Hippocampal GABAB(1a) Receptors Constrain Generalized Contextual Fear

Abstract

Many anxiety disorders are characterized by generalization of fear responses to neutral or ambiguous stimuli. Therefore, a comprehensive understanding of the mechanisms contributing to generalized fear is essential for formulating successful treatments for anxiety disorders. Previous research shows that GABA-mediated presynaptic inhibition has a critical role in cued fear generalization, as animals with genetically deleted presynaptic GABA_B(1a) receptors cannot discriminate between CS+ and CS- tones. Work from our laboratory has further identified that GABA_B(1a) receptors are necessary for maintaining contextual memory precision, thereby constraining generalized contextual fear. We previously found that GABA_B(1a) KO mice show generalized fear to a neutral context 24 h after training, but not 2 h after training. A similar pattern was observed with object location and recognition, suggesting that this receptor subtype affects consolidation and/or retrieval of precise contextual and spatial memories. Here we sought to specifically examine the involvement of GABA_B(1a) receptors in consolidation or retrieval of a precise fear memory. To do so, we infused a selective GABA_B(1a) receptor antagonist, CGP 36216, intracerebroventricularly (ICV), or locally into the dorsal hippocampus, ventral hippocampus, or anterior cingulate cortex (ACC), during consolidation and retrieval of context fear training. Blockade of GABA_B(1a) receptors through ICV, dorsal hippocampal, or ventral hippocampal infusions 'after' training (consolidation) resulted in fear generalization to the neutral context when mice were tested 24, but not 6 h after training. Post-training infusions of CGP into the ACC, however, did not promote generalized fear. In addition, ICV, dorsal hippocampal, ventral hippocampal, or ACC infusions immediately 'before' testing (retrieval) did not result in context fear generalization. These data suggest that GABA-mediated presynaptic inhibition is not critical for retrieval of precise contextual memory, but rather has an important role in the long-term consolidation of precise contextual memories and constrains generalized fear responses.

Figure 1

Infusion site verification for the dCA1, vCA1 and ACC. Local infusion site verification for all animals having cannula aimed at (a) the dCA1, (b) the vCA1, or (c) the ACC. For the ACC site verification is depicted unilaterally for ease, but the hemisphere for guide cannula implantation was varied across animals. Black dots represent the site of infusion for one or more animals. Drawings are adapted from Paxinos and Franklin, 2004.

Figure 2

Central inhibition of GABA_B(1a) receptors promotes generalized contextual fear. Animals received infusions into the lateral ventricle before training and were tested at different retention intervals. (a) Schematic representation of the experimental paradigm. Animals received infusions of vehicle or CGP 36216 into the lateral ventricle 30 min before context fear training and then were tested at distinct retention intervals in either the training or a neutral context. (b) Animals given infusions of CGP 36216 at 3 mM dose displayed impaired memory precision to the neutral context compared with vehicle-treated animals when tested 24 h after training. (c) When testing occurred 6 h after training, 3 mM CGP 36216 was not sufficient to impair memory precision. (d) Generalization was not due to differences in locomotor activity between vehicle and CGP 36216-treated mice. No differences were observed in distance traveled within an open field between vehicle and CGP 36216-treated mice. Values are displayed as mean (±SEM). Significance values were set at p<0.05. (*=p<0.05, **=p<0.01.)

Figure 3

Pre-testing or post-training inhibition of GABA_B(1a) receptors promotes generalized contextual fear. Animals received infusions into the lateral ventricle immediately following training or 30 min before testing. (a) Schematic representation of the experimental paradigm. Animals received infusions into the lateral ventricle immediately following training and were tested at 24 h. (b) Animals receiving infusions of CGP 36216 immediately following fear training, displayed impaired memory precision when tested 24 h later. (c) Schematic representation of the experimental paradigm. Animals received infusions into the lateral ventricle 30 min before memory testing. (d) When animals received infusions 30 min before testing, CGP 36216 did not impair memory precision. Values are displayed as mean (±SEM). Significance values were set at p<0.05. (*=p<0.05, **=p<0.01.)

Figure 4

Hippocampal GABA_B(1a) receptors constrain contextual fear generalization. Animals received infusions into the dorsal or ventral CA1 region of the hippocampus immediately after training or 5 min before testing. (a) Schematic representation of the experimental paradigm. Animals received infusions immediately following fear training and were tested 24 h later. (b) When infusions were given into the dorsal hippocampus, CGP impaired memory precision. (c) Infusions of CGP into the ventral hippocampus also impaired memory precision. (d) Schematic representation of the experimental paradigm. Animals received infusions 5 min before testing. (e) Infusions of CGP into the dorsal hippocampus before testing was not sufficient to impair memory precision. (f) Infusions of CGP into the ventral hippocampus before testing was not sufficient to impair memory precision. Values are displayed as mean (±SEM). Significance values were set at p<0.05. (*/#=p<0.05, **=p<0.01, ***=p<0.001.) (g) Generalization was not due to differences in locomotor activity between vehicle and CGP 36216-treated mice. No differences were observed in distance traveled within an open field between vehicle and CGP 36216-treated mice.

Figure 5

Inhibition of GABA_B(1a) receptors in the ACC does not promote generalized contextual fear. Animals received infusions into the ACC immediately after training or 5 min before testing. (a) Schematic representation of the experimental paradigm. Animals received infusions immediately following fear training and were tested 24 h later (b). When infusions were given into the ACC following fear training, CGP did not impair memory precision. (c) Schematic representation of the experimental paradigm. Animals received infusions 5 min before testing. (d) Infusions of CGP into the ACC before testing was not sufficient to impair memory precision. Values are displayed as mean (±SEM). Significance values were set at p<0.05. (*=p<0.05, **=p<0.01.)