Orbitofrontal activation restores insight lost after cocaine use

Abstract

Addiction is characterized by a lack of insight into the likely outcomes of one's behavior. Insight, or the ability to imagine outcomes, is evident when outcomes have not been directly experienced. Using this concept, work in both rats and humans has recently identified neural correlates of insight in the medial and orbital prefrontal cortices. We found that these correlates were selectively abolished in rats by cocaine self-administration. Their abolition was associated with behavioral deficits and reduced synaptic efficacy in orbitofrontal cortex, the reversal of which by optogenetic activation restored normal behavior. These results provide a link between cocaine use and problems with insight. Deficits in these functions are likely to be particularly important for problems such as drug relapse, in which behavior fails to account for likely adverse outcomes. As such, our data provide a neural target for therapeutic approaches to address these defining long-term effects of drug use.

Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1. Experimental timeline, task design and recording sites for in vivo recording experiment

a. Timeline and task design. Approximately 3 weeks after the end of self-administration, rats were trained in a Pavlovian over-expectation task while single unit activity was recorded in OFC. b. Drawings illustrate recording sites in OFC in sucrose (blue) and cocaine-trained (red) rats. Boxes indicate approximate location of recording sites in each rat, taking into account any vertical distance traveled during training and the approximate lateral spread of the electrode bundle.

Figure 2. Conditioned responding and cue-evoked activity summates at the start of compound training in sucrose but not cocaine-trained rats

a–b. Conditioned responding in sucrose (a) and cocaine-trained (b) rats at the end of conditioning (CP 1/2) and through compound training (CP 2/2 and CP2-CP4). Error bars indicate S.E.M. A 3-factor ANOVA (cue X phase X treatment) showed a significant interaction between treatment, cue and phase (F 1, 18 = 16.7, p = 0.0007), due to a significant increase in responding to A1 when it was paired with V in sucrose (*; p < 0.05) but not cocaine-trained (ns) rats. c–f. Population activity across all cue-responsive neurons to A1, V (c, d) and A2 (e, f) during the compound probe session; dark and light lines illustrate activity during the conditioning and compound phases of the session, respectively. Gray shading indicates S.E.M, and gray horizontal bars indicate the period of cue presentation. Two-factor ANOVA’s (treatment X phase) showed significant effects of treatment on the pattern of firing to A1 (F 1, 102 = 7.9, p = 0.0059) but not A2 (F 1, 102 = 1.17, p = 0.28), due to a significant increase in firing to A1 in the sucrose but not the cocaine group at the start of compound training (*; p < 0.05). g–j. Distribution of summation index scores for firing to A1 (g, i) and A2 (h, j) in the compound probe. Index scores were computed for each neuron based on the change in mean normalized firing to the relevant cue between conditioning and compound training, using the following formula: (firing CP 2/2 – firing CP 1/2)/(firing CP 2/2 + firing CP 1/2). Black bars represent neurons in which the difference in firing was statistically significant (t-test, p < 0.05). In sucrose-trained rats, the distribution of the scores for A1 shifted significantly above zero and was significantly different from the unshifted distribution for A2; A1 also differed significantly between groups (Mann-Whitney U test, p’s < 0.01). No shifts were observed in the scores from cocaine-trained rats. k–l. Scatter plots showing relationship between the change in behavior and neural activity to A1 in the compound probe session. Neural summation index scores were computed for firing to A1 as described above; behavioral summation index scores were computed similarly, for each session in which a cue-responsive neuron was recorded, but using conditioned responding instead of firing. Neural summation was correlated with behavioral summation in sucrose (k) but not cocaine-trained (l) rats.m–n. Line plots showing the ratio between normalized firing to A1 and A2 during each compound training session (CP – CP4). N’s indicate number of cue-responsive neurons in each session. Error bars indicate S.E.M. A 2-factor ANOVA revealed a significant effect of treatment on the A1/A2 ratios (F 4, 412 = 13.8, p < 0.0001), which increased significantly in the compound phase of the probe and then gradually decreased in sucrose (m) but not cocaine-trained rats (n). A similar effect was evident across trials within the compound probe session (inset, F 5,505 = 2.4, p = 0.036). *p < 0.05.

Figure 3. Conditioned responding and cue-evoked activity spontaneously declined at the start of extinction training in sucrose but not cocaine-trained rats

a–b. Conditioned responding in sucrose (a) and cocaine-trained (b) rats as a percentage of time in the food cup during each cue at the end of compound training (PB 1/2) and during the 8 trials of extinction (Trial 1–8 and bar graph showing means). Error bars indicate S.E.M., (*p < 0.05). A 3-factor ANOVA (cue X trial X treatment) revealed a significant interaction between treatment and cue (F 2, 36 = 4.61, p = 0.016), due to a significant decline in responding to A1 when it was separated from V in sucrose (*; p < 0.05) but not cocaine-trained (ns) rats. Notably both groups showed extinction of responding to A1 and A2 across trials due to reward omission, with cocaine-treated rats showing somewhat more pronounced effects.c–f. Population activity across all cue-responsive neurons to A1, V(c, d) and A2 (e, f) during the extinction probe session; light and dark lines illustrate activity during the compound phase and on the first trial (1T) of extinction during the session, respectively. Gray shading indicates S.E.M, and gray horizontal bars indicate the period of cue presentation. Two-factor ANOVA’s (phase X treatment) revealed significant effects of treatment on the pattern of firing to A1 (F 1, 118 = 25.4, p < 0.0001) but not A2 (F 1, 118 = 1.8, p = 0.17) due to a significant decrease in firing to A1 in the sucrose but not the cocaine group at the start of extinction (*; p < 0.05). g–j. Distribution of over-expectation index scores for firing to A1 (g, i) and A2 (h, j) in the extinction probe. Index scores were computed for each neuron based on the change in mean normalized firing to the relevant cue between compound training and the first trial of extinction, using the following formula: (firing PB 1T – firing PB 1/2)/(firing PB 1T + firing PB 1/2). Black bars represent neurons in which the difference in firing was statistically significant (t-test, p < 0.05). In sucrose-trained rats, the distribution of the scores for A1 shifted significantly below zero and was significantly different from the unshifted distribution for A2; A1 also differed significantly between groups (Mann-Whitney U test, p’s < 0.01). No shifts were observed in the scores from cocaine-trained rats.k–l. Scatter plots showing relationship between the change in behavior and neural activity to A1 on the first trial of extinction training. Neural over-expectation index scores were computed for firing to A1 as described above; behavioral over-expectation index scores were computed similarly, for each session in which a cue-responsive neuron was recorded, but using conditioned responding instead of firing. Neural changes were correlated with behavioral changes in sucrose (k) but not cocaine-trained (l) rats. m–n.Scatter plots showing relationship between the change in behavior on the first trial of extinction and neural activity to A1 at the start of compound training. Neural summation index scores were computed for firing to A1 as described in Figure 2. Neural summation was inversely correlated with behavioral over-expectation in sucrose (m) but not cocaine-trained (n) rats.

Figure 4. Reduced excitatory transmission in OFC pyramidal neurons in cocaine-trained rats

a. Timeline for slice recording experiment. Approximately 3 weeks after the end of self-administration, rats were trained in a Pavlovian over-expectation task illustrated in Figure 1.b–c. Conditioned responding in sucrose (b) and cocaine-trained (c) rats as a percentage of time in the food cup during each cue at the end of compound training (CP4) and during the 8 trials of extinction (Trial 1–8 and bar graph showing means). Error bars indicate S.E.M., (*p < 0.05). A 3-factor ANOVA (cue X trial X treatment) revealed a significant interaction between treatment and cue (F 2, 32 = 5.65, p = 0.008). Subsequent analyses showed that this was due to a significant decline in responding to A1 when it was separated from V in sucrose (*; p < 0.05) but not cocaine-trained (ns) rats. d. Traces show pharmacologically isolated, mEPSCs recorded in OFC pyramidal neurons in brain slices from sucrose and cocaine-trained rats.e. Mean cumulative probability distributions for mEPSC amplitude and frequency for cells from sucrose (n = 26 neurons, 9 rats) and cocaine-trained rats (n = 28 neurons, 9 rats), showing a reduction in mEPSC frequency (p < 0.0001, K-S test). Insets: mean mEPSC parameters: amplitude (*p = 0.0036, t - test) and frequency (p > 0.05, t-test). e. Mean mEPSC parameters: rise and decay times (p’s > 0.05, t-test). g. Rats that learned from over-expectation exhibited higher mEPSC frequencies (p < 0.05, t-test). White and black circles indicate mean mEPSC frequency from individual rats included in the cocaine and sucrose-trained rats, respectively. The mean and s.e.m. of mEPSC frequency for these groups is also indicated by black and white horizontal and vertical lines, respectively.

Figure 5. In vivo optogenetic activation of OFC neurons reverses the behavioral deficit in cocaine-trained rats

a. Timeline for the optogenetic experiment. Approximately 3 weeks after the end of self-administration (7 weeks post-virus injection), rats were trained in a Pavlovian over-expectation task illustrated in Figure 1. b. Locations of cannula tracks in ChR2 (left) and eYFP (right) rats. c. left panel: approximate location (box) of confocal fluorescence image (right panel) showing expression of ChR2-eYFP (yellow) ~8 weeks after virus injection into OFC in a representative brain slice. d. Sample traces (left) and mean data (right) of synaptic currents evoked by a 5–ms 473 nm light pulse into the OFC in vitro, in AAV-CAMKII-ChR2-YFP (n = 4 cells) and AAV-CAMKII-YFP (n = 4 cells) injected rats. Light-evoked currents were only observed in the AAV-CAMKII-Chr2-YFP-injected group, and they were abolished by NBQX (10 μM), an AMPA\kainate receptor antagonist. Dashed lines represent time of initiation of light stimulations. e–h. Conditioned responding in cocaine-trained rats as a percentage of time in the food cup during each cue at the end of compound training (PB 1/2) and during the 8 trials of extinction (Trial 1–8 and bar graph showing means) in ChR2-CS (e), and eYFP-CS (f), ChR2-ITI (g), and eYFP-ITI (h) groups. Error bars indicate S.E.M., (*p < 0.05). Three-factor ANOVA’s (cue X trial X treatment) revealed significant interactions between treatment and cue for both CS (F 2, 30 = 3.84, p = 0.032) and ITI stimulation (F 2, 30 = 4.02, p = 0.028). Subsequent analyses showed that this was due to significant declines in responding to A1 when it was separated from V in ChR2 (*; p < 0.05) but not eYFP (ns) rats.