Purpose: To describe serotonergic retinopetal axons in monkeys.
Methods: Whole macaque and baboon retinas, fixed in 4% paraformaldehyde, were labeled with antisera raised against serotonin (5-HT).
Results: Several large-diameter 5-HT-immunoreactive (IR) axons emerged from the optic disk. Most axons ran to the peripheral retina, where they branched extensively. Most terminated in the ganglion cell layer, but a few 5-HT-IR axons terminated in distal inner plexiform or within inner nuclear layer. Some axons branched extensively near the fovea, and a dense plexus of 5-HT-IR axons was also found around the optic disk. Varicose 5-HT-IR axons were also associated with blood vessels, especially in the central retina.
Conclusions: Immunoreactive serotonin is present in a distinct population of retinopetal axons in the monkey retina. Receptors for serotonin are present in the primate retinas, and based on physiological studies in other mammals, these retinopetal axons are expected to modulate neuronal activity and regulate blood flow.