Drug Administration

The GABA_B(1a) receptor antagonist, CGP 36216 (Sigma) was used to block presynaptic GABA_B(1a) receptors within specific brain regions during the learning and memory process of contextual fear generalization. CGP 36216 shows no activity for postsynaptic GABA_B1 receptors and is selective for presynaptic GABA_B1 receptors (Ong et al, 2001). For the initial experiment in which animals received infusions into the lateral ventricle, CGP 36216 was dissolved in 0.9% saline at 1 mM, 2 mM, or 3 mM concentrations. Subsequent experiments used a 3 mM concentration. Animals received infusions (1 μl infused at a rate of 0.2 μl per min for ICV; and 0.5 μl at a rate of 0.1 μl per min for site-specific infusions) of CGP 36216 or vehicle (0.9% saline from a 5 μl Hamilton syringe operated by a micro-infusion pump (Harvard Apparatus). Infusion needles were left in place for 1 min following infusion completion to ensure diffusion into the brain. During the infusion, mice were placed in a clean housing cage and allowed to walk freely with only brief restraint to place and remove infusion needles. Animals received infusions either 30 min before training, immediately following training, or 30 min before testing for ICV experiments. For experiments in which CGP 36216 or vehicle was locally infused into the hippocampus or ACC, animals received infusions immediately after training or 5 min before context fear testing. The timing for behavior following infusions was chosen to allow enough time for circulation throughout the brain (ICV) or to assess near-immediate effects locally within the hippocampus or ACC (site-specific infusions).