Protease-activated receptor 1 (PAR1) inhibits synaptic NMDARs in mouse nigral dopaminergic neurons

Protease-activated receptor 1 (PAR1) is a G protein-coupled receptor (GPCR), whose activation requires a proteolytic cleavage in the extracellular domain exposing a tethered ligand, which binds to the same receptor thus stimulating Gα_q/11-, Gα_i/o- and Gα_12-13 proteins. PAR1, activated by serine proteases and matrix metalloproteases, plays multifaceted roles in neuroinflammation and neurodegeneration, in stroke, brain trauma, Alzheimer's diseases, and Parkinson's disease (PD). Substantia nigra pars compacta (SNpc) is among areas with highest PAR1 expression, but current evidence on its roles herein is restricted to mechanisms controlling dopaminergic (DAergic) neurons survival, with controversial data showing PAR1 either fostering or counteracting degeneration in PD models. Since PAR1 functions on SNpc DAergic neurons activity are unknown, we investigated if PAR1 affects glutamatergic transmission in this neuronal population. We analyzed PAR1's effects on NMDARs and AMPARs by patch-clamp recordings from DAergic neurons from mouse midbrain slices. Then, we explored subunit composition of PAR1-sensitive NMDARs, with selective antagonists, and mechanisms underlying PAR1-induced NMDARs modulation, by quantifying NMDARs surface expression. PAR1 activation inhibits synaptic NMDARs in SNpc DAergic neurons, without affecting AMPARs. PAR1-sensitive NMDARs contain GluN2B/GluN2D subunits. Moreover, PAR1-mediated NMDARs hypofunction is reliant on NMDARs internalization, as PAR1 stimulation increases NMDARs intracellular levels and pharmacological limitation of NMDARs endocytosis prevents PAR1-induced NMDARs inhibition. We reveal that PAR1 regulates glutamatergic transmission in midbrain DAergic cells. This might have implications in brain's DA-dependent functions and in neurological/psychiatric diseases linked to DAergic dysfunctions.