Stressors and pain sensitivity in CFW mice. Role of opioid peptides

Effect of Acute Brief Social Isolation on Visceral Pain

Objective

The sensitivity of somatic pain could be affected by social isolation; however, few studies have examined the impact of social isolation on visceral pain. In the present study, the effect of acute brief social isolation on visceral pain response was investigated.

Methods

Adult male rats were either reared individually or grouped for 2 hours or 24 hours. Colorectal distention (CRD)-induced abdominal withdrawal reflex (AWR) score and pain threshold were used to assess visceral pain sensitivity. The amount of fecal bolus was used to determine the stress level.

Results

Acute brief isolation rearing for 2 hours significantly increased AWR score and reduced visceral pain threshold in rats when compared to group rearing. Similarly, acute isolation for 24 hours resulted in visceral hypersensitivity, as indicated by an increase in the AWR score and a decrease in the visceral pain threshold. Furthermore, the amount of fecal bolus in acute isolation rearing (2 or 24 hours) rats was considerably higher than in the control group rearing rats.

Conclusion

Acute short-term social isolation enhances visceral pain sensitivity, which could be related to an increase in stress levels.

Sleep and neighborhood socioeconomic status: a micro longitudinal study of chronic low-back pain and pain-free individuals

Individuals with chronic low back pain (cLBP) frequently report sleep disturbances. Living in a neighborhood characterized by low-socioeconomic status (SES) is associated with a variety of negative health outcomes, including poor sleep. Whether low-neighborhood SES exacerbates sleep disturbances of people with cLBP, relative to pain-free individuals, has not previously been observed. This study compared associations between neighborhood-level SES, pain-status (cLBP vs. pain-free), and daily sleep metrics in 117 adults (cLBP = 82, pain-free = 35). Neighborhood-level SES was gathered from Neighborhood Atlas, which provides a composite measurement of overall neighborhood deprivation (e.g. area deprivation index). Individuals completed home sleep monitoring for 7-consecutive days/nights. Neighborhood SES and pain-status were tested as predictors of actigraphic sleep variables (e.g., sleep efficiency). Analyses revealed neighborhood-level SES and neighborhood-level SES*pain-status interaction significantly impacted objective sleep quality. These findings provide initial support for the negative impact of low neighborhood-level SES and chronic pain on sleep quality.

Neural mechanisms of social homeostasis

Social connections are vital to survival throughout the animal kingdom and are dynamic across the life span. There are debilitating consequences of social isolation and loneliness, and social support is increasingly a primary consideration in health care, disease prevention, and recovery. Considering social connection as an "innate need," it is hypothesized that evolutionarily conserved neural systems underlie the maintenance of social connections: alerting the individual to their absence and coordinating effector mechanisms to restore social contact. This is reminiscent of a homeostatic system designed to maintain social connection. Here, we explore the identity of neural systems regulating "social homeostasis." We review findings from rodent studies evaluating the rapid response to social deficit (in the form of acute social isolation) and propose that parallel, overlapping circuits are engaged to adapt to the vulnerabilities of isolation and restore social connection. By considering the neural systems regulating other homeostatic needs, such as energy and fluid balance, we discuss the potential attributes of social homeostatic circuitry. We reason that uncovering the identity of these circuits/mechanisms will facilitate our understanding of how loneliness perpetuates long-term disease states, which we speculate may result from sustained recruitment of social homeostatic circuits.

Partner Loss in Monogamous Rodents: Modulation of Pain and Emotional Behavior in Male Prairie Voles

OBJECTIVE

Pain is modulated by psychosocial factors, and social stress-induced hyperalgesia is a common clinical symptom in pain disorders. To provide a new animal model for studying social modulation of pain, we examined pain behaviors in monogamous prairie voles experiencing partner loss.

METHODS

After cohabitation with novel females, males (n = 79) were divided into two groups on the basis of preference test scores. Half of the males of each group were separated from their partner (loss group), whereas the other half remained paired (paired group). Thus, males from both groups experienced social isolation. Open field tests, plantar tests, and formalin tests were then conducted on males to assess anxiety and pain-related behaviors.

RESULTS

Loss males showing partner preferences (n = 20) displayed a significant increase in anxiety-related behavior in the open-field test (central area/total distance: 13.65% [1.58%] for paired versus 6.45% [0.87%] for loss; p < .001), a low threshold of thermal stimulus in the plantar test (withdrawal latencies: 9.69 [0.98] seconds for paired versus 6.15 [0.75] seconds for loss; p = .037), and exacerbated pain behaviors in the formalin test (total number of lifts: 40.33 [4.46] for paired versus 54.42 [1.91] for loss; p = .042) as compared with paired males (n = 20). Thermal thresholds in the plantar test significantly correlated with anxiety-related behavior in the open-field test (r = 0.64). No such differences were observed in the males that did not display partner preferences (r = 0.15).

CONCLUSIONS

Results indicate that social bonds and their disruption, but not social housing without bonding followed by isolation, modulate pain and emotion in male prairie voles. The prairie vole is a useful model for exploring the neural mechanisms by which social relationships contribute to pain and nociceptive processing in humans.

Modulation of nociception by social factors in rodents: contribution of the opioid system

RATIONALE

The opioid system is involved in the regulation of several behavioral and physiological responses, controlling pain, reward, and addictive behaviors. Opioid administration, depending on drugs and doses, usually affects sociability reducing interactions between conspecifics, whereas some affiliative behaviors such as sexual activity, social grooming, and play behavior increase the endogenous opioid activity.

OBJECTIVES

The possible interaction between endogenous opioids released during socio/sexual behavior and their analgesic effect on pain response is reviewed in the rodent literature.

RESULTS

Direct evidence for socially mediated opioid changes resulting in increase in nociceptive threshold derives from studies exploring the effects of defeat experiences, social isolation, maternal, sexual behavior, and social reunion among kin or familiar animals in laboratory rodents. Indirect evidence for endogenous activation of the opioid system, possibly affecting pain sensitivity, derives from studies investigating the relevance of natural social reward using the conditioned place preference protocols or analyzing ultrasonic vocalizations associated to positive affective contexts. Finally, genetic and epigenetic factors that affect the opioid system during development are reported to be involved in modulating the response to social stimuli as well as nociception.

CONCLUSIONS

All studies highlight the relevance of affiliative contact behavior between conspecifics that is responsible for the activation of the endogenous mu-opioid system, inducing nociceptive threshold increase.

Alone but feeling no pain: Effects of social exclusion on physical pain tolerance and pain threshold, affective forecasting, and interpersonal empathy

Prior findings of emotional numbness (rather than distress) among socially excluded persons led the authors to investigate whether exclusion causes a far-reaching insensitivity to both physical and emotional pain. Experiments 1-4 showed that receiving an ostensibly diagnostic forecast of a lonesome future life reduced sensitivity to physical pain, as indicated by both (higher) thresholds and tolerance. Exclusion also caused emotional insensitivity, as indicated by reductions in affective forecasting of joy or woe over a future football outcome (Experiment 3), as well as lesser empathizing with another person's suffering from either romantic breakup (Experiment 4) or a broken leg (Experiment 5). The insensitivities to pain and emotion were highly intercorrelated.

Cholecystokinin and endogenous opioid peptides: interactive influence on pain, cognition, and emotion

It is well documented that stressful life experiences contribute to the etiology of human mood disorders. Cholecystokinin (CCK) is a neuropeptide found in high concentrations throughout the central nervous system, where it is involved in numerous physiological functions. A role for CCK in the induction and persistence of anxiety and major depression appears to be conspicuous. While increased CCK has been associated with motivational loss, anxiety and panic attacks, an increase in mesocorticolimbic opioid availability has been associated with coping and mood elevation. The close neuroanatomical distribution of CCK with opioid peptides in the limbic system suggests that there may be an opioid-CCK link in the modulation and expression of anxiety or stressor-related behaviors. In effect, while CCK induces relatively protracted behavioral disturbances in both animal and human subjects following stressor applications, opioid receptor activation may change the course of psychopathology. The antagonistic interaction of CCK and opioid peptides is evident in psychological disturbances as well as stress-induced analgesia. There appears to be an intricate balance between the memory-enhancing and anxiety-provoking effects of CCK on one hand, and the amnesic and anxiolytic effects of opioid peptides on the other hand. Potential anxiogenic and mnemonic influences of site-specific mesocorticolimbic CCK and opioid peptide availability, the relative contributions of specific CCK and opioid receptors, as well as the time course underlying neuronal substrates of long-term behavioral disturbances as a result of stressor manipulations, are discussed.

Why Does Social Exclusion Hurt? The Relationship Between Social and Physical Pain

The authors forward the hypothesis that social exclusion is experienced as painful because reactions to rejection are mediated by aspects of the physical pain system. The authors begin by presenting the theory that overlap between social and physical pain was an evolutionary development to aid social animals in responding to threats to inclusion. The authors then review evidence showing that humans demonstrate convergence between the 2 types of pain in thought, emotion, and behavior, and demonstrate, primarily through nonhuman animal research, that social and physical pain share common physiological mechanisms. Finally, the authors explore the implications of social pain theory for rejection-elicited aggression and physical pain disorders.

Effect of isolation on pain threshold and on different effects of morphine

1. The effect of three periods of isolation (8, 15 and 30 days) were studied in mice on the pain threshold and the sensitivity to morphine. 2. The pain threshold was unchanged after 8 and 15 days of isolation but increased after 30 days of isolation. 3. The analgesic effect of morphine was unchanged after 8 and 15 days of isolation but increased after 30 days of isolation. 4. The tolerance to morphine analgesia was unchanged after 8 and 15 days of isolation but increased after 30 days of isolation (morphine-induced analgesia was reduced). 5. The physical dependence on morphine induced by precipitated withdrawal was unchanged after 8 and 15 days of isolation but decreased after 30 days of isolation. 6. It is suggested that isolation may modify the metabolism the metabolism/absorption of morphine in a different way according as the treatment is unique or chronic.

The effects of laboratory handling procedures on naloxone-precipitated withdrawal behavior in morphine-dependent rats

Previous studies of opiate dependence in our laboratory have shown that opiate-naive (control) animals can display behaviors normally ascribed to withdrawal following naloxone challenge. The possibility arises that in these control rats, the stress of the induction of dependence, pharmacological challenge, and behavioral testing may result in the release of endogenous opioids such that on naloxone administration withdrawal behaviors are elicited. In the present study, we have utilized two differing experimental protocols for the induction of morphine dependence in rats that would represent the extremes of normal laboratory handling procedures and have then assessed withdrawal behaviors following naloxone challenge. In control animals, the results show that the stressors alone are insufficient to allow for the precipitation of naloxone-induced withdrawal. However, withdrawal behavior was generally greater in the opiate-dependent animals in the "high" rather than the "low" stress group, suggesting a summation of opiate effects. Similarly, a residual effect was noted in the control opiate-naive animals; naloxone rather than saline (control) challenged rats displayed a slightly higher incidence of withdrawal behavior. These results stress the importance of maintaining constant laboratory protocols, minimizing stressful procedures, and performing appropriate controls in assessing opiate dependence and withdrawal.

Endogenous opiates: 1990

This paper, an examination of works published during 1990, is thirteenth in a series of our annual reviews of the research involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence, eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, pregnancy, development, and aging; immunological responses; and other behavior.