Resumo

Objectives Human episodic memory refers to the capability of recollecting a specific event (what), as well as when and where it occurred. There are many studies in non-human mammals describing the role of the hippocampus in the formation of episodic memories, but most of the protocols used in these studies do not involve at the same time the three aspects of episodic memory. Dere et al have developed a three-trial object exploration task in rodents that can simultaneously access what, when and where (Neurobiol Learn Mem 84:214, 2005). The goal of this study was to evaluate the effects of the pharmacological inactivation of the dorsal hippocampus on the acquisition of this “what, where, when” object exploration task. Methods and Results Three-month male Wistar rats (250 – 350 g) were bilaterally implanted with intrahippocampal cannulas. After two days of post-surgical recovery, animals were subjected to the experiment. Muscimol (0,250 mg/mL/side; 0,5 µl) was used to inactivate the hippocampus and saline (0,9% NaCl; 0,5 µl) was used as a control. Both were injected fifteen minutes before training. The task consisted of two sample trials and one test trial, all 5 minutes long. In the first sample trial, the rat was placed in the open-field with four copies of object A. After 1 h, in the second sample trial, the animals were exposed to four copies of object B in a different spatial configuration. The test trial was held 24 h later. In this trial, 2 copies of each object encountered during the sample trials were present. One object of the first trial was in a different spatial location. All the other objects were in the same positions found during sample trials. In the first minute of the test trial, saline-injected rats showed increased exploration of old displaced object when compared to old stationary object (exploration rates, mean + SE: 0.44 + 0.06; 0.23 + 0.05). When the whole session was analyzed, saline rats spent more time exploring old objects than recent objects (0.59 + 0.04; 0.41 + 0.04). Muscimol-injected rats equally explored the old displaced object and the old stationary object (0.29 + 0.06; 0.41 + 0.06) within the first minute of the test trial. However, they spent more time exploring old objects than recent objects (0.57 + 0.03; 0.43 + 0.03) when the whole test session was considered. Conclusions Pretraining infusion of muscimol in the dorsal hippocampus impairs the acquisition of the combined what-where-when memory. However, integrity of the dorsal hippocampus does not seem essential, at least in this task, for the acquisition of the separate “what” and “when” components of episodic-like memories.