An EM of the neuropil of an adult visual cortex, showing the coexistence of three antigens within a single dendrite. Triple EM-ICC was achieved by combining SIG to immunolabel NR2A subunits of NMDA-type glutamate receptors (circles), 30 nm colloidal gold-PEG for β-ARs (small arrows), and 10 nm colloidal gold-PEG for the inhibitoryneurotransmitter, GABA (arrowheads). This result indicates that a GABAergic inhibitory interneuron is receptive to noradrenaline as well as l-glutamate. The dendrite is also receptive to GABA, since it is forming a contact with two GABAergic terminals (T), one of which is associated with a morphologically identifiable synapse (open arrow points to the postsynaptic membrane). The localization of the two receptors away from the plasma membrane may be an indication that the receptors are in a desensitized state because of synaptic transmission that occurred during or prior to tissue fixation. Calibration bar = 500 nm.