Dual innervation of the lamprey retina by GABAergic and glutamatergic retinopetal fibers: A quantitative EM immunogold study

Abstract

A quantitative electron microscopic analysis of glutamate and GABA immunocytochemistry, using the postembedding immunogold technique, was undertaken in the lamprey retina to determine the proportion of glutamate-immunoreactive (GLU-ir) centrifugal visual terminals which were identified by anterograde axonal transport after an iontophoretic deposit of HRP in the sectioned optic nerve. Single immunogold labeling carried out with two different GABA and GLU antibodies showed that about 45% of the retinopetal axon terminals were GABAergic, and that two types of GLU-ir terminals (GLU-ir/1 and GLU-ir/2) were observed in the inner plexiform layer. The former type showed a high density of gold particles (Neosystem: 19.38±0.74; Sigma: 106.26±5.70) which statistically differed from the GLU-ir/2 (Neosystem: 3.23±0.31; Sigma: 31.73±5.61). Subcellular estimates showed that gold particles were concentrated over the vesicular pool in the GLU-ir/1 terminals and over mitochondria in the second terminal type. Consecutive sections alternately processed for GABA and GLU revealed that the GABA-immunonegative terminals corresponded to the GLU-ir/1, whereas the GABA-ir terminals only contained few GLU-ir particles, mostly concentrated in mitochondria. The occurrence of GABAergic and glutamatergic retinopetal terminals in the adult lamprey retina is discussed in functional terms of their differential inhibitory and excitatory effects on ganglion cell activity and the possible role of the centrifugal visual system in visually-guided behavior.