Stress-induced hyperalgesia: animal models and putative mechanisms.

Stress has been shown to affect brain activity and promote long-term changes in multiple neural systems. A variety of environmental and/or stressful stimuli have been shown to produce analgesia, a phenomenon often referred to as stress-induced analgesia (SIA). However, acute and chronic stresses also produce hyperalgesia in various behavioral tests. There are now several animal models in which stress enhances nociceptive responses. The dysfunction of the hypothalamo-pituitary-adrenocortical axis (HPA axis) and multiple neurotransmitter systems in the central nervous system (CNS), including endogenous opioid, serotonergic and noradrenergic systems, has been reported. These stress-induced hyperalgesia models may contribute to a better understanding of chronic pain and provide a more rational basis for drug therapies in a variety of pain syndromes.