Changes in gene expression and integrin-mediated structural changes are associated with long-term plasticity of a spinal cord locomotor network.

Substance P evokes a long-term increase in the frequency of the lamprey locomotor network output. Although previous physiological experiments using protein synthesis inhibitors suggested separate transcription-independent and -dependent phases in the long-term network modulation, this conclusion was weakened by the non-specific effects associated with these inhibitors. We thus sought further evidence for substance P effects on gene expression here using differential display polymerase chain reaction. We found evidence for changes in gene expression in the potential transcription-independent and dependent phases. We sequenced the mRNAs that we detected, and found that one of the mRNAs shared homology with a portion of the lamprey fibrinogen alpha-2 chain. As fibrinogen is a component of the extracellular matrix, the change in fibrinogen gene expression suggested the potential involvement of structural changes in the effects of substance P. We have previously shown that the network effects of substance P are associated with a reduction of the synaptic gap at glutamatergic synapses. To determine if this ultrastructural change was related to potential fibrinogen-dependent effects we examined the effects of arginine-glycine-aspartate (RGD) peptides, which block integrin binding to fibrinogen. Electron microscope analyses showed that RGD peptides blocked the substance P-mediated reduction in the synaptic gap. In addition, physiological experiments suggested that RGD peptides blocked the maintenance of the long-term increase in the network burst frequency. These results thus support the involvement of changes in gene expression in the long-term network effects of substance P, and suggest the involvement of integrin-mediated changes in synaptic ultrastructure.