Organization of Serotonergic Cell Populations in the Brain and Spinal Cord of the Short-Lived African Turquoise Killifish

The African turquoise killifish is a short-lived vertebrate that is gaining interest as a model to understand degenerative processes associated with aging, including neuro-degeneration. However, due to its relatively recent adoption in laboratory research, detailed studies on the adult neuroanatomy of the killifish, particularly regarding the organization of neurotransmitter systems and their age-related changes, remain limited. Here, we study the organization of serotonergic cell populations in the brain and spinal cord of juveniles/young adult (1-3 months old) and in geriatric (5-6 months old) killifish using anti-serotonin immunofluorescence. Our findings reveal that the overall organi-zation of serotonergic cell populations in killifish closely resembles that of most teleosts, including the conspicuous hypothalamic and rapheal neuronal populations. Of note, adult killifish present a population of serotonergic neurons in the dorsolateral isthmus, which has been only described in a few other fish species. Aged killifish showed a similar organization of serotonergic cell populations to that of juveniles/young adults, with the major difference being the loss of serotonin immunoreactivity in pretectal cells of 6 month old fish. Our study comes to provide a neuroanatomical framework for future investi-gations on the serotonergic system in this emerging vertebrate model of aging.