The effect of social stress on chronic pain perception in female and male mice

Physiopathology of pain in systemic erythematosus lupus

Systemic lupus erythematosus is an autoimmune disease that promotes chronic inflammation, with periods of activation and remission. Pain is commonly one of the first symptoms reported by patients with lupus. It interferes with patients' quality of life, leading to a decrease in strength and difficulty in performing daily activities. Given the chronic nature of the disease, the high prevalence of pain and its negative impact on the quality of life of patients with lupus, the present literature review study aims to explain the pathophysiology of pain in systemic lupus erythematosus and the implications of the chronic process and contributing factors. During periods of systemic lupus erythematosus activity, pain is a significant symptom. Despite this, several studies show that severe pain can occur in patients with in mild to moderate disease activity. Also, in the early stages of the disease, the pain may be accentuated by the greater activity of the disease. However, even when advanced disease is under control, there can be comorbidities and accumulated damage that can also cause high levels of pain. This sensitivity is due to the overlap of primary, secondary, and tertiary pain pathologies, which feedback and make this symptom one of the main concerns of patients with lupus. Understanding the pathophysiology of pain in systemic lupus erythematosus, as well as its chronification and contribution factors, is essential to identify effective therapeutic alternatives in these patients at each stage of pain pathology (primary, secondary, and tertiary).

AMP-activated kinase and the endogenous endocannabinoid system might contribute to antinociceptive effects of prolonged moderate caloric restriction in mice

Background

Caloric restriction is associated with broad therapeutic potential in various diseases and an increase in health and life span. In this study, we assessed the impact of caloric restriction on acute and inflammatory nociception in mice, which were either fed ad libitum or subjected to caloric restriction with 80% of the daily average for two weeks.

Results

The behavioral tests revealed that inflammatory nociception in the formalin test and in zymosan-induced mechanical hypersensitivity were significantly decreased when mice underwent caloric restriction. As potential mediators of the diet-induced antinociception, we assessed genes typically induced by inflammatory stimuli, AMP-activated kinase, and the endocannabinoid system which have all already been associated with nociceptive responses. Zymosan-induced inflammatory markers such as COX-2, TNFα, IL-1β, and c-fos in the spinal cord were not altered by caloric restriction. In contrast, AMPKα2 knock-out mice showed significant differences in comparison to C57BL/6 mice and their respective wild type littermates by missing the antinociceptive effects after caloric restriction. Endocannabinoid levels of anandamide and 2-arachidonyl glyceroldetermined in serum by LC-MS/MS were not affected by either caloric restriction alone or in combination with zymosan treatment. However, cannabinoid receptor type 1 expression in the spinal cord, which was not altered by caloric restriction in control mice, was significantly increased after caloric restriction in zymosan-induced paw inflammation. Since increased cannabinoid receptor type 1 signaling might influence AMP-activated kinase activity, we analyzed effects of anandamide on AMP-activated kinase in cell culture and observed a significant activation of AMP-activated kinase. Thus, endocannabionoid-induced AMP-activated kinase activation might be involved in antinociceptive effects after caloric restriction.

Conclusion

Our data suggest that caloric restriction has an impact on inflammatory nociception which might involve AMP-activated kinase activation and an increased activity of the endogenous endocannabinoid system by caloric restriction-induced cannabinoid receptor type 1 upregulation.

Fear of pain and cortisol reactivity predict the strength of stress-induced hypoalgesia

Background

Acute stress can have an effect on pain sensitivity, yet the direction of the effect – whether it is hypoalgesic or hyperalgesic – is mixed across studies. Moreover, which part of the stress response influences pain sensitivity is still unclear. In the current experimental study, we aim to examine the effect of acute stress on heat pain thresholds and pain tolerance levels in healthy participants, while taking into account individual differences in stress responses.

Methods

Forty‐two healthy participants were randomly assigned to either a well‐validated stress paradigm: the Maastricht Acute Stress Task (MAST; combining physical and psychological stressors) or to a nonstressful version of the task. Heat pain thresholds and tolerance levels were assessed at three times: prior to the MAST, immediately after the MAST during the presumed sympatho‐adrenal medullary (SAM) response, and 15 min after MAST to cover the presumed hypothalamus–pituitary–adrenal (HPA) axis response. Stress responses were assessed both subjectively and physiologically.

Results

We observed that the acute stress induction led to increased heat pain thresholds, an effect that was present only in participants showing a cortisol response following stress induction and only in the presumed HPA axis time window. The strength of this hypoalgesic effect was further predicted by the change in cortisol and by fear of pain levels.

Conclusions

Our findings indicate that the HPA axis – and not the autonomic – stress response specifically underlies this stress‐induced hypoalgesic effect, having important implications for clinical states with HPA axis dysfunctions.

Significance

This experimental study shows that an acute stress induction – that combines physical and psychological stressors – increases heat pain thresholds, but not tolerance in healthy participants. Furthermore, the magnitude of this stress‐induced hypoalgesic effect is predicted by cortisol reactivity and fear of pain, revealing specific involvement of the HPA axis stress system and interactions with pain‐related psychosocial aspects.

Chronic Social Stress Time-Dependently Affects Neuropathic Pain-Related Cold Allodynia and Leads to Altered Expression of Spinal Biochemical Mediators

Clinical data have shown that chronic exposure to stress may be accompanied by an enhancement of inflammation-related pain sensitivity. In this context, little is however known on the impact of stress on neuropathic pain. In the present study we addressed this issue by combining the chronic constriction injury (CCI) model with an ongoing social stress (OSS) paradigm. Cold plate and von Frey tests were performed in 48 rats divided into four groups: OSS exposed to OSS, CCI subjected to chronic nerve constriction, OSS+CCI with a combination of neuropathy and stress and CON, a control group lacking any manipulation. While we did not observe any stress-related differences in mechanical sensitivity throughout the observation period, CCI rats were more sensitive to cold stimulation than OSS+CCI in the initial phase of neuropathy. A switch was observed at a later stage, leading to a hypersensitivity of the OSS+CCI compared to the CCI rats. At this time point we investigated the spinal mRNA expression of neuron and glia related molecules potentially involved in neuropathic pain and stress. The combination of psychosocial stress and neuropathic pain seemed to enhance glial cell activation, pro-inflammatory cytokine and neurotrophic factor mRNA levels, rather than glutamatergic transmission. Our data show that long lasting social stress may lead to time-dependent alteration of neuropathy-related cold pain sensitivity while mechanically-induced pain remains unchanged.

Indomethacin counteracts the effects of chronic social defeat stress on emotional but not recognition memory in mice

We have previously observed the impairing effects of chronic social defeat stress (CSDS) on emotional memory in mice. Given the relation between stress and inflammatory processes, we sought to study the effectiveness of the anti-inflammatory indomethacin in reversing the detrimental effects of CSDS on emotional memory in mice. The effects of CSDS and indomethacin on recognition memory were also evaluated. Male CD1 mice were randomly divided into four groups: non-stressed + saline (NS+SAL); non-stressed + indomethacin (NS+IND); stressed + saline (S+SAL); and stressed + indomethacin (S+IND). Stressed animals were exposed to a daily 10 min agonistic confrontation (CSDS) for 20 days. All subjects were treated daily with saline or indomethacin (10 mg/kg, i.p.). 24 h after the CSDS period, all the mice were evaluated in a social interaction test to distinguish between those that were resilient or susceptible to social stress. All subjects (n = 10–12 per group) were then evaluated in inhibitory avoidance (IA), novel object recognition (NOR), elevated plus maze and hot plate tests. As in control animals (NS+SAL group), IA learning was observed in the resilient groups, as well as in the susceptible mice treated with indomethacin (S+IND group). Recognition memory was observed in the non-stressed and the resilient mice, but not in the susceptible animals. Also, stressed mice exhibited higher anxiety levels. No significant differences were observed in locomotor activity or analgesia. In conclusion, CSDS induces anxiety in post-pubertal mice and impairs emotional and recognition memory in the susceptible subjects. The effects of CSDS on emotional memory, but not on recognition memory and anxiety, are reversed by indomethacin. Moreover, memory impairment is not secondary to the effects of CSDS on locomotor activity, emotionality or pain sensitivity.

Hypoalgesia Induced by Reward Devaluation in Rats

Reduced sensitivity to physical pain (hypoalgesia) has been reported after events involving reward devaluation. Reward devaluation was implemented in a consummatory successive negative contrast (cSNC) task. Food-deprived Wistar rats had access to 32% sucrose during 16 sessions followed by access to 4% sucrose during 3 additional sessions. An unshifted control group had access to 4% sucrose throughout the 19 sessions. Pain sensitivity was measured using von Frey filaments (Experiment 1) and Hargreaves thermal stimuli (Experiment 2) in pretraining baseline, 5 min, and 300 min after either the first (session 17) or second (session 18) devaluation session in the cSNC situation. Sucrose consumption was lower in downshifted groups relative to unshifted groups during postshift sessions-the cSNC effect. Hypoalgesia was observed in downshifted groups relative to unshifted controls when pain sensitivity was assessed 5 min after either the first or second devaluation session, regardless of the pain sensitivity test used. Both pain sensitivity tests yielded evidence of hypoalgesia 300 min after the second downshift session, but not 300 min after the first devaluation session. Whereas hypoalgesia was previously shown only after the second devaluation session, here we report evidence of hypoalgesia after both the first and second devaluation sessions using mechanical and thermal nociceptive stimuli. Moreover, the hypoalgesia observed 300 min after the second devaluation session in both experiments provides unique evidence of the effects of reward loss on sensitivity to physical pain 5 hours after the loss episode. The underlying neurobehavioral mechanisms remain to be identified.

Orexin-A and Endocannabinoid Activation of the Descending Antinociceptive Pathway Underlies Altered Pain Perception in Leptin Signaling Deficiency

Pain perception can become altered in individuals with eating disorders and obesity for reasons that have not been fully elucidated. We show that leptin deficiency in ob/ob mice, or leptin insensitivity in the arcuate nucleus of the hypothalamus in mice with high-fat diet (HFD)-induced obesity, are accompanied by elevated orexin-A (OX-A) levels and orexin receptor-1 (OX1-R)-dependent elevation of the levels of the endocannabinoid, 2-arachidonoylglycerol (2-AG), in the ventrolateral periaqueductal gray (vlPAG). In ob/ob mice, these alterations result in the following: (i) increased excitability of OX1-R-expressing vlPAG output neurons and subsequent increased OFF and decreased ON cell activity in the rostral ventromedial medulla, as assessed by patch clamp and in vivo electrophysiology; and (ii) analgesia, in both healthy and neuropathic mice. In HFD mice, instead, analgesia is only unmasked following leptin receptor antagonism. We propose that OX-A/endocannabinoid cross talk in the descending antinociceptive pathway might partly underlie increased pain thresholds in conditions associated with impaired leptin signaling.