Sprouting of synapses after partial denervation of frog cardiac ganglion

Abstract

SYNAPTIC connections in the adult nervous system of several species can be quite flexible and this characteristic has been termed neuronal plasticity. For example, when lesions are made in the axonal pathways of the brains of mature rats, surviving nerve terminals can proliferate and innervate the synaptic sites vacated by the degenerating axons1,2. This “sprouting” occurs in the central nervous system2–4, autonomic nervous system5 and peripheral nervous system6, and seems to be a characteristic response of nervous tissue to partial denervation. In only a few studies has sprouting been reported not to occur7–9. Sprouting of synapses has important implications for the maintenance of neuronal connections and the ability of the nervous system to recover from focal lesions. But several questions need to be answered. For example, are sprouted synapses functional, and if so, do they transmit normally? Is there any specificity in or control over the proliferation of new connections? What initiates and terminates the sprouting? We can now begin to answer these questions. Our studies on the para-sympathetic cardiac ganglion of the frog have shown that synaptic sprouting occurs very rapidly after partial denervation. Furthermore, the number of boutons per cell remains constant throughout sprouting and is the same as the number per cell in control ganglia.