Nociceptor Spontaneous Activity is Responsible for Fragmenting Non-Rapid Eye Movement Sleep in Mouse Models of Neuropathic Pain

Neurology Anesthesiology Medicine Basic Medical Sciences
nociceptor spontaneous activity non-rapid eye movement sleep mouse models chronic pain

Research Background and Motivation

Neuropathic pain is a complex and common clinical issue, accompanied by persistent spontaneous pain. Patients often report symptoms such as spontaneous pain, electric shock sensations, and burning pain. The physiological mechanisms behind this type of pain are complex, and reliable research models or biomarkers are hard to find, making the development of diagnostic tools and effective treatments particularly challenging. Current clinical and preclinical studies mainly rely on pain behavior assessments triggered by mechanical or thermal stimuli, but their translational value is limited because the primary symptom of neuropathic pain patients is spontaneous pain, not stimulus-induced pain.

Against this backdrop, Alexandre et al. conducted an in-depth study aimed at exploring how neuropathic pain affects Non-Rapid Eye Movement Sleep (NREMS) in mice and identifying the neural pathways driving this sleep fragmentation phenomenon.

Source

This research paper was co-authored by Chloé Alexandre, Giulia Miracca, and several other scholars from renowned research institutions such as Johns Hopkins University and Harvard University. The paper was published in the April 17, 2024, issue of Science Translational Medicine.

Research Process

Experimental Design

The research primarily utilized a series of experimental steps to reveal how neuropathic pain affects sleep and neural activities in mice. The specific process is as follows:

  1. Mouse Model Preparation: The study used several mouse pain models, including neuropathic pain (spared nerve injury and chronic constriction injury), inflammatory pain (Freund’s complete adjuvant and carrageenan injection), and chemical pain (capsaicin injection).

  2. EEG/EMG Recording: Electrophysiological techniques were used to record electroencephalograms (EEG) and electromyograms (EMG) in mice to thoroughly analyze sleep patterns under different pain models.

  3. Optogenetic Stimulation: Nav1.8 and TRPV1-positive neurons were optogenetically activated to investigate the effects of single and repeated light stimulation on awakening during Non-Rapid Eye Movement Sleep.

  4. Gene Knockout and Drug Testing: The role of specific neurons in regulating pain and sleep was elucidated through gene knockout and drug treatments.

Experimental Results

Optogenetic Operation

  1. Single Optogenetic Activation: During Non-Rapid Eye Movement Sleep, single light activation of Nav1.8 and TRPV1 neurons rapidly triggered mouse awakening, with an average awakening time of 86.9±3.9 milliseconds, and the awakening duration averaging 5.7±0.6 seconds.

  2. Repeated Optogenetic Activation: Repeated activation (ten laser pulses per 10 seconds) during the waking period significantly increased EEG Gamma wave (30-100Hz) power.

Impact of Neuropathic Injury on Sleep

  1. Spared Nerve Injury (SNI): In the SNI model, mice’s Non-Rapid Eye Movement Sleep was frequently interrupted by short awakenings, leading to fragmented sleep structures.

  2. NREMS fragmentation after nerve injury: Observations revealed that the frequency of short awakenings (lasting less than 16 seconds) significantly increased post-injury. These awakenings were highly correlated with the trough phase of the EEG Sigma band power (0.02Hz oscillations).

  3. Chronic Constriction Injury (CCI) and Sciatic Nerve Crush (SNcrush): These models also showed similar sleep fragmentation phenomena. Sleep fragmentation induced by CCI peaked at the fifth week post-injury, while that caused by SNcrush gradually disappeared as the nerve recovered.

  4. Impact of Inflammatory and Chemical Pain on NREMS: Unlike neuropathic pain models, inflammatory and chemical pain did not lead to NREMS fragmentation.

Gene Knockout and Drug Treatment Research

  1. Nav1.8::Tet-Tox and TRPV1::DTA Gene Knockout: The results indicated that mice with these gene knockouts did not exhibit NREMS fragmentation in the SNI model, highlighting the critical role of these neurons in pain-induced awakenings.

  2. Drug Treatment Effects: Experiments showed that anticonvulsant drugs gabapentin and carbamazepine effectively reduced NREMS fragmentation in neuropathic pain models, whereas morphine had no significant effect.

Discovery of CGPR-positive Neurons in the Lateral Parabrachial Nucleus

By microinjecting AAV-Flex-DTA into the lateral parabrachial nucleus of mice, the study found that deleting CGPR-positive neurons significantly reduced NREMS fragmentation in the SNI model, indicating the key role of these neurons in driving awakenings.

Conclusion

This study revealed how peripheral nerve injury leads to fragmented Non-Rapid Eye Movement Sleep (NREMS) through short awakenings and identified the key neural pathways involved. Specifically, ectopic activation of peripheral Nav1.8 and TRPV1-positive sensory neurons transmits signals through CGPR-positive neurons in the lateral parabrachial nucleus to the cortex, driving awakenings.

This sleep fragmentation may not only represent a symptom of spontaneous neuropathic pain but also serve as a reliable indicator for evaluating the efficacy of various clinical and pharmacological treatments. More importantly, this study aids in the understanding of the mechanisms behind sleep disorders associated with neuropathic pain and provides new targets and ideas for future treatment.

Research Significance

This study, through diverse experimental approaches, comprehensively analyzed and deeply validated the impact of neuropathic pain on Non-Rapid Eye Movement Sleep, providing valuable reference material for scientists and clinicians. Particularly in the research on accurately identifying and quantifying spontaneous pain, this work offers a new evaluation method and theoretical foundation, holding significant scientific and clinical application value.