Descending facilitation in chronic stress and chronic pain state]

The spino-thalamic tract consists of two systems; the lateral system terminates in the somato-sensory cortex, and participates in the sensory discrimination of pain, and the medial system terminates in the anterior cingulated cortex (ACC) and insular cortex (IC) to mediate affective components of pain. Persistent pain induces plastic changes in cortical neurons, especially in the ACC and IC. Activation of these neurons is transmitted to the periaqueductal gray and rostroventromedial medulla (RVM) (descending pain control system). This system has long been considered to exert descending inhibition, but recent studies revealed that it also causes facilitation in certain pathological conditions. A variety of stressful stimuli have been shown to affect pain sensitivity. We demonstrated that chronic restraint stress induced thermal hyperalgesia in rats, in which phosphorylated ERK and levels of tryptophan hydroxylase, a key enzyme of 5-HT production, were increased in the RVM. 5HT released from the bulbospinal neurons may exert facilitatory effects on spinal nociceptive processing probably through 5HT3 receptors. Patients suffering chronic pain originating from deep tissues, such as temporo-mandibular disorder, fibromyalgia, or low back pain, often complain of pain and tenderness in various parts of the body. We injected complete Freund's adjuvant into a temporo-mandibular joint of rats unilaterally, and then injected 5% formalin into the ipsilateral or contralateral masseter muscle 2 weeks later. Pain-related behavior and neuronal activation in the spinal trigeminal nucleus were enhanced on both sides compared to those in non-inflammatory controls. Systemic enhancement of pain and hyperalgesia induced by unilateral joint inflammation may have been caused by the central sensitization and descending facilitation.