Pain perception in relation to emotional learning

A novel conditioned nociceptive response in mice

Chronic pain tends to be intractable, regardless of whether the etiology has improved or is persistent. This intractability may be due, in part, to conditioning factors, but studies of the underlying mechanism are limited. We predicted that the body might learn pain sensation during sustained pain. In the present study, we sought to examine the prediction that nociceptive pain could be a conditioned response. After pre-exposing mice to the context box, we assessed hind-paw licking responses(s), an unconditioned nociceptive response (UCR), in the training phase for 30 min following each of two injections (24h apart) of formalin into the hind paws. Forty-eight hours later, in the test phase, we tested for a conditioned nociceptive response (CR) from paw injections of saline, with mice placed in either the same or a different visual context box. The results showed that the CR elicited in the same context box was significantly larger than one elicited in the different box. An audiovisual context, which is used in prototypical Pavlovian conditioning, augmented the CR. The CR diminished to baseline levels during repeated extinction procedures, in which saline alone was given in the same context box. However, the CR in animals injected with saline in their home cages was unchanged. Treatment with scopolamine, which has an antimuscarinic action, and thus induces an amnestic effect, did not affect the UCR, but reduced the CR. These results indicated that repeated nociceptive stimuli were sufficient to produce a CR.

Phantom headache: pain-memory-emotion hypothesis for chronic daily headache?

The neurobiology of chronic pain, including chronic daily headache (CDH) is not completely understood. “Pain memory” hypothesis is one of the mechanisms for phantom limb pain. We reviewed the literature to delineate a relation of “pain memory” for the development of CDH. There is a direct relation of pain to memory. Patients with poor memory have less chance to develop “pain memory”, hence less possibility to develop chronic pain. Progressive memory impairment may lead to decline in headache prevalence. A similar relation of pain is also noted with emotional or psychiatric symptoms. Literature review suggests that there is marked overlap in the neural network of pain to that of memory and emotions. We speculate that pain, memory, and emotions are interrelated in triangular pattern, and each of these three is related to other two in bidirectional pattern, i.e., stimulation of one of these will stimulate other symptoms/networks and vice versa (triangular theory for chronic pain). Longstanding or recurrent noxious stimuli will strengthen this interrelation, and this may be responsible for chronicity of pain. Reduction of both chronic pain and psychological symptoms by cognitive behavioral therapy or psychological interventions further suggests a bidirectional interrelation between pain and emotion. Longitudinal studies are warranted on the prevalence of headache and other painful conditions in patients with progressive memory impairment to delineate the relation of pain to memory. Interrelation of headache to emotional symptoms should also be explored.

Brain activity for chronic knee osteoarthritis: dissociating evoked pain from spontaneous pain

Chronic pain is a hallmark of osteoarthritis (OA), yet little is known about its properties and representation in the brain. Here we use fMRI combined with psychophysics to study knee pain in fourteen OA patients and nine healthy controls. Mechanical painful pressure stimuli were applied to the knee in both groups and ratings of evoked pain and related brain activity examined. We observe that psychophysical properties and brain activation patterns of evoked pain are essentially the same between OA patients and healthy subjects, and between worse and better OA knees. In OA patients, stimulus-related brain activity could be distinguished from brain activity associated with spontaneous pain. The former activated brain regions commonly observed for acute painful stimuli in healthy subjects, while the spontaneous pain of OA engaged prefrontal-limbic regions closely corresponding to areas observed for spontaneous pain in other chronic pain conditions, such as chronic back pain and post-herpetic neuralgia. Arthritis-related clinical characteristics of knee OA also mapped to prefrontal-limbic regions. In a subgroup of patients (n=6) we examined brain activity changes for a 2-week, repeat measure, cyclooxygenase-2 inhibitor (valdecoxib) therapy. Treatment decreased spontaneous pain for the worse knee and clinical characteristics of OA, and increased blood and csf levels of the drug which correlated positively with prefrontal-limbic brain activity. These findings indicate dissociation between mechanically induced and spontaneous OA knee pain, the latter engaging brain regions involved in emotional assessment of the self, and challenge the standard clinical view regarding the nature of OA pain.

Temporomandibular disorder modifies cortical response to tactile stimulation

Individuals with temporomandibular disorder (TMD) suffer from persistent facial pain and exhibit abnormal sensitivity to tactile stimulation. To better understand the pathophysiological mechanisms underlying TMD, we investigated cortical correlates of this abnormal sensitivity to touch. Using functional magnetic resonance imaging (fMRI), we recorded cortical responses evoked by low-frequency vibration of the index finger in subjects with TMD and in healthy controls (HC). Distinct subregions of contralateral primary somatosensory cortex (SI), secondary somatosensory cortex (SII), and insular cortex responded maximally for each group. Although the stimulus was inaudible, primary auditory cortex was activated in TMDs. TMDs also showed greater activation bilaterally in anterior cingulate cortex and contralaterally in the amygdala. Differences between TMDs and HCs in responses evoked by innocuous vibrotactile stimulation within SI, SII, and the insula paralleled previously reported differences in responses evoked by noxious and innocuous stimulation, respectively, in healthy individuals. This unexpected result may reflect a disruption of the normal balance between central resources dedicated to processing innocuous and noxious input, manifesting itself as increased readiness of the pain matrix for activation by even innocuous input. Activation of the amygdala in our TMD group could reflect the establishment of aversive associations with tactile stimulation due to the persistence of pain.

PERSPECTIVE

This article presents evidence that central processing of innocuous tactile stimulation is abnormal in TMD. Understanding the complexity of sensory disruption in chronic pain could lead to improved methods for assessing cerebral cortical function in these patients.

Predicting value of pain and analgesia: nucleus accumbens response to noxious stimuli changes in the presence of chronic pain

VIDEO ABSTRACT

We compared brain activations in response to acute noxious thermal stimuli in controls and chronic back pain (CBP) patients. Pain perception and related cortical activation patterns were similar in the two groups. However, nucleus accumbens (NAc) activity differentiated the groups at a very high accuracy, exhibiting phasic and tonic responses with distinct properties. Positive phasic NAc activations at stimulus onset and offset tracked stimulus salience and, in normal subjects, predicted reward (pain relief) magnitude at stimulus offset. In CBP, NAc activity correlated with different cortical circuitry from that of normals and phasic activity at stimulus offset was negative in polarity, suggesting that the acute pain relieves the ongoing back pain. The relieving effect was confirmed in a separate psychophysical study in CBP. Therefore, in contrast to somatosensory pathways, which reflect sensory properties of acute noxious stimuli, NAc activity in humans encodes its predicted value and anticipates its analgesic potential on chronic pain.

Observational fear learning involves affective pain system and Cav1.2 Ca2+ channels in ACC

Fear can be acquired vicariously through social observation of others suffering from aversive stimuli. We found that mice (observers) developed freezing behavior by observing other mice (demonstrators) receive repetitive foot shocks. Observers had higher fear responses when demonstrators were socially related to themselves, such as siblings or mating partners. Inactivation of anterior cingulate cortex (ACC) and parafascicular or mediodorsal thalamic nuclei, which comprise the medial pain system representing pain affection, substantially impaired this observational fear learning, whereas inactivation of sensory thalamic nuclei had no effect. The ACC neuronal activities were increased and synchronized with those of the lateral amygdala at theta rhythm frequency during this learning. Furthermore, an ACC-limited deletion of Ca(v)1.2 Ca(2+) channels in mice impaired observational fear learning and reduced behavioral pain responses. These results demonstrate the functional involvement of the affective pain system and Ca(v)1.2 channels of the ACC in observational social fear.

Observational fear learning involves affective pain system and Cav1.2 Ca2+ channels in ACC

Fear can be acquired vicariously through social observation of others suffering from aversive stimuli. We found that mice (observers) developed freezing behavior by observing other mice (demonstrators) receive repetitive foot shocks. Observers had higher fear responses when demonstrators were socially related to themselves, such as siblings or mating partners. Inactivation of anterior cingulate cortex (ACC) and parafascicular or mediodorsal thalamic nuclei, which comprise the medial pain system representing pain affection, substantially impaired this observational fear learning, whereas inactivation of sensory thalamic nuclei had no effect. The ACC neuronal activities were increased and synchronized with those of the lateral amygdala at theta rhythm frequency during this learning. Furthermore, an ACC-limited deletion of Ca(v)1.2 Ca(2+) channels in mice impaired observational fear learning and reduced behavioral pain responses. These results demonstrate the functional involvement of the affective pain system and Ca(v)1.2 channels of the ACC in observational social fear.

Cognitive impairment of prefrontal-dependent decision-making in rats after the onset of chronic pain

Forced choice between alternative options of unpredictable outcome is a complex task that requires continual update of the value associated with each option. Prefrontal areas such as the orbitofrontal cortex (OFC) have been shown to play a major role in performance on ambiguous decision-making tasks with substantial risk component, broadly named as "gambling tasks." We have recently demonstrated that rats display complex decision-making behavior in a rodent gambling task based on serial choices between rewards of different value and probability. This rodent task retains many of the key characteristics of the human Iowa Gambling Task (IGT), and performance in this novel task is also disrupted by OFC or amygdalar lesioning. In the present study we addressed if rat models of chronic pain would have impaired performance in this gambling task, since it is already known that the IGT response patterns of human pain patients are comparable to individuals with OFC lesions. We found that animals with a monoarthritic inflammatory model of chronic pain systematically preferred the lever associated with larger but infrequent rewards. In addition, we measured the neurochemical content of the OFC, amygdala and nucleus accumbens using HPLC, and found that in prolonged chronic pain animals there was a decrease in the tonic levels of dopamine, DOPAC (3,4-hydroxyphenyl-acetic acid) and 5-HIAA (5-hydroxyindole-3-acetic acid) in the OFC. This is the first report of the effect of chronic pain in rat decision-making processes and supports the notion that pain may have profound effects on the functioning of the reward-aversion circuitry relevant to strategic planning.

Emotional pain without sensory pain--dream on?

The article by Danziger and colleagues in this issue of Neuron evaluates empathy in a unique population--individuals with congenital insensitivity to pain. As such, it provides insights into the brain's ability to evaluate others' feeling to observed pain without having a specific sensory experience of pain itself.