Dorsal root potentials and dorsal root reflexes: a double-edged sword

The nature of dorsal root reflexes (DRRs) and their possible role in periph eral inflammation and the consequent hyperalgesia are reviewed. The history of DRRs and the relationship of DRRs to primary afferent depolarization an d presynaptic inhibition in pathways formed by both large and fine afferent s are discussed. Emphasis is placed on the mechanisms underlying primary af ferent depolarization, including the anatomical arrangement of the synapses involved, how depolarization can result in inhibition by decreasing transm itter release, the role of excitatory amino acids and GABA, the manner in w hich the equilibrium potential for chloride ions is determined in primary a fferent fibers, and forms of presynaptic inhibition that do not utilize GBB A(A) receptors. There is then a discussion of neurogenic inflammation, incl uding the role of the release of neuropeptides such as substance P and calc itonin gene-related peptide from sensory nerve endings. Evidence is reviewe d that links DRRs to a substantial part of the swelling of the knee joint i n acute experimental arthritis and to the flare reaction in the skin follow ing intradermal injection of capsaicin. Possible mechanisms by which the le vel of DRR activity might be enhanced following inflammation are suggested. The consquences of this increase in DRRs may include exacerbation of hyper algesia as well as of peripheral inflammation. The conversion of an inhibit ory process, presynaptic inhibition, to an excitatory one by DRRs can thus lead to pathological consequences.