Activity-dependent regulation of NMDA receptors in substantia nigra dopaminergic neurones

摘要

N-Methyl-d-aspartate receptors (NMDARs) are Ca²⁺-permeable glutamate receptors that play a critical role in synaptic plasticity and promoting cell survival. However, overactive NMDARs can trigger cell death signalling pathways and have been implicated in substantia nigra pars compacta (SNc) pathology in Parkinson's disease. Calcium ion influx through NMDARs recruits Ca²⁺-dependent proteins that can regulate NMDAR activity. The surface density of NMDARs can also be regulated dynamically in response to receptor activity via Ca²⁺-independent mechanisms. We have investigated the activity-dependent regulation of NMDARs in SNc dopaminergic neurones. Repeated whole-cell agonist applications resulted in a decline in the amplitude of NMDAR currents (current run-down) that was use dependent and not readily reversible. Run-down was reduced by increasing intracellular Ca²⁺ buffering or by reducing Ca²⁺ influx but did not appear to be mediated by the same regulatory proteins that cause Ca²⁺-dependent run-down in hippocampal neurones. The NMDAR current run-down may be mediated in part by a Ca²⁺-independent mechanism, because intracellular dialysis with a dynamin-inhibitory peptide reduced run-down, suggesting a role for clathrin-mediated endocytosis in the regulation of the surface density of receptors. Synaptic NMDARs were also subject to current run-down during repeated low-frequency synaptic stimulation in a Ca²⁺-dependent but dynamin-independent manner. Thus, we report, for the first time, regulation of NMDARs in SNc dopaminergic neurones by changes in intracellular Ca²⁺ at both synaptic and extrasynaptic sites and provide evidence for activity-dependent changes in receptor trafficking. These mechanisms may contribute to intracellular Ca²⁺ homeostasis in dopaminergic neurones by limiting Ca²⁺ influx through the NMDAR.