Localization of Melanocortin 1 Receptor in the Substantia Nigra

Recent findings have revealed that melanocortin 1 receptor (MC1R) deficiency leads to Parkinson's disease-like dopaminergic neurodegeneration in the substantia nigra (SN). However, its precise distribution and expressing-cell type in the SN remain unclear. Therefore, in this study, we analyzed the localization and characteristics of MC1R in the SN using histological methods, including in situ hybridization and immunohistochemistry. Our findings reveal that MC1R was slightly present in dopaminergic neurons in the ventral tier of SN pars compacta dorsal (vSNCD), a region particularly vulnerable to PD-related neurodegeneration. Notably, we discovered that MC1R is highly present in parvalbumin (PV)-positive neurons, which were also vesicular GABA transporter messenger RNA-expressing inhibitory neurons of the lateral SN pars reticulata (lSNR). Intracellular analysis demonstrated that MC1R was present not only in the plasma membrane but also in mitochondrial and endoplasmic reticulum membranes. Furthermore, MC1R co-localized with attractin (Atrn), a known MC1R modulator, in nearly all MC1R-positive neurons. Therefore, it has been suggested that MC1R and Atrn work together to regulate dopaminergic neurons in the SN through both direct expression and indirect modulation via PV-positive inhibitory neurons. These findings provide new insights into MC1R's role in the SN and its potential contribution to PD pathophysiology.