Research PapersStress hormones enhance retrieval of fear conditioning acquired either one day or many months beforeIzquierdo, L.A.a; Barros, D.M.b; Medina, J.H.c; Izquierdo, I.aAuthor Information aCentro de Memória, Departamento de Bioquímica, Instituto de Ciencias Básicas da Saúde, Universidade Federal do Rio Grande do Sul, Porto Alegre bDepartamento de Ciencias Fisiológicas, Setor de Farmacologia, Fundação Universidade Federal do Rio Grande, Rio Grande, Brazil cInstituto de Biología Celular y Neurociencia Eduardo De Robertis, Facultad de Medicina, Universidad de Buenos Aires, Buenos Aires, Argentina Correspondence to Luciana A. Izquierdo, Centro de Memória, Departamento de Bioquímica, Instituto de Ciencias Básicas da Saúde, Universidade Federal do Rio Grande do Sul, Ramiro Barcellos 2600-anexo, (90035-003) Porto Alegre, RS, Brazil. E-mail: [email protected] Paper dedicated to the memory of our good friend Béla Bohus. Received 21 January 2002 accepted as revised 29 April 2002 Behavioural Pharmacology: May 2002 - Volume 13 - Issue 3 - p 203-213 Buy Abstract It has been known for years that systemic administration of the stress hormones, adrenocorticotrophin (ACTH), lysine-vasopressin, adrenaline, or β-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, β-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. © 2002 Lippincott Williams & Wilkins, Inc.