Elemental signature of memory recall

(1) Physics Institute, Federal University of Rio Grande do Sul, Bento Gonçalves 9500, 91501970- Porto Alegre, Brazil
(2) Physics, Statistics and Mathematics Institute, Federal University of Rio Grande, Rua Barao do Cai, 95500000- Santo Antonio da Patrulha, Brazil
(3) Jožef Stefan Institute, Jamova Cesta 39, 1000-Ljubljana, Slovenia
(4) General Physics Department, University of São Paulo, Rua do Matão 187, 05315970-Sao Paulo, Brazil

The aim of this work is to obtain elemental maps of rat brain tissues during different stages of memory formation and recall. The experiment was performed considering four distinct situations related to different stages of Inhibitory Avoidance (IA) task according a protocol which we developed to study memory formation and maintenance in rats. Such situations correspond to: (1) control group – rats where we could found the baseline concentration of Cu, Fe, Zn, Ca, Cl, K, Mg, Na, P and S. Those Trace Elements (TE) are participating direct of memory formation and maintenance; (2) IA memory formation – rats passed through IA training session to established a new memory related to association between the electrical shock and the dangerous context in which the shock was applied; (3) IA weak memory recall - rats passed through IA training session to formed a memory and a day later, accessed the context in which the memory was formed with a single reactivation session of 15 seconds; (4) IA strong memory recall - the animals passed basically through the same experimental protocol as groups 2 and 3 with three long reactivation sessions (3 minutes each). All rats were sacrificed at the same time, under the same conditions. The tissues were immediately cryofixed in liquid propane cooled by liquid nitrogen and then freeze-dried. The samples were cross-sectioned with an ANCAP cryostat (ANCAP, São Paulo, Brazil). The cryostat chamber temperature was fixed at −15 °C and the sectioning  thickness was 90 μm. Samples were mounted between two layers of Formvar film. Micro-PIXE analysis was performed using the nuclear microprobe at the Jožef Stefan Institute (Ljubljana, Slovenia). A proton beam with energy of 3 MeV and a diameter varying from 1 μm to 1.5 μm at ion currents ranging from 40 pA to 500 pA was used. Our results indicated that the TE concentration pattern doubles after the acquisition of new memory and its subsequently formation. The only exceptions were observed for Ca and Fe, whose baseline have the inverse ratio. All TE showed the same concentration pattern at re-exposure in the test session. A single brief reactivation causes a decrease of the TE concentration. Moreover, TE levels restored to the same level presented during memory formation when reactivation was long and happened three times. These results may suggests that: (a) TE indeed participates in memory formation and maintenance processes and it can be studied by PIXE; (b) TE related to memory formation also display a hole in the recall process and their concentrations have a positive direct relationship with the total or partial IA MT recall; (c) the phenomenon of reconsolidation can actually mean something but a simple second consolidation based on TE concentration pattern.