What Controls the Timing of Striatal Spiny Cell Action Potentials in the Up State?
The excitatory and inhibitory components of synaptic conductance in the Up state of the striatal spiny neuron were studied in cells whose voltage-sensitive ion channels were blocked by intracellular application of cesium, QX-314, and D-890. The Up state was driven by a large synaptic conductance, ranging from 13 to 46 nS. From the reversal potential of the Up and Down states, it was possible to estimate the relative contribution of the excitatory and inhibitory conductance components as 85–67% and 15–33%, respectively. The frequency spectra of the membrane fluctuations produced by excitation and inhibition were estimated by holding the cells far from or near to the reversal potential for excitatory synaptic transmission. The spectrum for excitation was dominated by slow components, interpreted as the time course of correlations among cortical neurons. The spectrum for inhibition exhibited high-frequency components in the gamma range, not present in the excitatory input. Action potentials in the Up state were triggered from fast membrane potential events, whose time course matched that of inhibitory fluctuations rather than excitatory ones. It was proposed that the timing of action potentials is controlled by disinhibition events, perhaps arising from the pallido-striatal pathway.