GPR4 and Anxiety: GPR4 is expressed in additional anatomically and chemically distinct brain–body regions (Mahadevan et al., 1995; Hosford et al., 2018; Gonye et al., 2024), and these mice may also provide a useful tool to discover if direct proton sensing by this receptor contributes to additional behavioral (e.g., arousal, anxiety) or physiological (e.g., renal acid excretion) responses to CO2/H+.