Mechanism by which the paraventricular nucleus of the hypothalamus is involved in chronic pain and anxiety in mice with lumbar disc herniation

Abstract

BACKGROUND: Patients with lumbar disc herniation (LDH) often experience comorbid anxiety due to chronic pain and functional limitations, significantly affecting their quality of life. However, the mechanisms underlying the pain-anxiety comorbidity remain unclear. OBJECTIVE: To investigate the neural regulatory mechanisms of the paraventricular nucleus in the hypothalamus in a mouse model of lumbar disc herniation with chronic pain-anxiety comorbidity. METHODS: A total of 100 C57BL/6 mice were randomly divided into a normal group (24 mice) and a model group (76 mice). The lumbar disc herniation model was established in the model group using a needle puncture method. Seventy-two successfully modeled mice were randomly divided into the model group, oxytocin group, and oxytocin+Vasotocin group, with 24 mice in each group. Mice in the oxytocin group received a 200 nL injection of oxytocin (0.5 μg/μL) into the paraventricular nucleus of the hypothalamus. Mice in the oxytocin+Vasotocin group received a 200 nL injection of oxytocin into the paraventricular nucleus and a 20 μL intraperitoneal injection of Vasotocin (an oxytocin antagonist, 0.15 μg/μL). Anxiety-like behavioral changes were evaluated via the elevated plus maze and open field tests on day 20 after modeling. Mechanical paw withdrawal threshold and thermal paw withdrawal latency experiments were conducted for all groups before modeling and 21 days after modeling. On day 21 post-modeling, immunofluorescence staining was used to observe c-FOS expression in the paraventricular nucleus of the hypothalamus; qPCR was used to detect mRNA expression of inflammatory factors prostaglandin E2, tumor necrosis factor α, and interleukin-1β in dorsal root ganglion tissue; Western blot was used to detect oxytocin receptor and p-ERK1/2 protein expression in the paraventricular nucleus. RESULTS AND CONCLUSION: Compared with the normal group, the model group showed significantly decreased mechanical and thermal pain thresholds (P < 0.05), significantly reduced time and entries in the open arms of the elevated plus maze (P < 0.05), significantly reduced time and entries in the open field (P < 0.05), significantly increased mRNA expression of prostaglandin E2, tumor necrosis factor α, and interleukin-1β in dorsal root ganglion tissue (P < 0.05), significantly increased c-FOS expression in the paraventricular nucleus (P < 0.05), significantly decreased oxytocin receptor protein expression, and significantly increased p-ERK1/2 protein expression (P < 0.05). Compared with the model group, the oxytocin group showed significantly increased mechanical and thermal pain thresholds (P < 0.05), significantly increased time and entries in the open arms of the elevated plus maze (P < 0.05), significantly increased time and entries in the open field (P < 0.05), significantly decreased mRNA expression of prostaglandin E2, tumor necrosis factor α, and interleukin-1β in dorsal root ganglion tissue (P < 0.05), significantly decreased c-FOS expression in the paraventricular nucleus (P < 0.05), significantly increased oxytocin receptor protein expression, and significantly decreased p-ERK1/2 protein expression (P < 0.05). Compared with the oxytocin group, the use of Vasotocin reversed the beneficial effects of oxytocin on pain and anxiety, increased inflammatory factor expression, significantly decreased oxytocin receptor protein expression, and significantly increased p-ERK1/2 protein expression. These results indicate that oxytocin can significantly improve chronic pain and anxiety-like behavior in mice with lumbar disc herniation, inhibit dorsal root ganglion inflammation, and the mechanism may be related to activation of the ERK signaling pathway in the paraventricular nucleus and downregulation of inflammatory factor expression.

1. Introduction

With the aging population, the prevalence and disability rate of low back pain have been increasing year by year [1-2]. There are many causes of low back pain, including spinal stenosis, foraminal stenosis, lumbar disc herniation, joint injury, compression fractures, spondylolisthesis, and scoliosis, among which lumbar disc herniation is the most common [3]. Lumbar disc herniation commonly occurs in patients aged 20-64 years, most commonly in those aged 35-45 years, imposing a huge economic burden on society, families, and the healthcare system [4]. Patients with chronic pain due to lumbar disc herniation often have a long disease course; mild cases may have functional limitations, while severe cases may be bedridden and unable to care for themselves, often accompanied by anxiety, depression, fear, and somatization symptoms [5]. These negative emotions often interact during the development and progression of the disease, forming a vicious cycle of "pain-anxiety". However, current clinical treatment focuses mainly on pharmacological or surgical interventions for pain itself, lacking systematic research on the neuroendocrine mechanisms of comorbid anxiety, resulting in limited efficacy. Therefore, studying the anxiety disorder and its mechanisms in patients with chronic pain due to lumbar disc herniation is of great significance.

The paraventricular nucleus of the hypothalamus, located on both sides of the third ventricle, is an important neurosecretory nucleus involved in a wide range of pathophysiological processes, including visceral pain, inflammatory pain, and neuropathic pain, both centrally and peripherally. Oxytocin receptor is the main receptor of the oxytocin signaling pathway, and oxytocin is related to maternal behavior, reproductive behavior, learning and memory, and pain in the central nervous system. However, the specific role of the paraventricular nucleus and oxytocin signaling in the comorbidity of chronic pain and anxiety in lumbar disc herniation remains unclear. This study aims to investigate the neural regulatory mechanisms of the paraventricular nucleus in a mouse model of lumbar disc herniation with chronic pain-anxiety comorbidity, providing a theoretical basis for the treatment of pain-anxiety comorbidity.