Inhibitory synaptic potential of frog motor neurons

THE MECHANISM OF DISCHARGE PATTERN FORMATION IN CRAYFISH INTERNEURONS

Excitatory and inhibitory processes which result in the generation of output impulses were analyzed in single crayfish interneurons by using intracellular recording and membrane polarizing techniques. Individual spikes which are initiated orthodromically in axon branches summate temporally and spatially to generate a main axon spike; temporally dispersed branch spikes often pace repetitive discharge of the main axon. Hyperpolarizing IPSP's sometimes suppress axonal discharge to most of these inputs, but in other cases may interact selectively with some of them. The IPSP's reverse their polarity at a hyperpolarized level of membrane potential; they sometimes exhibit two discrete time courses indicating two different input sources. Outward direct current at the main axon near branches causes repetitive discharges which may last, with optimal current intensities, for 1 to 15 seconds. The relation of discharge frequency to current intensity is linear for an early spike interval, but above 100 to 200 impulses/sec. it begins to show saturation. In one unit the current-frequency curve exhibited two linear portions, suggesting the presence of two spike-generating sites in the axon. Current threshold measurements, using test stimuli of different durations, showed that both accommodation and "early" or "residual" refractoriness contribute to the determination of discharge rate at different frequencies.

EXCITABILITY CHANGES OF THE MAUTHNER CELL DURING COLLATERAL INHIBITION

Excitability changes during collateral inhibition of the goldfish Mauthner cell (M cell) were measured directly by stimulating the cell with current pulses applied through an intracellular electrode. Excitability was suppressed during the extrinsic hyperpolarizing potential (EHP) as well as during the collateral IPSP. The inhibitory effect of the EHP was shown to be comparable in intensity to the effect of the IPSP. Excitability changes in the M cell during collateral IPSP depended on changes in the membrane conductance as well as in the membrane potential. Some simple equations are advanced which describe the excitability change during the IPSP in terms of changes in membrane potential and conductance. It was also found that invasion of antidromic impulses into the M cell was suppressed during the EHP, but not during the collateral IPSP. Conductance increase during the IPSP did not interfere with the invasion of antidromic impulses.

The ionic mechanism of postsynaptic inhibition in motoneurones of the frog spinal cord

The ionic mechanism of postsynaptic inhibition in frog spinal motoneurones was studied with conventional and with ion-sensitive microelectrodes. In these neurones the inhibitory postsynaptic potential was depolarizing, its reversal potential being 15 mV less negative than the resting membrane potential. During the inhibitory postsynaptic potential the input resistance of the motoneurones was reduced to 20% of the resting value, indicating a strong increase of membrane conductance. The Cl- equilibrium potential calculated from intra- and extracellular Cl- activity measurements coincided with the reversal potential of the inhibitory postsynaptic potential to within a few millivolts. During repetitive inhibitory postsynaptic activity the intracellular Cl- activity decreased markedly, while the extracellular Cl- activity increased slightly. These changes of intra- and extracellular Cl- activities were no longer found after suppression of the inhibitory postsynaptic potential by strychnine. Blockade of an active, inward-going Cl- transport system in motoneurones by NH+4 led to a shift of the Cl- equilibrium potential and the reversal potential of the inhibitory postsynaptic potential towards the resting membrane potential. After prolonged action of NH+4, the Cl- equilibrium potential approached the membrane potential to within 5 mV, while the reversal potential of the inhibitory postsynaptic potential and resting membrane potential coincided. The difference between Cl- equilibrium potential and membrane potential after blockade of the Cl- pump is traced back to interfering intracellular ions, such as HCO-3 or SO42-, leading to an overestimation of intracellular Cl- activity and to the calculation of an erroneous Cl- equilibrium potential. Inhibitory amino acids like gamma-aminobutyrate or beta-alanine evoked depolarizations with reversal potentials similar to that of the inhibitory postsynaptic potential. These depolarizations were associated with a marked decrease of neuronal input resistance during inhibition. During the actions of these compounds a decrease of intracellular and a small increase of extracellular Cl- activity were found. The activities of other ions (K+, Ca2+ and Na+) did not change significantly, with the exception of extracellular K+ activity, which was slightly increased. Evidence is presented that the inhibitory postsynaptic potential, as well as the depolarizing action of inhibitory amino acids in motoneurones, is the result of an increase in membrane Cl- permeability and an efflux of Cl- from these cells, while other ions do not seem to be involved.

Differential sensitivity of spinal neurones to amino acids: an intracellular study on the frog spinal cord

Intracellular recordings from in vitro neurones of the frog spinal cord slice preparation were performed in order to examine the mechanism of action of gamma-aminobutyrate and glutamate on two distinct neuronal populations in the same region of the central nervous system. Amino acids were superfused at fast rate and low temperature (7 degrees C) to reduce their uptake process. On interneurones, the inhibitory action of gamma-aminobutyrate was characterized by a large input conductance increase while on motoneurones the conductance change was much smaller. Glutamate excited interneurones which greatly increased their input conductance and showed burst firing; motoneurones were also excited by glutamate but usually did not fire repeatedly nor showed large conductance changes. In spite of these differences the amplitude of depolarization in the presence of the same concentration of glutamate was similar for motoneurones and interneurones. It is suggested that amino acids (particularly glutamate) may act through different membrane mechanisms on two neuronal populations in the same region of the spinal cord.

Inhibitory postsynaptic actions of taurine, GABA and other amino acids on motoneurons of the isolated frog spinal cord

The actions of glycine, GABA, alpha-alanine, beta-alanine and taurine were studied by intracellular recordings from lumbar motoneurons of the isolated spinal cord of the frog. All amino acids tested produced a reduction in the amplitude of postsynaptic potentials, a blockade of the antidromic action potential and an increase of membrane conductance. Furthermore, membrane polarizations occurred, which were always in the same direction as the IPSP. All these effects indicate a postsynaptic inhibitory action of these amino acids. When the relative strength of different amino acids was compared, taurine had the strongest inhibitory potency, followed by beta-alanine, alpha-alanine, GABA and glycine. Topically applied strychnine and picrotoxin induced different changes of post-synaptic potentials, indicating that distinct inhibitory systems might be influenced by these two convulsants. Interactions with amino acids showed that picrotoxin seletively diminished the postsymaptic actions of GABA, while strychnine reduced the effects of taurine, glycine, alpha- and beta-alanine. But differences in the susceptibility of these amino acid actions to strychnine could be detected: the action of taurine was more sensitively blocked by strychnine compared with glycine, alpha- and beta-alanine. With regard to these results the importance of taurine and GABA as transmitters of postsynaptic inhibition on motoneurons in the spinal cord of the frog is discussed.

Excitation and inhibition of motoneurones in the tortoise

1. Intracellularly recorded responses of lumbosacral motoneurones in the tortoise are described. The preparation was decerebrate and unanaesthetized. Cells were tested by stimulation of the main branches of the ipsilateral sciatic nerve.2. The distribution of cells according to resting membrane potential is given; highest recorded values were around 70 mV.3. The action potential has a step on the rising phase and has a total duration in the region of 4 msec.4. The motoneurone input resistance, measured by the spike height method, was 5-8 MOmega.5. Orthodromic excitation of motoneurones occurred after a central latency of 2 msec.6. Inhibition, and inhibitory (hyperpolarizing) potentials were demonstrated. Peak amplitude of maximum response was frequently greater than 10 mV. The potentials were influenced by injected currents in a manner resembling IPSPs in mammalian motoneurones. Minimum central latency for inhibitory potentials was about 3 msec.7. Membrane potentials and dimensions of action potentials for motoneurones in the cat are given for purposes of comparison with the tortoise.

Microelectrode studies in the frog isolated spinal cord during depression by general anaesthetic agents

1. Extracellular and intracellular potentials have been recorded from the isolated spinal cord of the frog during depression of synaptic transmission by volatile and barbiturate general anaesthetic agents.2. Volatile agents did not impair conduction in presynaptic terminals in concentrations which completely blocked synaptic transmission.3. Methohexitone consistently impaired conduction in presynaptic terminals long before transmission through polysynaptic pathways was blocked.4. Volatile agents depressed the excitability of the motoneurone membrane, as evidenced by impaired antidromic invasion, reduced excitability to direct stimulation, depression of the synaptic potential and elevation of firing threshold. It is concluded that these actions are responsible for the depressant effect of volatile agents on spinal reflexes.5. Methohexitone produced an increase in the excitability of the motoneurone membrane, as evidenced by enhanced antidromic invasion, increased excitability to direct stimulation and potentiation of short latency responses. Despite this excitatory action, the polysynaptic pathways through the cord were depressed by an action of the drug on conduction in presynaptic terminals.6. It is suggested that the sensitivity of the motoneurone membrane to volatile agents may contribute to the good muscle relaxant properties of these drugs in clinical use.