Peripheral nerve injury triggers noradrenergic sprouting within dorsal root ganglia

In humans, trauma to a peripheral nerve may be followed by chronic pain syndromes which are only relieved by blockade of the effects of sympathetic impulse traffic. It is presumed that, after the lesion, noradrenaline released by activity of sympathetic postganglionic axons excites primary afferent neurons by activating alpha-adrenoceptors, generating signals that enter the 'pain pathways' of the central nervous system. The site of coupling is unclear. In some patients local anaesthesia of the relevant peripheral nerve does not alleviate pain, implying that ectopic impulses arise either within the central nervous system, or in proximal parts of the primary afferent neurons. In experimentally lesioned rats, activity can originate within the dorsal root ganglia. Here we report that, after sciatic nerve ligation, noradrenergic perivascular axons in rats sprout into dorsal root ganglia and form basket-like structures around large-diameter axotomized sensory neurons; sympathetic stimulation can activate such neurons repetitively. These unusual connections provide a possible origin for abnormal discharge following peripheral nerve damage. Further, in contrast to the sprouting of intact nerve terminals into nearby denervated effector tissues in skin, muscle, sympathetic ganglia and sweat glands, the axons sprout into a target which has not been partially denervated.