Expression of green fluorescent protein defines a specific population of lamina II excitatory interneurons in the GRP::eGFP mouse

Dorsal horn excitatory interneurons that express gastrin-releasing peptide (GRP) are part of the circuit for pruritogen-evoked itch. They have been extensively studied in a transgenic line in which enhanced green fluorescent protein (eGFP) is expressed under control of the Grp gene. The GRP-eGFP cells are separate from several other neurochemically-defined excitatory interneuron populations, and correspond to a class previously defined as transient central cells. However, mRNA for GRP is widely distributed among excitatory interneurons in superficial dorsal horn. Here we show that although Grp mRNA is present in several transcriptomically-defined populations, eGFP is restricted to a discrete subset of cells in the GRP::eGFP mouse, some of which express the neuromedin receptor 2 and likely belong to a cluster defined as Glut8. We show that these cells receive much of their excitatory synaptic input from MrgA3/MrgD-expressing nociceptive/pruritoceptive afferents and C-low threshold mechanoreceptors. Although the cells were not innervated by pruritoceptors expressing brain natriuretic peptide (BNP) most of them contained mRNA for NPR1, the receptor for BNP. In contrast, these cells received only ~ 10% of their excitatory input from other interneurons. These findings demonstrate that the GRP-eGFP cells constitute a discrete population of excitatory interneurons with a characteristic pattern of synaptic input.