Neural circuitry underlying effects of context on human pain-related fear extinction in a renewal paradigm.

OBJECTIVES: The role of context in pain-related extinction learning remains poorly understood. We analyzed the neural mechanisms underlying context-dependent extinction and renewal in a clinically relevant model of conditioned abdominal pain-related fear. EXPERIMENTAL DESIGN: In this functional magnetic resonance imaging study, two groups of healthy volunteers underwent differential fear conditioning with painful rectal distensions as unconditioned stimuli (US) and visual conditioned stimuli (CS(+) ; CS(-) ). The extinction context was changed in an experimental group (context group), which was subsequently returned into the original learning context to test for renewal. No context changes occurred in the control group. Group differences in CS-induced differential neural activation were analyzed along with skin conductance responses (SCR), CS valence and CS-US contingency ratings. PRINCIPAL OBSERVATIONS: During extinction, group differences in differential neural activation were observed in dorsolateral (dlPFC) and ventromedial (vmPFC) prefrontal cortex and amygdala, mainly driven by enhanced activation in response to the CS(-) in the control group. During renewal, observed group differences in activation of dlPFC and orbitofrontal cortex (OFC) resulted primarily from differential modulation of the CS(-) in the absence of group differences in response to CS(+) or SCR. CONCLUSION: The extinction context affects the neural processing of nonpain predictive safety cues, supporting a role of safety learning in pain-related memory processes.

Learning pain-related fear: neural mechanisms mediating rapid differential conditioning, extinction and reinstatement processes in human visceral pain.

BACKGROUND AND AIMS: There exists converging evidence to support a role of pain-related fear in the pathophysiology and treatment of chronic pain conditions. Pain-related fear is shaped by associative learning and memory processes, which remain poorly characterized especially in the context of abdominal pain such as in irritable bowel syndrome (IBS). Therefore, using event-related functional magnetic resonance imaging (fMRI), we assessed the neural mechanisms mediating the formation, extinction and reinstatement of abdominal pain-related fear in healthy humans. Employing painful rectal distensions as clinically-relevant unconditioned stimuli (US), in this fear conditioning study we tested if differential excitatory and inhibitory learning is evocable after very few CS-US learning trials ("rapid conditioning"), and explored the underlying neural substrates of these learning and memory processes. METHODS: In N=24 healthy men and women, "rapid" fear acquisition was accomplished by pairing visual conditioned stimuli (CS(+)) with painful rectal distensions as unconditioned stimuli (US), while different visual stimuli (CS(-)) were presented without US (differential delay conditioning with five CS(+) and five CS(-) presentations and a 80% reinforcement ratio). During extinction, all CS were presented without US. Subsequently, a reinstatement procedure was implemented, defined as the retrieval of an extinguished memory after unexpected and unpaired exposure to the US, followed by CS presentations. For each phase, changes in perceived CS-US contingency and CS unpleasantness were assessed with visual analogue scales and compared with analyses of variance. fMRI data were analyzed using whole-brain analyses (at p<.001 uncorrected) and in regions-of-interest analyses with familywise error correction of alpha (pFWE<.05). Differential neural activation in response to the CS during each experimental phase (i.e., CS(+)>CS(-); CS(+)

Towards understanding sex differences in visceral pain: enhanced reactivation of classically-conditioned fear in healthy women.

Sex differences in learned fear regarding aversive gastrointestinal stimuli could play a role in the female preponderance of chronic abdominal pain. In a fear conditioning model with rectal pain as unconditioned stimulus (US), we compared healthy males and females with respect to neural responses during aversive visceral learning, extinction and re-activation of fear memory (i.e., reinstatement). To do so, conditioned visual stimuli (CS(+)) were consistently paired with painful rectal distensions as US, while different visual stimuli (CS(-)) were presented without US. During extinction, both CSs were presented without US, whereas during reinstatement, a single, unpaired US was presented. In region-of-interest analyses, sexes were compared with respect to conditioned anticipatory neural activation (CS(+)>CS(-)). The results revealed that in late acquisition, CS+ presentation induced significantly greater anticipatory activation of the insula in women. During extinction, women demonstrated reduced activation of the posterior cingulate cortex. During reinstatement, the CS(+) led to greater activation of the hippocampus, thalamus and cerebellum in women. These group effects in neural activation during learning and memory processes were not accounted for by sex differences in pain thresholds, pain ratings, or stress parameters. In conclusion, this is the first study to support sex differences in neural processes mediating aversive visceral learning. Our finding of enhanced neural responses during reinstatement in key brain areas relevant for memory suggests enhanced reactivation of old fear memory trace in women. Sex differences in "gut memories" could play a role in the female preponderance of chronic abdominal pain.

Visceral sensitivity correlates with decreased regional gray matter volume in healthy volunteers: a voxel-based morphometry study.

Regional changes in brain structure have been reported in patients with altered visceral sensitivity and chronic abdominal pain, such as in irritable bowel syndrome. It remains unknown whether structural brain changes are associated with visceral sensitivity. Therefore, we present the first study in healthy individuals to address whether interindividual variations in gray matter volume (GMV) in pain-relevant regions correlate with visceral sensitivity. In 92 healthy young adults (52 female), we assessed rectal sensory and pain thresholds and performed voxel-based morphometry (VBM) to compute linear regression models with visceral sensory and pain thresholds, respectively, as independent variable and GMV in a priori-defined regions of interest (ROIs) as dependent variable. All results were familywise error (FWE) corrected at a level of PFWE<.05 and covaried for age. The mean (±SEM) rectal thresholds were 14.78±0.46mm Hg for first sensation and 33.97±1.13mm Hg for pain, without evidence of sex differences. Lower rectal sensory threshold (ie, increased sensitivity) correlated significantly with reduced GMV in the thalamus, insula, posterior cingulate cortex, ventrolateral and orbitofrontal prefrontal cortices, amygdala, and basal ganglia (all PFWE<.05). Lower rectal pain threshold was associated with reduced GMV in the right thalamus (PFWE=.051). These are the first data supporting that increased visceral sensitivity correlates with decreased gray matter volume in pain-relevant brain regions. These findings support that alterations in brain morphology not only occur in clinical pain conditions but also occur according to normal interindividual variations in visceral sensitivity.

Cerebellar contributions to different phases of visceral aversive extinction learning.

The cerebellum is increasingly recognized to contribute to non-motor functions, including cognition and emotion. Although fear conditioning has been studied for elucidating the pathophysiology of anxiety, the putative role of the cerebellum is still unknown. Fear conditioning could also be important in the etiology of chronic abdominal pain which often overlaps with anxiety. Hence, in this exploratory analysis, we investigated conditioned anticipatory activity in the cerebellum in a visceral aversive fear conditioning paradigm using functional magnetic resonance imaging. We extended and reanalyzed a previous dataset for different learning phases, i.e., acquisition, extinction, and reinstatement, utilizing an advanced normalizing method of the cerebellum. In 30 healthy humans, visual conditioned stimuli (CS(+)) were paired with painful rectal distensions as unconditioned stimuli (US), while other visual stimuli (CS(-)) were presented without US. During extinction, all CSs were presented without US, whereas during reinstatement, a single, unpaired US was presented. During acquisition, posterolateral cerebellar areas including Crus I, Crus II, and VIIb and parts of the dentate nucleus were activated in response to the CS(+) compared to the CS(-). During extinction, activation related to CS(+) presentation was detected in Crus I, Crus II, IV, V, VI, VIIb, IX, and vermis. Neural correlates of reinstatement were found in Crus I, Crus II, IV, V, and IX. We could show for the first time that the cerebellum is involved in abdominal pain-related associative learning processes. Together, these findings contribute to our understanding of the cerebellum in aversive learning and memory processes relevant to the pathophysiology of chronic abdominal pain.

Fear conditioning in an abdominal pain model: neural responses during associative learning and extinction in healthy subjects.

Fear conditioning is relevant for elucidating the pathophysiology of anxiety, but may also be useful in the context of chronic pain syndromes which often overlap with anxiety. Thus far, no fear conditioning studies have employed aversive visceral stimuli from the lower gastrointestinal tract. Therefore, we implemented a fear conditioning paradigm to analyze the conditioned response to rectal pain stimuli using fMRI during associative learning, extinction and reinstatement. In N = 21 healthy humans, visual conditioned stimuli (CS(+)) were paired with painful rectal distensions as unconditioned stimuli (US), while different visual stimuli (CS(-)) were presented without US. During extinction, all CSs were presented without US, whereas during reinstatement, a single, unpaired US was presented. In region-of-interest analyses, conditioned anticipatory neural activation was assessed along with perceived CS-US contingency and CS unpleasantness. Fear conditioning resulted in significant contingency awareness and valence change, i.e., learned unpleasantness of a previously neutral stimulus. This was paralleled by anticipatory activation of the anterior cingulate cortex, the somatosensory cortex and precuneus (all during early acquisition) and the amygdala (late acquisition) in response to the CS(+). During extinction, anticipatory activation of the dorsolateral prefrontal cortex to the CS(-) was observed. In the reinstatement phase, a tendency for parahippocampal activation was found. Fear conditioning with rectal pain stimuli is feasible and leads to learned unpleasantness of previously neutral stimuli. Within the brain, conditioned anticipatory activations are seen in core areas of the central fear network including the amygdala and the anterior cingulate cortex. During extinction, conditioned responses quickly disappear, and learning of new predictive cue properties is paralleled by prefrontal activation. A tendency for parahippocampal activation during reinstatement could indicate a reactivation of the old memory trace. Together, these findings contribute to our understanding of aversive visceral learning and memory processes relevant to the pathophysiology of chronic abdominal pain.

Acute experimental endotoxemia induces visceral hypersensitivity and altered pain evaluation in healthy humans.

Growing evidence suggests that systemic immune activation plays a role in the pathophysiology of pain in functional bowel disorders. By implementing a randomized crossover study with an injection of endotoxin or saline, we aimed to test the hypothesis that endotoxin-induced systemic inflammation increases visceral pain sensitivity in humans. Eleven healthy men (mean ± standard error of the mean age 26.6 ± 1.1 years) received an intravenous injection of either lipopolysaccharide (LPS; 0.4 ng/kg) or saline on 2 otherwise identical study days. Blood samples were collected 15 min before and 1, 2, 3, 4, and 6h after injection to characterize changes in immune parameters including proinflammatory cytokines. Rectal sensory and pain thresholds and subjective pain ratings were assessed with barostat rectal distensions 2h after injection. LPS administration induced an acute inflammatory response indicated by transient increases in tumor necrosis factor alpha, interleukin 6, and body temperature (all P<.001). The LPS-induced immune activation increased sensitivity to rectal distensions as reflected by significantly decreased visceral sensory and pain thresholds (both P<.05) compared to saline control. Visceral stimuli were rated as more unpleasant (P<.05) and inducing increased urge to defecate (P<.01). Pain thresholds correlated with interleukin 6 at +1h (r=0.60, P<.05) and +3h (r=0.67, P<.05) within the LPS condition. This report is novel in that it demonstrates that a transient systemic immune activation results in decreased visceral sensory and pain thresholds and altered subjective pain ratings. Our results support the relevance of inflammatory processes in the pathophysiology of visceral hyperalgesia and underscore the need for studies to further elucidate immune-to-brain communication pathways in gastrointestinal disorders.