BDNF: no gain without pain?

Injury to the adult nervous system promotes the expression and secretion of brain-derived neurotrophic factor (BDNF). Because it promotes neuronal growth, survival and neurogenesis, BDNF may initiate compensatory processes that mitigate the deleterious effects of injury, disease or stress. Despite this, BDNF has been implicated in several injury-induced maladaptive processes including pain, spasticity and convulsive activity. This review will concentrate on the predominant role of BDNF in the initiation and maintenance of chronic and/or neuropathic pain at the spinal, peripheral and central levels. Within the spinal dorsal horn, the pattern of BDNF-induced changes in synaptic transmission across five different, identified neuronal phenotypes bears a striking resemblance to that produced by chronic constriction injury (CCI) of peripheral nerves. The appearance of this "pain footprint" thus reflects multiple sensitizing actions of microglial-derived BDNF. These include changes in the chloride equilibrium potential, decreased excitatory synaptic drive to inhibitory neurons, complex changes in inhibitory (GABA/glycinergic) synaptic transmission, increases in excitatory synaptic drive to excitatory neurons and the appearance of oscillatory activity. BDNF effects are confined to changes in synaptic transmission as there is little change in the passive or active properties of neurons in the superficial dorsal horn. Actions of BDNF in the brain stem and periphery also contribute to the onset and persistence of chronic pain. In spite of its role in compensatory processes that facilitate the recovery of the nervous system from injury, the widespread maladaptive actions of BDNF mean that there is literally "no gain without pain".