Figure 7. Contribution of Cl⁻-dependent synaptic inhibition to sensory integration in MSNs.

A, synthetic projection micrograph (from a 120 μm-thick whole mount) of a striatal MSN recorded with a KCl electrode and intracellularly labelled with Neurobiotin. B, barrel cortex ECoG (upper trace) and simultaneously recorded intracellular activity of a Cl⁻-loaded MSN (bottom trace). Note the depolarized membrane potential and the elevated firing rate compared to MSN recorded with a KAc electrode (see Fig. 4C). As shown by the expanded record (inset, calibrations: 10 mV, 10 ms), the spontaneous firing is caused by large-amplitude and long-duration synaptic potentials, probably resulting from additional Cl⁻-dependent synaptic depolarizations. C, superimposition of 3 successive suprathreshold (Ca, the firing probability is indicated) or subthreshold synaptic responses (Cb) evoked in two different Cl⁻-filled MSNs by the same sensory stimulus (top, 40 p.s.i.). The bottom trace in Cb represents the average of 37 successive trials. D, comparison of the percentage of MSNs (and corresponding number of cells) without sensory responses (Non Resp.), exhibiting subthreshold (dPSP Resp.) or suprathreshold responses (AP Resp.) when recorded with KAc or KCl electrodes. E, activation of presumed striatal GABAergic interneurons by whisker deflection. Ea, microphotograph of a putative GABAergic striatal interneuron labelled by juxtacellular injection of Neurobiotin. This cell exhibited the morphological features (for a detailed description, see Results) of striatal interneurons with expanded dendritic field (see Kawaguchi, 1993). The inset shows the short-duration (0.58 ms) extracellular spike of the labelled neuron. Eb, four successive extracellular sensory responses of a putative GABAergic interneuron and corresponding post-stimulus time histogram of action potential discharges (50 collected sweeps, bin = 2 ms). The vertical dashed line indicates the onset of the air-puffs (top).