POSTEMBEDDING IMMUNOCYTOCHEMISTRY DEMONSTRATES DIRECTLY THAT BOTH RETINAL AND CORTICAL TERMINALS IN THE CAT SUPERIOR COLLICULUS ARE GLUTAMATE-IMMUNOREACTIVE

Although the excitatory neurotransmitter glutamate is known to be pres ent in the cat superior colliculus (SC), the types of synapses that co ntain glutamate have not been examined. We, therefore, studied the ult rastructure of synaptic profiles labeled by a glutamate antibody by us ing electron microscopic postembedding immunocytochemistry. In additio n, unilateral aspiration lesions of areas 17-18 were made at 5-28 days before death in order to determine whether degenerating terminals fro m visual cortex were glutamate immunoreactive (Glu-ir). Three types of axon terminal were glu-ir: 1) those containing large, round synaptic vesicles and pale mitochondria, characteristic of retinal terminals (R T profiles); 2) those containing small, round synaptic vesicles and da rk mitochondria (RSD profiles); and 3) those containing large, round s ynaptic vesicles and dark mitochondria (RLD profiles). Measures of mea n gold particle density revealed that RT, RSD, and RLD profiles had si milar average grain densities (11.3-12.7 particles/unit area). Other l abeled profile types included cell bodies, large-calibre dendrites, an d myelinated axons. Axon terminals containing flattened synaptic vesic les and vesicle-containing presynaptic dendrites, both of which contai n gamma-aminobutyric acid (GABA), had many fewer gold particles (3.6 a nd 4.8 mean particles/unit area, respectively). Following unilateral r emoval of visual cortex, normal RSD terminals were observed infrequent ly in the SC ipsilateral to the lesion. Synaptic terminals in the init ial stages of degeneration were heavily labeled by the glutamate antib ody, as were axon terminals and myelinated axons undergoing hypertroph ied or neurofilamentous degeneration. These results show that both maj or sensory afferents to the superficial layers of cat SC contain gluta mate-RT terminals from the retina and RSD terminals from visual cortex . The origin of RLD terminals is unknown. (C) 1996 Wiley-Liss, Inc.