Vitreal dihydroxyphenylacetic acid (DOPAC) as an index of retinal dopaminerelease

Dopamine is generally accepted as a major neurotransmitter associated with light-adaptive processes in the retina. However, little is known about its precise release pattern in vivo, largely due to the lack of an unambiguous method for the determination of dopamine release. We have found that vitrea l levels of dihydroxyphenylacetic acid (DOPAC) reflect the rate of dopamine release in chickens. Blocking re-uptake with nomifensine significantly low ered vitreal DOPAC and retinal dopamine, confirming the retinal origin and reliance of vitreal DOPAC on intact re-uptake mechanisms. Further, inhibiti on of monoamine oxidase with pargyline reduced vitreal as well as retinal D OPAC levels, confirming that the DOPAC detected is generated by monoamine o xidase. Finally, we found that DOPAC diffused freely into and out of isolat ed vitreous bodies and we found the vitreous to be metabolically inert with respect to DOPAC, supporting the idea that vitreal levels of DOPAC are con sequential to the retinal metabolism of dopamine. Exposure to light, which is known to increase retinal dopamine release, readily increased vitreal DO PAC levels. The accumulation of DOPAC in the vitreous over 6 h light fitted a mathematical model of DOPAC accumulation based on zero-order influx (pro portional to dopamine release rates) and diffusion driven, first-order effl ux.