Vitreal DOPAC as an index of retinal dopamine release

Abstract

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 vitreal levels of dihydroxyphenylacetic acid (DOPAC) reflect the rate of dopamine release in chickens. Blocking re-uptake with nomifensine significantly lowered vitreal DOPAC and retinal dopamine, confirming the retinal origin and reliance of vitreal DOPAC on intact re-uptake mechanisms. Further, inhibition of monoamine oxidase with pargyline reduced vitreal as well as retinal DOPAC levels, confirming that the DOPAC detected is generated by monoamine oxidase. Finally, we found that DOPAC diffused freely into and out of isolated vitreous bodies and we found the vitreous to be metabolically inert with respect to DOPAC, supporting the idea that vitreal levels of DOPAC are consequential to the retinal metabolism of dopamine. Exposure to light, which is known to increase retinal dopamine release, readily increased vitreal DOPAC levels. The accumulation of DOPAC in the vitreous over 6 h light fitted a mathematical model of DOPAC accumulation based on zero-order influx (proportional to dopamine release rates) and diffusion driven, first-order efflux.