THE DENSITY OF NNOS AND FOXP1 NEURONAL SUBPOPULATIONS IN THE MOUSE, RAT AND HUMAN SUBTHALAMIC NUCLEUS

The subthalamic nucleus (STN) is a small, ovoid structure essential for normal motor function and the treatment of Parkinson’s disease. Its molecular composition is highly complex and has been the subject of detailed investigation in recent years; however, quantitative data on STN neuronal subpopulations remain scarce. For instance, there are no existing data on STN neuron density in the mouse brain and only a single report for the rat brain. In this study, neuronal densities were quantified using immunohistochemistry and stereology, ensuring consistency across species. We analysed the density of two neuronal subpopulations—nNOS and FOXP1—as well as total neuronal density (using HuC/HuD as a pan-neuronal marker) in the STN of mouse, rat, and human brains. The highest density of STN neurons was observed in the mouse STN (149,282 ± 17,192 neurons/mm³), followed by the rat (67,371 ± 8,444 neurons/mm³) and human (4,942 ± 2,294 neurons/mm³) STN. FOXP1 represents a larger neuronal subpopulation in mouse (67,337 ± 19,103 neurons/mm³) and rat (34,467 ± 4,280 neurons/mm³) compared to the nNOS subpopulation (45,893 ± 10,859 neurons/mm³ in mouse STN; 27,713 ± 4,127 neurons/mm³ in rat STN). Interestingly, in the human STN, nNOS forms a significantly larger subpopulation than FOXP1 (nNOS – 3,832 ± 1,776 neurons/mm³; FOXP1 – 1,759 ± 628 neurons/mm³). These interspecies differences may reflect evolutionary adaptations in motor control circuitry, warranting further investigation into their functional significance and raise new questions about the role of specific neuronal subpopulations in motor function and disease. This study was supported by Croatian Science Foundation grant IP-2024-05-8297 (GS).