Inactive nose poke holes were largely ignored and EYFP control mice did not regularly nose poke into either hole (Figure 7D). The rate of behavioral responding in the ChR2-expressing mice steadily increased upon subsequent sessions and was similarly robust when tested 70 days later (Figure S6D). Together with the place preference experiment, this work shows

that both short and long stimulations of vHipp axons in the NAc are rewarding. Our examination of pathway-specific synaptic differences suggested that the only reason prefrontal cortex input to the NAc might not support self-stimulation is because it is a relatively weak input. We reasoned that if the optogenetic stimulus was robust enough, it might be possible that each excitatory input to NAc could reinforce instrumental behavior. We first tested this in the three-room chambers and, using Epacadostat mouse a 6 Hz pulse frequency contingent on mice being in the laser-paired room, www.selleckchem.com/products/pd-0332991-palbociclib-isethionate.html we found that mice preferred to spend time on the side of the chamber paired with the optical stimulation, regardless of which afferent pathway to the NAc was activated in each mouse (Figure 8A). In these same mice, optogenetic self-stimulation was also observed, although importantly we increased the

strength of the light stimulus to compensate for the weaker inputs (Figure 8B). We used 30, 60, and 90 pulses for the vHipp (20 Hz), basolateral amygdala (20 Hz), and prefrontal cortex fibers (30 Hz), respectively. These results raise the possibility that the specific excitatory pathway activated is not as important as how much glutamate is released into the NAc, at least in terms of generating motivated behaviors. It is surprising that discrete glutamate release facilitated reward seeking, since it has been hypothesized that the inhibition SB-3CT of NAc neurons is what encodes reward (Carlezon and Thomas, 2009; Carlezon and Wise, 1996; Roitman

et al., 2005; Taha and Fields, 2006). To directly test the role of NAc cell activity in modulating these behaviors, we gave mice the opportunity to self-stimulate medium spiny neurons. We indiscriminately targeted ChR2 expression to NAc projection neurons in the medial NAc shell and implanted optical fibers just above this area (Figure 8C). Robust self-stimulation was observed in these mice (Figure 8D), demonstrating that mice will work to obtain a nonselective activation of neurons downstream of dopamine signaling. This result shows that indiscriminate bulk activation of NAc neurons is sufficient to reinforce instrumental behavior. By comparing the innervation patterns and synaptic properties of vHipp, basolateral amygdala, and prefrontal cortex input to the NAc, we identified vHipp fibers as being uniquely concentrated in the medial NAc shell. In this region, vHipp input was predominant and selectively strengthened after cocaine injections.