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

Intracellular autogenetic and synergistic effects of muscular contraction on flexor motoneurones

1. Intracellular records have been taken from cat motoneurones innervating flexor muscles of the hind limb. Contractions of the ankle flexors tibialis anterior and extensor digitorum longus were elicited by stimulation of the peripheral end of the cut L 7 ventral root and the reflex effects of these contractions were recorded in silent and repetitively firing motoneurones.2. Contraction usually produces a hyperpolarizing response inside flexor motoneurones. This hyperpolarization is tension-sensitive in the sense that when, at constant muscle extension, the strength of the contraction is increased, the magnitude of the inhibitory response is augmented.3. Increasing the resting length of the muscles, while using a stimulus of constant strength to the ventral root, causes this inhibitory response to increase in some cells. More often, however, the hyperpolarization caused by contraction is gradually reduced in duration and/or amplitude as the muscles are extended.4. Even with the muscles slackened, so that they develop no tension at their ends, contraction usually produces prominent hyperpolarization of the motoneurones.5. By passing polarizing currents or injecting chloride ions through the intracellular micro-electrode, the hyperpolarizing potentials produced by contraction of the slack and extended muscles are shown to be, at least in part, genuinely post-synaptic inhibitory events.6. When the neurone is fired repetitively by injected current, the ;silent period' in contraction corresponds to the hyperpolarization of the post-synaptic membrane.7. Monosynaptic testing of the flexor motoneurone pool has been used to confirm the essential features of the intracellularly recorded activity.8. Acutely spinalizing the animal increases the magnitude of the inhibitory responses caused by contraction.9. Recordings from dorsal root fibres show that Golgi tendon organs of the ankle flexors are very sensitive to contraction and are indeed often activated by the internal forces developed in a contracting slack muscle.10. A number of muscle spindles of the ankle flexors are activated by stimulation of the ventral root at a strength submaximal or just maximal for the alpha-motor fibres, despite the simultaneous unloading effect of the contracting extrafusal fibres. This spindle activation, which occurs mainly during the phase of tension development in contraction, is favoured by an increased extension of the muscle. Attempts were made to establish whether it is due to alpha-motor innervation of the receptors or to some mechanical interaction between the intra- and extrafusal muscle fibres.11. The possible central and peripheral causes of the changes in motoneurone excitability produced by flexor muscle contraction are discussed. It is suggested that tendon organs of flexor muscles strongly inhibit flexor motoneurones and that alpha-motor innervation of muscle spindles is likely to play a more prominent role in flexors than in extensor muscles.

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.

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.

Enhancement of t-[35S]butylbicyclophosphorothionate and [3H]strychnine binding by monovalent anions reveals similarities between gamma-aminobutyric acid- and glycine-gated chloride channels

The characteristics of [3H]strychnine and t-[35S]-butylbicyclophosphorothionate ([35S]TBPS) binding to sites associated with glycine- and gamma-aminobutyric acid (GABA)-gated chloride channels were compared in the presence of a series of anions with known permeabilities through these channels. Good correlations were found between (a) the potencies (EC50) of these anions to stimulate radioligand binding and their permeabilities relative to chloride; (b) the affinities (KD) of these radioligands in the presence of fixed concentrations of these anions and their relative permeabilities; (c) the potencies (EC50) of these anions to stimulate [35S]TBPS and [3H]strychnine binding; and (d) the affinities (KD) of [3H]strychnine and [35S]TBPS measured at a fixed concentration of these anions. These studies support electrophysiological and biochemical observations demonstrating similarities between glycine- and GABA-gated chloride channels, and suggest that anions enhance [3H]strychnine and [35S]TBPS binding through specific anion binding sites located at the channels.

Post-synaptic inhibition of bulbar inspiratory neurones in the cat

Stable intracellular recordings from thirty-six bulbar inspiratory neurones revealed three centrally originating, rhythmic patterns of synaptic inhibition (i.p.s.p.s). (i) A declining pattern of i.p.s.p.s accompanying the early stages of inspiration (early inspiratory inhibition) was identified in a total of twenty neurones representing examples of each of the functional classes of bulbar neurones examined, i.e. six R alpha- and two R beta-neurones of the dorsal respiratory group and twelve R alpha-neurones of the ventral respiratory group. (ii) A transient pattern of i.p.s.p.s just preceding or coinciding with the cessation of inspiration (late inspiratory inhibition) was present in the remaining sixteen neurones which were tested, representing six R alpha-neurones and three R beta-neurones of the dorsal respiratory group and seven R alpha-neurones of the ventral respiratory group. (iii) An augmenting pattern of expiratory i.p.s.p.s was present in all thirty-six neurones. Late inspiratory and expiratory i.p.s.p.s in the same neurones showed a similar time course of reversal when chloride was injected or allowed to diffuse into the cells and were associated with similar increases in input conductance. Both patterns of i.p.s.p.s appear to arise at or close to the cell soma. Early inspiratory i.p.s.p.s required a relatively longer period of chloride injection for reversal to be accomplished. Input conductance changes were either absent or smaller than those associated with late inspiratory or expiratory inhibition. These i.p.s.p.s appear to arise at more distal dendritic sites. These patterns of i.p.s.p.s are discussed in relation to the mechanisms shaping the growth of central inspiratory activity, bringing this activity to an end, and suppressing its redevelopment during expiration.

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 role of intracellular chloride in hyperpolarizing post-synaptic inhibition of crayfish stretch receptor neurones

1. The intracellular Cl(-) activity (a(Cl) (i)) of isolated crayfish stretch receptor neurones was measured using liquid ion exchanger Cl(-)-selective micro-electrodes. The potential developed due to the difference between the normal extracellular Cl(-) activity (a(Cl) (o)) and a(Cl) (i) (V(Cl)) was compared with the simultaneously measured reversal potential of the inhibitory post-synaptic potential (E(i.p.s.p.)) to further clarify the ionic basis of the i.p.s.p..2. In normal Ringer solution, V(Cl) (63.3 +/- 2.3 mV) was found to be close to the resting membrane potential (E(m), 62.6 +/- 3.9 mV) while E(i.p.s.p.) (74.5 +/- 1.9 mV) was more negative than either. The V(Cl) value corresponds to an apparent a(Cl) (i) of 12.7 +/- 1.3 mM, which is about 4 mM more than required for a Cl(-) governed E(i.p.s.p.) of 74.5 mV.3. Reducing a(Cl) (o) caused smaller changes in V(Cl) than predicted for passive Cl(-) re-distributions. On complete removal of extracellular Cl(-) (Cl(o) (-)), V(Cl) increased to 84.6 +/- 2.7 mV, equivalent to an apparent a(Cl) (i) of about 5 mM-Cl(-). This value can be used as an estimate of the level of intracellular interference on the Cl(-)-selective micro-electrode.4. Increasing extracellular K(+) (K(0) (+)) decreased both V(Cl) and E(i.p.s.p.). Decreasing K(o) (+) had the converse effect. The time course of the changes in V(Cl) and E(i.p.s.p.) was much the same. The difference between V(Cl) and E(i.p.s.p.) decreased to about 3 mV in high K(o) (+), and increased to about 30 mV in low K(o) (+). This variation in the difference between E(i.p.s.p.) and V(Cl) is consistent with the assumption that anions other than Cl(-) contribute to the recorded V(Cl) rather than another ion contributes to the inhibitory current.5. Application of 5 mM-NH(4) (+) or of frusemide (6 x 10(-4) M) decreased V(Cl) and E(i.p.s.p.). The difference between V(Cl) and E(i.p.s.p.) was also decreased.6. We conclude that a(Cl) (i) is lower than predicted from a passive distribution and thus the chloride equilibrium potential (E(Cl)) is more negative than E(m). If a constant intracellular interference equivalent to about 4 mM-Cl(-) is assumed to contribute to the recorded V(Cl), E(Cl) was approximately equal to E(i.p.s.p.) in all the experimental conditions. Therefore we suggest that the i.p.s.p. is solely generated by Cl(-) ions.

Ammonium action on post-synaptic inhibition in crayfish neurones: implications for the mechanism of chloride extrusion

1. The reversal potential of the Cl(-)-dependent, inhibitory post-synaptic potential (E(i.p.s.p.)) was measured in the isolated crayfish stretch receptor neurone using two intracellular micro-electrodes. The difference between E(i.p.s.p.) and the resting membrane potential (E(m)), the i.p.s.p. driving force, was reversibly decreased by addition of NH(3)/NH(4) (+), and the mechanism of this decrease was investigated.2. The NH(3)/NH(4) (+)-induced decrease in i.p.s.p. driving force was dose-dependent with an onset at about 0.2 mM. E(i.p.s.p.) always remained more negative than E(m) or, when the neurone was spontaneously firing, the threshold potential. E(m) and resting membrane resistance (R(m)) also decreased in a dose-dependent fashion. Synaptic conductance (g(s)) increased with low doses, but decreased on application of 20 mM-NH(3)/NH(4) (+). All the effects were fully reversible on return to normal Ringer solution.3. Intracellular acidification (substitution of 50% Cl(-) by acetate compared with isethionate) considerably reduced the i.p.s.p. driving force. Simultaneous application of NH(3)/NH(4) (+) and acetate-substituted Ringer solution caused a similar decrease in the driving force to application of the same concentration of NH(3)/NH(4) (+) under normal conditions. Increasing the extracellular pH at which a given concentration of NH(3)/NH(4) (+) was applied caused a smaller decline in the i.p.s.p. driving force. These results suggest that intracellular acidification decreases the i.p.s.p. driving force and that the NH(3)/NH(4) (+)-induced decline is caused by an action of the ammonium ion.4. Elevation of extracellular K(+) (K(+) (0)) decreased the i.p.s.p. driving force, E(m) and R(m), and increased g(s). Reduction of K(+) (0) had the converse effects on all parameters.5. Application of Rb(+) or Cs(+) mimicked the effects of NH(3)/NH(4) (+). Substitution of K(+) (0) by Rb(+), Cs(+) or NH(3)/NH(4) (+) opposed or even reversed the increase in i.p.s.p. driving force when Na(+) was used as the substitute. The effectiveness of the various cations in decreasing the driving force was in the following order: Rb(+) > NH(4) (+) > K(+) > Cs(+).6. Inhibition of the Na pump by ouabain or K(+)-free Ringer solution caused a gradual reduction in the i.p.s.p. driving force. Since the driving force also decreased when the Na(+) gradient probably was increased (elevated K(+) (0)), this suggests a dependence on the K(+) gradient rather than the Na(+) gradient or the Na pump itself.7. Frusemide (6 x 10(-4) M) reversibly decreased the i.p.s.p. driving force and E(m), and increased g(s). R(m) was not significantly affected. Application of frusemide in the presence of 5 mM-Rb(+) and vice versa, caused a further reduction in the driving force. The recovery of the driving force on removal of either agent was slowed by the presence of the other.8. Application of 4,4-diisothiocyanostilbene-2,2-disulphonic acid (DIDS; 10(-4) M) caused spontaneous firing and reduced E(i.p.s.p.) to the threshold potential. R(m) and g(s) increased. The effects were slowly reversible on removal of the drug.9. It is proposed that the i.p.s.p. driving force is maintained by a K(+)-Cl(-) co-transport mechanism, driven by the K(+) gradient. The K(+) site exhibits the binding selectivity: Rb(+) > NH(4) (+) > K(+) > Cs(+) and the mechanism is inhibited partially by frusemide and completely by DIDS.

Electrogenic Na pump evidenced by injecting various Na salts into the isolated A-V node cells of rabbit heart

Electrogenicity of the Na pump was confirmed by injecting Na salts into a small cluster of A-V node cells. Injection of Na glutamate or Na acetate induced marked hyperpolarization, accompanying with cessation of spontaneous activity. The hyperpolarization exceeded EK in 27 mM K Tyrode solution and was inhibited by 10(-5) M strophanthidin. Injection of NaCl or NaI depolarized the membrane. These data showed that inward-going current carried by the injected anion antagonized the outward-going pump current and thus determined the net effect of the injection.

Voltage-sensitive outward currents in cat motoneurones

1. The soma membrane of cat motoneurones was voltage-clamped in vivo using intracellular current and voltage electrodes whose tips were separated by at least 5 micrometer. 2. Depolarization activates two separate, non-interacting K conductance systems whose rates of activation and decay differ by a factor of about 10. These conductances have a similar reversal potential, in the range of -6 to -21 mV (these and all subsequent voltages are expressed relative to the resting potential). Both conductances show linear 'instantaneous' current-voltage relationships. The steady-state magnitudes of both conductances increase with increasing depolarization. Neither conductance inactivates substantially during prolonged depolarizations. 3. The faster K conductance is similar to that described for squid axons and frog node. Activation begins at about +30 mV and is more than 90% complete within 5 msec of a depolarizing voltage step to +50 mV. Activation kinetics appear to be nonlinear. This fast K conductance contributes to the fast falling phase of the action potential. Following repolarization, this conductance decays with a time constant of 2-4 msec. 4. The slower K conductance activates during depolarizations of 10 mV or greater. The activation and decay of this conductance can be described by first-order exponential functions with time constants ranging from 20 to 50 msec. The slow K conductance underlies the prolonged hyperpolarization that follows motoneurone action potentials. Evidence from other studies suggests that this slow K conductance is regulated by intracellular Ca ions. 5. In addition to the two K conductance systems activated by depolarization, motoneurones exhibit another distinct conductance system that is activated by hyperpolarization. This third system has a reversal potential near the resting potential. Activation of this conductance during a hyperpolarizing voltage step can be fitted by a single exponential function with a time constant of 50-60 msec over the range -20 to -50 mV. This hyperpolarization-activated conductance accounts for some aspects of the anomalous rectification reported in cat motoneurones. 6. When the clamp circuit was turned off and the motoneurones were stimulated to discharge repetitively by depolarizing current steps, the apparent soma threshold voltage increased as the applied current (and discharge frequency) increased. 7. The basic features of the motoneurone action potential were reconstructed by simulations based on voltage clamp measurements of the voltage dependent conductance systems and previous measurements of passive membrane properties. These simulations assumed that the kinetics of the fast Na and K conductance systems in motoneurones can be described by equations of the same form as the Hodgkin-Huxley equations. These action potential reconstructions indicated that a major portion of the delayed depolarization following the action potential is attributable to capacitative currents from the dendrites...

A depolarizing inhibitory potential in neurones of the olfactory cortex in vitro

1. Stable intracellular recordings were obtained from neurones in slices of the guinea-pig olfactory cortex maintained in vitro. 2. Single stimuli applied to the lateral olfactory tract (l.o.t.) produced an excitatory post-synaptic potential (e.p.s.p.) usually generating a single spike. 3. The e.p.s.p. was followed by a long (200-500 msec) after-depolarization (l.a.d.) of peak amplitude 5-16 mV. This was accompanied by a very large conductance increase and was associated with an inhibition of the intracellularly recorded e.p.s.p. and of spike generation. 4. The l.a.d. was more susceptible than the e.p.s.p. to depression by (i) repetitive l.o.t. stimulation and (ii) raising external [Mg2+]. The l.a.d. could be generated without a preceding spike. 5. At an average resting membrane potential of -74 mV the average reversal potential for the l.a.d. (El.a.d.) was -63 mV.El.a.d. became more positive on reducing [Cl-]out or on using KCl-filled electrodes. 6. It is concluded that the l.a.d. represents a Cl- -mediated inhibitory post-synaptic potential, generated through deep-lying recurrent inhibitory loops.

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.

On the transmitter function of 5-hydroxytryptamine at excitatory and inhibitory monosynaptic junctions

1. Two symmetrical giant neurones located in the cerebral ganglion of Aplysia californica contain 4-6 p-mole 5-hydroxytryptamine (5-HT) and are able to synthesize it (Weinreich, McCaman, McCaman & Vaughn, 1973; Eisenstadt, Goldman, Kandel, Koike, Koester & Schwartz, 1973). Stimulation of each of these neurones evokes excitatory and inhibitory potentials in various cells of the ipsilateral buccal ganglion. In nine buccal neurones it evokes excitatory potentials, in other three, ;classical' inhibitory potentials and in one neurone an ;atypical' inhibitory potential.2. The connexion between the giant cerebral neurone and the cells receiving either an excitatory or a ;classical' inhibitory input from it are monosynaptic. TEA injection into the cerebral giant neurone, which prolongs the presynaptic spike, causes a gradual increase of both the excitatory and the inhibitory potentials. On the other hand, high Ca(2+) media, which block polysynaptic pathways, do not suppress these synaptic potentials.3. The iontophoretic application of 5-HT to the buccal neurones receiving excitatory input from the giant cerebral neurones evokes depolarizations showing the pharmacological properties of both A- and A'-responses to 5-HT (see preceding paper). Antagonists which block only the A-receptors (curare, 7-methyltryptamine, LSD 25) block partially the synaptic depolarizing potentials. Bufotenine, which blocks both the A- and A'-receptors, completely blocks the excitatory potentials. Thus, the post-synaptic membrane of these buccal neurones appears to be endowed with both A- and A'-receptors to 5-HT.4. The ;classical' inhibitory potentials elicited in three buccal neurones are hyperpolarizations which reverse at - 80 mV and are due to an increase in K(+)-conductance. The iontophoretic application of 5-HT to these post-synaptic neurones evokes hyperpolarizing B-responses which are also generated by an increase in K(+)-conductance. Antagonists which block the B-responses (bufotenine, methoxygramine) also block the inhibitory potentials.5. The ;atypical' inhibitory potential evoked in one buccal neurone consists in an hyperpolarization which increases in amplitude with cell hyperpolarization. Iontophoretic application of 5-HT to this buccal cell evokes an hyperpolarizing beta-response which also increases in amplitude with cell polarization and results from a decrease in both Na(+)- and K(+)- conductances. The monosynaptic character of the ;atypical' inhibitory potential is not yet fully proven.6. It can be concluded that the excitatory and inhibitory synaptic effects evoked in the buccal neurones by the stimulation of the 5-HT-containing-giant cerebral neurones are very likely mediated by 5-HT.

Active transport of chloride by the giant neuron of the Aplysia abdominal ganglion

Internal chloride activity, a(i) (Cl), and membrane potential, E(m), were measured simultaneously in 120 R2 giant neurons of Aplysia californica. a(i) (Cl) was 37.0 +/- 0.8 mM, E(m) was -49.3 +/- 0.4 mv, and E(Cl) calculated using the Nernst equation was -56.2 +/- 0.5 mv. Such values were maintained for as long as 6 hr of continuous recording in untreated neurons. Cooling to 1 degrees -4 degrees C caused a(i) (Cl) to increase at such a rate that 30-80 min after cooling began, E(Cl) equalled E(m). The two then remained equal for as long as 6 hr. Rewarming to 20 degrees C caused a(i) (Cl) to decline, and E(Cl) became more negative than E(m) once again. Exposure to 100 mM K(+)-artificial seawater caused a rapid increase of a(i) (Cl). Upon return to control seawater, a(i) (Cl) declined despite an unfavorable electrochemical gradient and returned to its control values. Therefore, we conclude that chloride is actively transported out of this neuron. The effects of ouabain and 2,4-dinitrophenol were consistent with a partial inhibitory effect. Chloride permeability calculated from net chloride flux using the constant field equation ranged from 4.0 to 36 x 10(-8) cm/sec.

The physiological role of three acetylcholine receptors in synaptic transmission in Aplysia

1. It is shown that a single presumably cholinergic presynaptic neurone can mediate, monosynaptically, multicomponent responses in a given cell and different responses in different cells.2. Complex responses (whether evoked synaptically or by ACh injection) are shown to be the result of the coexistence on a given post-synaptic neurone of more than one of three cholinergic receptor types previously described. Likewise, different responses in different cells are due to the fact that different post-synaptic neurones bear different combinations of these three receptors.3. Pharmacological analysis shows that the multicomponent nature of many of the responses is not always evident: what appears, under normal conditions, to be a single-component excitatory potential can be shown often to be a complex response consisting of superimposed e.p.s.p.s and rapid i.p.s.p.s which are sometimes, though not always, accompanied by a slow i.p.s.p.4. Although which and how many of the three receptor types is the major factor contributing to the type of response observed, in the case of some of the synaptic potentials certain other factors were found to contribute to the final response form. First, in the large cells of the visceral ganglion, as well as in the left giant cell of the pleural ganglion, there is a marked ;electrical separation' between the region in which the synaptic currents are generated and the point of recording. This ;electrical distance' often altered the inversion potential, and sometimes the form of the responses. Secondly, in some visceral neurones, activation of the cholinergic presynaptic neurone L10 causes (either directly or indirectly) a potential change which cannot be accounted for in terms of the activation of cholinergic receptors. This ;non-cholinergic' response (not imitated by an ionophoretic injection of ACh) is unmasked by the blocking of all three cholinergic receptors. It contributes differentially in different cells to the total response pattern produced by L10 under normal conditions, but its contribution is often characterized by a late hyperpolarizing phase which appears to be impossible to invert. This phase has been shown, however, to be dependent upon the potassium concentration in the extracellular space surrounding the synapse.4. It is tentatively suggested that this residual, non-cholinergic element of the synaptic response in some visceral cells be the result of the activation of an electrical synapse.

Inhibitory and excitatory effects of dopamine on Aplysia neurones

1. Electrophoretic application of dopamine (DA) on Aplysia neurones elicits both excitatory and inhibitory effects, which in many cases are observed in the same neurone, and often result in a biphasic response.2. The DA receptors are localized predominantly on the axons. Desensitization, which occurs after repeated injections or with bath application of DA, is more marked for excitatory responses.3. Tubocurarine and strychnine block the DA excitatory responses without affecting the inhibitory ones, which can be selectively blocked by ergot derivatives. It is concluded that the excitatory and inhibitory effects are mediated by two distinct receptors.4. The two DA receptors can be pharmacologically separated from the three ACh receptors described in the same nervous system.5. In some neurones the dopamine inhibitory responses can be inverted by artificial hyperpolarization of the membrane at the potassium equilibrium potential, E(K), indicating that dopamine causes a selective increase in potassium permeability.6. In other neurones the reversal potential of dopamine inhibitory responses is at a more depolarized level than E(K), but can be brought to E(K) by pharmacological agents known to block the receptors mediating the excitatory effects of DA.7. In still other neurones, the hyperpolarization induced by DA cannot be inverted in normal conditions, but a reversal can be induced by ouabain or by the substitution of external sodium by lithium. These results are discussed in terms of an hypothesis in which dopamine increases the potassium permeability of a limited region of the axonal membrane.8. It is concluded that a selective increase in potassium permeability probably accounts for all dopamine inhibitory effects in the neurones studied.

Spatial synaptic distribution of recurrent and group Ia inhibitory systems in cat spinal motoneurones

1. The reversal potentials of several types of inhibitory post-synaptic potentials (IPSPs) have been studied in cat spinal motoneurones with and without modification of intracellular chloride ion (Cl(-)) concentration. Single barrel intracellular micropipette electrodes have been used.2. When studied with potassium citrate filled micropipettes, the reversal potential for IPSPs evoked by stimulation of antagonist group Ia afferents is the same as that for recurrent IPSPs evoked by antidromic stimulation of motoneurone axon collaterals, confirming earlier observations (Araki, Ito & Oscarsson, 1961; Coombs, Eccles & Fatt, 1955).3. Studied with potassium chloride filled micropipettes. the reversal potential for the group Ia IPSP was found to be different from that for the recurrent IPSP when the amount of Cl(-) diffusing or iontophoretically injected into the motoneurone was small. The amount of difference in reversal potential varied from cell to cell but when present the group Ia IPSP reversed to a depolarizing potential more readily than the recurrent IPSP in all cases.4. Interaction between recurrent IPSPs and monosynaptic excitatory post-synaptic potentials (EPSPs) was also studied and the amount of non-linearity of potential summation was measured.5. The results are consistent with the hypothesis that the terminations of Renshaw cells responsible for the recurrent IPSP are located largely on the proximal dendrites of motoneurones, while the terminations of the interneurones generating the group Ia IPSP appear to be closer to or on the cell somata.

Membrane current and intracellular sodium changes in a snail neurone during extrusion of injected sodium

1. Sodium was injected into an identified snail neurone by passing current between two intracellular micro-electrodes, the membrane potential being recorded with a third micro-electrode.2. The injection of about 25 p-equiv Na, but not the injection of similar quantities of K or Li, caused a hyperpolarization of up to 20 mV. This response to Na injection was blocked by application of ouabain or removal of external K, indicating that it was due to the stimulation of an electrogenic pump.3. To measure the current produced by the sodium pump the output of a feed-back amplifier was fed into the cell via a fourth intracellular micro-electrode so as to keep the average membrane potential constant. The pump current, measured in this way, rose at a constant rate during, and declined exponentially after, an injection of Na, the decline having an average time constant of 4.4 min. The total charge transferred by the pump was between a third and a quarter of the charge passed to inject sodium.4. An intracellular Na-sensitive glass micro-electrode was used to follow changes in the concentration of intracellular Na ions. The results showed that both the pump current and the rate of Na extrusion were proportional to the concentration of intracellular Na ions above the normal level.5. It was concluded that about two thirds of the Na extruded was coupled to the active transport of other ions, probably to the uptake of K, the uncoupled third producing the electrogenic effect.

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.

Two different ionic mechanisms generating the spike "positive" afterpotential in molluscan neurons

The ionic bases of the "positive" afterpotential (ap) have been examined in the so-called DInhi neurons of the central nervous system of Cryptomphallus aspersa. In these cells E(K) has been determined and its value compared with the equilibrium, potential of the ap (E(ap)). It has been found that in half of the studied cells the E(K) value is very close to E(ap) whereas in another half, the difference (E(K) - E(ap)) is large and amounts to circa -10 mv. The effects of changes in the concentration gradients of K(+), Cl(-), and Na(+) were assayed in both groups of cells. When the [K(i)/[K](o) ratio is reduced in both groups of neurons, the ap amplitude and the E(ap) diminished. In cells displaying a large (E(K) - E(ap)), Cl-free Ringer's solution diminished the ap amplitude and E(ap), but produced no effect in the neurons with a reduced (E(K) - E(ap)). A similar effect was observed if [Cl], was increased by intracellular injection of NaCl. Changes in both [Na](o) and [Na](i) were ineffective. It is concluded that K(+) is the only ion involved in the origin of the ap in the groups of cells with a low value for (E(K) - E(ap)). On the contrary, the ap of the neurons presenting large (E(K) - E(ap)) is produced by a simultaneous increase in the fluxes of both K(+) and Cl(-).

Post-tetanic hyperpolarization and electrogenic Na pump in stretch receptor neurone of crayfish

1. Two types of after-potentials in the stretch receptor neurone of crayfish are described.2. A short-duration after-hyperpolarization associated with a single spike or a few spikes is diminished and reversed on applying hyperpolarizing currents. However, a much longer-lasting post-tetanic hyperpolarization (PTH) is enhanced by conditioning hyperpolarization; thus, no reversal potential can be obtained.3. No changes in membrane conductance occur during PTH.4. Reducing K concentration in the bathing fluid diminishes PTH, while it shifts the reversal potential of the short after-potential toward greater negativity.5. Replacement of Na with Li, or addition of 2,4-dinitrophenol in the bathing fluid suppresses PTH in a reversible manner.6. Electrophoretic injection of Na into the cell induces a long-lasting hyperpolarization.7. No change in K-equilibrium potential, as indicated by the reversal point of the short after-potential, is detected during PTH.8. It is concluded that the short after-potential is caused by a permeability increase for potassium ion, whereas PTH is produced by an electrogenic Na-pump.