Possible morphological substrates for GABA-mediated presynaptic inhibition in the lamprey spinal cord

Gamma-aminobutyric acid (GABA) neurons intrinsic to the lamprey spinal cord are known to modulate synaptic transmission from interneurons active during locomotion and from mechanosensory dorsal cells. Many of these physiological effects are presynaptic. To establish the morphological substrates for these axo-axonic interactions, an ultrastructural analysis was performed with an antiserum to fixed GABA. The GABA immunoreactivity (ir) was detected by postembedding peroxidase-antiperoxidase and immunogold techniques. GABA-ir terminals were found to make appositions with unlabelled axons located in the dorsal columns and in the ventrolateral aspect of the spinal cord. In the ventrolateral part of the cord, similar appositions between different GABA-ir terminals were also observed. The immunolabelled terminals contained spherical to pleomorphic synaptic vesicles, and also glycogen granules and dense core vesicles. In some cases, the fine structure of the contacts between immunogold-labelled terminals and unlabelled axons suggested a synaptic relationship. Such a relation was found in a relatively small proportion (2-3%) of the appositions studied. These specializations were always observed in close relation to an output synapse of the postsynaptic axon. It is suggested that the axo-axonal contacts described may provide an effective modulation of the synaptic transmission from axons in the lamprey spinal cord.