Putative GABAergic input to axons of spinal interneurons and primary sensory neurons in the lamprey spinal cord as shown by intracellular Lucifer yellow and GABA immunohistochemistry.

GABAergic phasic modulation of the membrane potential occurs in spinal interneurons during fictive locomotion in lamprey presumably indicating a presynaptic inhibition. GABA also modulates synaptic transmission from primary sensory neurons (dorsal cells) at a presynaptic site. From these findings GABA terminals would be expected to be in close contact with phasically modulated axons of spinal interneurons and/or dorsal cells and their axons. To test this supposition intracellular injections of Lucifer yellow into spinal interneurons or dorsal cells were combined with GABA immunohistochemistry. GABA-immunoreactive (ir) varicosities were found to be in close contact (less than 1 micron distance) with axons modulated during fictive locomotion as well as dorsal cell axons. Small GABA-ir bipolar neurons form processes, which are in close contact with the axons of dorsal cells.