Figure 1.

Electrophysiological properties of neostriatal and substantia nigra pars compacta neurons from BAC transgenic EGFP–TH⁺ mice. A, Typical striatal SPN. Note the extremely low input resistance, strong inward rectification, lack of spontaneous activity, and depolarizing ramp in response to depolarizing current pulse leading to delayed spiking (red trace). B, FSI characterized by relatively low input resistance, lack of spontaneous activity, nonlinear spiking in response to depolarizing current pulses, high maximal firing frequency, and large spike afterhyperpolarizations. C, Cholinergic interneurons were characterized by their spontaneous activity (F, top trace and red trace in C) and marked time-dependent sag in response to hyperpolarizing current injections attributable to activation of HCN channels. D, PLTS interneurons exhibited high input resistance and both low-threshold spike and plateau potential (arrow) at the offset of negative current injections. None of these neurons was EGFP⁺, and all exhibited characteristics identical to those described previously for electrophysiologically and neurochemically identified neurons in striatal slices from rat and mouse. E, Typical responses of a substantia nigra pars compacta (SNc) dopaminergic neuron, fluorescent for EGFP–TH⁺ (red arrow in inset). EGFP–TH⁺ nigral dopaminergic neurons exhibit slow regular spontaneous activity (F, bottom trace), long-duration action potentials, and a large time-dependent sag in response to hyperpolarizing current injections attributable to HCN. G, Single action potential from a SPN used to illustrate methods for measurements of action potential threshold (1), action potential duration at half-amplitude (2), afterhyperpolarization (3), and action potential amplitude (4). Scale bar in inset and Figs. 3–6, 20 μm.