Foot shock stress generates persistent widespread hypersensitivity and anhedonic behavior in an anxiety-prone strain of mice

Vagus nerve stimulation rescues persistent pain following orthopedic surgery in adult mice

ABSTRACT

Postoperative pain is a major clinical problem imposing a significant burden on patients and society. In a survey 2 years after orthopedic surgery, 57% of patients reported persisting postoperative pain. However, only limited progress has been made in the development of safe and effective therapies to prevent the onset and chronification of pain after orthopedic surgery. We established a tibial fracture mouse model that recapitulates clinically relevant orthopedic trauma surgery, which causes changes in neuropeptide levels in dorsal root ganglia and sustained neuroinflammation in the spinal cord. Here, we monitored extended pain behavior in this model, observing chronic bilateral hindpaw mechanical allodynia in both male and female C57BL/6J mice that persisted for >3 months after surgery. We also tested the analgesic effects of a novel, minimally invasive, bioelectronic approach to percutaneously stimulate the vagus nerve (termed percutaneous vagus nerve stimulation [pVNS]). Weekly pVNS treatment for 30 minutes at 10 Hz for 3 weeks after the surgery strongly reduced pain behaviors compared with untreated controls. Percutaneous vagus nerve stimulation also improved locomotor coordination and accelerated bone healing. In the dorsal root ganglia, vagal stimulation inhibited the activation of glial fibrillary acidic protein-positive satellite cells but without affecting microglial activation. Overall, these data provide novel evidence supportive of the use of pVNS to prevent postoperative pain and inform translational studies to test antinociceptive effects of bioelectronic medicine in the clinic.

Chronic stress induces wide-spread hyperalgesia: The involvement of spinal CCK1 receptors

Chronic primary pain (CPP) occurs in the absence of tissue injury and includes temporomandibular disorders (TMD), fibromyalgia syndrome (FMS) and irritable bowel syndrome (IBS). CPP is commonly considered a stress-related chronic pain and often presents as wide-spread pain or comorbid pain conditions in different regions of the body. However, whether prolonged stress can directly result in the development of CPP comorbidity remains unclear. In the present study, we adapted a 21 day heterotypic stress paradigm in mice and examined whether chronic stress induced wide-spread hyperalgesia, modeling comorbid CPP in the clinic. We found that chronic stress induced anxiety- and depression-like behaviors, and resulted in long-lasting wide-spread hyperalgesia over several body regions such as the orofacial area, hindpaw, thigh, upper back and abdomen in female mice. We further found that the expression of cholecystokinin (CCK)1 receptors was significantly increased in the L4-L5 spinal dorsal horn of the female mice after 14 and 21 day heterotypic stress compared with the control animals. Intrathecal injection of the CCK1 receptor antagonist CR-1505 blocked pain hypersensitivity in the subcervical body including the upper back, thigh, hindpaw and abdomen. These findings suggest that the upregulation of spinal CCK1 receptors after chronic stress contributes to the central mechanisms underlying the development of wide-spread hyperalgesia, and may provide a potential and novel central target for clinical treatment of CPP.

Social interactions and olfactory cues are required for contagious itch in mice

The phenomenon of contagious itch, observed in both humans and rodents, remains a topic of ongoing debate concerning its modulators and underlying pathways. This study delves into the relationship between contagious itch and familiar olfactory cues, a non-visual factor contributing to this intriguing behavior. Our findings showed that contagious itch in observer mice occurs during physical interaction with the cagemate itch-demonstrator but not with a stranger demonstrator or in a non-physical encounter condition. Notably, itch-experienced observer mice displayed an increased contagious itch behavior, highlighting the relevance of itch-associated memory in this phenomenon. Furthermore, anosmic observer mice, whether itch-naïve or itch-experienced, displayed no contagious itch behavior. These results demonstrate that the familiar olfactory cues, specifically cagemate body odors, are required for contagious itch behaviors in mice. In line with these behavioral findings, our study reveals increased activity in brain regions associated with olfaction, emotion, and memory during contagious itch, including the olfactory bulb, the amygdala, the hypothalamus, and the hippocampus, with this activity diminished in anosmic mice. In conclusion, our study unveils the critical role of familiar olfactory cues in driving contagious itch in mice, shedding light on the interplay between social factors, sensory perception, and memory in this phenomenon.

Parabrachial Calca neurons drive nociplasticity

Pain that persists beyond the time required for tissue healing and pain that arises in the absence of tissue injury, collectively referred to as nociplastic pain, are poorly understood phenomena mediated by plasticity within the central nervous system. The parabrachial nucleus (PBN) is a hub that relays aversive sensory information and appears to play a role in nociplasticity. Here, by preventing PBN Calca neurons from releasing neurotransmitters, we demonstrate that activation of Calca neurons is necessary for the manifestation and maintenance of chronic pain. Additionally, by directly stimulating Calca neurons, we demonstrate that Calca neuron activity is sufficient to drive nociplasticity. Aversive stimuli of multiple sensory modalities, such as exposure to nitroglycerin, cisplatin, or lithium chloride, can drive nociplasticity in a Calca-neuron-dependent manner. Aversive events drive nociplasticity in Calca neurons in the form of increased activity and excitability; however, neuroplasticity also appears to occur in downstream circuitry.

Parabrachial Calca neurons drive nociplasticity

Pain that persists beyond the time required for tissue healing and pain that arises in the absence of tissue injury are poorly understood phenomena mediated by plasticity within the central nervous system. The parabrachial nucleus (PBN) is a hub that relays aversive sensory information and appears to play a role in nociplasticity. Here, by preventing PBN Calca neurons from releasing neurotransmitter or directly stimulating them we demonstrate that activation of Calca neurons is both necessary for the manifestation of chronic pain after nerve ligation and is sufficient to drive nociplasticity in wild-type mice. Aversive stimuli such as exposure to nitroglycerin, cisplatin, or LiCl can drive nociplasticity in a Calca-neuron-dependent manner. Calcium fluorescence imaging reveals that nitroglycerin activates PBN Calca neurons and potentiates their responses to mechanical stimulation. The activity and excitability of Calca neurons increased for several days after aversive events, but prolonged nociplasticity likely occurs in downstream circuitry.

Vagus nerve stimulation rescues persistent pain following orthopedic surgery in adult mice

Postoperative pain is a major clinical problem imposing a significant burden on our patients and society. Up to 57% of patients experience persistent postoperative pain 2 years after orthopedic surgery [49]. Although many studies have contributed to the neurobiological foundation of surgery-induced pain sensitization, we still lack safe and effective therapies to prevent the onset of persistent postoperative pain. We have established a clinically relevant orthopedic trauma model in mice that recapitulates common insults associated with surgery and ensuing complications. Using this model, we have started to characterize how induction of pain signaling contributes to neuropeptides changes in dorsal root ganglia (DRG) and sustained neuroinflammation in the spinal cord [62]. Here we have extended the characterization of pain behaviors for >3 months after surgery, describing a persistent deficit in mechanical allodynia in both male and female C57BL/6J mice after surgery. Notably, we have applied a novel minimally invasive bioelectronic approach to percutaneously stimulate the vagus nerve (termed pVNS) [24] and tested its anti-nociceptive effects in this model. Our results show that surgery induced a strong bilateral hind-paw allodynia with a slight decrease in motor coordination. However, treatment with pVNS for 30-minutes at10 Hz weekly for 3 weeks prevented pain behavior compared to naïve controls. pVNS also improved locomotor coordination and bone healing compared to surgery without treatment. In the DRGs, we observed that vagal stimulation fully rescued activation of GFAP positive satellite cells but did not affect microglial activation. Overall, these data provide novel evidence for the use of pVNS to prevent postoperative pain and may inform translational studies to test anti-nociceptive effects in the clinic.

Maternal and postweaning probiotic administration alleviated footshock-induced anxiety in both sexes of adolescent Balb/c mice

Aim: Effects of maternal probiotics administered during pregnancy as well as consumption by offspring in the post-weaning period on anxiety behavior were examined.Methods: In addition to anxiety levels measured by EPM and OFT, the expression level of the hippocampal genes, and serum sex hormones in male and female mice that received foot shock stress were assayed in the pubertal period.Results: The results of this study showed that consumption of probiotics in the foot shock-stressed offspring in both sexes could significantly increase the length of stay in the EPM open arm compared to the control group, however, the offspring of the probiotic-treated dam did not. Consumption of probiotics by the pro-off group caused remarkable high expression of the 5HT_2AC receptor gene. In the pro-off group, consumption of probiotics led to a significant decrease in 5HT₁ receptor expression. Expression of GABRA2 was increased in probiotics-treated groups, thus the pro-off and the pro-dam group had a significant difference from the control group. Feeding offspring with probiotics by significantly increased progesterone concentrations compared to the control group, and maternal consumption of probiotics during pregnancy and lactation had no reducing effect on progesterone concentrations. This is due to electric shock stress. The consumption of probiotics by mice during infancy was shown to compensate for the decrease in progesterone concentration in them. Maternal use of probiotics during pregnancy and lactation did not affect this concentration.Conclusions: It is concluded that probiotics can protect against foot shock stress-induced anxiety, progesterone disturbance, and dysregulation of expression of some anxiety-related genes.

The biopsychosocial impacts of anxiety on overactive bladder in women

AIMS

Links between emotional state and the bladder have long been recognized, as psychological comorbidity is a common feature of overactive bladder (OAB). However, how psychological factors might contribute to the development and severity of OAB remains unclear. Therefore, we sought to examine the effect of anxiety on OAB with a specific focus on bladder hypersensitivity.

METHODS

In a sample of 120 adult women with OAB, we compared those with at least mild anxiety (PROMIS Anxiety score ≥55) to those with lower anxiety. Analyses focused on patient-reported questionnaires assessing urinary symptom severity and quality of life, psychological stress symptoms, general somatic symptoms, and results of quantitative sensory testing (QST), including temporal summation to heat pain (TSP). TSP was used to index elevated C-fiber responsiveness (i.e., central sensitization).

RESULTS

Thirty-six (30%) women had at least mild anxiety. While there were no group differences for urinary symptom severity, more anxious women reported worse OAB-specific quality of life, greater psychological stress burden, higher stress reactivity, and greater somatic symptoms. On QST, there were no differences between anxiety groups for pain threshold (43.6 ± 3.1°C vs. 44.0 ± 3.1°C, p = 0.6) and tolerance (47.3 ± 1.5°C vs. 47.4 ± 1.6°C, p = 0.7). However, those with anxiety had significantly higher TSP than those without anxiety (6.0 ± 4.8 vs. 3.7 ± 3.9, p = 0.006), indicating greater central sensitization.

CONCLUSIONS

Women with OAB and at least mild anxiety symptoms reported greater psychosocial burdens (i.e., psychological stress, stress reactivity, OAB-specific QOL) and somatic symptom severity and demonstrated greater central sensitization on QST than those without anxiety. These findings support the hypothesis that anxiety and psychological stress impact hypersensitivity mechanisms that may underlie and contribute to OAB, although further research is needed to better understand how and to what extent.

The Unmet Needs for Studying Chronic Pelvic/Visceral Pain Using Animal Models

The different definitions of chronic pelvic/visceral pain used by international societies have changed over the years. These differences have a great impact on the way researchers study chronic pelvic/visceral pain. Recently, the role of systemic changes, including the role of the central nervous system, in the perpetuation and chronification of pelvic/visceral pain has gained weight. Consequently, researchers are using animal models that resemble those systemic changes rather than using models that are organ- or tissue-specific. In this review, we discuss the advantages and disadvantages of using bladder-centric and systemic models, enumerating some of the central nervous system changes and pain-related behaviors occurring in each model. We also present some drawbacks when using animal models and pain-related behavior tests and raise questions about possible, yet to be demonstrated, investigator-related bias. We also suggest new approaches to study chronic pelvic/visceral pain by refining existing animal models or using new ones.

Neonatal cystitis alters mechanisms of stress-induced visceral hypersensitivity in rats

In rodent models, conditioning with acute footshock (AFS) has been demonstrated to produce bladder hypersensitivity which is more robust when rats, tested as adults, had also been pretreated with neonatal bladder inflammation (NBI). The spinal neurochemical mechanisms of pro-nociceptive processes in rats pretreated with NBI are not fully known and so the present study administered intrathecal (IT) opioid (naloxone) and NMDA receptor (MK-801) antagonists to determine whether these receptors' actions had been altered by NBI. Female Sprague-Dawley rat pups were intravesically pretreated on postnatal days P14-P16 with a 1% zymosan solution or with control procedures and then raised to adulthood (12-15 weeks of age). Bladder hypersensitivity was induced through use of an AFS paradigm. Visceromotor responses (VMRs; abdominal muscle contractions) to graded, air pressure-controlled urinary bladder distension were used as nociceptive endpoints. Immediately following AFS pretreatments, rats were anesthetized and surgically prepared. Pharmacological antagonists were administered via an IT catheter onto the lumbosacral spinal cord and VMRs determined 15 min later. Administration of IT naloxone hydrochloride (10 μg) to rats which had been pretreated only with AFS resulted in VMRs that were more robust than VMRs in similarly pretreated rats that received IT normal saline. In contrast, IT naloxone had no significant effect on rats that had been pretreated with both NBI&AFS, although MK-801 was inhibitory. These effects of IT naloxone suggest the presence of inhibitory influences in normal rats that are absent in rats pretreated with NBI. Absence of inhibitory influences produced by AFS was also demonstrated in rats pretreated with NBI&AFS using measures of thermal paw withdrawal latency (PWL): rats pretreated with only AFS had longer PWLs than rats pretreated with both NBI&AFS. Together, a reduction in anti-nociceptive mechanisms coupled with pro-nociceptive NMDA-linked mechanisms results in more robust nociceptive responses to distension in rats which had experienced NBI.

Stress reveals a specific behavioral phenotype for opioid abuse susceptibility

Susceptibility to stress has long been considered important for the development of substance use disorders. Nonetheless, behavioral and physiological responses to stress are highly variable, making it difficult to identify the individuals who are most likely to abuse drugs. In the present study, we employed a comprehensive battery of tests for negative valence behaviors and nociception to identify individuals predisposed to opioid seeking following oral opioid self-administration. Furthermore, we examined how this profile was affected by a history of stress. We observed that mice receiving foot shock stress failed to exhibit a preference for sucrose, showed increased immobility in the forced swim task, and exhibited mechanical hypersensitivity when compared to controls. When considering these behaviors in light of future fentanyl-seeking responses, we observed that heightened mechanical sensitivity corresponded to higher opioid preference in mice with a history of stress, but not controls. Moreover, we were surprised to discover that paradoxically high sucrose preferences predicted fentanyl preference in shock mice, while signs of anhedonia predicted fentanyl preference in controls. Taken together, these results indicate that stress can act as a physiological modulator, shifting profiles of opioid abuse susceptibility depending on an individual's history.

Spinal neurochemical mechanisms of acute stress-induced visceral hypersensitivity in healthy rats

Psychological stress has been demonstrated to increase reports of pain in humans with pelvic pain of urologic origin. In rodent models, conditioning with acute footshock (AFS) has been demonstrated to increase measures of stress/anxiety as well as bladder hypersensitivity. The spinal neurochemical mechanisms of this pro-nociceptive process are unknown and so the present study administered antagonists for multiple receptors that have been associated with facilitatory mechanisms into the spinal intrathecal space. Bladder hypersensitivity was induced through use of an AFS paradigm in which female Sprague-Dawley rats received a 15-min intermittent shock treatment. Visceromotor responses (VMRs; abdominal muscle contractions) to air pressure-controlled urinary bladder distension (UBD) were used as nociceptive endpoints. Immediately following AFS treatments, rats were anesthetized (inhaled isoflurane, IP urethane) and surgically prepared. Pharmacological antagonists were administered via an intrathecal (IT) catheter onto the lumbosacral spinal cord and VMRs to graded UBD determined 15 min later. Administration of IT naloxone hydrochloride (10 μg) and IT phentolamine hydrochloride (10 μg) resulted in VMRs that were more robust than VMRs in rats that received AFS and IT normal saline whereas there was no significant effect of these drugs on VMRs in rats which underwent non-footshock procedures. In contrast, a low dose of the NMDA-receptor antagonist, MK-801 (30 μg), significantly reduced VMRs in rats made hypersensitive to UBD by AFS, but had no significant effect on rats that underwent non-footshock procedures. This study suggests that pro-nociceptive effects of AFS in otherwise healthy rats involve a spinal NMDA-linked mechanism. The effects of IT naloxone and IT phentolamine suggest the presence of inhibitory influences that are opioidergic and/or alpha-adrenergic and that are masked by the pro-nociceptive mechanisms. Other agents with no statistically significant effect on VMRs include methysergide (30 μg), ondansetron (10 μg), mecamylamine (50 μg), antalarmin (24 μg), aSVG30 (12 μg), and SSR149415 (50 μg).

Neuroprotective potential of ocimum sanctum (Linn) leaf extract in preventing and attenuating stress induced substantia nigral neuronal damage in rats

BACKGROUND

In Ayurveda; an Indian system of traditional medicine, Ocimum sanctum is said to have remedial effect on hriddaurbalya (problems affecting the mind), aakshepayukta vikara (nervous disorders) and shiroroga (diseases of head). Hence, in Ayurvedic practice, it is profoundly used as an antistress medicine. Stress is known to affect neurons of functionally significant brain regions like substantia nigra. However, experimental evidence showing its effect on morphology of substantia nigral neurons is lacking. In addition, whether the O. sanctum treatment attenuates stress induced substantia nigral neuronal structural changes is not known.

OBJECTIVES

To know the effect of stress on morphology of substantia nigral neurons and the effect of O. sanctum fresh leaf extract (OSE) on substantia nigral neurons of stressed rats.

MATERIAL AND METHODS

Present study included three experiments. Experiment I: To study the effect of 3 and 6 weeks of foot shock stress in rats; Experiment II- To study the effect of 3 weeks of OSE treatment on 3 week-stress undergoing rats and on 3 week-stressed rats; Experiment III- To study the effect of 6 weeks of OSE treatment in 6 week-stress undergoing rats and in 6 week-stressed rats.

RESULTS

In experiment I, stress had significant deleterious effect on dendritic arborization of substantia nigral neurons. Experiments II and III showed prevention and attenuation of the stress induced dendritic atrophy of substantia nigral neurons in both 2 ml and 4 ml OSE treatment groups. Protective effect of OSE was more pronounced in rats which are treated for a longer duration.

CONCLUSIONS

Foot shock stress induces neuronal damage in the substantia nigra of rats. Treatment with fresh leaf extract of O. sanctum could prevent and attenuate the foot shock stress induced behavioral deficit and substantia nigral neuronal damage.

The impact of foot shock-induced stress on pain-related behavior associated with burn injury

Acute pain is prevalent following burn injury and can often transition to chronic pain. Prolonged acute pain is an important risk factor for chronic pain and there is little preclinical research to address this problem. Using a mouse model of second-degree burn, we investigated whether pre-existing stress influences pain(sensitivity) after a burn injury. We introduced a contribution of stress in two different ways: (1) the use of foot-shock as a pre-injury stressor or (2) the use of A/J mice to represent higher pre-existing stress compared to C57Bl/6 mice. C57Bl/6 and A/J mice were exposed to repeated mild foot shock to induce stress for 10 continuous days and mice underwent either burn injury or sham burn injury of the plantar surface of the right hind paw. Assessments of mechanical and thermal sensitivities of the injured and uninjured paw were conducted during the shock protocol and at intervals up to 82-day post-burn injury. In both strains of mice that underwent burn injury, thermal hypersensitivity and mechanical allodynia appeared rapidly in the ipsilateral paw. Mice that were stressed took much longer to recover their hind paw mechanical thresholds to baseline compared to non-stressed mice in both burn and non-burn groups. Analysis of the two mouse strains revealed that the recovery of mechanical thresholds in A/J mice which display higher levels of baseline anxiety was shorter than C57Bl/6 mice. No differences were observed regarding thermal sensitivities between strains. Our results support the view that stress exposure prior to burn injury affects mechanical and thermal thresholds and may be relevant to as a risk factor for the transition from acute to chronic pain. Finally, genetic differences may play a key role in modality-specific recovery following burn injury.