Indoleamine accumulation by retinal neurons exposed to blood

Exposing rabbit retinas for one minute to an incubation medium containing 10 microliters of blood diluted in 20 ml of medium was sufficient to produce serotonin-like immunoreactivity in some of the retinal indoleamine-accumulating neurons. Retinas from rabbits that had been perfused before the eyes were removed had no detectable immunoreactivity. Our results support the conjecture that the serotonin sometimes detected in the retina originates in the blood. Why the cells have a carrier for a molecule that they do not normally contain remains unclear.