EXTRACELLULAR DOPAMINE CONCENTRATION IN THE RETINA OF THE CLAWED FROG, XENOPUS-LAEVIS

Dopamine reaches targets in the outer retina of the clawed frog (Xenop us laevis) by diffusion from a network of dopaminergic cells and proce sses located predominantly at the junction of inner nuclear and inner plexiform layers. We obtained values for the steady-state release, upt ake, and extracellular concentration of dopamine in the retina by a co mbination of HPLC (with electrochemical detection), scintillation spec troscopy, and fast-scan cyclic voltammetry. Vitreal concentrations of dopamine varied from 564 +/- 109 nM in light-adapted eyes near the tim e of subjective dawn to 156 +/- 12 nM in dark-adapted eyes. The data a re consistent with a simple model for steady-state dopamine diffusion from an appropriately sited thin-sheet source. This model was used to generate a profile of extracellular dopamine concentration as a functi on of retinal depth. The model predicted an increase in the dopamine c oncentration from the vitreous to the layer of dopaminergic cells, rem aining constant from that layer to the distal tips of the photorecepto rs. This prediction was borne out by comparing fast-scan voltammetric measures of dopamine at the distal tips of the receptors with the vitr eal concentrations determined by HPLC using electrochemical detection.