HOMER1 and amyotrophic lateral sclerosis: Our main findings are that: (1) although GRPR cells apparently have low levels of expression of Homer, their excitatory synapses can be revealed by injecting an AAV coding for PSD95-tagRFP; (2) GRPR cells are predominantly innervated by excitatory interneurons, and receive a relatively sparse synaptic input from primary afferents; and (3) chemogenetic excitation of these cells results in activation of other dorsal horn neurons, including ALS projection neurons, particularly those located in lateral lamina V and the LSN.