Synaptic Currents in Thalamo‐cortical Neurons of the Rat Lateral Geniculate Nucleus

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Abstract
Thalamo-cortical neurons were identified in slices of the rat dorsal lateral geniculate nucleus and whole-cell currents were recorded using the patch-clamp technique. Postsynaptic currents occurring spontaneously, or elicited by extracellular stimulation in the vicinity of the recorded neuron, were analysed. Spontaneous postsynaptic currents were observed in every recorded neuron. At a holding potential of - 60 mV, and with a high internal Cl-, the currents were inward and had amplitudes ranging from < 10 to 425 pA. All the spontaneous currents were blocked by 10 microM bicuculline, indicating that they were due to the activation of postsynaptic gamma-aminobutyric acid (GABAA) receptors. The 10-90% rise time of these spontaneous GABAergic currents was 0.86 +/- 0.19 ms. Their time course of decay could be fitted to an exponential function with one time constant of 18.19 +/- 3.02 ms (mean +/- SD), or two time constants of 4.47 +/- 0.77 and 33.27 +/- 3.74 ms. This activity was frequently organized in bursts. Stimulus-evoked postsynaptic currents were recorded and shown to be due to the activation of glutamatergic receptors. Under similar experimental conditions a bicuculline-sensitive component was also recorded. These stimulus-evoked GABAergic currents had a 10 - 90% rise time of 1.93 +/- 0.54 ms. Their time course of decay could also be fitted to an exponential function with one time constant of 24.42 ms or two time constants of 10.26 +/- 2.46 and 49.30 +/- 10.98 ms. The difference in the time course between spontaneous and evoked GABAergic currents suggests that these responses may arise from synapses having different locations.