Journal of Neuroscience, Vol 11, 3907-3923, Copyright © 1991 by Society for Neuroscience
Spontaneous action potential activity and synaptic currents in the embryonic turtle cerebral cortex
MG Blanton and AR Kriegstein
Department of Neurology and Neurological Sciences, Stanford University School of Medicine, California 94305.
We used loose-patch and whole-cell recording techniques to study the
development of spontaneous action potential activity and spontaneous
excitatory and inhibitory synaptic currents in embryonic neurons in the
cerebral hemispheres of turtles. Sporadic action potential activity
appeared early in development at stage 17, soon after morphologically
identifiable pyramidal and nonpyramidal neurons were first observed in the
cortex. As the cortical plate matured in midembryonic stages, action
potential activity became more regular and fell into one of two distinct
patterns, tonic and intermittent high-frequency firing. Spontaneous
excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) appeared
at developmental stages 18 and 20, respectively, after action potential
activity was established. EPSCs and IPSCs exhibited characteristic ionic
dependence and pharmacology throughout development. EPSCs reversed in
direction at the equilibrium potential for cations and were sensitive to
6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist of the non-NMDA type of
glutamate receptor. IPSCs reversed at the equilibrium potential for
chloride and were sensitive to bicuculline methiodide, a GABAA receptor
antagonist. Spontaneous synaptic currents differed in their time course of
development and in waveform parameters. Spontaneous synaptic currents
differed in their time course of development and in waveform parameters.
Spontaneous EPSCs appeared at stage 18 and increased progressively in
frequency, from 0.2 +/- 0.1 Hz at stage 20 to 3.2 +/- 2.0 Hz at stage 26
(hatching), while spontaneous IPSCs appeared at stage 20 and surpassed
EPSCs in frequency, increasing to 7.1 +/- 1.6 Hz at stage 26. EPSCs
exhibited stable amplitudes during development, with a mean conductance of
126 +/- 20 pS at stage 26, while IPSCs increased in mean amplitude, from
180 +/- 12 pS at stage 18 to 260 +/- 44 pS at stage 26. The rise time to
peak conductance of both types of synaptic currents increased with
developmental time, for EPSCs increasing from 1.5 +/- 0.5 msec at stage 20
to 2.7 +/- 0.6 msec at stage 26 and for IPSCs increasing from 2.9 +/- 0.2
msec at stage 18 to 6.2 +/- 0.8 msec at stage 26. While the decay time
constants increased for EPSCs, from 3.9 +/- 1.2 msec at stage 20 to 8.7 +/-
2.3 msec at stage 26, decay time constants for IPSCs showed a decreasing
trend from 24.0 +/- 5.2 msec at stage 18 to 18.4 +/- 5.3 msec at stage 26.
The excitatory and inhibitory synaptic currents were sensitive to the
sodium channel blocker TTX and were thus dependent, in part, on spontaneous
action potential activity.(ABSTRACT TRUNCATED AT 400 WORDS)
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