Are Hippocampal NMDA and Muscarinic Receptors Required for Memory Consolidation? Open field exposure reverts scopolamine and MK801 instigated retrograde amnesia of inhibitory avoidance in the rat”

SNITCOFSKY MARINA; COLETTIS NATALIA; KORNISIUK EDGAR; SANTANA FABIANA; QUILLFELDT JORGE; JERUSALINSKY DIANA

Florencia

Congreso; IBRO Congress 2011; 2011

IBRO - International Brain Research Organization

We have studied the performance of adult Wistar rats in inhibitory avoidance (IA) by recording training and test latencies (24 h later) to step-down from an isolated platform onto a grid, where the animal gets a mild footshock. Learning criteria –i.e. significant difference between test (higher) and training latencies- was reached with 0.5mA or higher shocks. We then evaluated the influence of a previous exposure to an Open Field (OF) on rat’s performance in IA. It had been reported a promotor effect of a novel environment over IA trained with underthreshold stimulus (Moncada & Viola, 2007). Although we did not observe any facilitation in IA after a single exposure for either 3 or 5 minutes to the OF, rats twice exposed to the OF for 3 min in two consecutive days showing habituation to the arena, and trained in IA with a stronger stimulus (0.75 mA or higher), showed facilitation of LTM expression of IA in the test session 24 h later (Colettis et al., 2009). When rats were trained with a threshold shock of 0.5 mA, all groups either exposed or not to the OF reached the learning criteria and formed a LTM, but median of latencies were not significantly different between groups. We demonstrated that cholinergic system in dorsal hippocampus modulates memory consolidation of IA in a positive way (Jerusalinsky et al., 1993; Jerusalinsky y Harvey, 1994; Jerusalinsky et al., 1997), and when a muscarinic antagonist, scopolamine, is injected post training, memory is impaired. Similarly NMDA receptors of dorsal hippocampus are needed to EI consolidation, and its antagonist, MK-801, produces amnesia (Jerusalinsky et al., 1992). Therefore, we trained rats in IA with a threshold footshock of 0.5 mA and injected intrahippocampus the muscarinic antagonist scopolamine (8 µg per side) or the NMDA antagonist MK-801 (2μg per side) or the vehicle immediately after IA training; both drugs were amnesic. Nevertheless, if rats were exposed twice to the OF for 3 minutes previous to IA training and then were injected with either scopolamine or MK-801 immediately after training, they reached the learning criteria expressing a conspicuous LTM of IA 24 h later: their test latencies were not significantly different from saline injected or naive controls. The results corroborate the retrograde amnesic effect produced both by scopolamine and MK-801 when injected intrahippocampus post training in IA. On the other hand, these amnesic effects were reverted when animals where previously exposed twice to the OF. In conclusion, a non aversive experience, the exposure to the same arena in two consecutive days before training in an aversive associative task allowed to overcome retrograde amnesia instigated either by scopolamine or MK801.