['Actin rearrangements are induced in the dorsal hippocampus after conditioned morphine withdrawal, and involved in the formation of conditioned place aversion. In the present study, we investigated the mechanisms underlying the actin rearrangements in rat dorsal hippocampus induced by conditioned morphine withdrawal.', 'The RhoA-ROCK pathway inhibitor Y27632 (8.56 μg/1 μL per side) or the Rac1 inhibitor NSC23766 (25 μg/1 μL per side) was microinjected into the dorsal hippocampus of rats. Conditioned place aversion (CPA) induced by naloxone-precipitated morphine withdrawal was assessed. Crude synaptosomal fraction of hippocampus was prepared, and the amount of F-actin and G-actin was measured with an Actin Polymerization Assay Kit.', 'Conditioned morphine withdrawal significantly increased actin polymerization in the dorsal hippocampus at 1 h following the naloxone injection. Preconditioning with microinjection of Y27632, but not NSC23766, attenuated CPA, and blocked the increase in actin polymerization in the dorsal hippocampus.'] Is the small GTPase RhoA , but not Rac1 , essential for conditioned aversive memory formation through regulation of actin rearrangements in rat dorsal hippocampus?