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 (< 1-mu-m 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.