Figure 1. Characterization of striatal output neurons, astrocytes and NG2⁺ cells.

A, electrophysiological characteristics of MSNs: a hyperpolarized RMP, an inward rectification (illustrated by the steady-state I–V relationship, right panel) and a long depolarizing ramp to the action potential threshold leading to a delayed spike discharge. Raw traces show voltage responses to 500 ms current pulses from −130 pA to +140 pA with 30 pA steps (left panel) and to +30 pA above rheobase (middle panel). Ba, characteristic membrane properties of an astrocyte in voltage and current clamp modes. Hyperpolarizing and depolarizing steps (raw traces show individual responses to series of 500 ms voltage or current pulses from −200 pA or −90 mV with 100 pA or 30 mV increasing steps) activated large time- and current/voltage-independent membrane voltages/currents that resulted in a linear steady-state I–V relationship typical of astrocytes (right panel). Bb, example of biocytin intercellular coupling between astrocytes. An electrophysiologically identified astrocyte located in the dorsal striatum was filled with an intercellular tracer (biocytin) that diffused into a large number of neighbouring cells (326 cells in this example) through gap junctions (scale bar: 50 μm). Ca, membrane properties of a NG2⁺ cell: a hyperpolarized RMP, a high input resistance, an outward rectification (illustrated in the steady-state I–V relationship, middle panel) and a lack of spiking activity. Raw traces show individual voltage responses to series of 500 ms current pulses from −130 pA to +170 pA with 30 pA increasing current steps (left panel) or current responses to series of 500 ms pulses from −80 mV to +130 mV with 30 mV increasing voltage steps (right side). Cb, high-magnification view of an electrophysiologically identified complex cell filled with biocytin (scale bar: 50 μm); note the characteristic absence of dye-coupling. Complex cell showing a small cell body with restricted ramifications (inset, scale bar: 20 μm).