GPR4 regulates temporally dynamic and sex-dependent cocaine-induced locomotor sensitization.

Background and objectives: Cocaine-induced locomotor activation and sensitization are widely used models for investigating dopaminergic signaling and neuroadaptive plasticity relevant to substance use disorders. G protein-coupled receptor 4 (GPR4) is a proton-sensing receptor implicated in pH-dependent cellular signaling, but its role in cocaine-induced behavioral responses remains unknown. Moreover, sex-dependent variability in stimulant responsiveness may interact with genotype in temporally dynamic ways that are poorly understood. Methods: Locomotor responses were examined in 166 three-month-old wild-type (WT) and GPR4 knockout (GPR4KO) mice. Mice received daily saline or cocaine (20 mg/kg, days 1-5) during repeated locomotor testing, followed by a 14-day abstinence period and a cocaine challenge (10 mg/kg) to assess expression of locomotor sensitization. Distance traveled was recorded in 5-min bins during 150-min sessions and analyzed using factorial ANOVA models with Results: Baseline locomotor activity during the initial treatment sessions was higher in GPR4KO mice than in WT mice and higher in females than in males. During acute cocaine exposure, GPR4KO mice exhibited enhanced locomotor responses relative to WT mice, with genotype effects being most pronounced during early treatment sessions and progressively attenuating by day 5. Sex differences showed a biphasic within-session pattern, with females displaying greater early locomotor activity and males exhibiting greater late-session activity. Following abstinence, overall pre-challenge baseline activity did not differ according to prior cocaine exposure; however, genotype effects on baseline activity were observed in a sex-and treatment-dependent manner. After the cocaine challenge injection, females exhibited greater locomotor responses than males. Sensitization effects were strongest during the early phase of the challenge session and became sex-dependent during later intervals, whereas genotype × sex × sensitization interactions emerged selectively during the late phase of the session. Conclusion: GPR4 modulates cocaine-induced locomotor activation and sensitization in a temporally dynamic and sex-dependent manner. These findings identify GPR4 as a previously unrecognized regulator of stimulant responsiveness and implicate proton-sensitive signaling mechanisms in the neurobehavioral adaptations associated with repeated cocaine exposure.