Shock-induced hyperalgesia: evidence forebrain systems play an essential role

Neurobiology of stress-induced hyperalgesia

The intensity and severity of perceived pain does not correlate consistently with the degree of peripheral or central nervous system tissue damage or with the intensity of primary afferent or spinal nociceptive neurone activity. In this respect, the modulation of pain by emotion and context is now widely recognized. In particular, stress, fear and anxiety exert potent, but complex, modulatory influences on pain. Stress can either suppress pain (stress-induced analgesia) or exacerbate it (stress-induced hyperalgesia; SIH) depending on the nature, duration and intensity of the stressor. Herein, we review the methods and models used to study the phenomenon of SIH in rodents and humans and then present a detailed discussion of our current understanding of neural substrates and neurobiological mechanisms. The review provides perspectives and challenges for the current and future treatment of pain and the co-morbidity of pain with stress-related psychiatric disorders including anxiety and depression.

Modulation of nociceptive and acoustic startle responses to an unpredictable threat in men and women

The present study examined whether a moderately aversive abdominal threat would lead to greater enhancement in affect- and pain-related defensive responding as indexed by the acoustic startle reflex (ASR) and nociceptive flexion reflex (NFR) in women compared to men. We also predicted sex differences in threat-related autonomic arousal measured by skin conductance responses (SCRs) to acoustic startle and noxious sural nerve stimulation. Unpredictable threat was manipulated by alternating 30-second safe ("no abdominal stimulation will be given") and threat ("abdominal stimulation may occur at anytime") periods. The experiment consisted of 2 blocks, each containing 4 safe and 4 threat periods in which the ASR or NFR was randomly probed 9-21 seconds following period onset. Unpredictable abdominal threat potentiated both ASR and NFR responses compared to periods signaling safety. SCRs to acoustic startle probes and noxious sural nerve stimulation were also significantly elevated during the threat vs safe periods. No sex differences in ASR or startle-evoked SCRs emerged. However, nociceptive responding was moderated by sex; females showed significant increases in NFR magnitudes across both safe and threat periods compared to males. Females also showed greater threat-potentiated SCRs to sural nerve stimulation than males. Our findings indicate that both affect- and pain-related defense and arousal systems are strongly influenced by threat of an aversive, unpredictable event, a situation associated with anticipatory anxiety. Females, compared to males, showed greater nociceptive responding and pain modulation when exposed to an unpredictable threatening context, whereas affect-driven ASR responses showed no such sex differentiation.

Anticipation of pain enhances the nociceptive transmission and functional connectivity within pain network in rats

Background

Expectation is a very potent pain modulator in both humans and animals. There is evidence that pain transmission neurons are modulated by expectation preceding painful stimuli. Nonetheless, few studies have examined the influence of pain expectation on the pain-related neuronal activity and the functional connectivity within the central nociceptive network.

Results

This study used a tone-laser conditioning paradigm to establish the pain expectation in rats, and simultaneously recorded the anterior cingulate cortex (ACC), the medial dorsal thalamus (MD), and the primary somatosensory cortex (SI) to investigate the effect of pain expectation on laser-induced neuronal responses. Cross-correlation and partial directed coherence analysis were used to determine the functional interactions within and between the recorded areas during nociceptive transmission. The results showed that under anticipation condition, the neuronal activity to the auditory cue was significantly increased in the ACC area, whereas those to actual noxious stimuli were enhanced in all the recorded areas. Furthermore, neuronal correlations within and between these areas were significantly increased under conditions of expectation compared to those under non-expectation conditions, indicating an enhanced synchronization of neural activity within the pain network. In addition, information flow from the medial (ACC and MD) to the lateral (SI cortex) pain pathway increased, suggesting that the emotion-related neural circuits may modulate the neuronal activity in the somatosensory pathway during nociceptive transmission.

Conclusion

These results demonstrate that the nociceptive processing in both medial and lateral pain systems is modulated by the expectation of pain.

Gender differences in pain: do emotions play a role?

BACKGROUND

Research suggests that the influence of gender on the processing and experience of pain is a result of several mechanisms. One mediating variable is emotion, which may modulate pain through an interaction of valence (pleasant-unpleasant) and arousal (calm-excited).

OBJECTIVE

This review examines whether gender differences in the experience and processing of emotion contribute to differences in the modulation and perception of pain.

METHODS

An English-language search of MEDLINE and PsycINFO was conducted from 1887 to May 2005. Additional literature was obtained from reference lists of articles retained in the initial search.

RESULTS

Emotion appears to influence pain through a valence-by-arousal interaction. Specifically, negatively valenced emotions with low to moderate arousal (eg, anxiety) enhance pain, whereas negatively valenced emotions with high arousal (eg, fear) reduce pain. In contrast, positively valenced emotions always reduce pain, as long as minimal arousal is achieved. Some evidence suggests that women are more sensitive than men to threat-related stimuli and thus experience more negative affect than men. This would generally lead to enhanced pain perception in women. It is also possible that women are more likely than men to experience negative affect with high arousal (intense fear) and thus pain inhibition. However, the relatively lower base rate of intense negative emotions is not likely to contribute much to gender differences in pain. Evidence also suggests that men may be more sensitive to positive events, particularly sexual/erotic stimuli, which may lead to more positive emotion-induced pain reduction in men, relative to women.

CONCLUSIONS

This review suggests that gender differences in the experience of pain may arise from differences in the experience and processing of emotion that, in turn, differentially alter pain processing. Specifically, the system associated with negative affect may be more attuned to threatening stimuli in women, and the system associated with positive affect may be more attuned to pleasurable stimuli in men. However, there is a paucity of research directly addressing this issue; much of the research on this topic has failed to test a comprehensive model of emotion, failed to use adequate manipulation checks, or failed to use within-subject experimental designs that control for intra- and interindividual differences. Therefore, it is concluded that additional research is warranted.

Pain in a Balance: Noxious Events Engage Opposing Processes That Concurrently Modulate Nociceptive Reactivity

Studies have shown that noxious cutaneous stimulation engages physiologically different antinociceptive systems to inhibit a spinal reflex, tail withdrawal from radiant heat. Two experiments are reported that examine the relationship between the inhibition of the tail-flick response and brain-mediated responses to nociception. The induction of a spinally mediated antinociception was accompanied by an increase in latency to vocalize to a noxious thermal stimulus, suggesting pain inhibition. Physiological manipulations that eliminated the inhibition of the tail-flick reflex restored vocalization to thermal stimulation and revealed a concurrent sensitization that generally heightened behavioral reactivity. The results suggest that net pain is regulated by 2 opposing processes, a selective inhibition of nociceptive signals within the spinal cord and a general sensitization that heightens stimulus processing.

Individual differences in the emotional reaction to shock determine whether hypoalgesia is observed

OBJECTIVES

Our laboratory has shown that electrical shocks induce fear in human participants and subsequently inhibit pain on the hand receiving shocks. The present study examined whether pain modulation is bilateral, by testing pain on the hand contralateral to the one receiving shocks. We also evaluated whether individual differences in emotional response to the shocks influenced pain modulation.

DESIGN

Following baseline tests, 61 participants were randomly assigned to one of two groups. In the shock group, participants received three surprising electrical shocks. In the control group, stimulating electrodes were removed from participants' fingers, and no shocks were presented. Both groups received two more pain threshold tests 2 and 8 minutes later.

OUTCOME MEASURES

Similarly to the tail-flick test used in rodents, pain threshold was tested by measuring the latency of finger withdrawal to radiant heat. The following manipulation checks assessed the emotional state induced by shocks and the control procedure: Self-reported affect, skin conductance level (SCL), heart rate (HR), and blood pressure (BP).

RESULTS

Surprisingly, self-reported affect data indicate that some participants reacted to the shocks with humor and fear, while others reacted primarily with fear. Therefore, these groups were analyzed separately. Participants reacting with fear only exhibited hypoalgesia; however, participants reacting with mixed fear and humor showed no change in pain. This divergent effect was not mediated by arousal, because SCL, HR, and BP were similar in both shocked groups (fear only, fear + humor).

CONCLUSIONS

These data suggest that fear-induced hypoalgesia occurs bilaterally. However, humor experienced concurrent with fear appears to inhibit hypoalgesia. These findings may help explain individual differences in pain following traumatic events.

Negative affect: effects on an evaluative measure of human pain

Prior work indicates that exposure to fear-inducing shock inhibits finger-withdrawal to radiant heat in humans (hypoalgesia), whereas anxiety induced by threat of shock enhances reactivity (hyperalgesia; Pain 84 (2000) 65-75). Although finger-withdrawal latencies are thought to reflect changes in pain sensitivity, additional measures of pain are needed to determine whether pain perception is altered. The present study examined the impact of negative affect on visual analog scale (VAS) ratings of fixed duration thermal stimuli. One hundred twenty-seven male and female human subjects were randomly assigned to one of three emotion-induction conditions: (1) negative affect induced by exposure to three brief shocks; (2) negative affect elicited by the threat of shock without presentation; and (3) neutral affect, with no intervention. VAS ratings were tested before and after emotion-induction. Results suggest that both negative affect manipulations reduced pain. Manipulation checks indicated that the emotion-induction treatments induced similar levels of fear but with different arousal levels. Potential mechanisms for affect induced changes in pain are discussed.