Colocalization of glutamate and glycine in giant fiber synapses of the lamprey spinal cord.

Using electron microscope immunocytochemistry in serial and semi-serial sections, glutamate and glycine immunoreactivity was analyzed in various synapses of the lamprey spinal cord. Immunoreactivity to glutamate was observed over the synaptic vesicle clusters in the giant reticulospinal axons and dorsal/dorsolateral column fibers, both of which established "en passant" asymmetrical contacts. Most of giant axons (70%) were also observed to express an immunoreactivity to glycine. Glutamate or glycine immunopositive axon terminals were present throughout the spinal cord regions. Glutamate positive boutons made asymmetrical contacts and contained rounded synaptic vesicles. Glycine reactive axon terminals with rounded synaptic vesicles established asymmetrical contacts and those with flattened synaptic vesicles established symmetrical contacts. The possibility that these two amino acids could be released from the same reticulospinal synapse in the lamprey, with a glycine modulatory effect on NMDA receptors, is suggested.