Cholinergic mechanisms of the context preexposure facilitation effect in adolescent rats

Abstract

The context preexposure facilitation effect (CPFE) is a variant of contextual fear conditioning in which context learning, context-shock association, and expression of context conditioning occur in 3 separate phases-preexposure, training, and testing. During the preexposure phase, the CPFE is disrupted by hippocampal NMDA receptor blockade in juvenile rats (Schiffino et al., 2011), and a similar deficit is seen with a subcutaneous injection of the muscarinic receptor antagonist, scopolamine, in adult mice (Brown, Kennard, Sherer, Comalli, & Woodruff-Pak, 2011). As a foundation for further developmental research, the present study examined the role of cholinergic function in the CPFE in adolescent rats during each phase of the CPFE protocol. In Experiment 1, an i.p injection of either 0.5 or 1.0 mg/kg dose of scopolamine administered prior to all 3 phases of the CPFE protocol impaired the CPFE. Experiment 2 further showed that a 0.5 mg/kg injection prior to just 1 of the 3 phases of the CPFE also disrupted contextual fear conditioning. We further showed that the CPFE is impaired by localized scopolamine infusions into dorsal hippocampus on the preexposure day (Experiment 3a), training day (Experiment 3b), and test day (Experiment 3c). These findings demonstrate a role of cholinergic signaling in hippocampus during each of the 3 phases of the CPFE in adolescent rats. Implications for the development and neural basis of the CPFE are discussed. (PsycINFO Database Record

Figure 1

Mean (± SE) percent test freezing as a function of group (No-Pre, Low, High, or Saline). The animals in the Low, High, or Saline group were pre-exposed to the training and testing context on preexposure day. The No-Pre group was pre-exposed to an alternate context on the preexposure day (and was comprised of subgroups from the other treatment conditions, which were pooled because they did not differ). The saline group significantly differed from all three groups (p < 0.01).

Figure 2

Mean (± SE) percent test freezing as a function of group (Sal, Pre, Train, or Test). All animals were preexposed to the training and testing context on preexposure day. The Sal group significantly differed from all other groups (p < 0.01). Overall, scopolamine impaired the CPFE regardless of the day of administration.

Figure 3

Schematic representation of injection cannula tip placements in the dHPC in Experiment 3. Animals included in final analyses are represented by filled black dots while animals excluded as dHPC misses are black and white stripped. Placements ranged from −2.52mm to −4.68mm from Bregma. From The Rat Brain in Stereotaxic Coordinates (5th ed.), 140, 146, 152, 158, and 176 by G. Paxinos & C. Watson, 2005, New York, NY: Academic Press. Copyright 2005 by Elsevier Academic Press. Adapted (or reprinted) with permission.

Figure 4

Mean (± SE) percent test freezing as a function of group (No-Pre, Pre-Scop, Pre-PBS and Pre-UND) in Experiment 3a. Pre animals (Pre-Scop, Pre-PBS and Pre-UND) were preexposed to the training and testing context on preexposure day. No-Pre animals (pooled from Scop and PBS subgroups) were preexposed to the alternate context, however training and testing were in the same context as the Pre groups. The Pre-UND and Pre-PBS groups were significantly different from the (pooled) No-Pre and Pre-Scop (p < 0.05). Overall, scopolamine impaired the CPFE when administered intrahippocampally before the preexposure day.

Figure 5

Schematic representation of injection cannula tip placements in the dHPC in Experiment 4. Animals included in final analyses are represented by filled black dots while animals excluded as dHPC misses are black and white stripped. Placements ranged from −2.40mm to −4.92mm from Bregma. From The Rat Brain in Stereotaxic Coordinates (5th ed.), 138, 144, 152, 160, 172, and 180 by G. Paxinos & C. Watson, 2005, New York, NY: Academic Press. Copyright 2005 by Elsevier Academic Press. Adapted (or reprinted) with permission.

Figure 6

Mean (± SE) percent test freezing as a function of group (No-Pre, Pre-Scop, Pre-PBS and Pre-UND) in Experiment 3b. The No-Pre group combined Scop and PBS subgroups. The Pre-UND and Pre-PBS groups were significantly different from the No-Pre and Pre-Scop (p < 0.05). Overall, scopolamine impaired the CPFE when administered intra-hippocampally before the training day.

Figure 7

Schematic representation of injection cannula tip placements in the dHPC in Experiment 5. Animals included in final analyses are represented by filled black dots while animals excluded as dHPC misses are black and white stripped. Placements ranged from −2.16mm to −4.44mm. From The Rat Brain in Stereotaxic Coordinates (5th ed.), 134, 144, 152, 160, and 172 by G. Paxinos & C. Watson, 2005, New York, NY: Academic Press. Copyright 2005 by Elsevier Academic Press. Adapted (or reprinted) with permission

Figure 8

Mean (± SE) percent test freezing as a function of group (Pooled No-Pre, Pre-Scop, Pre-PBS and Pre-UND) in Experiment 3c. The No-Pre group combined Scop and PBS subgroups. The Pre-UND and Pre-PBS groups were significantly different from the No-Pre and Pre-Scop groups (p < 0.05). Overall, scopolamine impaired the CPFE when administered intra-hippocampally before the testing day.