Figure 3. Reward cue-inhibited rEPN neurons encode valence.

(A) Schematic of training paradigm. (B) After eight training sessions, rats show clear discrimination between reward and neutral cues, as measured by the probability of animals making nose pokes during the 2 s cue presentation. (C) We recorded in the rEPN, as delineated by retrograde labeling of CTb from the LHb and absence of parvalbumin immunoreactivity. (D) Schematic of recording paradigm, in which three different auditory tones signaled delivery of sucrose pellets, mild footshocks, or nothing. Animals exhibited ~80% accuracy in discrimination between different cues. (E) rEPN neurons were divided roughly equally between reward cue-inhibited and –excited responses. Amongst reward cue-inhibited neurons, the most common response was excitation to footshocks and shock cues. (F) Representative histograms of individual rEPN neuron responses to reward and shock trials, analyzed in 50 ms bins. (G) Responses of reward cue-inhibited subpopulation to reward, shock, and neutral trials, again in 50 ms bins. Excitation to shock cue was most prominent in first two bins (0–100 ms, upper solid black line), while inhibition to reward cue was most prominent 200–400 ms after stimulus onset (lower solid black line). Average firing rates from these two windows are shown in adjacent graphs. (H) In reward cue-inhibited rEPN neurons, responses to shock cues were positively correlated with individual neuron’s responses to footshocks. (I) Reward cue-inhibited neurons also showed excitation to reward omission (0–200 ms window) (J) Responses to reward omissions were positively correlated with responses to shock cues in omission-activated neurons. (K) Reward cue-inhibited neurons did not respond to shock omissions. Solid circles indicate neurons significantly excited by both stimuli.

Figure 3—figure supplement 1. rEPN neurons not inhibited by reward cues do not show valence-like encoding patterns.

24 of 53 rEPN neurons were reward cue-inhibited, leaving 29 that were not. Among the non-inhibited neurons, 15, 11, and three neurons showed reward cue phasic excitation (0–100 ms) (A-B), delayed excitation (100–1000 ms) (C), and non-response (D), respectively, to reward cues. Phasically excited neurons did not discriminate between cue types (A), while neurons showing delay-sustained excitation showed sustained responses to both reward-predictive and shock-predictive cues, but not neutral cues, consistent with salience-like encoding (B). Neurons lacking responses to reward cues nonetheless showed phasic excitations to shock-predictive cues but not neutral cues (C).