Location and mapping of the human rostromedial tegmental nucleus and associated midbrain inhibitory nuclei regulating dopamine neurons

Animal experiments reveal distinct GABAergic cell clusters within the dopaminergic midbrain regions in the rostromedial tegmental nucleus (RMTg) and retrorubral fields (RRF) that have yet to be clearly defined in humans. These neurons send prominent inhibitory projections to dopaminergic neurons in the substantia nigra and ventral tegmental area that impact motor, reward and threat processing. We have identified GABAergic RMTg and RRF cell clusters in 6 μm formalin-fixed paraffin-embedded transverse human midbrain sections from ten control cases obtained from the Sydney Brain Bank using immunohistochemistry for GABA and tyrosine hydroxylase. We determined the location and cell size of RMTg and RRF GABAergic neurons, further mapping these cell clusters in transverse 50 μm thick cresyl violet stained serial midbrain sections (every 750 μm) from previously published controls (Halliday et al. 1990). GABAergic neurons were cytoarchitecturally distinct, with the largest GABAergic neurons in the RRF, followed by RMTg neurons which were larger than GABAergic neurons in the well-defined interpedunclular nucleus (Kruskal-Wallis test, p < 0.0001). RMTg and RRF GABAergic neurons first appear in caudal transverse midbrain sections approximately 38 mm above the obex. RMTg moves rostrally and medially from underneath the decussation of the superior cerebellar peduncle to just lateral to the interpeduncular nucleus. The RRF cluster also moves rostrally and medially to the parabrachial pigmented nucleus (PBP) just under the red nucleus. The GABAergic neurons in RMTg and RRF/PBP that modulate dopamine neuronal excitability have distinct morphologies in humans. Identifying these inhibitory neurons is key to evaluating their role in neurodegenerative diseases.