Optogenetic activation of terminals from rostromedial tegmental nucleus (RMTg) neurons inhibited the firing of midbrain dopaminergic neurons.

<p>(A) Schematic of the experiment. Adeno-associated virus–channelrhodopsin-2 (AAV-ChR2) was injected into the RMTg of Sprague–Dawley rats, and responses were recorded in the midbrain (ventral tegmental area [VTA]/substantia nigra compacta [SNc]). (B) Representative images of dense mCherry<sup>+</sup> terminals (red) of RMTg neurons in VTA (top) and SNc (bottom). Tyrosine hydroxylase (TH, green) immunostaining was used to demarcate the VTA and SNc. (C) ChR2-mCherry expressing neurons showed fidelity of spiking (top) following 20 Hz blue light illumination in current-clamp mode. This depolarization coincided with inward current recorded under voltage clamp (bottom). (D, E) Fluorescence micrographs showing a recorded biocytin-filled neuron in the VTA (D) or the SNc (E) that expressed TH. (F) The location of connected biocytin-filled neurons across the rostrocaudal extent of the VTA (blue dots) or the SNc (red dots). (G, J) A typical cell-attached patch recording showed that brief blue light pulses totally suppressed spiking of DAergic neurons in the VTA (G) or SNc (J). (H, K) Latency (left axis) and amplitude (right axis) of light-evoked inhibitory postsynaptic currents (IPSCs) in the VTA (H) or SNc (K) TH-positive neurons (<i>n</i> = 8 cells for VTA or 11 cells for SNc from 6 rats). (I, L) Typical traces of postsynaptic currents from a VTA neuron (I) or a SNc neuron (L) evoked by blue light. The evoked currents were completely abolished by application of picrotoxin (PTX) (100 μM; black lines). Underlying data can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2002909#pbio.2002909.s001" target="_blank">S1 Data</a>.</p>