Influence of environmental enrichment on morphine-exposed neonate rats : effect on neurodevelopment and long-term memory

Abstract

The stressful stimuli exposure and the use of drugs in early life may affect neurodevelopment, altering behaviors, memory and nociceptive response. These factors have been related to the delay in recovery of patients after medical procedures in hospitals. In this study, we investigated the influence of environmental enrichment (EE) on morphine exposure effect on neurodevelopment and memory of neonate rats. 28 pups were divided into four experimental groups: saline + standard housing (SS), saline + environmental enrichment (SE), morphine + standard housing (MS) and morphine + environmental enrichment (ME). The newborns received daily (P8-P14) subcutaneous injections of saline (5 μl) or morphine (5 μg/5 μl), and they were submitted to manipulation or EE (P8-P21). The righting reflex and the negative geotaxis, the object recognition and the hot plate tests were used to evaluate neuromotor reflexes, long-term memory and thermal hyperalgesia, respectively. Animals that received morphine showed longer neuromotor reflexes response time compared to the saline group, an effect that was age-dependent. Animals that received morphine showed less ability to recognize a new object in the environment, an effect that was partially reversed by EE. Nociceptive response was not altered for morphine nor EE. These findings demonstrate that neonatal morphine exposure alters the neurodevelopment and long-term memory in pup rats, without affecting nociceptive response. Thus, EE can be a promising non-pharmacological treatment to the consequences of neonatal noxious stimuli.