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.

The African turquoise killifish is a short-lived vertebrate that is gaining interest as a model for understanding degenerative processes associated with aging, including neurodegeneration. 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 juvenile/young adult (1–3-months-old) and geriatric (5–6-months-old) killifish using anti-serotonin immunofluorescence. Our findings reveal that the overall organization of serotonergic cell populations in killifish closely resembles that of most teleosts, including pineal, pretectal, hypothalamic, rapheal and spinal cord neuronal populations. Of note, adult killifish also 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 provides a neuroanatomical framework for future investigations into the serotonergic system in this emerging vertebrate model of aging.