Sex differences in chronic pain-induced mental disorders: Mechanisms of cerebral circuitry

Role of Serum Brain-Derived Neurotrophic Factor as a Biomarker of Chronic Pain in Older Adults

BACKGROUND

Serum brain-derived neurotrophic factor (BDNF) has emerged as a promising biomarker for chronic pain (CP) research and treatment. Yet, most human studies have been limited by small sample sizes, inadequate control of confounders and a lack of focus on sex and mental health differences.

METHODS

This study included data from 1932 community-dwelling individuals aged ≥ 65 years, randomly sampled from the Spanish general population. Serum BDNF was quantified by ELISA. CP characteristics were assessed using the European Chronic Pain Survey and classified according to electronic medical records (ICPC-2 codes). Linear regression models-adjusted for sociodemographic, lifestyle and clinical factors-and stratified analyses by sex and depression status (defined by Geriatric Depression Scale score, recent physician diagnosis or antidepressant use) were performed.

RESULTS

Among 962 men and 970 women, mean BDNF concentrations were 18.55 (5.66) ng/mL and 19.39 (5.77) ng/mL, respectively. Most participants reported pain in multiple locations (median 3 sites, interquartile range: 2-4). In 511 participants with CP, probable musculoskeletal pain was predominant (n = 446), followed by nociplastic (n = 71), neuropathic (n = 54), visceral (n = 51) and vascular pain (n = 22). Notably, in non-depressed participants (n = 1639), women with severe or interfering pain showed lower BDNF concentrations [β coefficient (95% confidence interval) = -2.62 ng/mL (-5.03, -0.22) and -3.09 ng/mL (-4.71, -1.47), respectively] compared to those without CP-a pattern not seen in men. Conversely, among men with depression (n = 293), both severe [-5.12 g/mL (-9.26, -0.99)] and interfering [-4.95 g/mL (-8.29, -1.61)] pain were linked to lower BDNF, a trend absent in depressed women. Similar associations were observed in analyses of musculoskeletal and nociplastic pain subtypes.

CONCLUSIONS

While serum BDNF is a promising biomarker for CP, its reliability for gauging pain severity depends on patient sex and depression status. These factors must be considered to enhance the accuracy and clinical relevance of BDNF in CP evaluation.

SIGNIFICANCE

Our study is the first to reveal that the relationship between serum BDNF and chronic pain is distinctly modulated by sex and depression. This novel insight challenges one-size-fits-all biomarker approaches and paves the way for more personalised, precision-based strategies in chronic pain diagnosis and management.

Sex differences in the affective-cognitive dimension of neuropathic pain: Insights from the spared nerve injury rat model

Over 40% of neuropathic pain patients experience mood and cognitive disturbances, often showing reduced response to analgesics, with most affected individuals being female. This highlights the critical role of biological sex in pain-related affective and cognitive disorders, making it essential to understand the emotional and cognitive circuits linked to pain for improving treatment strategies. However, research on sex differences in preclinical pain models is lacking. This study aimed to investigate these differences using the spared nerve injury (SNI) rat model, conducting a comprehensive series of behavioural tests over 100 days post-injury to identify key time points for observing sex-specific behaviours indicative of pain-related conditions. The findings revealed that female rats exhibited greater mechanical and cold hypersensitivity compared to males following nerve injury and showed earlier onset of depression-related behaviours, while males were more prone to anxiety, social, and memory-related alterations. Interestingly, by the 14th week post-injury, females displayed no signs of these emotional and cognitive impairments. Additionally, fluctuations in the oestrous cycle or changes in testosterone and oestradiol levels did not correlate with sex differences in pain sensitivity or negative affect. Recognizing the influence of biological sex on pain-induced affective and cognitive alterations, especially in later stages post-injury, is crucial for enhancing our understanding of this complex pain disorder. PERSPECTIVE: This manuscript reports the relevance of long-term investigations of sex differences in chronic pain. It shows differential development of somatosensory sensitivity, negative affective states and cognitive impairments in males and females. It emphasizes the importance of including subjects of both sexes in the investigation of pain-related mechanisms and therapeutic management.

Activation of 5-HT6 Receptors in the Ventrolateral Orbital Cortex Produces Anti-Anxiodepressive Effects in a Rat Model of Neuropathic Pain

The comorbidity of anxiety and depression frequently occurs in patients with neuropathic pain. The ventrolateral orbital cortex (VLO) plays a critical role in mediating neuropathic pain and anxiodepression in rodents. Previous studies suggested that 5-HT6 receptors in the VLO are involved in neuropathic pain. Strong evidence supports a close link between 5-HT6 receptors and affective disorders such as depression and anxiety disorders. However, it remains unclear whether the 5-HT6 receptors in the VLO are involved in neuropathic pain-induced anxiodepression. Using a rat neuropathic pain model of spared nerve injury (SNI), we demonstrated that rats exhibited significant anxiodepression-like behaviors and the expression of VLO 5-HT6 receptors obviously decreased four weeks after SNI surgery. Microinjection of the 5-HT6 receptor agonist EMD-386088 into the VLO or overexpression of VLO 5-HT6 receptors alleviated anxiodepression-like behaviors. These effects were blocked by pre-microinjection of a selective 5-HT6 receptor antagonist (SB-258585) or inhibitors of AC (SQ-22536), PKA (H89), and MEK1/2 (U0126) respectively. Meanwhile, the expression of p-ERK, p-CREB, and BDNF in the VLO decreased four weeks after SNI surgery. Furthermore, administration of EMD-386088 upregulated the expression of BDNF, p-ERK, and p-CREB in the VLO of SNI rats, which were reversed by pre-injection of SB-258585. These findings suggest that activating 5-HT6 receptors in the VLO has anti-anxiodepressive effects in rats with neuropathic pain via activating AC-cAMP-PKA-MERK-CREB-BDNF signaling pathway. Accordingly, 5-HT6 receptor in the VLO could be a potential target for the treatment of the comorbidity of neuropathic pain and anxiodepression.

Factors associated with depression in patients undergoing rehabilitation for chronic pain: a cross-sectional analytical study at a referral hospital in Peru

Despite the well-known impact of depression on patients with chronic pain and its association, few studies have evaluated its related factors in Physical Medicine and Rehabilitation settings. The objective of the present study was to assess the factors associated with depression in adult patients undergoing rehabilitation for chronic pain at Hospital Rebagliati (HNERM) in Peru. A cross-sectional analytical study was conducted between June and August 2023, involving 212 adult patients with chronic pain undergoing rehabilitation at HNERM. Data were collected through a survey, including sociodemographic information, pain characteristics, and depression assessment using the Patient Health Questionnaire-9 (PHQ-9). Statistical analysis included descriptive statistics and generalized linear models to identify factors associated with depression. Among 212 participants, 17.9% had a depression diagnosis based on the PHQ-9 (cutoff score: 10 points). Factors associated with a higher frequency of depression included a time since pain diagnosis of 3-6 months [adjusted prevalence ratios (aPR): 1.15, 95% confidence interval (CI): 1.04-1.27], severe pain (aPR: 1.17, 95% CI: 1.04-1.32), comorbidities (for 1: aPR: 1.21, 95% CI: 1.08-1.35; for 2: aPR: 1.17, 95% CI: 1.06-1.29; for ≥3: aPR: 1.27, 95% CI: 1.10-1.47), use of ≥2 medications for pain management (aPR: 1.27, 95% CI: 1.13-1.42), and receipt of psychological therapy (aPR: 1.26, 95% CI: 1.09-1.46). Nonpharmacological interventions did not show an association with an increased prevalence of depression. These findings underscore the significance of adopting a comprehensive approach to chronic pain management, including the screening, assessment, and treatment of associated depression.

Sex Differences in Visceral Pain and Comorbidities: Clinical Outcomes, Preclinical Models, and Cellular and Molecular Mechanisms

Sexual dimorphism of visceral pain has been documented in clinics and experimental animal models. Aside from hormones, emerging evidence suggests the sex-differential intrinsic neural regulation of pain generation and maintenance. According to the International Association for the Study of Pain (IASP) and the American College of Gastroenterology (ACG), up to 25% of the population have visceral pain at any one time, and in the United States 10-15 percent of adults suffer from irritable bowel syndrome (IBS). Here we examine the preclinical and clinical evidence of sex differences in visceral pain focusing on IBS, other forms of bowel dysfunction and IBS-associated comorbidities. We summarize preclinical animal models that provide a means to investigate the underlying molecular mechanisms in the sexual dimorphism of visceral pain. Neurons and nonneuronal cells (glia and immune cells) in the peripheral and central nervous systems, and the communication of gut microbiota and neural systems all contribute to sex-dependent nociception and nociplasticity in visceral painful signal processing. Emotion is another factor in pain perception and appears to have sexual dimorphism.

Insula→Amygdala and Insula→Thalamus Pathways Are Involved in Comorbid Chronic Pain and Depression-Like Behavior in Mice

The comorbidity of chronic pain and depression poses tremendous challenges for the treatment of either one because they exacerbate each other with unknown mechanisms. As the posterior insular cortex (PIC) integrates multiple somatosensory and emotional information and is implicated in either chronic pain or depression, we hypothesize that the PIC and its projections may contribute to the pathophysiology of comorbid chronic pain and depression. We show that PIC neurons were readily activated by mechanical, thermal, aversive, and stressful and appetitive stimulation in naive and neuropathic pain male mice subjected to spared nerve injury (SNI). Optogenetic activation of PIC neurons induced hyperalgesia and conditioned place aversion in naive mice, whereas inhibition of these neurons led to analgesia, conditioned place preference (CPP), and antidepressant effect in both naive and SNI mice. Combining neuronal tracing, optogenetics, and electrophysiological techniques, we found that the monosynaptic glutamatergic projections from the PIC to the basolateral amygdala (BLA) and the ventromedial nucleus (VM) of the thalamus mimicked PIC neurons in pain modulation in naive mice; in SNI mice, both projections were enhanced accompanied by hyperactivity of PIC, BLA, and VM neurons and inhibition of these projections led to analgesia, CPP, and antidepressant-like effect. The present study suggests that potentiation of the PIC→BLA and PIC→VM projections may be important pathophysiological bases for hyperalgesia and depression-like behavior in neuropathic pain and reversing the potentiation may be a promising therapeutic strategy for comorbid chronic pain and depression.

Role of estrogen in treatment of female depression

Depression is a neurological disorder that profoundly affects human physical and mental health, resulting in various changes in the central nervous system. Despite several prominent hypotheses, such as the monoaminergic theory, hypothalamic-pituitary-adrenal (HPA) axis theory, neuroinflammation, and neuroplasticity, the current understanding of depression's pathogenesis remains incomplete. Importantly, depression is a gender-dimorphic disorder, with women exhibiting higher incidence rates than men. Given estrogen's pivotal role in the menstrual cycle, it is reasonable to postulate that its fluctuating levels could contribute to the pathogenesis of depression. Estrogen acts by binding to a diversity of receptors, which are widely distributed in the central nervous system. An abundance of research has established that estrogen and its receptors play a crucial role in depression, spanning pathogenesis and treatment. In this comprehensive review, we provide an in-depth analysis of the fundamental role of estrogen and its receptors in depression, with a focus on neuroinflammation, neuroendocrinology, and neuroplasticity. Furthermore, we discuss potential mechanisms underlying the therapeutic effects of estrogen in the treatment of depression, which may pave the way for new antidepressant drug development and alternative treatment options.

The anterior cingulate cortex controls the hyperactivity in subthalamic neurons in male mice with comorbid chronic pain and depression

Neurons in the subthalamic nucleus (STN) become hyperactive following nerve injury and promote pain-related responses in mice. Considering that the anterior cingulate cortex (ACC) is involved in pain and emotion processing and projects to the STN, we hypothesize that ACC neurons may contribute to hyperactivity in STN neurons in chronic pain. In the present study, we showed that ACC neurons enhanced activity in response to noxious stimuli and to alterations in emotional states and became hyperactive in chronic pain state established by spared nerve injury of the sciatic nerve (SNI) in mice. In naïve mice, STN neurons were activated by noxious stimuli, but not by alterations in emotional states. Pain responses in STN neurons were attenuated in both naïve and SNI mice when ACC neurons were inhibited. Furthermore, optogenetic activation of the ACC-STN pathway induced bilateral hyperalgesia and depression-like behaviors in naive mice; conversely, inhibition of this pathway is sufficient to attenuate hyperalgesia and depression-like behaviors in SNI mice and naïve mice subjected to stimulation of STN neurons. Finally, mitigation of pain-like and depression-like behaviors in SNI mice by inhibition of the ACC-STN projection was eliminated by activation of STN neurons. Our results demonstrate that hyperactivity in the ACC-STN pathway may be an important pathophysiology in comorbid chronic pain and depression. Thus, the ACC-STN pathway may be an intervention target for the treatment of the comorbid chronic pain and depression.

Intranasal oxytocin alleviates comorbid depressive symptoms in neuropathic pain via elevating hippocampal BDNF production in both female and male mice

The comorbidity of pain and depression is frequently observed in patients suffering from chronic pain and depression. However, the comorbid mechanism is not well elucidated and the therapeutic medication is still inadequate. Oxytocin is a neuropeptide synthesized in the hypothalamus. It has been reported to relieve chronic pain and depressive symptoms. However, the analgesic action and mechanisms of oxytocin have mainly been investigated using peripheral or spinal administration. Because of the advantage of intranasal delivery of oxytocin in crossing the blood-brain barrier, we investigated the effect of intranasal application of oxytocin on neuropathic pain and comorbid depressive symptoms in both female and male mice. In female and male mice receiving spared nerve injury (SNI) surgery, intranasal oxytocin (2.4 μg, daily for 28 days) attenuated depression-like behavior, but did not alleviate mechanical hyperalgesia. Intranasal oxytocin not only inhibited the activation of microglia and astrocytes, but also increased the downregulated oxytocin receptor (OTR) expression, reversed the elevated GluN2A, and restored the decreased BDNF expression in the hippocampus. SNI also decreased OTR expression in the spinal cord and increased spinal GluN2A and BDNF. However, intranasal oxytocin treatment did not change the expression levels of OTR, GluN2A, or BDNF in the spinal cord of neuropathic mice. The results suggest that the oxytocin signaling in the hippocampus is involved in the comorbidity of pain and depression, and intranasal oxytocin may have the potential to treat depressive symptoms in neuropathic pain patients.

Differences in the neural basis and transcriptomic patterns in acute and persistent pain-related anxiety-like behaviors

Background

Both acute and persistent pain is associated with anxiety in clinical observations, but whether the underlying neural mechanisms differ is poorly understood.

Methods

We used formalin or complete Freund's adjuvant (CFA) to induce acute or persistent pain. Behavioral performance was assessed by the paw withdrawal threshold (PWT), open field (OF), and elevated plus maze (EPM) tests. C-Fos staining was used to identify the activated brain regions. Chemogenetic inhibition was further performed to examine the necessity of brain regions in behaviors. RNA sequencing (RNA-seq) was used to identify the transcriptomic changes.

Results

Both acute and persistent pain could lead to anxiety-like behavior in mice. The c-Fos expression indicates that the bed nucleus of the stria terminalis (BNST) is activated only in acute pain, whereas the medial prefrontal cortex (mPFC) is activated only in persistent pain. Chemogenetic manipulation reveals that the activation of the BNST excitatory neurons is required for acute pain-induced anxiety-like behaviors. In contrast, the activation of the prelimbic mPFC excitatory neurons is essential for persistent pain-induced anxiety-like behaviors. RNA-seq reveals that acute and persistent pain induces differential gene expression changes and protein-protein interaction networks in the BNST and prelimbic mPFC. The genes relevant to neuronal functions might underline the differential activation of the BNST and prelimbic mPFC in different pain models, and be involved in acute and persistent pain-related anxiety-like behaviors.

Conclusion

Distinct brain regions and gene expression patterns are involved in acute and persistent pain-related anxiety-like behaviors.