Crayfish Stretch Receptor Neuron Research Articles

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.

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.

Dose-conductance relationships for GABA agonists and the effect of uptake inhibitors in crayfish stretch receptor neurons

The interaction of GABA (gamma-aminobutyric acid) and structurally-related compounds with postsynaptic GABA receptors was studied quantitatively by measuring receptor-mediated increases in membrane input conductance in isolated crayfish stretch receptor neurons (SRN). The following compounds, in order of decreasing potency, were effective agonists: muscimol greater than GABA greater than isoguvacine greater than (-)gamma-amino-beta-hydroxybutyric acid greater than beta-gu anidinopropionic acid greater than 3-aminopropanesulfonic acid greater than (+)gamma-amino-beta-hydroxybutyric acid greater than isonipecotic acid greater than THIP. A highly significant correlation was found between the log potencies for GABA agonists that were obtained in the SRN and those obtained in our laboratory using mammalian GABA receptor binding assays. Hill plot analyses of the log concentration-conductance data from the SRN indicated a Hill slope (nH) of approximately 2 for all agonists except GABA and guanidinopropionic acid (nH greater than 2), two compounds known to be actively accumulated by cellular GABA uptake processes. Nipecotic acid, guvacine, and L-alpha, beta-diaminopropionic acid, blockers of GABA uptake processes, had essentially no effect by themselves on the SRN membrane input conductance at concentrations up to 5 mM, however, they potentiated the effects of sub-maximal concentrations of GABA and decreased the steepness of the log concentration-conductance curve, and consequently nH, for GABA. The effects of muscimol, however, were not affected. When the influence of uptake processes was considered, it appeared that all agonists tested acted by the same cooperative mechanism which required at least two molecules of agonist to activate a receptor-ionophore unit.

Pentobarbitone interference with inhibitory synaptic transmission in crayfish stretch receptor neurones.

1. The effect of pentobarbitone (PB) on GABA-ergic inhibition was investigated in the isolated crayfish stretch receptor. The soma of the slowly adapting neurone was impaled with two micro-electrodes to give an accurate determination of membrane conductances. 2. Application of PB in concentrations from 10(-6) to 10(-3) M increased the rise time constant of the inhibitory post-synaptic potential (i.p.s.p.). The i.p.s.p. percentage amplitude and decay time constant were also increased in eight out of twelve neurones. On prolonged exposure, the percentage amplitude declined at a rate dependent upon the dose and the frequency of stimulation until the i.p.s.p. became undetectable. 3. The response to ionophoretically applied GABA remained essentially unaltered in the presence of PB, but the falling phase was prolonged by up to 8% in four of the ten neurones tested. Resting membrane conductance, i.p.s.p. driving force (i.p.s.p. reversal potential minus resting membrane potential), and parameters of the anti- and orthodromic action potential were not significantly affected. 4. Removal of PB after prolonged exposure usually caused an immediate increase in i.p.s.p. percentage amplitude but the i.p.s.p. rising phase remained slowed. 5. Application of excess extracellular GABA only affected the i.p.s.p. percentage amplitude after it had been reduced by PB. It transiently increased the attenuated i.p.s.p. percentage amplitude in the presence of PB, and after the removal of PB permanently increased the amplitude to its original value. 6. Nipecotic acid and cis-1,3-aminocyclohexane carboxylic acid, inhibitors of GABA re-uptake, slightly increased the i.p.s.p. percentage amplitude, and prolonged the falling phase but did not affect the rising phase. The percentage amplitude declined on prolonged exposure. 7. We conclude that PB has no electrophysiologically demonstrable post-synaptic action in the crayfish stretch receptor neurone, but it inhibits the presynaptic release and re-uptake of GABA.

A model of excitatory synaptic interactions between pacemakers. Its reality, its generality, and the principles involved.

This is a model of the steady-state influence of one pacemaker neuron upon another across a synapse with EPSP's. Its postulates require firstly the spontaneous regularity of both cells, whose intervals are E and N, respectively. In addition, they require a special shortening or negative "delay" of the interspike interval by one or more EPSP's, with a V-shaped dependence of the delay on the position or "phase" of the EPSP's in the interval; the minimum of the delay function corresponds to the earliest EPSP arrival phase (?) that triggers a spike immediately. Finally, they impose on the variables certain bounds. The model's behavior has two main features. The first is a zig-zag relationship with an overall increasing trend between the steady-state pre- and post-synaptic discharge intensities (Fig. 7). The zig-zag is formed predominantly, if not exclusively, by segments with positive slopes that are rational fractions. Passage from one such segment to others is negatively-sloped ("paradoxical"), involving staggered positively-sloped segments whose details are unclear for weak presynaptic discharges and discontinuities for intense discharges. The same postsynaptic intensity may result from several presynaptic ones; the maximum postsynaptic intensity may reflect refractoriness, or the earliest instants of immediate triggering. The second main feature is the "locking" of the discharges in an invariant forward and backward temporal relation. With at most one EPSP per postsynaptic spike, locking is always present. If the presynaptic interval E is in the closed {rN+?,(r+1)N} range, locking is 1:r+1, either stable at a greater-than-? phase or unstable at a smaller one; arrivals at integral multiples of N do not affect the postsynaptic intensity. If E is in {rN, rN+?} (r>0), locking is at other ratios (e.g., 2:3) and less apparent. With more than one EPSP per spike, when E is below bounds that depend on the interspike interval and the point of earliest triggering, locking happens in the simple s?:1 ratio (s?=2,3, ...) and is stable; when E is above those bounds, there are E ranges where locking is in other ratios (e.g., 3:2) and ranges where behavior is unclear. The validity of any model is based jointly upon an a priori judgment as to whether postulates depart reasonably little from nature, and upon an a posteriori experimental comparison of modelled and real behaviors. The model's domain of applicability depends on the specific embodiment, each of the latter tolerating characteristically each departure. The present model will be evaluated in the crayfish stretch-receptor neuron (Diez-Martinez et al., in preparation). The model is applicable to any physical system that complies with its postulates, and evidence compatible with this notion is available in many disparate fields. It illustrates the modelling path to a scientific proposition, other paths being inference from experimentation, or deduction from premises acceptable at other approach levels (in this case, for example, from that of synaptic mechanisms). The periodicity postulates set this model within the category of those for oscillators. The notion of an oscillator has a far broader applicability than appears at first sight, since all physically realizable systems have some predominant output frequency, i.e., to a certain extent are oscillators.

Effects of temperature and deuterium oxide on crustacean stretch receptor

1. The receptor potential was recorded intracellularly from the isolated stretch receptor neuron of crayfish. Total substitution of D2O for H2O in the bathing solution reversibly decreased the amplitude of the receptor potential to a level of 34% of the control. The input resistance of the neuron was slightly increased by the D2O treatment, while the resting potential and the input capacity were virtually unchanged. The viscoelastic property of the receptor muscle was not altered by the D2O substitution. Thus, D2O seems to inhibit directly the activation process of the receptor potential. 2. Temperature change over the range of 25-5 degrees C did not affect the amplitude of receptor potential. However, the input resistance of the neuron was increased about twofold by the lowering of temperature from 25 to 5 degrees C. 3. We conclude that when the temperature is lowered, the receptor current is diminished, but this decline is just about compensated for by the increase in membrane resistance, resulting in a near constancy of the receptor potential. This is the cellular basis for the relative constancy of the transduction process of the stretch receptor in the face of large temperature changes. On the other hand, in the case of D2O substitution the compensation mechanism of the resistance increase does not work, and the sensory transduction is inhibited substantially.

Voltage clamp analysis of anticonvulsant potentiation of GABA responses of crayfish stretch receptor neurones

Voltage clamp analysis of anticonvulsant potentiation of GABA responses of crayfish stretch receptor neurones

Avermectin B1a irreversibly blocks postsynaptic potentials at the lobster neuromuscular junction by reducing muscle membrane resistance.

Avermectin B1a, a macrocyclic lactone with broad spectrum anthelmintic activity, affects neuromuscular transmission in the lobster stretcher muscle. Perfusion of the muscle with 1-10 microgram of the drug per ml eliminates inhibitory postsynaptic potentials within a few minutes. Intracellularly recorded excitatory postsynaptic potentials are gradually reduced in amplitude over 20-30 min, and their falling phases become faster; there is no effect, however, on extracellularly recorded excitatory potentials. Avermectin B1a reduced the input resistance of the muscle fibers with a time course similar to that of the reduction of excitatory potentials. Washing for up to 2 hr with drug-free solution fails to reverse the drug's effects. However, perfusion with 20 microgram of picrotoxin per ml results in recovery of the excitatory potentials and input resistance. Avermectin B1a also blocks the firing of the crayfish stretch receptor neuron, and this block is also reversed by picrotoxin. We hypothesize that the reduction in excitatory postsynaptic potentials after avermectin B1a treatment is caused solely by reduction in membrane resistance; additional experiments suggest that the reduction in membrane resistance is due to the opening of membrane Cl- channels, perhaps including those regulated by gamma-aminobutyric acid at the inhibitory synapse.

Excitation parameters of the repetitive firing mechanism from a statistical evaluation of nerve impulse trains.

Repetitive firing of single tonic neurones is modeled to include in detail both membrane excitation kinetics and electrotonic effects due to membrane non-uniformities in the impulse encoder region. The model is evaluated dynamically and compared with similar data obtained from the crayfish stretch receptor neuron. Two dynamic techniques utilizing small amplitude sinusoidal signals are employed. One technique is used to fix the values of two parameters which relate to the electrotonic control of membrane potential in the interspike interval and to the relaxation time of the K-conductance during repetitive firing. The other technique is employed as a consistency check. The dynamics are particularly sensitive to the K-channel relaxation time in the interspike interval.

Action of Ammonium Ions on the Resting Membrane of Crayfish Stretch Receptor Neuron

In the crayfish stretch receptor neuron, extracellularly applied NH4+ acted on the nonsynaptic membrane and produced not only a depolarizing shift of EIPSP but also depolarization and conductance increase. Under certain conditions these actions may become the main cause of the disinhibitory effect of NH4+ on the inhibitory synapse.

Nonlinear systems analysis of computer models of repetitive firing.

Simulated white noise analysis experiments on a simple integrate-and-fire neuron model with adaptation yields Wiener kernels comparable to those found for a crayfish stretch receptor neuron, for low modulation depths (linear range). At high modulation depths (nonlinear range), this model corresponds well to the neuron only if the "membrane potential" variable is constrained to positive values. An alternative kind of neural model considered is one in which spike initiation processes are ignored, and instead a time-continuous spike frequency variable is used. Such an analytic differential equation model can be represented by a half-wave rectifier with low-pass feedback; simulated white noise analysis of this model shows good correspondence with the stretch receptor, except at the higher frequencies approaching the cell's carrier frequency. The analytic system model is amenable to mathematical analysis using linear and nonlinear systems theory, resulting in equations which relate features of Wiener kernels (peaks, undershoots, time constants, etc.) to previously described features of neurons (threshold, pacemaker sensitivity, adaptation or post-inhibitory rebound).

The mechanism of action of diphenylhydantoin on invertebrate neurons. II. Effects on synaptic mechanisms

The effect of diphenylhydantoin (DPH) on certain synapses of neurons in the abdominal ganglion of Aplysia and on the γ-aminobutyric acid (GABA) mediated inhibitory synapse of the crayfish stretch receptor neuron (SRN) has been studied. DPH decreases the amplitude of the excitatory postsynaptic potential, but is ineffective on the “short” ACh mediated, Cl − dependent, inhibitory postsynaptic potential (IPSP) in Aplysia. However, it facilitates the “long” ACh mediated, K + dependent, IPSP, in this same group of neurons. DHP has a profound effect on the GABA mediated, Cl − dependent, inhibitory synapse of the SRN. The time course of the IPSP is prolonged up to 10 times control values, due to an increased and prolonged postsynaptic conductance. Similar results have been obtained in the SRN with iontophoretic application of GABA. This selective effect of DPH on synaptic mechanismis, especially in view of the role of GABA and ACh as putative transmitters in the mammalian an nervous system, may play an important role for the anticonvulsant action of DPH.

The mechanism of action of diphenylhydantoin on invertebrate neurons. I. Effects on basic membrane properties

The effect of diphenylhydantoin (DPH) has been studied on certain membrane properties of the crayfish stretch receptor neuron (SRN) and of neurons in the abdominal and buccal ganglia of Aplysia. DPH decreases the amplitude of post-tetanic hyperpolarization of the SRN, which is thought to be an expression of the electrogenic pump, and does not antagonize the effect of ouabain on this activity. DPH decreases the membrane resistance of all the different types of neurons studied, with little or no change in the resting membrane potential. It decreases the overshoot of the action potential in some of the neurons studied and prolongs the falling phase and the undershoot in other neurons. DPH also decreases repetitive firing. These effects have also been observed at different external concentrations of potassium. It is concluded that DPH, in the different preparations studied, does not have any effect on or decreases the electrogenic pump, but produces changes in other membrane properties which are consistent with its anticonvulsant action.

Simulation of phase-dependent pattern changes to perturbations of regular firing in crayfish stretch receptor

A comparison was made of responses of a repetitively firing neuron to perturbations imposed in an ongoing steady firing pattern with those predicted by a model.termed the ‘active pacemaker model’ and by a simpler ‘integrate-and-fire’ model. The active pacemaker model simulates the kinetics of processes such as active response generation and electrogenic sodium pumping which govern the membrane potential trajectory between a reset-point following an impulse and threshold for the next impulse. Responses of the crayfish stretch receptor neuron (MRO) and of both models were studied for both abrupt steps and square pulses of current, both hyperpolarizing and depolarizing, as a function of phase of the start of the stimulus in the normal interspike interval. Physiological results are consistent with a substantially weaker effect of a perturbation delivered earlier in the interspike interval as compared to the same one given later. The active pacemaker model successfully simulates several of the prominent results of the physiology (though by no means all), while the linear integrate-and-fire model is less successful.

Relation between trains of action potentials across an inhibitory synapse. Influence of presynaptic irregularity.

The correspondence between pre- and postsynaptic fluctuations in generation of action potentials was studied using the inhibitory synapse on the crayfish stretch receptor neuron. The presynaptic discharges were imposed by an appropriately controlled stimulator modulated with triangular or sine waves at 1/60 to 10 cps by±1 to 10 ips and around an average rate of 5 to 10 ips. The rate of the regular postsynaptic discharge was set, in the absence of IPSPs, at 5 to 20 ips by the degree of constant stretch on the receptor organ. Cycle histograms were constructed, i.e. discharges were averaged across several cycles, presynaptically on the one hand and postsynaptically on the other. This exploration led to the recognition of some of the rules whereby the correspondence between spike trains is established across a synapse with IPSPs. The mapping or coding from the presynaptic to the postsynaptic cycle histogram was complex, and its main facets were: i) the possibility of the transfer of a broad spectrum of frequencies, at least from 1/60 to 10 cps; ii) a general tendency for faster presynaptic discharges to be associated with slower postsynaptic ones (i.e. a negative overall slope in the presynaptic rate-postsynaptic rate graph); iii) interspersed consistently located segments of appreciable size where inhibitory accelerations led to faster postsynaptic discharges (i.e. positively-sloped “paradoxical” segments); iv) a sensitivity to whether the presynaptic cell was accelerating or decelerating (hysteresis); and, finally, v) minimal consequences of changes around extremes where the receptor was either hardly affected or halted by very low or high rates, respectively (saturation). The type, magnitude and ubiquity of the deviations (iii, iv, v) from a simple and negatively-sloped linearity (suggested by ii) makes them integral and practically important facets of inhibitory transfer. The form of the correspondence depended on such issues as the overall pre- and postsynaptic rates (e.g. 5 versus 10 ips), the modulation frequency (e.g. 1/60 versus 2 cps) and depth (e.g.±1 versus 10 ips), etc. When restrictions were placed upon these variables, the correspondence adopted special forms whose particular descriptions (e.g. linear in- or out-of-phase, rectifier-like, etc.) were acceptable only if conditional to those restrictions. A rapid presynaptic irregularity which increased the varibility of individual cycles without altering the average cycle reduced, sometimes markedly, deviations from linearity. This demonstrates the applicability to the living synapse, represented here by a system whose input and output spike trains are assimilated to point processes, of a concept developed for system whose input or output are continuous: namely, that the addition at the input of a high frequency “dither” reduces the complexities of the element. This effect has interesting physiological possibilities and explains apparent discrepancies between earlier publications.

Rhythmic firing of deiters' neurons and synchronization by cerebellar inhibition in lidocaine-treated cats

Cerebellar inhibition of spontaneous or homocysteic acid-induced activity iof dorsal Deiters' neurons was studied in cats anesthetized with chloralose-urethane. Following intravenous lidocaine (5–10 mg/kg), cerebellar inhibition was not reduced but postinhibitory peaks (PIPs) appeared in the postimulus histograms. The number of PIPs increased as variability in firing rate decreased after lidocaine. The frequency of PIPs observed in the histograms approximated the monitored firing rates of the cells. The peaks observed in autocorrelograms constructed after lidocaine had the same frequency, structure and variability as the PIPs. In ‘dot diograms’, only one spike per corresponding PIP was observed. Similar PIPs were observed in poststimulus histograms constructed during inhibition of crayfish stretch receptor neurons which fired regularly in the absence of lidocaine. These studies indicate that the postinhibitory peaks and silent periods were due to synchronized firing of Deiters' neurons induced by cerebellar inhibition, which was detectable only after lidocaine regularized the firing of Deiters' neurons.

Factor I and GABA: Resolution of a long-standing problem

1. 1. Factor I, the inhibitory agent(s) present in certain nerve extracts which is responsible for the inhibition of crayfish stretch receptor neurones, is up to 300 times more effective than can be accounted for by the contained γ-aminobutyric acid (GABA). 2. 2. The inhibition caused by extracts of crab nerves is almost exclusively due to amino acids (aspartate, glutamate, taurine) other than GABA, except in the case of extracts of isolated inhibitory axons. 3. 3. The sensitivity of stretch receptor neurones to aspartate, glutamate and taurine varies independently from that to GABA. As a result of this, there are great quantitative variations in the inhibitory effects of nerve extracts. 4. 4. The mechanism of action of aspartate, glutamate and taurine appears to be the same as that of GABA: all these amino acids activate Cl channels and their action can be blocked by picrotoxin. 5. 5. When tested on the isolated hindgut of the vas deferens of crayfish, aspartate, glutamate and taurine can act like GABA. 6. 6. Extracts of mammalian brain contain a far greater proportion of GABA than do crustacean nerve extracts. GABA thus accounts for about 35 per cent of the inhibitory effect of the brain extracts. Aspartate, glutamate and taurine account for another 30 per cent. The remainder-according to other authors-is due to guanidino compounds, possibly also to γ-amino-β-hydroxybutyric acid. 7. 7. An extensive discussion resolves apparent contradictions in the literature concerning the identity and significance of Factor I, and its relation to GABA.

Action of ammonium on a chloride pump. Removal of hyperpolarizing inhibition in an isolated neuron.

The actions of intra- and extracellularly applied ammonium ions were studied on membrane potentials of the isolated crayfish stretch receptor neuron. In accordance with results obtained from cat spinal motoneurons, ammonium produced a gradual and reversible shift of the IPSP reversal potential towards the resting potential following both kinds of application. In the concentrations tested extracellularly (3 to 15 mM/l), passive membrane properties, spike generation and inhibitory synaptic transmission showed no alterations. The transient shifts of resting potential and spike amplitude that followed intracellular ammonium injection are comparable with those of injected sodium ions in some respect.

Computer Simulation of After-Inhibition in Crayfish Slowly Adapting Stretch Receptor Neuron

A theoretical model is described which is able to mimic the responses of slowly adapting stretch receptor neurons of crayfish to applied currents. Its principal feature is postspike inhibition, in which each nerve impulses produces a small inhibitory current that decays with a simple exponential time-course that is long compared with normal interspike intervals. The model was simulated with both an analogue and a digital computer. Parameters for particular model neurons were determined both by an analysis of experimental data obtained from adaptation to constant injected currents, and by matching computer output to the data. Parameter values estimated by the two techniques agreed within +/-10%. Model parameters determined from adaptation data successfully predicted the magnitude and time-course of posttetanic hyperpolarization (PTH) in the stretch receptor neuron. In addition, model neurons were able to reproduce posttetanic depression (PTD) as seen in stretch receptor cells.

The action of veratridine on the membrane potential of the crayfish stretch receptor neurone.

The results fully support the current idea that veratridine acts on the sodium system of regenerative membranes (see review of Ulbricht, 1969). Evidence against an action of the alkaloid on generator membrane elements comes from the results (5) and (6), where no alteration of the generator potential could be observed after the addition of the drug. Therefore the results also support the hypothesis of Paintal (1964).