A Arte de esquecer

Iván Izquierdo

Instituto de Pesquisas BiomédicasCentro de Memória

Pontifícia Universidade Católica de Rio Grande do SulPorto Alegre, RS, Brasil

• “O aspecto mais notável da memória é o esquecimento” – James McGaugh

• De fato, esquecemos a imensa maioria das informações que adquirimos

• Os mecanismos da memória se saturam

• Funes o Memorioso – Jorge Luis Borges

• Há memórias que nos perturbam (medos, humilhações, maus momentos)

• Há outras que nos prejudicam (fobias)ou nos perseguem (estresse pós-traumático)

• Há memórias que nos impedem adquirir outras novas ou recordar outras antigas, mais importantes

• ...novamente Funes o Memorioso: “...incapaz de esquecer para poder pensar, e para pensar é necessário esquecer para poder fazer generalizações” (Borges)

• Formas de “esquecimento”

• Extinção

• Repressão voluntária e involuntária

• Memórias que não ultrapassam a memória de trabalho OU AS FASES INICIAIS DA MEMÓRIA DE CURTA DURAÇÃO; só duram poucos minutos ou horas.

• Memórias que duram poucos dias e depois desaparecem

• Esquecimento real (as memórias desaparecem por atrofia sináptica (falta de uso).

One-trial, step-down, inhibitory avoidance (IA)

Características:

One-trial, step-down, inhibitory avoidance (IA)

1. La memoria asociada con esta tarea es adquirida en apenasuna sesión de entrenamiento, lo que la hace ideal para el estudiode las cascadas bioquímicas activadas durante la consolidaciónde memorias, sin la contaminación proveniente del proceso deexpresión que ocurre durante el aprendizaje de paradigmas desesiones múltiplas.

2. Representa una forma rápida y simple de aprendizaje queinvolucra una forma “universal” de memoria. Esto permite quelos eventos biquímicos iniciados por el entrenamiento puedan serseguidos de manera precisa y que los resultados obtenidospuedan ser extrapolados a otros sistemas.

ENTRENAMIENTOCS US+

TESTECS US+

24 h?

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The inhibition of acquired fearNeurotoxicity Research, 6; 2004

Fig 1 : Extinction of IA memory requires the normal functionality of several signaling pathways in thehippocampus and amygdala.

Animals with cannulae implanted in the CA1 region of the dorsal hippocampus or into the baso-lateral amygdala were trained (Tr) in IA using a 0.5 mA, 2 s footshock and submitted to 4 daily extinction sessions (T1 to T4).

Fifteen min before T1 the animals received through the implanted cannulae bilateral infusions of the RNA pol II inhibitor, DRB, theprotein synthesis inhibitor, anisomycin (ANI), the blocker of ERK1/2 activation, PD098059 or the PKA inhibitor, Rp-cAMPs.

The entorhinal cortex plays a role in extinctionNeurobiology of Learning and Memory, 86, 192-197, 2006

Animals trained in IA were submitted to 4 non-reinforced test sessions at 24, 48, 72 and 96 h after training (T1 to T4). Immediately after T1 animals received bilateral infusions of 10% DMSO in saline (VEH), AP5 (25 nmol/side), anisomycin (ANI; 300 nmol/side), PD98059 (5 nmol/side) or KN-93 (10 nmol/side) into the entorhinal cortex. Values are expressed as median interquartile range of step-down latency. n=14-22 per group; **p<0.01 and ***p<0.001 vs T1 in Dunn’s multiple comparisons after Friedman test for repeated measures. Please note in the upper right corner the schematic drawing taken the Atlas of Paxinos and Watson (1986) showing the location of the infusion sites in the entorhinal cortex (light gray)

Inhibition of mRNA and protein synthesis in the CA1 region of the dorsalhippocampus blocks reinstatement of an extinguished conditioned

fear response Journal of Neuroscience, 23; 2003

Fig 2: Presentation of the CS after enhanced extinction does not induce spontaneous recovery of the original CR.

Unimplanted animals were trained (T) in IA and tested for 5 consecutive days (TT1 – TT5; first test 24 h after training). During testsessions, animals were allowed to freely explore the training box for 30 sec after they stepped down from the platform.

To evaluate the spontaneous recovery of the avoidance response, animals were tested once again 8 d after the fifth extinctionsession (TT6). Data are expressed as median ± interquartile range of the step down latency (i.e. the time the animals spend

on the platform before stepping down to the grid). *p<0.01 vs T in Dunn’s poc hoc comparison after Kruskal-Wallis test.

Inhibition of mRNA and protein synthesis in the CA1 region of the dorsalhippocampus blocks reinstatement of an extinguished conditioned

fear response Journal of Neuroscience, 23; 2003

Fig 3: Retrieval enhancers are unable to recover the expressionof the original CR after enhanced extinction.

A. Animals bilaterally implanted with cannulae aimed to the CA1 region of thedorsal hippocampus were trained in IA and tested without reinforcer for 5consecutive days (TT1-TT5; first test 24 h after training). Fifteen minutesbefore TT5 the animals received bilateral intra-CA1 infusions of either saline(VEH), Sp-cAMPs (Sp), SKF38393 (SKF),or oxotremorine (OXO), or anintraperitoneal administration of saline (Sal) or ACTH1-24. B. Animals bilaterally implanted with cannulae aimed at the CA1 region weretrained as in A and, 15 min before a test session performed 24h after trainingthey received bilateral infusions of VEH, Sp, SKF or OXO, or an intraperitonealadministration of SAL or ACTH. *p<0.05 vs TT1 in a Mann-Whitney two-tailedtest.

Inhibition of mRNA and protein synthesis in the CA1 region of the dorsalhippocampus blocks reinstatement of an extinguished conditioned

fear response Journal of Neuroscience, 23; 2003

Fig 4: Intrahippocampal ANI and DRB block reinstatement of the avoidance response after enhanced extinction.

Animals bilaterally implanted with cannulae aimed to the CA1 region on the dorsal hippocampus were trained in IA and tested for 4consecutive days (TT1 – TT4; first test 24 h after training. After that the animals were randomly assigned to four different groups.

Fifteen minutes before the fifth session (TT5), each experimental group received bilateral intra-CA1 infusions of saline (Sal),anisomycin (ANI), 0.1% DMSO in saline (VEH) or DRB. During this session, instead of being allowed to freely explore the training

box, rats received a scrambled electric footshock equal to that received in the training session immediately after they stepped downto the grid. Retention was measured in a subsequent test session performed 24 h later (TT6). *p<0.001 vs VEH or Sal groups

at TT6 in a Mann-Whitney two-tailed test.

LTMLTMA

LTMLTMSTMB

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LTMLTMC

Relationship between short- and long-term memory andshort- and long-term extinction Neurobiology of Learning & Memory, 84:25-32, 2005

(A) Animals were trained in IA and submitted to 6 non-reinforced test sessions at 24.0, 25.5, 27.0, 48, 49.5 and 51 h post-training. (B) Animals trained in IA received bilateral intra-CA1 infusions of vehicle (VEH), ANI (80 g/side) or AP5 (5 g/side) 15 min before the first of 3 non-reinforced test sessions at 24, 48 and 72 h after training. (C) Animals were trained in IA and 15 min before the first out 6 non-reinforced test sessions at 24.0, 25.5, 27.0, 48, 49.5 and 51 h post-training received bilateral intra-CA1 infusions as in (B); the arrow indicate the time of the infusion . (D) Animals were trained in IA and 15 min before the third out 6 non-reinforced test sessions at 24.0, 25.5, 27.0, 48, 49.5 and 51 h post-training received bilateral intra-CA1 infusions as in (B); the arrow indicate the time of the infusion. Values are expressed as median interquartile range. *p<0.005 vs test latency at 24 h after training in Mann-Whitney U test and #p<0.01 vs VEH in Dunn’s comparisons after Kruskal-Wallis test. (E) Drawing showing the location of all the infusion sites in the CA1 region of the dorsal hippocampus and photomicrograph showing the infusion site.

• Extinction is generated at the first CS-no US test through mechanisms involving NMDA receptors, CaMKII, PKA, ERKs, gene expression and protein synthesis in CA1, and PKA, gene expression and protein synthesis in the BLA and in the entorhinal cortex.

• In fear-motivated tasks protein synthesis and cell firing in the medial prefrontal cortex also plays a role in extinction. In other tasks other brain regions may also be involved (insula: CTA); in some aversive tasks the hippocampus is not involved.

• Extinction can be increased by enhancing the “no US” component. This is useful for psychotherapeutic purposes. It causes a suppression of the original task that may be viewed as very close to forgetting.

• There is a short form of extinction. There is continuity between short- and long-term extinction, established when short-term extinction begins.

• Extinction is the treatment of choice for fobias and PTSD

Conclusions on extinction

REPRESSÃO• O cérebro costuma reprimir a evocação de memórias

prejudiciais, ruins ou desagradáveis, de maneira inconsciente (without awareness)

• Também podemos conseguir isto de forma consciente , “forçando-nos” voluntariamente a esquecer esse tipo de memórias (“não quero lembrar da cara dessa pessoa”, etc.)

• Em ambos tipos de repressão intervém provavelmente o córtex prefrontal medial (“locus”da memória de trabalho e do “gerente geral de informações” na hora da aquisição ou evocação de memórias.

• Esse córtex age inibindo a atividade do hipocampo, principal “locus”da evocação de memórias

• Extinction and repression are in general processes which cause a reduction of retrieval, not real forgetting

• They might, however, reduce retrieval so much that they effectively function as forms of forgetting (or pseudo-forgetting)

• What about real forgetting due to a rapid decay of WM or STM? Many memories are retained for only a few hours: one often “forgets” what happened the day before, or a few hours ago. This is easily explained by deficient LTM formation.

• Other memories are retained for a few days and then forgotten: Recent experiments suggest that there are delayed post-acquisition maintenance processes that also depend on hippocampal protein synthesis.

Delayed memory processing

Early and delayed memory processing:Effects of antibody against BDNF

• Loss of memory after a few days may be viewed as real forgetting and does not (necessarily) result from synaptic loss or atrophy.

• It might instead obey to a failure or deficit of further, delayed posttraining hippocampal protein synthesis- and BDNF-mediated events.

Martin Cammarota (PUCRS)Monica Vianna (PUCRS)Lia Bevilaqua (PUCRS)Janine Rossato (PUCRS)Juliana Bonini (PUCRS)Jorge Medina (UBA)Pedro Beckinschtein (UBA)Lionel Müller Igaz (UBA)• FONCYT (Argentina), FAPERGS, CNPQ

(Brasil)