Comparison of NMDA Receptor Subunit RNA Expression in the Spinal Dorsal Horn Across Species (Mice, Rats, Humans), Sex, Laminar Location and Neuronal Subtypes using Single Cell RNA Sequencing Data and qRT-PCR

Abstract

The hyperexcitability and central sensitization of spinal pain processing circuits is mediated by N-methyl-d-aspartate receptors (NMDARs) in the dorsal horn. Distinct circuits of neuronal subtypes in the superficial dorsal horn (SDH) versus the deep dorsal horn (DDH) primarily integrate pain and other somatosensory signals, respectively.Unique combinations of NMDAR subunits (GluN1/GRIN1, GluN2A -D/GRIN2A -D, and GluN3A -B/GRIN3A -B) confer different functional NMDAR properties and have differing effects on synaptic plasticity. In this study, we investigated NMDAR subunit expression across sex, species, spinal region, and neuronal subtypes using single cell/nucleus RNA sequencing data from the mouse and human spinal cord and performing qRT-PCR on adult male and female rat and human spinal cord tissue. We found high GRIN1, GRIN2A, GRIN2B, and GRIN3A expression and moderate GRIN2D expression in the SDH. Unlike in the rat dorsal horn, GRIN2C is highly expressed in the human dorsal horn. Both GRIN2C and GRIN2D are more highly expressed in inhibitory than excitatory neurons of the dorsal horn, but to a greater extent humans than in mice.Grin3a is more highly expressed in excitatory than inhibitory neurons of the mouse SDH.Compared to the other NMDAR subunits, GRIN2A and GRIN3A have variable expression across neuronal subtypes, but could play different roles in synaptic plasticity. Taken together, our results suggest that select NMDAR subunit genes play prominent and specialized roles in SDH somatosensory processing and different neuronal subtypes may have different patterns of GRIN gene expresssion.