Serotonergic neuronal sprouting as a potential mechanism of recovery in multiple sclerosis

Experimental allergic encephalomyelitis (EAE) is widely considered as an animal model of multiple sclerosis (MS). Damage to the bulbospinal serotonergic (5-HT) neurons occurs in the early paralytic stages of EAE in rats with the severity of neurologic signs corresponding to spinal serotonergic depletion. Neurologic recovery of EAE rats is associated with reestablishment of spinal 5-HT transmission possibly through sprouting of undamaged axons and nerve terminals. Damage to the bulbospinal serotonergic fibers also occurs in patients with MS (as reflected by reduced lumbar CSF 5-HIAA levels) and may contribute to several manifestations of the disease including autonomic dysregulation, sensory symptoms (i.e., paresthesias, pain) and motor symptoms (weakness, spasticity, clonus). Spinal serotonergic neuronal sprouting with regeneration of 5-HT nerve terminals may also occur in the early stages of MS and may be associated with spontaneous remission of MS symptoms following an acute relapse. Sprouting of serotonergic neurons may also explain the disparity in MS between the extent of demyelinating plaques and clinical signs of the disease. The chronic course of MS may be associated with progressive axonal degenerative changes with reduction of serotonergic nerve terminals and loss of their sprouting capability. It is proposed that the beneficial effects of treatment with AC pulsed electromagnetic fields on the symptoms and course of the disease in patients with chronic progressive MS may be related in part to renewed sprouting of serotonergic neurons.