Exposure to enriched environment decreases neurobehavioral deficits induced by neonatal glutamate toxicity

Abstract

Environmental enrichment is a popular strategy to enhance motor and cognitive performance and to counteract the effects of various harmful stimuli. The protective effects of enriched environment have been shown in traumatic, ischemic and toxic nervous system lesions. Monosodium glutamate (MSG) is a commonly used taste enhancer causing excitotoxic effects when given in newborn animals. We have previously demonstrated that MSG leads to a delay in neurobehavioral development, as shown by the delayed appearance of neurological reflexes and maturation of motor coordination. In the present study we aimed at investigating whether environmental enrichment is able to decrease the neurobehavioral delay caused by neonatal MSG treatment. Newborn pups were treated with MSG subcutaneously on postnatal days 1, 5 and 9. For environmental enrichment, we placed rats in larger cages, supplemented with different toys that were altered daily. Normal control and enriched control rats received saline treatment only. Physical parameters such as weight, day of eye opening, incisor eruption and ear unfolding were recorded. Animals were observed for appearance of reflexes such as negative geotaxis, righting reflexes, fore- and hindlimb grasp, fore- and hindlimb placing, sensory reflexes and gait. In cases of negative geotaxis, surface righting and gait, the time to perform the reflex was also recorded daily. For examining motor coordination, we performed grid walking, footfault, rope suspension, rota-rod, inclined board and walk initiation tests. We found that enriched environment alone did not lead to marked alterations in the course of development. On the other hand, MSG treatment caused a slight delay in reflex development and a pronounced delay in weight gain and motor coordination maturation. This delay in most signs and tests could be reversed by enriched environment: MSG-treated pups kept under enriched conditions showed no weight retardation, no reflex delay in some signs and performed better in most coordination tests. These results show that environmental enrichment is able to decrease the neurobehavioral delay caused by neonatal excitotoxicity.

Figure 1

Daily changes in body weight (*: p < 0.05, **: p < 0.01, ***: p < 0.001 control monosodium glutamate (MSG) vs. all other groups).

Figure 2

(A) Development of physical parameters: Eye opening, incisor eruption and ear unfold; (B) Ear twitch reflex and eyelid reflex.

Figure 3

Development of reflex performance. (A) Gait performance; (B) Air righting reflex appearance; (C) Surface righting reflex performance; (D) Negative geotaxis performance (*: p < 0.05).

Figure 4

Motor coordination tests I: Grid walking and footfault test. (A) Number of total steps; (B) Number of total faults; (C) Total faults/total steps ratio; (D) Number of forelimb faults (*: p < 0.05).

Figure 5

Motor coordination tests II. (A) Rope suspension test; (B) Inclined board test; (C) Rota-rod test; (D) Walk initiation test—inner circle.