, 1984 and Stephenson et al., 2005), and lesions of the EP greatly reduce these markers in the LHb and thalamus (Penney and Young, 1981 and Vincent et al., 1982). Recently, it was found that most LHb-projecting pallidal neurons have

reward-modulated activity that begins before that of LHb neurons themselves, consistent with upstream control of LHb neurons (Hong and Hikosaka, 2008). Surprisingly, LHb-projecting pallidal neurons display antireward characteristics, similar to LHb neurons (Hong and Hikosaka, 2008). This finding suggests that either inhibitory projections out of the basal ganglia disynaptically disinhibit LHb neurons or a previously unidentified excitatory projection exists from the basal ganglia to the LHb. Here we test the hypothesis that an excitatory projection exists from the EP to the LHb that signals Autophagy Compound Library cell line aversive events. We use a combination of optogenetics and immunohistochemistry to show that the projection from the EP to the LHb is predominantly excitatory, glutamatergic, and aversive. We also show that the excitatory projection

from the EP to the LHb is suppressed by low concentrations of serotonin, providing a link between aversive signaling in the LHb and a neuromodulator involved in mood disorders. To test the hypothesis that the projection from the Pfizer Licensed Compound Library cost EP to the LHb is excitatory, we injected AAV-driving expression of the light-inducible cation channel channelrhodopsin-2 3-mercaptopyruvate sulfurtransferase (Boyden et al., 2005), tagged with yellow fluorescent protein (ChR2-YFP), into the EP in vivo. Two weeks after injection, we prepared coronal brain slices, which displayed localized fluorescence in neuronal cell bodies

in the EP (Figure 1A and see Figure S1 available online) as well as fluorescent fibers in the projection region, the lateral aspect of the LHb (Figure 1B and Figure S1). To test for excitation of LHb neurons by EP inputs, we obtained whole-cell current-clamp recordings from neurons in the lateral aspect of the LHb and stimulated the EP inputs to the LHb with brief (0.5–5 ms) pulses of 470 nm light through an LED-coupled optic fiber placed over the LHb. Consistent with the EP providing excitatory input to the LHb, light stimulation produced depolarizing synaptic responses at resting potentials (Figures 1C and 1D; 13/13 cells depolarized). To maximize detection of any hyperpolarizing synaptic response, we injected depolarizing current and raised the membrane potential close to 0mV. Even in these conditions, the slope of the postsynaptic response remained positive for 12 out of 13 cells (Figures 1C and 1D), indicating dominant depolarizing synaptic input. Bath application of NBQX largely blocked the excitatory response, indicating its mediation by AMPA-type glutamate receptors (Figure 1E), although we could detect GABA-mediated currents when cells were clamped at positive holding potentials (Figure S1).