Reversal properties of climbing fiber potential in cat Purkinje cells: an example of a distributed synapse

1. The electrophysiological properties of the EPSP generated in Purkinje cells by the activation of CFs were studies in the cat cerebellar cortex. 2. CF-EPSPs were evoked by electrical stimulation of the cerebellar white matter and recorded intracellularly from the soma of the Purkinje cells. 3. Current was injected into the Purkinje cells via the recording micropipette using a bridge amplifer in order to study the reversal properties of the EPSP. 4. The CF-EPSP reversal was biphasic with the early portion reversing first. 5. The reversed EPSP waveform was not a mirror image of the EPSP, but displayed a briefer time course. 6. A four-compartment computer stimulation showed that the reversal properities of the CF-EPSP were explicable in terms of a distributed synapse on a cable. 7. The biphasic reversal and asymmetry were shown to be due to the spatially nonuniform potential distribution created by the somatic current injection, which predominantly reversed the proximal part of the distributed synapse. Delayed rectification may also have contributed to the reversal asymmetry. 8. The advantages of a distributed synapse over a point synapse are discussed and the reversal properties of the CF-EPSP compared to those of the Ia-evoked EPSP in motoneurons.

Theory of current source-density analysis and determination of conductivity tensor for anuran cerebellum

The theoretical basis of current source-density (CSD) analysis in the central nervous system is described. Equations relating CSD, the current flow vector, and the extracellular field potential are given. It is shown that the CSD provides superior resolution of neuronal events when compared to conventional field-potential analysis. Expressions for the CSD in rectangular Cartesian coordinates are derived, including the general case of anisotropic, inhomogeneous conductive tissue, and a coordinate system rotated with respect to the principal axes (APPENDIX). The minimum number of spatial dimensions for accurate CSD analysis is discussed. The conductivity tensor was experimentally measured in frog and toad cerebella. All three principal components of the tensor were evaluated and their spatial gradients determined to be negligible. It was also shown that the conductivity was independent of potential. Thus the anuran cerebellum is anisotropic, homogeneous, and ohmic. On the basis of these results the appropriate mathematical expression for the CSD was selected.

Experimental optimization of current source-density technique for anuran cerebellum

This paper represents a systematic, semirigorous attempt to optimize the technique of current source-density (CSD) analysis experimentally. We compared different spatial differentiation formulas in terms of accuracy, aliasing, and smoothing, and provide experimental and theoretical rationale for their use. Sources of error have also been investigated. Expressions were derived to enable one to estimate the relative magnitude of errors due to electrical noise, uncertainty in tip position of recording electrodes, and error in the conductivity tensor. Corresponding experiments illlustrating the validity of such estimates are also presented. Methods to determine the optimum interelectrode spacing are given, based on computations of spatial energy-density spectra in the anuran cerebellum. The application of the technique of CSD analysis developed in this, and the accompanying paper, to the vestibulocerebellar input in the toad cerebellum provided significantly better temporal and spatial resolution of neuronal events than conventional field-potential analysis. Considerations germane to the optimum application of this technique to other neural structures are also discussed.

Calcium role in depolarization-secretion coupling: an aequorin study in squid giant synapse

Aequorin, a protein that emits light in the presence of calcium, was injected in the presynaptic terminal of the squid giant synapse. This injection was preceded by intracellular tetraethylammonium administration, which prolonged the duration of the presynaptic action potential. After this procedure light emission was evoked by single presynaptic spikes capable of releasing synaptic transmitter. In a second set of experiments, presynaptic tetraethylammonium injection was followed by the administration of tetrodotoxin extracellularly, which abolished the presynaptic action potential. Under these conditions artificial depolarization of the presynaptic terminal triggered the release of synaptic transmitter, in a graded manner. However, as previously reported by other authors, membrane potential steps to an internal positive value of approximately plus 90 mV (the suppression potential) produced a blockage of transmitter release for the duration of the imposed potential. Synaptic transmission recurred, nevertheless, as the current injection was terminated. A similar set of experiments, performed after the intracellular injection of aequorin in the presynaptic fiber, demonstrated that the aequorin light response was evoked by membrane potential steps capable of releasing synaptic transmitter. If the membrane potential was made positive to the "suppression" level, no light response was evoked but the light emission appeared, as did transmitter release, at the end of the current pulse. These experiments demonstrate that release of transmitter is directly correlated with intracellular calcium concentration and that the suppression potential is compatible with the existence of a calcium equilibrium potential at the presynaptic terminal.

Equilibrium potential for the postsynaptic response in the squid giant synapse

The reversal potential for the EPSP in the squid giant synapse has been studied by means of an intracellular, double oil gap technique. This method allows the electrical isolation of a portion of the axon from the rest of the fiber and generates a quasi-isopotential segment. In order to make the input resistance of this nerve segment as constant as possible, the electroresponsive properties of the nerve membrane were blocked by intracellular injection of tetraethylammonium (TEA) and local extracellular application of tetrodotoxin (TTX). Thus, EPSP's could be evoked in the isolated segment with a minimal amount of electroresponsive properties. The reversal potential for the EPSP (EEPSP) was measured by recording the synaptic potential or the synaptic current during voltage clamping. The results indicate that EEPSP may vary from +15 to +25 mV, which is more positive than would be expected for a 1:1 conductance change for Na(+) and K(+) (approximately -15 mV) and too negative for a pure Na(+) conductance ((+)40 mV). This latter value (E(Na)) was directly determined in the voltage clamp experiments. The results suggest that the synaptic potential is probably produced by a permeability change to Na(+) to K(+) in a 4:1 ratio. No change in time-course was observed in the synaptic current at clamp levels of -100 and +90 mV. The implications of a variable ratio for Na(+)-K(+) permeability in subsynaptic-postsynaptic membranes are discussed.

Calcium transient in presynaptic terminal of squid giant synapse: detection with aequorin

Microinjection of aequorin, a bioluminescent protein sensitive tocalcium, into the presynaptic terminal of the squid giant synapse demnonstrated an increase in intracellular calcium ion concentration during repetitive synaptic transmission. Although no light flashes synchronous with individual presynaptic : tion potentials were detected, the results are considered consistent with the hypothesis that entry of calcium into the presynaptic terminal triggers release of e synaptic transmitter substance.

Preferred centripetal conduction of dendritic spikes in alligator Purkinje cells

Dendritic action potentials in alligator Purkinje cells tend to have a unidirectional preference which favors centripetal over centrifugal propagation. This unidirectional tendency funnels the peripherally evoked dendritic spikes into the lower dendrites and soma of these cells, and it allows the peripheral dendritic branches to operate to a certain extent as partially independent functional units.

Dendritic spikes and their inhibition in alligator Purkinje cells

Alligator Purkinje cells generate action potentials in the peripheral dendritic tree, after synaptic depolarization via superficial parallel fibers. These action potentials are inhibited at the dendrite level by preceding parallel-fiber volleys at close intervals. We conclude that this inhibition is produced by the activation of the inhibitory interneurons of the molecular layer, the stellate cells, which establish synaptic contacts with the dendrites of the Purkinje cells.