Ultrastructural analysis of ectopic synaptic boutons arising from peripherally regenerated primary afferent fibers

The central axons of peripherally regenerated A beta primary sensory neuron s were impaled in the dorsal columns of alpha-chloralose-anesthetized cats 9-12 mo after axotomy.. The adequate peripheral stimulus was determined, an d the afferent fibers intracellularly stimulated while simultaneously recod ing the resulting cord dorsum potentials (CDPs). Fibers that successfully h ad reinnervated the sl;in responded to light tactile stimulation. and evoke d CDPs that suggested dorsally located boutons were stained intracellulary: with horseradish peroxidase (HRP). Two HRP-stained regenerated A beta affe rent fibers were recovered that supported large numbers of axon collaterals and swellings in laminae I. II0 and II1. Sections containing the ectopic c ollateral fibers and terminals in the superficial dorsal horn Here embedded in plastic. Analyses of serial ultrathin sections revealed that ectopic pr ojections front both regenerated fibers supported numerous synaptic boutons filled with clear round vesicles, a few forge dense con vesicles (LDCVs) a nd several mitochondria (>3), All profiles examined in serial sections (19) formed one to three asymmetric axo-dendritic contacts. Unmyelinated portio ns of ectopic fibers giving rise to en passant and terminal boutons often c ontained numerous clear round vesicles. Several boutons (47%) received asym metric contacts from axon terminals containing pleomorphic vesicles. These results string, suggest that regenerated A beta fibers activated by light t actile stimuli support functional connections in the superficial dorsal ham that have distinct ultrastructural features. In addition. the appearance o f LDCVs suggests that primary sensory neurons are capable of changing their neurochemical phenotype.