Three monoclonal antibodies have been raised against the ganglion cell layer of the adult mouse retina. The first antibody, R3, labeled optic axons in the inner retina, and with colchicine pretreatment somata and dendrites of large ganglion cells could be seen. A small number of other processes, including fibers projecting to the retina from elsewhere (efferent fibers), were also labeled in the inner retina. In the outer plexiform layer R3 stained the axonless class of horizontal cells. R3 recognized a 185,000- to 200,000-dalton polypeptide which is most probably the heaviest of the neurofilament subunits. Antibodies R4 and R5 labeled filamentous components mainly in glia and cells of mesenchymal origin. The antigens appeared in most but not quite all locations morphologically closely related to the intermediate filament protein vimentin. In the retina both antibodies labeled strongly the regularly spaced Müller glia. The astroglia of the optic fiber layer was stained with R5 but not R4. Although the two antigens were in general not expressed in neurons, they were both present in axonless horizontal cells in the outer plexiform layer, coexisting with neurofilaments in this neuron.

Cat primary visual cortex has been used as an immunogen to produce monoclonal antibodies that detect subpopulations of neurons. When tested by immunofluorescence on tissue sections of areas 17 and 18, 2 of these antibodies, VC1.1 and VC5.1, outlined a rare subpopulation of neurons located mainly in layer 4 but also in layers 5 and 6. Double-labeling immunofluorescence experiments in area 17 revealed that all VC1.1-reactive cells were also VC5.1-reactive and 83% of VC5.1-reactive cells were VC1.1-reactive, suggesting that the antibodies were reacting with the same subpopulation of cells. Both antibodies labeled similar or identical subpopulations of cells in other areas of the cat CNS, including the superior colliculus, parts of hippocampus, cerebellar deep nuclei, and rostral spinal cord. Neither antibody labeled cell bodies in the lateral geniculate nucleus. In the retina, VC1.1 labeled cell bodies and processes of some horizontal and amacrine cells, whereas VC5.1 labeled only ganglion cell axons. In the cerebellar cortex, the most prominent labeling of VC1.1 was of Purkinje cells, whereas that of VC5.1 was of Lugaro cells. Immunoblotting analyses of cat cortical homogenates demonstrated that VC1.1 recognized a major polypeptide band of Mr 95,000-105,000 and additional bands of Mr 145,000 and Mr 170,000. VC5.1 recognized bands of Mr 97,000 and Mr 150,000. Subcellular fractionation and extraction studies showed that the VC1.1 antigens were integral membrane proteins preferentially located in a synaptosomal plasma membrane fraction. The VC5.1 antigens were preferentially located in a soluble cytoplasmic or extracellular fraction. The results indicate that antibodies VC1.1 and VC5.1 recognize unique epitopes in the cat CNS and define a previously unrecognized subpopulation of cells in cat visual cortex.

The distribution of an epitope recognized by the monoclonal antibody JONES has been studied immunohistochemically in the developing nervous system of the rat. In the present report, we survey selected regions of the fetal, postnatal, and adult rat nervous system to test the hypothesis that JONES binding is invariably associated with neural cell migration and axon growth in the developing rat. A series of selected developmental stages extending from embryonic day (E) 9 to adult were used in this investigation. The distribution of JONES binding was examined using indirect immunofluorescence, as well as the immunogold procedure. Particular attention was paid to regions where the positions and timing of cell and axon migrations have been well described for the rat. JONES immunoreactivity first appears at E11-12, when it is localized to the lamina terminalis, the telencephalic-diencephalic junction, the midbrain, and the rhombic lip regions of the cytologically undifferentiated neural tube. In all the regions studied, during embryonic and early postnatal life, the labeling is very intense in the ventricular zone and shows a radial array in the adjacent intermediate and marginal zones. The expression of JONES epitope correlates particularly with times of cell migration in the retina, superior colliculus, cerebellum, and telencephalon and in regions undergoing neurite extension, such as the developing optic tract, the white matter of the cerebellum, the dorsal roots, the trigeminal system, and olfactory nerve. JONES binding becomes progressively restricted in the postnatal period. In the adult brain, immunoreactivity is present only in the retina and cerebellum. In the retina, JONES labeling is present in the outer plexiform layer and optic fiber layer. The labeling in the optic fiber layer extends to the optic nerve head and stops abruptly outside the orbit. In the cerebellum, JONES shows a radially oriented pattern throughout the molecular layer and delineates the cell bodies in the Purkinje cell layer. The only non-neural regions that show JONES immunoreactivity are the adrenal medulla and the kidney glomeruli. We conclude that the antigens recognized by the JONES monoclonal antibody are associated with the migration of subsets of cells and axons within the developing rat nervous system and, consequently, may play a role in conveying selectivity to these processes.

VC1.1 is a monoclonal antibody generated against cat area 17, which selectively outlines subsets of cortical neurons (Arimatsu et al., 1987). This study was conducted to determine the ultrastructural distribution of the VC1.1 antigen and to identify the particular subclasses of cortical neurons that were labeled. In the light microscope, VC1.1 delineated the surfaces of neurons located mainly in layer IV but also in other layers. The staining surrounded neuronal cell bodies and dendrites in a periodic or meshwork pattern but did not label axons. VC1.1-labeled neurons were morphologically heterogeneous and included multipolar, bipolar, and bitufted classes. In the electron microscope, VC1.1 immunoreactivity surrounded presynaptic membranes of terminal boutons and intersynaptic sections of postsynaptic membranes, but was not present within terminal boutons or synaptic clefts. Both asymmetric and symmetric synapses were immunoreactive. Labeling was also observed intracellularly on VC1.1-outlined neurons, associated with perisynaptic portions of plasma membranes. Tract-tracing methods were used in conjunction with immunocytochemistry to determine whether VC1.1 identified projection neurons, local circuit neurons, or a combination of both types. Layer V and VI corticogeniculate and corticotectal projection neurons were retrogradely labeled with rhodamine fluorescent latex microspheres. In a large sample of retrogradely labeled neurons, none were VC1.1-positive, suggesting that VC1.1 stained a population of local circuit neurons. Additional immunocytochemical double-labeling studies with an antiserum to GABA and VC1.1, revealed that VC1.1-positive neurons were immunoreactive to GABA. These were a major subset of the GABAergic neurons in area 17 and tended to have medium to large cell bodies. It is concluded that VC1.1 identifies a new, immunologically distinct subset of GABAergic neurons in area 17. The restricted distribution of this antigen on perisynaptic portions of GABA-containing cells and surrounding terminal boutons onto these cells suggests that this antigen may play an important role in inhibitory cortical circuits.

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