The central amygdala recruits mesocorticolimbic circuitry for pursuit of reward or pain

The study investigates how the amygdala, specifically the central nucleus of the amygdala (CeA), interacts with mesocorticolimbic circuitry to assign motivational significance to stimuli, potentially producing maladaptive attractions characteristic of disorders such as addiction and self-harm. Maladaptive attraction is operationally defined as excessively intense motivation, narrowly focused on one target among alternatives, and carrying adverse consequences. Prior work showed that optogenetic excitation of CeA can intensify appetitive motivation and narrow pursuit to a paired reward. Here, the authors test whether CeA ChR2 pairing can arbitrarily bias choice between natural (sucrose) and drug (cocaine) rewards, create attraction to a noxious shock rod, endow shock-associated cues with incentive value, and whether the valence of CeA-driven motivation can switch to negative in a Pavlovian fear context or become neutral with innocuous stimuli. The overarching hypothesis is that CeA activation can flexibly recruit mesocorticolimbic circuits to generate positive or negative motivational salience depending on context and paired stimuli.

Background literature indicates the amygdala participates in assigning motivational value to rewards and threats (e.g., Baxter & Murray, LeDoux). CeA involvement has been implicated in both appetitive and aversive learning and in focusing incentive salience on cues. Earlier optogenetic studies showed CeA excitation can amplify and narrow motivation for specific rewards and increase cue-directed consummatory behaviors. The mesocorticolimbic network (VTA, NAc, OFC/insula) computes incentive salience (wanting) distinct from hedonic liking and can be modulated by physiological and contextual states. CeA microcircuits and projection-defined subpopulations (e.g., SOM+, PKCδ+, CRF+ neurons) have dissociable roles in fear and reward processes, suggesting potential mechanistic substrates for valence-dependent effects. This work builds on these findings to test how CeA activation paired with different unconditioned stimuli can create maladaptive attraction or fearful salience and how distributed circuitry is recruited.

Subjects: 55 rats (female Sprague-Dawley n=37; male Sprague-Dawley n=6; female Long Evans n=12), housed on reverse light/dark cycle with ad libitum food/water. All procedures approved by University of Michigan IACUC.

Surgery and viral expression: Under isoflurane anesthesia, bilateral CeA microinjections of AAV5-hSyn-ChR2-eYFP (n=39) or control AAV5-hSyn-eYFP (n=16) were delivered (0.75 µl per side; AP -2.4, ML ±4.0, DV -7.6 mm). Bilateral optic fibers were implanted 0.3 mm above CeA. Some rats later received jugular vein catheters for cocaine self-administration.

Behavioral tasks:

• Sucrose vs. cocaine instrumental choice: After separate single-outcome training to a criterion of 50 rewards each, rats underwent 2-h daily two-choice sessions with both portholes available (up to 10 choices/session). For each rat, CeA laser (473 nm, 10 mW, 25 Hz, 8 s) was paired only with its designated outcome (sucrose pellet or IV cocaine 0.3 mg/kg/infusion); the alternative outcome was never paired with laser. Nosepokes, latencies, perseveration, and consummatory interactions with portholes were recorded.
• Shock rod encounters: In a Plexiglas chamber with bedding, an electrified metal rod protruded 9 cm; voluntary contact delivered 0.2–0.5 mA shock. CeA laser (473 nm, 10 mW, 40 Hz) was triggered when rats approached within 2 cm of the rod, bracketing encounters. Sessions repeated across 3 days; day 4 was a laser-extinction test (no laser) with the rod still electrified. Measures included rod approaches/touches, defensive treading/burying, proximity heatmaps, and consummatory actions (sniffing, nibbling, biting the rod). Control tests included a dummy unelectrified rod and an inedible wooden block.
• Barrier test: After 5 min of access, an opaque barrier was inserted requiring a 13-cm climb to reach the rod, assessing motivated pursuit under obstacle.
• Conditioned reinforcement: In separate chambers without the rod, rats could nosepoke to earn 4-s presentations of an auditory CS+ previously paired with rod encounters and laser, or a CS− presented in homecage sessions. Active vs. inactive nosepokes were recorded across counterbalanced CS+ and CS− sessions.
• Pavlovian fear conditioning: Naive rats received tone CS+ (10 s) paired with footshock UCS (0.75 mA, 0.5 s); CeA laser (473 nm, 10 mW, 40 Hz, 10 s) accompanied each CS+ during training. Contextual CS+ odor (almond or lemon) was also paired with the shock context; CS− odor presented in homecage. Tests assessed freezing CRs to CS+ with and without concurrent laser and place avoidance of CS+ odor vs. CS− odor.
• Laser self-stimulation: In a spout-touch task, one empty-metal spout delivered CeA laser (1 s or 8 s; 25–40 Hz, 10 mW) on FR1; an inactive spout yielded nothing. Criteria for self-stimulation: >2:1 laser:inactive touches and >50 touches.

Histology and neural activity mapping: Fos immunohistochemistry quantified local “Fos plumes” in CeA around optic fibers and distributed Fos expression across mesocorticolimbic regions (VTA, SNc, NAc shell/core, ventral pallidum, lateral hypothalamus perifornical area, OFC/insula, BLA, BNST, PAG). Fos counts were compared to eYFP and homecage controls.

Statistics: Mixed/repeated-measures ANOVAs, paired/unpaired t-tests with Bonferroni corrections where appropriate, nonparametric tests for non-normal data, and effect sizes (Cohen’s d). Significance threshold p<0.05.

• Arbitrary focusing of reward pursuit: Pairing CeA ChR2 with sucrose or cocaine caused exclusive pursuit of the paired outcome when both were available. Preference for the laser-paired outcome reached 87±4% by day 4 (~10:2 ratio) vs. eYFP controls ~49±13% (near 1:1). ANOVA main effect of virus on earned rewards: F1,10=4.6, p=0.046; paired comparisons within ChR2 showed robust laser-paired vs. non-laser responses (t10=7.5, p<0.001, d=4.39); day 4 preference t14=3.4, p=0.006, d=1.81.
• Enhanced vigor and perseveration: ChR2 rats initiated nosepokes >30× faster into the laser-paired porthole (3±0.3 s; median 2) vs. non-laser porthole (97±12 s; median 40) on single-choice trials (Wilcoxon Z=2.8, p=0.005). They made 5±1 perseverative pokes during 8-s timeouts and showed doubled nibbling/biting of laser-paired portholes.
• Creation of attraction to a noxious shock rod: Unlike eYFP rats that avoided the rod and emitted defensive treading/burying, CeA ChR2 rats repeatedly approached and touched the electrified rod (mean touches: day1≈5, day2≈7, day3≈8), often hovering near it and receiving multiple shocks. They displayed consummatory interactions with the rod: on day 3, 66–71% chewed/nibbled, with cumulative chewing 81±24 s per session; no eYFP rat chewed. Group differences in rod touches and chewing were significant (e.g., touches: t23=3.6, p=0.002; chewing: t18=-3.3, p=0.004). Defensive treading was greatly reduced in ChR2 vs. eYFP when laser was present (t20=2.4, p=0.03) and emerged on no-laser extinction day (~5 s bouts).
• Dependence on concurrent CeA activation: On a no-laser extinction day, ChR2 rats reduced rod approaches/touches (>50% reduction in 7/8 rats) and nearly ceased chewing; brief defensive treading appeared, indicating recognition of aversiveness and that full attraction depends on simultaneous laser pairing rather than solely on enduring revaluation.
• Motivated pursuit under obstacle: With an opaque barrier blocking view/access, ChR2 rats climbed over repeatedly (5±1 crosses; received 3.36±1 shocks/15 min) vs. eYFP 0–1 crosses; barrier crosses t9=3.0, p=0.02. ChR2 rats typically resumed chewing once at the rod.
• Incentive value to shock-associated cues: In conditioned reinforcement, ChR2 rats nosepoked to earn CS+ (shock-rod-paired sound) >300% more than for CS−, discriminating CS+ vs. inactive and CS− (CS type × virus interaction F1,12=3.84, p=0.04; CS+ group difference p=0.03, d=1.35). eYFP rats showed low, non-discriminative responding.
• Mesocorticolimbic recruitment: During pursuit of laser-paired sucrose/cocaine, ChR2 rats showed robust Fos elevations: VTA >800%, rostromedial NAc shell >700%, posterior insula >500%, dorsolateral neostriatum >500%; decreases in ventrolateral PAG and BLA (>200% below baseline). Shock-rod attraction similarly elevated Fos: caudal VTA >400%, SNc >200%, rostral medial NAc shell >180%, dorsolateral neostriatum ~200%, perifornical lateral hypothalamus >200%, medial OFC ~175%, posterior insula >250%. In contrast, eYFP “rod avoiders” showed elevated ventrolateral PAG (>400%), BLA (>240%), and BNST (>125%).
• Valence reversal in Pavlovian fear: In a standard fear conditioning paradigm with unavoidable footshock, concurrent CeA laser potentiated freezing CR expression in ChR2 rats (trial type × virus interaction F1,11=5.07, p=0.036; within-ChR2 laser vs. non-laser trials p=0.035). ChR2 rats also avoided the CS+ odor context more than CS− (virus × CS interaction F1,11=6.06, p=0.03; CS+ vs. CS− time in ChR2 p=0.04).
• CeA laser is not a strong reinforcer by itself: In self-stimulation tests, only a minority of ChR2 rats met criteria (7/29 across groups). Group-level analyses showed no significant virus × laser effects (e.g., sucrose/cocaine group: 1 s F1,15=1.71, p=0.21; 8 s F1,15=2.06, p=0.18; shock-rod group: 1 s F1,18=0.07, p=0.8; 8 s F1,18=1.93, p=0.18). Self-stimulation did not predict stronger overall attraction, though the strongest self-stimulators in the shock-rod group showed more rod chewing (140±68 s vs. 31±16 s; t16=2.4, p=0.029).

The findings demonstrate that CeA activation can flexibly impose motivational salience onto concurrently encountered targets, recruiting mesocorticolimbic circuitry to drive intense and narrowly focused pursuit. When paired with sucrose or cocaine, CeA stimulation steers choice almost exclusively to the paired option, increasing vigor and cue-directed consummatory actions—hallmarks of enhanced incentive salience. Strikingly, pairing with a noxious shock rod induces maladaptive attraction to a pain-associated object, supporting repeated voluntary contacts and consummatory interactions, while concurrently activating dopamine-rich midbrain (VTA/SNc), rostral medial NAc shell, and cortical hotspots (mOFC/posterior insula) typically linked to positive valence motivation. In contrast, avoidance in controls was associated with defensive circuitry (PAG, BLA, BNST), underscoring distinct network engagement for fearful vs. incentive states.

Context critically shapes valence: in a standard Pavlovian fear setting with unavoidable, inescapable footshock, CeA activation enhanced acquisition/expression of defensive freezing and contextual avoidance, indicating a switch to negatively valenced fearful salience. Thus, CeA-driven motivation is plastic, with situational features (controllability/localizability and intensity of aversive stimuli) tilting the balance between incentive and fearful salience. The lack of robust self-stimulation further suggests CeA activation gains motivational impact primarily when bound to affective unconditioned stimuli that already engage mesocorticolimbic circuits, rather than acting as a strong reinforcer in isolation.

Psychologically, the pattern supports an incentive salience account: paired targets and their cues become attention-grabbing and ‘wanted’, eliciting approach and cue-seeking even when the primary outcome is aversive (suggesting ‘wanting what hurts’ rather than ‘wanting to be hurt’). Clinically, the ability of CeA activation to produce focused, persistent pursuit despite adverse consequences parallels key features of addiction and may offer insight into certain forms of self-harm where pain-associated targets acquire motivational pull.

This study shows that optogenetic activation of central amygdala, when paired with affective encounters, can arbitrarily and powerfully focus motivation on natural rewards, drugs, or even aversive pain-associated objects, by recruiting mesocorticolimbic circuits. CeA-driven motivational salience exhibits valence plasticity: it supports positive incentive attraction in controllable, localizable contexts, flips to enhance negative defensive responses in standard fear conditioning, and is largely neutral when paired with innocuous stimuli or presented alone. These results dissociate ‘wanting’ from ‘liking’ and highlight CeA as a context-gated controller of mesocorticolimbic motivation.

Future directions include dissecting CeA neuronal subpopulations and projection-defined circuits (e.g., SOM+, PKCδ+, CRF+, D1/D2) that mediate valence-specific effects, identifying circuit conditions that determine valence switches (controllability, spatial localizability, shock intensity), and testing translational relevance to human maladaptive motivations and self-harm. Understanding these mechanisms may inform targeted interventions that modulate maladaptive ‘wanting’.

• Cell-type specificity: ChR2 under hSyn promoter broadly infected CeA neurons, preventing attribution of effects to specific subpopulations; future work should parse SOM+, PKCδ+, CRF+, and projection-defined circuits.
• Context dependence: Shock-rod attraction depended on concurrent CeA stimulation and diminished without laser, indicating limited persistence of learning and strong state dependence.
• Stimulus eligibility: Neutral stimuli (dummy rod, wooden block, empty spout) generally failed to acquire attraction, suggesting CeA effects require pairing with affectively salient UCS that engages mesocorticolimbic circuits.
• Self-stimulation variability: CeA laser alone was generally not a robust reinforcer; individual variability complicates generalization.
• Aversive intensity and controllability: Differences between shock-rod (0.2–0.5 mA, controllable, localizable) and Pavlovian footshock (0.75 mA, uncontrollable) may confound direct comparisons of valence; precise parameter mapping remains to be done.
• Species/generalizability: Findings in rats may not directly translate to humans; clinical implications remain speculative.