SUBSTANCE-P - A NEUROTRANSMITTER OF AMACRINE AND GANGLION-CELLS IN THE VERTEBRATE RETINA

A short history and summary of the occurrence of substance P in the ve rtebrate body is presented. Substance P is now generally accepted to b e a neurotransmitter and can be visualized by immunocytochemistry to o ccur in various nerve cells in the CNS. In the retina, substance P-imm unoreactivity (SP-IR) occurs in amacrine cell populations in all the s pecies so far studied. In some vertebrates retinas SP is also apparent in one or more ganglion cell types. Anatomical investigations have re vealed the morphology and connectivity of SP-IR amacrine cells: they b ranch in several strata of the inner plexiform layer receiving input f rom bipolar and amacrine cells and making synapses upon bipolar and ga nglion cells, Most commonly SP-IR amacrines emit axon-like process tha t pass to both the outer plexiform layer and the ganglion cell and ner ve fiber layers. These processes often end upon the retinal vasculatur e. SP-IR ganglion cells have been described in turtle, rabbit and huma n retinas. In turtle, intracellular dye injection has revealed the mor phology of one type of SP-IR ganglion cell as being a large-field mono stratified cell with a branches in the outer stratum of the inner plex iform layer. It may correspond to a ''Dogiel cell'' type. Intracellula r investigation of SP-IR amacrine cells in turtle reveal their physiol ogical responses to be ON-OFF in nature with some color-coding charact eristics. In general SP acts as an excitatory neurotransmitter raising the spontaneous activity level of ganglion cell responses. The SP-IR ganglion cell is an OFF-center unit in the turtle retina and may be dr iven in the center of its receptive field by luminosity bipolar cells and in its surround by amacrine cells with color-opponent properties.