Threat and Bidirectional Valence Signaling in the Nucleus Accumbens Core

Abstract

Appropriate responding to threat and reward is essential to survival. The nucleus accumbens core (NAcc) is known to support and organize reward behavior. The NAcc is also necessary to fully discriminate threat and safety cues. To directly reveal NAcc threat firing, we recorded single-unit activity from seven female rats undergoing pavlovian fear discrimination. Rats fully discriminated danger, uncertainty, and safety cues, and most NAcc neurons showed the greatest firing change to danger and uncertainty. Heterogeneity in cue and reward firing led us to identify distinct functional populations. One NAcc population signaled threat, specifically decreasing firing to danger and uncertainty cues. A separate population signaled Bidirectional Valence, decreasing firing to the danger cue (negative valence), but increasing firing to reward (positive valence). The results reveal the NAcc to be a source of threat information and a more general valence hub.SIGNIFICANCE STATEMENT The nucleus accumbens core (NAcc) is synonymous with reward. Yet, anatomy, neurotoxic lesions, and optogenetic manipulation implicate the NAcc in threat. Here, we directly revealed NAcc threat firing by recording single-unit activity during multicue fear discrimination. Most cue-responsive NAcc neurons markedly altered firing to threat cues. Finer analyses revealed a NAcc population signaling threat, specifically decreasing firing to danger and uncertainty cues; and a NAcc population signaling Bidirectional Valence, increasing firing to reward but decreasing firing to the danger cue. The results reveal the NAcc to be a source of threat information and a valence hub.

Keywords: associative learning; fear; single unit; ventral striatum.

Figure 1.

Fear discrimination, histology, and behavior. A, Pavlovian fear discrimination consisted of three auditory cues, each associated with a unique probability of footshock: danger (p = 1.00, red), uncertainty (p = 0.25, purple), and safety (p = 0.00, blue). B, Each trial started with a 20 s baseline period followed by 10 s cue period. Footshock (0.5 mA, 0.5 s) was administered 2 s following the cue offset in shock and uncertainty shock trials. Each session consisted of 16 trials: 4 danger trials, 2 uncertainty shock trials, 6 uncertainty omission trials, and 4 safety trials with an average intertrial interval (ITI) of 3.5 min. C, Mean ± SEM suppression ratios to danger (D; red), uncertainty (U; purple), and safety (S; blue) cues are shown for the initial eight fear discrimination sessions. D, Example of a Nissl-stained NAcc (outlined in black) section showing the location of the recording site within the boundaries of the NAcc. E, Histologic reconstruction of microelectrode bundle placements (n = 7) in the NAcc are represented by pink bars, bregma levels indicated. F, Mean (bar) and individual (data points, n = 7) baseline nose poke rate (B; black) is shown for each rat. G, Mean (bar) and individual subject (data points, n = 7) suppression ratio for each cue (D, red; U, purple; S, blue) is shown. ⁺95% bootstrap confidence interval for differential suppression ratio does not contain zero. H, Correlation matrices among individual baseline nose poke rate (B), cue suppression ratio (D, red; U, purple; S, blue), and overall discrimination (danger suppression ratio – safety suppression ratio) are depicted. Color scale for correlation coefficient (R) is shown to the right, with perfect positive correlation dark red (R = 1) and perfect negative correlation dark blue (R = −1). *Pearson's correlation, p < 0.05. Mean individual suppression ratios are shown in Extended Data Figure 1-1, and session × session individual suppression ratios are shown in Extended Data Figure 1-2.

Figure 2.

Heat plot of cue-responsive neurons. Mean normalized firing rate for each cue-responsive neuron (n = 193) for each of the three trial types (danger, uncertainty, and safety) in 250 ms bins (2 s before cue onset to 2 s following cue offset), as well as reward (2 s before to 2 s following reward delivery). Cue onset (on), offset (off), and reward are indicated by black arrows. All cue-responsive neurons are sorted by their responses to cues [ThE, Threat Excited (n = 5, black); ThI, Threat Inhibited (n = 26, gray); DE, Danger Excited (n = 55, pink); BV, Bidirectional Valence (n = 91, tan); NS, Non-Selective (n = 16, black)]. Color scale for normalized firing rate is shown to the left. A normalized firing rate of zero is indicated by the color black, with the greatest increases indicated by light red and the greatest decreases indicated by light blue. Alternative clustering heat plots are shown in Extended Data Figure 2-1.

Figure 3.

Firing and waveform characteristics of cue-responsive neurons. A, i, ii, Histograms depict the distribution of the firing rate (in hertz, Hz) during a 10 s baseline period just before cue onset for Threat Inhibited (n = 26, gray), Danger Excited (n = 55, pink), and Bidirectional Valence (n = 91, tan) neurons. B, i, ii, Identical graphs made for the coefficient of variance for Threat Inhibited, Danger Excited, and Bidirectional Valence neurons. C, i, ii, Histogram (left) and bar graph (right) depicts the distribution of the waveform peak–valley duration (in milliseconds, ms) for Threat Inhibited, Danger Excited, and Bidirectional Valence neurons. Black dashed lines divide neurons into narrow (small values) and wide (large values) waveforms.

Figure 4.

Differential firing by cue-responsive neurons. A, Mean normalized firing rate to danger (D; red), uncertainty (U; purple), and safety (S; blue) is shown from 2 s before cue onset to cue offset for the Threat Inhibited neurons (n = 26, gray). Cue onset and offset are indicated by vertical black lines. SEM is indicated by shading. B, Mean (bar) and individual (data points) normalized firing rate for Threat Inhibited neurons during onset (the first 1 s cue, left) and late cue (the last 5 s cue, right) are shown for each cue (D, U, S). Colors maintained from A. C–F, Identical graphs made for Danger Excited (n = 55, pink) and Bidirectional Valence neurons (n = 91, tan), as in A and B. ⁺95% bootstrap confidence interval for differential cue firing does not contain zero; ^‡Pitman–Morgan test, p < 0.05.

Figure 5.

Threat Excited and Non-Selective NAcc neuron firing. A, Mean normalized firing rate in response to danger (D; red), uncertainty (U; purple), and safety (S; blue) is shown from 2 s before cue onset to cue offset for the Threat Excited neurons (n = 5). Cue onset and offset are indicated by vertical black lines. SEM is indicated by shading. B, Mean (bar) and individual (data points), normalized firing rate for Threat Excited neurons during onset (the first 1 s cue, left) and late cue (the last 5 s cue, right) are shown for each cue (D, U, S). Colors are as in A. C, D, Identical graphs made for Non-Selective neurons (n = 16), as in A and B. ⁺95% bootstrap confidence interval for differential cue firing does not contain zero.

Figure 6.

Temporal firing similarity by cue-responsive neurons. A–C, Firing similarity matrices between cue pairs [D, danger (red); U, uncertainty (purple); S, safety (blue)] are depicted for Threat Inhibited (n = 26, gray; A), Danger Excited (n = 55, pink; B), and Bidirectional Valence (n = 91, tan; C) neurons. Color scale for correlation coefficient (R) is shown, with the greatest firing similarities shown in black (R = 1) and the least firing similarities shown in white (R = 0). D–F, Mean (bar) and individual (data points) correlation coefficients (R): danger versus uncertainty (DU), uncertainty versus safety (US), and danger versus safety (DS) are shown for each population. *Bonferroni t test, p < 0.05; ^‡unpaired t test, p < 0.05.

Figure 7.

NAcc tuning curves. Mean ± SEM beta coefficients are shown for cue pattern regressor, during the 10 s cue presentation, for each uncertainty assignment from 0 to 1 in 0.25 increments (0.00, 0.25, 0.50, 0.75, 1.00), for the Threat Inhibited (n = 26, left), Danger Excited (n = 55, middle), and Bidirectional Valence (n = 91, right) neurons.

Figure 8.

NAcc neurons do not signal fear output. A, Mean ± SEM beta coefficients are shown for each regressor [ET, exaggerated threat (dark green); FO, fear output (gray)], from 2 s before cue onset to cue offset in 1 s intervals for the Threat Inhibited neurons (n = 26). Cue onset and offset are indicated by vertical black lines. B, Mean (bar) and individual (data points) early cue (the first 5 s cue, left) and late cue (the last 5 s cue, right) beta coefficients for each regressor (ET and FO) are shown for Threat Inhibited neurons (colors are as in A). C–F, Identical graph made for the Danger Excited neurons (n = 55) and Bidirectional Valence neurons (n = 91), as in A and B. ⁺95% bootstrap confidence interval for differential beta coefficient does not contain zero. ⁺95% bootstrap confidence interval for beta coefficient does not contain zero (colored + signs).

Figure 9.

Reward firing by cue-responsive neurons. A, Mean ± SEM normalized firing rate to reward is shown 2 s before and 2 s after reward delivery (advancement of feeder) for the Threat Inhibited (n = 26, gray), Danger Excited (n = 55, pink), and Bidirectional Valence neurons (n = 91, tan) neurons. Reward-related firing was extracted from intertrial intervals when no cues or footshocks were presented. Feeder advance is indicated by the black arrow. SEM is indicated by shading. B–D, Mean (bar) and individual (data points) normalized firing rates for Threat Inhibited (B), Danger Excited (C), and Bidirectional Valence (D) neurons are shown during the 250 ms interval prior (pre) to and 250 ms interval after (post) reward delivery. ⁺95% bootstrap confidence interval for differential reward firing does not contain zero. ⁺95% bootstrap confidence interval for normalized firing rate does not contain zero (colored + sign). E–G, Mean normalized firing rate to cues (danger, 10 s cue, red; safety, the first 1 s cue, blue) versus reward is plotted for Threat Inhibited (E), Danger Excited (F), and Bidirectional Valence (G) neurons. H–J, Mean normalized firing rate to danger (10 s cue) versus safety (the first 1 s cue) is plotted for Threat Inhibited (H), Danger Excited (I), and Bidirectional Valence (J) neurons. Trendline, the square of the Pearson correlation coefficient (R²) and associated p value are shown. Fisher r-to-z transformations (Z) are shown. Colors are as in A.

Figure 10.

NAcc neurons are not responsive to nose poke cessation. A, Mean ± SEM normalized firing rate is shown 2 s before and 2 s after nose poke cessation for the Threat Inhibited (ThI; n = 26; gray), Danger Excited (DE; n = 55; pink), and Bidirectional Valence (BV; n = 91; tan) neurons. Nose poke cessation is indicated by the black arrow. SEM is indicated by shading. B–D, Mean (bar) and individual (data points) normalized firing rates for Threat Inhibited (B), Danger Excited (C), and Bidirectional Valence (D) neurons are shown during the 250 ms interval prior (pre) to and 250 ms interval after (post) nose poke cessation. E–G, Mean normalized firing rate in response to cues (danger, 10 s cue, red; safety, the first 1 s cue, blue) versus nose poke cessation is plotted for Threat Inhibited (n = 26, gray; E), Danger Excited (n = 55, pink; F), and Bidirectional Valence (n = 91, tan; G) neurons. Trendline, the square of the Pearson correlation coefficient (R²) and associated p value are shown. ^cz95% confidence interval contains zero.