The plasticity of descending controls in pain: translational probing

• Published in: The Journal of physiology 2017-07, Vol.595 (13), p.4159-4166
• Main Authors: Bannister, Kirsty, Dickenson, A. H.
• Format: Article
• Language: English
• Subjects: Analgesics; Analgesics - pharmacology; Analgesics - therapeutic use; Animals; Brain - drug effects; Brain - metabolism; Brain - physiology; Brain stem; Conditioning; CPM; descending; DNIC; Excitability; Humans; Mesencephalon; modulation; Neural networks; Neuronal Plasticity; Nociception; Noradrenaline; Norepinephrine; Pain; Pain - drug therapy; Pain - metabolism; Pain - physiopathology; Peripheral neuropathy; Pyramidal Tracts - drug effects; Pyramidal Tracts - metabolism; Pyramidal Tracts - physiology; Receptors (physiology); Serotonin; Serotonin receptors; Serotonin S3 receptors; Serotonin S7 receptors; Spinal cord; Symposium Review; Symposium section reviews: Top‐down control of pain; Translation
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/JP274165

Descending controls, comprising pathways that originate in midbrain and brainstem regions and project onto the spinal cord, have long been recognised as key links in the multiple neural networks that interact to produce the overall pain experience. There is clear evidence from preclinical and clinical studies that both peripheral and central sensitisation play important roles in determining the level of pain perceived. Much emphasis has been put on spinal cord mechanisms in central excitability, but it is now becoming clear that spinal hyperexcitability can be regulated by descending pathways from the brain that originate from predominantly noradrenergic and serotonergic systems. One pain can inhibit another. In this respect diffuse noxious inhibitory controls (DNIC) are a unique form of endogenous descending inhibitory pathway since they can be easily evoked and quantified in animals and man. The spinal pharmacology of pathways that subserve DNIC are complicated; in the normal situation these descending controls produce a final inhibitory effect through the actions of noradrenaline at spinal α2‐adrenoceptors, although serotonin, acting on facilitatory spinal 5‐HT3 receptors, influences the final expression of DNIC also. These descending pathways are altered in neuropathy and the effects of excess serotonin may now become inhibitory through activation of spinal 5‐HT7 receptors. Conditioned pain modulation (CPM) is the human counterpart of DNIC and requires a descending control also. Back and forward translational studies between DNIC and CPM, gauged between bench and bedside, are key for the development of analgesic therapies that exploit descending noradrenergic and serotonergic control pathways.  The supraspinal modulatory influences on pain processing are well connected, complicated and fervently researched. Descending drives originating from superficial dorsal horn NK1‐expressing neurons (themselves activated by a primary afferent input) will relay through the parabrachial, a vital component of the machinery that processes ascending nociceptive information. From these nuclei inputs may relay to the limbic brain, hypothalamus and the periaqueductal grey (PAG) with which they make extensive reciprocal connections. The parabrachial also has a functional connection with the rostroventral medial medulla (RVM), allowing the spinobrachial pathway to access descending control systems as part of a recurrent circuit. Serotonergic projections from the RVM will ha