Dorsal raphe nucleus receives retinal projections of morphologically distinct fibers in the common marmoset (Callithrix jacchus): A subunit B cholera labeling.

Non-image forming (NIF) pathways, a specialized branch of retinal circuitry, play a crucial role supporting physiological and behavioral processes, including circadian rhythmicity. Among the NIF regions, the dorsal raphe nucleus (DRN), a midbrain serotonergic cluster of neurons, is also devoted to circadian functions. Despite indirectly send photic inputs to circadian centers and modulating their activities, little is known about the organization of retina-DRN circuits in primate species. To enhance our understanding of the intrinsic organization of NIF circuits and identify retinoraphe innervation in the common marmoset (Callithrix jacchus), a diurnal non-human primate model, we employed an anterograde tract-tracing method to labeling terminal/fibers with cholera toxin subunit B (CTb) and characterized the morphology of their projections. Our analysis revealed that sparse CTb(+) retinal terminals are predominantly located in dorsal subdomain of the DRN, displaying two morphological types, such as simple en passant and R2-like terminals. This anatomical evidence suggests a phylogenetic stability of the retina-DRN projections in diurnal primate species, potentially serving as a significant source of photic modulation on the serotonergic profile in the DRN. However, functional significance in primate models remains uncertain. Our data provide a crucial anatomical foundation for understanding the functional aspect of this circuitry in primates, contributing to the comprehension of the phylogenetic pathways used by NIF functions, such as circadian rhythmicity.