Subcellular distribution of AMPA and NMDA receptor subunit immunoreactivity in ventral posterior and reticular nuclei of rat and cat thalamus.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) selective glutamate receptors mediate excitatory neurotransmission in the somatosensory thalamus, but morphological localization of the receptors at identified thalamic synapses has been lacking. The authors used electron microscopic immunocytochemistry to localize AMPA selective GluR 2/3 subunits (GluR2/3) and NMDA receptor subunit 1 (NMDAR1) in rat and cat ventral posterior lateral nucleus (VPL) and in the associated sector of the reticular nucleus (RTN). Light microscopy showed that GluR2/3 and NMDAR1 immunolabeled neurons are homogeneously distributed in both nuclei. The relationship between glutamate receptor labeled profiles and glutamate or gamma-aminobutyric acid (GABA) labeled synapses was revealed by combining preembedding and postembedding immunostaining at the electron microscopic level. GluR2/3 and NMDAR1 immunoreactivity was located in somata and in proximal and distal dendrites of VPL relay cells and of RTN cells. Immunoreactivity was concentrated in postsynaptic densities of glutamatergic synapses and absent from postsynaptic densities of GABAergic synapses. In the cat, GluR2/3 and NMDAR1 immunoreactivity was also localized in GABAergic interneurons, including their presynaptic dendrites (PSD). Of the GluR2/3 and NMDAR1 labeled thalamic synapses observed, 10-29% were lemniscal (RL) type synapses in VPL; 60-70% were corticothalamic (RS) type synapses in the VPL and RTN. In the cat, 7-19% were identified as PSD profiles, and more NMDAR1 labeled PSD were found in the VPL than in the RTN. The main findings were as follows: 1) AMPA selective GluR2/3 and NMDAR1 share similar distribution patterns in the rat and cat somatosensory thalamus, 2) both glutamate receptors are likely to be colocalized at postsynaptic densities of both RL and RS synapses, and 3) localization of the glutamate receptor proteins in GABAergic dendrites in the cat thalamus indicates that glutamatergic transmission to GABAergic neurons is also mediated by both NMDA and non-NMDA receptors.