cell impaled by the microelectrodes.

Intracellular ion activities and equilibrium potentials in motoneurones and glia cells of the frog spinal cord

Intra-and extracellular ion activities were measured with ion sensitive microelectrodes in motoneurones and glia cells of the spinal cord of the frog. These data were corrected for cross sensitivities of the ion exchangers to intracellular interfering ions, and equilibrium potentials for K +, Na +, Ca2 + and C1- (EK, ENa, ECa and EC1) were calculated. In motoneurones with membrane potentials exceeding -60 mV the following mean equilibrium potentials were determined. ENa = + 29.4mV, EK = -87.9 mV, ECa = + 52.6 mV, EC1 = -34.1 mV. The corresponding values for glia cells were: ENa = + 40.5 mV, EK = -84.0 mV, ECa = + 35.7 mV, EC1 = -59.7 mV. The intracellular ionic milieu is probably disturbed by the impalement of the cells. This transiently decreases the intracellular K + and increases intracellular Na +. These effects were estimated and their origin is discussed. The results of the experiments suggest a non-passive transmembrane distribution of K +, Na + and Ca2 + in motoneurones and glia cells, a non-passive transmembrane distribution of C1- in motoneurones, and a passive transmembrane distribution of C1- in glia cells.

Biochemical and electrophysiological characteristics of mammalian GABA receptors

The concept that GABA is a neurotransmitter in the mammalian CNS is supported by both electrophysiological and biochemical data. Whereas the electrophysiological studies are essential for demonstrating a specific functional response to GABA, the biochemical approach is useful for characterizing the molecular properties of this site. As a result of these studies the concept of the GABA receptor has progressed from a simple model of a single recognition site associated with a chloride channel to a more complex structure having a variety of interacting components. Thus, both electrophysiological and biochemical data support the existence of at least two pharmacologically distinct types of GABA receptors, based on the sensitivity to bicuculline. Also, anatomically, there appear to be two different types of receptors, those located postsynaptically on the soma or dendrites of a neighboring cell and those found presynaptically on GABAergic and other neurotransmitter terminals. From biochemical studies it appears that the GABA receptor may be composed of at least three distinct interacting components. One of these, the recognition site, may exist in two conformations, with one preferring agonists and the other having a higher affinity for antagonists. Ion channels may be considered a second component, with some of these regulating the passage of chloride ion, whereas others may be associated with calcium transport. The third major element of GABA receptors appears to be a benzodiazepine recognition site, although only a certain population of GABA receptors may be endowed with this property. In addition to these, the GABA receptor complex appears to contain substances that modulate the recognition site by influencing the availability of higher affinity binding proteins. It would appear therefore that changes affecting any one of these constituents can influence the characteristics of the others. While increasing the complexity of the system, this arrangement makes for a more sensitive and adaptable receptor mechanism. Thus the GABA receptor can be envisioned as a supramolecular complex of interacting sites, all of which contribute to the functional expression of receptor activation. Because of this complexity, GABA receptors can theoretically be modified in a variety of ways by drug treatment or disease. Accordingly, it may be possible to develop selective agonists and antagonists that may act at one of the basic components, as well as agents that may alter the receptor modulators. Conversely, a disorder of any of these entities may result in an alteration of GABA receptor function, which in turn could contribute to the symptoms of a variety of neuropsychiatric disorders.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of ions on the binding of agonists and antagonists to muscarinic receptors

1 There are no selective effects of Na+, K+, Ca2+, Mg2+ or Cl- on the binding of antagonists or agonists to muscarinic receptors in rat brain. A decrease in affinity related to ionic strength is found for all these ions. 2 Larger effects were produced by T1+, La3+, and some transition metal ions.

Characterization and ionic basis of GABA-induced depolarizations recorded in vitro from cat primary afferent neurones

1. Responses of single cells in the isolated cat spinal ganglion to GABA applied by superfusion or by iontophoresis were recorded using intracellular micro-electrodes. 2. Of the twelve structurally related compounds investigated, GABA was the most effective in its ability to produce a depolarization of the cell membrane. 3. Studies determining concentration-response relationships indicate that two to three molecules of GABA are required to combine with the GABA receptor for activation. 4. Bicuculline and picrotoxin, each act in a non-competitive manner to antagonize the GABA-induced membrane current. 5. The equilibrium potential for iontophoretically induced GABA depolarizations (EGABA) was found to be -23.5 plus or minys 6.1 mV. EGABA was independent upon [cl-]o, but independent of [Na+]o, [K+], or [Ca2+]o. 6. Intracellular injection of twenty antions (Br-, I-, NO2-, NO3-, ClO4-, SCN-, Bf4-, HS-, OCN-, ClO3-, BrO3-, F-, HCO2-, HSO3-, HCO3-, CH3CO2-, SO42-, C6H5O73-) indicated that the activated GABA receptor membrane was permeable to those anions whose hydrated diameter is no larger than that of ClO-3. 7. Restoration of the GABA depolarization to its control level after augmentation by Cl- injection had a mean time constant of 27.8 plus or minus 2.6 min. Picrotoxin did not alter this value. 8. When foreign anions were exchanged for Cl- in the perfusion solution, the ten anaions smaller or equal to ClO3-, decreased the GABA depolarization by 50-90% and increased its time course 1.5-2.0 x control. The only exception having a small radius was Br- which augmented the amplitude 10-30%. 9. The ten anions larger than ClO3- produced a biphasic effect, i.e. an initial augmentation followed by a marked (up to 100%) depression of the response. Experiments with CH3COO-, CH3SO4-, or HOCH2CH2SO3-, indicated that this depression was non-competitive.

The ionic mechanisms concerned in generating the i.p.s.ps of hippocampal pyramidal cells

A variety of experimental tests has been initiated in order to discover whether the large hyperpolarizing inhibitory postsynaptic potentials (i. p. s. ps) of hippocampal pyramidal cells are generated by the influx of Cl¯ down a gradient maintained by an outward pumping of Cl¯ across the membrane, as has been proposed by Lux, Llinás and associates for other i. p. s. ps. Intravenous infusion of NH 4 acetate or intracellular NH 4 acetate caused little depression of extracellular field potentials and of i. p. s. ps recorded intracellularly, i. e. there was no evidence for the blockade of an outwardly directed chloride pump. The recovery time constants in seconds from an increase in intracellular chloride, either by chloride injections (22.7 ± 6.9) or by passage of depolarizing current through K + salt-filled microelectrodes (20.6 + 6.8) did not differ from the time constant of recovery from depleted intracellular chloride by passage of hyperpolarizing current through electrodes containing K + salts of impermeant anions (21.1 + 5.4). Depletion of the intracellular K + concentration following sodium injections caused a long-lasting depolarizing shift in the i. p. s. p. with a recovery time constant of almost 70 s. These results are identical with those obtained in spinal moto­neurons, where the very slow recovery was explained by an inward KCl pump triggered by low internal K + . Our results suggest that an outward Cl¯ pump dependent on internal Cl¯ concentration does not exist in hippocampal neurons or at least on their somatic membrane. Two alternative hypotheses are given to account for our negative findings with respect to NH 4 acetate action on the hyperpolarizing i. p. s. ps and on the rate of Cl¯ movements across the membrane. First, the original hypothesis as proposed by Eccles and collaborators, in which conductance increases to both Cl¯ and K + ions produce the hyperpolarizing i. p. s. ps of hippocampal neurons. However, we have no positive evidence for the involvement of K + ions. Secondly, an outward Cl¯ pump keeps the E Cl more negative than the resting potential and the i. p. s. p. is solely caused by Cl¯ as postulated by Lux, Llinás and associates. This pump is located remotely in the dendrites and is resistant to the action of NH 4 acetate. This pump would have to be effective in a background manner so that it did not interfere appreciably with the diffusional exchange of Cl¯ ions across the soma membrane.

The ionic permeability changes during acetylcholine-induced responses of Aplysia ganglion cells

ACh-induced depolarization (D response) in D cells markedly decreases as the external Na(+) is reduced. However, when Na(+) is completely replaced with Mg(++), the D response remains unchanged. When Na(+) is replaced with Tris(hydroxymethyl)aminomethane, the D response completely disappears, except for a slight decrease in membrane resistance. ACh-induced hyperpolarization (H response) in H cells is markedly depressed as the external Cl(-) is reduced. Frequently, the reversal of the H response; i.e., depolarization, is observed during perfusion with Cl(-)-free media. In cells which show both D and H responses superimposed, it was possible to separate these responses from each other by perfusing the cells with either Na(+)-free or Cl(-)-free Ringer's solution. High [K(+)](0) often caused a marked hyperpolarization in either D or H cells. This is due to the primary effect of high [K(+)](0) on the presynaptic inhibitory fibers. The removal of this inhibitory afferent interference by applying Nembutal readily disclosed the predicted K(+) depolarization. In perfusates containing normal [Na(+)](0), the effects of Ca(++) and Mg(++) on the activities of postsynaptic membrane were minimal, supporting the current theory that the effects of these ions on the synaptic transmission are mainly presynaptic. The possible mechanism of the hyperpolarization produced by simultaneous perfusion with both high [K(+)](0) and ACh in certain H cells is explained quantitatively under the assumption that ACh induces exclusively an increase in Cl(-) permeability of the H membrane.