Sex differences in choice-based thermal nociceptive tests in adult rats

Analgesic Effect of Oxytocin in Alcohol-Dependent Male and Female Rats

INTRODUCTION

Chronic alcohol exposure in humans and rodents causes tolerance to the analgesic effects of alcohol, and enhances pain sensitivity during alcohol withdrawal (i.e., hyperalgesia). The available literature suggests a bidirectional enhancement between chronic alcohol consumption and chronic pain sensitivity. We previously found that oxytocin administration could reduce alcohol consumption in alcohol-dependent rats, and now hypothesize that oxytocin, through analgesic action in the central nervous system, could ameliorate the hyperalgesia induced by alcohol-dependence. To test this hypothesis, we assessed the ability of central and peripheral oxytocin administration to alter thermal (Hargreaves assay) and mechanical (Von Frey assay) pain sensitivity, in male and female rats, made alcohol dependent through repeated cycles of chronic-intermittent ethanol-vapor exposure (CIEV; compared to air-exposed controls).

METHODS

Male and female cohorts of Wistar rats were surgically prepared with an ICV cannula and assigned to two groups matched in terms of initial response in the Hargreaves assay. Rats in the alcohol dependent group were exposed to chronic-intermittent alcohol-vapor, while air-exposed control rats were exposed only to room air and served as the control group. The thermal nociception sensitivity of all rats was monitored via weekly Hargreaves assay to determine alcohol-dependence-induced hyperalgesia in dependent rats. Next, rats were ICV administered oxytocin (0, 0.5, or 5 μg in 2.5 μL saline) prior to Hargreaves testing (Experiment 1) or Von Frey testing (Experiment 2). Finally, rats were IP administered oxytocin (0, 0.1, or 1 mg/kg) prior to Hargreaves testing (Experiment 3) or Von Frey testing (Experiment 4). In a follow-up experiment, female rats were tested to directly compare three methods for applying the Von Frey test.

RESULTS

Male and female alcohol-dependent rats developed hyperalgesia, observed in the Hargreaves assay (Experiment 1 & 3), however, hyperalgesia was not so readily observed when the same rats were tested in the Von Frey assay (Experiments 2 & 4, with the exception of female rats in Experiment 4; follow-up testing indicated that the method of Von Frey test employed is likely important to explain this discrepancy). In both the Hargreaves and Von Frey assays, and in both male and female rats, following central or peripheral administration, oxytocin produced analgesia similarly in both alcohol dependent rats and air-exposed controls.

CONCLUSION

Together, these data suggest the oxytocin system could be targeted to produce therapeutic action in disease that produce hyperalgesia such as in alcohol dependence. We discuss methodological considerations and future experiments that could further elucidate a role for oxytocin in the overlapping neurobiology of alcohol dependence and chronic pain.

An improved conflict avoidance assay reveals modality-specific differences in pain hypersensitivity across sexes

ABSTRACT

Abnormal encoding of somatosensory modalities (ie, mechanical, cold, and heat) are a critical part of pathological pain states. Detailed phenotyping of patients' responses to these modalities have raised hopes that analgesic treatments could one day be tailored to a patient's phenotype. Such precise treatment would require a profound understanding of the underlying mechanisms of specific pain phenotypes at molecular, cellular, and circuitry levels. Although preclinical pain models have helped in that regard, the lack of a unified assay quantifying detailed mechanical, cold, and heat pain responses on the same scale precludes comparing how analgesic compounds act on different sensory phenotypes. The conflict avoidance assay is promising in that regard, but testing conditions require validation for its use with multiple modalities. In this study, we improve upon the conflict avoidance assay to provide a validated and detailed assessment of all 3 modalities within the same animal, in mice. We first optimized testing conditions to minimize the necessary amount of training and to reduce sex differences in performances. We then tested what range of stimuli produce dynamic stimulus-response relationships for different outcome measures in naive mice. We finally used this assay to show that nerve injury produces modality-specific sex differences in pain behavior. Our improved assay opens new avenues to study the basis of modality-specific abnormalities in pain behavior.

Evaluating Pain Behaviours: Widely Used Mechanical and Thermal Methods in Rodents

Globally, over 300 million surgical procedures are performed annually, with pain being one of the most common post-operative side effects. During the onset of injury, acute pain plays a protective role in alerting the individual to remove noxious stimuli, while long-lasting chronic pain without any physiological reason is detrimental to the recovery process. Hence, it created an urgent need to better understand the pain mechanism and explore therapeutic targets. Despite the hardship in performing human pain studies due to ethical considerations, clinically relevant rodent pain models provide an excellent opportunity to perform pain studies. Several neurobehavioural tests are used to assess the drug efficacy in rodents to determine avoidance behaviour latency and threshold. This review article provides a methodological overview of mechanical (i.e. von Frey, Mechanical Conflict System) and thermal (i.e. Hargreaves Assay, Hot and Cold Plate, Temperature Place Preference) tests to assess pain in clinically relevant pain rodent models. We further discussed the current modifications of those tests along with their use in literature, the impact of confounding variables, advantages and disadvantages.

IL-1 Mediates Chronic Stress-Induced Hyperalgesia Accompanied by Microglia and Astroglia Morphological Changes in Pain-Related Brain Regions in Mice

Chronic stress causes several pain conditions including fibromyalgia. Its pathophysiological mechanisms are unknown, and the therapy is unresolved. Since the involvement of interleukin-1 (IL-1) has been described in stress and inflammatory pain but no data are available regarding stress-induced pain, we studied its role in a chronic restraint stress (CRS) mouse model. Female and male C57Bl/6J wild-type (WT) and IL-1αβ-deficient (knock-out: IL-1 KO) mice were exposed to 6 h of immobilization/day for 4 weeks. Mechanonociception, cold tolerance, behavioral alterations, relative thymus/adrenal gland weights, microglia ionized calcium-binding adaptor molecule 1 (IBA1) and astrocyte glial fibrillary acidic protein (GFAP) integrated density, number and morphological transformation in pain-related brain regions were determined. CRS induced 15–20% mechanical hyperalgesia after 2 weeks in WT mice in both sexes, which was significantly reduced in female but not in male IL-1 KOs. Increased IBA1+ integrated density in the central nucleus of amygdala, primary somatosensory cortex hind limb representation part, hippocampus cornu ammonis area 3 (CA3) and periaqueductal gray matter (PAG) was present, accompanied by a cell number increase in IBA1+ microglia in stressed female WTs but not in IL-1 KOs. CRS induced morphological changes of GFAP+ astrocytes in WT but not in KO mice. Stress evoked cold hypersensitivity in the stressed animals. Anxiety and depression-like behaviors, thymus and adrenal gland weight changes were detectable in all groups after 2 but not 4 weeks of CRS due to adaptation. Thus, IL-1 mediates chronic stress-induced hyperalgesia in female mice, without other major behavioral alterations, suggesting the analgesic potentials of IL-1 in blocking drugs in stress-related pain syndromes.

Targeting the chemokine ligand 2-chemokine receptor 2 axis provides the possibility of immunotherapy in chronic pain

Chronic pain affects patients' physical and psychological health and quality of life, entailing a tremendous public health challenge. Currently, drugs for chronic pain are usually associated with a large number of side effects and poor efficacy. Chemokines in the neuroimmune interface combine with their receptors to regulate inflammation or mediate neuroinflammation in the peripheral and central nervous system. Targeting chemokines and their receptor-mediated neuroinflammation is an effective means to treat chronic pain. In recent years, growing evidence has shown that the expression of chemokine ligand 2 (CCL2) and its main chemokine receptor 2 (CCR2) is involved in its occurrence, development and maintenance of chronic pain. This paper summarises the relationship between the chemokine system, CCL2/CCR2 axis, and chronic pain, and the CCL2/CCR2 axis changes under different chronic pain conditions. Targeting chemokine CCL2 and its chemokine receptor CCR2 through siRNA, blocking antibodies, or small molecule antagonists may provide new therapeutic possibilities for managing chronic pain.