Exposure to an enriched environment selectively increases the functional response of the pre-synaptic NMDA receptors which modulate noradrenaline release in mouse hippocampus

We evaluated the impact of environmental training on the functions of pre-synaptic glutamatergic NMDA and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and nicotinic receptors expressed by hippocampal noradrenergic nerve terminals. Synaptosomes isolated from the hippocampi of mice housed in enriched (EE) or standard (SE) environment were labeled with [H-3] noradrenaline ([H-3]NA) and tritium release was monitored during exposure in superfusion to NMDA, AMPA, epibatidine or high K+. NMDA-evoked [H-3] NA release from EE hippocampal synaptosomes was significantly higher than that from SE synaptosomes, while the [H-3] NA overflow elicited by 100 mu M AMPA, 1 mu M epibatidine or (9, 15, 25 mM) KCl was unchanged. In EE mice, the apparent affinity of NMDA or glycine was unmodified, while the efficacy was significantly augmented. Sensitivity to non-selective or subtype-selective NMDA receptor antagonists (MK-801, ifenprodil and Zn2+ ions) was not modified in EE. Finally, the analysis of NMDA receptor subunit mRNA expression in noradrenergic cell bodies of the locus coeruleus showed that NR1, NR2A, NR2B and NR2D subunits were unchanged, while NR2C decreased significantly in EE mice as compared to SE mice. Functional up-regulation of the pre-synaptic NMDA receptors modulating NA release might contribute to the improved learning and memory found in animals exposed to an EE.