Colocalization of glycine and GABA in synapses on spinomedullary neurons

Spinomedullary neurons of the postsynaptic dorsal column pathway in adult cats were retrogradely labelled with horseradish peroxidase. Postembedding immunogold reactions were performed with antisera which recognise GABA or glycine to determine if synaptic boutons in contact with these neurons contain both transmitters. Analysis of series of ultrathin sections revealed that synaptic profiles with strong immunogold reactions for GABA usually also displayed strong immunogold reactions for glycine. Pre-embedding immunocytochemistry was performed on sections containing labelled cells with a monoclonal antibody which recognises the glycine receptor-associated protein, gephyrin. Many synapses onto postsynaptic dorsal column neurons were associated with gephyrin-like immunoreactivity and these typically contained irregularly shaped vesicles. Immunogold reactions showed that synaptic profiles apposed to gephyrin-immunoreactive junctions contained GABA and glycine. The evidence suggests that glycine is a neurotransmitter at synapses on spinomedullary neurons and that it is colocalized with GABA.