Stress hormones enhance retrieval of fear conditioning acquired either one day or many months before.
It has been known for years that systemic administration of the stress hormones, adrenocorticotrophin (ACTH), lysine-vasopressin, adrenaline, or beta-endorphin, enhances retrieval of aversive behaviours acquired one or a few days before. Here we show that the pre-test i.p. injection of the hormones in rats can also enhance retrieval when given months after the original training. The effectiveness of the treatments changed with time. When animals were tested 3 months after training the hormones enhanced retrieval only at doses five times higher than those needed 1 day after training. Between 6 and 9 months from training the hormones either lost their effect (vasopressin, beta-endorphin) or actually inhibited retrieval (ACTH, adrenaline). The effects of the hormones cannot be explained by a decrease in locomotor activity: none of the treatments had such an effect, as measured in an open field. However, when the animals were tested between 12 and 19 months after training, the hormones once again became as effective as they had been 1 day after training. This was so in spite of the fact that control retention levels became very low with age, probably as a result of extinction. The oscillation of the sensitivity of retrieval to the hormones does not appear to depend on changes in anxiety levels with ageing or to effects of the hormones on locomotor activity.
Izquierdo
L A
LA
Centro de Memória, Departamento de Bioquímica, Instituto de Ciencias Básicas da Saúde, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil. izquier@terra.com.br
Barros
D M
DM
Medina
J H
JH
Izquierdo
I
I
D000324:Adrenocorticotropic Hormone / Q000494:pharmacology; D000818:Animals; D001007:Anxiety / Q000523:psychology; D001362:Avoidance Learning / Q000187:drug effects; D003213:Conditioning, Psychological / Q000187:drug effects*; D004597:Electroshock; D004837:Epinephrine / Q000494:pharmacology; D005108:Extinction, Psychological / Q000187:drug effects; D005239:Fear / Q000187:drug effects*; D006728:Hormones / Q000494:pharmacology*; D008236:Lypressin / Q000494:pharmacology; D008297:Male; D008568:Memory / Q000187:drug effects*; D009043:Motor Activity / Q000187:drug effects; D051381:Rats; D017208:Rats, Wistar; D013315:Stress, Psychological / Q000503:physiopathology*; D001615:beta-Endorphin / Q000494:pharmacology
D006728:Hormones; D008236:Lypressin; D001615:beta-Endorphin; D000324:Adrenocorticotropic Hormone; D004837:Epinephrine