Lesions of lateral habenula attenuate win-stay but not lose-shift responses in a competitive choice task

Exploration-Exploitation and Suicidal Behavior in Borderline Personality Disorder and Depression

Importance

Clinical theory and behavioral studies suggest that people experiencing suicidal crisis are often unable to find constructive solutions or incorporate useful information into their decisions, resulting in premature convergence on suicide and neglect of better alternatives. However, prior studies of suicidal behavior have not formally examined how individuals resolve the tradeoffs between exploiting familiar options and exploring potentially superior alternatives.

Objective

To investigate exploration and exploitation in suicidal behavior from the formal perspective of reinforcement learning.

Design, Setting, and Participants

Two case-control behavioral studies of exploration-exploitation of a large 1-dimensional continuous space and a 21-day prospective ambulatory study of suicidal ideation were conducted between April 2016 and March 2022. Participants were recruited from inpatient psychiatric units, outpatient clinics, and the community in Pittsburgh, Pennsylvania, and underwent laboratory and ambulatory assessments. Adults diagnosed with borderline personality disorder (BPD) and midlife and late-life major depressive disorder (MDD) were included, with each sample including demographically equated groups with a history of high-lethality suicide attempts, low-lethality suicide attempts, individuals with BPD or MDD but no suicide attempts, and control individuals without psychiatric disorders. The MDD sample also included a subgroup with serious suicidal ideation.

Main Outcomes and Measures

Behavioral (model-free and model-derived) indices of exploration and exploitation, suicide attempt lethality (Beck Lethality Scale), and prospectively assessed suicidal ideation.

Results

The BPD group included 171 adults (mean [SD] age, 30.55 [9.13] years; 135 [79%] female). The MDD group included 143 adults (mean [SD] age, 62.03 [6.82] years; 81 [57%] female). Across the BPD (χ23 = 50.68; P < .001) and MDD (χ24 = 36.34; P < .001) samples, individuals with high-lethality suicide attempts discovered fewer options than other groups as they were unable to shift away from unrewarded options. In contrast, those with low-lethality attempts were prone to excessive behavioral shifts after rewarded and unrewarded actions. No differences were seen in strategic early exploration or in exploitation. Among 84 participants with BPD in the ambulatory study, 56 reported suicidal ideation. Underexploration also predicted incident suicidal ideation (χ21 = 30.16; P < .001), validating the case-control results prospectively. The findings were robust to confounds, including medication exposure, affective state, and behavioral heterogeneity.

Conclusions and Relevance

The findings suggest that narrow exploration and inability to abandon inferior options are associated with serious suicidal behavior and chronic suicidal thoughts. By contrast, individuals in this study who engaged in low-lethality suicidal behavior displayed a low threshold for taking potentially disadvantageous actions.

PACAP-expressing neurons in the lateral habenula diminish negative emotional valence

The lateral habenula (LHb) is a small, bilateral, epithalamic nucleus which processes aversive information. While primarily glutamatergic, LHb neurons express genes coding for many neuropeptides, such as Adcyap1 the gene encoding pituitary adenylate cyclase-activating polypeptide (PACAP), which itself has been associated with anxiety and stress disorders. Using Cre-dependent viral vectors, we targeted and characterized these neurons based on their anatomical projections and found that they projected to both the raphe and rostromedial tegmentum but only weakly to ventral tegmental area. Using RiboTag to capture ribosomal-associated mRNA from these neurons and reanalysis of existing single cell RNA sequencing data, we did not identify a unique molecular phenotype that characterized these PACAP-expressing neurons in LHb. In order to understand the function of these neurons, we conditionally expressed hM₃ Dq DREADD selectively in LHb PACAP-expressing neurons and chemogenetically excited these neurons during behavioral testing in the open field test, contextual fear conditioning, sucrose preference, novelty suppressed feeding, and conditioned place preference. We found that Gq activation of these neurons produce behaviors opposite to what is expected from the LHb as a whole-they decreased anxiety-like and fear behavior and produced a conditioned place preference. In conclusion, PACAP-expressing neurons in LHb represents a molecularly diverse population of cells that oppose the actions of the remainder of LHb neurons by being rewarding or diminishing the negative consequences of aversive events.

Chronic unpredictable stress shifts rat behavior from exploration to exploitation

Chronic stress evokes wide-ranging behavioral alterations, including risk avoidance, increased motoric output, and reduced consummatory behaviors. These are often interpreted as dysfunctions, but they may subserve adaptations for coping with existential threats. We tested this in a cohort of rats previously exposed to mild unpredictable stress for 5 weeks. Previously stressed rats exhibited the typically increased avoidance of open field and altered responses to predator odor, suggesting enhanced sensitivity to threatening contexts and cues. Interestingly, these animals collected rewards at a higher rate than controls, because they locomoted faster, spent less time in off-task (exploratory) behavior, and committed fewer licks at feeders. Further, they were not impaired in flexibly shifting choice as reward probabilities changed among feeders, suggesting that behavioral adaptations are not simply of transference to behavioral control to neural systems insensitive to reward (e.g. habits). These data add to a small but growing body of evidence indicating that stress shifts responses away from exploration and toward exploitation of resources, possibly to reduce threat exposure.HighlightsRats with a history of stress collected reward at a higher rate than controls on an operant task, owing to increase locomotion speed, reduced off-task behavior, and reduced time licking at feeders.Previously stressed rats exhibited increased win-stay responses than controls, suggesting the involvement of neural circuits related to goal-directed responding.Previously stressed rats performed equally to controls on a task requiring a shift of preferences based on reward probability, suggesting that they are not simply relying more on habit-based neural systems.