Spontaneous action potential activity and synaptic currents in the embryonic turtle cerebral cortex.

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)