title: Neurochemical comparison of synaptic arrangements of parvocellular, magnocellular, and koniocellular geniculate pathways in owl monkey (Aotus trivirgatus) visual cortex.
abstract: As in other primates, the lateral geniculate nucleus (LGN) of owl monkeys contains three anatomically and physiologically distinct relay cell classes, the magnocellular (M), parvocellular (P), and koniocellular (K) cells. M and P LGN cells send axons to the upper and lower tiers of layer IV, and K cells send axons to the cytochrome oxidase (CO) blobs of layer III and to layer I of primary visual cortex (V1). Our objective was to compare the synaptic arrangements made by these axon classes. M, P, and K axons were labeled in adult owl monkeys by means of injections of wheat germ agglutinin-horseradish peroxidase into the appropriate LGN layers. The neurochemical content of both pre- and postsynaptic profiles were identified by postembedding immunocytochemistry for gamma-aminobutyric acid (GABA) and glutamate. Our key finding is that the synaptic arrangements made by M, P, and K axons in owl monkey exhibit more similarities than differences. They are exclusively presynaptic, contain glutamate and form asymmetric synapses mainly with glutamate-positive dendritic spines. The majority of the remaining axons synapse with glutamatergic dendritic shafts. There are also differences between LGN pathways. M and P terminals are significantly larger and more likely to make multiple synapses than K axons, although M and P axons do not differ from each other in either of these characteristics. Of interest, a larger percentage of M and K axons than P axons make synapses with GABAergic dendritic shafts. Cells directly postsynaptic to M and K axons are known to exhibit orientation selectivity and, in some cases, direction selectivity. Cells postsynaptic to P axons do not show these properties, but instead tend to reflect their LGN inputs more faithfully; therefore, it is possible that these physiologic differences seen in the cortical cells postsynaptic to different LGN pathways reflect the differential involvement of inhibitory circuits.
authors.affiliation: Department of Cell and Developmental Biology, Vanderbilt University, Nashville, Tennessee 37232, USA.
authors.forename: Yuri
authors.initials: Y
authors.lastname: Shostak
authors.forename: Yuchuan
authors.initials: Y
authors.lastname: Ding
authors.forename: Vivien A
authors.initials: VA
authors.lastname: Casagrande
mesh_terms: D000818:Animals; D001029:Aotus trivirgatus; D001369:Axons / Q000737:chemistry*; D005829:Geniculate Bodies / Q000737:chemistry; D018698:Glutamic Acid / Q000032:analysis*; D007124:Immunoenzyme Techniques; D008854:Microscopy, Electron; D013569:Synapses / Q000737:chemistry* / Q000648:ultrastructure; D014793:Visual Cortex / Q000737:chemistry* / Q000648:ultrastructure; D014795:Visual Pathways / Q000737:chemistry* / Q000648:ultrastructure; D005680:gamma-Aminobutyric Acid / Q000032:analysis*
chemical_list: D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid