Cognitive impairment is commonly associated with the pain experience. This impairment represents a major obstacle to daily activities and rehabilitation, especially in the chronic pain population. Here we review clinical and preclinical studies that have investigated pain-related alterations in cognition. These include impaired attentional, executive and general cognitive functioning. We describe the anatomical, neurochemical and molecular substrates common to both cognitive processing and supraspinal pain processing, and present the evidence for their involvement in pain-related cognitive impairment. We also examine the added complexity of cognitive impairment caused by analgesic medications and how this can further impact on morbidity in chronic pain patients. The need for a better understanding of the mechanisms of both pain-induced and treatment-related cognitive impairment is highlighted. Further research in this area will aid our understanding of patient symptoms and their underlying pathophysiology, ultimately leading to increased provision of guided therapy.

Abbreviations: SIA, stress-induced analgesia; FCA, fear-conditioned analgesia; GABA, Ȗ-aminobutyric acid; HA, high analgesia; LA, low analgesia; fMRI, functional magnetic resonance imaging; DNIC, diffuse noxious inhibitory control; PAG, periaqueductal grey; RVM, rostroventral medulla; 5-HT, 5-hydroxytryptamine; CB 1 , cannabinoid type 1; CB 2 , cannabinoid type 2; NMDA, N-methyl-D-aspartic acid; AMPA, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid; mGluR, metabotropic glutamate receptor; NK1, neurokinin 1; SP, substance P; 2-AG, 2-arachidonoylglycerol; FAAH, fatty acid amide hydrolase; MGL, monoacylglycerol lipase; HPA, hypothalamo-pituitary-adrenal; ACTH, adrenocorticotropic hormone 3 ABSTRACT For over 30 years, scientists have been investigating the phenomenon of pain suppression upon exposure to unconditioned or conditioned stressful stimuli, commonly known as stress-induced analgesia. These studies have revealed that individual sensitivity to stressinduced analgesia can vary greatly and that this sensitivity is coupled to many different phenotypes including the degree of opioid sensitivity and startle response. Furthermore, stress-induced analgesia sensitivity can vary a great deal depending on age, gender, and prior experience to stressful, painful, or other environmental stimuli. Stress-induced analgesia is mediated by activation of the descending inhibitory pain pathway.Pharmacological and neurochemical studies have demonstrated involvement of a large number of neurotransmitters and neuropeptides. In particular, there are key roles for the HQGRJHQRXV RSLRLG PRQRDPLQH FDQQDELQRLG Ȗ-aminobutyric acid and glutamate systems. The study of stress-induced analgesia has enhanced our understanding of the fundamental physiology of pain and stress and has been a useful approach for uncovering new therapeutic targets for the treatment of pain and stress-related disorders.4

The importance of the modulation of pain by emotion is now widely recognised. In particular, stress and anxiety, depending on their nature, duration and intensity, can exert potent, but complex, modulatory influences typified by either a reduction or exacerbation of the pain state. Exposure to either acute or chronic stress can increase pain responding under experimental conditions and exacerbate clinical pain disorders. There is evidence that exposure to chronic or repeated stress can produce maladaptive neurobiological changes in pathways associated with pain processing, resulting in stress-induced hyperalgesia (SIH). Preclinical studies of SIH are essential for our understanding of the mechanisms underpinning stress-related pain syndromes and for the identification of neural pathways and substrates, and the development of novel therapeutic agents for their clinical management. In this review, we describe clinical and pre-clinical models used to study SIH and discuss the neural substrates, neurotransmitters and neuromodulatory systems involved in this phenomenon.

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Hyaluronic acid hydrogel alleviated inflammatory pain, altered glycosylation, and regulated matrix components for disc repair.