MORPHOLOGIC ANALYSIS OF RAT RETINOCOLLICULAR NEURON TERMINALS CONTAINING MONOAMINE-OXIDASE

The retino-collicular neuron terminals containing type A monoamine oxi dase (MAO-A) in the stratum griseum superficiale of the rat superior c olliculus were analyzed to provide a morphologic basis for the physiol ogic role of these neurons in the visual pathway. A computer-assisted, three-dimensional reconstruction of the terminal complex associated w ith the MAO-A-positive terminals was performed. MAO-A-positive termina ls originated in the retina and terminated in the stratum griseum supe rficiale. This was confirmed by tract tracing and enucleation experime nts. The terminals were densely grouped in clusters of irregularly sha ped swellings. Electron microscopy revealed that the MAO-A-positive te rminals were located in a glomerulus-like structure. In this terminal complex, a significant proportion of the axonal profiles (42.96%) syna psed with the MAO-A-positive terminals. Most of the profiles (24.16%) resembled presynaptic dendrites, which represent intermediate elements between the retinal terminals and conventional dendrites. unlike the glomerulus in the dorsal lateral geniculate body, the MAO-A-positive t erminal swellings were not located in the central part of the terminal complex. The terminals had an irregular shape and were located in the complex, The terminal complex was partially ensheathed by glial proce sses. Furthermore, the membrane surfaces exhibiting synaptic specializ ations were very small compared with the total surface of the terminal swellings. The membrane length of the synaptic specialization was 5.3 8% of the total perimeter of the MAO-A-positive terminals.