Environmental Enrichment and Postnatal Handling Prevent Spatial Learning Deficits in Aged Hypoemotional (Roman High-avoidance) and Hyperemotional (Roman Low-avoidance) Rats

The p r e s e n t s tudy invest igated the e n d u r i n g effects of pos tna ta l h a n d l i n g ( admin i s t e r ed d u r i n g the first 21 days of life), a n d e n v i r o n m e n t a l e n r i c h m e n t (for a pe r i od of 6 m o n t h s s tar t ing 3 weeks after w e a n i n g ) o n spatial l e a rn ing in 2 4 m o n t h o l d h y p o e m o t i o n a l (Roman h igh-avoidance , RHA/Verh) and h y p e r e m o t i o n a l (Roman low-avoidance, RLANerh) rats. Two g roups of 5 -month -o ld rats f rom bo th l ines were also i n c l u d e d in the e x p e r i m e n t as y o u n g controls . The R o m a n l ines p e r f o r m e d dif ferent ly in the Morris water maze: Path l eng ths of RLANerh rats we re sho r t e r and they swam at lower speed than R H A N e r h rats, showing quicker and m o r e eff ic ient learning overall. Postnatal h a n d l i n g i m p r o v e d l ea rn ing ma in ly in RHA/Verh rats, w h e r e a s e n v i r o n m e n t a l e n r i c h m e n t was able to p r e v e n t the deficits s h o w n by aged cont ro ls of bo th l ines. Young, en r i ched , a n d h a n d l e d p lus e n r i c h e d an imals exh ib i t ed be t te r p e r f o r m a n c e than i m p a i r e d aged controls , to the p o i n t tha t aged e n r i c h e d and h a n d l e d p lus e n r i c h e d an imals did n o t differ f rom y o u n g contro ls . Thus , bes ides ind ica t ing that R I ~ N e r h rats are be t t e r l ea rne r s t h a n R H A N e r h rats in the Morris water maze, this s tudy d e m o n s t r a t e s that e n v i r o n m e n t a l e n r i c h m e n t p reven t s the cogni t ive loss associated wi th aging, over the long te rm. Finally, the posi t ive effects ob t a ined wi th postnata l h a n d l i n g w e r e d e p e n d e n t on the rat l ine. I n t r o d u c t i o n Early exposure of animals to enriched environment leads to an enhancement of brain growth and increases in the density of neurons and synaptic contacts, as well as the size of neuronal nuclei and the number of dendritic branches (Diamond 1988; Greenough et al. 1990). Several behavioral studies have also demonstrated that enriched rats are high explorers and good performers in several labyrinths and conflict tasks (Renner and Rosenzweig 1987; Escorihuela et al. 1994), two properties that might be related with the modifications induced by early enrichment in neuronal substrates underlying memory and learning (Paylor et al. 1992) or on neuroendocrine factors at the hippocampal level (Olsson et al. 1994). Nevertheless, there are only a few studies on the effects of environmental enrichment as a procedure for preventing the learning deficits associated with aging (Van Gool et al. 1985). On the other hand, neonatal handling during the first 21 days of life has been shown to reduce emotional reactivity as measured by defecation, ambulation, or exploratory activity in novel situations (DeNelsky and Denenberg 1967; Levine et al. 1967), by eating measures in the hyponeophagia test (Ferr¢~ et al. 1995) and by the ability to cope LEARNING & MEMORY 2:40-48 © 1995 by Cold Spring Harbor Laboratory Press ISSN1072-0502/95 $5.00