Figure 1.

RMTg stimulation induces inhibition of VTA DA neuron spontaneous activity. (a; top panel) The trace shows typical superimposed spike waveforms of a DA neuron recorded in the VTA. Below, the diagram illustrates the stimulation protocol used in all in vivo experiments. The RMTg was stimulated with rectangular current pulses (0.5 mA, 0.5 ms) delivered at 1 Hz, and responses of DA cells to RMTg stimulation were recorded extracellularly. Recording location for a DA neuron in the posterior VTA (the triangle indicates the pontamine sky blue dye; bottom panel; left). Right: electrical stimulation site in the RMTg. The black asterisk indicates the area lesioned by the tip of the electrode, the black triangles indicate the track of the recording electrode. DR, dorsal raphe; IP, interpeduncular nucleus; RN, red nucleus; SNr, substantia nigra pars reticulata; xspc, decussation of the superior cerebellar peduncle. Scale bars, 0.5 mm. (b) Percentages of different types of DA cell responses to RMTg stimulation. Traces acquired from a digital oscilloscope represent typical examples of excited, inhibited, and unresponsive DA cells. A PSTH of the inhibited cell is displayed (100 consecutive sweeps) and shows that RMTg stimulation produced a full suppression in firing activity of a VTA DA neuron for ∼90 ms. Note that the inhibitory response occurred immediately after the stimulus (arrowhead).