Immunogold localization of AMPA and NMDA receptors in somatic sensory cortex of albino rat

We performed an electron microscopic study of S-l cortex by using postembed ding immunogold histochemistry to examine the subcellular distribution of a lpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors (asses sed with an antibody recognizing the glutamate receptor 2 and 3 [GluR2 and GluR3] subunits) and to compare this distribution with that of N-methyl-D-a spartate (NMDA) receptors (assessed with an antibody for the NR1 subunit). Both receptors were concentrated at active zones of asymmetric synapses, of ten directly apposed to presynaptic dense bodies. GluR2/3 showed a bias for long active zones, whereas short active zones expressed GluR2/3 at substan tially lower levels; in contrast, labeling for NR1 was independent of synap tic size. Particle counts suggested that synaptic labeling was Poisson dist ributed and implied that the majority of synapses express both receptors. Q uantitative analysis indicates that approximately one-half of synapses expr ess high levels of GluR2/3 and that the remainder express GluR2/3 at a much lower level. Approximately three-fourths of synapses express NR1 at a unif orm level; the remainder, which may lack NR1 completely, include synapses w ith especially large active zones. The present results suggest that the sma llest active zones may play a special role in synaptic plasticity. J. Comp. Neurol. 412:292-302, 1999. (C) 1999 Wiley-Liss, Inc.