Synaptic localization of ionotropic glutamate receptors in the rat substantia nigra

Glutamatergic neurotransmission in the substantia nigra pars compacta and p ars reticulata is mediated through N-methyl-D-aspartate and alpha -amino-3- hydroxy-5-methyl-4-isoxaline propionic acid/kainate (AMPA) type receptors a s well as other glutamate receptors and is critical for basal ganglia funct ioning. A major glutamatergic input to the substantia nigra originates in t he subthalamic nucleus, and the long-lasting stimulation of the dopaminergi c cells of the substantia nigra pars compacta by the subthalamic neurons ha s been implicated in the pathophysiology of Parkinson' a disease. The objec tives of the present study were to determine the subcellular and subsynapti c localization of subunits of the N-methyl-D-aspartate and AMPA receptors i n the substantia nigra, and also to determine whether co-localization of N- methyl-D-aspartate and AMPA receptor subunits occur at individual synapses. To achieve this, pre-embedding and post-embedding immunocytochemistry was applied to sections of substantia nigra using antibodies that recognize the NR1 and NR2A/B subunits of the N-methyl-D-aspartate receptor, and GluR2/3 subunits of the AMPA receptor. In both regions of the substantia nigra, immunolabelling for each of the su bunits was observed in numerous perikarya and proximal dendrites. At the su bcellular level, silver-intensified immunogold particles localizing N-methy l-D-aspartate and AMPA receptor subunits were most commonly present within dendrites where they were associated with a variety of intracellular organe lles and with the internal surface of the plasma membrane. Post-embedding i mmunogold labelling revealed immunoparticles labelling for NR1, NR2A/B and GluR2/3 to be enriched at asymmetric synaptic specializations, although a l arge proportion of asymmetric synapses were immunonegative. Double immunola belling revealed, in addition to single-labelled synapses, the colocalizati on of subunits of the N-methyl-D-aspartate receptor and subunits of the AMP A receptor at individual asymmetric synapses. Similarly, double immunolabel ling also revealed the co-localization of the NR1 and NR2A/B subunits of th e N-methyl-D-aspartate receptor at individual asymmetric synapses. Labellin g for NR1 and GluR2/3 was, on average, relatively evenly distributed across the width of the synapse with a gradual reduction towards the periphery wh en analysed in single sections. In summary, the present results demonstrate that AMPA and N-methyl-D-aspart ate receptors are selectively localized at a subpopulation of asymmetric sy napses in the substantia nigra pars compacta and reticulata and that the tw o receptor types, at least partially co-localize at individual synapses. It is concluded that glutamatergic transmission in the substantia nigra pars compacta and pars reticulata occurs primarily at asymmetric synapses and, a t least in part, is mediated by both N-methyl-D-aspartate and AMPA receptor s. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.