Neurotrophin-dependent plasticity of neurotransmitter segregation in the rat superior cervical ganglion in vivo.

Neurons are able to segregate transmitters to different axon endings. Segregation is a plastic neuronal feature; it can be modulated by synaptic environment. We have demonstrated that neurotrophin and other cellular factors regulate segregation in sympathetic neurons in culture. Herein we tested the hypothesis that sympathetic neurons in vivo are also capable to exhibit neurotrophin-dependent plasticity of segregation. To explore the effect of neurotrophin on segregation, we reduced ganglionic NGF content by the transection of postganglionic nerves (axotomy) of the superior cervical ganglia. By immunohistochemistry, Western blot, and PCR analyses, we explored the effect of axotomy on the NGF and BDNF content of ganglionic neurons, and on the segregation extent of vesicular acetylcholine transporter (VAChT) and methionine enkephalin (mENK) in pre-ganglionic varicosities. We analyzed NGF-dependence of the changes found by applying exogenous NGF. Axotomy reduced ganglionic NGF and BDNF content, increased NGF transcripts, and increased VAChT-mENK segregation. Axotomy also increased the number of VAChT immunopositive varicosities, and caused the appearance of a population of VAChT-, mENK- or SV2-containing varicosities lacking Synaptophysin (Syn). Administration of NGF prevented changes in NGF content, kept NGF transcripts increased, and counteracted changes in segregation and in the number of cholinergic varicosities. The exogenous NGF did not preclude change in BDNF content or in the occurrence of the VAChT- or mENK-containing varicosities lacking Syn. Data demonstrate that segregation of transmitters in vivo is plastic and it is modulated by environmental signals like NGF. We propose a possible functional correlate of segregation plasticity in the sympathetic ganglia.