Combined spikes induced by Ca and Na currents in cultured cerebellar neurons from the chick embryo

Expression and electrophysiological function of actin in chick cerebellar neurons

Among several monoclonal antibodies obtained by immunizing Balb/c mice with cerebellar synaptic membrane fractions from E20 chick embryos, the antibody, named M35, suppressed Ca-spikes in immature cultured chick cerebellar neurons. M35 immunoprecipitated 43 kDa protein from a 125I-labeled embryonic crude cerebellar membrane fraction. Immunohistochemically, the M35 antigen was expressed most intensively in Purkinje cells, but its expression was limited to highly motile structures at developmental neuronal remodeling. Electrophysiologically, M35 facilitated current responses to AMPA and inhibited the responses to GABA in cultured cerebellar Purkinje neurons. The several peptides derived from the affinity-purified 43 kDa protein were found to have homologous amino acid sequences to non-muscle actins. These results suggest that the antigen recognized by M35 may play an essential role probably as membrane ion channels modulating synaptic functions in not only the development and growth but also the neuronal activity of chick cerebellar Purkinje cells.

Calcium currents in rat cerebellar Purkinje cells maintained in culture

Calcium permeabilities were examined in large cerebellar neurons maintained in culture, and morphologically identified as Purkinje cells. When cells were supplied with a Dulbecco Minimum Eagle's Medium with 10% horse serum added (5-10 days), somatic recordings revealed complex spikes and these were shown to be generated by Na and Ca components, the Na one being tetrodotoxin-sensitive. At the dendritic level, Ca currents were better resolved than at the soma. In dendrites, Ca entry was shown to occur through at least two distinct currents. The first was a low-threshold transient current (elicited above -60 mV from a holding potential of -80 mV) which was reduced by almost 30% by 50 microM cadmium. The second was a high-threshold current (above -20 mV) which gave rise to (1) a transient component exhibiting a steady-state inactivation and so requiring holding potentials at -80 mV, and (2) a sustained component. Both components were suppressed by 50 microns cadmium. We measured a total Ca current at the dendritic level reaching values of up to 1 nA. In another culture medium (Leibovitz medium) known to allow expression of three types of calcium currents in nodose cells we observed the development of the dendritic tree of Purkinje cells but with no simultaneous expression of the high-threshold Ca current.

Effect of calcium ions on the sensitivities of cultured cerebellar neurons to glutamate and aspartate

Iontophoretically applied glutamate and aspartate induced depolarizations in immature (6-13 days in culture) and mature (25-45 days) cultured chick cerebellar neurons, immature neurons being less sensitive. The input resistances of the neurons were variously changed by these amino acids. Reversal potentials of the depolarizations induced by both amino acids were similar in either immature or mature neurons. The population of amino acid-sensitive neurons increased with maturation. In mature neurons, the amplitude of glutamate- or aspartate-induced depolarization was decreased by addition of 10 mM Ca2+ to normal Tyrode's solution, aspartate responses being decreased more greatly. In low-Na+ solution (2.7 mM), however, high Ca2+ significantly enhanced amino acid-induced depolarizations. In immature neurons, on the other hand, the amplitude of glutamate- or aspartate-induced depolarization was drastically and consistently increased when 10 mM Ca2+ was added either to normal solution or to the low-Na+ solution. These enhancing actions of Ca2+ were abolished by Mn2+, but only partially by 10 mM glutamic acid diethylester or 1 mM D-alpha-aminoadipate, though responses to both amino acids in normal solution were blocked by these antagonists at the same concentrations. These results suggest that calcium ions enhance the effect of glutamate and aspartate in immature neurons, possibly by interacting with the ionophores involved in amino acid responses.

Development of sensitivity to GABA and glycine in cultured cerebellar neurons

The effects of iontophoretically applied gamma-aminobutyric acid (GABA) and glycine on developing cerebellar neurons cultured for 7-40 days were intracellularly investigated. All neurons tested dose-dependently responded to both GABA and glycine. In mature neurons (after 25 days in culture) these amino acids inhibited spontaneous spikes, decreased the membrane input resistance and induced either hyperpolarization or depolarization of membrane potential. The mean reversal potential was -47 mV for GABA and -43 mV for glycine. Immature neurons, 7-12 days in culture, which were not spontaneously firing, also behaved in a similar manner as the mature ones, though the membrane resistance was not so largely changed by GABA or glycine and the reversal potential was more positive (-39 mV for GABA, -37 mV for glycine). These reversal potentials were shifted toward 0 mV by lowering the external Cl- concentration in either mature or immature neurons. The effects of GABA and glycine on mature or immature neurons were more or less inhibited by all of picrotoxin, bicuculline and strychnine. The effective concentrations of these antagonists, however, were lower in general in immature neurons. In mature neurons, picrotoxin and bicuculline became more selective to GABA than glycine and strychnine became more selective to glycine than GABA. These results suggest that sensitivities to GABA and glycine differentiate into selective types in the course of maturing of cerebellar cultured neurons.