Studies investigating the neural mechanisms by which associations between cues and predicted outcomes control behavior often use associative learning frameworks to understand the neural control of behavior. These frameworks do not always account for the full range of effects that novelty can have on behavior and future associative learning. Here, in mice, we show that dopamine in the nucleus accumbens core is evoked by novel, neutral stimuli, and the trajectory of this response over time tracked habituation to these stimuli. Habituation to novel cues before associative learning reduced future associative learning, a phenomenon known as latent inhibition. Crucially, trial-by-trial dopamine response patterns tracked this phenomenon. Optogenetic manipulation of dopamine responses to the cue during the habituation period bidirectionally influenced future associative learning. Thus, dopamine signaling in the nucleus accumbens core has a causal role in novelty-based learning in a way that cannot be predicted based on purely associative factors.
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All data in the manuscript or the supplementary material are available from the corresponding author upon request. Correspondence and requests for materials should be addressed to Erin S. Calipari.
Codes used for the analysis of the fiber photometry data are available at github.com/kutlugunes.
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This work was supported by NIH grants no. KL2TR002245 to M.G.K.; nos. DA055380 and DA048931 to E.S.C.; no. GM07628 to J.E.Z.; and no. DA045103 to C.A.S.; as well as by funds from the VUMC Faculty Research Scholar Award to M.G.K.; the Pfeil Foundation to M.G.K.; the Brain and Behavior Research Foundation to M.G.K., E.S.C. and C.A.S.; the Whitehall Foundation to E.S.C.; and the Edward Mallinckrodt, Jr. Foundation to E.S.C. We thank J. Dunning for his technical support. The opinions expressed in this article are the authors’ own and do not reflect the views of the NIH/DHHS.
The authors declare no competing interests.
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a, Diagram showing the methods used for calculating area under the curve, peak height, time to baseline and tau. These analyses have been used extensively for defining the kinetics and dynamics of dopamine signals previously59. Area under the curve (AUC) is the total area from stimulus onset to the return to baseline. Peak height is the maximal amount of dopamine that is evoked by the stimulus over the entire trace. Time to baseline is the time in seconds that it takes for the signal to return to baseline following the peak. Tau is the time it takes to return to 2/3 of peak height. b, Representative traces for 470-nm excitation (dLight) and 405-nm excitation (isosbestic control) channels in an individual animal at baseline. c, Representative ΔF/F trace showing dopamine transients in the nucleus accumbens core.
Session 2 dopamine signal to repeated white noise presentations (6–7 presentations per animal; n = 5 mice). The first presentation of the neutral stimulus in session 2 evoked a smaller dopamine response compared to the first presentation of the neural cue in the first session (peak height for the first presentation of session 1 versus session 2; two-sided paired t-test, t4 = 2.429, P = 0.07, n = 5 mice). # P = 0.07. Data represented as mean ± s.e.m.
Extended Data Fig. 3 Pre-exposure to stimuli decreases positive dopamine responses during subsequent fear conditioning without affecting shock responses.
a, Averaged dopamine response (z-scores) during the CS+ and pre-exposed CS+ cues and footshocks in the first fear conditioning session. The music note represents the cue onset and the lightning symbol denotes the footshock onset. b, Fold change (in AUC) from average CS− values across 6 trials (two-sided nested ANOVA, F(2, 83) = 2.10, P = 0.1287). c, Percent change (in peak dopamine response) from CS− values across 6 trials (two-sided nested ANOVA, F(2, 83)= 3.91, P = 0.0239). Pre-exposure to the predictive cue does not affect dopamine response to the subsequent footshock. d, Averaged dopamine signal to footshocks following the CS+ and pre-exposed CS+ on fear conditioning session 1. e, Peak dopamine response to the footshock following a pre-exposed or non-pre-exposed cue during session 1 (two-sided nested ANOVA F(1,54) = 0.13, P = 0.3738), f, time for the signal to return to baseline following peak evoked by the footshock across trial types did not differ (two-sided nested ANOVA F(1,54) = 0.10, P = 0.7475) and g, tau also did not differ between groups (two-sided nested ANOVA F(1,54) = 0.71, P = 0.4040). Data represented as mean ± s.e.m. * P < 0.05. ns = not significant.
Extended Data Fig. 4 Latent inhibition: Dopamine responses to non-pre-exposed and pre-exposed stimuli do not differ in the absence of latent inhibition and converge following extensive experience.
a, Dopamine responses did not differ between the CS+ and pre-exposed CS+ for the animals that did not show latent inhibition. b, The peak heights (two-sided nested ANOVA, F(1, 21) = 0.61, P = 0.4449, n = 30 presentations; n = 5 mice), c, the time to return to baseline (two-sided nested ANOVA, F(1, 21) = 0.30, P = 0.5888, n = 30 presentations; n = 5 mice) and d, tau were not different between the CS+ and pre-exposed CS+ (two-sided nested ANOVA, F(1, 21) = 0.60, P = 0.4467, n = 30 presentations; n = 5 mice). e, In the mice that showed latent inhibition, the behavioral and dopamine differences disappeared. f, Freezing responses to the pre-exposed CS+, non-pre-exposed CS+ (CS+) and non-pre-exposed CS− (CS−) were measured on session 2 of a two session fear conditioning paradigm (RM ANOVA pre-exposure main effect, F(1.466,5.863) = 19.99, P = 0.0032), the difference between the CS+ and pre-exposed CS+ disappeared on the second conditioning session (Tukey post-hoc, P = 0.9979). Both the CS+ (Tukey post-hoc, P = 0.0034) and the pre-exposed CS+ (Tukey post-hoc, P = 0.0037) yielded a stronger freezing response compared to the CS−. g, Averaged dopamine responses to the CS+ and pre-exposed CS+ during session 2 over all trials. h, Dopamine responses did not differ between the CS+ and pre-exposed CS+ (nested ANOVA, F(1, 54) = 0.42, P = 0.8901, n = 30 presentations; n = 5 mice). i, The time to return to baseline was not different (nested ANOVA, F(1, 54) = 0.07, P = 0.7864, n = 30 presentations; n = 5 mice). j, Tau is another measure of dopamine clearance and is defined by the time in seconds for the signal to return to 2/3 of peak height. Tau was not different between the CS+ and pre-exposed CS+ (unpaired t-test, t58 = 0.27, P = 0.78, n = 30 presentations; n = 5 mice). In the absence of the latent inhibition effect, dopamine response to the pre-exposed and novel CS+ do not differ. Data represented as mean ± s.e.m. ** P < 0.01, ns = not significant.
Extended Data Fig. 5 Fear conditioning with additional trials yielded a negative dopamine response to the fear cues.
a, Averaged dopamine signal to fear cues during the first two versus last two CS+ trials in a separate group of C57BL6/J mice (n = 4). b, Dopamine response to the CS+ (area under the curve, AUC) following 6 trials of the latent inhibition experiment compared to the dopamine response to the CS+ in an additional group with extensive fear conditioning trials did not differ for the first 6 trials (RM ANOVA group × trial interaction F(2, 14)= 0.52, P = 0.60; main effect of group F(1, 7) = 0.12, P = 0.20) before becoming a negative response after the 9th trial. Data represented as mean ± s.e.m. ns = not significant.
a, Mice (n = 8; 4 males, 4 females) received two sessions of pre-exposure rather than four. b, Fewer pre-exposure sessions still produced a latent inhibition effect (two-sided paired t-test t7 = 3.314, P = 0.0129). Data represented as mean ± s.e.m., * P < 0.05.
Optogenetics studies were designed to test whether the latent inhibition effect is controlled by the NAc core dopamine response to the pre-exposed fear cue. a, Representative images showing the expression of ChR2 and TH in the VTA dopamine cell bodies. AAV9.rTH.PI.Cre.SV40 and AAV5.Ef1a.DIO.hchR2.eYFP or AAV5.Ef1a.DIO.eYFP was injected into the VTA to achieve specific expression of Chr2 in dopamine neurons. Specifically, AAV9.rTH.PI.Cre.SV40 injections resulted in Cre expression in all Tyrosine Hydroxylase (TH) positive cells within the VTA. By placing a fiberoptic above the NAc core, we were able to stimulate dopamine release from VTA projecting dopamine terminals in the NAc core. b, Representative images showing the expression of NpHR and TH in the VTA dopamine cell bodies using the same approach as described. AAV9.rTH.PI.Cre.SV40 and AAV5.hSyn.eNpHR.3.0.eYFP or AAV5.Ef1a.DIO.eYFP were injected into the VTA and a fiberoptic was placed in the NAc core. c, Schematic showing histologically verified fiber optic placements for all mice (n = 21 mice, 9 males, 12 females). d, Cell counts were completed within the VTA from the experiments using the TH-specific excitatory/inhibitory opsin strategy. About 75% of the Cre+ cells in the VTA were also TH + suggesting a significant portion of the ChR2 and NpHR cells were dopaminergic (two-sided paired t-test t22 = 8.96, P = 0.00000001). Data represented as mean ± s.e.m., **** P < 0.0001.
Extended Data Fig. 8 Optogenetic stimulation, but not inhibition, of dopaminergic terminals during inter-trial interval abolishes latent inhibition.
a, Mice (n = 12; 5 males, 7 females) underwent four sessions of pre-exposure where they received unpaired stimulations (ChR2) or inhibitions (NpHR) during inter-trial interval windows. b, Unpaired stimulation of the NAc core dopamine response abolished latent inhibition (two-way ANOVA cue × group interaction F(1,10) = 4.078, P = 0.071; Bonferroni multiple comparisons: ChR2 pre-exposed versus non-pre-exposed P = 0.973; NpHR pre-exposed versus non-pre-exposed P = 0.023) while inhibition of the terminals resulted in a latent inhibition effect. Data represented as mean ± s.e.m., * P < 0.05, ns = not significant.
Extended Data Fig. 9 The effect of the optogenetic inhibition and excitation of dopaminergic terminals disappears with additional fear conditioning training.
Freezing response to the CS+ and pre-exposed CS+ did not differ in the eYFP or ChR2 groups on the second session of fear conditioning (multiple comparison Ps > 0.05). Freezing to the CS+ was still greater than the freezing response to the pre-exposed CS+ at the end of the session 2 (two-way ANOVA cue × group interaction F(2,36) = 4.31, P = 0.02; multiple comparisons: NpHR pre-exposed CS+ versus CS+ P = 0.04). This suggest that the freezing response to all cues (pre-exposed and non-pre-exposed) reached the asymptotic level with additional training but the enhancing effect of dopamine inhibition during pre-exposure on latent inhibition persisted beyond the initial fear conditioning session. Data represented as mean ± s.e.m., * P < 0.05, ns = not significant.
Extended Data Fig. 10 Optogenetically stimulating VTA dopamine cell bodies during cue pre-exposure enhances subsequent associative learning for that stimulus.
a, Representative histology showing ChR2 expression in the VTA dopamine cells in the TH-Cre rats. Histology maps showing ChR2 and eYFP expression and fiber placements in the VTA. b, These experiments were designed to look at the effects of dopamine stimulations during the pre-exposure period when the cues are novel and have not yet acquired value. Ventral tegmental area (VTA) dopamine neurons were stimulated using a blue laser at the time of the cue presentation during pre-exposure sessions. c, Rats received 2 sessions of stimulus pre-exposure followed by a single session of appetitive conditioning without any stimulation. In the pre-exposure session, the auditory cue was presented in the absence of an outcome whereas in the conditioning sessions, both the pre-exposed and non-pre-exposed cues were followed by the delivery of a food pellet. d, Averaged responses (appetitive response = CS response − preCS response) for the eYFP group throughout the 6 conditioning trials (repeated measures session × group interaction ANOVA F(5,80) = 0.78, P = 0.56). e, The difference between the first trial responses to the pre-exposed and non-pre-exposed cues trended towards significance in the eYFP group (paired t-test, t8 = 2.13, P = 0.06, n = 9 rats). f, There was no difference between pre-exposed versus non-pre-exposed cue responses during the last 3 trials of the conditioning session in the eYFP group (paired t-test, t8 = 0.25, P = 0.80, n = 9 rats). g, Averaged responses for the ChR2 group throughout the 6 conditioning trials (repeated measures session × group interaction ANOVA F(5,70) = 2.42, P = 0.04). h, The difference between the first trial responses to the pre-exposed and non-pre-exposed cues did not differ in the ChR2 group (paired t-test, t7 = 1.11, P = 0.30, n = 8 rats). i, The pre-exposed cue responses were significantly higher compared to the non-pre-exposed cue responses during the last 3 trials of the conditioning session in the ChR2 group (paired t-test, t7 = 0.008, P = 0.02, n = 8 rats). This demonstrates that stimulation of the VTA dopamine cell body response to stimuli during pre-exposure enhances the learning of cue-reward associations in the subsequent appetitive conditioning training. Data represented as mean ± s.e.m., # P = 0.056, ** P < 0.01, ns = not significant.
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Kutlu, M.G., Zachry, J.E., Melugin, P.R. et al. Dopamine signaling in the nucleus accumbens core mediates latent inhibition. Nat Neurosci 25, 1071–1081 (2022). https://doi.org/10.1038/s41593-022-01126-1