Blocking Dopaminergic Signaling Soon after Learning Impairs Memory Consolidation in Guinea Pigs

Abstract

Formation of episodic memories (i.e. remembered experiences) requires a process called consolidation which involves communication between the neocortex and hippocampus. However, the neuromodulatory mechanisms underlying this neocortico-hippocampal communication are poorly understood. Here, we examined the involvement of dopamine D1 receptors (D1R) and D2 receptors (D2R) mediated signaling on memory consolidation using the Novel Object Recognition (NOR) test. We conducted the tests in male Hartley guinea pigs and cognitive behaviors were assessed in customized Phenotyper home cages utilizing Ethovision XT software from Noldus enabled for the 3-point detection system (nose, center of the body, and rear). We found that acute intraperitoneal injections of either 0.25 mg/kg SCH23390 to block D1Rs or 1.0 mg/kg sulpiride to block D2Rs soon after acquisition (which involved familiarization to two similar objects) attenuated subsequent discrimination for novel objects when tested after 5-hours in the NOR test. By contrast guinea pigs treated with saline showed robust discrimination for novel objects indicating normal operational processes undergirding memory consolidation. The data suggests that involvement of dopaminergic signaling is a key post-acquisition factor in modulating memory consolidation in guinea pigs.

Fig 1. Phenotyper cages from Noldus used to conduct novel object recognition tests.

In (A) is shown the top view of the Phenotyper cage configured as an open arena for animal habituation prior to the start of the object recognition testing as described in the main text. This was proceeded with the addition of two familiar objects (B) followed by replacement of one familiar object and one novel object (C). Note that the objects were placed centrally in all cases. Furthermore, the different zones in the cage were digitally demarcated using EthoVision XT 7 acquisition and analyses software. Automated recordings could be conducted continuously unobtrusively and with experimenter out of sight. In (D) is shown the experimental protocol that was followed to conduct the NOR tests as described in detail in the text.

Fig 2. Activity profiles recorded from representative guinea pigs from the treatment groups.

Each row presents data from the same guinea pig from the indicated treatment group whereas columns reflect the different behavior phases assessed. In each case, EthoVision XT 7.0 software tracked the guinea pig activities using a three point detection system (i.e. nose, center of body, and rear) but only the nose tracks (light blue with arrow head denoting head direction) are shown in the records. In (A-C) note the heavy ‘traffic’ in peripheral regions relative to the central portions of the cage during habituation. During the familiarization phase (D-F) guinea pigs explored two similar objects equally well and this was prior to drug administrations. In (G-F) are shown track marks following drug treatments–i.e. soon after familiarization phase–and then a 5-hour interval. Note that the guinea pig treated with saline spent more time with the novel object (G), whereas the guinea pig treated with sulpiride (H) or SCH23390 (I) did not show preferences for the novel object (star-shaped object) relative to the familiar object (bell-bar object). Abbreviations: SAL, saline group; SUL, sulpiride group, and SCH, SCH23390 group.

Fig 3. Time spent with two similar objects during familiarization phase.

In (A) is group data showing the amount of time guinea pigs spent with the left (unfilled circles) and right (filled squires) objects of the same type. Data is given in 30 second bins and the recording was for 3 minutes. Each plotted point is mean ± SEM. In (B) is summary of group data showing there were insignificant differences between the total time spent exploring the left and right objects during the 3 minute observation period (paired t(41) = 0.8845, p > 0.05).

Fig 4. Blocking D1R and D2R with selective antagonists impairs object discrimination.

Guinea pigs from the saline group (A) spent a significantly greater proportion of time exploring novel objects within the initial 30 seconds time interval but this tapered down over the 3-minute test window. By contrast, guinea pigs from both the SCH23390 group (B), or the sulpiride group (C), did not exhibit this characteristic feature during the initial 30 second time interval or any other interval over the 3-minute observation period. Note that the object exploration time does not evolve with time in the SCH23390 animal group which reflects the failure of habituation response during the test. Only guinea pigs in the saline group spent significantly more cumulative time with the novel object versus the familiar object (D), whereas there were insignificant differences in time spent between the novel and familiar objects in the SCH23390 group (E) or the sulpiride group (F). Group data (G) for calculated discrimination index revealed significant preferences for novel objects relative to familiar objects in guinea pigs injected with saline, but not SCH23390 or sulpiride [F(2, 32) = 8.791; p = 0.0008; within subject 1-way ANOVA with Tukeys post-hoc tests]. Only guinea pigs that exhibited the same preferences for FAM1 and FAM2 objects during the familiarization phase were used here. Asterisks indicate statistical differences and further discussed in text.

Fig 5. Changes in locomotor activity in the three treatment groups.

Data shows that the total distances travelled during the testing phase versus the testing phase was significantly less in the saline group, but not in both the SCH23390 group and sulpiride group. The findings are suggestive of the wearing down of the ‘novelty’ for the new object in the control guinea pigs with progression of time in the observation cage.